Is our universe the only universe? - Brian Greene

a few months ago the Nobel Prize in Physics was awarded to two teams of astronomers for a discovery that has been hailed as one of the most important astronomical observations ever and today after briefly describing what they found I'm going to tell you about a highly controversial frame it for explaining their discovery namely the possibility that way beyond the earth the Milky Way and other distant galaxies we may find that our universe is not the only universe but is instead part of a vast complex of universes that we call the multiverse now the idea of a multiverse is a strange one I mean most of us were raised to believe that the word universe means everything and I say most of us with forethought as my four-year-old daughter has heard me speak of these ideas since she was born and last year I was holding her and I said Sofia I love you more than anything in the universe and she turned to me and said daddy universe or multiverse but but barring such an anomalous upbringing it is strange to imagine other realms separate from ours most with fundamentally different features that would rightly be called universes of their own and yet speculative though the idea surely is I aim to convince you that there's reason for taking it seriously as it just might be right I'm going to tell the story of the multiverse in three parts in part one I'm going to describe those Nobel prize-winning results and highlight a profound mystery which those results revealed in part two I'll offer a solution to that mystery is based on an approach called string theory and that's where the idea of the multiverse will come into the story finally in part 3 I'm going to describe a cosmological theory called inflation which will pull all the pieces of the story together okay part 1 starts back in 1929 when the great astronomer Edwin Hubble realized that the distant galaxies were all rushing away from us establishing that space itself is stretching it's expanding now this was revolutionary the prevailing wisdom was that on the largest of scales the universe was static but even so there was one thing that everyone was certain of the expansion must be slowing down that as much as the gravitational pull of the earth slows the ascent of an apple tossed upward the gravitational pull of each galaxy on every other must be slowing the expansion of space now let's fast forward to the 1990s when those two teams of astronomers I mentioned at the outset were inspired by this reasoning to measure the rate at which the expansion has been slowing and they did this by painstaking observations of numerous distant galaxies allowing them to chart how the expansion rate has changed over time here's the surprise they found that the expansion is not slowing down instead they found that it's speeding up going faster and faster that's like tossing an apple upward and it goes up faster now if you saw an apple do that you want to know why what's pushing on it similarly the astronomers results are surely well deserving of the Nobel Prize but they raised an analogous question what force is driving all galaxies to rush away from every other at an ever quickening speed well the most promising answer comes from an old idea of Einsteins you see we are all used to gravity being a force that does one thing pulls objects together but in Einstein's theory of gravity his general theory of relativity gravity can also push things apart how well according to Einstein's math if space is uniformly filled with an invisible energy sort of like a uniform invisible mist then the gravity generated by that mist would be repulsive repulsive gravity which is just what we need to explain the observations because they're repulsive gravity of an invisible energy in space we now call it dark energy but I've made it smoky white here so you can see it it's repulsive gravity would cause each galaxy to push against every other Drive the expansion to speed up not slow down and this explanation represents great progress but I promise you a mystery here in part one here it is when the astronomers worked out how much of this dark energy must be infusing space to account for the cosmic speed up look what they found this number is small expressed in the relevant units it is spectacularly small and the mystery is to explain this peculiar number we want this number to emerge from the laws of physics but so far no one has found a way to do that now you might wonder should you care maybe explaining this number is just a technical issue technical detail of interest to experts but of no relevance to anybody else well it surely is a technical detail but some details really matter some details provide windows unto uncharted realms of reality and this peculiar number may be doing just that as the only approach that so far made headway to explain it invokes the possibility of other universes an idea that naturally emerges from string theory which takes me to part two string theory so hold the mystery of the dark energy in the back of your mind as I now go on to tell you three key things about string theory first off what is it well it's an approach to realize in Stein's dream of a unified theory of physics a single overarching framework that would be able to describe all the forces at work in the universe and the central idea of string theory is quite straightforward it says that if you examine any piece of matter ever more finely at first you'll find molecules and then you'll find atoms and subatomic particles but the theory says that if you can probe smaller much more than we can with existing technology you'd find something else inside these particles little tiny vibrating filament of energy a little tiny vibrating string and just like the strings on a violin they can vibrate in different patterns producing different musical notes these little fundamental strings when they vibrate in different patterns they produce different kinds of particles so electrons quarks neutrinos photons all other particles will be united into a single framework and they would all arise from vibrating strings it's a compelling picture a kind of cosmic symphony where all the richness that we see in the world around us emerges from the music that these little tiny strings can play but there's a cost to this elegant unification because years of research have shown that the math of string theory doesn't quite work it has internal inconsistencies unless we allow for something wholly unfamiliar extra tensions of space that is we all know about the usual three dimensions of space and you can think about those as height width and depth but string theory says that on fantastically small scales there are additional dimensions crumpled to a tiny size so small that we have not detected them but even though the dimensions are hidden they would have an impact on things that we can observe because the shape of the extra dimensions constrains how the strings can vibrate and in string theory vibration determines everything so particle masses the strengths of forces and most importantly the amount of dark energy would be determined by the shape of the extra dimension so if we knew the shape of the extra dimensions we should be able to calculate these features calculate the amount of dark energy the challenges we don't know the shape of the extra dimensions all we have is a list of candidate shapes allowed by the math now when these ideas are first developed there are only about five different candidate shapes so you could imagine analyzing them one by one to determine if any yield the physical features we observe but over time the list grew as researchers found other candidate shapes from five the number Brewin to the hundreds and then the thousands a large but still manageable collection to analyze since after all graduate students need something to do but then the list continued to grow into the billions and the billions until today the list of candidate shapes has soared to about 10 to the 500 so what to do well some researchers lost heart concluding that with so many candidates shapes for the extra dimensions each giving rise to different physical features string theory would never make definitive testable predictions but others turn this issue on its head taking us to the possibility of a multiverse here's the idea maybe each of these shapes is on an equal footing with every other each is as real as every other in the sense that there are many universes each with a different shape for the extra dimensions and this radical proposal has a profound impact on this mystery the amount of dark energy revealed by the Nobel prize-winning results because you see if there are other universes and if those universes each have say a different shape for the extra dimensions and the physical features of each universe will be different and in particular the amount of dark energy in each universe will be different which means that the mistery of explaining the amount of dark energy we've now measured would take on a wholly different character in this context the laws of physics can't explain one number for the dark energy because there isn't just one number there are many numbers which means we have been asking the wrong question is that the right question to ask is why do we humans find ourselves in a universe with a particular amount of dark energy we've measured instead of any of the other possibilities that are out there and that's a question on which we can make headway because those universes that have much more dark energy than ours whenever matter tries to clump into galaxies the repulsive push of the dark energy so strong that it blows the clump apart and galaxies don't form and in those universes that have much less dark energy will they collapse back on themselves so quickly that again galaxies don't form and without galaxies there are no stars no planets and no chance for our form of life to exist in those other universes so we find ourselves in a universe with a particular amount of dark energy we've measured simply because our universe has conditions hospitable to our form of life and that would be that mystery solved multiverse found now some find this explanation unsatisfying we're used to physics giving us definitive explanations for the features we observe but the point is if the feature you're observing can and does take on a wide variety of different values across the wider landscape of reality then sinking one explanation for a particular value is simply misguided an early example comes from the great astronomer Johannes Kepler who was obsessed with understanding a different number why the Sun is 93 million miles away from the earth and he worked for decades trying to explain this number but he never succeeded and we know why Kepler was asking the wrong question we now know that there are many planets at a wide variety of different distances from their host star so hoping that the laws of physics will explain one particular number 93 million miles well that is simply wrong headed instead the right question to ask is why do we humans find ourselves on a planet at this particular distance instead of any of the other possibilities and again that's a question we can answer those planets which are much closer to a star like the Sun would be so hot that our form of life wouldn't exist and those planets that are much farther away from the star well they're so cold that again our form of life would not take hold so we find ourselves on a planet at this particular distance simply because it yields conditions vital to our form of life and when it comes to planets and their distances this clearly is the right kind of reasoning the point is when it comes to universes and the dark energy that they contain it may also be the right kind of reasoning one key difference of course is we know that there are other planets out there but so far I've only speculated on the possibility that there might be other universes so to pull it all together we need a mechanism that can actually generate other universes and that takes me to my final part part 3 because such a mechanism has been found by cosmologists trying to understand the Big Bang you see when we speak of the Big Bang we often have an image of a kind of cosmic explosion that created our universe and set space rushing outward but there's a little secret the Big Bang leaves out something pretty important the bang it tells us how the universe evolved after the bang but gives us no insight into what would have powered the bang itself and this gap was finally filled by an enhanced version of the Big Bang's here it's called inflationary cosmology which identified a particular kind of fuel that would naturally generate an outward rush of space the fuel is based on something called a quantum field but the only detail that matters for us is that this fuel proves to be so efficient that it's virtually impossible to use it all up which means in the inflationary theory the big bang giving rise to our universe is likely not a one-time event instead the fuel not only generate our Big Bang but it would also generate countless other Big Bang's each giving rise to its own separate universe with our universe becoming but one bubble in a grand cosmic bubble bath of universities and now if you meld this with string theory here's the picture roulette - each of these universes has extra dimensions the extra dimensions take on a wide variety of different shapes the different shapes yield different physical features and we find ourselves in one universe instead of another simply because it's only in our universe that the physical features like the amount of dark energy are right for our form of life to take hold and this is the compelling but highly controversial picture of the wider cosmos that cutting edge observation and theory has now led us to seriously consider one big remaining question of courses could we ever confirm the existence of other universes well let me describe one way that might one day happen the inflationary theory already has strong observational support because the theory predicts that the Big Bang would have been so intense that as space rapidly expanded tiny quantum jitters from the microworld would have been stretched out to the macro world yielding a distinctive fingerprint a pattern of slightly hotter spots and slightly colder spots across space which powerful telescopes have now observed going further if there are other universes the theory predicts it every so often those universes can collide and if our universe got hit by another that collision would generate an additional subtle pattern of temperature variations across space that we might one day be able to detect so exotic as this picture is it may one day be grounded in observations establishing the existence of other universes I'll conclude with a striking implication of all these ideas for the very far future you see we learned that our universe is not static that space is expanding that that expansion is speeding up and that there might be other universes all by carefully examining faint pinpoints of starlight coming to us from distant galaxies but because the expansion is speeding up in the very far future those galaxies will rush away so far and so fast that we won't be able to see them not because of technological limitations but because of the laws of physics the light those galaxies emit even traveling at the fastest speed the speed of light will not be able to overcome the ever widening gulf between us so astronomers in the far future looking out into deep space will see nothing but an endless stretch of static inky black stillness and they will conclude that the universe is static and unchanging and populated by a single central oasis of matter that they inhabit a picture of the cosmos that we definitively know to be wrong now maybe those feature astronomers will have records handed down from an earlier era like ours attesting to an expanding cosmos teeming with galaxies but with those future astronomers believe such ancient knowledge or would they believe in the black static empty universe that their own state-of-the-art observations reveal I suspect the latter which means that we are living through a remarkably privileged era when certain deep truths about the cosmos are still within reach of the human spirit of exploration it appears that it may not always be that way because today's astronomers by turning powerful telescopes to the sky have captured a handful of starkly informative photons a kind of cosmic telegram billions of years in transit and the message echoing across the ages is clear sometimes nature guards her secrets with the unbreakable grip of physical law sometimes the true nature of reality beckons from just beyond the horizon thank you very much Ron thank you the range of ideas you've just spoken about are dizzying accelerating incredible how do you think of where cosmology is now in a sort of historical side are we in the middle of something unusual historically in your opinion well it's hard to say when we learn that astronomers of the far future may not have enough information to figure things out the natural question is maybe we're already in that position and certain deep critical features of the universe already have escaped our ability to understand because of how cosmology evolves so from that perspective maybe we will always be asking questions and never be able to fully answer them on the other hand we now can understand how all the universes we can understand how to understand the data from the microwave background radiation that was set down 13.7 billion years ago and yet we can do calculations today to predict how it will look and it matches holy cow that's just amazing so on the one hand it's just incredible where we've gotten but who knows what sort of block to may find in the future you're going to be around the next few days maybe some of these conversations can continue thank you my pleasure thank you thank you Brian you

Channel: TED-Ed

Views: 293,475

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Keywords: \Brian Greene\, TED, \TED, talk\, universe, multiverse, physics, \big, bang\, \what, caused, the, big, \how, many, universes, are, there\, theory, astrophysics, TED-Ed, Ed\, TEDEducation

Id: Vx2RcUQNh6Q

Length: 21min 48sec (1308 seconds)

Published: Fri Apr 19 2013