Was the Big Bang the Beginning? Reimagining Time in a Cyclic Universe

[Music] the daily rising and setting of the Sun the annual transition between winter spring summer and fall the repeated stages of development of living systems from birth growth maturity aging and eventual death much of the world unfolds through processes like these that are [Music] cyclical so might such rhythmic Cycles extend beyond those we directly observe and encounter perhaps being relevant to the cosmos as a whole some ancient cultures surely thought so Hindu tradition developed a complex series of time Cycles whose durations were hundreds of thousands to millions of years and Beyond other Traditions by contrast develop the view that the Universe came into existence through some kind of onetime creation event and some also imagin that it would end through a one-time [Music] Apocalypse in the early years of the 20th century scientists w in with yet another perspective that the very concept of a beginning might only apply to objects in the universe but not to the universe as a whole thus suggesting that the Universe always was and always will be this in fact was Einstein's view but only until about 1930 when observations by Edwin Hubble reveal that the fabric of space is expanding and so running the cosmic film back in time suggesting a singular event from which everything emerged in a violent swelling of space the Big Bang the big bang and its further refinements has become the dominant scientific Paradigm for cosmology but in the 1940s the physicist Richard Tolman introduced the scientific possibility of a cyclic Universe imagining that the Big Bang might be followed by an era of spatial expansion that would One Day end end leading to spatial contraction and the recreation of a dense universe that then expands a new through another big bang and the Cycles repeat further research taking into account the Relentless rise in entropy or disorder dictated by the second law of Thermodynamics reveal that such Cycles could not have happened infinitely far into the past the Cycles themselves must have had a beginning but an incarnation of a cyclic Universe has more recently been proposed and its proponents argue that the Cycles in this version could have been happening arbitrarily far back indicating that the Universe would not have had a beginning and if their calculations prove relevant to reality our current cycle may come to an end sometime in the far future this new perspective on time and cosmological development offers a controversial competitor to mainstream cosmological thinking and if correct will hearken back to ancient wisdom that encourages us to reimagine time and consider a reality not subject to the usual organization of beginning middle and end a reality that in this view May surprisingly be our reality good evening thank you you know we in science we work often within Paradigm certain established ways of thinking about this or that quality of existence and these paradigms can be quite useful quite fruitful quite powerful in guiding our research what we focus on and the way in which we analyze and interpret data but every so often we learn that the Paradigm needs to be looked at a new sometimes we need to take a kind of Sledgehammer to the Paradigm and and break it and start fresh to free up our thinking and when it comes to the big questions of cosmology how did the universe as we know it come to be we may potentially be in root to that kind of rethinking at least According to some of the scientists that we will have on the stage here in just a moment because as we just saw in that piece even Albert Einstein went through a radical change in thinking initially anticipating that on the largest of scales the universe was fixed Eternal static and unchanging and then ultimately through observations that actually comported well with his own general theory of relativity Einstein didn't about face and came to recognize this idea of a beginning as being relevant to reality as a whole but just because Einstein believed it and just because there's been a great deal of evidence supporting it it doesn't mean that we don't want to entertain alternate possibilities that might fit the data just as well and indeed that's going to be the subject that we are going to explore here this evening with some deep thinkers who have spent a lot of time thinking about these issues first up we have Peter Gallison who is the pelo University professor of the history of Science and of physics at Harvard University he's a leading historian of science whose research explores the interaction of experimentation instrumentation and theory in physics welcome Peter nice to see you Brian next we have Anna Yos who is a senior research scientist at New York University and a principal investigator of the Simon initiative new directions in cosmology and gravitational theory in her research she studies the origin structure and evolution of the universe great to see [Applause] you and finally we have Paul steinhardt who is the Albert Alber Bert Einstein professor in science and director of the center for theoretical science at Princeton University and a prime architect of our leading cosmological theories in 2002 his contributions to science steinhardt was awarded the prestigious dur medal good to see you good to see you all right let's just maybe begin with a big sort of overview of the human urge to understand understand Origins I mean we see it across cultures there's almost no culture doesn't come with some kind of creation myth creation story where does this urge come from Peter I think that the attempt to understand the structure of the universe as a whole to see the stars and to look at the Earth the plants the objects the peoples around us has always seemed to cultures something that had to be accounted for and sometimes that's done with images sometimes with with stories or accounts sometimes in books in poems uh but it is a universal drive to understand why we're here in the case of of the scientific tradition that we're in mesh in right now the study of the universe as a whole is a fairly recent Affair and it's really with Einstein and and one of the things that's so surprising is that he finished his general theory of relativity in 1915 1916 and it was only a year after that that he began to write down uh theory of the whole of saying this this account about gravity and our experience of it our solar system perhaps larger structure still was something that could be thought of as explaining and accounting for the universe widely considered not looking at the little clumps of matter that make up Earth or Saturn or the Sun but imagining mass and energy spread out over the whole and thinking of this as having a geometry this is really the birth of of a modern observational science of cosmology and that's so we're still in early days we're just over a hundred years into a history of cosmology even if the drive to understand the Universe goes back many Millennia yeah so so Anna you're in your scientific career you spent a lot of time trying to push the boundaries of our understanding of cosmological thought when you think back on the eras that preceded ours with people like the Matra and fredman and Einstein we'll perhaps talk a little bit more about their contributions do you see yourself following in their footsteps or do you see yourself as trying to blaze a completely new path oh definitely the first one definitely the first one so maybe I'll get to that but you know um and I'm sure that Paul will talk about it when cosmology really took off um in the 80s right um then it became your field it particle physics made it to an experimental science and I believe you know a few years ago and we will get back to that too we might have reached you know the limit how far we can push the boundaries of cosmology and our understanding by just using particle physics and what I have been doing a few years ago is actually going back and seeing maybe we missed something what we can learn by using general relativity especially using the mo modern to of computer computer simulations and maybe understand more so I definitely see myself much closer to the matter and tomman and Einstein then maybe most of my colleagues who are doing um particle physics inspired cosmology that was founded by Paul uh uh are doing so definitely the first one yeah now that's an important distinction to make it is the case now that cosmology is a field that brings in people from all over but certainly particle physics gravitational physics and in the early days it wasn't so obvious that someone working on the elementary particles would have something to say about the largest structures in the universe so Paul before we get into sort of the more modern story of cosmology where you make a tremendously important Mark can you just give us a thumbnail sketch going from Einstein's new view of gravity in terms of a flexible SpaceTime that can stretch and swell and so forth the ideas that then take that picture and turn it into a cosmological story can you just give us a feel for how that went sure I mean it's curious that Einstein had a theory that space was elastic and could change with time but came up with a static model which it was as boring as possible right so kind of weird contradiction and um when a few years later Alexander fredman showed that the same equations the same Einstein equations allowed for the possibility for an expanding universe or a universe that would expand for a period and Rec and Rec and contract and maybe a bouncing universe that would uh repeat this cyclically Einstein rejected that he he he he thought it was well first of all he thought it was mathematically incorrect he even wrote a little piece that argued that it was mathematically wrong and had to retract it that's right yeah that's right and obviously it was because it bothered him philosophically yeah so the early days of cosmology we also have to respect the fact that the early days of cosmology were not well informed by observations yeah at that time we didn't even know there were galaxies that was first around contemporaneous with fredman and then the idea that there were Millions no billions of galaxies out there and they're moving in all kinds of complicated ways is something that developed over the course of the next decade yeah and showed us that the uh that in fact fredman's idea and fredman and lra's idea that the Universe might be expanding was once hot and dense and has been expanding and cooling seems to be consistent with the observations at that time yeah and then over the course of the century we've obtained more and more observational information that reaffirms that part of the story over and over again yeah and so so Peter just want to Dr down a little bit and I don't want to psychoanalyze Einstein per se but um here's this radical thinker who gives this radically new theories Paul was mentioning where gravity is now who would have thought gravity is geometry warps and Curves in SpaceTime it's flexible and yet when the math suggests that flexibility applies to the entirety he recoils now there is some evidence and I and I wonder if you have a sense of this that Einstein did not like the idea a of a dynamic Universe because it suggested a beginning and a beginning to him felt too religious too theological you know I think the idea of a beginning has been attached to religious accounts for a long time yeah and in the dispute for example between those who advocated for a steady state universe that matter was popping into existence all the time versus the idea that there was a big bang which was only resolved with uh discoveries in the last decades this cosmological microwave background that suffuses the Universe of echo of the Big Bang um was a big shock and in fact got theological endorsement the the the big bang by the pope and more recently Again by by Fran by Pope Francis that that somehow the idea of a beginning was consonant with Biblical Genesis so these things had a deep root in culture and philos philosophy I think for Einstein too Einstein did care about experiment in observation and the standard view was the stars are fixed and you know it wasn't until the work of Edwin Hubble that you really started to have an idea that things were expanding so I think Einstein thought he was saving the phenomena as they say he he was he was doing what he had to do to the theory to make it consonant with what he thought was standard observation at the time and certainly was the case when the observations were overwhelming he didn't stick to his guns he absolutely turned around very very quickly on that point so Anna by you know 1930 there's general or by 1933 let's say there's General consensus that the universe is expanding Einstein's theory in Broad brush Strokes agrees with this idea and you could even say predicts it although it wasn't accepted in that particular particular temporal order but as people begin to think more deeply about this idea of a big bang beginning and everything swelling from that point cracks begin to appear or at least issues begin to arise they often go by the name of say flatness problem Horizon problem can you just give it take take one of those and just give us a sense of what the issue is so this is tough this is this is we still haven't through solve this problem so this is really these are the the problems at the heart of modern cosmology uh that you just mentioned and let's just start with with the big bang so I don't pick up your your problems first but let's just start with the big bang and say why doesn't expanding Universe imply a big bang as a beginning yes how do we know that well the answer is I think we all agree we actually don't um we are all being thoughted that if the universe started to expand 13.8 billion years ago then it had to have a beginning but that's an extreme extrapolation just because Universe at some point started to expand it doesn't imply that it had to have a beginning because the extrapolation from when the first element started when the universe was already cool enough back to something that we might call the beginning is actually bigger in temperature in energy in everything and we actually don't know the theory that would describe that phase because that would require the the marriage of quantum physics and and and general relativity and we don't have that theory so just to quickly jump in just to clarify I mean the normal the usual thinking or the usual lore is if the universe is expanding today you just kind of wind the cosmic film in Reverse right and going back it gets smaller and smaller and smaller and so so it does suggest that there was some beginning where everything was small everything is on top of everything else but I gather you are suggesting that that's taking that winding the film backward maybe too far yeah that's the point because just because something if you wind back the film to 13.8 billion years ago it looks okay but only because we observe it it could have turn turned out that we shoot up the satellites look at space because when we look far enough that means we look back in time um and we see something else that's why we are scientists we assume something but then we check we check by observation and if something else comes out then we would have to conclude we were wrong so we know that because we check that's what we know so we know how uh cold and vast the universe is today uh that has overall a temperature of 2.7 Kelvin it's super super cold and then we ex backwards in time in 13.8 billion years then we see how hot it was we see the 1 second Mark so 1 second after the beginning of expansion it was 10 billion Calin but what you say now is it's actually worth looking when the next slide comes by how much further in energy or in temperature we would have to ex plate backwards from the first second Mark to what we call a big bang well you see it it's a million times a million times a million times a million times Calin so it's a huge extrapolation it's much bigger extrapolation between whatever the beginning might be and the first second so if you say oh it is just a little bit going backwards no that's just being thought wrong right uh okay so so you actually it's further from the first second to whatever it takes plus we don't have the theory so maybe just the first thing and now if we get to your question well actually before I love the direction you're taking this can we just follow on that I think I think it's actually great so you've now encouraged us to question the intuitive lore that just because we're expanding today it had to all come back together early on so that raises the question of um there may have been a singular time for a big bang perhaps there's something else and you're going to suggest something else as we go forward not to uh spoiler alert here but I'd like to continue on the more traditional trajectory in order that the new ideas that you're going to suggest have a counterpart to bounce off of especially because Paul you're one of the architects of the other theory that your new theory is going to try to undercut so it's a very you know interesting situation that we find ourselves in so when it comes to of of the Big Bang if there was one certainly one of the things that we would need to explain is what drove the outward expansion and certainly the approach that you developed inflationary cosmology at least in principle supplies an answer to what that outward push would be so if you're willing to play along on the old theory that you developed that you no longer are in favor of if you could just give us a sense of what the inflationary Theory says um so what was the reasoning behind the inflationary Theory well the problem that the big Bank uh presents to us is that uh as the universe first emerges we expect there to be large Quantum effects uh that create wild variations in SpaceTime itself wild variations in uh the distribution of energy and radiation um and and also wild variations in the Curve of the universe yeah so if you simply went from that point forward like the diagram was suggesting you'd expect by the time you get to the 1 second Mark you have a universe that's still very curved with a very wild distribution of matter and energy um and remarkably that's not the case so whereas in general relativity the least likely thing to have is a smooth and flat universe it should be very elastic and wildly varying we actually need to explain why when we get to the 1 second Mark some somehow has to have made it smooth and flat and observationally we know that at that Mark and now we can say that we couldn't say that in the 1980s when inflation was invented but now we can say in fact looking at the microwave background we actually see that large scale homogeneity compared to the size of the UN part of the universe that would be causal contact at that time that could possibly have we're going to come we're going to come to the microwave background and the issue of homogeneity and the temperatures being uniform on scales that are hard to explain but can we first Peter just give us a thumbnail what is the cosmic micro background radiation gives me a vital tool in the conversation so as the universe develops at a certain point the electrons and the and the light begin to interact and it begins to you we can you can see the Echoes of that radiation uh even today I mean it's somewhere in the television channels uh Channel 69 in the usual 1% of the static or so so when you turn on your television you're seeing some echo of that and it can be studied much more finely first it was discovered in the 60s and then it was with satellites of increasing accuracy we've been able to look at finer and finer structures on this noisy background in the sky and it amazingly enough enough it looks pretty much the same in all the different parts of the sky and at different scales and if the universe was developed in isolation from each other then why should it why should it have the same structure it's like we we see this room the temperature is about the same in different parts we can explain that because the hot parts and the cool Parts come to equilibrium but if they've never been in causal contact as would seem to be if we run the film backwards from today then it's a great mystery how did they get to be so homogeneous in scale and and space so that's where perhaps we want to take up we have a little visual just to emphasize that particular puzzle if you imagine looking out from Earth and you took away all the stars and everything else that was contributing any other heat what would be left over is the heat from The Big Bang that Peter was just making reference to and the point is the temperature at widely separated points in this background radiation is almost identical to Fantastic accuracy but yet if you look at a signal that attempts to establish contact between these widely separated regions there isn't enough time for Those Distant regions to have communicated so how in the world did they get to the same temperature so as Anna was describing we have this Theory from early on expanding Universe it seems to match the fact that the galaxies are all moving apart but there's a real issue that there are unexplained qualities of the world and and and and Paul this motivated you certainly in the 1980s to try to solve that problem right so the first thing you might try to do as a theorist is try to keep everything in place that was there before but add something that would solve this problem and that's what inflation uh how inflation was introduced so something between the big bang out of which would come some wild dis you know uh wild Universe something that would tame it that would smooth out flatten the universe well before we reached the point where the temperature was around the temperature of the center of the sun when observations tell us it already was smooth and flat and the microwave background reaffirms it so that was that was that was the idea um it didn't really explain why the universe started expand expanding that's not exp in fact it needs some it's not a complete Theory inflation itself it needs something before it that explains how you got to inflation so the expansion already had to be happening for some reason um in order for inflation to have any chance of Ever Getting start but you did have a at least in some versions there's a mechanism by which the expansion is driven if you have some energy that gets uniformly distributed through a patch of space and even in Einstein's basic general theory of relativity it gives rise to repulsive gravity something we've never experience in everyday life because we only experience clumpy things like tables and chairs and planets and stars and so forth but in Einstein's theory you leverage the fact that it can have an outward push it could accelerate the expansion which accelerates the expans compared to what you would have gotten straight from the Big Bang and the concept was that if you can accelerate this expansion this would take a very uneven universe and curvy universe and smooth and straighten it out that was the concept but before we go on can we just show a little visual on that because it's sort of a key idea so if you imagine just having a shape that has some curvature to it a simple shape it looks like a ball but just a sort of a storybook version of it if we make this bigger and bigger through rapid expansion it looks flatter and flatter and so even something that have some curvature you it's why we think that the Earth was flat you know you know 10,000 years ago right because it does look flat locally so if you can have this accelerate expansion your theory together with Alan Guth and Andre Lind and your student I guess Andreas albre as well you had this proposal for how you get rid of the wild undulations get rid of the curvature as well as find that these distant locations would be able to be in causal contact early on before they got rapidly separated so it starts out really small and in equilibrium expands faster than it would have under the big bang and then it begins its more stately March through time right so it's some it'll be important for our later discussion that embedded in our minds was the idea that the only way to smooth the universe yeah was to make sure that distant regions were at causal contact much closer together in the past than the Big Bang model so you got them really close together in the past in in the inflationary Theory and then inflation sort of lets them catch up to where they otherwise would have been so they can start closer together so Anna you must have learned about the inflationary Theory as a as a student I mean how was it presented to you as as you know the theory it was the way of thinking about how the universe evolved from the earliest fraction of a second yeah you ask actually at the right time because so so I am younger than inflation so I didn't have a chance to contribute to it and I'm younger than it uh and so when I came to it first time it was over 10 years ago I was a student um and it was textbook material so I was fairly young and um everything what was just said now was presented in class and not as possibility but as something that's been established and what just por I explained is something that I remember it I when I first heard about first heard about the theory I just thought this is nuts because it's really nuts because these guys tell me that they explain the smoothness by assuming it and that somehow sounds good because if if you remember what Paul said well we just wanted to put things into causal contact and how inflation operates is and it is pretty clear how it's being taught is that you take things out of causal contact very rapidly and then when inflation stops and the expansion starts a slow expansion and these things come back again into causal contact and it looks like that everything uh um is the same and the reason would be because it was the same but you correct me the idea was though and that's why we started with the big bang that we have to explain this smoothness um this uniformity uh starting from arbitrary conditions starting from conditions that quantum gravity would give us and that's not smooth that's that's it just it just seemed to me like these guys actually assumed what they want to explain and they just made a little trick by saying okay we assume that some part of space looked already right and we try to blow that up over many many many parts of space and then we somehow have the right answer but and this is actually what I came back to then later 10 years later to work on it really doesn't work so it really what and and we will get back to that later it really was the you know my impression as a student I don't want to work on this I don't want to stay away from this because somehow people came to believe that if they start from the right condition then they can explain the same conditions on larger scales but at the same time they also believe that somehow generic that somehow comes out of the big bang and for me this connection this long connection what's up there was always missing so I decided I will not work on this I will just do something completely different I don't want to have anything to do with cosmology and I actually really started to work like that until I met Paul uh three years later in Harvard so we'll pick up your story of where you ultimately did come back to cosmology in just a moment but but Peter can you Channel if you would just to give us an accurate snapshot of where people's minds are in cosmology you today how is the inflationary Theory again you know developed by Alan Guth Andre Lind Paul and so forth how is that viewed in the community right now I'd say the ma the majority view is that some version of it is right although we may not have the right model there may be things that have to be adjusted or changed but that basically something like that so the general Paradigm that there was a early period of Rapid expansion that can do these tricks of bringing things into calls of contact and pushing them apart that that's the basic yeah I what honest said it it pushes it it pushes the problem back towards the Big Bang Yeah and allows us to have an account for things like the causal connection that otherwise seems impossible to explain but um it comes at a cost and we'll be talking about that cost momentarily and so I'd like to give you guys the floor in just a moment to go in a in a direction that you think may be more fruitful to filling in that part of our past but again from a more sociological standpoint I mean just thinking historically is it common that there are periods of time where everybody thinks X but it's really y I mean is that a rarity you know and and how would we fit into that historical Rhythm so I mean certainly have been periods where the consensus was against special relativity against general relativity against it because it seemed philosophically impossible or it had too much mathematics this what to the physicists was unimaginably difficult differential geometry uh uh which is now taught as part of mechanics and the ordinary course of things but at the time was shocking the development of quantum mechanics upset people hugely including uh are Einstein uh there have been times when major so there certainly have been times when majority views have been disrupted by alternative views once a view is established though it's hard to get rid of it without an alternative and I think one thing that I mean just to anticipate a little bit is I think having an alternative is always helpful to the articulation of where we are in science so Einstein's theory for a long time didn't really have any alternative Brans and dicki came up with this alternative Theory I'd say it's a minority quite significant minority of physicists who today think that's the way to go yeah but the very fact that there was an alternative pushed people to think about testing to look at what is the what are the fragile parts of Einstein's theory and where where can we push it with observation and and and inquiry in in different forms so I as I say to to anticipate a little bit I think there's a an absolute good in having an Al alternatives to a dominant Theory whether it's general relativity or inflation or the Big Bang uh because it it forces us to articulate where we are yeah and what the different aspects of the theory really ought to be so so Paul Anna mentioned and showed that in that nice graphic of just how significant an extrapolation it is to go from things that we know to things that we want to understand but don't have direct observational evidence for and so on so if you're going to imagine something other than a bang what is the next option that comes to mind than that you've been developing well an alternative that people even thought back thought about back in the 1920s is a bounce that the Universe going backwards in time would be contracting okay and at some some point would at some stage would reverse to expansion yeah in fact I guess Richard Tolman I guess is among the more well-known of people there may have been others who were thinking about like fredman as well but um so tomman my understanding is he tried to make that idea work this notion of a of a bounce work it was it was freedman's idea and he was trying to um explore Freeman's idea and whether it could be consistent with the laws of thermodynamics the second law of Thermodynamics and how was that received back then when it was initially introduced it was a big blow to the idea of a cyclic universe so the so the so the idea of a cyclic Universe had been around since fredman uh and when we discovered that the universe is expanding that was on the list of possibilities that that the Universe could then begin Contracting and and and bounce um and that this would get rid of the problem of having a beginning of time time if it were cyclical forever in the forever into the past uh and that intrigued people but Tolman pointed out because you wouldn't have to explain why there's a universe at all you just say there always was a universe it's just been expanding Contracting forever it's another logical possibility yeah maybe we're spending a lot of time trying to explain something the Big Bang which never happened so maybe there's another logical possibility to be explored and ultimately you want to turn that into a theory that has testable consequences so tomman was exploring exploring that idea I should I should emphasize that the idea of bouncing in those days was that you would bounce until all of SpaceTime disappeared and then reappeared and and then uh expanded and contracted and disappeared again so it was this kind of bouncing universe and it led to this kind of problem that Tolman pointed out that uh if you think about the disorder in the universe the entropy or disorder in the universe whatever disorder you had in one cycle would be Rec contracted or reconcentrated as the universe approached the bounce came to zero size and would be there when the universe began the next bounce but it would influence through its gravity gravitational effect that next bounce so instead of that next bounce being identical to the one before it now has whatever disorder it's going to produce plus the sum of all the disorders of all earlier cycles and then he showed that that meant that the Cycles couldn't be identical obviously there's something you can measure about the universe that would the amount of disorder that would distinguish one cycle to the other and the Cycles in fact must be growing in time going a growing in duration going forward in time which means if you go back backwards in time they were Contracting rapidly to zero so you didn't get rid of the beginning issue you got back to the beginning uh and so that um discouraged that and other other historical facts discouraged people from thinking about cyclic models for a number of decades yeah basically nobody really took the idea seriously until the work that you have recently pioneered right there hasn't been much in cylic cosmology since tomman right uh well in bouncing cosmology there was a movement in the 50s and 60s where people began to think about the idea that the Universe could have undergone a per period of contraction into a bounce and then expansion um it ran into yet other problems whenever you concentrated the universe to a point uh that meant all the matter and radiation that existed in the before the bounds was being scrunched together and when you scrunch that much energy together uh you can't really predict what's going to happen next there's no particular reason why you it would bounce when you because of its gravity strong gravity holding it together yeah so that that was a concern and then another concern was as the universe contracted in those models um there was a instability in which you would begin to um change shape in wild ways depending upon which point in space you were observing from uh and that essentially it was DEH homogenizing the universe it was messing it up as much as you could possibly imagine so you had three problems with cyclic models tolman's entropy problem what do you do about crunching all this stuff together how do you survive that and ever have a bounce and what do you do about the inhomogenity you would naturally create during the kind of contraction uh kind of contractions that people were thinking about in the 50s and 60s right so I think that's a a good motivation to uh think now more fully about the work that you and Anna and other collaborators have been working on because I gather you claim that in your version of this cyclic cosmology you have resolved the majority if not all of the problems that beset it early on so however best to explain it Anna Paul just give us uh a sketch and then I think you have some nice simulations that will both argue for your approach and also against the inflationary approach trying to bolster the case for this reimagining of cosmology sure I can start yeah go ahead okay so so I think you know the three problems that Paul mentioned they really affect two phases actually three because so one point we call it cyclic cosmology but one point that we often tend to forget is and that was what Paul was talking about in the 50s and 60s there really are two different scenarios we could imagine a universe that started to contract bounced and now expands forever maybe that will be I think we have to fix first that scenario is that possible if that's not possible you will not get a cyclic cosmology and it's easier to fix in a certain sense because then because that doesn't suffer from all of let's say the torment problems what happens if the expansion stops and goes over to contraction can can you do that just get one cycle to work get go get one cycle done and this this is really where we started and again even the cycle the only thing what we know how to describe today is the expanding part we know that nobody debates about it that's what we call the consensus mod or Lambda CDM so what we have to fix is can we describe this Contracting phase that in particular that it approaches the bounce without this crazy chaotic Behavior because if and and at the same time Smooths the universe so that once we bounce we see exactly the universe as we observe it in the microwave background so that's the one question is there a good theoretical model for the Contracting phase yeah and we will start with that and then the second part is and I imagine Paul we tell more about it it's a different story can we really let's say the Contracting phase works and we will see that does that's easy we don't have to bring in new physics we don't have to do anything about quantum gravity we just we just really use general relativity and some new computational tools but then the second question is once everything is fine can we balce that will be much harder so that's the second part and the third part is if we can contract pounds expand can we start again to repeat over and over without suffering from the entropy problem so I think these are the three points we should go through um all right so can you get the Contracting phase to work yeah so one thing that we um that I think that we should um talk about is that there's um not just a single kind of contraction and not just a single kind of Bounce so up until our work when people were thinking about bouncing universes they were thinking something that would be analogous to a balloon that was collapsing to zero and expanding and collapsing to zero and expanding so the thing that was cycling was what we call this amount of stretching or the scale factor yes that was the important cycl cycling uh Factor but there's another thing in general relativity you have to keep track of in cosmology which is what we call the horizon or the h volume how much is in causal contact those things that can actually send signals to each other and perhaps get to the same temperat like what you were drawing in in the figure before and it the the the Hubble Horizon is how far you can see so an observer in the universe like us can only see as far as their Horizon they can't see the entire space so think about a little circle on our on our uh balloon which would represent if you were standing in the middle of it the Horizon how far you could see yeah well the thing is the how much how fast the universe expands or stretches is different from how much the Hubble Horizon stretches so whereas before we were thinking about a universe in which uh the entire balloon was shrinking and expanding let's imagine instead just the observable universe were shrinking expanding in a cyclical way yeah space would always exist in fact space could be continually expanding but the amount you could see if you're an observer in the Universe since you can only see a limited region of the universe would look the same from cycle to cycle if you looked in your environment same as if you were on if you imagine that on a planet that was expanding or Contracting but you had a limited Horizon so that means you don't have to have space shrink to zero to have a cyclic Universe it's really a fundamental different kind of cyclic universe that means space is ordinary during the bounce uh it also means anything that was in your horizon before the contraction began like the amount of disorder in entropy has been stretching out while the Horizon is contracted so yes there's lots of disorder out there but it's been spread out uh to parts of space that you cannot observe so this is really a fundamental different kind of cyclic universe that evades the disorder problem okay it also happens to be that this condition also prevents the kind of chaotic effects we're talking about uh that that were discussed in the 50s and 60s and you never have crunching because you know on average space is always expanding I see so if you're focusing your attention on the part of space that we observe we don't have to use the language that perhaps you would have invoked in an ordinary cyclic Theory everything is crushed together and then re-expands and in that way you're avoiding the more thorny parts of how a bouncing cosmology would need to to be developed yeah you're getting rid of the crunch what we sometimes call Crunch it's a crunchless bouncing you still need to ensure that during that phase of contraction you're setting us up for Success yes after that phase concludes and we go into the more ordinary expanding phase so are you I mean is it the case that you can set up the model in such a way that the conditions at the end of this contraction phase will yield a universe like the one that we see on I can I can jump in because because I I think it's it's it's actually worth rephrasing just once more in the language that we used earlier what Paul said because it's really a fundamental feature of Relativity that inflation Ed that we used to explain themas the the consensus model also contraction that given what fills the universe the rate at which your caal connectedness the radius of coal connectedness evolves can be very different from the rate at which space expand BS or contracts so it's all the same trick so what we see um um let's say the the slowly expanding Universe we know that space expands slower than the region of coal connectedness so stuff comes into us that's that's that's the consensus model that's why we needed inflation now it's very natural to say well can we uh find a model can we find some stuff energy density that during a Contracting phase we make the the region the space contract much slower than the region of coal connectedness because what happens is you sh a huge amount of space and instead of doing much to space so let's say if you start with a w space a universe like today full of black holes and you ask um what kind of contraction do we need then we would need a type of contraction in which black holes stay approximately at the same distance they come a little bit closer but not much at the same time though the radius of coal connectedness sh drinks very rapidly and that's what you need and the nice thing is that we can do that in relativity so what this model needed to be fixed is uh is something that people in the 20s 30s 50s 60s 70s would never have thought about we as a matter of fact inflation and all the ingredients that that people in the 80s invented um something what we know from the hixus scaler field from particle physics that can actually uh Trigger or excert this behavior that space tracks very slowly but the causal Horizon shrinks very rapidly and this is exactly what can be done so that solves the problem of no chaos and everything is smooth and uniform as space contracts and goes to the uh goes towards the bounds moreover it's really important to note we don't need new physics so it's all general relativity uh quantum physics only enters on small scales and no quantum gravity is required and all ingredient what's require bi is that we are very familiar with in physics it's a scalar field that dominates this energy density and is this a a simulation that can give us some numerical evidence for what it is that you were referring to do you want yeah so so let's go back let's not start the simulation so I think we should pan back a little bit and just talk about the role simulations plays yeah so cosmology started first as pencil and paper yeah then we went to particle physics observations and then then what came up really the last 20 to 30 years is we actually can rerun the universe without putting up satellites to look at our current universe but we can simulate the universe and I think before we you know describe maybe the features maybe Peter want to comment yeah I mean how have we gotten to a place where there's you know a third way I mean there's Theory there's observation experiment simulation I mean what role is simulation playing in these Cutting Edge ideas well I I think that starting in the period at the end of and just after World War II you had physicists thinking about the hydrogen bombs this is what Oppenheimer much on everyone's Minds these days uh was concerned about he objected to its building teller wanted to build it Stan ulam a mathematician began to think about a way of doing it but the hydrogen bomb unlike the atomic bomb used basically all of our physics how radiation is transported hydrodynamics nuclear physics the whole works and you couldn't do it with pencil and paper or even with the sort of old adding machines that were available at Los Alamos and so ulam and Von noyon began to think well maybe there's a kind of game you could play where you know if you wanted to simulate the diffusion of a red dye in a thin tube of water you sort of could flip a coin and go to the right if it was heads and left if it was tails and do this over and over over again and you could model how the diffusion worked and that's a red molecule that's going right or left exactly and you could see how they accumulated and what the distribution was and suddenly people began to think well maybe we could use this to simulate these very complicated processes where radiation is coming in and and we use it more and more it's been a big slow process over the 50s and 60s and 70s I remember times when you see an astronomer would say well you made a simulation that showed that such and such an effect works but what's the real equations that tell us this but at a certain point people stopped asking that question the simulations were what you needed to understand Galaxy formation and neutron stars these very complicated objects that are like the nucleus of an atom but much bigger and you have thermonuclear explosions going on in the surface and new forms of matter at the inside and there's so many things in astrophysics that require this but not just astrophysics particle physics what goes on at CERN or firy laab or or or or any of the other accelerator Laboratories what goes on in the plasma physics lab these experiments to do nuclear fusion all of this requires simulations so when it came time to look at colliding black holes you needed these numerical methods that could take you from Pure description of independent black holes to this merging process and those numerical techniques um have then been adapted by Anna to to to to look at the early universe and those techniques were vital to the success of Lio right vital you like I mean Lio was like jumping out of a window from a high building hoping you could invent just Lio is the gravitational wave detector I think most people are familiar finding the first ripples in the fabric of space but when the signal goes by you don't know what it means if you don't have some library of simulations that you can compare the signal to so without those Sim ulations it would have been lost on us that the signal absolutely crucial and in so many areas of Science and in ligo you need these numerical simulations so there is a kind of there's experiment or observation there's Theory and there's this terium quid this third thing which is simulations and that's the world of science that we live in now in more and more areas so I think that's a kind of background to the important work that Anna's been doing uh to try to give us a picture of this very this this this these gravitational uh intense moments in the early universe and how it works so this this quick little simulation that I think we we already saw we can run it again tell us what it's meant to show us so um just just one you know two sentences maybe because we swallow it so often so we should appreciate that we have relativity for over 100 years yeah but it took almost 80 years since we had relativity to put the equations on the computer so this is really difficult it's it's not because that's one thing that we didn't immediately have the computers but it required a lot of math and a lot of computational skills so it's amazing that we have it yeah and then we first did it for black HSE and then what we also figured and this really actually relates back to where I was stuck as a student with inflation and there simulations are crucial so can we really check that the theory starting from arbitrary condition arbitrary arit conditions arbitrary distribution of energy arbitrary initial geometry so we can test what would be after the bang and we could test how what kind of condition a Contracting Universe could start from and this is exactly what we can do so that's one example of this simulation of this this is what we see there we can see it repeatedly but I briefly describe what happened so we take the Einstein equations yeah and put them on the computer and the feature of the Einstein equations is that you can give them some initial distribution of matter and initial distribution of geometry so it can be and that's what we see there so we see that these omegas they just represent distribution of curvature we talked about curvature that would be if it's flat then it's good if there is no curvature then it should be at zero so there and there that should be a sheet and the same same is uh uh true for this uh Omega s which is an isotropy so curvature would tell you gradients how directed uh the the the geometry is how much uh uh directedness how much it matters uh which direction you look and U an isotropy would tell you how much sheer there is in in the universe so we know that there are almost infinitely many so geometries possible at the beginning of the Contracting phase and it's a good theory if whatever we give to the universe lots of sheer lots of curvature it all goes away right it should go away and we only can do this only can test all these universes by simulations so we cannot rerun our universe and that's what it shows so if you it's it's very quick so it's it it is and this is amazing because that's one of the thing is so this is a representative example of a very curved and uh a universe which is full of sheer so we thought people thought in the 50s 60s that will definitely lead to this chaos but now we put in this third ingredient that we talked about the scalar field yeah that should do this magic that while space hardly contracts the caal connect region of causal connectedness just shrinks down and if that's true then what one would see is one sheet up there which is blue completely uniform and the sheet of curvature and an isot Shear just vanishes at zero no contribution to the energy density so this boring outcome of the simulation is actually our universe so we do a lot of work to see something boring at the end I always so the fact that it's nice and flat at the end tells you that regardless of what chaos you had before it got to the end of the contraction phase it gets resolved wiped out and you've got a nice Placid starting point for the expanding phase now it also shows it's really fast it does go a little bit fast so on the one hand it's disappointing you'd like to see a simulation which has a lot of action yeah but there's actually a really important message there that this slow contraction compared to inflation is much you know is very fast in smoothing the universe so it's very effective at doing what inflation itself is claiming contract so the universe only has to contract by a factor of two to produce enough smoothing and flattening to take us from a wildly inhomogeneous Universe to a universe as smooth as what we see in the microwave so you view this as a strong piece of of simulated theoretical evidence that a period of slow contraction does the work necessary to set us up for an eventual Evolution that yields a universe like the one we're familiar with you have some simulations that I gather also take a look at the inflationary model to see whether it does the work that it was claimed to have done so I think we have that on hand before we do that I want to make one other point uh because we discovered something with these simulations that we didn't anticipate okay so when you saw that originally curvy Treet that's that's spanning directions uh regions of space which are not in causal contact and because the contraction is so little in fact they never become in causal contact yet somehow magically we ended up with the smooth Universe this changes our assum this goes back to our assumption that we thought we needed to put things in causal contact in order to smooth them out that's true in an expanding universe that's your only hope in an expanding universe but in a Contracting Universe what happens is that different regions of space will independently smooth of each other even though they're out of causal contact in fact what happens is because the Horizon shrinks so little very quickly All Points become disconnected from one another which means that the equations that they follow now don't depend on their neighbors they only depend upon what's left after you ignore your neighbors that's the same for every point and that's why they converge so rapidly to the same final thing this is a new phenomenon we call it ultral locality the context in this context so they're all dynamically going to the same state as opposed to interacting in order to negotiate coming to the same they don't have to negotiate and it's really important one point because you know we tend to always think Newtonian but you would think if something contracts it collapses so it's the opposite of what's happening so it becomes more messy not local but what we discovered is that's really Newtonian and now you see when you simulate relativity how different the universe that's govern where gravity is governed by relativity and not neonian physics is because contraction doesn't have to be collapsed it's actually you know the gradi the the directionality this mess vanish it C all curvature immediately goes down so that also represents that you know our picture of of gravity really has to be radically revised whenever we we whenever relativity is important so contraction is not collaps and that's why the previous simulation show and that that's the reason why uh um everything gets smooth because you know you just the Contracting phase is exactly the opposite as you would expect it doesn't mess things up but smooth things and and you cannot help it no matter where you are in every little uh causally connected space will look the same because it cannot do it otherwise and it's it's done to relativity and that's very different as Paul will I think explain an inflation and it and and part of one of the things that happens there I said is that neighbors become unaware of one another so in terms of general relativity that means that what we call gradients the variation from point to point those become irrelevant every point is operating independently right it's a feature that's specifically for contraction so now we can turn well what about expansion yeah what happens there well our intuition had about inflation had been if we began with some very inhomogeneous Universe okay then essentially what in inflation does is stretch it yeah and as it stretches and if you think about a curvy rubber sheet it's and winkly rubber sheet you think it's going to smooth it out but that's a very bad metaphor for general relativity because in a rubber sheet those wrinkles don't resist the expansion but in general relativity A Wrinkle In SpaceTime has stress energy that gravitates itself and wants to fold it out itself up more so what you really have is a competition between whatever wild initial conditions you have that want to fold themselves up versus whatever is trying to inflate and the question is which one wins okay so now we've taken the same techniques that Anna developed for contraction and then she repeated it for expansion again we're beginning with a universe like before with the green represents how much sheer there is in the universe so it's almost all sheer in the universe the red represents how much matter and the blue represents how much curvature you have and so if you go back to the beginning go back to the the beginning again now you begin with a different vantage point because in this case the opposite happens than what we saw in the contraction case where everything smoothed right now what happens is things get Wilder and Wilder and Wilder and that's because you're it's just with the effect I talked about if your fluctuate if your initial variations are wild enough they actually resist the expansion prevent inflation from taking hold in fact they grow in intensity becoming more and more resistant to the expansion again that's only possible to learn when we have simulations we could not have gotten that out of solving with p pencil and paper equ and so the proponents of inflation uh what what is their reaction to these simulations they're overcome they're overcome by the variations in shear and and curvature they can they can never take hold relative to them because what you you see when the uh when you see the curvature going yes you know but I think you're answering what happens to the uh structure of SpaceTime I was actually talking about the people who um who who are in the inflationary mindset when when when you've presented like these simulations what's the what's the response um I think it's a little early to say because what I've shown you here is only a few months old so the word hasn't really gotten out to see see uh to see every all the all the work that we've done on the inflationary case and to contrast the two so I think this is something which I hope will be one of the things that contributes to people opening up their minds to the idea that slow contraction really is a powerful smoother for this subtle reason that I call Ultra locality this subtle feature of general relativity and the converse of that is that it's a challenge or problem for inflation that inflation unless you begin very close to smooth to begin with can never take hold and um in any theory of course you're making a variety of assumptions and we're not looking under the hood right now can you give us a feel for what kind of assumptions go into this are are are you making a variety of choices of uh the behavior of these fields that are driving part of the Dynamics here how how generic would you say that these results are yeah the the the basic thing you that the basic uh quantity that you're uh besides the shear and curvature which are already part of general relativity the the component and just give people a sense of sheer it's the way in which space is in some sense pulling on itself stretching this way while pulling this or something like that the same effect the gravitational waves produce when they pass through a detector they stretch it in some directions and contract other directions and so that's a kind of you know uh unevenness yes to SpaceTime and would make an unevenness in uh Cur uh in um the distribution of matter and energy and how everything behaves so the the thing the component that you're adding for inflation was a scaler field which had a property that it induced accelerated expansion yeah and here you're doing something analogous you're producing introducing a scalar field its details are different but it has the property that it causes this slow contraction um and in terms of uh the ingredient it's it's very similar it's a scale it's some field um it's just the details of how its energy depends on its strength field strength that what I was sort of driving at do you have to fine-tune that in any way I mean certainly you have to fine-tune it inflation to make sure that this ingredient gives rise to that energy we were talking about that can give the repulsive push that drives inflation how much tuning do you have to do within the theory to make this behave as we've just seen um I mean we have we have studied that you can run the simulations with varying also the ingredients so we call that the potential energy of the ingredient and what's interesting it's actually relatively insensitive but I should say that in inflation it's Al so the two pictures that we saw they are fairly generic for both scenario so we did just on the scale of hundreds and thousands so it's very Qui quick to do very quick to run the simulations you know what you saw it's maybe 30 minutes uh for contraction so we could run during the last few years hundreds of simulations so this is very generic um you don't really I I don't think that this at this level it involves the type of tuning that you might need to fit the little Wiggles in the microwave background and then in inflation what happens is because it's so complicated it actually takes much longer to run the simulation sometimes the same simulation the same initial conditions takes a week or more just just for for comparing until it runs and but I would say that that's also fairly generic so we have Nan also there hundreds of simulations with different types of potentials and the outcome what you see this contrast that the one Smooths and the other one not that's not tuned in either case and where tuning comes in I think is and maybe they'll get back to this what the quantum effects how we moderate the quantum effects in both theories now now we called this session reimagining Time Peter because part of the idea is if a bounce can happen and I'll turn to you guys in a moment maybe many bounces can happen if many bounces happen maybe time doesn't start maybe time doesn't have a beginning how how do you if that's where this goes how do you think that begins to realign people's thinking on on how we fit into some grander Cosmic order you know I think in the largest frame we're caught in science with an implicit conflict of philosophy on the one side we don't want to be special we'd like to have a machine that predicts a theoretical machine that predicts the universe without having to tune many different knobs exactly right uh and have everything fall apart in its predictive power if they weren't tuned just right it would all fall AP we we don't want a machine that requires such exact tuning and by machine I mean the theoretical apparatus yeah um we you know even from the time of cernus uh I mean cernus said we're we're not special we're not at the center of the universe and this was you know then oand wrote this preface saying he's just kidding basically read it it's really great mathematics it's great for predicting where the planets will appear to be but he doesn't really mean we're not at the center of the universe because this was a fundamental anxiety that's a kind of spatial specialness and then there's the temporal specialness we you know we have this long tradition in in uh judeo-christian thought and in others that there's a moment of creation yeah and that's been objected to by scientists at different phases who didn't want there to be a special uh time of the beginning and that maybe the universe was popping into existence all the time and just expanding as I mentioned before that got defeated by the early measurement of the cosmic microwave background but these two conflicting desires to to to have to to have a point of origin uh in space and in time a specialness Freud once said there were three great blows to our Collective understanding that cernus took us out of the center of the universe Darwin took us out of the center of species and psycho took us out or that the ego is no longer master of its own house as he once said and I I think that those blows to specialness looking for something that's constant is a is a theme that comes up in cosmology often and I think you know there is a sense of you know that I think lies behind the thinking about the cyclic Universe of saying we don't want to have a one special moment we want to be a process that exists over time so I think these these these con these conflicting desires I mean there's a philosophy that people can make and say oh let's make special philosophical inquiries about things and then there's a philosophy that's embedded in the practice of science itself yeah and you know we are driven towards these questions is there an absolute time is there an absolute space is there a beginning is I mean is our galaxy special uh all of these questions keep coming back and usefully so they Propel us to think imag atively and to think otherwise to think alternatively to say maybe not the big bang maybe something cyclic so I think these questions really are they run deep and long in the history of science yeah so given that as sort of the philosophical backdrop you've argued so far that there is this framework in which if the universe contracts and then expands it can very easily according to your simulations give rise to a universe that we're familiar with you've also argued that the previous theories have a hard time doing it now can you repeat this trick and have that expanding phase go into a Contracting phase and thereby perhaps not have a beginning to time if these Cycles were to continue eternally sure um yes it all depends on the shape of this potential curve for this scaler field and the potential curve again is just how much energy this field has when it has a particular Val in space so it's kind of changing its value and then that changes its potential energy and how that rate of change change how that changes with time determines uh rates of expansion or contraction how the Hubble volume or how the causal volume changes relative to the expansion rate so in the moment we're in an interesting situation in the universe where the expansion rate is accelerating we discovered that back in the in the in 1998 yeah the expansion of the universe is speeding up up at present now you might wonder then how do we ever return to a universe that is Contracting uh if you're trying to make a bounce in the future the answer is you can imagine that what's happening at the moment is that this potential energy curve is right now it has a positive value but it's decreasing in strength it's decreasing as the field evolves and if it decreases to the point that it goes below zero becomes negative this is exactly the condition you need to get the slow contraction condition so it actually is not hard as you might think at first to go from today's accelerated expansion to the end of acceleration as we hit the zero potential to slow contraction it's like rolling off a log in a sense you actually have no choice in such a scenario as to what happens to the universe it will definitely enter a period of of contraction uh and then you could ask a the question well how far in the future might that be I was actually going to ask that question yes let's see what time is it I know well the answer could it depends on how steep you would make this curve um and you might think that well maybe it takes billions of years before this might happen but it could be surprisingly shorter than that it could be as short as a 100 million years and maybe even shorter and the reason why it can be as soon as that which I call it soon well compared to the age of the dinosaurs I picked that number intentionally that's roughly the time since the dinosaurs to the present could be how far to the Future So cosmically speaking that's actually a very short period of time yeah know it's a blink of an eye cosmically speaking so so so one of the things we have to appreciate when we say the universe is accelerating what information are we using to get to get that conclusion we're measuring the expansion rate at Early times when Supernova certain Supernova at very large distances were exploding and we're using those Supernova at Great distances which emitted their light long ago to infer that we've gone through some acceleration but if you ask what's happening now we have no information that tell telling us exactly what's happening now and that's why it could be that we're already well on the road to end of acceleration and contraction and we wouldn't know it we very we wouldn't know it so there could be a cult forming soon the end of times are near the end of our cycle is near well for us that would be the end of of of times can't have everything and a and so is this a point of view that you view sort of abstractly I mean this is a radically different future in the conventional story of the our future right you know we love to extrapolate in science and we talk about you know what happens in you know 10 to the 20 years from now maybe the Earth will spiral into the sun because it lost energy through gravitational radiation 10 to the 30 years we imagine Stars cycling into the black holes in the center of their galaxies 10 to the 38 years we imagine protons disintegrating all matter falling apart 10 to the 50 years the end of thought the universe can't absorb the from the process of thinking itself 10 to the 68 years we talk about solar mass black holes evaporating 10 to the hundred years we think about enormous Galactic scale black holes evaporating you're telling us that those are ridiculous time scales potentially to even be thinking about yeah it could be it could exactly there's another there's another possible story and we'd have no way of refuting it given what we what we know at present wow and it could be short which by the way has an interesting implication which is one of the things we'd like to understand is what's special about now yeah what's now mean around this time what's special around this time is that this is the uh period in time when the most number of galaxies will have formed in a cycle and if you like the most number of intelligent species will have formed in a cycle before we end before we enter a period of contraction which as you say will bring our demise uh because we'll we'll go into a state of high temperature once again during the bounce so in a sense it's a kind of interesting uh answer to the what's special about why are we here now rather than at sometime in the future when there because there was no time in the future that we could be formed so in a sense it we have a a specialness within our cycle but if the Cycles have always been and will always be we're just again not special so in some sense maybe we get a mixture of the philosophical perspective that that Peter you were describing so we're we're pretty much at a time I just like to get sort of a a final word maybe can just go down the line where do you see this kind of cosmological science going in the next few years are there going to be experiments that can start to adjudicate between these competing ideas well there's there's one observational matter that there was a s of false alarm on a few years ago and that is that if you look at polarized light when you have polarized sunglasses and you tilt it you can see blocking and letting in light at different angles off of reflected light off the window of a car for instance and if you look at the cosmic microwave background it's polarized too and there's a pattern to it inflation predicts that there sort of a handedness to this that the gravitational waves will create a certain kind of twist in that pattern and the cyclic model does not so as these observations get better and better eventually they'll be you we'll either see it or we won't see it and you won't see it with better and better accuracy or you will see it uh that that would be something that would in addition to theoretical concerns of the type that Paul's raised and the simulation concerns Anna's raised that would be an observational a remarkable observational consequence and and you know and so Anna if we were to see that particular signature what how would that affect your thinking on these ideas well first of all it would be great right I mean if we saw something like that that would mean that a whole bunch of models is probably out namely you know this this Contracting model is probably not the right story yeah and you know I have been always asked especially because I saw started to work on this when I was young I didn't have tenure and all that stuff why are you working on something that could be wrong you know that's um but you know as a scientist you should only work on things that should be wrong so I think it's great we learned something could be wrong no no no sorry should be wrong no but but really you know I mean I I just think that um we have learned something I mean what you want to learn something and if you hadn't explored this that adds definitely to the value of the experiment we should name it it says the Simon Observatory experiment this is up in the mountains already in Chile this was just built up so it will be running we will see first light in a few months and within 5 to 10 years we will know so I think this idea you know just to pan back to what Peter was saying and and just we were talking about that you can just within 100 years or thousand years go from philosophical thinking to to are we special does time have a beginning a space extended to actually we can check it we can write on theories we can pen land paper we can simulate it and then we can go up on the mountain in Chile and we can check it that's a huge progress so I am happy either way because we learned something yeah and but but definitely you know if if it if if it detect those signals then this idea is wrong and that's just it yeah but that's very important because we just know me important yeah so Paul final word I mean again if if the particular polarization pattern that Peter made reference to that seems to be uh a a likely outcome of inflation but not actually a definite outcome it's perhaps worth noting it depends there are definite outcomes okay fair enough yeah that's part of your other argument but if we do see this which is not compatible with your cyclic model where will that leave your thinking on cosmology will you go will you try to make inflation work go back and try to fix it or are you on to something else yeah I having worked on inflation for a number of years I think it's not fixable I think the problems with inflation have just grown more and more Ser more and more in number and more and more in seriousness as we've come to understand those problems better so I don't see I personally don't think uh it can surv I I I I think it's in Ser very serious trouble now and I don't see it being fixed myself right so we need yet another idea and you'd want an but we have learned something I think is very profound which is there exists a smoothing mechanism we never imagined before and it Smooths everywhere in the universe and we happen to observe our universe has this remarkable homogene isotropy we can't lose that I I mean that's a that's a that really changes my own thinking about how I think about these two different theories and they're no longer really in my view competitive because one inflation produces a Multiverse of every possibility in homogenity curvature anything you like and doesn't really predict any particular one being likely and the other one actually produces a universe which matches what we see powerfully and and doesn't allow you any flexibility from that so to me that's a uh that's a GameChanger in thinking and so if I were going to look for another theory I somehow have to I I want to save that idea because it's remarkable uh and it may not be slow contraction maybe there's some other way producing it but it's not going to be inflation that's what we saw in the inflation example it does the opposite if you get if you start off not smooth enough it just runs away the nons smoothness runs away well it'll be an exciting 5 10 years as we wait for Simon's Observatory to give some insight and uh once that comes through maybe we'll join up again and analyze where we've gotten so please join me in thanking the panel [Music] here [Music]

Channel: World Science Festival

Views: 381,609

Keywords: NYC, World, Science, Festival, John Templeton Foundation, Big Ideas Series, Brian Greene, Paul Steinhardt, Anna Ijjas, Peter Galison, Big Bang, Cyclic Universe, expanding universe, The beginning of time, Multiverse, new kind of cosmology, what is time?, When did time begin?, The science of time, Time science

Id: MO3w2D3uDTw

Length: 86min 2sec (5162 seconds)

Published: Fri Dec 22 2023