Is Dark Energy Decaying?

Welcome everyone, today we are having a conversation about the cosmos, about a quality of the cosmos that we have had many conversations about over the years in the World Science Festival programs because in the last 20 years, 25 years, It is perhaps the most surprising. of all cosmological results, which is simply that not only is the universe expanding, but it is expanding more and more rapidly, it is accelerating its expansion. This is something we learned in 1998, as we will discuss in our conversation here today, but the reason we are focusing on the so-called

dark

energy

that we believe may be responsible for the accelerated expansion of space is that, as you may have read, there are New observations, remarkably new observations, are still provisional, to be sure, but there is at least one h in the data. that we may need to rethink aspects of

dark

energy

that we have had for a long time.

I don't think I'm giving anything away, but a spoiler alert where we're going to get to in this conversation is at least the possibility that dark energy is not constant and can change over time and that would be a real revolution in the way we We think about cosmology as we discuss it. It would have profound implications for the distant future of the cosmos, how things will be far beyond. any cosmological era that we have direct access to, but when you're trying to understand the totality of reality, knowing things like what's going to happen in the far future is something that really matters and all of these clues or all of these hints come from a truly remarkable technological Fe in the work in observational astronomy that has now allowed us - and by we I don't mean me I mean the team whose director we're going to talk to in a moment - measure the properties of distant galaxies with fantastic precision on a scale that is truly unprecedented in cosmology and that's really what has allowed these new results to emerge, so it's a great pleasure for me to include in the conversation Micha Leevy, a senior scientist at Lawrence Berkeley National Laboratory in the physics division there. and he is the director of what is known as Daisy the dark energy spectroscopic instrument which is the technological development that has allowed the results to reach the four and as we will discuss these new observations they can disappear over time or they can change everything we think . about how the cosmos has evolved, so Michael, please join us and thank you for being here today, where are you right now?

Well, thanks Brian, from Well, I'm currently in my office at Lawence Berkeley National Laboratory, it's on up. in the Berkeley hills, right above the UC Berkeley campus, and that is the epicenter of the daisy experiment, right, the experiment itself is not there. I guess it's in Arizona, right, and this is where all the data analysis happens and look, I want to get into the details of the actual experiment and the equipment that you guys have built, but I think it's really good to give it to our audience, although many Some of them are well versed in some of the cosmological ideas I just briefly referenced, but I think it's always nice to hear it again, so I don't know what the right time is in cosmological history to come back to, but maybe Around 1930 is when you know we had the first observations that began to give us an idea of ​​the expansion of space.

That was, of course, Edwin Hubble and um um one of the assumptions that I think most people led to that result okay, it's mind-blowing and maybe unexpected, except for a handful of people, that space is expanding, but everyone thought the expansion would be slowing. right, I don't think anyone questions it, no and I think because of the gravity we assume, in fact, I remember talking to my colleague Sal Pearlmutter, whose office is down the hall and I know you interviewed him and of course he was One of the people in the discovery of the accelerating universe, of course, was on a quest to measure the um, the number associated with how we were slowing down and probably returning to a major crisis.

I mean, just and of course that was the answer and I think we had settled on a kind of model where the universe was just going to drift and little by little almost come to a stop, you know, in infinite time. , in infinite time it would just stop or or go back and make a crack and I don't think we knew it and that was the interest in experimentation was okay, okay exactly how that is, how we end up, we end up in another crunch where everything Rec collapses In itself, is there enough matter for that to happen?

It is inconceivable that it was expanding at an accelerated rate, which of course was a surprising discovery in 1998 and I think all the years before that, that I can remember for my entire professional career. That, we just assumed that, you know, the ending was going to be in a fiery collapse, something like that. I think it was kind of an Assumption, so when you first heard about Saul's outcome and of course it was too. Adam Reese and Brian Schmidt shared the 2011 Nobel Prize for this work when you first heard that their results seemed to suggest that expansion was accelerating.

I mean, did you believe it at first? The initial measurements were, you know, they may not be public, but you know we heard, we certainly heard the rumors through the walls here, yeah, um, about a smaller number of supernovae, well, you know, these are difficult measurements. To do, in particular, you are, requires measurement. the brightness of a standardized candle-shaped object and um, so you count on the type 1A supernova collapsing in a way that it always emits the same amount of light and then you can calibrate it against that that's a standard candle, so that was the Assumption Well, maybe there's something wrong with that assumption, there's dust in the universe, which makes it harder for you to make that measurement and it's getting harder to correct. there may be something wrong and these are subtle measurements and they are difficult to do and you are doing it over huge time spans, across wavelengths for an instrumental perspective, it is harder to do when it comes to wavelengths, so These are difficult measurements. and of course it was the first one and and um and the answer was wrong, you know?

I mean, this would have completely dumbfounded everyone, so of course it required more data and it got more data and then. um uh after 40 42 Supernova and through a lot of testing, you know they published and of course they had colleagues who also published uh you mentioned the multiple Nobel Prize winners, I mean, they're two groups, the, the, both groups. Having the same response, you know, was a big help in calming the community's concerns about the experiment. This was correct. I mean, I think if it had been just a group, then you would say yes, but is it true? but having two different, different experimental approaches, I think that was very, very important, yes, I think that's why in some ways I'm a theorist and not an experimenter, because at least in theory, once you make your assumptions and the you make clear. then it's just deduction and as long as you don't make an actual error in the calculations you're on solid ground, whether or not it's relevant to the world whatever you found is a different question.

I don't think I would ever do it. having the confidence to get an amazing result as if the expansion was accelerating and trusting it enough to go out into the world and make a statement. I think you have to be a little paranoid to be in our business, yeah, because you have to. I have to correct those mistakes. I mean, I think you're right that it's a scary business. In fact, we just published some articles that we are going to talk about. I can assure you that all. before you know you press, you press send uh, you're pretty worried, you know, we did all the checks we needed to do and the list is meters long of those types of checks and so, yeah, you try, you try. to eliminate potential sources of error or just, you know, outright errors, and then you know we have methods to do that and of course our experiment we tried to find something that was also like a standard sail like what Saul had with the Supernova, but something that was even simpler that we were able to find, so that we could do a more, even more refined search for this question, so that's what we did and of course, simple, simple, physical.

I'm a physicist, I'm not a physicist, I'm sure physicists are reductionists, you know, we like really simple approaches, uh, simple, simple answer, simple ways of doing things and, so, we think we found one that was pretty good and, you know, so you have, you have. less less opportunities to make mistakes, right, so that's very important and of course that's part of the art, yeah, yeah, you know, my physics homework, you know, I got those two error factors too, so, Well, we all know them too. Well, but just to give everyone an idea again, I think most of our audience is familiar with the idea of ​​standard candles, but you quickly know that if you're looking at something with a telescope very far away, determining its distance is not trivial, I naively say well. the dimmer it is, the further away it is, well yes, sure, but it could also be dim because it's inherently dim or it could be dim because it's very far away and that makes it look dim, so if you have these standard candles, you know how bright ones are like a 100 watt light bulb and based on its apparent dimness you can calculate how far away it is, so you referenced the Nobel Prize winning work that they use supern noi because its physics was well understood, so its brightness it's good. understood in terms of you know how your brightness changed over time etc., we don't have to get the details, but what was your standard candle, then you mentioned that you looked for one, what was so good, so we have a standard. rule, so instead of it being um the intensity of the Light, which you know, you have to calibrate it against something um uh, we have a yardstick, a cosmic yardstick and it turns out that it's related to the Big Bang and uh in a moment of time about 400,000 years after the big bang, uh, all this very hot plasma that existed after the big bang of the electrons, all the protons floating around, they recombined into neutral hydrogen atoms and all the light came out of all of them .

Suddenly this became transparent and we have this moment in time that we can look back on. A large number of experiments have looked back at this moment in time that is reasonably well understood and we found that at this uh in At that moment you have basically uh uh bubbles in a sort of shape that has the shape of the universe at that moment in the time and it's called baryon acoustic oscillations imagine the universe had this uh it was like a uh a drum uh you know someone was hitting it yeah it was a big explosion hitting the drum and saying here I am and it forms uh uh you can think of a kind of bubbles or foam that are of a standard. size that represents the size of the universe in that 400,000 year period and now that is imprinted on the distribution of galaxies, so where was a hot spot?

In this, in these, you get more galaxies where there is more density. you get more galaxies and where there is low density you get less Y and you can find that in the data through correlations it's not a huge effect, it's five or 10 Perc, but it's definitely there, that's our standard candle because that thing is now a object that then stretches with the universe as the universe gets larger and then we can follow that unusual distribution of matter that we call, as I said, we call it baryon acoustic oscillations acoustic oscillation Barons in this case are hydrogen and again only for La meat is out of contact, then you have the barans, you have the photons, the photons want to flow out, the barans want to fall gravitationally inward and then you get this oscillatory behavior and then you know that, around 400,000 years, you say the barons become bond into neutral atoms, the photons are released and at that point the acoustic oscillations freeze and you measure the size of those that are shaped so you know it's pretty simple, um, and that freezes.

It froze a long time ago. Nothing else happened between others. so these are now separating, you know all about galaxy formation, but everything is separating, so that phenomenon was trapped for a long, long time, you know, in the year 400,000 and okay, so we ran this movie towards forward 13.6 billion years to today and So what we're trying to do with our experiment is be able to play that movie, go back in time and see this, you know, these bubbles expand with the expanding universe, so that's our criteria, um, you can. sit down, as a theorist, you can sit down and say how big that yardstick should be and you know there's a number and we can just go to the computer and see well what we should do, how this should propagate given the different theories of cosmology and what?

I can think of that oneThey were devouring, you know, millions of stars other stars in the early history of the universe and we're the brightest objects of all time, so we can pick them up, yeah, um, and so, uh, uh, these are the kind of things. to which we were Ed to collect our data and then this breaks. in seven different time periods, what you have to look for is Flatline, it was supposed to be the correct answer. Flatline right in the middle, right in the middle, it's called Lambda CDM Lambda being uh the cosmological constant, yeah and the rest is cold Dark Matter, so the Lambda CDM universe is the current standard model of cosmology, but that says that You know that Lambda is a static number, yes, the curve, the dashed line that goes through the data points, is the one.

That says, oh, it's actually the best fit for a time-varying expansion rate, so which fits best and that's how you can tell it's about two and a half Sigma because those data points have error bars that individually don't seem that significant, but when you put them all together, that's how you get two Sigma and medium and if I'm interpreting it correctly, this suggests that the accelerated expansion, the acceleration is slowing down over time, is that H, how is this? actually tells us about the acceleration being so deep, so of course we explain in the paper that we know we incorporate other things into our data set, including the Supernova, uh, and the plonc and other other CNB measurements, and the we put all together now we have about a three sigma effect, maybe even a three and a half sigma effect and then we ask what exactly is going on here and it seems a little bit as if from the beginning there is a little bit of extra pressure. the universe separates and then as it evolves there's a little less, um, so that would be very interesting because there are some theories that weren't predicted as well not long after this was first discovered, one of them It's called quintessence, but there are a lot of other theories that are floating around that might suggest you know maybe this is a potential model.

You have to know that your viewers are going to be watching and then squint at this like I really see it. It's what it means to be two and a half Sigma and that's why we call it intriguing, but we're not there yet if you take those air bars and reduce them by a bunch, suddenly, oh, yeah, you've got it. you have a curve and it wouldn't be consistent with a line or maybe the data points will move, they will be consistent with a line and not that curve, right, we don't know and that's what's so fun, that's why Me I love doing physics because it's a detective novel that you can read and discover and it was written by the universe, it's fabulous, so if this holds up, you know, obviously theorists jump into everything very quickly, so, yes, a lot . of ideas are already emerging and yes, but do you have an idea of ​​the explanation?

I mean, what would this mean for the distant future of the universe? If this holds up well, look, I mean, I would start with the distant past, I mean. maybe this is all related to the Big B connected to inflation, well we think so, we think you know right, we are on our current theory that the Universe started with an accelerated inflationary expansion, I mean, we, we, we . it went from nothing in 10 to minus 32 seconds we got to everything yeah um that was quite a lot that was a pretty powerful acceleration right there so it's not like we haven't seen this before I mean theoretically we have seen it yeah like that that maybe connects.

To that maybe there is something very fundamental in this and I actually find it very attractive that we are living in an epic that is somehow linked to the beginning of time, yes, yes, okay, where are we going with this? Well, ah, it depends. In which of those models you know, maybe it's cyclical, so it could be that's what we're seeing, what we actually saw, that little acceleration, deceleration and then deceleration, maybe that's oh, that's the first wave of an oscillation of the operating system, uhh. yeah, we're going to go back to you know and then that's going to change and it's going to come back and we're going to know that history will repeat itself yeah, I don't know, but I like this, I really find the notion that we're going back to the beginning um yeah , that would be very exciting and more appealing because I think that could provide a direction of where we should be heading with this yeah and again, you just know most people in Our audience are familiar with it, but the inflationary period.

That's the dominant way of thinking about the Big Bang, it involves energy in space that changes over time and only lasts a small fraction of a second, causing the rapid expansion of space that produces the universe. that we are familiar with and then we have had this last period of energy filling the space and most people have seen them as separate for a long time, but as you say, if they connect in some way, how wonderful would that story be? . Look, I have papers here on my desk, you know, there are so many different models, um, and you know one of them is called Mirage and the other one, you know, I mean, they're just, they're, they're. defrost and freeze defrost defrost and yeah, you know, tell me, now okay, now we hand this to you, Brian, tell us the answer, um, we're just the, we're just the experimentalist, but look, you know we always say experimentalist we say that the data speaks for itself, yeah, and so the data is the data is speaking uh, we haven't, we haven't, it's going to speak a little bit more as we're very excited about it.

I have to tell him. We, you know, the fact that we're sitting on this, shouldn't say that we have this data that we're now moving through our computers, that's going to give us another factor of three on this. in terms of statistical power, um, it's really incredibly ex, I have to wait for it, we all have to wait for it, of course, again, it's going to be a blind analysis, so this is, you know, I love it, I love the excitement of this, It's just It's so exciting that you can answer, you know, like ask the universe these questions, um, about the answer like you and but, and it's starting to talk to us.

I mean, that's what I, as an expert, always say, we have to ask the universe. question and he answered us and then, how was it? I'm wondering in the room, you said there was a revelation on a particular date, at a particular time, when people saw, I guess at that very moment there was a suggestion of something that was not in keeping the cosmological constant being constant. if there were oo and oz in the room it was me how it was well, they are different they are different I would say that there was a different collection of oo and Oz and depending on how you accentuate it it could be or uh or it could be ah, so, there was, there was , it was true, oo, and it had oozed, and then, yeah, this is it and I'm very grateful that we, we, like I said, blinded him for it, okay. this is really exciting and interesting and we need to bring it to light yeah this is really worrying and what did we do wrong you know so you have the range of emotions and I think you even have As human beings you can have those same emotions at the same time time about it, so yeah, I definitely heard that in the room absolutely right, it's so wonderful and exciting, it's so exciting to know that within a year or whatever length of time it takes you to get it right we'll know if this is just a coincidence or if it is telling us something profound about our cosmology.

The connection to the past and how the future may differ from what the standard model has told us. It's exciting, Michael. Thank you very much for joining us in these wonderfully clear explanations of the work that you and your colleagues have been doing, are doing and will do and we will just sit here in the side lines of identification with bated breath waiting to hear what the Universe has to tell us next. through your data is so wonderful and exciting that maybe we can talk again. You know, the next iteration of your data analysis is delivered to the public so well that It would be wonderful, thank you very much, well, thank you, okay, everyone, that's our conversation today, an exciting possibility and look, that's what In the history of science there have been many results, you know, three sigma or more that have disappeared, but who knows? , this may be one that increases in importance over time and reaches that golden threshold of five sigma.

If that is the case, it will change our cosmology. models and look, you know, as Michael said, that's what observers and experimenters live for and it's what we theorists live for too because if you eliminated the cosmological constant as a constant and we know that dark energy varies with time, that's a Kind of a gold mine of opportunities for theorists to jump in and propose new, more robust models of the universe. How the Universe began and how it will end. That's what this story is about. That's why all this matters. That's what it's about. so stay tuned for more chapters of this particular story, sign up, join our YouTube channel.

Channel Subscribe to the World Science Festival newsletter. You'll be alerted to these conversations as they happen and we're waiting for the next big development. will be announced in this Arena, as well as many other topics we cover in these conversations, so thank you very much for joining us until next time. I'm Brian Green at the World Science Festival in New York, saying goodbye. See you next time.