“If there's nothing wrong with me... maybe there's something wrong with the universe.”

 — Dr. Beverly Crusher

I do so love it when Star Trek gets the science right. That quote from Dr. Crusher (“Remember Me”, STTNG, season 4 episode 5) is a classic, and dovetails nicely with some astronomy news: the Universe is doing something weird, and it’s looking more and more like it’s not a problem with our observations, but in the cosmos itself. 

OK, let’s do a quick synopsis: The Universe is expanding. I covered this topic in an episode of Crash Course Astronomy, if you want to take a look, including the evidence for it and how that works. We’ve known this fact for more than a century now, and no professional astronomer seriously doubts it’s happening. 

The work that underlies this discovery can be grouped into two different volumes of space: ones that look relatively nearby, out to roughly a billion light-years, and the others which look at the extremely distant Universe, mapping out the cooling fireball of the Big Bang itself. For a long time, observations in both these realms were not hugely accurate, and had a lot of uncertainty, but both methods more or less agreed on the expansion rate, which we call the Hubble Constant, or H₀ (pronounced “H naught”).

The expansion is happening everywhere, which means that a galaxy at a given distance will be moving away from us at a certain speed, and one twice as far will be moving away from us twice as rapidly. That means the velocity at which something is moving as the Universe expands depends on distance, so the rate of expansion is given as a velocity over distance: usually in units of kilometers per second per megaparsec (abbreviated Mpc; a parsec is 3.26 light-years, so a megaparsec is 3.26 million light-years). The expansion rate of the Universe determined by various methods is very roughly 70 km/s/Mpc. That means a galaxy 1 Mpc away will recede from us at 70 km/s, and one 2 Mpc away at 140 km/s, and so on.

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But then in the 1990s things got a bit sticky. We started getting a lot better at the measurements, and the uncertainties got smaller. And what happened was that the two groups of observations started to disagree with each other. The ones using measurements of the local Universe got a number of about 73 km/s/Mpc, but ones looking at the distant Universe got a lower number, about 68 km/s/Mpc. That’s close, but the uncertainties got small enough that the two numbers did not agree, and that indicates there’s something going on.

But what? Are the observations wrong, or did the Universe behave differently way back when versus what it’s doing now?

New observations show that it’s the Universe’s fault.

Observations of the local Universe use what’s called a distance ladder; we get measurements of nearby stars in our galaxy, extrapolate that to more distant stars, use that to look at stars in other galaxies to get their distances, and so on. There are actually quite a few methods used to do this.

One of the key methods is to look at a specific kind of variable star, called a Cepheid variable, which changes its brightness in a regular pattern. The time it takes to do this depends on how luminous the star actually is (how much energy it radiates). By timing that cycle and then measuring how bright the star appears, the true distance can be found. That method can be done to galaxies at a decent distance from us using Hubble.

It gets better, though: We also can measure the distance to a certain type of exploding star, called a Type Ia supernovae, as well, measuring how bright it gets and how long it takes to decay. If we see Cepheids and Type Ias in the same galaxy, we can then calibrate them against each other, making sure we have the correct distances calculated for them. We can only see Cepheids in relatively nearby galaxies, but Type Ia supernovae can be seen for over a billion light years! That is a very important step in the distance ladder.

A key galaxy in this process is NGC 4258. It hosts some astronomical phenomena that allow a very precise distance measurement for it, independent of everything else. So, a team of astronomers (led by my colleague Adam Riess) used Hubble Space Telescope to observe Cepheids in NGC 4258, as well as red giant stars, which also can be used to determine distances. They found that using Hubble these distances agree pretty well. Critically, they then looked at JWST data of NGC 4258 as well, to see if the measurements made in the infrared also match Hubble’s [link to journal paper].

And… they do. Very well, getting H₀ equal to 72.6 ± 2.0 km/s/Mpc with JWST, versus the Hubble data which yielded 72.8 ± 2.0 km/s/Mpc. Those numbers are equal given the uncertainties.

This is the first time that this step of the distance ladder has been confirmed using Hubble and JWST data together, and shows that these measurements are very accurate. They also show that the disagreement with the cosmological measurements that show a lower Hubble constant appear to be real, and the Hubble tension really does exist (at least on the “nearby observations” end).

There’s more work to be done, of course. The JWST sample of measurements had fewer stars than Hubble’s so there’s a problem with small number statistics, which means this work needs to continue with a lot more JWST observations. But they also show this is unlikely to make a huge difference, and the two telescopes getting the same number is significant.

So, now what? Well, it’s looking more and more like the universal expansion was different when it was young than it is now. That isn’t hugely unexpected; things change over time. Even the cosmos. When it was young it was denser, galaxies closer together, and conditions were just different. I’d kinda expect the Hubble constant to not be so constant over 13 billion years.

If this tension is real it’s in the theorists’ hands to figure out why. What changed specifically to cause the cosmic expansion rate to change over time? We know that dark energy plays a huge role here (in fact, it was Adam Riess’s team in part that discovered dark energy and announced it in 1998), causing the expansion to speed up, but how it does that isn’t clear. Heck, we don’t know what dark energy even is, we just call it dark energy because we don’t know what else to call it.

But observations like this one are important. They show the tension in the first place! But we also need to know if our methods of measuring things is affecting the results we’re getting. Does the fault lie in the stars or in ourselves?

It’s looking like it’s the stars, or more properly, in the Universe. Dr. Crusher was right!

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