To be fair, all scientific models are in some sense wrong. They’re all approximations of reality. They’re all mathematical models that we use to describe and understand our observations and measurements. And like I said, the LCDM model has, over the course of almost a quarter century, proven to be enormously resilient, flexible, and powerful when describing broad swaths of nature.
But we know we’re missing something. We can debate whether LCDM is wrong or not, but we can’t debate that it’s incomplete.
That’s because, among other things, we have no idea what dark energy IS. What we have is a set of observations, most importantly observations that tell us that the expansion of the universe is accelerating. We CALL this accelerated expansion dark energy, and we have a very simple model of what COULD be causing that accelerated expansion – the cosmological constant, a background source of anti-gravity that pervades the entire universe.
But in the years since Einstein’s discovery-slash-blunder, we’ve come to understand something about the cosmological constant that gives us exactly what we need to potentially understand how dark energy might be changing.
Perhaps the greatest cosmologist you’ve never heard of is Yakov Zel’dovich, a Soviet scientist who enjoyed running, swimming, joking around, probably drinking, and certainly revolutionizing entire aspects of astrophysics and cosmology. When you trace back in time the intellectual threads of our current understanding of the universe, most of them intersect – and even start – at Zel’dovich.
Zel’dovich realized that there’s more to the cosmological constant than just a number stuck awkwardly into the equations. There’s a SOURCE for it. And that source is the quantum vacuum energy that pervades spacetime. In quantum field theory, which is the best of the field theories, what we call particles are really just local excitations, or vibrations, of an associated quantum field. I know it sounds all woo-woo and that I’m soon going to launch into a discussion of the health benefits of healing crystals, but it’s the real deal (and it deserves its own episode).
The quantum fields have a fundamental amount of energy built into them, because in quantum mechanics you can never totally get rid of all vibrations. We call this the zero point energy or vacuum energy, and Zel’dovich realized that the quantum vacuum energy is RESPONSIBLE for the cosmological constant. When you talk about one you’re automatically talking about the other.
And a cosmological constant accelerates the expansion of the universe. So, here we go: accelerated expansion -> cosmological constant -> vacuum energy -> dark energy. It’s a hypothesis that vacuum energy is ultimately responsible for the accelerated expansion. It’s grounded in known science, based on facts we know from other aspects of physics, and provides a plausible mechanism for, you know, generating the accelerated expansion.
Except its way too strong. Naïve calculations of the amount of dark energy are something like a hundred-plus orders of magnitude too strong to account for the observed accelerated expansion. So something in this chain of reasoning has to break.
And DESI may be what we need to put the pieces back together. There are hints in the data that dark energy may not be constant. It’s not a super-strong, iron-clad observation. We often use “sigma” to denote how strong a result is (it’s actually way, way more complicated and nuanced then that but I am definitely not getting into that today). Typically in physics we need at least 3 sigmas to start being interested in a result, and at least 5 sigmas before we start believing it. The DESI preference for evolving dark energy over a cosmological constant is only around 3.5 sigmas.
So…significant, but barely.
The great news is that if this result holds up, we now have no much more information about what dark energy could be. Trust me, there are a lot of theories out there. We’ve been living with dark energy for about 25 years now, and that’s been more than enough time for theorists to come up with all sorts of theories, models, and hypotheses if what might be powering dark energy (guilty as charged).
But without better data, or some sort of sign that dark energy is anything other than a cosmological constant, we didn’t have much to go on, and all this theorizing was the intellectual equivalent of twiddling your thumbs waiting for something interesting to happen.
Well, now something interesting is maybe happening.
One way to get dark energy to change is to reassign what’s responsible for it. Instead of all the quantum fields in the universe working together to combine into a single unified acceleration, maybe dark energy is due to only a single quantum field. We call this hypothesis “quintessence,” which takes its name from the idea in ancient alchemy for a “fifth essence” in the universe. And if you’ve seen the movie The Fifth Element then you know that already that the fifth essence is…us, love, or something, I think.
Anyway, in the quintessence picture this new quantum field is dynamically evolving, like a ball rolling down a hill. It started in one state and is continuing into another, which we would see as changes to the acceleration rate. Why did this quantum field start at one position and end at another with this precise rate? Why does this new quantum field exist at all? And hey whatever happened to all the OTHER quantum energies, are we just pretending those don’t exist anymore?
All excellent questions.
Moving on.
Another possibility is that dark energy is somehow connected to matter. You know, now that I think about it, it is a rather strange coincidence. Dark energy takes up around 70% of all the stuff in the universe, and dark matter takes up around 25%. That’s not the same, but it’s oddly close…like, if we were to just pick random numbers out of a hat, then why should they even be the same order of magnitude?
Well maybe they’re connected to each other. Maybe there’s a fifth force of nature that only operates in the dark sector, that somehow ties the evolution of dark matter and dark energy together, and what we’re seeing in the DESI results is a manifestation of that connection. But why would this fifth force only operate in that way, and not manifest anywhere else? And what drives this connection, and why does it have this kind relationship, and not any other?
All excellent questions.
Moving on.
Maybe the problem isn’t on the dark energy side, but on the matter side. Maybe some strange process creates dark energy at the expense of matter. There have been some wild proposals that maybe dark energy is produces at black hole event horizons through – insert waving arms here – process. So what we’re seeing is actually dominated by the destruction of matter in the universe, rather than what’s happening to dark energy direction. But what about…?
You get the idea.
