WSU Master Class: History and Mysteries of The Universe with Max Tegmark

it's a great honor and pleasure to be here today so i'm going to share with you some of the most exciting discoveries in my opinion in cosmology the study of our cosmos and i'm going to do it in two in four parts first we're going to talk about what we've learned about space then about our place in time then about the origin of it all the theory of inflation and finally the future both of the cosmos and of the study of the cosmos now to understand our universe we need to think big so how big is our cosmos we humans have again and again underestimated not only the size of our cosmos realizing that everything that we thought existed was just a small part of a grander structure a planet a solar system a galaxy a universe and maybe even a hierarchy of parallel universes but equally importantly we've repeatedly underestimated the power of our human minds to understand our cosmos and that's an inspiring thought that we're going to come back to again and again and again let's begin in the himalayas here heading off into space and as we do this i want to thank the american museum of natural history for putting together this beautiful video which is not just eye candy but perfectly scientifically accurate with everything to scale now when our ancestors first started exploring the earth by foot later with sailboats they kept discovering that it was much bigger than they thought until finally over 2000 years ago eratosthenes was able to measure exactly how big our beautiful round planet is how did he do that not by flying around the earth but just by letting his mind fly and very creatively using angles he realized that while the sun was straight overhead and one part of our planet on the summer of solstice it was seven and a half degrees away from overhead somewhere else and he correctly inferred from that that those two parts must be 7.2 degrees away from each other therefore around the earth which was 794 kilometers so we figured that the way to go the distance all the way the way around the earth then is simply equal to 794 kilometers times all of the 360 degrees divided by those 7.2 degrees which gives 39 700 kilometers which is just remarkably close to the modern measurement of 40 075 kilometers it's better than one percent he was a bit ahead of his time columbus later completely bungled this and underestimated the distance of his travel to india by a factor five so he wouldn't even have gotten funding for that if he had used the correct values but this is very very impressive and since we knew the fact the size of the earth it was very natural to ask well what's next when we look farther out into space when you look at the moon for example we must ask ourselves is the moon something which is relatively small and nearby or is it something really huge and far away either way it might subtend the half degree in the sky that we observe today of course we have very high tech ways of figuring out these things we can for instance using not mental power but the rocket power that we got from the mental power put satellites into orbit we can bounce radar off of the moon and simply measure how long it takes to come back and by this radar bouncing method we can measure the distance to the moon now to better than a millimeter and measure the distances to mars and venus and other planets but already way back in ancient greeks aristarchus was able to figure out roughly the distance to the moon just with human ingenuity again and the clever clever use of geometry he noticed that during a lunar eclipse when the earth casts its shadow on the moon the shadow has a curved edge and as you can see here this immediately tells you that earth is much bigger than the moon and our starcraft realized that actually from this it's 3.7 times bigger than the moon so then we since we know the size of the earth now from our tostenas now we know the size of the moon and we know the angled moon subtends in the sky so we know the distance to the moon and indeed our starches correctly figured out it's 30 earth diameters away a huge distance now as we keep flying out into space to even greater distances what about the other things that we encounter what about the sun how far away is the sun is it some it subtends about the same half degree angle in the sky as the moon is it just a little bit farther away than the moon enough to explain why the moon goes in front of the sun during a solar eclipse or is it vastly larger and vastly farther away well aristarchus actually figured this out too by just considering this triangle here which has it at its corners the sun the moon and the earth he knew that during last quarter moon when the moon lift looks half full this is of course the bottom angle is a 90 degree angle this angle between the moon and the sun he was able to measure to be a little bit less than 90 degrees and now since he knew both the shape of the triangle and also the length of this earth moon leg of the triangle he could figure out this leg the distance to the sun he figured out that the sun was much much farther away than the moon therefore much bigger and that's why he came to the conclusion that we were probably orbiting around the sun unfortunately although he was a genius back in those days they weren't very good at error analysis and estimating the uncertainties in their measurements and both he and then copernicus and others when these with similar cleverness what angles figured out the distances to the various planets and built a nice scale model of the solar system were all off by a factor 20. only over two about only a couple thousand years later did we realize that our whole solar system was actually 20 times bigger than we thought that's like confusing a doll's house with a real house now as we keep flying out farther and farther away exploring the full vastness of space we see our solar system and then we see these stars in the distance people always wondered about stars great myths were created for what they might be maybe there were holes punched in a sphere that light shone through maybe they were something else but i think as smart as they were most ancients who thought about this must have felt a little bit melancholy knowing that they would never really know what the stars were and in fact when this famous song was written twinkle twinkle little star how i wonder what you are they still didn't know what the answer was only 30 years after the song was published was this mystery actually convincingly solved through work by the astronomer of bessel and others and how was he able to do this how could he figure out how far away these things were well by a method known as parallax and here's how it works i'd like you to all hold your thumb up like this at arm's length and alternate blinking your left eye and right eye and then with your other hand hold it up in the air if you can see your thumb jumping back and forth as you do this this is parallax now move your thumb in so it's much closer and keep blinking you see how it jumps by a larger angle now the so-called parallax angle is larger this is how your brain figures out 3d vision right this is how you know how far away things are so effortlessly because it's all hardwired into your neural circuitry this parallax method if you were a giant and you had your eyes much much farther apart your 3d vision would be much better and fortunately we can act like giants because if we take pictures from the earth during the summer and during the winter when it's on opposite sides of the sun it's like being that big and having your eyes that far apart we get fantastic 3d vision for and if you take a picture for example of the 61 cygni system and you look at it and you flash the july first picture versus the january first picture for instance you notice that almost everything looks the same except there are there's something that's moving back and forth just like your thumb was here right that must be closer that binary star than the other things and by measuring the angle with which it jumps we can measure its distance and vessel really impressed his colleagues because this was a vast distance he measured the distance to the sun is so large that it takes eight minutes for light to reach us from there but most of the stories you see in the night sky here in new york are so far it takes hundreds of years for light to reach us from hundreds of years so someone there looking at us wouldn't see the world science festival they might see for example the sale of manhattan being negotiated with the native americans and moreover bestial and the other astronomers knew that how bright something looks depends on how far away it is with an inverse square law right so if you move it 10 times as far away it looks 100 times dimmer and he figured out that if you take something as bright as this as a star and you move it you were to move it to be as close as the sun since it looks if it's for if it's for argument's sake 10 million times farther than the sun and it's 10 to the power of 14 times fainter well when you move it 10 to the 7 times closer it's going to look 10 to the 14 times brighter and it will look approximately as bright as the sun and this gave the first convincing answer to the question of what the stars are there are these things which are about as bright as the sun they're distant suns just like jordana bruno had speculated back in the 1600s when he got burnt at the stake for his um as a reward for that and and by looking farther at this at the data from distant stars by looking carefully at the spectrum of light that came out in different colors people were able to not only establish that these are distant suns but also look at what they were made of and see that they were made of the same kind of gases that the sun is made of hydrogen and other things with and would measure the temperature and so on and these mysterious white dots you know which had really foiled science for so many decades even when the telescope was invented and people looked at the white thoughts they saw white thoughts again right but now there had been this fantastic break and we knew that these were physical objects even though we couldn't go there by letting our minds fly we could understand let's continue our journey even farther out into space raise your hand if you know anybody who was born before 1925. i'm thinking of my grandma right now and when she went on her first date you know this was her universe the solar system and a bunch of stars because it wasn't until 1925 that the mystery of galaxies was actually solved by edwin hubble until then this beautiful nebulae that you can see in vangog's starry night painting for instance it was not known what they were some people thought they were just silly little gas clouds out there between the stars but edwin hubble for the first time was able to discern that there were actually individual stars in what was known as the andromeda nebula and he had this idea that this was actually an agglomeration of billions and hundreds of billions of stars which he called an island universe which we now call a galaxy and by using the same idea we just talked about that farther away things look dimmer and figuring out how many watts of light those stars he saw were actually shining with and comparing that with how bright they were he could measure the distance and again totally shattered all distance records we now know that the andromeda galaxy is so far away it takes about three million years for its light to reach earth and that was just the beginning we can now look at galaxies much farther away even if we can't see most garden variety stars in there sometimes the star explodes in a supernova explosion which is so violent and powerful that it can out shine the whole galaxy for for some days and we can see it and we can figure out by looking in detail at its spectrum and its time dependence how many watts that was and then infer how far away this is and by using methods like supernovae explosions and a lot of other clever techniques we've been able to actually now figure out how far away vast numbers of galaxies are from us and build up enormous three-dimensional maps of the cosmos and i had a great fortune and look in my career to get to work on the sloan digital sky survey that you see here which is one of the most ambitious three-dimensional sky mapping projects to date and again we've learned that everything we thought existed now our galaxy was just a part of an even grander structure groups clusters of galaxies superclusters and even enormous filaments like the sloan great wall stretching a billion light years and even this is still not the end of space even this is still part of an even grander structure where we affectionately know as our universe the spherical region of space from which light has a time to reach us so far during the 13.8 billion years since our big bang so this begs the question is that it or is there still more is space still bigger does space even go on forever this is something that i used to wonder a lot about as a kid and it used to seem to me that just from logic alone space had to go forever because how could it not now if it didn't would there be some sort of a sign you would come to saying like warning space ends here mind the gap that just sounded way too silly and moreover what would be beyond the sign then right but then as i grew older i learned that einstein had given us much less silly ways for space to not be infinite for example space could be curved like the surface you see here of the sphere on the left if you're the ant here you can just keep walking walking walking what seems to you to be a straight line and eventually you would just come back to where you started maybe it's the same with our space that you just go really far in this direction and just like in the old space invaders computer game find yourself coming back from the other side of the screen it's possible according to einstein and it's an experimental question not a philosophical question now the exciting thing that you see here is that this ant can actually figure out whether it's on a curved surface or not without going all the way around by just drawing a triangle if he sees that the angles add up to more than 180 degrees like he learned in his aunt high school then it's curved and i've had a lot of fun with my friends and colleagues doing exactly this experiment in our physical space making the biggest triangle you can imagine which goes from earth over to the end of the edge of the universe off the side and back and the most accurate measurements we have show so far no hint of any departure from 180 degrees for the sum of the angles so we have no evidence there that space is not infinite there are other clever experiments we can do as well no evidence that it's not infinite and moreover the best theory we have for what created our space the inflation theory that i'll talk about later which stretched out and created this awesome cosmos perhaps that we're in it actually predicts that not only that space is big but it generically predicts is even infinite now in summary then we've repeatedly underestimated both our ability to understand the cosmos and also the sheer size of it and the edge of what we saw here in our explorations of of space is the edge of our observable universe but what is the deal here what is all what are all these you get red yellow and blue spots that adorn the surf the edge of what we can see to understand this it's not enough to just talk about our place in space we also have to understand our place in time and that's what we're going to turn to next so let's explore the 13.8 billion year history of our universe in 15 minutes so i'll have to talk fast we saw that as we explored farther and farther out into space the farther we looked the more stuff we found very intuitive but finally beyond all these beautiful galaxies we saw that the edge of what we could see the edge of this spherical region that we call our universe from the sphere from which light has had time to reach us during the 13.8 billion years since our big bang is this strange radiation coming with these strange yellow blue and green colorations here what is that how can we understand this so let's think more about our place in time fortunately we can learn a lot about our place in time just from looking into the sky because the sky is like a time machine when i look at you i see you the way you were about six nanoseconds ago when i look at the sun i see the sun the way it was eight minutes ago and when i look at some of these galaxies here in the hubble ultra deep field i see them the way they were 10 billion years ago so what have we learned by observing the universe at various stages of its cosmic history we have learned something really really surprising and to make you fully appreciate just how surprising it is i want you for a moment to pretend that each one of you is a galaxy and i'm looking at you with my telescope and i see something really strange i see that those of you near me here are in your 90s looking good but in your 90s and you guys look are in your 80s you're in your 70s you're in your 60s 50s 40s here's a bunch of teenagers over there and then a bunch of toddlers and in the second last rows there it's an infants and the very last robe at the back of the room there is completely empty why on earth have did you guys decide to sort yourself by age when you filtered into the classroom and compounding my confusion the rear wall of the classroom is glowing at me with a strange microwave glow and as if that weren't weird enough you were all blushing you guys near the front are just a little bit pink in the face you guys near the back are like seriously tomato red so what's going on here is is it something i said and and well we can understand this exactly because the farther away i look right the farther into the past i'm seeing so when i look at nearby galaxies from from my vantage point over here i see them they were the way they were pretty recently when they had had time to mature and become big and modern looking when i look farther into the past i'm seeing galaxies long ago that hadn't had time yet to grow up and become big galaxies and really far away all i see is baby galaxies that had not had time to do much growing at all and beyond that corresponding to the very last row here in the classroom i see no galaxies at all because i'm looking so far back in time that i'm witnessing the epoch before there were any galaxies that had had time to form all i'm seeing there is the building the raw material out of which the galaxy is later formed namely hydrogen gas which looks black here because it's transparent what's with the blushing why do you all look red in the face well if you go down to the highway you hear the cars go that's the doppler effect right something receding from you it has a lower frequency cars do not sound they go lower frequency when they're going away and it's exactly the same way not just with sound but with light so a galaxy flying away will have its light shift at the lower frequencies which here corresponds to the colors towards the red so we call this redshift in astronomy the more redshifted something is faster it's flying away and edwin hubble discovered that pretty much all the galaxies he looked at were redshifted everything seemed to be flying away from us and this is what we mean when we say that our universe is expanding now if a galaxy is flying away from here how long ago was it near us that's the same question basically is if you have a bank robber flying away driving away from a bank with some velocity v a distance d from the bank we can figure out this when the crime happened basically by just taking the distance and dividing it by the velocity and then correcting for acceleration and deceleration right we can play the exact same game with these galaxies and say that take the distance divided by the velocity that tells us how long ago something rather dramatic happened where this galaxy was kind of over here and what edwin hubble discovered when he made these fantastic measurements in the late 1920s was that things weren't expanding just in some random haphazard way but in a very organized way pretty much all the galaxies were moving effectively straight away from earth much like what you would see if you were looking for the vantage point of a chocolate chip and a muffin that was rising in the oven you would see all the other chocolate chips moving straight away from you and moreover the farther away they were the faster they receded if they were five times as far away they receded five times as fast so the velocity of recession for a galaxy is was simply equal to the distance to it times this constant which hubble got named after him we now call it the hubble constant of the hubble parameter now if we combine that with a bank robber analogy and simply ask okay how long ago was a particular galaxy here right it's if you take the distance divided by the speed but the speed is given by the hubble's formula above here so let's plug that in distance divided by hubble's constant times the distance now we can cancel out the distance and we get the same answer for every single galaxy regardless of how far away it is about and when you plug in the numbers here it gives you about 14 billion years when we do this a little more accurately to take into account acceleration and deceleration we get 13.8 billion years so what this is saying is that about 13.8 billion years ago something really crazy happened because pretty much everything was on top of everything else things were extremely dense okay in other words we've been witnessing now an expansion very much like what happens in the oven when you bake everything is receding from everything else in this very orderly fashion where the patterns stay the same it's just that all distances keep getting increased by the same factor that also gives you an alternative way of thinking about the expanding universe that some of you might find more intuitive instead of thinking about a static space where these galaxies are flying apart like this you can think of all these galaxies actually just sitting still in a space that's just expanding because the relative patterns are all the same suppose we look at these rulers and we just re-label all the millimeters into centimeters and then we change the centimeters into meters and so on now all distances have increased by the same factor that's what we mean when we say that space is expanding and it's allowed to do that according to einstein's general relativity theory now another one of the really surprising things we've discovered about the history of our universe from looking out when i look at you galaxies here was the far wall was glowing with microwaves why would it do that well since everything is expanding so is the gas that fills space right we all know that if you expand the gas it will cool off that's how air conditioners and refrigerators work so if we run this backwards and imagine going backward in time the gas is getting more compressed which means it's getting hotter and hotter and hotter and when a gas if you heat up a liquid it turns into gas steam if you turn it if you heat up a gas what does it turn into eventually a plasma that's right that's exactly right so eventually beyond all of these galaxies we're going to see a plasma screen of hydrogen plasma and that's opaque so it's going to look to us like there's an opaque wall there of plasma and it's going to look that way in whatever direction i choose to look i'm going to see galaxies i'm going to see nothing and then there's going to be a plasma screen so it looks to us actually like we're surrounded by a spherical plasma screen that we're looking at from inside and we can and we in fact have photographed this with these beautiful and gotten these beautiful images from the wilkinson microorganisms probe and in fact maya do you have our universe can you pass it around to the audience so you can be gentle and loving with our universe it's treated us pretty well so far these are remarkable pictures of what these are baby pictures of what a universe looked like just 400 000 years after our big bang 13.8 billion years it took the light to reach us here okay did they get it right well last year the plonk satellite released even better images of this going from three megapixels to 50 megapixels and yes look how well they agree it's just fantastic except now it's an even sharper image it's really spectacular that we've been able to get this sort of images this is these are the most these are photos of the most distant thing you can photograph in science now another striking thing about these images is they reveal that the baby universe was very boring because the color scale here has been stretched by five or by ten to the five for you to even see anything it would look gray otherwise or uniform what's shown here is simply how hot the plasma screen is in different places and it differs by only a thousandth of a percent from place to place which means that the plasma was only a thousandth of a percent denser in some places than other places that's about as uniform as the air in this room i mean raise your hand if you can hear me that's because there are sound waves propagating here right so the density is different in different places and if i speak about this loud it varies by about a thousandth of a percent about a foot for my mouth so this is how boring our universe was early on like the air here how did it get so interesting how did you get these clumps like stars and planets in the world science festival well because over time in addition to this process of expansion that we've now talked a great deal about that you can see in the super computer simulation there was also a separate process of clustering that transformed the boring into interesting and we can see this better with another supercomputer simulation here where i've removed the expansion so you can focus on the transition from smooth and and dull to clumpy and interesting it's hundreds of millions of light years on the side here and gradually it gets more and more interesting why because if you have a little bit more stuff over here than in its surroundings then the gravitational pull from that will draw in more stuff and you get a bigger clump which is even more able now to draw in more matter from the surroundings get a still bigger clump still bigger clump and before you know it you form clumps about the size of galaxies so imagine a galaxy forming in each one of these halos or clumps that are seen in this picture let's zoom in a lot here now and uh take a look at how how you can form a solar system we have again same thing happening on a smaller scale you have a cloud of gas gravity again is trying to squish it together make it even clumpier but it's spinning so it can't clump it into a point that would violate the so-called conservation of angular momentum instead it squashes it down into a pizza shape disc but in the center it keeps getting denser and denser and hotter and hotter until the gas gets so hot that nuclear fusion ensues and a star is born and in the meantime stuff in the outer parts of this disc has formed planets which become revealed when the nascent star blows away the disk out of which it was formed and it's no surprise that all of the planets are orbiting around our sun in the same sense which is also the same sense in which the sun itself is rotating roughly once a month because all of this spin all of this angular momentum came from that initial cloud so that's a very short version of of how the world science festival got here so to summarize we have seen that during the past 13.8 billion years there were two quite different processes that went on there was the process of expansion which transformed our universe from small to big from hot to cold from dense to more verified but in parallel with that there was also this transition from smooth and uniform and boring to clumpy and interesting and it's interesting that the different forces of nature have actually taken turns tag-teaming driving this clustering the strong interaction early on clumped together triplets of quarks to form protons and neutrons and then bound together these protons and neutrons into atomic nuclei then the electric force took over took the lead and started clumping together nuclei with electrons to form atoms and then handed over the baton to the gravitational force which clumped enormous numbers of atoms into stars 10 to the 57 atoms or so in a typical star and also formed planets and ultimately people and and the world science festival now although we understand quite well then what happened during the past 13.8 billion years what happened before this why did we start out with all this hot stuff flying it flying apart in this very orderly and boring way that's what we're going to turn to next let's explore inflation this is a very fun time to talk about the theory of inflation because just two days ago inflation was awarded the kavli prize right here at the world science festival and this is a very happy day not only for alan guth and andre linde and alex says sterbinsky who won the cavalier prizes but also for the many other people who contributed in various ways to formulating and developing this fascinating theory of inflation so what is inflation then it's the most popular theory right now for what actually happened early on for what created our big bang and what is it that the theory says well it has only one single assumption that you need to make you need to assume that there is a tiny speck of matter vastly smaller than a proton is plant enough that has a strange property that it's very hard to dilute so that if you expand it into a larger volume its density stays almost the same that's very different from everyday stuff like air where if you put it in a larger volume the density drops because the total mass is the same if you have this kind of infiltrator inflation substance you have one kilogram of it you put it in twice the volume well now you got two kilograms if you are willing to assume that such a substance exists then you can prove by just plugging it into the theory of general relativity that it will create a big bang for you specifically what comes out of the equations very beautifully and caused alan guth to write on his notes spectacular realization when he figured this out is that it will just keep doubling and doubling and doubling itself at regular intervals in the simplest models it doubles every ten every hundredths of a trillionth over trillions of trillionths of a second okay so you have one little speck of the stuff then you have twice as much twice as much twice as much twice as much and you all know that repeated doubling rapidly creates an enormous amount of things and it's not just the amount of stuff that goes up vastly the number of kilograms but also the speeds involved keep doubling as you can see from the doubling of the length of these arrows right so even if the motions are very slow and insignificant pretty quickly you have enormously high speeds if you plot here how big this clump is as a function of time you realize actually that our universe is birth according to inflation was very similar to our own because we started out being one cell then we were two cells and four cells and eight cells 16 32 64 and so on fortunately for our mommies we didn't keep doubling for nine months because then the delivery would have been a little bit painful we would have had more mass than the mass of our universe instead what happened is that when we were about this big this dublin stopped and gave rise to more leisurely growth which is why we're about this size at nine months and interestingly our universe did exactly the same through some strange coincidence it's the same y-axis on both of these plots when our universe was about this big it stopped inflating the horizontal axis on the other hand is very different we humans we doubled about once per day our baby universe and the simplest model doubled every hundredth of a trillionth of a trillionth of a trillionth of a second then did so maybe 80 times perhaps a lot more okay you only need one assumption to get this but you get a lot of predictions out this is a theory that gives much more out than you put into it what does it predict well first of all it predicts that there will be a big bang we just saw how this repeated doubling actually causes the big bang okay so it solves the bang problem in the sense that in traditional cosmology if you're asking well what caused the big bang you're like uh awkward silence because there is no explanation the equations simply assume that it's happened and it happened it's very unsatisfactory that you have to start out with an infinite space which somehow for some weird reason all started expanding exactly uniformly all at the same time all with the same temperature all the same density basically you need to start with some sort of miracle inflation on the other hand says no there's no miracle at all you just need this puny speck less mass than that of an apple much smaller than a proton and then it predicts our big bang making everything we see here in our cosmos it also predicts something else it predicts that what we see should look almost exactly the same in all the different directions so you the keeper of our universe if you look closely at the universe there you'll see the temperature is varying by a thousandth of a percent from place to place but that's not much right why is it not 50 times hotter on some side on one side than the other you might think well when you pour cold milk into warm coffee and wait the while it also all acquires the same lukewarm temperature so maybe that's the explanation for why our universe kind of has the same temperature everywhere except in traditional cosmology that's an epic fail it doesn't work at all because the light from the region a has has only now just barely reached us here at the halfway point at the same time as the light from the other side has just barely reached us so there's been no time at all for any communication between these two parts let alone more time for energy to flow back and forth and equilibrate the temperatures to all be the same right this is what alan guth called the horizon problem how did these parts know to have the same temperature well inflation solves that and predicts that everything should have the same temperature because it says that early on there was this tiny speck it could have been hanging around doing whatever it did for a long time reaching kind of uniform temperature and then we're all inflated at a vast speed and those temperatures were the same because the conditions had already been set up to be kind of the same throughout the tiny little speck inflation also predicts that the space we observe in our universe should be almost exactly flat that the angles of a large triangle in space should add up to 180 degrees why does it predict that well if you take something like a curved space here that the ant is on right and you inflate it up to an enormous size very quickly light doesn't have time to go very long distances in that short time so what the ant is going to call its universe you know the part that light reach light has reached it from is just a tiny fraction of the whole sphere which was going to look very flat to the ant just like if you're out in the middle of the ocean the ocean is going to look pretty flat to you because you're seeing a small small part of earth so inflation predicts that space should be very flat which also solved the so-called flatness problem because it was well known that if you just tweak the density of the universe a billionth of a second old by just changing the last decimal place here a little bit the whole thing would have collapsed together in a black hole long ago or totally diluted itself away long ago yet somehow it had exactly the magic density exactly the magic flatness to last this long well inflation explains that inflation predicts stillmore stuff it also predicts that our universe was not completely uniform early on but had these tiny little fluctuations which we previously just had to assume were there right if our universe had started completely uniform we wouldn't be having this conversation today because then it would have to stay completely uniform and we would never have formed this large-scale structure the galaxies the stars the planets or the world science festival what was it that created those seed fluctuations what was it that created effectively the cosmic dna these early 10 to the minus five level fluctuations these patterns that indicated like a blueprint where the galaxies were going to form and where we would instead form giant voids where did that come from well inflate inflation predicts that they were there and the prediction it gives is actually in my opinion one of the most beautiful ideas in all of science by connecting two things which seem completely unrelated inflation says that they come from quantum mechanics from the heisenberg uncertainty principle which says that you cannot have anything complete the uniform the heisenberg uncertainty principle says that if you have something completely uniform you're not actually allowed to know exactly then the velocities of your particles and pretty quickly they're going to start moving and become non-uniform but how could possibly these quantum fluctuations which we know to be important today only on tiny scales have anything to do with the scales of galaxies well because inflation stretches space by this stupendous factor right so these tiny quantum fluctuations get stretched out the scales of galaxies and beyond and predict that actually the ultimate origin of the largest structures we see in our universe like super clusters comes from quantum fluctuations in the subatomic realm and this isn't just a bunch of nice sounding blah blah blah we can turn this into equations and make very rigorous predictions for exactly how clumpy our universe should be on different scales and this is something i've spent many years of my own career doing precision measurements of with my colleagues and it agrees beautifully beautifully with the predictions from inflation not only for the galaxy clustering but also if you look in detail at these hot and cold spots in the cosmic microwave background these baby pictures of our 400 thousand year old universe if you want to know if you make a histogram you want to know how many spots are there of different sizes it agrees beautifully with the prediction of inflation finally the most audacious prediction of all from inflation is that there should be gravitational waves in the simplest inflation models ripples in the very fabric of space itself that are huge of what are billions of light years in size and this has for many years been considered sort of the holy grail to look for this and uh something that would be viewed as a smoking gun of inflation if it were found and then just this spring it was announced by the bicep team that they had found it and there were a lot of smiles all around including at mit here by alan guth and andre linde who happened to be visiting the little dark cloud over their heads here as a dust cloud because there's there's an interesting discussion going on right now about whether this is really what the signal is that they've seen or whether contamination by radio noise from dust in our galaxy might play an important role we're going to have to wait until the end of this year with a lot more data coming in to know for sure but this is another example of a spectacular prediction it's a very audacious one too it's the most audacious extrapolation ever of known physics to 100 billion times higher energy than we've tested in the in the large hadron collider and 38 orders of magnitude earlier in time and it's so if this holds up it'll be just absolutely spectacular now that's the good news about inflation for as long as i've been in cosmology i've also heard a lot of my college leagues criticize inflation and and you'll also hear an interesting balance assessment of this from the next speaker paul steinhardt here today uh some of the most common complaints about inflation are first of all you know this assumption the one assumption that there could be a substance that isn't diluted by expansion that sounds kind of nutty how could you possibly have that except that now we know there is such a thing dark energy which was discovered and got the nobel prize it's precisely that kind of thing so that argument is kind of moot now inflation is simply dark energy but much higher density so things double much faster then people have made the objection well okay the most popular physical model for what such a substance could be is called a scalar field it's like a cousin of the magnetic field which instead of having a strength and a direction at every point in space like the magnetic field does has only a strength it's like a number everywhere in space and people will say well this is kind of speculative because we haven't found any scalar fields in nature except that now we have as of last year the higgs boson the higgs field is exactly a scalar field finally people have said that well inflation doesn't really make testable predictions but actually specific models of inflation certainly make testable predictions in fact the simplest model of all where the thing that specifies the so-called potential energy function is simply a parabola look at all the predictions it makes loads of things the business about space being flat is parameters by a number called omega inflation predicts it should be one i and many of my colleagues have been involved in measuring it it's 1.001 plus minus 0.007 so it agrees with the predictions of one percent accuracy that's pretty awesome two smiley faces for that and a lot of other predictions that are very good agreement and other predictions where we don't really have the measurements yet the people are working on making the measurements better so this is science at its best inflation sticks its neck out makes a lot of predictions and we're working on testing them so i think we should take we don't do we know that inflation happened no we don't is it a scientific theory absolutely should we take it seriously yes absolutely and if we do then we also have to take seriously what it predicts not just for these numbers but also conceptually one of the things we have to take seriously is that we need to change the way we talk about our big bang for example i cracked up when i read this article in new york times here where it says that actually inflation caused this rapid expansion in the instant after the big bang no inflation didn't happen after the big bang it makes no sense at all to say that the beginning or the early stages of inflation were a hot big bang or after the hot big bang because at that time it was neither particularly hot or particularly big or particularly bangy but it got much much hotter hundreds of thousands of times it got a thousand times hotter once inflation stopped in the beginning of inflation it was not that big it was less massive than apple by less than a billionth the size of a proton and it was really not much of a bang because the expansion speeds back then were a trillion trillion times smaller than after it ended okay instead a much better way of saying it is that the early stages of inflation was a cold little swish and then eventually it picked up steam and this cold little swish caused our hot big bang let's turn now from the past to the future although we understand now a great deal about what happened during the past 13.8 billion years many mysteries remain what was the ultimate origin what's our ultimate fate what is our universe really made of the most popular theory for how things began inflation we first want to test with experiments whether that really took place then we would like to know what the detailed physics of it is and what if anything happened before that if we look in the other direction towards the future we're forced to face up to this question of you know what's going to happen which is linked to what our universe is made of and we're in this very embarrassing situation now though we still have no clue what 95 percent of our universe is made out of we used to think it was all atoms but now we know that our atoms make up only five percent of the cosmic matter budget the remainder the stuff we have no clue about we've come up with two cutesy words for it for this you know but these dark matter and dark energy are really just fancy words for ignorance right now so how is it going to end then if we take our best theories of general relativity and so on and plug them in there are five possible cosmocalypse scenarios for the distant future of our universe which have been talked about to various degrees the big chill big crunch big rip big snap and death bubbles if we write down the equation at the bottom here the friedman equation that governs the expansion rate of our universe this h that we talked about before which links the expansion rate of the velocity of something to how far away it is right for the simplest case of a flat universe it says that this expansion rate is simply governed by g which is the gravitational constant which we know and the density the total density of all the stuff in our universe so it matters what the stuff is if the stuff is such that the density always stays positive this equation shows that the units will always have a positive expansion it'll keep expanding forever giving us the big chill here on the left if instead this dark energy the dark horse in the waist can go negative which it might then it can bring the total density in this equation to zero so h equals zero the expansion stops and eventually everything recollapses and a big crunch making a black hole and a huge mess if instead this dark energy sort of undilutes and gets higher and higher density and reaches infinite density at some finite time in the future that'll cause the big rip where everything first galaxies then stars and eventually even the atoms that we're made of get ripped apart if space itself turns out to not be infinitely stretchy much like your rubber band is not infinitely stretchy because it's ultimately made of something of some sort of granular stuff atoms then just like when you stretch a rubber band too much when you stretch space too much something bad might happen the big snap and finally if it turns out that the dark energy is an unstable substance that can undergo some kind of radioactive decay or some other sort of decay we could end up with these death bubbles just like if you have water which is hyper cooled to lower than zero celsius but you haven't touched it it's still liquid all of a sudden if you disturb it a little bit you get a little bit of ice you have an expanding bubble of ice this tune percolates and fills it freezes everything we could have a death bubble like this where space itself freezes effectively and turns into a different kind of space that doesn't support life which would be a bit of a bummer that's the fifth scenario here what's going to happen first of all i wouldn't lose too much sleep over this because most of these things are very unlikely based on the research we've done to happen any time within the next billion years or so you need to worry about more about human self-inflicted disasters before that but if you want to know what's going to happen you notice that pretty much all of these scenarios hinged on what the dark energy does so interestingly to know our destiny we need to understand dark energy now i'm a kind of impatient guy so to figure out quickly what dark energy it was in time for meeting with you i i went to the source of all knowledge of course the google image search and discovered that dark energy is actually a bioblend nutrient additive and if you read the fine print that says it stinks but it rocks and this is true because i actually ordered one on amazon and i had to move it outside the house because it smelled so bad but it also stinks but it rocks really summarize very nicely how i feel about dark energy because on one hand it rocks because if you just assume that there is a substance which is completely constant it fits all our data beautifully but it really stinks that we don't know what it is and therefore can't predict our future properly so the billion dollar question in our quest to understand this better is is it constant is it constant space like it seems to be having exactly the same density everywhere wherever we look and is also constant in time in the sense that it run doesn't dilute at all and this keeps density the same well there are fortunately a lot of different experiments going on right now trying to look more carefully at data to see if it is constant or not and if anybody discovers any way in which the dark energy is not constant either in space or in time i'm pretty confident they're going to win a free trip to my hometown of stockholm and a particularly powerful way of learning about dark energy is to simply look at the billions of objects that adorn our universe and for each one of these luminous objects measure just two numbers measure how dim that is and therefore learn its distance and then measure how red-shifted it is and therefore know how fast it's flying away from each other and then just plot the distance against the the recession velocity like this to measure this expansion rate this h the hubble parameter that we've kept talking about here since we can see these objects many different times in the past we're therefore able to measure exactly how fast their universe was expanding at different stages in the cosmic history and from that we can infer exactly what the density of dark energy was at various times in the past to see if it was really constant so let's turn now from dark energy that big mystery to another another big mystery what is dark matter we have four roads to learn about dark matter we can either catch it make it infer it or weigh it so far the best evidence we have for dark matter comes from weighing it that's how it was first discovered weighing things in astronomy is perfectly doable even if you can't see the thing you're weighing all you need to do is find something which is affected by its gravity if you want to weigh the sun just look at earth for example and look at the orbit little period of it around the sun look at the distance the radius of the circle plug it in to the laws of gravity it tells you how many kilograms of mass there is now the shocker was that when vera rubin first did this to our galaxy and figured out how much how many how much mass is there actually in the galaxy in different parts of it she found that there is much more stuff there than there is in the stars that in fact most of the kilograms in our galaxy are in an invisible form you might want to think of dark matter really as being invisible matter there is about and there's a you know in the ballpark of five six times more dark matter in our universe than all the atoms combined another way you can weigh invisible matter dark matter is by exploiting the fact that gravity bends light which was a prediction of general relativity so if you take a photo of this cluster of galaxies here light from other galaxies in the background have been so bent by the gravitational force of the dark matter that's here that some of them have had gone really warped out of shape if you see these strange long arcs here they're actually relatively round galaxies that have been so bit warped out of shape as your face might be if you go to an amusement park and look in one of these funny mirrors and if you look more generally a lot of galaxies here actually look more elongated elliptical than they really are so if you have a lot of dark matter around things end up looking more oblong than they really are and in a correlated fashion so what we can do is we can look for these elongation correlations these ellipticity correlation patterns and infer from that how much dark matter there is in different places a very spectacular example of that is this image here so what you see first of all in the black and white part of it is just a regular photo with an optical telescope which shows that there are two galaxy clusters there's one here and another one there they have actually collided with each other in the past and once they missed each other and come out on the other side kind of unscathed but the gas that was there between the galaxies couldn't interpenetrate it made a huge traffic accident which can be seen here in the pink which is an x-ray photo overlaid so the gas got kind of stuck here in the collision region as the galaxy is continued out and what about the blue the blue is where the dark matter is in firm inferred from this gravitational lensing technique the way in which the matter has bent light and you can see that the dark matter didn't get stuck in the middle with a gas it's moved along interpenetrating all the other dark matter particles and coming happily out on the other side so what this and all the other data we have on dark matter shows is that dark matter whatever it is is a very shy kind of substance it doesn't like to interact much with atoms so if a dark matter particle strikes my hand it'll typically you hit it go through hit the earth go through earth and out on the other side most of my colleagues think that dark matter is some kind of new particle or maybe several kinds of new particles but in our best physical models even though they're very shy kind of like the neutrino ghost particles that have already been detected they do still sometimes once in a blue moon bounce off of an atom and that's the idea behind trying to catch them in a laboratory many labs around the world have these fantastically sensitive detectors where they're trying to wait until the dark matter particle bounces off and they can detect it and it's a very exciting time now it's very competitive and the experiments have finally gotten sensitive enough that it wouldn't be too surprising to me if if you read on the front page of the new york times in a year or in a few years the big bold letters the dark matter detected another thing we can try to do is to infer the presence of dark matter indirectly for example if you take photos of the middle of the milky way galaxy with a gamma-ray camera you see these crazy bubbles that are 50 000 light years from side to side they're known as fermi bubbles and people are arguing passionately about what's causing them but perhaps the most one of the most popular explanations is that they're actually caused by dark matter by dark matter particles in the middle of the galaxy where the dark matter density is a densest bumping into each other and annihilating and producing gamma rays which make these huge bubbles this is this and other indirect evidence is very exciting and active field right now to look forward to and in addition to that we can make dark matter perhaps in the large hadron collider when we smash particles together at this near the speed of light a lot of this energy in the collision can be arranged according to e equals m c squared perhaps into dark matter particles perhaps the lightest supersymmetric particle is a dark matter particle for instance and finally the last but not least weighing it that's how we first discovered it and that's something we can do so much better by more accurately mapping out all the gravitational forces out there in the cosmos so far we've only used that technique to learn one number about the dark matter it's density but i would love to see if we can measure more maybe we can measure if the dark matter has a temperature some sort of viscosity or any other properties that can give us a grip on what it is to explore on this theme a little bit more both to learn more about dark matter also to learn more about dark energy and about inflation and our cosmic future it'll be wonderful to map out in more detail what the gravitational forces are and in general what's going on in our universe so to end on an optimistic note let's take a look at how little we've accomplished how much information is out there still for us to harvest let's peel off the surface of this picture and see that these great microwave background baby photos of our universe actually come from only a tiny fraction much less than a percent of the volume of our universe and those grand 3d galaxy maps that we flew through also i'm mainly probing a tiny fraction of the volume how can you measure how can we map out the rest there's more than 100 times more volume out there that we haven't really charted yet it's much like when people first started exploring australia for example they'd sailed around the coasts but didn't know much about what was in the middle we cannot no matter how big telescopes we build see galaxies out there because there aren't any right this is so far back in time that galaxies haven't had time to form yet but everywhere in this volume there is the hydrogen gas out of which the galaxy is later formed and a wonderful fact about hydrogen gas is it gives off radio waves that are 21 centimeters long so with a really good radio telescope we can map out how much hydrogen there is in different directions we can also figure out how far away these hydrogen signals are coming from because these 21 centimeter waves get stretched with a stretching of space they get longer and longer so if we get one detected to be 210 centimeters long for example we know that that came from so far away that our universe was 10 times smaller than so with a really good radio telescope we can make three dimensional maps of our universe which would blow away what we have so far in cosmology and overtake even the cosmic microwave background is the most sensitive cosmological probe at least potentially so there's a race going on now in the world to try to do exactly this this goes by the geeky name of 21 centimeter cosmology and we at mit have had a lot of fun being part of this race and i want to share with you just how much fun and how easy it is to build really state-of-the-art radio telescopes you can actually do it in just two minutes if you move fast [Music] [Music] do [Music] [Music] [Music] this is one of the reasons it's so much fun for me to get to work at mit because there's so many awesome students it's just a treat to be with and what you saw here is the fundamental reason why this is this new generation of telescopes is so cheap to build is there are no moving parts if you tell your friend that you need a one square kilometer radio telescope dish that has to be able to point with a big motor that person won't be your friend anymore and it's also even prohibitively expensive right however here all we do is we mass produce these cheap antennas just measure the volts coming from the sky and then we leverage off of the huge investment of the computer industry just figure out what the sky must look like and what's so cool about this is we don't just get an image from one part of the sky at the time the computer can figure out what the whole sky looks like all at once and moreover by putting the antennas in a particularly clever pattern we can make this design vastly cheaper than previous designs have been and we're so happy with how this technology demonstrator that we did worked out that we've teamed up now with a bunch of other universities across the u.s to put in a proposal to take these ideas and build something much bigger which would cover about almost 0.1 square kilometers you can see for reference this is a pretty big thing here and this is would cost not billions of dollars but 15 million for an experiment which would actually have the potential to be something which can ultimately overtake the best measurements we've done of any other sort in cosmology so to summarize we don't know the future of our universe but we do know that the future of the study of our universe the future of cosmology is very exciting because we have plenty of wonderful mysteries to pursue and we have a lot of really exciting experiments that are giving us fantastic new clues about these mysteries so thank you [Applause]

Channel: World Science Festival

Views: 101,350

Rating: 4.7297659 out of 5

Keywords: World, Science, Festival, Brian Greene, World Science U, University, New York City, Physics, WSU, #worldsciu, best science talks, max tegmark, the hubble constant, history of the universe, origin of the cosmos, cosmology, cosmic history

Id: SxmLdJvJ8lc

Length: 66min 56sec (4016 seconds)

Published: Tue Oct 27 2020