DESI Drives a Dark Energy Debate
In April, I finished the last bit of filming for an upcoming video series being produced with the Perimeter Institute, called Cosmology 101. It’s a follow-up to our very successful series from last year, Quantum 101, but even more fun for me to do because it’s so much more in my own research wheelhouse. On the final day, after successfully getting all the scripts recorded, I settled happily into a seat at the cosmology lunch table at Perimeter’s in-house Black Hole Bistro to catch up with my colleagues on the latest developments in the field. The group was
unusually animated – the first results from the Dark Energy Spectroscopic Instrument (DESI) had just been announced, and they held a surprise. Rather than presenting yet another result confirming our standard cosmological model, the DESI team’s result contained a hint that dark energy might not be as constant as we thought. An exciting result for cosmologists, and great fun to discuss around the lunch table, but a bit inconvenient for me, as I had just, moments before, recorded a video in which I stated that all the data so far is consistent with dark energy being a cosmological constant.
Let’s quickly go through the physics here. There are a couple of things we’re pretty darn certain of, observationally. The universe is expanding, and that expansion is speeding up. Accelerated expansion is not what anyone would expect from a universe in which only the usual kinds of matter and energy are present, in the context of our (very well tested) gravitational laws – Einstein’s General Relativity. The acceleration has to be due to SOMETHING weird, and whatever that weird thing is, we call it “dark energy.”
But as weird anomalies go, acceleration at least didn't leave us entirely clueless. As soon as it was discovered, cosmologists realized that a good candidate for dark energy could be a proposed cosmic ingredient that was originally written into Einstein’s equations but later removed – a “cosmological constant.” In Einstein’s time, it was added to sort of counteract gravity just enough to keep the universe from collapsing on itself. (He removed it when he found out the universe is expanding, which made a lot of things work better than they would in a static universe.) In a universe that's expanding but that has no cosmological constant, expansion should always be slowing down, because after the initial kick of the Big Bang, the only
relevant force is gravity, and gravity always pulls things together. The gravity of all the galaxies in the universe should be putting the brakes on expansion. If expansion is accelerating instead of decelerating, then there must be some other force acting to make the expansion speed up. A cosmological constant works because it adds a sort of inclination for swelling into every little bit of space, so once the universe gets big enough for the gravitational pull of the matter to be relatively weak, the swelling becomes the dominant factor, and the expansion starts speeding up. The “constant” thing about a cosmological constant is how much of it exists in every volume of space. So while the expansion can change speed, the amount of cosmological constant (along with its strength) stays the same. The alternative to this is “dynamical dark energy” – something that causes accelerated expansion in a way that changes in time, either to get more or less
powerful.
The announcement from DESI presents some evidence that dark energy is NOT a cosmological constant, but is getting less powerful over time. It’s not the first hint we’ve seen, and it is only really at the level of being enticing when the DESI galaxy survey data are combined with supernova data, but it’s enough to get astronomers and physicists excited about potential alternative dark energy theories.
There have been lots of news articles written arguing that the DESI announcement is shaking up the world of cosmology and drastically changing our understanding of the universe – especially its future. (Unfortunately two of the outlets I generally find most reliable for science news, Quanta Magazine and the New York Times, both contained some rather jarring errors in their explanations of the basic physics of dark energy, but there’s a reasonable summary here.) Personally, while I certainly think the result is intriguing, I don’t think it’s anywhere near
at the level of confirmation it would need to be to justify throwing out the cosmological constant completely. As for what it would mean for the future of the cosmos, that’s a little less clear. It wouldn’t mean that we escape the Heat Death completely, but it would result in an end that looks somewhat different from any of the scenarios I described in my book, “The End of Everything (Astrophysically Speaking).” And, in a way that doesn’t get covered in the press on account of being weird and technical, it could challenge our understanding of how energy conservation worked in our cosmic history. (Briefly, the same reason theorists don't like the idea of a Big Rip ending to our universe -- the fact that the kind of dark energy driving it would violate some energy conditions
we think should remain un-violated -- is a reason to not like what the DESI result seems to be suggesting about dark energy in our cosmologically recent past.)
Ultimately, we just need more data. There are other upcoming galaxy surveys that will provide important independent checks, and future data releases from DESI itself will also help us better understand how to interpret this early hint.
As a theorist, I can say with confidence that dark energy being something other than a cosmological constant would make my life and that of my colleagues much more exciting, even if – really, especially if – it messes up some important long-assumed-inviolable energy conditions. But as a person who has seen intriguing anomalies in cosmology and particle physics come and go like seasonal fashions, I’m going to try to avoid getting my hopes up.
After all, even if we’re stuck with the boring old cosmological constant, fortunately, we don’t understand that either – there’s still plenty of work to be done.
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