Dark matter and the nature of black holes are two of the biggest mysteries in astrophysics, and now a new theory has linked the two together: Dark matter might actually be composed of a type of black hole.
Dark matter, an invisible substance that scientists think makes up 27% of the universe, is a mystery. Scientists aren't sure what it is. One leading theory is that it's made of particles that we simply haven't detected yet. But more and more experiments attempting to detect those particles are coming up empty-handed.
"These studies are providing increasingly sensitive results, slowly shrinking the box of parameters where dark matter particles can hide," NASA astrophysicist Alexander Kashlinsky said in a statement. "The failure to find them has led to renewed interest in studying how well primordial black holes — black holes formed in the universe's first fraction of a second — could work as dark matter."
So dark matter might be made of primordial black holes. And new research shows this wacky theory actually lines up with what astronomers have observed. If the theory checks out, it means we're surrounded by black holes.
"If this is correct, then all galaxies, including our own, are embedded within a vast sphere of black holes each about 30 times the sun's mass," Kashlinsky said.
Why the answer might be black holes: Astronomers recently noticed a background glow in a region of space that appeared weirdly patchy in infrared light. When they looked at it in X-ray light, they noticed the same strange pattern:
"The only object we know of that can be sufficiently luminous across this wide an energy range is a black hole," NASA explains. So that weird background patchwork suggests tons of primordial black holes were mixed in among some of the earliest stars that formed after the Big Bang.
There's more to the puzzle, though. Back in February, scientists at the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced they detected gravitational waves coming from a pair of merging black holes.
Those black holes quickly sparked a lot of interest beyond gravitational wave research. If primordial black holes are real, their masses are probably near the masses of the merging black holes that LIGO found.
"Depending on the mechanism at work, primordial black holes could have properties very similar to what LIGO detected," Kashlinsky said. "If we assume this is the case, that LIGO caught a merger of black holes formed in the early universe, we can look at the consequences this has on our understanding of how the cosmos ultimately evolved."
Kashlinsky has a theory about how all this might fit together. If dark matter really is made of primordial black holes, their gravitational pull would have warped all the matter in the early universe. Right after the Big Bang, the primordial black holes probably collapsed into small halo shapes, and normal matter started to cool around it and form into the stars and galaxies we see today. Those halos could account for the patchiness that astronomers observed.
"Future LIGO observing runs will tell us much more about the universe's population of black holes, and it won't be long before we'll know if the scenario I outline is either supported or ruled out," Kashlinsky said.
So we might finally be making some progress in figuring out what the heck dark matter is.