Rick Robinson

Oct 13th 2021

Gravity Portals May Turn Dark Matter Bright


Most of the mass in the universe takes a form that we cannot see, interact with or detect with any instrument we possess.

Physicists call it dark matter, but this moniker doesn’t quite capture the full picture. Whatever it is, dark matter isn’t really “dark” in the usual sense that it absorbs or blocks light. Instead, in all normal circumstances, dark matter simply does not interact with light at all.

But some physicists have now suggested that in certain extreme conditions, dubbed gravity portals, dark matter can turn bright. More precisely, particles of dark matter can annihilate each other, producing a burst of gamma rays — a high-energy cousin of ordinary light.

What makes all this more than speculation is that gravity portals could explain the powerful gamma rays that flood out from the center of our galaxy — and which physicists have been unable to otherwise explain.

A theory that explains one mysterious phenomenon is good. But when a theory explains two mysterious phenomena that previously seemed unrelated, scientists sit up and take notice. That is what happened when physicists Sun Xu-Dong and Dai Ben-Zhong posted their research report, “Dark matter annihilation into leptons through gravity portals.”

An Elusive Elephant in the Room

The dark matter mystery, as LiveScience explains, goes back nearly half a century to the 1970s, when astronomer Vera Ruben made a puzzling discovery. Studying the rotation of galaxies, she found that they were rotating far too fast.

The rotation speeds she measured would easily overcome the combined gravity of a galaxy’s visible stars, gas and dust (plus lesser bits and pieces such as planets), and cause galaxies to fly apart as soon as they formed. But real galaxies have held together for billions of years, indicating that they weigh more — a lot more — than the combined mass of the stuff we can see.

In fact, the stuff we can’t see — what physicists dubbed dark matter — must account for more than 80% of the total mass of galaxies, and indeed of the universe as a whole. Like an iceberg, the universe is mostly hidden from view.

For people in the field of particle physics, this has been a frustrating problem. Most of the universe is made of particles they can’t detect at all, except for the indirect evidence of enormous mass.

The Gravity Problem

But as sometimes (in fact, often!) happens in science, one puzzling problem can help solve another. Gravity itself has been a puzzling problem going all the way back to Einstein.

The other basic forces in the universe — magnetism, and two short-range forces known as the strong and weak nuclear force — all play well with each other. And they play well with quantum mechanics, which complements Einstein’s relativity theory by explaining small scales while relativity explains the cosmic large scale.

Gravity, however, does not play well. On small scales, it is incredibly weak, trillions of times weaker than even the “weak” nuclear force, as points out. But on a large scale, it is powerful enough to shape the universe. All of this hints strongly to scientists that gravity has properties we have not yet explored.

But wait — there’s more!

Too Many Gamma Rays

Gamma rays are the most energetic form of electromagnetic radiation, followed by X-rays, ultraviolet, visible light, infrared, microwaves and (at the low-energy end) ordinary radio waves. Only extreme events, such as powerful nuclear reactions, can produce gamma rays, as NASA notes.

The central region of our galaxy has more than its share of extreme events, but not enough of them to explain the powerful bursts of gamma rays that we observe coming from the galactic core. Something else — something that we cannot directly detect — must be contributing to the observed flux of gamma rays.

And the galactic core has something else that we cannot directly detect: a hefty share of the massive dark matter that holds the galaxy together.

The concept of gravity portals brings all these threads together, as LiveScience points out. The other basic forces can transform particles into other particles or bursts of energy (and vice versa). While we don’t fully understand gravity, the principles of particle physics suggest that it may also be able to trigger such transformations.

The galactic core, with plenty of gravity and plenty of dark matter, provides an optimal environment for dark matter particles to collide, annihilating each other and ultimately producing the energetic gamma rays we observe. The term “gravity portal” refers not to fixed locations in space (as with the popular interdimensional portals of science fiction) but to the momentary environments created when dark matter particles randomly collide and interact.

In the current proposal, what the gravity portals emit are very high-energy electrons, according to LiveScience. These in turn collide with ordinary light photons, amping them up into the gamma rays that we observe coming from the galactic core.

So far, the idea of gravity portals transforming dark matter particles into detectable forms such as electrons or gamma rays is only an initial proposal or suggestion. A great deal of work will need to be done to develop the idea — both theoretical work on possible mechanisms and experimental work on pinning down the observed galactic gamma rays.

But a door may have been cracked open to bring dark matter out into the light.

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