All of the proposals floating around out there for invoking dynamical dark energy are a little on the weak side. In many cases, they raise more questions than answers.
Right now you could probably cook up your own theory to explain the DESI results. But I can almost promise that any idea you create would end up running afoul of dozens of OTHER cosmological probes, like our observations of the cosmic microwave background. There is no theory out there, even after twenty five years of trying, that can beat LCDM in terms of explaining ALL the data. Every hypothesis we have for evolving dark energy ends up breaking something, somewhere, somehow.
Which is why as cosmologists we’re so hungry for high-quality data and why we’re willing to invest in survey programs that last for years, just to squeeze as much juice out of the cosmological fruit as we can. Because any new bit of information helps us.
DESI is not done – it’s got many more years of survey data to explore. And it’s not alone. There are also two space-based instruments, the European Space Agency’s Euclid Telescope, and NASA’s Nancy Grace Roman Space Telescope. Euclid is already up and in its commissioning phase. The Roman is nearly complete but it’s future is a little more uncertain. Those two telescopes will do what DESI has done, but with many more galaxies.
And on the ground, we have the gigantic Vera Rubin Observatory and its Legacy Survey in Space in Time. That survey will cover tremendous swaths of the universe, but it will have less precision than DESI and Euclid when it comes to measuring distances to galaxies. Still, the sheer numbers involved (we’re talking hundreds of millions of galaxies) will give us an unprecedented view of what the wider universe is up to.
Perhaps with more data the DESI result will simply fade away. It’s happened before, where a promising new result just gets eaten by more data and better analysis. Or perhaps it will get stronger, and confirmed by these different surveys, targeting different areas of the universe and employing their own techniques. At this stage, it’s too early to tell.
But I can tell you one thing. There is no cosmologist alive who is afraid of this result, or is willing to “defend” LCDM with their dying breath. As a field, cosmologists tend to be conservative. No, not “red hat” conservative, but “slow to change our beliefs” conservative. That’s because the LCDM is so successful and simple, two hallmarks of a great scientific theory. It will take a LOT of evidence to justify moving to another paradigm. But we’ve done it before, and we’re willing to do it again.
We’re not helpless, and we’re not panicking. What’s happening now is good. It’s fun. It’s interesting. It’s obvious that nature just might be trying to tell us something. We’re not exactly sure what that is, but there are enough hints in the data that we feel like we’re on the cusp of a new cosmological revolution. We know that LCDM has its shortcomings, its mysteries, its unanswered questions. And we know that any proposed replacement for LCDM has even more shortcomings, more mysteries, and more unanswered questions.
Cosmology is the ultimate big picture challenge, which is one of things that’s always motivated me over the years. And when it comes to dark energy, we’re talking about the ultimate fate of the universe.
If dark energy stays a constant, then our universe as we know it is doomed. All dark energy does is tear things apart. The cosmic web, the largest pattern found in nature, is slowly unraveling. In time almost every single galaxy will be plucked away from you, pulled faster than the speed of light past our cosmological horizon. Billions of years from now, it will just be the local group – the Milky Way, Andromeda, Triangulum, and a dozens of smaller galaxies – in the entire observable universe. After that, stars will fade, the universe will turn to darkness, and there will be no energy left for anything.
But that’s only if dark energy stays constant. If it changes, then that opens up brand new possibilities. Maybe dark energy will fade away and leave the cosmic stage. The universe still grows old and cold, but much slower. Maybe it will decay, or transform, breathing new life into the future cosmos. Maybe it will even trigger a recollapse, a Big Crunch.
Maybe something else. Maybe something we’ve never even thought of yet. Yes, all the models of evolving dark energy are kind of janky, but if one of them rises to the top and explains the data, both old and new, then we can start poking at that, and maybe learn something new about the universe that we never knew before.
LCDM was not the first time that we’ve had to dramatically rewrite our understanding of the universe. And I hope it’s not the last.
