Black holes, those gravitational monsters so named because no light can escape their clutches, are by far the most mysterious objects in the universe.
But a new theory proposes that black holes may not be black at all. According to a new study, these black holes may instead be dark stars home to exotic physics at their core. This mysterious new physics may cause these dark stars to emit a strange type of radiation; that radiation could in turn explain all the mysterious dark matter in the universe, which tugs on everything but emits no light.
Thanks to Einstein’s theory of general relativity, which describes how matter warps space-time, we know that some massive stars can collapse in on themselves to such a degree that they just keep collapsing, shrinking down into an infinitely tiny point — a singularity.
Once the singularity forms, it surrounds itself with an event horizon. This is the ultimate one-way street in the universe. At the event horizon, the gravitational pull of the black hole is so strong that in order to leave, you’d have to travel faster than light does. Since traveling faster than the speed of light is utterly forbidden, anything that crosses the threshold is doomed forever.
Hence, a black hole.
These simple yet surprising statements have held up to decades of observations. Astronomers have watched as the atmosphere of a star gets sucked into a black hole. They've seen stars orbit black holes. Physicists on Earth have heard the gravitational waves emitted when black holes collide. We’ve even taken a picture of a black hole’s "shadow" — the hole it carves out from the glow of surrounding gas.
And yet, mysteries remain at the very heart of black hole science. The very property that defines a black hole — the singularity — seems to be physically impossible, because matter can’t actually collapse down to an infinitely tiny point.
That means the current understanding of black holes will eventually need to be updated or replaced with something else that can explain what's at the center of a black hole.
But that doesn’t stop physicists from trying.
One theory of black hole singularities replaces those infinitely tiny points of infinitely compressed matter with something much more palatable: an incredibly tiny point of incredibly compressed matter. This is called a Planck core, because the idea theorizes that the matter inside a black hole is compressed all the way down to the smallest possible scale, the Planck length, which is 1.6 * 10^ minus 35 meters.
That's … small.
With a Planck core, which wouldn’t be a singularity, a black hole would no longer host an event horizon — there would be no place where the gravitational pull exceeds the speed of light. But to outside observers, the gravitational pull would be so strong that it would look and act like an event horizon. Only extremely sensitive observations, which we do not yet have the technology for, would be able to tell the difference.
Radical problems require radical solutions, and so replacing “singularity” with “Planck core” isn’t all that far-fetched, even though the theory is barely more than a faint sketch of an outline, one without the physics or mathematics to confidently describe that kind of environment. In other words, Planck cores are the physics equivalent of spitballing ideas.
That’s a useful thing to do, because singularities need some serious out-of-the-box thinking. And there might be some bonus side-effects. Like, for example, explaining the mystery of dark matter.
Dark matter makes up 85% of the mass of the universe, and yet it never interacts with light. We can only determine its existence through its gravitational effects on normal, luminous matter. For example, we can watch stars orbit the centers of the galaxies, and use their orbital speeds to calculate the total amount of mass in those galaxies.
In a new paper, submitted Feb. 15 to the preprint database arXiv, physicist Igor Nikitin at the Fraunhofer Institute for Scientific Algorithms and Computing in Germany takes the “radical singularity” idea and kicks it up a notch. According to the paper, Planck cores may emit particles (because there’s no event horizon, these black holes aren’t completely black). Those particles could be familiar or something new.
Perhaps, they would be some form of particle that could explain dark matter. If black holes are really Planck stars, Nikitin wrote, and they are constantly emitting a stream of dark matter, they could explain the motions of stars within galaxies.
his idea probably won't hold up to further scrutiny (there’s much more evidence for the existence of dark matter than just its effect on the motion of stars). But it’s a great example of how we need to come up with as many ideas as possible to explain black holes, because we never know what links there may be to other unsolved mysteries in the universe.
Originally published on Live Science.
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Paul M. Sutter is a research professor in astrophysics at SUNY Stony Brook University and the Flatiron Institute in New York City. He regularly appears on TV and podcasts, including "Ask a Spaceman." He is the author of two books, "Your Place in the Universe" and "How to Die in Space," and is a regular contributor to Space.com, Live Science, and more. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy.