Today's post is about a pair of new scientific papers that have the potential to shake up the world of cosmology in a big way, but first, some background.
I'm sure you've all heard of dark energy, the mysterious energy that permeates the entire universe and acts as a repulsive force, propelling everything (including space itself) outward. The most astonishing thing is that it appears to account for 68% of the matter/energy content of the universe. (The equally mysterious, but entirely different, dark matter makes up another 27%, and all of the ordinary matter and energy -- the stuff we see and interact with on a daily basis -- only comprises 5%.)
Dark energy was proposed as an explanation for why the expansion of the universe appears to be speeding up. Back when I took astronomy in college, I remember the professor explaining that the ultimate fate of the universe depended only on one thing -- the total amount of mass it contains. Over a certain threshold, and its combined gravitational pull would be enough to compress it back into a "Big Crunch;" under that threshold, and it would continue to expand forever, albeit at a continuously slowing rate. So it was a huge surprise when it was found out that (1) the universe's total mass seemed to be right around the balance point between those two scenarios, and yet (2) the expansion was dramatically speeding up.
So the cosmological constant -- the "fudge factor" Einstein threw in to his equations to generate a static universe, and which he later discarded -- seemed to be real, and positive. In order to explain this, the cosmologists fell back on what amounts to a placeholder; "dark energy" ("dark" because it doesn't interact with ordinary matter at all, it just makes the space containing it expand). So dark energy, they said, generates what appears to be a repulsive force. Further, since the model seems to indicate that the quantity of dark energy is invariant -- however big space gets, there's the same amount of dark energy per cubic meter -- its relative effects (as compared to gravity and electromagnetism, for example) increase over time as the rest of matter and energy thins. This resulted in the rather nightmarish scenario of our universe eventually ending when the repulsion from dark energy overwhelms every other force, ripping first chunks of matter apart, then molecules, then the atoms themselves.
The "Big Rip."
I've always thought this sounded like a horrible fate, not that I'll be around to witness it. This is not even a choice between T. S. Eliot's "bang" or "whimper;" it's like some third option that's the cosmological version of being run through a wood chipper. But as I've observed before, the universe is under no compulsion to be so arranged as to make me happy, so I reluctantly accepted it.
Earlier this year, though, there was a bit of a shocker that may have given us some glimmer of hope that we're not headed to a "Big Rip." DESI (the Dark Energy Spectroscopic Instrument) found evidence, which was later confirmed by two other observatories, that dark energy appears to be decreasing over time. And now a pair of papers has come out showing that the decreasing strength of dark energy is consistent with a negative cosmological constant, and that value is exactly what's needed to make it jibe with a seemingly unrelated (and controversial) model from physics -- string theory.
(If you, like me, get lost in the first paragraph of an academic paper on physics, you'll get at least the gist of what's going on here from Sabine Hossenfelder's YouTube video on the topic. If from there you want to jump to the papers themselves, have fun with that.)
The upshot is that dark energy might not be a cosmological constant at all; if it's changing, it's actually a field, and therefore associated with a particle. And the particle that seems to align best with the data as we currently understand them is the axion, an ultra-light particle that is also a leading candidate for explaining dark matter!
So if these new papers are right -- and that's yet to be proven -- we may have a threefer going on here. Weakening dark energy means that the cosmological constant isn't constant, and is actually negative, which bolsters string theory; and it suggests that axions are real, which may account for dark matter.
In science, the best ideas are always like this -- they bring together and explain lots of disparate pieces of evidence at the same time, often linking concepts no one even thought were related. When Hess, Matthews, and Vine dreamed up plate tectonics in the 1960s, it explained not only why the continents seemed to fit together like puzzle pieces, but the presence and age of the Mid-Atlantic Ridge, the magnetometry readings on either side of it, the weird correspondences in the fossil record, and the configuration of the "Pacific Ring of Fire" (just to name a few). Here, we have something that might simultaneously account for some of the biggest mysteries in cosmology and astrophysics.
A powerful claim, and like I said, yet to be conclusively supported. But it does have that "wow, that explains a lot" characteristic that some of the boldest strokes of scientific genius have had.
And, as an added benefit, it seems to point to the effects of dark energy eventually going away entirely, meaning that the universe might well reverse course at some point and then collapse -- and, perhaps, bounce back in another Big Bang. The cyclic universe idea, first described by the brilliant physicist Roger Penrose. Which I find to be a much more congenial way for things to end.
So keep your eyes out for more on this topic. Cosmologists will be working hard to find evidence to support this new contention -- and, of course, evidence that might discredit it. It may be that it'll come to nothing. But me? I'm cheering for the bounce.
A fresh start might be just what this universe needs.
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