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Using a vast dataset of observations of the Universe, an international team of astronomers just announced they’re seeing more evidence that dark energy — the mysterious substance that pervades the cosmos and causes the expansion to accelerate — is not a constant over time, but may instead be weakening [link to journal paper].

The evidence is interesting for sure, but not yet quite enough to be positive about it. Still, this is a big deal. Let’s see why.

The Universe is expanding. We’ve known this for a century now, ever since observations showed that galaxies outside our own are rushing away from us, and the more distant they are, the faster they’re moving. This is indicative of an explosion: over time, the “shrapnel” (in this case, galaxies) that is moving fastest has moved farthest away from the explosion. Run the universal clock backwards and this implies there was a moment when the cosmos got its start, and burst forth in rapid expansion. We call this model of universal birth the Big Bang. 

It’s not really an explosion, and galaxies aren’t really moving away from us — space is actually expanding, carrying galaxies along with it. Even that’s not a perfect analogy, but it’s close enough to get the picture across.

Now to be clear, we are extremely confident this idea is correct. The evidence is quite overwhelming. The problem is in the details. Some aspects of this model are difficult to pin down.

For example, we also know dark matter exists: material that has gravity, but does not interact with normal matter (protons, neutrons, and all the stuff that makes up us), nor does it emit light. This material by mass outweighs normal matter by a factor of about five to one, so in fact calling visible matter “normal matter” may be a bit biased. Anyway, dark matter plays a huge role in how the Universe behaves, including its expansion.

There’s also dark energy. This weird stuff is everywhere, but we don’t really know what it is or how it behaves. What we do know is that it acts almost like a pressure, forcing the expansion of space to accelerate. For about a century after the idea of the Big Bang was put forward, it was assumed the expansion rate was constant. But in 1998 astronomers announced that the rate was accelerating — the expansion was getting faster all the time — and proposed dark energy as the cause.

At this point, we have a ton of observations and theoretical data (equations of how the Universe works, essentially) that lead us to a model called Lambda-CDM: the lambda represents dark energy and CDM means Cold Dark Matter, which means that dark matter is made of something (likely subatomic particles, though that’s not clear) and that it doesn’t have a lot of energy (it can clump together via gravity to form larger structures — if it were hot it wouldn’t be able to do that). 

That lambda though… it’s been assumed this whole time that dark energy is a constant, applying a force everywhere that’s been the same throughout time. That’s the simplest version of it, and this stuff is complex, so that assumption makes the math as easy as it can be (which is still hard).

But it’s an assumption. We don’t like assumptions, we like evidence. So astronomers have been building observatories to try to measure lambda and see how dark energy behaves, especially over time.

One of these projects is the Dark Energy Survey, or DES. It uses a 4-meter telescope in Chile with a massive instrument called the Dark Energy Camera to observe huge chunks of the sky at a time, using five different filters that can give a decent estimate of a galaxy’s distance by a process called photometric redshift. It also looked at very distant supernovae, exploding stars, which can be used to get distances to distant galaxies and measure dark energy.

In the end the DES builds up a 3D map of the Universe, because it can measure the position of a galaxy on the sky and its distance. This survey is very deep (meaning sensitive to very faint and distant galaxies) and accurate. It covers a staggering one-eighth of the sky, a huge amount of celestial real estate.

One thing it measures is called baryon acoustic oscillations. That’s basically a fancy way of saying “ripples in a pond”. Kinda. The Universe was initially much hotter than it is now. After about a second or so post-Big Bang it cooled enough to allow protons and neutrons—collectively called baryons—to form, and as they did so they created pressure waves that moved through the plasma around them. These are essentially sound waves, also called acoustic waves. Plasma behaves differently than neutral gas; when the Universe cooled more these waves froze in place. They left an imprint on the matter embedded in them though, with slightly higher density of baryons at the wave crests. These ripples grew as the Universe expanded, and the slightly higher density at the crests of these waves means more galaxies formed there.

Here’s a NASA video that explains this:

Importantly, there is a characteristic wavelength (distance between crests) to these waves, which depends on their density and more. Dark energy also affects that wavelength, so by looking at where galaxies are in physical space (and looking for slightly higher numbers of them at different locations) we can get a handle on the size of that wavelength.

Remember, too, that light travels at a finite speed. So, the more distant a galaxy is the farther back in time we see it. DES looks at so many galaxies — millions of them — that by seeing where they clump up we can actually measure the acoustic waves at different eras in the Universe. From that, we can get a handle on dark energy behaved in the distant past.

At the present epoch of the cosmos, the characteristic wavelength of the plasma oscillation is about half a billion light-years, a huge distance. In the early Universe it was smaller. How this distance changes over time is what DES is designed to measure. These observations can be combined with others that measure other properties of the Universe over distance to see what dark energy is doing.

(OK, still with me? Nearly there.)

Last year, astronomers announced that after looking at a big set of supernova observations from DES, there were hints that dark energy may not be so constant. They had 1,500 exploding stars in their set, a large number (mind you, the original dark energy announcement in 1998 used only a few dozen supernovae). What they found was interesting: when they graphed their data, they found that the Lambda-CDM model didn’t do a great job fitting it. It predicts a certain behavior of supernova versus distance, but that prediction was off from the observations. Not enough to say anything definitive, but still: suspicious.

Now though, things have changed. Using DES observations of an incredible 16 million galaxies, they find the plasma oscillation predicted by Lambda-CDM is also off! The scale of that characteristic wavelength is smaller by about 4% than what the model predicts. That’s not much, but it’s enough to give scientists pause, and moreover it agrees with the supernova data. 

If true, it means dark energy has changed over time, getting weaker! That’s a huge deal. It changes how we think the Universe behaves in a fundamental way. We can lump all of matter and energy together and take an assessment of the contents of the Universe. In that budget, dark energy accounts for about 70% of everything in the Universe, so if we’re off in our estimate then that affects a lot of the equations, and those equations are how we understand the Universe.

The thing is, the astronomers can’t say for sure their measurements are accurate. They’re somewhat confident they are: there’s only very roughly a 1% chance that the data they obtained would show these results due to random fluctuations (so, they’re about 99% confident it’s real). It’s possible it’s not real, but unlikely.

Still, scientists like to be even more confident before they say they’ve actually discovered something like this (they want 99.9999% confidence, in general, depending on what kind of discovery it is). What they need is more observations, more galaxies and supernovae and other ways of seeing what the Universe was like a long time ago, and how it’s evolved over time. Those types of observations are ongoing.

What’s next? Well, let’s say dark energy does change with time. How? Is it linear, just dropping with time, or is it something more complicated like an exponential, fading rapidly at first then slowing later?

And the big question: what the hell is dark energy? Is it an actual thing, like an energy pervading the cosmos, or is it a fundamental property of spacetime itself, woven into the fabric of existence? Answers for these deeper questions still await us. Patience is the key here. We still have a ways to go before we can dig more deeply.

In the meantime, the cosmos keeps expanding, galaxies still interact, stars are born and die, and astronomers keep a constant eye to the sky to measure them all. 

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