When the Universe makes no sense, look at a pretty galaxy

What do you do when you find out there's something wrong with the Universe?

In the case of my old friend and colleague Adam Riess, you point Hubble Space Telescope at a gorgeous spiral galaxy.

There may be a wee bit more to say about that, but you know if I use the phrase "gorgeous spiral galaxy" then I'm going to show you, well, a gorgeous spiral galaxy.

The spiral galaxy NGC 3254, seen by Hubble in 2019. Credit: ESA/Hubble & NASA, A. Riess et al.; CC BY 4.0

Oof. Told ya.

That is NGC 3254, a spiral about 65 million light years from Earth. That's decently close as galaxies go, so Hubble has a fantastic view of it; tens of thousands of individual stars can be seen in this image. Which is the point.

Here's the problem with the Universe: We know it's expanding, and we know that expansion is accelerating, increasing with time. But by how much is posing something of an issue. When we measure very distant objects, like the background radiation left over from the Big Bang, we get one number for that rate of expansion, but when we look relatively closer, like at stars exploding in galaxies a billon or two light years away we get a different number. The numbers are close, but the difference is irritating.

Both calculations rely on a series of complex observations that themselves are sensitive to how well we understand what we're doing. When it comes to the one with exploding stars, we're looking at galaxies that are still pretty distant, and knowing just how far away they are is an important part of the process. One way to do that is to look for the same kind of supernovae in much closer galaxies to us where we have independent ways of measuring their distance, then extrapolating that to the more distant exploding stars.

Ground-based Pan-STARRS observatory image of the spiral galaxy NGC 3254. Credit: Aladin/Pan-STARRS

This particular kind of supernova, called a Type Ia, happens often enough in our neighborhood galaxies that we can use Hubble to take a look at the host galaxy and search for a special kind of star in them called Cepheid variables. These literally pulsate, changing their brightness over weeks and months. The rate at which their light changes depends on how luminous they actually are, which is the key to everything. If we can measure that pulsation, we know how bright the star really is. Then we compare it to how bright it appears (because the farther away a star is the dimmer it appears) and boom: We have the distance to that galaxy.

Adam Riess is one of the leaders in the astronomical community in trying to understand the universal acceleration, and has a program on Hubble to look at these galaxies; observing them over and again over time makes it possible to find these variable stars and get their periods.

And that's why Hubble observed NGC 3254. In 2019, after traveling since around the time an asteroid took out the dinosaurs, the light from a Type Ia supernova reached Earth from the galaxy. Called SN 2019NP, it was pretty bright (for a supernova dozens of millions of light years away), and you can easily see it in the Hubble image as a bright blue star near the top of the image:

The supernova SN 2019NP (centered) blew up in the galaxy NGC 3254, and Hubble was used to observe both it and the stars around it to more accurately gauge its distance. Credit: ESA/Hubble & NASA, A. Riess et al.; CC BY 4.0

Wow. These images were taken in March 2019, a couple of months after the supernova reached peaked brightness and started to dim. It was about 1/10,000th as bright as the faintest star you can see with the naked eye, but then Hubble is a decent 'scope. The star is pretty obvious.

So these observations go on the pile of many other galaxies observed to add a few rungs to what we call the distance ladder: Get the distance to nearby Cepheids in the Milky Way, use that to get distances to them in more distant galaxies, use that to calibrate Type Ia supernovae explosions in those galaxies, use those to calibrate much more distant ones, and then use those to figure out how rapidly the Universe is expanding.

And that is why Hubble looked at a gorgeous spiral galaxy. In the end the problem almost certainly isn't with the Universe — it generally knows what it's doing — but with our observations of it. Looking at NGC 3254 may help with that.

Though, to be honest, I do rather hope the problem turns out to be real. If those two methods give two different numbers because the Universe itself behaves differently very very far from us, that would be extremely cool. It means more things to figure out, more fun science! Hopefully in a few years we'll know.