Inflationary cosmology on trial

okay so I'll just say again it's a beautiful sight it's a real pleasure to be here and it's great that you're all out here this evening so yeah I have some remarks prepared and if you want to interrupt at any time please go ahead and do so there's a lot of ideas here which are kind of subtle and so it's best if you ask questions as you go along what I'm going to be talking to tell you tonight as a tale of two universes or if I want to be more precise I'm going to be telling you two tales about one universe one of these is supposed to be the correct story and one of them can't be the correct story but we just don't know which ones which that's what the that's what the essence of the story is one of these is a theory you've probably heard about since you were in school it's called what's conventionally called the Big Bang Theory although nowadays we often call it the Big Bang inflationary theory and the other and that's a theory that's been developed since the 1920s and is the current champion it's the theory that most astronomers and most astrophysicists would be pointing to today as the leading theory and then I'm going to tell you about a alternative a radical alternative theory a second tale which is fairly recent it's only been developed in the last few years so it's a child of the 21st century uses a lot of new and recent ideas and at the moment both these theories are able to explain all the observations we have about the universe at present to the same degree of exquisite precision so we have a lot of data it's impressive that you can have even one theory that could match the data but I'm telling you is we have two theories which can match the data equally well yet these two theories can't be more different in terms of what they're telling us about how the universe began and what its future is going to be so we have two very different histories of the universe producing nearly the same outcome now it's not precisely the same outcome fortunately because otherwise you'd be this would be an ambiguity forever if we there are very fine differences which we can hope to measure in just the next few years that will allow us to sting which these two ideas we just haven't quite develop the technology to do it yet well I can't even say that we have a technology to do it some of these experiments are underway could wake up tomorrow morning and discover that the measurement pollution measurements been made but probably it's going to take a few two or three years to resolve it but what that means is that in the meantime you're kind of in a very interesting state in which the science isn't certain usually when you hear science stories with already decided what the winning idea is here we don't know what the winning idea is so you're capturing science an unusual stage where it could go either way you're seeing science and real action so theorists like me and my community pillai work with and the people we compete with in different ideas you know are all taking our risks and pooping in developing our ideas experimentalists and taking their risks and in trying to develop tests to distinguish these ideas and the end there we know it'll all be history we'll see what the answer turns out to be but at the moment you're seeing it you know really in action it's kind of fun to capture a science at that very special stage you really get that opportunity I think it's nice to have this discussion now as you see over the next few years these developments occur in the papers you'll know what it means why it's important why people are emphasizing it is really important to distinguish these ideas not just because it tells us something about the universe which is pretty important but you'll see that it really cuts to even more profound issues about what is the nature of science how much about the universe is ultimately knowable and how much can we you know how much are we limited from knowing certain things so it cuts to the the essence of of knowledge and so really this would make a difference what your philosophical outlook or religious view is there's no way this discussion this debate and the way it resolves itself can't have a profound influence so I'm going to begin by talking about the conventional theory we'll talk about that a bit and then we'll switch to the alternative tale of the universe so the conventional theories will be as I said what you've probably all learned in school as being the Big Bang Theory and it sort of represented this cartoon as being the idea that suddenly for reasons we don't explain suddenly there was a universe created that there was nothing before and by nothing I mean no matter no radiation no energy not even space or time and then suddenly there was something okay and that's the Big Bang and that's something an instant later look like spacefill for the hot full of a hot fire ball of matter radiation and it was stretching he was expanding and as space stretched the matter spread out and it cooled and a little over time it condensed into all the structure we observe today that's kind of the simple idea a very simple sounding idea that we think of as we think of as a Big Bang model so the first point I want to make about the Big Bang model is that the Big Bang model of 2011 isn't that model so when people say the Big Bang model that myelitis described definitely fails if that's all you say about it we have to fix the Big Bang model we have to add things to it to make it work and that's important when we're going to be judging two different theories you kind of want to keep track of how each Theory evolves and how it has to develop pieces to it because it's you know as it becomes so sometimes theories become way down by having to add more and more bits to them over time and that's part of what you want to include in your judgment factor so this model doesn't work and the reason why it doesn't work is because if the universe really begins in this sudden event that the matter in the there's there's there's no chance for the matter to arrange itself in a smooth pattern it's going to arrange itself in a very disordered pattern where it's very non-uniform over this space space itself in fact isn't going to be very uniform in Einstein's theory of general relativity space itself can stretch wiggle her warp and so space itself when it's produced in the sudden event is going to be very curved and warped and and rippled and and disordered so you end up with the most disordered possible universe coming out and then as the universe stretches if it if all it contains it is matter and radiation what happens is that more and more if you're an observer if you imagine yourself as an observer in at that initial time you get at first you see almost nothing of the universe and over time you see more and more of it so you just become more and more aware of how it's disordered in a non-uniform and curved and Mort yet one thing we know for sure from observations of the universe especially those of the lab ten years is that the universe is remarkably uniform and remarkably smooth in its geometry so somehow there has to be something to take the universe from this very disordered state to something - that would cause it to order and so for that we have to introduce an idea that comes right after the bang and which is known as inflation so the term inflation tells you almost already a little bit about what you're going to do to solve this problem inflation is a kind of super stretching of the universe a stretching of the universe had an extraordinary exponential rate much faster than the Big Bang regional Big Bang Theory would have ever told you to expect so if you only have a universe which has ordinary matter and radiation in it that matter radiation through itself gravity tends to resist the expansion of the universe and tend to slow it down whereas inflation is actually a speeding up of the expansion of the universe so inflation is a period when the universe goes through a accelerated expansion in which it doubles in size roughly once every billions billions billions billions of a second and it sustains it for a fairly short period okay maybe it doubles in size a hundred thousand times over maybe a million times over that only takes you know a few about a million billions of billions billions of a second so it's a very short period of time that we're modifying the Big Bang Theory in terms of time but in terms of stretching that means we've expanded the universe by two to the hundred thousand power or two to the millions power in each of three dimensions an extraordinary expansion more expansions happen in that brief instant that all the expense that happened between then and now according to this theory so since that time the universe has doubled in size in a roughly 80 times or something like that but during that time its orbit expand this that brief in that took 14 billion years but that brief time expanded a hundred thousand times or a million times and what does that do if you stretch it well imagine some incredibly wrinkled sheets curved sheet and you stretch it very very rapidly it's going to smooth it out and if there's any matter radiation that existed before it stretches it and spreads it out so much that is essentially irrelevant the universe returns to almost perfect vacuum the only thing left in the universe at the end of this stretching is the energy which drives the inflation itself so that energy can't be I said can't be matter and radiation has to be a special form of energy I'm not going to go into the issue the details the micro physics that explains what that energy can be I'm just going to call it inflationary energy and you can ask me about it later what what are the candidates for inflationary and gee we don't have a unique idea we have you know a number of possible ideas we imagine that some kind of inflationary energy which has the property that instead of being like matter and radiation resisting the expansion of the universe it actually promotes the expansion of the universe we know that's physically possible we have many candidates for doing it we assume that inflationary energy is this now that's not enough we not only have to assume in order to make the model work so we've already had to make one assumption the big bang assumption an unproven assumption now you have to make a second assumption that there's this inflation now we have to assume it's third thing its inflation is unstable we better not have this inflationary energy last forever or we won't be here okay because it'd only be this inflationary energy we're not made of it we made of ordinary matter so this inflation has to end and the way this inflationary energetic end is that the energy has to be unstable it has to be able to decay and went decayed it should decay into the matter radiation we know so there's a little bit like the energy stored in the uranium nucleus decaying into highly energetic particles something analogous can happen to this inflationary energy where it decays through a quantum process very much very very much the same physics the details are different but very much conceptually the same physics as a sort of quantum decay process where it can last for a long time long enough to cause this smoothing but at the end it eventually will decay just like if I had a block of your 80 reactive uranium the nuclei will eventually decay now if you think of that analogy we know that one thing about radioactive decay is that it's a and like all quantum physics it's inherently random process so I can't point to uranium nucleus and say exactly when that one will decay or that will multi K or that one that will decay I can measure average properties I can say on average half of them will decay in X amount of time that's what we call the half-life okay so you know if it's ten half-lives I can say it's you know to the tenth power of them will you know it'll bit they'll have you know ten times over at that time I can measure things about average properties but if you point to any one piece of uranium one atom rhenium atom or one group of them I can't say then those particular ones will decay quantum physics has that inherent randomness associated with it so that means when the inflationary energy decays it doesn't quite it's not like all of a sudden you have an else and you don't have it what's going to happen is certain parts will decay than other parts and other parts so it will sort of be little islands of decay that eventually will meld into finishing the process and because they decay in this slightly random way this slightly uneven way it will lead to a universe which isn't perfectly smooth but will have a spectrum a small you know a spectrum of hot spots and cold spots as you actually look carefully over the space since it's a quantum process it's a weak process those spots will you know the difference between the hot spots and cold spots might be small but they'll be there and they'll look something like oh so without them I should say without them the universe wouldn't be smooth if we didn't have this quantum decay process be perfectly smooth could get together at the same same time but actually would be too smooth because it was so smooth that you wouldn't have anything to make Galit you wouldn't have any in homogeneous on which to form galaxies so actually we'll be getting something like but we actually don't get this super smooth universe we get a universe which has these slight inhomogeneities in it with say you think about the red as being the slightly spots are slightly more dense or sloppy hotter than average and blue we're slinging slightly colder or slightly less dense and average you get you get some distribution which if which is nearly smooth but with these small little wrinkles in the distribution of matter and radiation that's what inflation predicts now the picture I'm showing you though I should emphasize is not the theoretical prediction could just as well could be but actually it's the physical measurement it's something we've actually measured about the universe this is actually a measurement of radiation from the very early universe when the universe was in its infancy only a few hundred thousand years old there was emitted then has been flowing through the universe and which preserves the imprint which according to the inflationary theory was imprinted first at even earlier on within the first second of the universe if you were to work out the mathematically the predictions of inflation they would produce a pattern which would be indistinguishable from this one up to the level we've been able to measure so that's really impressive this is something that inflation predicted that we not only smooth and flatten the universe out but left slight wrinkles in it and those wrinkles have the statistical properties that agree with what we actually observe I should say in case you're wondering the inflationary theory doesn't tell you where the hot spots and cold spots are it tells you what the distribution statistical distribution should be on average how many hot spots compared to cold spots you should have on average it could have many different possible renditions of that but this agrees a beautiful precision with that with that prediction so this is the reason why many it's important to keep in mind this is the reason why many astronomers astrophysicists and physicists believe that inflation is the right answer because nowadays it's smooth and flat in the universe but so far if you be able to measure so far it gets this pattern dead right - so far great precision we'll talk about later the works there are other satellites up now and other ground experiments now to even do better than this measurement that we have now which will further add tests to it but already this is impressive now the idea when inflation was first developed about and this is the 30th anniversary I should say at the publication of the first paper and inflation so the idea when they I do when the idea was first invented was that well if we assume something about the Big Bang and then we add this idea of inflation oK we've made added a number of assumptions we should be through we should be able to explain everything else about the history of the universe well we know that's not true there's at least one other major amendment we have to make to the theory we not only have to mend something about its past we have to meant something about its present and future and that is this a new thing which which we've called dart which isn't often known as dark energy so if you just took the Big Bang inflation then assume the inflation to clayed into ordinary matter and radiation it turns out it explains the next 9 billion years of the universe up to the formation of the Milky Way and even the earth ok but then it begins to fail because what that would give you is a universe which after inflation after it's finished this accelerated expansion after inflation has ended would be slowing down its expansion more and more but we've discovered by measuring light from distant galaxies and particularly from exploding stars known as supernovae and from other measurements including those micro those measurements I showed you of the microwave background we've discovered is actually the expansion of the universe has been speeding up for the last five billion years what that means is that most the stuff of the universe can't be matter as we know it or even dark matter or radiation or most of the forms of matter that we've discovered all the forms of matter we've discovered in the laboratory because those all have the property of being gravitationally self attractive and they slow the expansion of the universe it means we must have a situation that reminds us a lot of the inflationary phase where there's some form of energy which is causing it to speed up again and that's what we call dark energy now it's different than inflation in many ways first of all inflation was supposed to have occurred early and then stopped the stark energy began late and still going secondly inflation caused the expansion of the districts celebrated this rate of doubling in size you remember once every billions billions billions billions of a second the stark energy is a much weaker form of this accelerating energy so it's only causing the expansion of the universe to double in size once every 10 billion years or so now 10 billion years so the universe is 14 billion years old today so only 5 billion years into this period of accelerated expansion and so it's just beginning to note it's just barely noticeable effect but just to give you some impression if you were to come back in a trillion years that's roughly a hundred doublings if you spread out the matter matter the universe 100 doublings universe is almost entirely empty now it doesn't pull apart the galaxy our galaxy would stick together and stars my galaxies some of them will you know can live a trillion years or there'll be more generations of stars for a trillion years so there will still be a galaxy where we are we will have merged with Andromeda by that time we're in the process of fully merging with Andromeda at that time but everything beyond Andromeda will be gone because of this accelerated expansion so from our point of view if you're from a civilization of a civilization were first evolving from that point of view they would see themselves living in an island universe you know just an island of star is separated and nothing beyond it which is kind of ironic because back in the early 20th century that actually was the picture of the universe people had at that time was called the island universe theory and then it got later replaced by our modern idea of cosmology general relativity and the big bang theory and all that but if you were grew up at that later time in trillion years from now first evolved that later time you wouldn't have a clue that there had been billions of galaxies out there in the in the past in fact you'd have nothing to observe they could even tell if the universe was accelerating away from you now this dark energy has to be put in by hand into your model in order to make it fit the data those the data that forced us to add it it wasn't required by inflation it wasn't required by the Big Bang model it's only required you're putting it in by hand in order to fit the data so what you begin to get sees what's happening to the model is that as we learn more about the universe we're having to add more and more pieces to it so and the since they're not tightly related to one another the theory become is begin to become more of a patchwork quilt rather than a single coherent idea that we began with so it's important to bear that in mind a second thing to note about it is that once you've had this dark energy period Pleasant once you have this inflation dark energy it leads to a very different view of the universe this conceptual viewer philosophical view of the universe than the original Big Bang Theory the original Big Bang Theory you began with this fireball of stuff and it would condense into stuff into more and more interesting stuff stars and galaxies and life and things like that and become ever more complex going to the future so the universe was it's more likely way we think of the oven you know if you want to think about this way the evolution of life you know is going from simple to complex and just we expect it I'll go we want more and more complex in the future well in this new version of the Big Bang in the 2011 version of the Big Bang you can't really say that's the story the story is there's the Big Bang then there's inflation and what inflation does evacuate the universe of everything that existed before then there's the stars and galaxies that form after the inflationary energy decays and now we're headed back to vacuum again this the dark energy takes over so the universe as we know it is just like a brief in cosmological times it's sort of a brief interval of time when matter as we know it in stars as we know it exists before and after which the universe has complete vacuum and in the future it would just be vacuum forever this becomes a eternal wasteland yep yes cue me outside our field of view yeah so we're so um the lumpiness of matter can keep other matter in orbit around it so we're in orbit around so first of all we are held together the universe is expanding right now but when you're not expanding as far as I can tell and that's because you're being held together by your inter atomic forces and earth is not being stretched from the Sun because the Sun has a strong enough gravitational pull to resist this overall stretching of the universe so it's only when you take objects which are so far apart from one another that the stretching effect is stronger than the gret then they're you know sort of direct gravitational effect that they get stretched apart from one another so that's why I said Andromeda is already in orbit around us it's not run it's not going to escape our orbit or we not react xscape each other's orbit but things beyond Andromeda the gravitational force is weak enough that we'll lose those things so well the universe will look very very different yes they can't be there how could be dumb questions on this subject can't have it oh okay yeah okay so yeah so this is this is a you know we don't know how to represent artistically what we mean when we say the universe is stretching so this is supposed to this cartoon is only supposed to is a is just I don't know a way of visually representing the idea that you begin at a point at the top with almost no space and if you think about cutting this you know the amount of space undergoes a sudden growth you see as you go from the go from that very top let me go up here as you go yeah so here's the beginning and then there's sudden growth and then it's a more steady growth it's just trying to convey the idea that you have a bang that a rapid growth which is inflation and then it's it's increasing its steady rate but you notice that is begin to flare out again that supposed to represent the dark energy this is not to be taken literally at all it's all it's doing is just summarizing the story I told you other questions yeah yes an inflation airy in a single direction that in fact the universe is made up of not just the inflation but another process as well and these things are now coming together so so we're trying to so so it's an observed fact about the universe that it's there's no preferred direction so we look at that microwave that that globe I was showing you there's nothing that prefers one direction on that globe to another that's an important observational fact about the universe it was a speculation back in 1920 but now we can point to an observation that says it must be so an inflation has the property that it tends to convert a universe that was originally uneven into a universe which the it is an omnidirectional process it would tend overtime to drive the universe to look more and more isotropic or more and more the same in all directions so it's built into them it's automatic in the picture to construct an inflation they would only inflate in one direction that we actually don't I don't I don't how to do that that's not so easy to do so so one problem with this the one issue is its causes patchwork property and other properties it's kind of a different view of the universe but the thing I want to emphasize is which is important for comparing the theories is that the way the theory really works is more subtle than it's taught in most textbooks and so I want to tell you a little bit of truth about inflation and then because it's important to figure into our into our discussion so the way inflation is usually taught in most textbooks is that you begin with a universe which emerges from a Big Bang and looks something like a Jackson Pollock painting you know very disordered and things like that inflation comes along and makes it everywhere uniform and inflation ends creating a turning that patch and filling it with hot mattered radiation the scale isn't right the inflation is supposed to be you know a million times increase in the size of the square but I can't fit it on my powerpoint but very very uniform and then that uniform stuff is supposed to you know over 50 14 billion years become the stars and galaxies we see today that's the way the picture is usually come but the reality is more subtle than that the reality is that we begin with this Jackson Pollock painting this disordered beginning inflation does the stretching and when I said it ended it decayed I should have been more careful instead of decayed almost everywhere just like the uranium atoms would have if they wait ten half-lives or decay almost everywhere but not quite everywhere there's always be some little bit left over that hasn't decayed now for the uranium atoms I you know there's few enough of them I don't care but unfortunately in this case you have to care because the little regions left over which I'm representing so I'm representing the regions pictorially gray means it's still inflating so when it ended it mended almost everywhere and I don't mean to mean take it literally I just mean there's parts are called red a good fraction is decayed little parts left over that haven't behaved yet okay that is going to happen naturally in any kind of quantum process that always be something left behind that hasn't decayed yet but the problem here is at that part that hasn't decayed yet is still inflating so even if it was small to begin with it doesn't remain small within an instant it's most of the stuff of the universe most of space it inflates while the stuff that ended the red region I stopped inflating again the scale is all wrong here the gray stuff should be imbued as exponentially bigger than the red spot I just can't fit it on my transparency I might have my slide okay so now that means most of the universe is still inflating well a small patch has ended and begun to may be made in go on to form stars but the process doesn't end there because the inflation area is still unstable so it's going to try again so it's going to create another one of these patches another one and another one always there'll be some little patch left behind it's still inflating but that patch that's still inflate will always then take over the volume of the universe so pictorially the picture is really not that we ended inflation everywhere that we only invented it and they ended it in certain patches or bubbles I've drawn them as kind of bubbles here where it's full of full of hot matter radiation but with inflating stuff in between but again the scale is wrong the space between the bubbles should be exponentially huge and only a few the bubbles are very rare now this by itself isn't a problem of any sort because you might say well we would live inside one of those bubbles those bubbles are big enough to encompass everything we see and so we just made more than one bubble what's wrong with that as long as they're all the same well the problem is they're not all the same the problem is you for the same quantum reasons that are creating this whole bubbling effect the fact that quantum physics is inherently uncertain these bubbles are not going to all be have the same character this inflationary energy decay process can do all kinds of weird things to the bubble before it forms it some of them might be smooth and flat just the way you wanted them but some of them will not be some of them because of the random quantum fluctuations will end up being rather significantly curved and not flat or warped or they won't have that nice spot pattern that I showed you that agreed so well with the data they'll have something different maybe sharp features or unlike what we see in the in that picture at all and all the other predictions of the and all the other features that we thought of inflation is predicting all those will occur sometimes but other times not now I said sometimes inflation is eternal so these bubbles go on forever so some really means an infinite number of times an infinite number of times you'll get things that look like us an infinite number of times you won't get something look like us now that's a problem because now what do I mean that the theory predicts something if it produces anything in fact what will happen in this theory allowed to evolve over time is it anything that can physically conceivably happen will happen it will happen infinite number of times so inflation just doesn't won't let go this is this is that this is a serious problem for the theory what means that in such a universe which people now haven't come to call the multiverse okay you hear about a lot in the press these days you were except if you accept that idea accepting the idea that number one where we are is not a not in any sense be described as typical and in fact it's kind of random and there's nothing special about where we are and all the properties of our universe are not special at all compared to other places in the universe's there's no prediction or special or explanation to what we have it's just a random chance that's a big concession given where we were starting from so we began with a theory which we thought was simple natural and explained a lot we end up with a theory which actually isn't that simple it's kind of this patchwork of ideas it's not natural in the sense that the ideas don't fit together naturally after each one has to be put in and put in just so I haven't described how you put them in but mathematically what I mean is they have to be adjusted just in a very special way for which you have just have to assume that in order to make the model fit the data and then finally does it explain well maybe not if they flash and once it starts as eternal and anything can happen then you can't disprove the theory but you also can't prove such a theory since we only observe one patch of the universe yeah yes that's inflating space that's what that's a bit so inflationary energy only whatever energies do it hasn't yet decayed yet so it's still full that it's just stretching and being creating more space with more and more inflationary energy well nothing that you would recognize no atoms no molecules no electrons just this other form of energy which you're waiting to decay and only when it decays you end up getting quarks and electrons and photons and stuff we're familiar with so the Reds the bubbles or stuff that could potentially have stuff that we know but but the grey ones can't they just there's flow this inflationary energy well it's whatever energy it's what I said avoiding what this energy form is you know if you want to sometimes use a language of quantum could be due to quantum fields that are whose energy is dominating the universe there's various candidates for the form of energy it might be whatever it is that's driving the inflation that's what's there okay but nothing we'd recognize so the only potentially levels livable spots are the bubbles but I've tried to represent that even among those only some of them will be like what we are yes that's a good question actually it's the other way around the bubbles push out actually eat up more and more of the inflating space because if everything fellating space is unstable so you put stable stuff next to under stable stuff along the boundary the stable stuff is going to win and eat up more and more of the inflationary stuff the only problem is the inflationary regions are stretching faster than the bubbles can grow and eat up the inflationary phase so physically the bubbles are growing and occupying more space but the space between them is stretching so fast that it just can't keep up so yeah so this so obviously this process does not conserve inflationary energy because we're creating more and more space and we're in each bit of space has the same concentration of energy so there's must be more total energy so inflation is and is an energy generating it is inflationary energy generating process and a space generating process where is the energy coming from it's coming from gravity so then your question boils down to is there a limit to how much energy I can draw from a gravitational field and convert it into inflation or energy and the answer is no all other forms is something special about gravity which we all should have learned in school but never did I mean I try to teach it to my students now but it's a very basic fact which is all the other formats of energy we know have a bottom to them if you draw the energy down they eventually hit zero or some minimum where you can't go below it it's not true for gravity gravity is the unique form of energy which is bottomless and if you you know those who've taken physics before and are familiar with gravitational potential energy knows that the grand potential energy curve actually B goes to negative infinity as two objects are brought to notice close arbitrarily close to one another so if you can find some way of tapping that energy without continuously there's no end to how much you can do it an inflation happens to be a mechanism which does it automatically so we think of energy is being conserved it is conserved but if a sense you can think of gravitational energy can be arbitrarily negative so you can always create more negative energy and balance it with positive inflationary energy and there's no end to that your total would be conserved well as other energies are in some sense positive and so you can't draw them that you create an infinite amount of them okay so go back to where we were so that is the status of the Big Bang Theory as we know it today whoops sorry and what I want to emphasize is that we were led to this line of reasoning beginning with one idea that the idea of the Big Bang was a beginning Big Bang is a beginning then it would have to be a sudden event if it's a sudden event I had to have this inflation then I have to explain the later data idea the dark energy that was the line of arguments okay there's not we know there's no way out of this line of argument as long as you consider the Big Bang is the beginning so if you want to get out of it if you're not satisfied with it I think there's only one way to go and that is to go back and rethink that idea we don't know that the Big Bang is the beginning that was always a hypothesis that was never we don't know that's true from general relativity we don't know that it's true from understanding of quantum gravity it's just a hypothesis that if you take the universe and stretch it back and follow it back in time it'll eventually converge to a well if you believed Einstein's theory all the way back to that point that converged to a point and then somehow there would be a beginning but maybe it's not we don't know that we know that in fact Einstein's theory is incomplete it doesn't even include quantum physics so we know as you reach that point there's some point where quantum physics comes in but we don't know yet what we don't have yet a quantum theory of gravity and hands so we can't say for sure what happens at that point so it's reasonable if you don't like what happened if you're not satisfied with it to contemplate the alternative possibility the Big Bang is not the beginning and then see where that leads you and that is the line of reason that's eventually led to this alternative we want to talk about now which is this idea of a cyclic universe now the idea of a cyclic universe is certainly not the concept of a cyclic universe is certainly not new it's pretty ancient goes back in the east Heraclitus and there stoic philosophers and that's why Neeson and the West their colitis and their at least stop Risa pre-socratics of Lots pre-socratic philosophers that's what I meant to say or an Eastern philosophy Hindu hism is a very complex cyclic universe very detailed cyclic version of the universe so the idea of a cyclic universe has been out there along with the idea of a created universe or the idea of a static universe there's always been these kind of three competing ideas out there and in modern cosmology it's also not the first time people have discussed this idea shortly after general relativity was invented the first thing that Einstein invented was a static universe when Hubble discovered that the universe was expanding couldn't be static that idea had to be ditched two other ideas were born one is the one we known as the Big Bang universe the created universe and the other was a cyclic universe and people like Lemaitre who was one of the fathers of the Big Bang model and Einstein who was the father of general relativity and the static model themselves thought about this idea of a cyclic universe they called it an oscillatory universe and it was the idea you again probably learned in school of a universe which is expanding and stretching expanding and stretched and expanding and stretching so if there was enough matter in the universe to whose whose self gravity is enough to overcome the expansion of the universe it would the universe would kind of breathe it would expand to a certain point the matter would great cause it to wreak elapsed to a point and then it could bounce and happen over and over again so it produced a universe that looks something if you're looking at the size the universe we would sort of graph its size it would look something like this bouncing universe that we're showing over here now this idea has been abandoned and is dead several times over by the present time for various reasons the first reason why was killed was a sort of theoretical argument posed by this fellow it was trained as a chemist but became a cosmologists his name is Richard Tolman and although he liked the idea of a cyclic idea he saw that it would violate a fundamental law of thermodynamics which is the law the second law famous second law which says that entropy is always going in the universe so if you try to make a cyclic universe which is like this well what's happened what's actually happening between what does this represent this little bumps here well they represent the universe expanding creating stars and galaxies and then be collapsing again well when you create stars and galaxies those are their irreversible processes that increase the entropy or the amount of radiation or the amount number of degrees of freedom in the universe and when you bring the universe back together again you have all the entropy that existed from previous cycles but now whatever new stuff you just added so when you come back and bring things back together again and collapse it you have not the same entropy you had the cycle before not the same amount of radiation or temperature you had before you have additional stuff from what you just created and that'll get more additional stuff the next time more additional stuff the next time and that additional stuff affects how you bounce you don't bounce the same way if you have more concentration of entropy you actually change the bounces so they go like this instead they actually get longer and longer and longer going forward in time so the fact that you're we concentrating is a problem they go for more and for forward in time going backwards in time going forward in time that's not a problem just get longer and longer and longer but your goal was to try to get rid of this idea of a beginning of universe and that you fail to do because going back in time they converge to zero size in a finite time so you thought you got rid of the idea of a beginning of time but actually all you do is you push it back a few bounces actually so this was disappointing to people like Einstein who were motivated to think about didn't like the idea of the Big Bang being a beginning and we're looking for an alternative this said the alternative failed and nowadays we'd say this model also fails because it required that to be enough matter to close the universe to cause it to come back again we've now measured the amount of matter in the universe it's not enough not enough by at least a factor of three more like a factor of four furthermore we know the expansion the universe isn't slowing down it's actually speeding up so it's the opposite of what this theory predicts so this theory cannot be saved this theory is dead so if we talk about a cyclic universe it's going to have to be a new kind of cyclic universe and it's going to have to use some new ideas that get around these problems so in terms of outline form it's going to be a universe which has a bang I haven't called it a Big Bang it's not quite as big a bang in the sense that the Big Bang we say it usually means a point of infinite temperature and density this bang will have a large very large temperature and density but it will be finite so it'll be you know billion times hotter than the Sun but not infinitely hot at its hottest will go from the bang will immediately have expansion and cooling the formations and stars and galaxies but we don't have any inflation so we still have to explain why the universe is smooth and flat but it won't be through inflation the idea is to avoid inflation all together so it's really a very big turnip in this sense we'll have dark energy we need to have dark energy to explain the observations but actually you also need it for this theory it plays a key role in the theory if you try to make a universe which is cyclic let's think about the universe for a moment in the regional Big Bang picture you begin with it's hot fireball it condenses to form stars and galaxies if you're going to make something cyclic you somehow have to get back to a simple beginning again what do you do with all the stars and galaxies you just made we have to get rid of them well the best way to get rid of them is to stretch them apart from one another and turn the returned the universe to vacuum and dark energy is a very effective way of doing it it turns it universe almost everywhere to vacuum where it doesn't turn the universe to vacuum those regions might be spoiled we know we might not form galaxies there at the next time around but everywhere else where the universe is vacuum we're back to a simple pristine state where we can get back to a simple beginning again so dark energy isn't just required observation is actually required to make this theory work then it goes through a period of contraction it's not going to be contraction quite in the sense that I was describing before I'll describe it in a moment but it's a special period of contraction which even further in doubt to further smooth and flat in the universe although not quite so it'll leave slight inhomogeneities in the after it's done then the universe will crunch and rebound and bouncing to a bang and the process will again over and over again but the smoothing and flattening does not occur after the bang just after the banger inflation it actually occurs well in each cycle beginning now as the dark energy takes over and between now and the next Big Bang the Big Bang doesn't just happen once but it happens over and over again and it's what happens between now and the next crunch that sets up the structure for the universe to come so it's not just cyclic but the cyclic what's happening one cycle is actually affecting the next cycle which is affecting the next cycle etc so it's a very interesting more intricate kind of more subtle kind of cyclic model than even the ancient ideas one had before now to explain it and yet one more level of detail well I have to do one of two things one is I could introduce some mathematical equations and didn't think that was the hour to do that and the other is to introduce some ideas that will seem strange at first especially if you're hearing about them for the first time but then afterwards if you get used to them I think you'll see it provides a very natural geometric picture of what could be going on here so the ideas I want to introduce don't come from cosmology but come from more fundamental physics physics attempt to unify fundamental forces with gravity namely the theory known as string theory or a version of it known as M theory so okay that's that's a little jarring let me relax you a little bit into it and string theory what makes us think think that's a good idea well what makes us think that's a good idea is it has a very simple unifying principle to it we know that we in the laboratory observe there are many types of particles that exist in nature we have a catalog of pop-top some types of particles in string theory that can all be understood of actually because all actually being the very same entity a vibrating string if you had a super microscope they could have looked down two scales of 10 to the minus 30 centimeters you would see that what you thought was a point particle was really a little piece of vibrating string and what made different particles different from one another was the fact that they were vibrating with different tones that's a very pretty idea so just one byte entity could explain everything that we observe every particle we observe can be understood as a vibrating string of one sort or another including gravity itself and now it's a quantum string that means it's not allowed to vibrate any way you want there's certain rules quantum rules about any vibrates in certain ways but that's what gives you the discretely different kinds of particles you see so that's a really already a beginning idea powerful idea in fact it's the only theory which is a finite theory of quantum gravity we have at the presence our best hope of a of a unifying theory of gravity now to make this idea work one needs to introduce extra more than the dimensions we know in our everyday lives so the point is that to have the string vibrate in all the ways you need to explain the particles you see you have to assume not just three spatial dimensions but additional spatial dimensions now of course we don't we only see three dimensions we want experience three dimensions so we have to do something to hide those extra dimensions from if you can make the theory consistent with what we observe you have to do something to hide those extra dimensions what can we do to hide them well one thing you can do is you can take an extra dimension you can kind of close in a circle so some dimensions can be think of a soda straw it has one long dimension and one small dimension okay so if you were if you were a flea walking on the straw you see there's a two dimensional straw but if I show you the straw from a distance it looks one dimensional because you can't notice the extra dimension so it could be that some of the dimensions of space and that's the basic idea of string theory are wrapped up curled up in such a way that you just can't see them and there are six of those dimensions in EM theory this yet one other additional dimension that gets hidden in a different way it gets kind of sandwiched and that's kind of what I'm trying to show here so think of let's ignore those first six dimensions let's think of our three-dimensional world as being like a three dimensional world which I've drawn out here as a flat a piece of flat paper but it should be three dimensional and there's an extra dimension a fourth dimension of fourth spatial dimension which we can't reach out into along which on the other side there's another similar kind of world now identical world but similar in the sense that it has three spatial dimensions is details as particles might be different than ours the space the extra dimension only exists between these two walls or membranes in there these are called brains BR ane or brain worlds the laws of m-theory say that particles that we know happen to have the property they are stuck like flies on flypaper on just one end we're allowed to move in three dimensions but we just can't move in the fourth dimension we just just forbidden we just don't have that capability of moving in that direction similarly there's particles in the other end that only capable of moving on that end but there's space in between but no space beyond so the universe in terms this extra dimension is a sandwich the whole thing is the universe with our three dimensions which I'm kind of I should really draw them in three dimensions but I'm drawing it this way and the X fourth spatial dimension in between this theory ends up being able this version of string theory ends up being able to describe a lot of details about that we actually observe in the laboratory about elementary particles so although it seems kind of bizarre it actually unifies in a very powerful way lots and lots of different measurements and that's why people are interested in this idea okay this is also if this theory is correct then our theory of cosmology has to be explainable in terms of this picture so let's think about the Big Bang model explained in terms of this picture so the Big Bang model says you first have nothing that's what they're showing there on the Left far left nothing and then suddenly have to have the universe so the universe would mean you have nothing then suddenly there emerged these two brane worlds okay and they have to merge in the same place like a and then they something those so quantum fluctuation they suddenly appear and they move apart then they must contain the right kind of inflation air and energy to cause them to stretch at this inflating rate then that's inflation and energy decays it leaves little wrinkles in them the wrinkles end up at least on our side forming galaxies and stars we don't know what they form on the other side etc so that would be the Big Bang Theory expressed in the language of string theory now if you look at that sequence you might come to invent an alternative picture that would fit almost as well which is you know why not that why not instead of the universe suddenly beginning from nothing it's almost asking you inviting you to say well actually that that picture over here there's what I call the Big Bang might not be the beginning it might be that these two brain worlds existed before collided and then bounced apart again so what would that look like it would look like a collision of two objects and when two things collide they eat each other up they'd fill it with matter and radiation and then once they come apart again they begin to expand it would look just like a Big Bang so it leaves you the idea once you see this idea you can see geometrically there's two possibilities you began from nothing or a Big Bang as a collision that's a big difference because a collision means something's this before as well as during as well as after so the Big Bang instead of being beginning there now is exists a prehistory to the bang and as far as we can tell we can't tell which of these pictures is correct from anything we know from string theory at the present time they're both logical possibilities but the second possibility is very interesting because once you have that idea once well why would they collide well they could collide because they were thrown at one another although who does the throwing more likely they would be colliding cuz there's a weak force kind of spring-like force that's drawing them together but it's a weak spring-like force that's drawing them together why should they only collide once they can do it again and again and again and so without so you're pretty much led you're pretty soon led to a picture of the universe which looks something like this little cartoon so this doesn't represent everything in the theory the timing isn't right but it represents conceptually what we're talking about a collision it creats the universe with hot matter and radiation the hot met that once the hot matter radiation is produced it causes the universe to stretch us like in the Big Bang picture the universe becomes the darknet well hmm interesting our my computer broke this morning so we haven't uses backup which is not quite behaving the way I'd hoped it would that's I just need it behave differently this time and see if it continues to go I'm just going to shut itself off again if it is I'll try a different tool to say something a little bit different spread itself out again no it is okay well okay well we were supposed to show is that happened again and again and again okay so you could after imagine that it happens again and again and again so in fact what happens in this theory is that the spring-like force has with it associated with it an energy just like if I have a spring and I stretch it there's energy stored in the spring that energy acts just like dark energy so when the brains are apart and stretched there's dark energy they're held in that spring-like force there's also matter and radiation at first the matter of radiation is much more abundant and much stronger than this energy much greater in energy than it so the spring like spring just remains stretched but when the universe expands enough like at the present time the spring begin then this energy begins to dominate the universe causing the stretching of the brains to accelerate and become emptier and emptier which is just what we're seeing now but it doesn't last forever because it's associated with this force was just trying to bring them together and as the so when as it brings them together the spring is going to unwind and as it does so the energy has to be released it's being converted from connecticut from potential energy to kinetic energy just like in an ordinary spring so that in this picture the dark energy can't last forever because it's created by the force that's trying to draw these two worlds together so what happens is they color is converted to kinetic energy which when they collide gets converted partially into matter and radio nation which then gives us the matter of radiation we need to start the cycle again and then the spring-like force can continue with the help of gravity can continue to do this process forever cyclically whether it's literally forever or many many times or exponentially many times we don't know there's no limit no physical limit to how long it can go that we know of but this ku cycles could be going at least for a very very long time and during this process of contraction when the and during this process of contraction the universe gets smoother and smoother and flatter and flatter except so now I'll have you look at the thing again I'm sorry I can't ki I can't keep it going notice this is supposed to represent the moment in the future after the universities become empty and there's almost only these flat if you like whirls next to one another this is really for and now what happens as they move towards one another is you might notice the big the picture represents of the fact that they wrinkle up so once they stop stretching because the dark energy begins to the spring begins to unwind what happens as quantum fluctuations will begin to take over just like they did inflation and with the quantum fluctuations do is they cause the brains to wrinkle and it caused it because it causes them to wrinkle when they hit into one another they actually don't hit each other at exactly the same place at same time everywhere they're not hitting like that they actually you're hitting slightly unevenly so when they come apart again different regions have bounced before other regions so different regions are hotter than other regions afterwards because they've they bounced later and heated up later than other regions so you can now compute on the back of the envelope again like we did for inflation what expect what do we expect the distribution of radiation to look like after this collision and the answer is that it looks almost exactly like inflation and exactly like we observe even though the physics is completely different than inflation for completely different reasons it produces almost exactly the same outcome to the level we've measured indistinguishable so that's really interesting inflation decaying uneven bang produce the same outcome can't produce the same outcome yes we have a theory of the age of the universe yes yes if you can where you can calculate all those things so when we say we have a theory of the age of the universe what we really have now we have to rethink it we have a theory of how long it's been since the last bang so if there's many bangs all it's telling us is we're fourteen billion years since the last bang tells us nothing about what existed beforehand so if you figure the four reasons various reasons we estimate that the lifetime of the cycles is roughly a trillion years plus or minus a factor of 10 or something like that or a few factors of 10 but truly you know that would be a typical number then we're sort of early on into one cycle yeah and then it turns out this prey phase of contraction itself lasts roughly 10 billion years so it's very very slow inflation was very fast the whole thing lasted 10 the minus 30 seconds this is ultra slow it lasts about 10 billion years yes in the back good good it's a very important question because we said there was this problem in the original cyclic models the old oscillatory models that you had this entropy problem which has related to the fact of the matter and entropy we're all being brought together so again I'm sorry the movie didn't stay keep going but the idea is that during the phases of expansion the brains are stretching and they're emptying out and all the matter and radiation that was produced from previous cycles others be produced the previous cycle is being diluted the universe becomes almost empty vacuum anywhere the contraction though remember is not a contraction along our three dimensions it's a contraction along the extra dimension so we're taking explicit advantage of this idea of extra dimension say the contraction that's important here is the contraction of that extra dimension so in that two brands come together the diluted matter and radiation entropy is already still diluted there's no big concentration of it so you have the same situation of two empty brains colliding before as you did a cycle ago so the result is the same there's no reason why the cycles one should be you know to within small uncertainties exactly the same now if you think about that a bit that means that this is not really in some sense a cyclic model is to say if I'm following the sight the volume of the brains are constantly growing more and more entropy is being produced from cycle to cycle however we as observers don't observe the entire universe we want to observe a patch of it so the kinds of things we're able to measure things like the temperature the density the density of galaxies the density of black holes those will all be the same okay the number of black holes the number amount of matter will be different if you could see everything but since we only get to see a finite part of it we to us locally it seems like things are cyclic so it's seem cyclic even though in principle it's not quite cyclic yeah out there well the first thing we want to know is uh yeah so you're getting ahead of the game we first of all just want to ask the question when we look at that picture of that globe of the microwave background radiation the first question when asked is was that created after the bang was that created before the bang and that's the now once I known that question then your second question becomes meaningful which is how would I know there existed yet an earlier bang or a future bang but first of all we want to answer this first question we haven't even answer the first question yet observational II what doesn't so we don't the answer is obviously we haven't answered the question yet we don't have evidence yet yes rocky hmm so yeah so the answer is not stretching and made stretching is stretching you can create more stuff by stretching it so that's why I can answer that question is say imagine you begin with an infinite rubber sheets okay and it's and you line it like graph paper okay so there's little squares it's broken up into little squares and now imagine I stretch it such that each square grows by a factor of two now it asks the question where has it stretched into does may have any meaning it's just intrinsic it's just stretching right and if I ask well there was an infinite amount of space before how much is there now infinite okay so the answer is that those kinds of global questions if you like actually don't begin to make sense of this context it makes sense to say what's the density of things things have spread out you can talk about that but you can't and you can talk about space being created there's more space in some sense and there was before that's where the space that made up that's why the squares are bigger but it doesn't make sense to ask what about the totality yes so what what ah oh okay so hold I think huge but let me let me save that question for just a bit let me do talk about one other issue first which is how we just how we might distinguish these theories which I think you might want to know I promised you that this isn't just metaphysical discussion but we actually can do measurements that will decide which of these theories is correct so let me hit that and then we'll come back to your question so I said the two theories agree with everything we observe up to the present time about galaxies about microwave radiation etc but if we were to make finer scale measurements especially about this radiation in the early universe we would they were formed in the early universe it turns out we could distinguish these two theories that's where they begin to break apart in their predictions in particular one Porton one has to do with the existence of what are called gravitational waves gravitational waves not produced by the collisions of black holes where you probably heard about them but collision but produced by inflation itself or produced by the collisions between these brains themselves and these gravitational waves are ripples in space-time that if they were to move past you would kind of squeeze you in one direction stretch you another and then the next moment string squeeze you in that direction stretch you in the other that we kind of have this shearing effect as they move past you that's what the space does to them they're extraordinarily weak but they're not so strong enough that they would leave if there was they were powerful enough they could leave an imprint on the microwave background they leave a certain pattern the microwave background radiation in particular they would cause the background radiation to become polarized and this is kind of a cartoon to represent looking with Polaroid lenses at the microwave background of this radiation from the early universe you would find they would leave a certain polarization pattern of radiation if they were there in the sky now inflationary model predicts that there should be strong gravitational ways the cyclic model this collision between brains has the property that it produces almost zero strength gravitation ways so one of the key things you want to do is check whether these gravitational waves exist which is to say check whether or not the characteristic polarization pattern you would expect to exist in the microwave background is there or not so if you look at the sky with these microwave this radiation from the early universe with polarized glasses they should produce a kind of swirling pattern of radiation okay that we should be detectable that's what we're looking for so they're experiments in this in space Planck set called the Planck experiment in space which I think gave a picture here the Planck satellite experiment which was launched last year and which is begin to make measurements now which is looking to see if they can with polarized detectors detect whether this is gravitational wave imprint there or not if it sees it then the cyclic ideas are dead because they can't produce that kind of effect and if they don't see it then that adds support to the cyclic picture it kind of points us in that direction so this is an example of a key test there's a second test which I won't describe but there's a two different tests we can do and this is one of them so we don't have to wait around forever to decide which this is correct because between the Planck satellite and experiments being built on the ground on mountaintops and launched and being launched in BAE balloon and in Antarctica we hope to really learn the answer in just the next few years but the mean time we sort of caught up between we don't know which one is correct so let me just return to your question and say you know what's the difference between these or two ideas why do we care about these two different ideas well of course one reason is well we're interested in the history of the universe and we want to learn something and because which of these theories is correct tells us something about what happened in the Big Bang and the fundamental physics of gravity and funding and and the nature of quantum gravity whether it the Big Bang is necessary of beginning or not but more generally just stepping back you might say we're kind of just debating between two different views of the nature of time is time having a beginning and sort of go linearly forward or is time effectively cyclic I mean not mean things exactly repeat but in some sense average properties of the universe repeat and the universe and is the so the average properties the University repeating over and over again that's really what we're contrasting and what we've seen from this talk I think is that if this in this linear theory of the universe we end up with the universe with a number of interesting and properties first of all in such a theory because of this period of inflation that occurs at the beginning it wipes out all information about what existed before inflation our vision is in a certain sense limited by limited I mean if you want to know what happened to the Big Bang you've lost all that information all got inflated away so we're not answer able to answer directly observational E or experimentally anything about what happened at the Big Bang the future is in a certain sense bleak the universe in this picture is accelerating away and will become an empty wasteland within the trillion years and just continue to be more of that in the future the universe has this structure in which it's inherently random and where we are can't be argued to be typical is in some sense a typical and its properties are entirely random not under control not predictable and you might contrast that with the alternative theory we've developed which has the property that well your vision extends beyond the Big Bang what you're looking at when you see the same microwave background image this is that same globe just stretched out here on this oval when you're seeing this microwave background image in this interpretation you actually are seeing the Big Bang itself you're seeing the splats produced by the bang itself and the rip and the fact that it wasn't particularly even it owing to effects that occurred before the bang so you're actually able to study the bang and even study what happened before the bang what the conditions were like leading up to it so a completely different physical interpretation the future is hopeful if you regard it as hopeful in the sense that the patch of space where we are now won't be a term is not evolving into an eternal wasteland it's going to expand for a while it's going to empty out but then they'll be new matter and radiation created and the cycle from now the you know new galaxies and stars created so it isn't wasteful I mean the the the the inflationary picture and dark energy picture is in some sense very waste lose a lot of space which has is completely useless and in terms of making stars and galaxies here it's if you like very much more efficient and every bit of space is used over and over again so it's economical in the sense in the inflationary picture you have this eternal runaway and so different patches of the universe and a pet can end up you introduce an infinite number of patches and you end up producing patches with an infinite number of possibilities and the cyclic picture you don't have this runaway picture at all the universe sort of keeps itself more or less in sync and every bit of the universe that collides end up producing stars and galaxies and having the same physics as we observe so you end up reducing universe which is which is the properties we observe should be the properties you would see anywhere in the universe so we are able to learn something about the universe as a whole and the laws of physics are in some sense and the property physical properties are universal truly universal so they're two very different outlooks on the universe to different connect to different implications for fundamental physics in terms of what happens at physics of the Big Bang but we don't know at the present time which is correct so over the next two years we'll have you all of us will have fun thinking about these two different possibilities and debating them and deciding which ones you like better but which one you like better doesn't really count nature gets the nature gets to decide which one is the winner and we'll find the answer of that a few years as well yes so that question no did the Big Bang happen and things like that is something that really bothers astronomers and astrophysicists and like because when we ask that question we often mean two different things okay so that's one be careful that before I answer your question I know which one you meant by that's one of the careful distinguish them first there's first of all the idea of the universe was once hot and it's been expanding and cooling and space is stretching for that we have overwhelming evidence that it's true and we actually have enough overwhelming evidence to describe it in full detail with full observational evidence beginning from one second all the way up to the present so that idea is definitely the case and that is what many astrophysicists and astronomers say the Big Bang Theory that's what they mean now if you pin them and ask the question does that mean you're confident from your observations or whatever reason that the Big Bang was a beginning most of us say it's a common view but we don't necessarily don't know it to be true we have no proof to be true because in fact what's happening is we're extrapolating to points where we can't observational II test using this theory which at a certain point we know can't be properly extrapolated for too far back because you go too far back we know quantum effects become important and Einstein when he developed his general theory didn't know about quantum well wasn't developed in include quantum physics so we know the theory we're extrapolating isn't correct and we don't yet have a replacement and improved theory of gravity yet I told ascribe the one which is the leading alternative and it's not even developed enough to tell us for sure what happened so the answer is we don't know that part is not proven so we don't have the idea of beginning from nothing to something now you could ask the question is it based on laws of physics we present low conceivable that that could happen do we have even you know the Inklings of mathematics that would give us explain how that would happen the answer is yes people have developed that ideas so-called quantum cosmology where you could have sort of a sort of a decay from no space and matter and radiation to suddenly something you can go from nothing to something it's related to what I was telling you about gravity before you might think you normally think you can't go from something from nothing to something because it doesn't conserve something we have some idea in it but think about what I told you to settle for about gravity beginning with zero I can create positive matter and negative gravitational energy it still sums to zero but I have something if space and matter in it so there's nothing we know about the laws of physics that forbids us from doing this it's conceivable that we can do this it's mathematically conceivable physically conceivable based on what we know although it's stretching our it's pushing our quantum physics in a domain that we've we've ever seen space created we've seen you know you're waiting atoms decay or particles decay we've never we're using that same object the same ideas for space we've never seen it in that case we never seen the Big Bang but we don't know anything that blocks us from doing it so we're allowed you know we're allowed to imagine it and so part of the important issue you asked why it's important is it would if it would approve the inflationary theory correct well that would be kind of forcing us to say oh okay that's beginning idea must somehow be correct okay push us into that corner whereas otherwise you might not be pushed into that corner you might say oh okay maybe there is something yet to be discovered why that I did doesn't work so the experiments can help us answer that kind of interesting things we can measure something the polarization and answer something very deep in you you think is far beyond the reach of thought of science which to answer a question like that go back first not come to you Hammond go ahead mmm-hmm how's the next six it predicts the next sitcom I think well that's part of why I go to our book or why I tell the story because this is actually part of the fun of doing science people think of often especially young people they sometimes have the view that science is very remote robotic and you know you sort of follow you know that they don't understand the really human element of it the emotional element of it that it's a to develop a theory or to develop really this theory didn't come out of no way didn't like I sat down and said okay here's what's going to be it's gonna be all this it didn't develop that way at all developed in a sort of very unpredictable human process bit by bit and then suddenly began to see patterns and did some mathematical calculations and you got really excited about it then you got really disappointed because it looked like something was going to fail then you discovered no no I see a way out of it that's the way science really works and part of it is just your human natural human Drive and nature and emotions all built into it so it's important to tell the story of how we do our science and that's actually the reason why I thought was more important to tell the story before we know the answer rather than afterwards afterwards the competitions does it's like telling you what happened in the Vanderbilt game today you know describing what was like to be in the Vanderbilt game now after the game is over well it means a lot more human it's much more inspiring if you during the middle of the game you don't know the outcome that's when you want to know how people were feeling that after you know the outcome so so I don't know that's that's that's the best way I know how to go about it that I can yeah you can ask a question so no so so to answer questions about what precisely exists on the brains isn't going to be so much a statement about cosmology as a statement about the fundamental physics that told you there were the might be these brains they're string theory so what it's telling us so far as first there's no reason why they have to or should be the same the nature of particles are likely to be very different now the two worlds although we think of them as separate actually it's really better to think of as a sandwich they're actually always relating to one another in one way or another even if we can't reach out into the fourth dimension if you make a clump of stuff on one brain it gravitates and it's gravitation is felt everywhere including across to the other side so it make a clump of stuff on our side it's going to draw it matter to it if you pick a clump of stuff on the other side even though you can't see it you can't touch it it's going to grab bring stuff to it to it so in fact illogical is not required but logical possibility is what we call the dark matter the stuff that clusters it makes most of the matter of galaxies could be stuff on the other side that was produced in the in the collision along with the stuff we see and we can't touch and feel and see it because it's sitting over there and we're over here but when it gravitationally clusters it will it could lead the way to the clustering of the matter on our side drawing it to a cluster along with it those are logical possibilities so but we have much fewer constraints about what is on the dark side then that is on our side the visible side yeah so as the acceleration is happening now like you said our galaxy will stay together grams yes survival classes so can you describe what it would look like from say within our galaxy as we approach the next balance and sure so so the the structure of this universe where these two brains that we're living on one of them has the property that the laws of physics as we understand it are sensitive to the distance between brains so the strength of gravity the strength of the electromagnetic forces the strength of the weak force the nuclear forces are sensitive to it so we think of those forces being described by fundamental constants of nature but an EM theory they're not fundamental constants they're constants they depend in part among other things on the distance between brains they seem to us to be fundamental constants because we've been living through a period of the universe's history where these brains have been fixed at a fixed distance so we don't notice that they're dependent upon the distance between them but if those regains begin moving towards one another you'll the first effect you'll notice is the fundamental constants of nature which you thought were the fundamental constants of nature will begin to slowly change and then as the brains accelerate to each other more and more rapidly change so you will see you'll get more now when the change is slow you'll you'll first recognize it when the change is fast well even your instruments for measuring the change the things that make them operate involve fundamental laws so your instrument won't remaining it won't be remaining behaving the way it was after a certain point you will begin to especially near the end the last two instance that things will be changing so fast you'll have no idea how to physically interpret that in terms of you know energy conservation and things like that and then the last instant which is the collision then suddenly you know it'll seem like it'll seem like a lot of energy is being pumped interpret it but suddenly whatever is causing the concepts of change will be suddenly turned into matter and radiation whatever was causing that the universe will be become you know hot again evaporating if you were around then evaporating you and anything in new space and then when the brains went back to where they were again the kind of laws would return to the conditions we observe them today it could be different and so yeah so it's always it's easy to make them different actually this hard thing about is to make ones in which they would be same so but it turns out this model allows them to be the same if you can add things to the theory they would make them different but the points are not required to so the net the simplest outcome is that because the conditions of the collision from one cycle of the next are nearly the same the outcome after the collision including the laws should be nearly the same so the same thing that smooths and flattens the universe also ensures that one collision of the next is nearly the same now over many many cycles it could be some time something in some area where of space where things are violently different random quantum fluctuations some rare event would cause them to collide in some atypical way in those regions you might find space gets spoiled it doesn't collide it sticks together it forms a black hole and it never undergoes anymore collision in that region any longer but the meantime you produce lots of other space where things go on and they keep colliding over and over again reproducing the same conditions yes yes well in both there if string theory I was barring an idea from string theory which is motivated by things have to do with elementary particle physics fundamental forces and that's what forces you to introduce extra dimensions and then I'm using that idea okay which is I was saying so with a big bit so and there's two independent ideas here string theory and extra dimensions and cosmology one sort or the other so what I did is I use this idea of string theory and extra dimensions to explain both the Big Bang model in that language and the alternative so if that part turns out to be correct not to be correct you'd have to replace you know there'd be some other elements that would replace it in both cases and there would be a little bit harder to explain other examples in which you can accomplish the same thing using other using just say three spatial dimensions and other devices on this scalar fields you can do the same thing as I'm doing here just that doesn't have the nice geometrical interpretation using using this idea well yeah well you don't you yeah yeah so I guess that's a no the following analogy is so useful but imagine that you have you're standing on a rug which is not cut pile so it has the loops in it okay on top of it when you walk along it you think you're walking along a two-dimensional surface so you say I'm only observing I only live in a to space facials plus time continuum as long as I'm on that rug on the other hand if you were an ant walking on that rug you could walk around a loop or you could walk through loops so you would actually see it as three-dimensional so you only as a large being can't fit between the these two dimensions and no you can you go across between them you can't walk around the extra dimensions but the elementary particles inside you that compose you are in fact vibrating along those extra dimensions right now and electron wouldn't be an electron and a cork wouldn't be a cork unless that we're happening right at this moment they would have different physical properties if the universe only had in this picture three dimensions so you have to be small enough to fit into those extra dimensions to notice them otherwise you can't notice them I think we've entered a universe of endless questions so I think Paul will be here for a little while you sure I'm happy to come and talk to him with individual questions we do have someone from the bookstore who has a limited number of books if you're interested in getting copies of the endless universe and I'm sure Paul would be willing to scribble in for you you pick one up but thanks for a wonderful talk thank you thanks for all the great questions you

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Keywords: vanderbilt university, big bang, astronomy, universe, space

Id: IcxptIJS7kQ

Length: 83min 56sec (5036 seconds)

Published: Mon Apr 04 2011