Are we living in a multiverse? - Anthony Aguirre (SETI Talks)

[Music] [Music] okay so welcome along ladies and gentlement you're a weekly City seminar series today we're very fortunate to be joined by Anthony agua has come across to us from UC Santa Cruz Anthony got a BS in mathematics and physics from Brown University and then moved across to Harvard to do his PhD and then after Harvard was at the Institute of Advanced Studies at Princeton and then in 2003 came across to UC Santa Cruz where he has his current position Anthony is interested in the early universe and physics of the early universe and everything associated with that as such is also the co-founder and co-owner of an organization called the foundation for foundational questions Institute for a foundational questions which you can get at fqx org which is a great granting institution as well and Anthea is very interested in gravitational physics dark matter and the first stars heavy metal in the intergalactic medium and galaxy formation and of course black holes so today he's going to talk to us about eternal inflation and then cosmic collisions so if you'll join me in welcoming Anthony [Applause] Thanks I'm very grateful to you for the invitation to come up here and see the SETI Institute is my first time here it's a pretty impressive operation and nice to visit so so maybe we could start with the lights just a little bit lower so I always like to start controversal subjects with with things we can all agree on and this is one I think the universe is big in fact even for those of us who study it professionally it's kind of mind-boggling to imagine that it you know if we take this whole Sun and shrink it down to a grain of sand and the earth is just this mote of dust next to it that the nearest star is 10 miles away I mean to really try to visualize it and to think of the way that those tiny sand and so separated objects are arranged into something much bigger our Milky Way galaxy containing of order a hundred billion of those stars it gets to be truly baffling how to think about it and this Milky Way galaxy of course is just one of many with its hundred billion stars it's arranged in a small local group of dozens of objects and those local groups are then further agglomerate into a sort of distribution of hundreds of billions of galaxies of which we've mapped out millions with our largest galaxy surveys and those are sort of arranged as you can see in these complicated voids and filaments and sheets that we've managed to trace out now as we go back in time or sorry as we look farther away we're also going back in time and if we go back in time far enough by looking far enough away we come to an age of the universe where the universe was much smaller much more dense and where it actually was so dense that matter was a plasma and light didn't propagate so the time and the distance from which that light came to us as it travels to us it gets shifted into larger wavelengths it comes to us in microwave wavelengths but that surface from which that light came to a basically constitutes the boundary of the observable universe such as far as we can see and that's it that ball has mapped onto it the the picture of that early phase of the universe and the Microwave Background fluctuations that's the observable universe there it is and it's really big there's no doubt about it it's pretty colossal but you have to ask when you look at it this way is that really all that there is is all of creation everything just that new that's a new version of the pale blue dot right or could it be that there's something more and the fascinating thing that's happened over the past 20 years I think is that we've come to a place where we think that there very well may be more and that we have actually good reasons to think that there might be more and that's the story that I'd like to tell you today so as with a lot of good physics stories that starts with Albert Einstein and in the early part of the 20th century he of course invented general relativity which told us that gravity is not so much a force between things as an effect of the structure of space-time which is in turn determined by matter and this theory as well as explaining gravity and in a beautiful way allowed us to do something no gravity theory before it in particular Newton's could do would just talk about the whole universe at once this is the first theory that could be applied to cosmology and he immediately did so and he didn't do it quite in the right way at the beginning and there was kind of a history of working out you know Hubble discovered the cosmic expansion and and over time we came to assemble a cosmological model but from that cosmological model or from that theory we generated this cosmological model that the universe on very large scales is uniform that is although we see all these small fluctuations like galaxies and groups of galaxies on small scales when we take a sufficiently large view of things the universe is more or less uniform and it's expanding and this model which we call the Big Bang model had these wonderful triumphs it explained Hubble's results that the farther away a galaxy is the for the faster it appears to be moving away from us so this is explained in the sense that the if we imagine space-time itself is expanding and sort of carrying along the stuff that's in space-time with it then we see that in a fixed amount of time things move apart and they move apart farther say this one in this one move apart less far than these two depending on how far apart they originally are so over a fixed amount of time the far the things are apart the farther they move away and so they appear to be moving away from each other faster the farther away they are so is a beautiful explanation of Hubble's result and also showed that no matter which galaxy you're sitting on they all look like they're moving away from you it also explained in a beautiful way observations of the chemical content to the universe so at these early and when the universe was very small and hot it essentially constituted a big nuclear reactor and since we know nuclear physics quite well we can predict the outputs of that reactor which because of the high temperatures relative to the the density of stuff turned out to be mostly light elements like helium and lithium and deuterium and so on mostly hydrogen and helium and we can compare those predictions in detail to regions of the universe that we think are more or less pristine and leftover from from this early phase and haven't been polluted by stuff like stars and so on and it works out very nicely for a given value of what the what the universe's density was at some early times temperature it's on high say billion degree temperature it also predicted that from this very early hot phase there should be this leftover radiation just the heat of the early universe that once the universe becomes transparent to that radiation travels pretty much unimpeded to us and that radiation who is predicted would have a precise form the blackbody spectrum so this is the theoretical blackbody spectrum just plotting the the intensity of the radiation versus wavelength and this is not only the theoretical prediction this is also the observational data from the Coby satellite you can't see the actual observational points or error bars because they're too small to be seen past the theoretical curve it completely covers them over so this is a beautiful confirmation that this relic afterglow is there that this basic Big Bang model of the of the universe was right but this Cosmic Microwave Background has done even better than that it so as I mentioned this is a map just if you look in all directions you can think of it as the surface of the sphere so this is the variations in temperature that are about a part in a hundred thousand mapped on that sphere you probably many of you have seen pictures like this and what that tells us is what the variations in the density of the early universe was at the time when the light last left last was interacting with the matter now if we take those density fluctuations and use our understanding of gravity and how those fluctuations would evolve parts that are more dense will tend to to attract more stuff to them parts that are less dense will attract less stuff so those fluctuations will grow in their amplitude those are the things that can potentially lead into lead to formation of galaxies and those sheets of galaxies and so on and it turns out that it does a beautiful job in fact if we take this picture put it through the computer codes that evolve that sort of initial state forward in time you get just the sort of distribution of galaxies and clusters of galaxies and so on that we then observe in say galaxy redshift surveys and and optical data and so on so these tryouts really tell us that we've got a very clear picture of how the universe expanded for the last 13.7 billion years or so there are many interlocking pieces of data and theoretical understanding that fit together into what I think is safe to say is it is a consistent model that has really no serious conflicts with the data at this time there are things that we certainly do understand details of galaxy formation more small-scale questions like is there life elsewhere in the galaxy but we don't have large-scale problems with the basic Big Bang Theory now that being said there are a couple of extra ingredients that had to be thrown in to make things work there's Dark Matter you've heard of this stuff this is stuff that doesn't interact with ordinary matter except by gravity so we can see its gravitational effects but we can't detect it so far in any way this is strange and a little bit disturbing but not terribly disturbing because particle physicists have come up with dozens of actual you know fundamental particles that could be dark matter several of them actually quite natural and predicted on the basis of other considerations so for example the the so called lightest supersymmetric particle is something that comes naturally out of other theories supersymmetry and string theory with about the right mass this is something we might detect you know in the next couple years at the LHC so dark matter is weird but it's not terribly weird and there's a good reason to think that although we haven't found it yet we might well do so in the next five or ten years there's also something more disturbing dark energy so this is another component of the universe that presently takes up about 70% of the matter or the energy density of the universe it's weird stuff it causes anti-gravity it is stuff that Einstein actually first stuck into his cosmological model and and called his biggest mistake ever because he introduced this stuff to make the universe static when in fact the universe was expanding but it's come back we we see it we see the the universe accelerating and its expansion that's caused by some kind of stuff that we've called dark energy nobody knows what it is nobody knows its fundamental nature it's very disturbing but you know you could you could worry that we've now introduced these two things just to make all this stuff work out so we you know how can we really trust our theory if we had to stick in the you know ninety-five percent or something of the universe and this stuff though that we don't understand it's disturbing but at the same time the properties of these things are very simple dark matter is just a collisionless particle that gravitates dark energy is a smoothly distributed stuff that anti gravitates and that's it so once you assume those properties of that stuff then you can explain many many many different astronomical observations and and a kind of little representative picture of that is here where these I'm not going to explain all the symbols for those who aren't familiar with them the but basically this is plotting that the amount of dark energy the amount of geometric curvature of the universe which I haven't mentioned but is something that the universe could be and the amount of matter and you can use all kinds of different experiments to measure these three fundamental aspects of the universe and what's interesting is that all these measures are consistent with a particular value of these things here and this is just three sets of data there are actually many many more than you could plot on this diagram that didn't have to happen right it could be that there simply wasn't any simple explanation there wasn't any point where all those different sets of physics converged but they do another example is the amount of baryons the normal protons and neutrons that were made out of they constitute about 4% of the universe and there are about four or five completely different ways of measuring this using astronomical observations all of which give four or five percent so it's not that so although these things are weird I think it's true that we have a self-consistent and sort of beautifully cross-checked understanding of the universe up to this time that's about 13.7 billion years ago but even as far as back as the the early 70s or late seventies rather people understood that although we sort of understand how this initial fireball evolved into the universe we see now in great detail then they just kind of hoped that it would but they recognized even back then that there were some strange things about this model one for example is that if you just you know we sort of extrapolated back the universe was smaller and earlier times it was denser it was hotter etc if you keep doing that for another couple of minutes you come to a time when the universe would have had infinite density and infinite temperature and all the laws of physics break down 'la Metra who sort of first realized this back in the 1920s called that the primordial atom so he thought of it actually is an atom but nowadays we think of it as the Planck time when quantum gravity becomes important and the theories that we don't know are the ones that tell us what to do and so on so we have no idea what happened at that putative Big Bang singularity the primordial atom another question is why is the universe expanding at all I mean it's certainly allowed to be expanding or contracting it can't really sit there forever very well but you know just as you you know a ball if you stick it here will just sit there if you see a ball flying up it makes you want to know who threw the ball you know why is it flying up rather than just sitting on the ground or falling you know so what why is the universe expanding no no real answer from the Big Bang model another question is why is it so uniform but with with just the right density perturbations so you know if this is a picture of a sort of uniform sky say of the microwave background a very troubling thing is that since this primordial atom until the time at which this microwave background light first started propagating to us light could only have travelled a finite distance there was only a couple of hundred thousand years there and in that amount of time it can travel a distance that would take up a part of the sky it's actually smaller than these spots about a degree on the sky now if nothing can travel faster than light no information can travel faster than light then how did say this this region of the sky and that region the sky agreed to on what temperature did be to within a part in a hundred thousand they couldn't signal each other they just evolved from kind of the primordial atom forward yet they happen to have at this time a few minutes later the same temperature to a part in 100,000 how did they how did they do that more / even if you explain how they did that you know perfectly they didn't do it quite perfectly it's only a part to a to a pot in a hundred thousand and those little ripples that are left over quite vital those are responsible for our existence because those are what grew into galaxies and stars and planets and so on so how did it have just the right density fluctuations and then finally why is the universe so uncurbed so einstein's theory tells us that space time can be and is curved in the presence of gravitating stuff the universe on large scales could be curved it could be curved positively like a sphere or it could be flat like a sheet of paper or negatively curved kind of like a bowl why is it but what we measure is that the universe is uncurbed now why is that a mystery well it turns out that if you look at the equations for how that sort of curvature would evolve the universe tends to get more curved as time goes on so if this universe started out a little bit curved it would have very quickly essentially either diluted away into nothing or rika elapsed because it was either so positively or so negatively curved another way to look at it is if you just try to think what is the scale over which the universe is curved like the radius of the sphere or something the only natural scale that comes out of fundamental physics is the Planck scale which is so microscopic that that it's not even worth talking about here so why is the universe so uncurbed why is it so big there was no explanation for that either so these initial conditions the conditions the universe had at this very early stage were very strange in a lot of senses no in the in the late 70s and early 80s there was also an idea however those would nicely put together by Alan Guth this is one of the few pictures of him wearing a pink scarf that exponential expansion at very early times would take the universe that was kind of expanding at some level and then suddenly it expands it doubles in size say a hundred times or so from maybe about a billionth of the size of a proton for the observable universe up to about the size of a grapefruit so a grapefruit isn't that big for the observable universe but that's the that's how big it was at some point but inflation took it from this vastly tinier size expanded by a factor of a billion billion billion up to the size of a grapefruit so why is that so important what does that do well whatever was driving that exponential expansion was something like an antigravity for something very much like in fact the dark energy that we're seeing now and whatever that is that anti-gravity force that's the thing that actually gave us the expansion so that's the thing that threw the ball up in the air that's the force to move the ball up in the air that's what put the bang in the big bang so it explains naturally why the universe is expanding it also explains the flatness in exactly the way picture here if you start with something that's curved like a sphere and you make it really big then relative to some fixed size scale like human say then suddenly it looks flat just like we can't see that the earth is particularly curved because we're small compared to it if the universe got really big that tends to drive the curvature that you actually measured down it also explained the uniformity in the sense that they could take one of these little regions that had an opportunity to agree on what temperature that it should be and took one of those and basically blew it up to some big size so that it could cover our whole sky so it gives the opportunity for a causally connected region that can actually agree on its temperature to become big and explain why everything looks the same and finally perhaps most elegantly it it has a natural explanation built into it for where the fluctuations in those microwave background temperatures and and density fluctuations came from and those fluctuations apart in ten to the five ones there first was first seen by Kobe the the same satellite that measured the spectrum and it looked like this it was kind of this patchy stuff it looks a lot like noise which in some sense it is but it but it's a real signal from the early universe and when people computed you could do compute sort of Statistics out of this like what is the amplitude of these fluctuations on a given over a given size on the sky see the these are kind of big patches of several degrees wide follow-up experiments to Kobe did the same thing on kind of smaller angular scales on the sky so when I came into grad school around 1995 this was the state of affairs this is plotting smaller angular scales to the right and the power the amplitude of those variations is the vertical axis and they're all these points clearly these things were being detected but what exact shape they would have what what this would look like was pretty unclear from this plot at least the theorists meanwhile the ones who were making predictions from inflation models were drawing a picture that looked like this all these bumps and Wiggles coming from the physics sort of the physics of inflation and also the physics of how the universe kind of had those problems those perturbations evolve in the universe later on depending on how much matter and curvature and stuff there is but there were plots with all these bumps and Wiggles there's called acoustic peaks in the parlance of this game and when I was in grad school I was thinking there is no way we're going to see all these I mean they're never gonna see all these bumps and Wiggles you know it's going to something else is gonna go on it would just be unbelievable that these cosmologists would know what they were doing that well now here we are some years later and those are the data points that we've now observed so some of these are the W map satellite seven-year data that was launched in 2003 there's some terrestrial experiments at smaller scales but all the bumps and Wiggles are there they're beautifully there and they're there at the level that we can then use this also to constrain information about the the contents of the universe as I showed on that earlier slide that it's 70 percent dark energy and 30 percent dark matter and so on those are just a few numbers but as you can see there are hundreds of these points that are fit beautifully by this curve so it's very you would never be able to fit this just by twiddling numbers alone the theory really is telling you something and it seems to be right so this inflation theory has not only explained a number of things that were puzzling about the big bang theory but it's also made predictions genuine predictions that have come true and there are further predictions that that have sort of started to come true with that W map data that may be verified by the Planck satellite which is now flying and taking data and we'll see how it turns out but it's at the point where the great majority of cosmologists now think something like inflation happened in the very early universe before the kind of early fireball now back to the subject of this talk so the universe got to the size of a grapefruit or something that doesn't seem so big how big exactly could the universe be well if you think about exponential expansion it it grows small things big very quickly and we can ask how much universe could inflation have created and the answer there is just that it depends on exactly how long inflation happened for we actually know that if inflation is the explanation of those Microwave Background fluctuations that inflation had to create a bigger universe than the universe we see so if it just created the universe we see there would be sort of these anomalies on large scales that we don't see and we can actually compute that our observable universe has to be about a millionth of the actual universe at best so inflation had to create at least about a billion I'm sorry a million similar copies to our observable volume that that blue ball but that's just a minimum we can ask how much could it have been and that depends on how long inflation lasts for so let's think about how inflation how do we end inflation well inflation probably is driven by something like vacuum energy that repulsive force but it can't just be a fixed force like the cosmological constant Einstein came up with or that we may be observing now because an inflation never would have ended anywhere it has to be something that can change so that it can let inflation stop and let regular cosmological evolution begin so if we think of sort of a number that throughout space and time we can think of that is a field that's a field is just a number that assigned something that assigns a number to each space-time point and we can think of that in each space time point there's a field value and there's a value of the dark energy associated with that now it's really nice about this is that the physics of such a thing is very simple as it turns out if we draw the dark energy the sorry the vacuum energy is a function of the field value it can make some curve and at any point the evolution of the field looks exactly like the physics of a ball on a slope there's gravity force pulling it towards lower vacuum energy values there turns out to be a friction force so we can imagine it gravity with with air or something sitting around and so the evolution that we would expect out of picture like this is the universe at that point the field is up here there's lots of vacuum energy the universes inflating it slowly rolls down here then it maybe oscillates back and forth giving up some of this energy to friction and eventually stops down here where there's very little vacuum energy and regular cosmological evolution can happen so this is a kind of simple model of how inflation would start go on for a while and end and how it how long it goes on just depends on exactly the physics of what a ball does on this slope but now we can let this we can think a little bit more about what kind of slopes it might be might actually be describing the physics of the this inflationary field one slightly simpler and scape we can draw is where there's the slope down here but there's also this little region up here where where a ball could get stuck now in classical physics if you put a ball here you can just sit for as long as you want nothing's gonna happen to that ball it's just gonna sit there but quantum mechanics tells us that things that seemed classically impossible actually aren't to say the ball stays there you have to say it has a certain amount of energy but you can't know the energy exactly so the energy might fluctuate a little bit or another way of saying it is that the ball might tunnel through this barrier just like a radioactive atom has a has a particle that tunnels through a barrier an energetic barrier and flies off so in this case you could have a tunneling event and what that turns out to actually look like when you analyze the physics of it is that there's a little bubble that forms so the universe is inflating with this value vacuum energy a little bubble forms of a lower vacuum energy that bubble grows and inside that bubble the field can then relax down to where inflation gives way to regular evolution so that's an interesting other way that inflation can end but but this has a twist because if you take some region that's at the high the inflating vacuum energy it's growing exponentially so even though there's bubble forms inside it and expands as it turns out at the speed of light as that bubble expands the background that it's in is expanding even faster so even if that bubble keeps growing and another bubble forms maybe the background keeps growing even faster so it turns out that by this process these bubbles form and they keep forming and keep growing but because the background region is expanding exponentially the new phase can never take over so as long as this process goes on spawning these little bubbles actually that get quite big the inflation keeps going on it can never be completely killed off everywhere so this cool this process that this sort of analysis gives rise to has been called everlasting or eternal inflation and and it leads us to this picture where there's this expanding sea of inflation and these pockets pocket universes sometimes people call them or or bubbles or bubble universes they they form and they fill in the gaps but then and they grow but then the background grows even faster so the whole thing just keeps going forever so here's a kind of visualization of what that might look like these bubbles keep forming but the whole thing keeps expanding so you can see that I can just keep adding you know infinitely many bubbles in here now if you wait long enough one of those bubbles might just overtake you and you might find yourself inside it then depending on what that that potential that landscape kind of looked like there might be other bubbles forming inside there that it would have lower vacuum energy you can see that they're expanding a little bit more slowly because the expansion is actually related to the vacuum energy so the the global picture of the universe would look like that and our observable universe would just be contained inside one of these little bubbles now it might seem a little bit artificial to draw you know this particular bumpy thing that I drew but as it turns out there are other ways that you can get exactly the same behavior another one is called topological eternal inflation and it basically says that if you start some region of the universe here you know part of the universe might go this way and part of it might go that way but because this field has to be continuous there has to be part of the universe that's always up here that is you can't have a you know you can't have that side of the room be 10 degrees and that side of the room be 20 degrees without there being some part of the room that's 15 degrees and if 15 degrees is the sweet spot for having inflation it means that no matter what the room does you're always going to have part of the room inflating no matter what so that's another way that eternal inflation can happen there's a third way which is that even if you have just a simple picture like this the same quantum mechanics that gives you that tunneling process also gives you little jitters in the classical physics so as long with rolling it's actually kind of jittering up and down those jitters can actually be sufficient that the ball on average or no matter how many regions make it to the bottom there will always be regions where the jitters are sufficient to keep the ball up the hill far enough that inflation keeps going this is called stochastic eternal inflation because it's driven by this kind of random process so the bottom line is sort of that that no matter what you draw you know if it's the balls at a minimum or a maximum or just on a slope as long as there's some region of the potential this inflationary potential where that is fairly flat this eternal inflation process is going to happen so although people invent an inflation to solve a particular set of problems and make a particular set of predictions it has this weird side effect it's kind of a genie that you let out of the bottle that takes everything over this fairly generic side effect is that the observable universe is not everything that there is and that the universe keeps going on and it's very interesting and complicated way beyond that so the big picture of the inflationary multiverse is not just that there are you know kind of millions of regions the same size as our universe but that there may be infinitely many actually infinitely infinitely many in a sense because there will be infinitely many sort of times in this global picture at which these bubbles could form and each time their enormous Li or infinitely many of these bubble or pocket universes that form because the universe is huge at every time so it forms many of these and now here's the real kicker each one of these bubbles is infinite inside now this seems really strange right like like what are the shipping costs how can something that starts out as a bubble the infinite inside so this is weird enough and fun enough that it's that it's worth just taking a few minutes to to try to explain how that is it it has to do in a fundamental way with Einstein's theory of relativity and kind of taking it to its real logical limit so in Newton's theory of space and time when we say something's happening now say me snapping my fingers and a supernova going off halfway across the galaxy that has an absolute meaning when I snap my fingers either the supernovas going off or not it's one of the other and we call everything that's happening at the same time you know one time we label it you know now or five minutes ago or whatever and so there's a separation in space and then this this these surfaces where we say everything is at the same time just kind of March on up the screen but Einstein of his special theory of relativity told us that this is not correct that depending on our state of motion what we call simultaneous will change so in fact if I'm here snapping my fingers and there's a supernova near the center of the galaxy it turns out that just by my walking at a few a meter per second or so the frame that I define in my motion means that that supernova may have gone off an hour ago or an hour earlier so I really can't say whether the supernovas going off now or an hour ago or an hour earlier you know an hour ago hour earlier it totally depends on the frame of reference so there's no absolute meaning to saying that something is happening now okay so this is strange but but kind of abstract and we don't let it bother us because on a day-to-day level you know things we don't notice this because signals travel so quickly on in our experience that we don't notice anything like this now Einstein's general relativity takes it one step further and it says things are even more vague you can basically call any curve now as long as it doesn't have the property that it can send a signal to itself so as long as so these are little light cones these are the regions where this point can communicate with everything in here and can see get signals from everything out here but nothing else so this curve is a valid definition of something that's happening now because nothing that's happening now can influence something else that's happening now that's kind of what we mean by now in general relativity now this means that we have a lot of latitude actually in saying what the universe looks like right now so now here's a here's a strange thing we can do so here's a regular flat space here it's you know one time the next time the next time the next time is Newton would have liked to draw it now let's draw another weird way of drawing those equal time surfaces I'm drawing them like these hyperbolas and in more dimensions these would actually these would be hyperboloids these have the interesting property that they are valid equal time surfaces that is no two points on one of these hyperbolas can communicate with each other so they're okay I can say this is time one this is time two this is time three they also have the strange property that they are all nestled inside this one curve here the dotted line that's actually the light cone the future light cone of this point that means that if I were at this point I could send a signal to any point on any one of these curves the third interesting thing is that see if this light cone can keep going forever so can these okay this curve can go up as far as I want so now look what interesting thing has happened we have something that when we define time one way looks like just this little segment or and more dimensions would actually look like a little sphere a little ball if you include the interior that's expanding so as we go up this axis the ball goes bigger but inside there's a perfectly valid description where this at this time the universe is infinite at this time the universe is infinite and at this time the universe is infinite so just by the way you slice up space and time into space and time separately which I ensign tells you you have all the freedom in the world to do you can tell you can say that the universe is either finite and growing or infinite at all times and this is not just a mathematical curiosity that I've drawn here this is it turns out is exactly the structure of one of these inflationary bubbles it starts out at some size it grows but inside if you look at the surfaces where the universe has the same density those are exactly these hyperboloids that are always infinite and so the evolution inside looks like the universe starts out infinite and uniform at a later time it's infinite in uniform with a little bit lower density and then a lower density and so on and that's just exactly what an expanding homogeneous infinite universe looks like so each of these bubbles so this is the same picture in one more dimension so you can kind of see how it's a circle here each one of these bubbles is exactly of this structure it has this wall and we see the expansion of that wall is this kind of growing sphere but it's tracing out this structure inside of which are these hyperbolas nestled so connecting back to this the kind of big picture so looking at a particular to one of those the first definition of time where things look like the sphere this it looks like a bunch of these spheres each of which is expanding right if we look in this other view of time this space time view then this is just the bottom of one of these things which later on grows and forms galaxies and so on so this is a space-time view in a fixed time view of how these bubbles form and grow and inside each of them is it looks finite here but that's due to graphical design and considerations each one of these is in fact infinite so we've got this triply infinite universe even if we so the that inflation has led us to think about we can we can think about how we might test that but let's think about some of the implications first what does this mean well it tells us that our that that blue dot are enormous observable universe is really a kind of bit part in this huge infinite eternal drama in particular our observable universe sits inside one of these bubbles that is infinite and has everywhere similar properties so there are infinitely many copies of something that looks pretty much like our observable universe but that there are other regions other bubbles perhaps that might have profoundly different properties now we can think we can look at each of those in turn and they have their own implications so first let's think about some of the weird implications of this one that they're infinitely many regions out there that are kind of like our observable universe well each of the here's an interesting thing each of those regions we'll call it a sub regions properties they're set by physics and inflation that kind of created it and some random component that's the quantum mechanics that led to those fluctuations in the microwave background and other stuff now we also physics also tells us that there's every reason to believe that if we take a amount of stuff and a finite volume there are only a finite number of different configurations it can have their finite number of ways of arranging say the atoms or whatever particles it's the stuff is made out of so this finite possible set of configuration some region can be it can have so if you just imagine you know picking universes or these sub regions you know here's one version here's another version here's another version you keep going eventually you're going to come to one of the ones that you already had there's going to be duplicates so any given realization any given configuration will be repeated elsewhere if they're infinitely many of these things so good news for SETI is that there is intelligent life as we know it elsewhere right it's out there in fact exactly as we know it out there somewhere is a copy of this lecture happening right now or one you know every variation I could be wearing a pink bow tie and one of them sporting a nice unicorn horn and another one maybe a little less rare version etc and there are all kinds of other strange roads you can go down there they're all possible variations on our world in general so there could be somewhere you know the Nazis won the war or where the Mongols won the war and and all sorts of interesting things will be happening some of your lives you'll be there too in various copies some of them will be faring better than others so some of them you might be happier some of them in that be sadder then again there's some of us where no matter where we are you know our happiness or sadness is kind of determined if the if they're the same as us so we might as well just at least try to make the best of the universe we're in but so you can go down all kinds of strange through the looking-glass kinds of thought process he's thinking about these I'll let you do that on your own but we can also ask you know aside from the kind of personal level of duplicates of us what what about the second implication that the regions other regions of the universe might be quite different that is the fact that all of these pockets are bubble universes need not be exactly the same and this can happen in a few ways so one way for example is you know suppose the the simple you know potential I drew before is a little bit more complicated so say from if you're inflating here you might be able to form bubbles like this or like this so then you just have two different kinds of bubbles they might have different histories of inflation this might have a long period of inflation and then a nice hospitable universe this one would just keep on inflating at this at high energy for a long time until it maybe decay to this one over here so there can be lots of different sorts of bubbles there can also be fields the the acción is a kind of dark matter and and it's pretty easy to write down models where this accion field that determines how much dark matter there is varies from place to place even within a bubble or across bubbles so you could have say different dark matter amounts in different bubbles even with very very much the same physics behind all of them but it can go even farther than that and this takes us to another interesting part of the story which is about an old dream that is of unifying the fundamental forces through geometry so after Einstein developed general relativity he liked it but he immediately thought well if gravity's due to geometry maybe some of the other forces are due to geometry too so he spent a long time trying to understand if electromagnetism and then maybe even the quantum mechanics could be made part of geometry and he was enormous Lee encouraged by something that mathematicians Colusa and climb did which was that they found you know we live in this four-dimensional world three spatial in one time dimension but suppose we added an extra dimension it was a five dimensional world that seems totally crazy because you know where is it but we can hide that fifth dimension by making it small in a particular sense so so this is an illustration of that where there's a tightrope walker for the tightrope walker there's basically two directions he can go on the tightrope forward or backward so that's a one-dimensional system but for the aunt there's two directions while two dimensions this way and that way so to the aunt who's small it's a two dimensional world to the tightrope walker it's a one dimensional world so as long as the extra dimension is wrapped up in a way that is very small compared to Howard terrestrial experience we won't notice it's there we can't get around in that extra dimension now the really really neat thing that they found was that if you took a five dimensional curve time curved space-time with one dimension curled up small into a little circle and just worked out what the physics of it was it turned out that it's exactly four dimensional curved space-time that's Einstein's gravity plus an electromagnetic field voila gravity and electromagnetism were unified so this was really neat Einstein loved it it led to a sort of interesting physics where there's a relationship between the strength of electromagnetism that's the so-called fine structure constant and the size of that extra circle in the fifth dimension so the radius of that circle basically translated into a strength of the fine-structure concept was strength of electromagnetism and you could imagine that this radius was maybe determined by something like a potential so there might be something that keeps this extra dimension that some particular radius and if you could figure out what that physics was then that would explain what the value of the fine-structure constant was it could it could be other numbers but it would be observed to be that number if you knew what that potential was okay now string theory is a theory that was invented also to try to unify gravity with other fundamental forces and it does so by adding extra dimensions essentially string theory is more ambitious than includes a client theory so it adds six or seven extra dimensions instead of just one but it says the same thing where all these extra dimensions have to be curled up and small so that we don't see them now as you can imagine you know there aren't that many ways to curl up one hidden dimension but there are many ways to curl up six or seven the geometries that they can take are called klutz our sorry collaborates there are many of them and just like the radius of this circle determined the physics of electromagnetism the strength of electromagnetism all the different parameters that would determine this shape its size how many folds it has there other things that are so-called fluxes that can you can stick on this thing all the parameters that make this up determine what our four-dimensional physics well we can't get into these small scales would look like and just like there might be a potential for this size of the Colusa klein circle there there's also a sort of landscape of possibilities for all these parameters the difference is that while this is sort of a one-dimensional thing just alpha this is a two-dimensional plot that already looks complicated this would be a plot with like 500 dimensions so it's really really complicated but the physics is essentially the same that that the ball wants to roll to the bottom if it gets stuck it might tunnel and so on and where the ball is at any particular time determines what the physics that an observer on a larger scale would experience so it could be that this um little region corresponds to something like our universe but with ten times the cosmological constant and stronger electromagnetism one part might be totally unified physics and a huge cosmological constant no electromagnetism at all another part might be just like our observable universe so people are out there trying to figure out you know how we might get something like this but this leads to a very interesting picture because now each bubble you know as the the field as all these fields are kind of working their way through that landscape by forming bubbles there are bubbles with all these different properties different fundamental physics laws on or different physics laws on you know laboratory scales different constants like different fine structure constants or even regions that might have light and other ones that don't and so on you can even have different numbers of dimensions so you could have some regions that have four large dimensions of have three others have five so you get a really complicated messy thing which I think this is just fanciful but again but it gives kind of a picture of it and that adds to this huge diversity to this complicated landscape that's there but also leads to some disturbing questions like what do we do with this this big diverse beautiful mess in particular what do we do for science because each of these regions gives different initial conditions for the big bang and so we have to ask if we have this theory which universe do we compare to ours I mean I show these nice comparisons with between Kobe and or the W Maps satellite and the predictions of inflation and so on but now we have it seemingly all kinds of different predictions all different values of the fine-structure constant of low energy physics and so on what do we do and in fact we have to really start to think about what questions can we ask and what can't wait so one question we no longer allowed to ask is what value of the observables will we observe there simply isn't an answer to that there's no unique answer to it that's off the table we can ask something like what values would be chose and what would be observed in some randomly chosen universe if we just put our finger down somewhere in this big messy bubble distribution what would be there right and it might be that some values are more common than others or we could ask what values would be seen by a random point in space or by a random baryon or by a random observer now these are all seemingly kind of reasonable questions that we might ask but they're quite different questions or what value should we see that is an observer that's just like us who knows everything that we know about the universe now if so that once I've chosen one of these that I can ask the question suppose that I am a randomly chosen one of these things universe baryon observer etc then what will I observe so that adds a probability to it that says that regions that you know are more common are more likely to be observed by me and I can compute those probability and see if our values make sense so it could be that you know almost all universes have huge cosmological constants so it's pretty strange that we have a small cosmological constant on the other hand it may be that most observers are in universes with a cosmological constant just about the value that we see and so if we chose X as observer this would make perfectly good sense if we chose X as universe then we would say that our universe is very unlikely so you can see that which X we choose is going to change our prediction and make us more or less likely to say that this theory of the universe matches our observation so it's very very tricky business to figure out how we should think about this it has some implications for SETI I think the bad news is that you know even if life is extremely rare in the galaxy we don't profess shock that we find ourselves on a life-supporting planet you know couldn't really be otherwise right we're not going to be on some planet that doesn't support life the sort of flipside of that is the fact that there's life on Earth doesn't allow us to infer that it's common throughout the universe it could be that you know there's just one habitable world in the universe and we would necessarily be on it and that's kind of the frustration of SETI you simply don't know what some of those numbers are likewise a multiverse makes it perfectly plausible that the fact that life arises could be ultra rare even in universes so it could be that that you know there's a one chance in 10 to the 1,000 that life could ever get started on a planet we've got plenty of volume for that to happen in in the multiverse and we would just happen to be on the on the incredibly vastly rare region where that happened and that would be depressing but there's it's perfectly consistent with this picture the good news though I think is that you know if we take this more the if string theory or something like that we're right and there are lots of versions of physics and the Krenim Ellicott's and so on and so on if some of them have life that forms very commonly and easily and not in others then these sort of counting arguments might suggest that it's more probable that we would in one of these nice hospitable you know universes that has lots of life arising all over the place and so if you like that argument then that should say that there really should be lots of independent origins of life that should be evolving and so on all around because we probabilistically should happen to be in a universe that's very hospitable to life so it's not clear now it turns out that there are more difficulties even if we choose the X we need to a way to count this counting turns out to be really hard essentially because you're comparing infinities so even if they're infinitely many say red universes infinitely many blue universes it's hard to say whether there are more red universes or more blue universes right and and it turns out that there are just horrible mathematical difficulties in defining what those relative numbers are but so after spending a couple years thinking about that horrible problem I got encouraged to think about well maybe there's something a little bit more direct that we could see so suppose well something that you that may have passed by too quickly in the earlier universe if in the earlier model and movie is you see right here one of these bubbles is actually run into one of the other ones and in the in the kind of opening diagram the space-time diagram that would look something like this where this blue bubble is here this red one runs into it it kind of gives it a couple bruises here the the blue one kind of wins the battle between these two bubble universes the blue one takes over the red one but at the cost of this little perturbation of it early on so the question is could we look for that little perturbation could we actually see the imprint of one of these other bubbles running into ours in say the microwave background and as it turns out the answer is that maybe we could we kicked this off in a paper a few years ago and what would it look like it would look like some kind of you know a sort of disk in the microwave background Scot the intersection of two those two balls is a disc that would be some perturbation that's discs like there it turns out that if you go through the calculations you could expect some signature like this if the bubbles formed fast enough so that there'd be a good chance that one runs into our bubble in a place that we can see if inflation inside our bubble doesn't wipe out the signal inflation is good at wiping stuff out and making the universe smooth so if it doesn't do that quite well enough then we might see it and and we can actually use this already in cosmology to rule out some models so there's some versions of bubble collisions that would stop inflation in some region and basically leave a big empty hole in the sky we don't see that so we can we can now rule out some versions of kind of bubbling universes in there and their collisions but the more fun question of course is are there such observable impacts in the CMB so you can take a look maybe the eye is actually pretty good at picking these things out you'll notice this thing here that's actually pretty interesting in some sense now this is a tricky thing to do looking for anomalies in the CMB is very hard case in point Stephen Hawking or any of you that have the initials s H or H s his initials and yours are in the microwave background radiation right here right those is that some cosmic meaning probably not there are lots of random patterns you will see if you're just looking for random patterns so it's very important that you decide first what you're looking for so if you decide I'm going to look for Stephen Hawking initials in the microwave background with such-and-such angular skies sighs and then you found this then you'd be pretty excited then you have to carefully you know once you've chosen what you want to look for carefully where the evidence for that versus that thing arising by chance so even if you saw this you might say well what are the chances that I would have seen this you know just in a random chance configuration they might be pretty good but there'll be a lot smaller if you would chose this before you you looked and this has now been done by a former grad student of mine and some others on the W map data and they've published a paper where they basically found that there were a few sort of a little anomalous looking things like that big cold spot that you saw there not enough to rise to any level of like wow there's something interesting going on here but but they're a little bit out of the ordinary and and we can look with the next Planck data this didn't stop reports like this that their first evidence of universes that exist alongside our own from being in the press but but you know probably not but we will see there are some intriguing things there and we'll see with the further data whether any of them actually resolves into something really interesting or not so this is fun I think it's proof that in principle we can actually learn about these other universes they're not sort of forever in the realm of science fiction or speculation they can have observable effects and and we're thinking about other observable effects that interactions between universes are in general this process of eternal inflation might leave that would allow us to distinguish it from regular inflation so where does that leave us well if this picture is right then you know somewhere within a bubble within a bubble maybe within a bubble within a bubble there maybe so that you know there's some region of the universe where the laws of physics are such that complex things can arise galaxies can form structures of galaxies can swarm planets can form and I find it quite remarkable that do you know it may be that just like we we discovered to our great amazement that there were other planets just like ours that there were around and other galaxies just like ours and and finally in the past you know 10 or 20 years really understood the full scope of the observable universe and how large it is we may really be in in this period on the threshold of another change that is at least as big is figuring out that that you other stars were Suns just like our own or that other planets where planets just like our own it may be just as bigger even bigger and and may fundamentally change our view of where we fit into this big picture so we're here in the center of it all trying to figure it out and I think the sort of short version of what I've told you today is just that we do understand the universe in the past 13.7 billion years at this point I can't take much credit for that I came a little late to the game but so I can just tell you as someone who understands it that it's really amazing those guys did a great job okay so we understand the universe for the last thirteen point seven years billion years really well and but the current view is that the Big Bang the sort of primordial fireball is just the local end of a previous inflationary period so the large-scale structure of the universe is quite different than our observable universe and this science this marriage of observation and theory has led us sort of by accident and and for many people led them kicking and screaming to this suggestion that there's this really hugely mind-boggling ly diverse multiverse out there that were just a tiny part of and what's even better I think is that it is true that although it'll be hard and we'll have to be lucky it's conceivable at least that these other universes are not just forever banished to science fiction stories but they might actually leave some invert observable traces and imprints on our own that would allow us to actually see whether the universe is just really big or something even bigger so thanks for your time it's been a pleasure [Applause] Anthony just to kick-off the questions roger penrose in and a colleague of his have suggests have found in the WM data circular structures that they've suggested do your previous universes is there a chance that we can use the data to find out which model is correct yeah so there's a model that Penrose has put in putting forth where the universe goes through cycles and there could be remnants in our cycle from previous cycles that are essentially caused by black hole mergers that give off gravity waves that create these rings in the sky now a bunch of concentric rings are kind of similar to a a disk actually saw Roger Penrose a couple weeks ago we discussed this so I I pointed this out to him that it's not you know we have to be careful and do the analysis very carefully the the theory is an interesting one that I haven't managed to study in quite enough detail to really have a strong opinion on the observational facts I think my colleagues who have done this very very careful analysis they're not particularly happy with the way that that analysis was was done and a lot of others are in the community are not that happy that the evidence that they're claiming is much much much stronger than the level of evidence that that my colleagues have seen looking for these spots so you know my guess is that that analysis has to be done much more carefully and when it's done more carefully you could in principle distinguish between you know the spots that we're looking for they have a particular profile versus a set of concentric rings which is what they're looking for so broadly they'd be similar but you could distinguish them in detail um just be clear you mentioned these sub regions of a bubble actually one of which is our observable universe these sub regions are separated by anything other than the speed of light rights right you only talk about an area within your you know the age of the universe times that many like yours exactly exactly the bubbles the different bubbles are separate tend to be separated by by something physical which is a so-called domain wall but the sub regions within a bubble are just causally defined yeah the last scientific American had an article about multiverse and mentioned the yet that our universe is 42 billion light years I guess that was diameter or radius radius radius and I guess that comes from some analysis that shows that there's millions of times more than just our observable universe that's a different thing so they're talking about our observable universe so that calculation takes the part of the universe that so we look back in time and we see some region which is that that surface of that sphere WM say they're calculating how big that got in the 13.7 billion years and you can wonder how did something get so much bigger than 13.7 billion light years but it's essentially because as well as expanding at the speed of light the background spacetime is also expanding so it's not just that so if it was just light propagating into a fixed space-time it would be exactly 13.7 billion light years in radius but because the background is expanding also the actual current size of that of the observable universe is you know a few times larger than that so you know here so the interesting thing that's happened I think is the age of the universe is 13.7 billion years plus a number between zero and infinity and you know the for the reasons I've talked about the size of the universe is you know forty billion light years plus a number between you know um you know a million times that and infinity right so so while I've been gushing about the success of cosmology at the same time we have to admit that those numbers that are quoted in the media is slightly glossing over some uncertainties so one thing in the model that seems that you're proposing that seems to be uniform is the direction of time and so if I mean it makes sense to have the multiverse start with a singularity which also kind of begs the question of you know if you understand where the Big Bang came from oh we're just budding off a multiverse like well where did the multiverse come from it is one question and are you assuming that the multiverse the time runs in the same direction then the whole shebang is getting bigger and if so why yeah so the answer is so great questions it I think once you go to this internal inflation picture there is no longer any particular reason to think that there's an initial singularity so you could now this is to debate within the you know a handful of people who really think about this stuff but my personal position is that there's no there's no real reason to believe that and you could just have in a universe that has always been inflated there are also very interesting models that are just as you suggested that the arrow of time changes so there could be not a singularity but some earliest time so let's you know if we're drawing it on a page the earliest time might be here and then time runs this way on the page but also that way so there can be I think changes in the arrow of time that could come out of this and so so although many people thinking about this do carry around this idea of the direction of time going the same way every direction those who think about it the most I think will agree that that that's not clear that there could be different at least something that is hard to Don Regina drawn on one piece of paper there will be arrows good of time going in all kinds of different directions yeah it now I'm assuming here that the arrow of time is defined by entropy increasing so you can have some region where the entropy was simply increasing you know this way and that way and you would call the arrow of time pointed those other ways if if time sort of goes beyond the second law or something like that then it's a different picture but but if you think of the arrow of time is entered increase that I think there certainly are models where it goes different directions we can't observe so you say these bubbles have a wall to them at the main something you called it yeah which we can't observe obviously because it's outside of our causality but if I understand it right there are regions of these bubbles where parts of universe sub regions of the universe are within observable distance of the edge of the bubble and if so what would they observe so what we can essentially see so suppose we go to that picture I had where the bubble is our bubble and another one that runs into it so in between there's this domain wall but that domain wall if it's going into the other universe our universe will all be sort of to the future of that domain wall it's not a place that we can really go to it's more like a time so we can't get back to it any more than we can travel in time but we can look back on it and and and on what it influences so so that domain wall would basically constitute part of the kind of initial conditions surface for our universe and insofar as it's different from the conditions in other directions that's exactly where that spot would come from so that spot on the microwave background would come precisely from that is the direction in which we're kind of looking back at a different kind of domain wall than in all the other directions and that the physics of that has caused a little perturbation in the initial conditions that we're looking back on that has then affected the microwave background in a way that we can see so but the actual domain wall has causality up outside the direct observation of any portion inside the bubble though well in a sense we're see we're seeing it the domain wall is in our past light cone but what we actually see are things like atoms you know and all of those are outside of the domain but you can't get to it it's outside you cannot get to it because yeah it's travelling away at basically the speed of light so you certainly can't go there it can come to you and that's another thing I didn't talk about which is bad so let's just hope that doesn't have I'll just shown a throne a quick one so the fact that it's intersecting with our universe does that mean that it's it's in some sense related to our universe are they in universes that are more closely together in this space in that sense well those related universes yeah yeah so in the in this sort of potential space they're connected in in the sense that you know we I drew this this picture with sort of the squiggly line with multiple minima to it so these would be bubbles that are you know one hop away from ours that are running into us and then there'd be that region the hill kind of in-between is what makes up the domain wall so we will tend to not see some bubble you know from way over somewhere else in the landscape so so we are getting hit by bubbles that are close to us in the kind of potential landscape that doesn't unfortunately mean that their properties will be similar to the ones of our universe though the nearby infield space doesn't necessarily mean nearby in properties so so we can't really say that the nearby ones would be like ours in any real way I've noted found a list of about a half-dozen theoretical physicists that claim that the universe could arise out of a quantum fluctuation with negative energy being gravity and positive energy forming everything else what is your opinion on that yeah so I think it's there are two points there there's forming the universe out of nothing and there's the belt that there's a sort of explanation of how the energy works the explanation of how the energy works I think is probably just true in the sense that you know there's a there's a funny question you might ask about inflation you start out with this little thing you end up with a huge or infinite amount of stuff what about energy conservation and energy conservation gets tricky in general relativity but one way of thinking about it is just the way that you've described that gravitational energy can be negative and that that can balance out positive energy of rest mass and kinetic energy and so on to give you zero so energetically you can tell a story like that you can also just see in Einstein's equations that the physics is make sense so there are consistent solutions to Einstein's equations that do exactly that so so I think it's true that the that energetically there's no barrier to creating a universe from nothing if you assume that nothing has no energy and the universe has no energy there's no problem there the problem is more I think in saying exactly what you mean by nothing you know when they say nothing they don't mean nothing there's already laws of physics and and there's a notion of space-time and all the stuff it's a very simplified model where there's kind of a universe of a finite size and when you take that size to zero you call that nothing and what they're describing is a transition from zero size to finite size but you've still got lots of stuff in some sense with a zero size universe you've got all the laws of physics you've got space-time dimensionality all these things so so I think a lot more thinking and clarifying would have to be done in really making a model like that answer kind of the ultimate question of where did it come from okay yeah run out a little bit over time so if anyone has any more questions I'm sure you showing you how after such a mind-boggling talk then please approach Anthony now Anthony we have a special setting mark as a memento of your talk so please join me in thanking Anthony [Applause] [Music] you [Music]

Channel: SETI Institute

Views: 39,777

Rating: 4.7545128 out of 5

Keywords: space science, carl sagan, seti institute, astronomy, cosmology, big bang, Anthony Aguirre, multiverse, bubble collisions, eternal inflation, inflation

Id: G59zIL2nacI

Length: 76min 46sec (4606 seconds)

Published: Sun Aug 21 2011