Dark Energy Survey Reveals New Clues About the Expanding Universe

<v Speaker 1>Welcome to Bedtime Astronomy. Explore the wonders of the cosmos
<v Speaker 1>with our soothing Bedtime Astronomie podcast. Each episode offers a
<v Speaker 1>gentle journey through the stars, planets, and beyond, perfect for
<v Speaker 1>unwinding after a long day. Let's travel through the mysteries
<v Speaker 1>of the universe as you drift off into a peaceful
<v Speaker 1>slumber under the night sky.
<v Speaker 2>Imagine a map. But I don't want you to picture,
<v Speaker 2>you know, a folded paper map in a glove box,
<v Speaker 2>or even Google Map.
<v Speaker 3>No, this is this is something else entirely.
<v Speaker 2>I want you to imagine a map so colossal, so
<v Speaker 2>incredibly detailed, that it covers an entire eighth of the
<v Speaker 2>sky above your head. And on this map, you're not
<v Speaker 2>looking for roads or cities. You are looking at galaxies.
<v Speaker 3>Hundreds of millions of them. It's a staggering number to
<v Speaker 3>even try and hold in your head.
<v Speaker 2>It is. And we aren't just looking at where they
<v Speaker 2>are like pushpins on a court. We're looking at a
<v Speaker 2>map that tells the story of how they got there
<v Speaker 2>over billions of years.
<v Speaker 3>Exactly.
<v Speaker 2>This is the deep dive. And today we are tackling
<v Speaker 2>something that has been what twenty five years in the making.
<v Speaker 2>At least as of yesterday, Thursday, January twenty second, twenty
<v Speaker 2>twenty six, the Dark Energy survey Or DEES collaboration has
<v Speaker 2>finally released the analysis of all six years of their
<v Speaker 2>survey data.
<v Speaker 3>This is it. This is the big one. I mean,
<v Speaker 3>we've had the year one results, the year three results,
<v Speaker 3>which were exciting.
<v Speaker 2>Sure there were great appetizers.
<v Speaker 3>They were, but this is the full data set. This
<v Speaker 3>is the final word from this particular experiment.
<v Speaker 2>So the mission here, it sounds so simple on paper,
<v Speaker 2>but I know it's just mind bendingly complex. The mission
<v Speaker 2>is to understand the invisible force that makes up seventy percent.
<v Speaker 3>Of our universe dark energy dark energy exactly, and to
<v Speaker 3>understand why this specific release is so huge. You have
<v Speaker 3>to look at the sheer precision. This isn't just another
<v Speaker 3>star chart. It is the culmination of a quarter century
<v Speaker 3>dream to really nail down the history of the universe's expansion.
<v Speaker 3>And these results, the constraints they provide are twice as
<v Speaker 3>tight as any analysis we've done before.
<v Speaker 2>Twice as tight. I don't want to pause on that,
<v Speaker 2>because that really is the headline, isn't it. In physics?
<v Speaker 2>Narrowing down those error bars is just as exciting and
<v Speaker 2>sometimes more exciting than finding a whole new particle.
<v Speaker 3>Oh absolutely, because you're effectively running out of places for
<v Speaker 3>alternative theories to hide. When your constraints are loose, you
<v Speaker 3>can fit all sorts of wild ideas into the data.
<v Speaker 3>You can say, maybe gravity works backwards on Tuesdays. And
<v Speaker 3>if your data is fuzzy enough, you can't prove it wrong.
<v Speaker 2>You can't rule it out.
<v Speaker 3>You can't. But when you tighten those constraints, when you
<v Speaker 3>drill down into that precision, you start squeezing those other
<v Speaker 3>theories out of existence.
<v Speaker 2>But and I always love a good butt in a
<v Speaker 2>science story, there is a twist.
<v Speaker 3>There's always a twist universe. It rarely gives up its
<v Speaker 3>secrets easily.
<v Speaker 2>So while our standard model of physics held up and
<v Speaker 2>held up remarkably well again this massive data set, there
<v Speaker 2>is this persistent, nagging, little mystery in the numbers. It's
<v Speaker 2>about how clumpy the universe actually is, and it's refusing
<v Speaker 2>to go away.
<v Speaker 3>That is the part that has everyone, myself included, scratching
<v Speaker 3>their heads today. It's a subtle discrepancy. We call it
<v Speaker 3>the S eight tension, or the clustering gap. But in physics,
<v Speaker 3>you know subtle discrepancies. That's where the next revolution begins.
<v Speaker 2>It's the loose thread on the sweater.
<v Speaker 3>You pull on that and who knows what unravels.
<v Speaker 2>Okay, let's unpack all of this before we get to
<v Speaker 2>the clumpy mystery and the invisible forces. We need to
<v Speaker 2>set the stage, because this story really begins about one
<v Speaker 2>hundred years ago, doesn't it.
<v Speaker 3>It does. To understand why DEES is such a big deal,
<v Speaker 3>you have to understand what we thought the universe was
<v Speaker 3>doing before.
<v Speaker 2>All this, So rewind into the nineteen twenties.
<v Speaker 3>Right back then. That's really the baseline for our modern
<v Speaker 3>understanding of the cosmos.
<v Speaker 4>Yeah.
<v Speaker 3>Before that, the general idea was that the universe was static, eternal, unchanging.
<v Speaker 3>It was just there. Yeah. But then Edwin Hubble and
<v Speaker 3>his peers, they made this startling.
<v Speaker 2>Realization galaxies are moving.
<v Speaker 3>They're moving, and not just moving randomly like you know,
<v Speaker 3>bees swarming out of a hive. There was a specific relationship.
<v Speaker 3>The farther away a.
<v Speaker 2>Galaxy is the faster it's moving away from.
<v Speaker 3>Us, the faster it's receding. This is the discovery of
<v Speaker 3>cosmic expansion. The actual fabric of space itself is stretching
<v Speaker 3>and it's carrying the galaxies along for the ride.
<v Speaker 2>Okay, so the universe is getting bigger. I think most
<v Speaker 2>people are familiar with that concept, but for decades there
<v Speaker 2>was this logical assumption that came with it, right, a
<v Speaker 2>prediction about what would happen next.
<v Speaker 3>Yes, the logic trap, so I like to call it,
<v Speaker 3>And it makes perfect sense intuitively. Think about gravity. Gravity
<v Speaker 3>pulls things together. Mass attracts mass. So if you have
<v Speaker 3>a universe full of stuff, stars, gas, galaxies, dark matter,
<v Speaker 3>all of it pulling on each other, you would naturally
<v Speaker 3>logically assume that this initial expansion from the Big Bang
<v Speaker 3>should be slowing down.
<v Speaker 2>It's like throwing a ball in the air. You give
<v Speaker 2>it that initial push that's the Big Bang, but as
<v Speaker 2>it flies up Earth's gravity is constantly pulling it back,
<v Speaker 2>it slows down Exactly.
<v Speaker 3>Everyone assumed the universe was that ball For about seventy years.
<v Speaker 3>The only question was how fast is it slowing down?
<v Speaker 3>Is there enough mass to stop it completely and cause
<v Speaker 3>a big crunch where everything collapses back in on itself.
<v Speaker 2>Or does it just coast forever, slowing but never quite stopping?
<v Speaker 3>Sisily We even had a name for it, the deceleration parameter.
<v Speaker 2>We were so sure it was decelerating we named the
<v Speaker 2>parameter after it. That's some serious confidence.
<v Speaker 3>We're very confident.
<v Speaker 2>Yeah.
<v Speaker 3>But then came nineteen ninety eight, the year everything we
<v Speaker 3>thought we knew just got flipped upside down.
<v Speaker 2>This is the nineteen ninety eight shock. I remember reading
<v Speaker 2>about this. For the scientific community. This must have been like,
<v Speaker 2>I don't know, like finding out the Earth is actually flat.
<v Speaker 3>It was. It was that level of paradigm shift. Yes,
<v Speaker 3>you had two independent teams of cosmologists, the Supernova Cosmology
<v Speaker 3>Project and the High Zy Supernova Search Team, and they
<v Speaker 3>were both looking at the same thing.
<v Speaker 2>Distant type YA supernova.
<v Speaker 3>Distant type YA supernova.
<v Speaker 2>We should probably define that because TYPEA comes up a
<v Speaker 2>lot in the DS papers. Why are these specific explosions
<v Speaker 2>so important? Why not just look at any old star?
<v Speaker 3>Because the type of supernova is a very special, a
<v Speaker 3>very standardized kind of death for a star. It happens
<v Speaker 3>in a binary system.
<v Speaker 2>So two stars orbiting each other.
<v Speaker 3>Right, and one of them is a white dwarf, the
<v Speaker 3>super dense core of a dead star, and it's stealing
<v Speaker 3>matter from its companion star.
<v Speaker 2>That cosmic vampire.
<v Speaker 3>That's a great way to put it. It just siphons
<v Speaker 3>off gas, and it keeps doing that until it hits
<v Speaker 3>a very very specific mass limit, the chanderse Car limit,
<v Speaker 3>about one point four times the mass of our Sun.
<v Speaker 3>And because that mass limit is the same every single time,
<v Speaker 3>the explosion has roughly the same intrinsic brightness every single time.
<v Speaker 2>So it's a standard candle.
<v Speaker 3>It is the perfect standard candle. Think of it like
<v Speaker 3>a specific brand of sixty watt light bulb. If you
<v Speaker 3>see that bull bright in front of your face, it's blinding.
<v Speaker 3>If you see it a mile away, it's just a
<v Speaker 3>dim point of light.
<v Speaker 2>But since you know it's a sixty watt bulb, and
<v Speaker 2>calculate exactly how far away it is, just by how
<v Speaker 2>dim it looks exactly.
<v Speaker 3>These supernovae are our cosmic light bulbs. They let us
<v Speaker 3>measure vast distances across the universe.
<v Speaker 2>So these two teams they use these cosmic light bulbs
<v Speaker 2>to measure the distance to far off galaxies. And what
<v Speaker 2>they found was not what they expected, not at all.
<v Speaker 3>They found the supernovae were dimmer than they should have
<v Speaker 3>been if the universe was slowing.
<v Speaker 2>Down, dimmer mint farther away.
<v Speaker 3>Dimmer meant they were farther away than all our calculations predicted.
<v Speaker 3>The only way they could be true is if the
<v Speaker 3>expansion wasn't slowing down.
<v Speaker 2>It's accelerating.
<v Speaker 3>It's accelerating.
<v Speaker 2>I always love the car analogy for this. It's like
<v Speaker 2>you're driving down my way. You take your foot off
<v Speaker 2>the gas to coast, and you expect the car to
<v Speaker 2>slow down.
<v Speaker 3>Right because of friction and wind resistance.
<v Speaker 2>But instead, without you touching anything, the car suddenly floors
<v Speaker 2>it and starts speeding.
<v Speaker 3>Up, and you have no idea who is pressing the pedal.
<v Speaker 3>That is exactly the situation we found ourselves in in
<v Speaker 3>nineteen ninety eight. And to explain this acceleration, physicists realized
<v Speaker 3>there must be some form of energy FILLINGMPTY space, pushing
<v Speaker 3>everything apart, and.
<v Speaker 2>Since we didn't know what it was, we gave.
<v Speaker 3>It a placeholder name dark energy.
<v Speaker 2>And it's not just some tiny rounding error, right, It's
<v Speaker 2>not a one percent effect you have to squint to see, no.
<v Speaker 3>Far from it. Our current estimates suggests dark energy makes
<v Speaker 3>up roughly seventy percent of the mass energy density of
<v Speaker 3>the universe.
<v Speaker 2>Seven zero seventy percent dark matter, the invisible stuff holding
<v Speaker 2>galaxies together. That's another twenty five percent, the stuff we're
<v Speaker 2>actually made of, atoms, stars, planets, you me, this conversation,
<v Speaker 2>all of it, it's barely five percent.
<v Speaker 3>That is just it's humbling. We are the rounding error,
<v Speaker 3>wh're the debris.
<v Speaker 2>We are the foam on the cosmic ocean. And yet,
<v Speaker 2>despite being seventy percent of reality, we know almost nothing
<v Speaker 2>about it. Is it constant? Does it change over time?
<v Speaker 2>Is it a property of space itself? This is what
<v Speaker 2>the Dark Energy Survey was built to find out.
<v Speaker 3>Okay, so let's talk about the tool. Because you don't
<v Speaker 3>just go out to the backyard with a pair of
<v Speaker 3>binoculars to find dark as energy. You need the.
<v Speaker 2>DECAM, the Dark Energy Camera. It is a beast of
<v Speaker 2>a machine, truly, one of the most advanced instruments ever
<v Speaker 2>pointed at the sky.
<v Speaker 3>I was looking at the specs on this thing. It's
<v Speaker 3>a five hundred and seventy megapixel camera. I mean, my
<v Speaker 3>phone is a decent camera, maybe forty eight megapixels if
<v Speaker 3>I'm lucky, five seventies. Just it's absurd.
<v Speaker 2>It has to be. It was fabricated by the US
<v Speaker 2>Department of Energy for this specific purpose. It uses sixty
<v Speaker 2>two separate charge coupled devices CCDs, and they're all specifically
<v Speaker 2>designed to be super sensitive to red light. Why red
<v Speaker 2>light specifically.
<v Speaker 3>Because of redshift. As the universe expands, the light from
<v Speaker 3>distant galaxies gets stretched out during its journey to US.
<v Speaker 2>Light waves get longer, exactly.
<v Speaker 3>They shift towards the red end of the spectrum. So
<v Speaker 3>if you're looking at galaxies that are billions of light
<v Speaker 3>years away, their light is going to be very, very red.
<v Speaker 3>The camera needs to be able to see that faint,
<v Speaker 3>stretched out light.
<v Speaker 2>And where did they put this thing?
<v Speaker 3>It's mounted on the Victor M. Blanco four meter telescope,
<v Speaker 3>which is at the Sarotololo Inter American Observatory.
<v Speaker 2>In Chilling, high up in the Andy. Why Chili, why not?
<v Speaker 2>I don't know Arizona or why.
<v Speaker 3>The Andes offer some of the most stable atmosphere conditions
<v Speaker 3>on the planet. You're high up, so the air is thin,
<v Speaker 3>it's incredibly dry, so there's almost no water vapor to
<v Speaker 3>distort the light. And it's super remote so there is
<v Speaker 3>zero light pollution. It is the perfect place on Earth
<v Speaker 3>for staring into the deep dark, and.
<v Speaker 2>The scope of what they did with this camera is
<v Speaker 2>just immense. We're talking about data collected over seven hundred
<v Speaker 2>and fifty eight.
<v Speaker 3>Nights between twenty thirteen and twenty nineteen. That's six years
<v Speaker 3>of just staring. And in that time they recorded information
<v Speaker 3>from six hundred and sixty nine million galaxies.
<v Speaker 2>I can't even conceptualize that number. Six hundred and sixty
<v Speaker 2>nine million galaxies and each one of those has billions
<v Speaker 2>of stars and we're looking at objects that are billions
<v Speaker 2>of light years away.
<v Speaker 3>It creates a volume of data that is it's hard
<v Speaker 3>to comprehend. We're talking petabytes of information. Yeah, and this
<v Speaker 3>wasn't a small team either. This is a huge international collaboration,
<v Speaker 3>over four hundred astrophysics from thirty five institutions across seven countries.
<v Speaker 2>It's a global effort to solve a universal mystery. So, Okay,
<v Speaker 2>they have the camera, they have the data, but how
<v Speaker 2>do you actually weigh something invisible like dark energy. You
<v Speaker 2>can't see it.
<v Speaker 3>Directly, No, you can't. You can only see its effects
<v Speaker 3>on this stuff you can see. And this brings us
<v Speaker 3>to the methodology, which is what makes this year's really
<v Speaker 3>so special. For the first time, they've combined all four
<v Speaker 3>of the probes or methods of measurement that were originally
<v Speaker 3>proposed when DS was just an idea twenty five years ago.
<v Speaker 3>The Big four, the Big four. So briefly, you have
<v Speaker 3>uryan acoustic oscillations or BAO that uses the scale of
<v Speaker 3>sound waves from the early universe as a sort of
<v Speaker 3>standard ruler.
<v Speaker 2>Like a cosmic yardstick frozen into the sky from the
<v Speaker 2>beginning of time.
<v Speaker 3>That's a great description. Then we have the type of
<v Speaker 3>supernova which we talked about, the standard candles. Then we
<v Speaker 3>have galaxy clusters. You literally just count how many massive
<v Speaker 3>clusters exist at different points in history.
<v Speaker 2>And if gravity is winning, you get more clusters. If
<v Speaker 2>dark energy is winning, you get fewer exactly.
<v Speaker 3>And finally, the heavy hitter for this analysis weak gravitational lensing.
<v Speaker 2>Okay, I want to double click on weak lensing because
<v Speaker 2>reading through the reports, it really seems like this is
<v Speaker 2>the bread and butter of the DEES analysis. This is
<v Speaker 2>where the magic happens.
<v Speaker 3>It is absolutely fundamental. It's arguably the most powerful tool
<v Speaker 3>we have for mapping dark matter, which in turn tells
<v Speaker 3>us about dark energy.
<v Speaker 2>So explain this to us. What is weak lensing. It
<v Speaker 2>sounds like a vision problem.
<v Speaker 3>It does, doesn't it, But it's actually a direct consequence
<v Speaker 3>of Einstein's general relativity. Einstein told us that mass bends space.
<v Speaker 3>It curves the very fabric of the universe. So imagine
<v Speaker 3>light traveling from a very very distant galaxy towards us
<v Speaker 3>here on Earth, Okay, on its long journey here, that
<v Speaker 3>light has to pass by massive objects. It could be
<v Speaker 3>clusters of other galaxies, huge clouds of invisible dark matter, and.
<v Speaker 2>The gravity from those objects bends the path of the
<v Speaker 2>light right like a glass lens bends light.
<v Speaker 3>To exactly so the gravity acts like a lens. Now,
<v Speaker 3>sometimes this lensing is strong. You see these weird, beautiful
<v Speaker 3>arcs or rings or multiple images of the same galaxy.
<v Speaker 3>That's spectacular, but it's rare. Weak lensing is much much
<v Speaker 3>more subtle. It's a tiny distortion in the shape of
<v Speaker 3>the background galaxy.
<v Speaker 2>So just it squishes it a little.
<v Speaker 3>Bit, a tiny amount. We're talking about a distortion of
<v Speaker 3>roughly one percent.
<v Speaker 2>Wait one percent, how can you possibly tell? I mean,
<v Speaker 2>galaxies are already kind of blobs, some are spirals, some
<v Speaker 2>are elliptical, some are just messy, irregular shapes. How do
<v Speaker 2>you know if a galaxy is distorted by one percent
<v Speaker 2>or if it's just naturally shaped like that?
<v Speaker 3>That is the genius of it. You hat you can't
<v Speaker 3>tell for a single galaxy. It's totally impossible.
<v Speaker 2>Yeah, what's the trick.
<v Speaker 3>The trick is statistics. If I showed you just one galaxy,
<v Speaker 3>you couldn't tell me if it was lensed or just
<v Speaker 3>naturally oval shaped. But if you look at millions of galaxies,
<v Speaker 3>patterns emerge. In a random universe, galaxies should be oriented randomly,
<v Speaker 3>some pointing up, some down, some left, some right, just
<v Speaker 3>a total jumble.
<v Speaker 2>So the average orientation should be zero, no preference precisely.
<v Speaker 3>But if you look at a certain patch of sky
<v Speaker 3>and you see that the galaxies there are all slightly
<v Speaker 3>aligned or distorted in a similar way, a phenomenon we
<v Speaker 3>call cosmic shear, that tells you something.
<v Speaker 2>It tells you there's a massive amount of matter in
<v Speaker 2>front of them.
<v Speaker 3>Yes, a huge lens distorting the wallpaper in the universe
<v Speaker 3>behind it. And that's how we map dark matter. If
<v Speaker 3>the light is bent, there must be mass there. Even
<v Speaker 3>if we can't see the mass itself, we can create
<v Speaker 3>a three D map of the invisible scaffolding of the
<v Speaker 3>universe based purely on how it warps our view of
<v Speaker 3>the stuff behind it.
<v Speaker 2>The source material mentioned The Bullet Cluster is a famous
<v Speaker 2>example of this in action.
<v Speaker 3>The Bullet cluster is the classic smoking gun. It's two
<v Speaker 3>galaxy clusters that collided a massive cosmic car crash about
<v Speaker 3>three point seven billion light years away. When we look
<v Speaker 3>at it with weak lensing, we can see where all
<v Speaker 3>the mass is, and.
<v Speaker 2>It wasn't where the normal matter was.
<v Speaker 3>Not at all. The normal matter, the hot gas a
<v Speaker 3>crashing got stuck in the middle due to frick. But
<v Speaker 3>the dark matter, the bulk of the mass, it just
<v Speaker 3>passed right through itself like a ghost. It proved that
<v Speaker 3>dark matter is real and it's separate from the stuff
<v Speaker 3>we're made of. It doesn't interact except through gravity.
<v Speaker 2>That is just wild. So DEES is doing this, but
<v Speaker 2>on a massive scale.
<v Speaker 3>On the scale of the entire survey. They are measuring
<v Speaker 3>the probability of any two galaxies being a certain distance
<v Speaker 3>apart and how they are distorted. By doing this, they
<v Speaker 3>can reconstruct the distribution of matter over six billion years
<v Speaker 3>of cosmic history.
<v Speaker 2>So they're building a three D movie of the universe's structure.
<v Speaker 2>They can see how clumpy the dark matter was six
<v Speaker 2>billion years ago, five billion years ago.
<v Speaker 3>Four billion, precisely. And that finally tells us about dark energy,
<v Speaker 3>because remember this is a war. It's a cosmic tug
<v Speaker 3>of war. Dark energy is pushing things apart, trying to
<v Speaker 3>smooth everything out. Gravity is pulling things together, trying to
<v Speaker 3>make them clump up. By measuring the clumpiness over time,
<v Speaker 3>we can see which force was winning at different eras.
<v Speaker 2>Okay, so we have the arena set gravity versus dark energy.
<v Speaker 2>The data is in six hundred and sixty nine million
<v Speaker 2>galaxy is analyzed. What's the verdict, Well, it.
<v Speaker 3>Comes down to a battle of the models. In one
<v Speaker 3>corner you have the heavyweight champion LAMB.
<v Speaker 2>To CDM, the standard model of cosmology.
<v Speaker 3>The vanilla option. Yeah. In this model, Lambda stands for
<v Speaker 3>the cosmological constant. This is an idea Einstein originally had
<v Speaker 3>then through away then we had to bring back. It
<v Speaker 3>says that dark energy is a constant for us. It
<v Speaker 3>doesn't change. It's just a property of empty space.
<v Speaker 2>So every cubic meter of space has a set amount
<v Speaker 2>of this energy.
<v Speaker 3>Exactly, if you make more space you get more energy.
<v Speaker 3>It's simple, it's elegant, and honestly, it's a little boring.
<v Speaker 2>And in the challenger's corner.
<v Speaker 3>We have a model we call WCDM. In this model,
<v Speaker 3>dark energy isn't necessarily constant. It has this parameter we
<v Speaker 3>just call it W that describes its it's called its behavior.
<v Speaker 3>If W is exactly minus one, it's constant. It's lambdas okay.
<v Speaker 3>But if W is different from minus one, or if
<v Speaker 3>it changes over time, then dark energy evolves. It could
<v Speaker 3>be getting stronger, which might lead to a big rip
<v Speaker 3>where atoms eventually get torn apart, or it could be
<v Speaker 3>getting weaker.
<v Speaker 2>So after all that, who won.
<v Speaker 3>The standard model LAMB to CDM.
<v Speaker 2>Einstein is still right.
<v Speaker 3>Einstein is still right. The analysis from all the paper
<v Speaker 3>is submitted to Physical Review D shows that the data
<v Speaker 3>is perfectly consistent with a constant dark energy density. The
<v Speaker 3>value of W they found is effectively minus one, well
<v Speaker 3>within the error bars.
<v Speaker 2>Now I can hear some listeners groaning, oh man, the
<v Speaker 2>standard model one. That's so boring. We want new physics.
<v Speaker 3>I know, I know, we all want warp drives and
<v Speaker 3>new dimensions. But you have to understand the triumph here.
<v Speaker 3>The real victory is in the precision. The report says
<v Speaker 3>these results are consistent with the standard model, but the
<v Speaker 3>constraints are twice as tight as previous.
<v Speaker 2>Analyzes, meaning that window of error has been cut in
<v Speaker 2>half drastically.
<v Speaker 3>It means that if there is some weird evolving dark
<v Speaker 3>energy out there, it has to be hiding in a
<v Speaker 3>very very tiny sliver of possibility. We are squeezing the
<v Speaker 3>space where those alternative theories can live. The data fits
<v Speaker 3>the evolving model, sure, but not any better than the
<v Speaker 3>simple one. So Ockham's razor applies.
<v Speaker 2>You go with the simpler explanation.
<v Speaker 3>You go with the simpler explanation.
<v Speaker 2>So dark energy appears to be constant. But and here
<v Speaker 2>comes that. But we teased at the top of the show,
<v Speaker 2>there is something in the data that doesn't quite fit.
<v Speaker 3>The clustering gap, or as we call it, the say tension.
<v Speaker 2>This is the plot twist. Tell us about this gap.
<v Speaker 3>Okay, so we have two different ways to figure out
<v Speaker 3>how clumpy the universe should be today. Method one we
<v Speaker 3>look at the very beginning.
<v Speaker 2>Of the universe, the baby picture.
<v Speaker 3>The baby picture exactly the cosmic microwave background or CMB.
<v Speaker 3>This is the afterglow of the Big Bang, measured with
<v Speaker 3>incredible precision by the Plank satellite. We can measure how
<v Speaker 3>clumpy things were back then these tiny fluctuations in density.
<v Speaker 3>Then we use our standard model LAMB to CDM to
<v Speaker 3>fast forward thirteen point eight billion years and predict how
<v Speaker 3>clumpy the universe should be.
<v Speaker 2>Now, okay, so that's the prediction based on the baby picture.
<v Speaker 2>We predict what the adult should look like.
<v Speaker 3>Right based on how you looked at age one, and
<v Speaker 3>knowing how humans grow, you should be six foot two.
<v Speaker 3>That's the prediction. Now metcha two. We use the Dark
<v Speaker 3>Energy Survey to actually go out and measure how clumpy
<v Speaker 3>the universe is right now. Using weak lensing, we take
<v Speaker 3>a photo of the adult. We take a photo of
<v Speaker 3>the adult and did they match? They don't. Not quite
<v Speaker 3>the universe we see today. The adult photo is slightly
<v Speaker 3>smoother than the prediction says it should be. It's less clumpy.
<v Speaker 3>The parameter we use to measure this, s eight comes
<v Speaker 3>in lower in the lensing data from DEES than it
<v Speaker 3>does in the prediction from the CNB.
<v Speaker 2>Wait, so the math says gravity should have pulled things
<v Speaker 2>into tighter, denser clumps by now, but the actual picture
<v Speaker 2>says nope, it's a little more spread out.
<v Speaker 3>That's it exactly. It's like predicting someone will be six
<v Speaker 3>foot two, but when you measure them they're five foot eleven.
<v Speaker 3>It's close. Yeah, but it's definitely not the number we expected.
<v Speaker 2>Is it a mistake? Systematic error? Did someone forget to
<v Speaker 2>carry a one somewhere?
<v Speaker 3>That is the multi billion dollar question. In the previous
<v Speaker 3>analyzes des year one, year three, we saw a hint
<v Speaker 3>of this, but people thought, Okay, maybe it's just noise,
<v Speaker 3>a statistical fluke, maybe we didn't account for galactic dust perfectly.
<v Speaker 3>But now we have the full six year data set,
<v Speaker 3>the most precise map ever made, and the gap didn't
<v Speaker 3>go away. In fact, it widens slightly.
<v Speaker 2>It's stubborn.
<v Speaker 3>It is very persistent, and it's showing up in other
<v Speaker 3>weak lensing surveys too, like the Kilo degree survey. Now
<v Speaker 3>to be scientifically responsible here, the discrepancy is roughly two
<v Speaker 3>point five to three sigma.
<v Speaker 2>Translate that from statistics speak for us. What does that mean?
<v Speaker 3>It means there's roughly a one in one hundred or
<v Speaker 3>one in a few hundred chance that this is just
<v Speaker 3>bad luck, a random fluctuation. It's not yet at the
<v Speaker 3>five sigma level, which is the gold standard in physics
<v Speaker 3>for claiming a discovery, so we can't definitively say the
<v Speaker 3>standard model is broken.
<v Speaker 2>But it's enough to make people sweat a little.
<v Speaker 3>It's enough to keep cosmologists up at night because if
<v Speaker 3>this game app is real, if the universe really is
<v Speaker 3>smoother than our physics predicts it should be, it implies
<v Speaker 3>that something fundamental is missing from our understanding.
<v Speaker 2>What could it be? I mean, if it's not a mistake,
<v Speaker 2>what kind of physics could cause the universe to resist
<v Speaker 2>clumping up?
<v Speaker 3>That's the fun part. This is where the theorists get to play.
<v Speaker 3>It could be well, it could be that gravity behaves
<v Speaker 3>differently on these massive scales than Einstein predicted. Maybe gravity
<v Speaker 3>gets a little weaker over huge distances. It could be
<v Speaker 3>that dark energy does interact with dark matter in some
<v Speaker 3>way we haven't thought of. Maybe it gives dark matter
<v Speaker 3>a little push, preventing it from clumping as tightly. Or
<v Speaker 3>maybe there's a new type of particle, like a new
<v Speaker 3>kind of neutrino that's just zooming around and smoothing things
<v Speaker 3>out like a cosmic iron.
<v Speaker 2>So this is the potential aha moment. The standard model
<v Speaker 2>survived the main test, the expansion rate. But this little gap,
<v Speaker 2>this is the loose thread. You pull on it, the
<v Speaker 2>whole thing might just unravel.
<v Speaker 3>That is the most exciting thread to pull on in
<v Speaker 3>all of cosmology. Right now, we confirm the basics, but
<v Speaker 3>in doing so we might have found a crack in
<v Speaker 3>the foundation.
<v Speaker 2>So, speaking of the future, the Dark Energy Survey has
<v Speaker 2>finished its data collection. The survey is done.
<v Speaker 3>What happens next, Well, the baton is being passed des
<v Speaker 3>paved the way it proved that these techniques like weak
<v Speaker 3>lensing reconstruction on a massive scale actually work. It was
<v Speaker 3>the training ground. Now we are leveling up in a
<v Speaker 3>big way.
<v Speaker 2>Enter the VERI C Reuben Observatory.
<v Speaker 3>Yes, the NSF DOE ver C Ruben Observatory. It's being
<v Speaker 3>built in Chile right now, not too far from the
<v Speaker 3>Blanco Telescope actually, and it's going to conduct what's called
<v Speaker 3>the Legacy Survey of Space and Time or LSST.
<v Speaker 2>I love that name, Legacy Survey Space and Time. It
<v Speaker 2>sounds so epic.
<v Speaker 3>It is appropriately epic. Just think about the scale difference
<v Speaker 3>des looked at six hundred and sixty nine million galaxies
<v Speaker 3>over an eighth of the sky. Impressive, right, extremely, The
<v Speaker 3>Ruben Observatory is going to catalog about twenty billion galaxies.
<v Speaker 2>Whoa from six hundred million to twenty billion, and it.
<v Speaker 3>Will cover the entire Southern Hemisphere sky. It's not even
<v Speaker 3>a static map. It's going to scan the whole sky
<v Speaker 3>every few nights. It will essentially make a ten year
<v Speaker 3>time lapse movie of the universe.
<v Speaker 2>That's just that's an exponential leap.
<v Speaker 3>It's a generational leap. It'll take the constraints we just
<v Speaker 3>got from ES and tighten them even further. If that
<v Speaker 3>clustering gap is real, Ruben will find it. It will make
<v Speaker 3>it undeniable. It will turn that two point five sigma
<v Speaker 3>hint into a five sigma gold standard discovery.
<v Speaker 2>Or it will prove it was just noise all along.
<v Speaker 3>Or it will prove it was just noise and we
<v Speaker 3>can all go back to sleep. But either way we'll know.
<v Speaker 3>It's like Dees built the foundation and now Ruben is
<v Speaker 3>going to build the sky scraper on top of it.
<v Speaker 2>It really feels like we're entering a golden age of
<v Speaker 2>this kind of work. You know, when we started this,
<v Speaker 2>we talked about a map, and what's amazing is that
<v Speaker 2>for most of human history, mapping the universe meant drawing constellations,
<v Speaker 2>connecting dots to make pictures of bears and hunters.
<v Speaker 3>We were just projecting our own stories onto the stars,
<v Speaker 3>trying to make sense of the chaos by drawing familiar line.
<v Speaker 2>Right, And now we are mapping the invisible scaffolding of
<v Speaker 2>reality itself. We're weighing the dark matter that holds galaxies
<v Speaker 2>together and measuring the dark energy that's ripping them apart.
<v Speaker 2>We are not just drawing bears anymore. We are drawing
<v Speaker 2>the forces that created the universe the bears live in, and.
<v Speaker 3>We are doing it with the level of precision that
<v Speaker 3>would have just seemed like magic a century ago. Imagine
<v Speaker 3>telling Hubble in nineteen twenty nine that one day we'd
<v Speaker 3>be measuring the one percent shape distortion of six hundred
<v Speaker 3>million galaxies to weigh the vacuums space. He would have
<v Speaker 3>thought you were insane.
<v Speaker 2>It really is an incredible human achievement. Four hundred people
<v Speaker 2>working for over a decade just to verify a theory
<v Speaker 2>proposed before some of them were even.
<v Speaker 3>Born, and in the process to find that one little
<v Speaker 3>loose thread that might lead to something entirely new. That's
<v Speaker 3>what science is. It's not about having all the answers,
<v Speaker 3>It's about finding the next better question.
<v Speaker 2>So as we wrap up this deep dive, here's the takeaway.
<v Speaker 2>We looked at an eighth of the sky. I'll last
<v Speaker 2>hundreds of millions of galaxies, and we confirm that, yes,
<v Speaker 2>the universe is expanding under the influence of a constant
<v Speaker 2>dark energy.
<v Speaker 3>Einstein is still the boss.
<v Speaker 2>But the universe is smoother than it should be, And
<v Speaker 2>that is the thought I want to leave you with today.
<v Speaker 2>The universe is smoother than our math predicts based on
<v Speaker 2>how it started. So if the beginning and the present
<v Speaker 2>don't quite match up, what happened in the middle. Is
<v Speaker 2>gravity changing its rules when we aren't looking, or is
<v Speaker 2>there a piece of the puzzle floating right in front
<v Speaker 2>of our faces that we just haven't learned how to
<v Speaker 2>see yet.
<v Speaker 3>The standard model survived this test, but that little gap
<v Speaker 3>that might just be the door to a whole new physics.
<v Speaker 2>Thanks for listening to the deep dive. Keep looking up.
<v Speaker 4>The days. SA