Paul M. Sutter is an astrophysicist at The Ohio State University, host of Ask a Spaceman and "Space Radio," and author of "Your Place in the Universe." Sutter contributed this article to Space.com's Expert Voices: Op-Ed & Insights.
The true nature of dark energy continues to evade our understanding, despite two decades of investigation. Frustratingly, most attempts to explain dark energy fail extremely stringent tests of particle physics.
But new research shows how a hypothetical form of dark energy might be made inside the sun and could be detected here on Earth. In fact, we may have already seen it.
A secret in the dark
In 1997, astronomers surprised themselves and the world with the discovery that the expansion of the universe is accelerating. They gave this accelerated expansion a cool name — dark energy — because they did not understand what was causing this strange cosmological effect.
The simplest explanation for dark energy is that it's a "cosmological constant," an extra number tossed into Einstein's theory of general relativity. In other words, in this view, the expansion of the universe accelerates because … the expansion of the universe accelerates — which isn't exactly a satisfying explanation. Physicists have tried to connect this cosmological constant to the quantum vacuum energy found throughout space-time, but their calculations estimate a strength for the accelerated expansion about 120 orders of magnitude too big.
So perhaps dark energy isn't embedded directly in space-time itself. Maybe there's some new force, field or particle at work in the universe — something previously unknown to the Standard Model of particle physics. This entity would explain the accelerated expansion, but theoretical models run into issues here, too.
The problem is that once you introduce a new force, field or particle into the mix of the ingredients of the cosmos, that force, field or particle will start interacting with all of the other forces, fields and particles known to physics. And because we have no hints of any new physics in any of our high-energy (let alone low-energy) physics experiments, that doesn't seem like a viable option.
Behind the screen
Maybe dark energy has one more trick up its sleeve. Maybe there is some entity that generates the accelerated expansion at large cosmological scales, and something within that entity screens it from interacting with known physics at small solar system scales.
It's a bit of a stretch, but since we have no idea what's creating dark energy, it's worth investigating. But how do we find something in our experiments that's designed to be hidden from our experiments?
Some theorists have proposed that dark energy might be caused by some new kind of particle. This hypothetical particle wouldn't be able to interact with other particles from the Standard Model (like electrons and top quarks), because it would have been produced in great quantity in the center of the sun, where the densities and temperatures are high enough for interactions between dark energy and the Standard Model to occur. The production of dark energy particles inside the sun would mess up its thermal equilibrium, altering its light output, temperature and lifespan. And because the sun's behavior lines up with exactly what we expect from the Standard Model, it can't be producing dark energy particles in its core, according to those theories.
But a recent paper posted to the preprint database arXiv.org proposes another possibility: Perhaps dark energy doesn't connect directly to any Standard Model particles but does connect to photons.
Deep inside the sun, there is a region known as the tachocline, where the sun's magnetic field is extremely strong. Magnetic fields are carried by photons, so tons of photons are produced in the tachocline. If dark energy somehow connects to photons, then this is where dark energy particles might be produced, too.
Now you see it
The upshot is that there might indeed be a way for the sun to spit out dark energy particles. These dark energy particles would then shoot through the remainder of the sun's bulk, through empty space, and stream through you right now.
There's another entity in the universe that might be silently streaming through you right now: dark matter. Dark matter is the invisible form of matter that makes up over 80% of the mass of the universe. Like this hypothetical form of dark energy, dark matter is likely made of a particle currently unknown to the Standard Model of particle physics and streams through the universe. And it, too, is completely mysterious.
Several experiments scattered across the Earth are hunting for elusive dark matter particles, as scientists hope to catch a glimpse of an extremely rare interaction. Recently, one of those experiments, XENON1T, witnessed a signal that it couldn't fully explain — the first slight hint of such a detection.
And there's a chance, based on the latest research, that what the experiment detected may not have been dark matter, but dark energy. It's not a very strong detection, and it's far from a confirmation of the nature of dark energy. But a hint is a hint. It's been over two decades since the original discovery of dark energy, and theory and experiment alike have failed to make much headway.
Scientists hope that future dark matter detection experiments, like XENONnT and PandaX-4T, will provide more data and finally give us our first glimpse of the dark universe.
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