Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of How to Die in Space. He contributed this article to Space.com's Expert Voices: Opinions and Insights.
We don't know what's behind dark energy, the name we give to the current era of accelerated expansion in the universe. Many theorists favor some sort of quantum field as the driver of dark energy, but these ideas are hard to reconcile with insights from string theory.
But new research proposes a radical solution: What if there is more than one cosmological agent for dark energy? This mixture would have strange effects in our universe, making it potentially detectable with upcoming surveys.
Into the darkness we go
Not only do we live in an expanding universe — we learned that about 100 years ago, with Edwin Hubble's observations of galaxies receding away from us — but we live in a universe whose expansion is accelerating. For about the past 5 billion years, the expansion rate of our cosmos has been increasing, ramping up the growth of the universe with every passing day.
To put it mildly, we have no idea what's causing this accelerated expansion. We first noticed it about 20 years ago when studying distant supernovae, and since then a plethora of independent observations (including, but not limited to: the cosmic microwave background, baryon acoustic oscillations, cosmic voids and more) have all confirmed that accelerated expansion is the real deal.
In place of an actual explanation, cosmologists have instead given accelerated expansion a cool nickname: dark energy. Even though we don't know what dark energy is, over the years theorists have made some feeble attempts to potentially explain it (because, obviously, it must have some physical source, and explaining physical sources of things we see in nature is the job of theorists).
One of the most attractive ideas floating around out there is that there is some sort of quantum field that is responsible for dark energy and keeping the pedal to the cosmological metal. Quantum fields are pretty handy — they soak every bit of space-time and are responsible for generating the forces and particles that make up our everyday existence — and so it's not so crazy to imagine that there's a new quantum field (one never before known to science) that has just the right properties to trigger accelerated expansion.
Draining the swampland
So, maybe dark energy is caused by some quantum field that soaks all of space-time. Got it, no biggie. While it does indeed sound a) simple and b) attractive, there are some downsides to this hypothesis. But to talk about the downsides, I have to talk about string theory.
String theory is an attempt to unite all the forces of nature under one mathematical roof — a theory of everything. In string theory, every particle and every force is really a manifestation of super-duper tiny vibrating strings. But to explain all the richness and variety of the physical universe, these strings can't just vibrate in three dimensions. They need more room. To make string theory work, our universe needs a few extra dimensions, all tiny and curled up on themselves, where the strings can do their business and give rise to physics. This all happens at the tiniest of scales, which is why we haven't noticed it yet.
One of the biggest headaches facing string theory, and why it isn't a complete theory of nature, is that we have no idea how those extra dimensions are curled up. There could be as many as 10^200 possible configurations, and each configuration of curled-up dimensions gives a new set of physics. Since we only live in one universe with one set of physics, only one of these configurations can be ours. But which one?
String theorists have made some attempts to separate the stringy wheat from the chaff, and at least reject some potential configurations of the curled-up dimensions as Definitely Not This Universe.
One problem: at first glance, it seems like universes that allow for dark energy as caused by a quantum field aren't compatible with other things we know about string theory. In the stringy lingo, dark energy seems to live in the "swampland," the possible configurations of curled-up dimensions that simply don't work.
Related: Putting string theory to the test
An extra pinch of darkness
One way to look at this conundrum is to point to the reality of dark energy and say that string theory is wrong — after all, if string theory can't predict a universe that has dark energy, then it's probably not a good fit as a theory of everything.
Or maybe we're not thinking about dark energy in the right way, as claimed by a recent paper appearing in the preprint journal arVix. It could be that there isn't just one quantum field responsible for dark energy, but several working together in concert. This isn't as crazy as it may seem; just think of all the complicated physics that go into tying your shoes, and remember that nature is under no obligation to be simple and straightforward.
By allowing for multiple quantum fields to generate dark energy, it might be possible for string theory to still be relevant in our universe, as these models may not be stuck in the "swampland."
But that means we have to find evidence that there is more than one agent responsible for dark energy.
It turns out that in these models of multiple dark energy, it's possible for dark energy to clump up on itself, meaning you can travel around the universe and find patches of less-than-average and greater-than-average dark energy. In total, across the cosmos, the large-scale effect is still the same (namely, accelerated expansion), but an excess of dark energy here or a deficit there could affect how the biggest structures in the universe, like clusters of galaxies and the great cosmic voids, grow and evolve.
We don't yet have the sensitivity to measure these differences, but future experiments like NASA's Nancy Grace Roman Space Telescope could provide some insights, helping us to determine if we really do live in the swampland or not.
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