Science Friday
Hosted by Flora Lichtman
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Researchers built the largest 3D map of our universe yet. What they found supports the idea that dark energy could have evolved over time.
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This is Science Friday. I'm Flora Lichtman. Today in the podcast, new dark energy research begs the question, just how much do we understand about how the universe works?
I mean, I think it's the most ambitious effort of humanity, really, is to try to understand the big everything.
One of the mysteries of the universe is why does it expand at the rate it does? Back in 1998, two teams of researchers observed that not only was the universe expanding, but the rate of expansion was increasing. That observation was the basis for a concept now known as dark energy. And in the years that followed, cosmologists have been trying to get a better handle on how dark energy works and where it comes from. This week, researchers on a project called DESI, the Dark Energy Spectroscopic Instrument, released results based on their first three years of data. And when you line up their results with those from other measurements,
it hints that possibly dark energy, whatever it is, has changed over the lifetime of the universe. The so-called cosmological constant is not a constant. Joining me now to try to explain are my guests. Andre Cuchot is a postdoctoral research fellow at the Lawrence Berkeley National Laboratory, and he's part of the DESI project. And Adam Riess, Bloomberg Distinguished Professor at Johns Hopkins University and the Space Telescope Science Institute, and co-winner of the Nobel Prize in 2011 for those first expansion measurements I mentioned. Welcome to you both.
Thank you. Thank you, Flora. It's great to be here.
Adam, in your universe, how big of a deal are these findings?
They're really exciting. I mean, if these hold up, and you know, this is already the second go around where they have maintained, this would be the biggest clue we have about the nature of dark energy since it was discovered, I would say, in about 25 years.
Andre, what did the data show? What does he find?
Right. So in our case, we're using one particular tracer that looks at the large-scale structure of the universe. And we're looking at these ripples that were left frozen in this large-scale structure in the universe. And then these ripples were free to expand with the universe, which means it acts sort of like a ruler, a ruler that expands with the universe. And then we go and we measure this ruler at different periods, looking back in time. And this allows us to map the expansion history of the universe for about the last 12 billion years. And when we take our measurements and we combine
them with other results from other collaborations, from other types of tracers, we start to see some very strong hints that dark energy might not be a cosmological constant, that in fact, it might be evolving over time.
Okay, Adam, big picture, a constant is not a constant. Why does that matter? What are the implications?
Well, it sounds like we're just talking about some number. Is that number changing or not? What we're really talking about is our fundamental understanding of the physics of the universe and of gravity itself. Because if it's a constant, if it's what we call the cosmological constant, then the physics of that is likely that there is energy built into space and time that is sort of ever-present and continues to appear as the universe grows. If instead, as these results suggest, that it's not constant, then we have to think about a very different picture in physics. It might be that this energy is kind of like a field, but that it has been changing over time, in which case, the story of the past
and future of the universe is very complex. It might be that we don't understand how gravity operates on the largest scale. That is, we may have finally broken Einstein's theory of general relativity. And so, while it sounds like we're just talking about some number, is this number, you know, constant or is it changing? What we're really talking about is the big picture and how we understand the universe.
Can we back up a second? How should I be thinking about dark energy? Is it a force like gravity? Is it a thing?
So I think the first thing that I want to emphasize is that what we're measuring is essentially the acceleration of the expansion of the universe. That's the direct thing that we're measuring. And then we have this dark energy that is meant to explain that acceleration. And then what this new data is indicating is that this acceleration doesn't quite behave like we would expect it to behave if this dark energy was a constant. And this implies that there is more complex behavior going on. And this, just like Adam said, opens up a large space of possibilities that could explain this behavior.
So I want to stay with this for a minute more. Adam, you know, when I think about dark energy, I think of it as a thing. Should I actually just be thinking of it as a placeholder for like, we're seeing a thing that doesn't make sense with our physics and we're calling that dark energy?
Yes, the answer is both of those. So I will say the idea of dark energy, it's not a completely made up concept. So in, I'll just take you back to high school. In physics, in Newtonian physics, right? If you have matter, matter attracts other matter. There's attractive gravity. That's nice and simple. In Einstein's theory of gravity, things get more complicated. We need to know about the basic physics of the kind of material. And in particular, if there is energy in empty space, unlike matter, it can have repulsive gravity. It can push things further apart. And so that already is a pretty exotic concept, but we didn't make up that idea.
Okay. What we did see 25 years ago and now here is that the expansion of the universe is accelerating. And so matter wouldn't do that. So we went to, you know, Einstein's dumpster and we pulled out the cosmological constant and we said, okay, well, you know, that is a possibility for sure. And so we went. With that in some ways, because scientists like to shave with Occam's razor. We like to take the simplest approach possible. But now, as the the DESI team is showing, if there is greater complexity there, if it is not just constant, then that might not be the right story. And so we're just barely, you know, understanding the physics of dark energy.
We're just getting sort of perhaps the first wrinkles in that story.
Did these measurements get us closer to figuring out what dark energy is? Or get us closer to understanding what's broken about our current understanding? Like, oh, maybe our understanding of gravity isn't quite right. Or, you know, can we drill down to any specificity about what needs revising?
Well, yeah, we can, actually. I mean, if you take these at face value, they suggest that dark energy is weakening in some way. And of course, you know, a big question has always been, what is the ultimate fate of the universe? If it's accelerating and you have this dark energy. And in the past, if we thought it was a constant, it meant the universe would expand forever. In which case, everything would become more and more dispersed. We would lose energy. It would be kind of a cold, empty future for the universe. But these new results open up other possibilities, including still the possibility for a recycling universe where things recollaps in the future. And we're sort of on the end cycle of a continuously reforming process.
So, you know, again, these results, I think, do bring us closer, at least to getting real answers for some of these questions.
Andre, I heard you use the word, you know, this data strongly hints. How confident is the team that this is a real effect that you're seeing?
Right, so that's a that's a great question. When we had our first data release last year, then we were seeing hints when we combined our data with with what was available back then. And what happened now is we got significantly more data. So the precision of our measurements increased.
And then what we did is, besides combining with the probes that that we checked last year, we also went back to the drawing board and tried many different types of combinations with either subsets of those probes or with entirely new probes that have appeared since then. And what we're seeing is a coherent picture where whichever combination we try sort of falls in the same region that hints at these deviations from a cosmological constant. Now, the level of that deviation depends on the exact combination. And again, no combination gets to the level of of claiming a discovery. But we're definitely seeing a lot stronger hints this time because we're trying many different
combinations. And again, they're all hinting at the same region.
Adam, what would you want to see to make you, you know, as confident as you could be in this result? Is it more data? Is it data from a different project?
You know, I would say, as an outside observer for DESI, I would say, you know, the work they've done is outstanding. This is absolutely state of the art instrumentation. The team that's doing it is really the A-team. This is the best people working on this problem. And this is their second iteration after hearing from the community, you know, all their ideas on the first iteration and doing very important tests. So this is certainly, you know, impressive and it is probably more than a hint. And I think that there are many experiments coming up over the next few years that should be able to really dig into this. I will also say that there has been sort of growing evidence
that we may indeed be seeing cracks in what we call the standard model, this mixed dark energy, dark matter model. The expansion rate of the universe doesn't seem to match between the early universe and the late universe, a problem known as the Hubble tension. And that has even greater statistical significance than the present result. The present result has been growing. So, you know, once you start to see more and more cracks in the model, you know, while we don't yet understand what's going on, it does kind of build some level of momentum for, hey, you know, there's something a little more complicated here than we thought.
This seems like very humbling work.
It is. I mean, you know, this is the ultimate, I would say, cosmic humility that, you know, you're trying, you know, as these tiny little creatures on a tiny pale blue dot somewhere, you know, in Nowheresville in the universe, and you're trying to understand everything. And you can't even go to all these places. So you look with telescopes and, you know, like watching some complex chess game. You're trying to figure out the rules of the game. Sometimes you think you have it, then things surprise you. One of the challenges, of course, is that 96% of the universe, the dark energy and dark matter, are fundamentally different than us. And so we cannot use our own intuition because most of the universe isn't like us. It doesn't follow our intuition. So we have
to use physics. And it's a challenging problem.
I love that idea, that most of the universe is totally incomprehensible to us.
I mean, I think it's the most ambitious effort of humanity, really, is to try to understand the big everything. And there's been tremendous progress in this area in the last several decades.
Andre, talk a little bit about how you actually make these measurements. What kind of instruments are involved?
What DESI is, it's both a collaboration that that actually does this experiment, but it's actually an instrument on that that we mounted on this telescope in Arizona at at Key Peak, that essentially the the main goal is we want to build a 3D map of the universe. And to do that, the most challenging part is the the third dimension, which is distance between us and those galaxies that we're trying to map. And to measure this distance, we we measure it through through these things called redshifts. And to do that, we have to take spectra of each of those galaxies. So we have an army of 5,000
robots. And for each observation, each of the robots is essentially targeting a single object, a single galaxy, and taking collecting the light from that one galaxy. And then this 5,000 Fibers that collect that light get fed into 10 spectrographs that actually measure those spectra. And then we go and we look at each of the spectra and we measure the distance to those galaxies. So our analysis actually takes a long time. And for the for the first year analysis, it took us about two years from the moment where we finished collecting data to the moment where we finished all of the analysis and were ready to announce the results. And now with with the second analysis, it took us a lot less. It took us about a year.
So we finished collecting this data set in April 2024. So almost exactly one year ago.
After the break, how do we wrap our heads around a part of the universe that's so very, very different from ourselves?
These are big questions, and we have now tools to answer them.
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Andre, I heard that there was a reveal, like a big reveal of this data. Will you tell me about that?
Right, yes. So, an important part of our analysis is that we blind ourselves to the result. And this is because we want to avoid human biases. You know, everyone has their preferred model, but that shouldn't influence the science and the results, right? So, to achieve this, when we do the analysis, we inject something that essentially shifts the final result, which means that we see the result, but we don't know if that's the real result or, you know, which way it's been shifted. So, you know, for most of the last year, we saw this result, but it was sort of in the wrong spot. Or, we didn't know where exactly it really is. But it was beautiful. It was a really precise measurement.
And then, we spent most of this time, you know, digging into our pipeline, digging into our analysis, and trying to validate each step of the process and making sure that, you know, there's no choices that were made along the analysis that have a significant impact on the result. So, essentially, we're trying to prove that our result is robust. And once we were happy with that process and we're happy with all of the tests that we've done, in December, at our collaboration meeting, we actually sat down all together and we unblinded the result. And what that means is, for the first time, we removed that thing that we injected and we looked for the first time at the actual result. And that was, that was an incredible moment.
Should I be imagining, like, people around a conference table with a velvet curtain around a computer screen? What was it like?
So, yeah, it was, it was about 200 people in a room and then one person essentially presenting the result. So, that person, you know, a couple of hours before that presentation, they actually ran the code to produce the results. And then, you know, all of the plots were quickly made and put in a presentation. And then, together, we take in those results as a collaboration.
Were there gasps?
Yes, there were. There were some gasps. I think the best way to describe it is, you know, the first few minutes, everyone was just sort of taking in the results. It was a lot of silence. Everyone was just staring and, you know, trying to think about the implications of what we're seeing. But, you know, after those few minutes, there were a lot of excitement and applause and yeah.
Adam, you know, you were one of the researchers to discover dark energy, which basically told us, you know, that we that we had a lot more to understand about the universe. This may be a funny question. Did that initial discovery or these sort of subsequent discoveries that add complexity to this picture, have they changed you or how you see the world?
You know, I think that it instilled in me and still does this great excitement for science, the kind of work that Andre's described, where there is great mystery out there, but we want to understand it. And we expect to learn things, but we also expect to be surprised that, you know, this is not just adding, you know, another digit to pi or something like that. These are big questions and we have now tools to answer them through rigorous sort of accounting, hard science, experimental data. And so to me, it's always made this just a really exciting endeavor. And when you see new results like this, it's sort of like Christmas.
You know, you get to unwrap a package and you're like, oh, my gosh, I had no idea. You know, we would receive this. And so I think it really injects the whole field with a lot of energy, not just dark energy.
Thank you both for joining me today.
My pleasure.
Thank you.
Andre Cuchot is a postdoctoral research fellow at the Lawrence Berkeley National Laboratory and Adam Rees, Bloomberg Distinguished Professor at Johns Hopkins University and the Space Telescope Science Institute.
And that is about all we have time for. Lots of folks helped to make this show happen, including Shoshana Buxton. Kathleen Davis. Diana Plasker. Beth Ramey. I'm Flora Lichtman. Thanks for listening.
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