What Would It Take to Overthrow Lambda-CDM? | Saul Perlmutter

[00:00:00]Saul there is a cottage industry and seems to be growing that challenges every aspect of uh current cosmology the standard model so-called lambda CDM uh even the big bang itself um and let me just list some of the ones that I've been familiar with or people have written to me about uh and then ask you as a hardcore observationalist and experimentalist the kinds of data that would be required to justify any of these challenging the current standard model or affirming these in some positive sense.

[00:00:42]So here [clears throat] are some no order of importance just as they occurred to me you know way past midnight. Um the first is so-called tired light where the red shift is called into question where it's not the expanding universe that's uh shifting into the red. It's just the light's getting tired. It's uh it it it has some energy and that over time it just like you and I we get tired and so it gets tired and kind of stretches out. um the classical steady state. Some are bringing that back the that was you know theoretically to totally uh eliminated by the cosmic microwave background. But some some say this some say the speed of light can vary. It's not just it gets tired. Some say it uh it it varies the speed of light just like as you know you and others have shown recently the the dark energy may vary. So maybe the speed of light varies and that special relativity was not as super strong as we thought. Then there are cyclical or ice um um universes that oscillate uh models. There's the specific uh so-called e pyroic where brains br not not these kind of brains but brains of uh of three-dimensional universes collide in some fourthdimensional space that creates a heat and then uh Roger Penrose famously has this conformal cyclical cosmology where the universe expands and then after a time if I get this I uh with the the the universe kind of forgets how big it is and so it's expanded into vastness and totally dispersed but that becomes the hot big bang or the the hot plasma of a new of a new universe and then of course all the modified gravity theories. We have all these different theories that challenge fundamental aspects of uh of the standard model of cosmology. So again I want to ask you as a observationalist when you hear of these things one way or the other what are the kinds of data that are um utilized to to defend these uh and that what is your analysis of it?

[00:03:08]First of all, you know, this is of course what science loves, right? That we we want to be looking at a wide range of of uh theories. Um, and the trickiest part is the fact that it's so easy to make up a theory and so difficult to test a theory that um that you have to spend a lot of your time trying to figure out how do you um sort of winnow uh what looks like more uh important classes of theories to check and which ones um can you just say well that sounds like another theory we've heard before and we've tested that already.

[00:03:45]Let's not spend a lot of time on that one. Um maybe you know it's give us some examples. I mean something like um tired light was a a you know a very interesting concept back in its day. Um but it was something that you could actually test relatively straightforwardly because u for example you know the the time scale of the supernova that we observe. Um if the if they're looking fainter and and if there's red shift just because of tired light um then they the uh you should not see any dilation of the time scale of the event. So um a supernova has a very characteristic time that rises and falls and um it would be the same time rises fall just fainter if it was tired light.

[00:04:30]But in fact when we go out and we make a measurement we see that um the higher the red shift the um the more that what we call time dilation the slower the uh the rise is and the slower the fall exactly proportional to how you would expect if it was due to a stretch of so um tired light right now is we I think is ruled out in any of its simple forms by something like that. Um varying speed of light um is a very uh uh tricky one.

[00:05:00]um if if you are imagining um you know how how would you be able to tell if it's happening um but of course there are many um physical processes that depend on the speed of light um for for things to be running um the way they do and the fact that these vast balls of gas that explode um the supernova that we see um uh and where it can you know in principle light takes time to go you through these these these events. Um you would not expect the the entire physics of this large explosion to look the same at at different times back in history um as much as they do if uh if um speed of light was changing. Now um that one you have to do a little more work to turn that into a limit on the change in the speed of light. Um, but it's an example of the many things that people have been trying where they say, "Okay, well, if that's the case, we should be able to limit that effect." Um, and it's one of the it's one of the ones where if it were to change, of course, it has all vast consequences for many parts of the physics story because the speed of light built into special relativity and then there's many things that uh you would imagine would look different. Um so there you know people you might be able to argue well perhaps if everything is changing along with the speed of light maybe it gets hidden. Of course if you change enough things it doesn't matter that you change it's basically the same the same theory as we currently have. Um it's just everything is is scaled you know. Um all right. So then we get to these uh more uh more um I think in play concepts um that people are actually you know wondering about um you know something like a a uh a a um a universe that's constantly that you know has constantly creating stuff is um is a little bit definitely would not fit our current model. On the other hand, it's also true that um the current picture of the expanding universe is constantly creating extra vacuum and the vacuum has has stuff in it. The vacuum is a buzzing uh you know sea of virtual particles. Um so uh so you know th those stories at least you you have to understand you what would it look like if you created extra vacuum energy um extra dark energy with an expansion. um how will that how will that play in in a in a constant in a constantly expanding universe? um I so far I have not seen any models that take advantage of that particularly as their mechanism to to to deal with things whereas the cyclic um models of course um they are in uh very much uh ones that people are are trying very hard now um and this is uh some of the work that you know uh Paul Steinhard and his colleagues have been doing where they've been trying to ask um can you uh can you get the same effect as the inflationary period but do it using um a mechanism that they think might be more natural than a a very specific uh inflaton potential that you would have to draw for inflation and can they get just happen with every cycle of a of a cyclic model. Um, and I think those, you know, are are fascinating uh comparison points. They're the data sets that you want to look at. Um, we would depend on what they can predict. And so far, uh, they're just beginning to try to ask, could they possibly be seen in a slightly different history of the expansion of the universe in relatively recent times? Um, and that would look different than what you would have gotten with a ordinary inflation model.

[00:08:50]um not yet something that uh anybody has given a a hard prediction. It's possible that it will be only very lucky if you happen to be able to uh uh catch it in the act of of of of being in one of those for places in the cycle that we can see. Um but you know it's it's something that people are are are working on. Um maybe that's the the the feel of of of the general feel. But I will say that almost all of these um uh you know concepts um they go from a interesting philosophical possibility that just expands range of how we think um to being something that we really take seriously in the field just when they can make a very specific prediction and when it they don't fail some other natural um tests that you could do that you know they they sound good in nearby galaxies but as soon as you look clusters of galaxies, it doesn't work. Then we're not interested as much as if it actually works for everything we know and it makes a very specific prediction that we didn't make otherwise.

[00:09:54]>> Yeah. Most of the challenges have to do with uh u basic laws or regularities that change. And so whether it's the speed of light that changes or gravity changes or dark energy changes, once you allow the possibility of a change, then um then all bets are off, right?

[00:10:18]>> And so the question is how legitimate are some of these changes? You've discussed the change in the speed of light, which you believe is can be rejected, but how about modified gravity because that shows itself in many different ways. Uh is it sacraank that the the general relativity is that uh universally correct throughout time and throughout distance or could there be some modification?

[00:10:51]the measurements that we make the or the entities that we study uh that are easy to measure that we measure in many many ways in many many locations at many many distances and times those are the ones that I think are harder to have them change. Um the ones that we have very little way to directly measure like dark energy um sure um you know a change in dark energy why not you know because uh we we aren't we're not we don't make direct measurements of it we make measurements of its effect and so um it's much easier for a model to uh propose a change in dark energy than it is to a change in the speed of light.

[00:11:30]gravity is a little bit u is a little bit in between um in the sense that it's um we have lots of measurements of gravity in many many scenarios and many scales and times um and so it's not trivial to have uh have you modified gravities that change with time um that you could have modified gravities that you know maybe change with scale um and that's of course like what the Mand uh you know me measurement does um And uh but uh it's it's a constrained area. It's it's just that gravity we always work on that we're usually measuring saying a little bit that's a little bit more removed and so it's a little easier to hide a a effect on gravity. um but not trivial because in the end it's built into general relativity and general relativity has so many um perfect symmetries uh that that work out when you when you look at the observations that you'd have to get all those to keep working even if you change that >> yeah it's very very robust and if you make a change you have to explain a lot of other changes which general relativity explains so I absolutely agree u just to take a step back uh The motivation of people who uh would like to undermine uh the uh standard model or even the big bang has to do I think with the fact that a a eternal static universe of some kind is a an easier explanation in the sense that once you have that you don't need to explain it. It's self-explanatory.

[00:13:12]It's there. it's eternally and it's there. Whereas, if you have the standard model that had a a beginning, whether it's an inflation, you need some sort of a of a kernel of energy, you know, I don't know how many times smaller than a proton, but you know, many many many orders of magnitude smaller than a protein. But you need something to begin with and and then you can have your theory, and that's the standard model. But then you have the problem of explaining that initial condition which you do not have with a standard wi-i with a static universe. So I think philosophically not not the you know >> we're putting aside any theological but just philosophically it's an easier simpler explanation to have a static eternal universe then you have nothing else to explain. Uh so >> let let me try two two cuts at that question. Uh so one is um there are ways of describing a universe that has a beginning. Um where uh which which basically describes it a little bit more like the um if time was thought of as as being like the uh the the lines of long of longitude on a on a globe and you go back to the point which is the North Pole and you say, "Huh, it's incredible.

[00:14:36]You can't go any further back." you know, it it all begins there, you know, but of course, you know, it's just a sphere, you know, and it depends on where you put the lines, you know, and then it has to do with our mental um relationship to some lines versus other lines. And some of them to us feel, you know, primary uh and others feel like, you know, they're the primary space as opposed to time. And since we see them in very different ways um for us then that origin and time seems hard to explain. So that's one way J >> James Hardle and Stephen Hawking they had their their model of that sort of rounded beginning of of time.

[00:15:17]>> Exactly. So it's it's one way to explain your way out of it. Now um I I I you know I can feel the pull both ways on that one. So I I so let me go back to the other uh when you know way of thinking about this which is that um I realized at some point or other when I was starting to think about um what makes a good philosophically satisfying explanation of something is it more satisfying if it's a explanation that's cyclic versus more satisfying one that has what could be an origin um I started realizing that there's a problem about what counts as satisfying which is that as I go back and I said Okay, suppose somebody was to tell me the answer to anything I want to know about how this has all happened. Um, I would realize that I get really stuck very quickly and figure out what the question is. Um, I it's hard even to describe what is it that I want to know. Um, once I get back to the point of saying, okay, let's imagine that we have uh these equations and we have these initial conditions.

[00:16:20]Um, then what do I want to know? I I guess I want to know you know could there be any other equations or could there be any other positions are they required and if I say well in what sense required do I need something to explain the requirement um of of them and you very quickly you you you reach the point that you're realizing you're asking questions where you don't even know what would be a satisfying answer what could somebody possibly tell me that would make me ah now I understand and is it any different those questions that I would ask if it were the kind that went back to a initial condition or for the kind that went back to a a um a continuous uh you know always in play uh cyclic condition. Either one I think I would find myself asking the same questions and not knowing even how a satisfying answer would look. Um and so it's a little bit hard to complain about one if I can't even explain what a satisfying answer would be. Um so that that's that's I I think in some why for me I don't end up strongly favoring one or the other as the natural um you know feeling explanation >> you're you're dissatisfied with both of them because you can't imagine what that [clears throat] either one would would give you that philosophical fulfillment >> and and I and I and I start realizing that's my problem to some extent not the problem of the explanations you know in once I get to that because I can't even think of what would count as the right question to ask. Um that that that if somebody answered it, I would say now I'm done. I you know, I get the whole thing. And so the best I can do is say, you know, let's try to get as much of the story as complete as possible and then maybe it might lead us to some better way of understanding what question to ask.