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(ESA/Hubble & NASA)

What if It's Not Dark Matter Making The Universe's Extra 'Gravity', But Light?

7 MAR 2019

We've been looking for decades for dark matter, yet the mysterious stuff remains undetectable to our instruments. Now, astrophysicists have explored an intriguing possibility: what if it's not dark matter that's affecting galactic rotation after all. What if it's the mass of light instead?


In a 1980 paper, the American astronomer Vera Rubin pretty conclusively proved something really weird about galaxies: their rims are rotating far faster than they should be.


This is called Keplerian decline, or decreasing rotation curve, and it can be observed quite neatly in planetary systems like our own Solar System. But most galaxies don't actually do this.


So astrophysicists hypothesised dark matter. We don't know what it is, and we can't detect it directly. But unless all our current understanding about the physical Universe (and all the data we've collected on the phenomenon is wrong), something out there is definitely making extra gravity.



In a new paper, they lay out an argument that light particles (photons) are at least partially the source of the phenomenon - causing an effect that isn't gravity, but behaves a heck of a lot like it.

"The hypothetical effect we are investigating is not the result of increased gravity," Budker said.


The effect they describe is a sort of "negative pressure" caused by electromagnetic stresses related to the photon mass.


So, yes, it's all purely hypothetical at this point. And it's not perfect.



But longer-lived stars create a problem. The Sun, for example, is around 4.6 billion years old, and orbits the galactic centre once every 230 million years, so it's had a few turns on the roundabout. According to the team's calculations, it should have a highly elliptical orbit under Proca stresses.

And yet it does not. So the theory would need a bit of work to be compatible with our actual observations of the Universe. For now, dark matter is still king. But there's no harm, and potentially a lot of good, in looking for other explanations too.

"We don't currently consider photon mass to be the solution to the rotation-curve problem. But it could be part of the solution," Budker said.


The research has been published in The Astrophysical Journal.