The Universe Has Been Expanding Since the Big Bang. Just Not How We Thought.

[00:00:00]Last year, the Dark Energy Spectroscopic  Instrument—DESI—made headlines around the world. Its enormous map of the Universe hinted  that dark energy—the mysterious something causing the expansion of the Universe to accelerate—might  not actually be constant. And just last month, another giant cosmology experiment released  its final results. The Dark Energy Survey.

[00:00:21]DES. Its results are intriguing. But  did they confirm DESI? We’ll, we'll see, but there’s also a much more interesting  question— which is why are completely different scientific studies so much more  powerful in combination than any one study on its own? The answer brings us to  the heart of what science really is.

[00:00:50]Before we get started, a couple of quick  announcements. First, we have some new data.

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[00:01:28]This month marks an incredible milestone: 50  years since humanity first landed on Mars. On July 20, 1976, Viking 1 became the first  spacecraft to successfully touch down. To celebrate, we've created a limited-edition  50 Years on Mars collection featuring the iconic Viking 1 lander as a UV glow t-shirt and  hoodie. We also have a new desktop and gaming Matt that shows the actual location of every Mars  lander. If Mars exploration has inspired you as much as it's inspired us, check it out at the  Space Time merch store. Now on to the episode.

[00:02:01]Did DES confirm DESI? Seems like a reasonable  question, no? New data is supposed to reduce the uncertainty on whatever number we’re trying  to measure. Tighten the error bars and we confirm the measurement. Science sometimes works that  way—like when we first measured the existence of dark energy in the first place. But modern  cosmology has reached the point where this doesn’t work any more. The signals we’re looking  for are so subtle—so tangled together—that it’s no longer about directly measuring the  parameters of this universe—it’s about ruling out countless hypothetical universes that  we’re not in. And DES was ruthless in eliminating universes. So what did it discover? Despite their nearly identical names, the Dark Energy Survey, or DES, and the  Dark Energy Spectroscopic Instrument, DESI, were built to tackle the same grand  mystery from almost opposite directions.

[00:03:02]DESI's mission is beautifully focused. It  measures the distances to tens of millions of galaxies, constructing the most detailed  three-dimensional map of the Universe ever made.

[00:03:13]Hidden within this vast cosmic atlas is a faint  fossil pattern left behind by sound waves that rippled through the hot plasma of the infant  Universe nearly 14 billion years ago. These are the Baryon Acoustic Oscillations, which  we’ve covered before. BAOs gives us a cosmic ruler that grow with the expanding universe,  and so allows DESI to trace the history of that expansion with extraordinary precision. DES took a much broader approach. This truly international collaboration of some 400 scientists  took control of the Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory in  the Chilean Andes. After building a dedicated 570 megapixel camera, they scanned an enormous  region of the southern sky for six years, building an extraordinarily deep portrait  and movie of 300 of million galaxies.

[00:04:10]This became the foundation for several  completely different ways of probing the cosmos.

[00:04:16]DES measures the tiny distortions in distant  galaxy shapes caused by the subtle warping of intervening space by massive objects. Through  this weak gravitational lensing, DES maps the invisible distribution of dark matter and tracks  how cosmic structure has grown over time. It also measures how galaxies have clustered together  over cosmic time due to their mutual gravity, giving a completely different perspective on  this same structure growth. By combining those two measurements through galaxy-galaxy lensing,  DES connects the visible galaxies to the much larger halos of dark matter that surround them. And DES also discovered and monitored thousands of Type Ia supernovae, which allowed it to  measure the expansion history of the Universe by determining the distances to these exploding white  dwarf stars. And, by the way, this is exactly how dark energy was first discovered.And finally DES even measures the same ancient baryon acoustic oscillation pattern as DESI—but with a completely  different observing strategy. So DES is really an entire laboratory of cosmological probes built  from one extraordinary survey of the sky. DESI, meanwhile, takes one of those probes and  pushes it farther than anyone ever has.

[00:05:37]Why try to measure dark energy by so many  different methods? It’s not because we’re worried that any one of those methods might be  wrong—at least, not mostly. It’s because each one tells us something slightly different about  the universe. And it’s only in the combination of all of our “cosmological probes” that we  find the answer that we were looking for.

[00:06:00]The truth is, there’s no one experiment  that actually measures dark energy. Instead, we observe galaxies and their distances to  each other and tiny distortions in their shapes and the faint flashes of exploding  stars. We observe the miniscule temperature fluctuations in the oldest light in the  Universe—the cosmic microwave background.

[00:06:21]None of these directly measure dark energy.  Instead, each measurement is influenced by a set of unknown cosmological parameters that  together determine the way the universe expands and how structure forms, which in turn  drive the observables that we can collect.

[00:06:37]It’s these cosmological parameters that we  really want—things like the proportions of dark energy to dark matter to regular matter  to light to neutrinos. Or The expansion rate at the beginning and the slowing and speed of  that rate due to gravity and dark energy. The parameters of the equation of state that  determine if and how dark energy changes over time. The subtler details of how light  and atoms and dark matter and dark energy interact with themselves and with each other. Each tiny variation in the configuration of these parameters represents an entire possible  universe. Cosmology is about figuring out which of these infinite possible universes we are  in. But we can’t do that just by measuring the cosmological parameters one by one—there is no  observation that will give us these parameters.

[00:07:36]Instead, we use our observations to determine  which hypothetical universes we are NOT in, over and over until we’re left with the universe  we are in, like a cosmic game of 20 questions.

[00:07:51]The cosmological parameters that are of most  interest to DES and DESI are the parameters of the dark energy equation of state. This is perhaps  the simplest way to describe a changing dark energy—we have the baseline dark energy strength,  omega-0, and the rate of change of dark energy, omega-a. Before we ever discovered dark energy,  as far as we knew we might have been any one of a vast distribution of possible universes in  the space defined by these parameters. With the first observations of white dwarf supernovae  we culled huge numbers of these universes to demonstrate that we’re in a universe with  at least some dark energy described by omega-0.

[00:08:34]The supernova studies had ask the universe a  question: they asked how has cosmic expansion dimmed distant supernovae? The universe  answers in a way that many possible universes couldn’t have answered. So we  were able to eliminate those universes.

[00:08:53]Then the cosmic microwave background  studies asked a different question: what did the early universe look like? The  answer rejects different universes. The baryon acoustic oscillation studies like DESI  ask: how did the fossil parameters from the early universe expand? Again, different  hypothetical universes are eliminated.

[00:09:15]DES asks some of the same with new data, and  also asks how dark matter has warped galaxy images and drawn galaxies together against that  expansion. Again, counterfactual realities were slain by DES in this case. In the end, we’re  left with a much smaller pocket of possible universes where the cosmological parameters  are now much more tightly constrained than they were before we began DES.

[00:09:41]Here’s an analogy. I’m thinking of two whole  numbers between one and ten and you can ask a series of questions to figure them out, but  you can’t ask what the actual numbers are.

[00:09:52]Let’s say you ask what the sum of the numbers  that you're thinking of. I tell you it’s 10.

[00:09:58]You immediately know that the numbers are 1  and 9 or 2 and 8 or 3 and 7 or 4 and 6 or 5 and 5. That's five possibilities assuming  that order of the numbers doesn’t matter.

[00:10:11]Or you could instead ask what the product  is of the two numbers. I tell you it’s 24—so that could be 4 and 6, 3 and 8—two  options. But only one of the answers to the second question is consistent with the  answer to the first question. My numbers had to be 4 and 6. Only by asking multiple  questions can we infer the original numbers.

[00:10:35]In this analogy, the numbers are our cosmological  parameters, and the function—the sum or product—is one aspect of the complex physics that turns  those numbers into observables. For example, one “function” might be “how do the cosmological  parameters govern dark matter halo growth and so warp the shapes of distant galaxies through weak  lensing” or “how do they determine baryon acoustic oscillation formation and then expansion”. Unfortunately, we don’t get to choose these “functions” or questions. They’re what  the universe offers us. When we come up with a cosmological probe, we’re identifying  observables that best pry the cosmological parameters in the context of all the other  available probes. What we’re trying to do is to minimize what we call degeneracies.  In the context of our cosmological probes, degeneracies are the multiple plausible  cosmological parameters that could have led to the same observations. Like how there  are several combinations of whole numbers that sum to 10. Sometimes a probe is good at  constraining one parameter over others, sometimes a probe contains a combination of  parameters without giving any single constraint.

[00:11:55]We want a combination of probes that whittle  down the degeneracies and leave us with tight constraints on the parameters we’re interested in. All we need to do is combine the datasets from these different probes. These various  cosmological probes are not independent of each other and so we can’t combine the  constraints they give in a simple way.

[00:12:22]These different probes may observe many of the  same galaxies, or rely on the same potentially biased distance calibrations, or they make similar  assumptions about how galaxies form and evolve.

[00:12:34]That means some of their uncertainties are also  shared. In statistics, those shared uncertainties are called covariance. Ignoring covariance is  a bit like polling a hundred people who all got their news from the same rumor—you think you have  a hundred independent opinions, but really you've only measured the same bias a hundred times. So  before cosmologists compare two experiments, they have to figure out exactly how much information  is genuinely new, and how much is information they've effectively counted already. It turns out  that understanding those relationships between datasets is just as important as understanding  the datasets themselves. In modern cosmology, the challenge isn't simply collecting more  evidence—it's making sure every new piece of evidence is genuinely asking a different question. But in the end, reality is self-consistent. There aren't separate Universes for DESI and DES, or for  supernovae and the CMB. There’s just one universe, and it will always give consistent answers  to the questions we ask of it. We just need to choose the right questions that allow us to  efficiently rule out the universes that could not have provided the answers that we receive. So, what happened when DES finally asked its questions of the universe? Analyzed on its own,  the new DES results are beautifully consistent with the standard picture of cosmology.  A Universe dominated by a cosmological constant still provides a perfectly good  description of everything DES observed. It very slightly favors a changing dark energy.  DESI is similar—consisstent with the standard cosmology—there are surviving universes whose  physics is exactly what we thought. But there are slightly more surviving potential universes  whose dark energy is actually diminishing.

[00:14:35]Remember, DES wasn't built to verify DESI or vice  versa. Both were built to interrogate the Universe in different, complementary ways. That means  that together they have the power to eliminate more possible universes than either in isolation. And when the DES results are compared with DESI, and the cosmic microwave background, and supernova  observations, many of the same Universes do survive, but not all. Further universe-culling  pushes us slightly further towards favoring a decreasing dark energy. Not enough to rule out  physics-as-we-thought-it, but the intrigue does increase. DES independently points to the same  exciting region of parameters space as DESI, which is extremely important to rule out  bias, while also shaving the edges because DES has different degeneracies to DESI. The  several different questions asked by DES and DESI were answered consistently by a smaller  and smaller fraction of possible universes.

[00:15:46]Perhaps the most exciting “science result”  from DES is that the intriguing hints from DESI weren’t immediately ruled out or disfavored.  That could’ve happened, and would have meant that some bias in the DESI method was probably at  work. No, that island of intriguing universes with decreasing dark energy is still in play. So what’s next? We ask more questions of the universe. We give those counterfactual  universes more chances to fail our grilling and get eliminated. That's why the next decade  of cosmology is going to be incredible. The Rubin Observatory will discover billions  of new galaxies and measure the changing Universe with unprecedented depth and monitor  it over a decade. Euclid is already in space, mapping a vast patch of the sky with incredible  spatial resolution. The Roman Space Telescope will perform this exquisitely sharp imaging  over a smaller scale, but with even better calibration and will also monitor over  time. DESI will continue expanding the largest three-dimensional map of the Universe ever  assembled. And we'll continue culling universes.

[00:16:59]Modern cosmology is really a beautiful example  of how humans have always learned about nature.

[00:17:06]We know in our hearts that there’s a world out  there and that the world is self-consistent. And when we ask questions of it—with our senses and  with our experiments—it’ll tell us the truth. We believe in the model of the world that never fails  a test—never answers wrong. Every independent line of evidence needs to agree on the same story. And even when we think we have the story, we keep asking questions. At the frontier of science,  the hardest part isn't discovering something new, it's convincing ourselves that we haven't fooled  ourselves. Our current cosmological models, along with all of our best scientific theories  in their broad strokes—from evolution to climate change to germ theory to the Big Bang—are  the explanations of a reality that have withstood every question that we've so far  asked of it. But these also still represent families of possible universes that still need  whittling down. So we keep asking questions, bringing us closer to pinpointing in  parameter space this singular space time.