A hundred years ago, astronomers figured out that  the universe hadn’t always been the same size.

More specifically, it’s getting bigger. But if you ask a bunch of astronomers  how fast the universe is getting bigger, you won’t get a single, satisfying answer.

Estimates tend to hover  around two different values. And it’s such a huge issue, some  people call it The Crisis in Cosmology. So when the shiny new James Webb  Space Telescope launched in 2021, there were hopes that the  crisis would be resolved.

But the JWST may have actually made it worse, leading to an even bigger mystery that  might reveal brand new physics at work. [♪ INTRO] Our story starts in 1929, when American  astronomer Edwin Hubble published a paper announcing that a bunch of galaxies in  the night sky are moving away from us. In other words, the universe was expanding. Not the galaxies themselves,  but the space between them.

The rate of that expansion has varied over time. And astronomers refer to the current rate  of expansion as the Hubble constant, H naught. Hubble even provided us with an estimate for 

H0: 530 kilometers per second per megaparsec. Which doesn’t mean a whole lot to  someone who isn’t an astronomer,   so let’s take a moment to break it down. A megaparsec is a unit of distance. One megaparsec is about three million  light years, or 31 quintillion kilometers.

For context, the distance between  the Milky Way and Andromeda… the next major galaxy over…is  about 0.8 megaparsecs. And the entire observable universe is  about thirty thousand megaparsecs wide. So if the value for H0 were really 530, it’d  mean that a galaxy that’s one megaparsec away from us would be flying away from us  at a speed of 530 kilometers a second.

A galaxy that’s two megaparsecs away would be  moving away from us at 1,060 kilometers a second. And so on. But over the years, it turned  out Hubble’s estimate was off.

Like, way off. Thanks to better telescopes and better datasets, we know H0 is really somewhere around  70 kilometers per second per megaparsec. But the exact value?

That’s where the problem lies. And we’ve got a cosmological crisis  on our hands because different groups   are trying to measure H0 using different methods. They should all give the same answer,  but right now, they simply aren’t.

And since the problem surrounds the Hubble  Constant, some call it the Hubble Tension. There are two main ‘camps’  that are in conflict here. Let’s call them the ‘CMB’ camp  and the ‘distance ladder’ camp,   for reasons that we'll make clear.

The CMB camp says H0 is about 67. The Distance Ladder camp claims it’s about 73. Compared to 500, that doesn’t  sound like a big difference at all.

But it is when you realize  the error bars don’t overlap. That’s a fancy way of saying that both  camps have rigorously checked all the   possible sources of random error  and noise in their measurements, and strongly believe that their  number is basically correct. For instance, the CMB crew quote their  value for H0 as 67.4 plus or minus 0.5, which means they’re confident that the real,  final value is somewhere between 66.9 and 67.9.

But different distance ladder teams quote their  number as being around 73, plus or minus about 1. And 73 minus 1 is still larger than 67.9. Both camps are claiming that  random errors alone can’t   explain the difference between their numbers.

Hence, a crisis in cosmology. Which, well, okay maybe you  wouldn’t quite call this a crisis,   but trust me when I say this is  high drama in astrophysics circles. Because the stakes are high:   the Hubble constant is crucial for  understanding how old the universe is, how fast it expanded in the past, and what  the expansion will look like in the future.

The biggest of big picture stuff. And every observation we  make builds on the others. So if the story we’re telling about the  universe’s expansion isn’t consistent, we might need to doubt the conclusions  we’ve made in other areas of astronomy.

Having two conflicting H0 values also  makes it harder to look for new physics… like trying to solve the long-standing  mysteries of both dark matter and dark energy, which our universe seems to be full of. But before we get into how these camps got their  different H0 values, we’ve got to pay some bills. Thanks to Brilliant for  supporting this SciShow video!

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You’ll get 20% off an annual premium Brilliant  subscription and the first 30 days for free. So what exactly are the two camps, and the  two techniques they’re using to calculate H0? For the distance ladder teams,  the idea is to look right at   galaxies a few megaparsecs away and  measure their distance and speed.

But getting accurate distances and speeds for  objects that far away is easier said than done. So astronomers start with more reliable  distance calculations from our cosmic backyard… those are the bottom rungs  of the so-called ladder… and then use those rungs to help  you work out distances farther away. The next rungs up on the  ladder are standard candles: things that always shine with the same  intensity, no matter how far away they are.

So in theory, all you have  to do to calculate their   distance is measure how bright they appear to be. You also have to know precisely how these objects   function to know how bright they should  be, which is also easier said than done. For standard candles, astronomers tend to rely   on certain kinds of stars and  certain kinds of supernovas.

Hubble himself relied on Cepheid  variable stars, which visibly   pulse super consistently depending on their mass. We’ll be coming back around to those, later. For the past few decades, astronomers  have also really relied on the Hubble   Space Telescope to study those standard  candles at all distances across the sky.

But the other camp in this cosmological crisis   is relying on data from  a different telescope entirely… studying light from a completely  different point in history. The CMB camp derives their H0 number  from the oldest light in the universe,   the Cosmic Microwave Background. It was emitted just a few hundred  thousand years after the Big Bang, and our best view of it comes to us courtesy  of the now defunct Planck Space Telescope.

But a few hundred thousand years after the  Big Bang is over 13.6 billion years ago. And H0 is the rate of expansion now. So these astronomers actually aren’t  directly calculating H0 at all.

Instead, they’re creating  computer simulations on   the known laws of physics and their CMB data, and plugging in numbers for a bunch of different  properties to try and recreate our universe. Alongside a bunch of other values, like the  abundance and type of dark matter in the universe, an H0 value around 67 seems to work the best. So in a way, the Hubble tension represents  just how well physicists understand the   history and composition of the whole cosmos.

Or rather, don’t. Because as is, the models are not predicting  what astronomers are actually observing. With its superior resolution, the James  Webb Space Telescope was supposed to provide more accurate observations, and therefore  a more accurate distance ladder.

And that extra accuracy would,  hopefully, resolve this crisis. But in 2024, the first JWST results came  in and sort of…spawned a new crisis. Two research groups analyzed  data from JWST and came to   totally different conclusions about what it meant.

And it’s significant because both groups  are hugely respected in their field, having produced gold-standard distance  ladder results before JWST came along. First is the team led by Dr. Adam Riess, who shared a Nobel Prize for discovering that the   universe isn’t just expanding,  but expanding faster over time.

In February 2024, their results were published  in The Astrophysical Journal Letters. Their research analyzed over  a thousand Cepheid variables, and concluded there was no significant difference  between a Hubble and JWST-derived distance ladder. And with no new distance ladder,   there was no new H0 that would be  closer to what the CMB models predict.

Then, a couple of months later, veteran cosmologist Dr. Wendy Freedman presented her team’s research  at the annual American Physical Society meetup. They had compared Cepheid data with data from  two other kinds of standard candle stars, meaning they had three kinds of  standard candles that could be   calibrated and cross-checked against each other.

And after all that calibration, they calculated a   new value for H0 that agreed much  more closely with the CMB camp. In other words, they were claiming that the  Hubble tension had basically been solved. But come summer, Riess’s team argued those  results could be biased by the specific objects that Freedman and her team chose to include in  their analysis…and which objects they did not.

Now, this is still hot-off-the-press drama. As of our recording, Freedman’s results haven’t  made it through the peer review process, yet. Meanwhile Riess’ criticisms have, but  haven’t actually been officially published.

So the jury continues to be out when it  comes to the latest results from JWST. But what does this bit of  academic beef mean for now? Remember those error bars that don’t overlap?

Well, an error bar is only as  good as your error accounting,   and there’s one type we need to acknowledge here. These are called systematic errors:  small assumptions you make in your data   processing that skew every number  consistently in one direction. And everyone’s numbers…be  they be from Riess or Freedman, or anyone else looking at standard  candles or the CMB models,   involve a lot of assumptions  about how the universe works.

From how stars evolve, to how  subatomic particles really act. So most cosmologists believe  that the Hubble tension is   caused by some accounting trick gone wrong. Some faulty assumption in the data pipeline  that’s messing up someone’s results.

But if Reiss’s team is right, and the JWST  can’t produce an H0 that’s closer to the   CMB predictions, it could mean there’s  some new kind of physics to uncover. There are still a lot of  cosmological mysteries out there. Like why the universe is expanding  faster and faster over time.

Scientists blame dark energy, but  no one really knows what it is. Right now, the strength of dark  energy is considered to be constant. It’s one of the many variables that go into  the CMB models that predict what H0 should be.

So instead of hinting at an accounting error, the Hubble tension could be showing us that dark  energy actually varies in strength over time. If that’s the case, it would  be one of the biggest, most   surprising new discoveries in physics in decades. But before anyone throws  away their astro textbooks,   we should wait for more data to come in.

The JWST is only a couple of years  into its fact-finding mission. So it may yet provide the data we need to solve  the tension without invoking new laws of physics. Because despite their opposite conclusions,   Riess and Freedman’s teams have  come to agree on at least one thing: the rung on the cosmic distance ladder  that corresponds to nearby galaxies.

So with a bit more back-and-forth, there  might be more consensus on other rungs. But JWST isn’t the only telescope  that could help solve the crisis. By imaging the whole southern sky every few  days, the upcoming Vera Rubin Observatory will be able to spot millions of supernovas,   providing a more robust catalog  of those standard candles.

And gravitational wave detectors like LIGO could   act as a sort of neutral  third party in the debate… providing a completely different  kind of dataset that astronomers   could cross reference against  the CMB and standard candle data. So, many cosmologists think it’s  only a matter of time before the   tension is resolved, one way or the other. Whatever the solution ends up being, it’s a  nice reminder that our story of the cosmos is never finished being written,   and that science is at its most vibrant  when there are big mysteries to solve. [♪ OUTRO]