# Is there really a 'crisis' in cosmology?

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of "Ask a Spaceman" and "Space Radio," and author of "How to Die in Space." Sutter contributed this article to Space.com's Expert Voices: Op-Ed & Insights.

You may have heard about the "cosmology crisis:" Different methods of measuring the age of the universe are giving different results, and cosmologists have no idea why.

As an astrophysicist — and a cosmologist, to boot — I have my own take on this so-called crisis.

But first, let's see what the problem is.

## In one corner: the cosmic microwave background

To measure the age of the universe, you have to know its expansion history. And to get the expansion history, you need to do some mathematical modeling. Mathematical modeling is pretty common in science (indeed, it basically is science), and in the case of the entire universe, the mathematical model is Einstein's theory of general relativity

When you apply general relativity to the whole universe, you get the Friedmann equations, a set of equations that connect the contents of the universe at any one time — say, the relative amounts of dark matter, dark energy, radiation and anything else — to the expansion rate at that time.

So, measure the stuff, get the age.

The best way to measure how much stuff is in the universe is through the cosmic microwave background (CMB), the light left over from when the universe was only 380,000 years old. We have exquisite maps of the CMB thanks to efforts such as the European Space Agency's Planck mission, and those maps give us extremely detailed breakdowns of the contents of the universe.

The one thing the CMB maps don't give us is the amount of dark energy. Dark energy wasn't really a thing back when the CMB was generated, so in our modeling, we have to use other observations and add in the amount of dark energy by hand. But once you do that, you can plug everything into the Friedmann equations and get the expansion rate at any point in time (including today) and, from there, determine the age of the universe.

## In the other corner: supernovas

While it's an extremely reliable accounting of the contents of the universe, the CMB isn't exactly immediate; it was created billions of years ago. Another way of measuring the expansion rate of the universe is to … well, measure the expansion rate of the universe.

One way is through Type Ia supernovas,a kind of stellar explosion in which one star spills its atmosphere onto a neighboring white dwarf, the dense remnant of a dead star. Once a critical threshold is reached, the white dwarf goes boom, and we get a supernova.

Because it's pretty much the same scenario across the universe, we can use supernovas as standard candles. In other words, we know how bright they're supposed to be, and we can compare that to how bright they appear. We can then use that information to estimate the expansion rate of the universe at the time of the supernova.

Astronomers used this method in the late 1990s to discover dark energy, and it has been a cornerstone of cosmological measurements since then. It's a handy technique because it allows us to measure the expansion rate in the nearby universe (something called the Hubble constant, even though it really isn't constant — but that's another tale). That said, it is pretty limited; there aren't exactly a lot of supernovas available to us, and we can use them to peer into the more distant universe.

The universe: Big Bang to now in 10 easy steps

## Fight!

In the two decades since astronomers discovered dark energy, we've come upon a little hitch: Measurements of the expansion rate of the universe (and so its age) from both the CMB and supernovas have gotten ever more precise, but they're starting to disagree. We're not talking much; the two methods are separated by only 10 million or 20 million years in estimating the 13.77-billion-year history of the universe. But we're operating at such a level of precision that it's worth talking about.

One way to solve this is by saying that our measurements of the CMB are flawed. But the Planck CMB measurements stand as some of the most precise measurements ever taken in science, and multiple tests and checks in the years since the initial data release in 2013 have only solidified the calculations.

But because we need to add dark energy to the CMB data by hand, maybe we're missing something about that force. It is a giant mystery anyway, so it's not crazy. Maybe dark energy changes with time, or is connected to dark matter, or something else. This option, in particular, keeps theorists busy as they write paper after paper exploring the enticing possibility.

And lastly, maybe the supernova measurements are a little off. Sure, they're a more direct route to the present-day expansion rate, but they're not foolproof. Stars blowing up is somewhat complicated, and if we don't perfectly understand the messiness of the physics, we can't use supernovas as a precise cosmological probe. All it would take is a dash of uncertainty in modeling supernovas for the entire crisis to go away.

Personally, I think we don't understand supernovas as much as we think we do — and I'm not just saying that because I was a member of the Planck mission and helped in the CMB analysis. The "crisis" is a good excuse to keep writing papers, because we've been stumped by dark energy for over two decades, with a lot of work and not much understanding. In a sense, many cosmologists want to keep the crisis going, because as long as it exists, they have something to talk about other than counting down the years to the next big mission.

If it's interesting, it's probably wrong. The most boring way to solve the crisis is to admit that supernovas aren't as powerful a probe of the expansion rate as we would hope. But that wouldn't produce nearly as many papers as a crisis, and our cosmological conferences would be a lot less entertaining. And so, at least for now, the crisis will continue.