For years, scientists have been trying to solve the mystery of dark matter, an unexplained substance that accounts for the majority of the universe's mass. Though dark matter does not emit detectable light, scientists know that it exists due to its gravitational effects on galaxy clusters and other radiant objects in space.
A dazzling range of hypotheses have been proposed to explain dark matter, from speculative particles called axions to undiscovered dimensions in physics. Now, scientists have proposed that primordial black holes, hypothetical objects that date back to the universe’s infancy, “are a viable candidate for dark matter,” according to a study by researchers in the U.S., Japan, and Taiwan, published last month in the journal Physical Review Letters.
What’s more, the new study hints at the existence of a multiverse, in which our universe is just one of many in a broader and more complex structure. If there is an “outside” to our universe, then an external observer might see the entirety of everything we know—stars, galaxies, and billions of light years of space—as a puny primordial black hole.
“We still don't know what dark matter is made of, but, since the black holes are known to exist, it is natural to ask whether dark matter could be composed of black holes that could form before the stars and galaxies formed,” said Alexander Kusenko, an astrophysicist at UCLA who led the new study, in an email.
Previous studies have already pointed to primordial black holes, or PBHs, as a solution to dark matter, but Kusenko’s team outlines a novel PBH formation scenario involving hypothetical “baby universes'' that may have emerged in the early cosmos. In other words, tiny cosmic offshoots of our universe might be both seeds of PBHs and they key to unlocking the multiverse.
For the moment, these concepts are all speculative, though Kusenko and his colleagues suggest ways to observationally constrain the mind-boggling hypothesis with sophisticated telescopes in the coming years.
Scientists think PBHs were born during cosmic inflation, the period after the Big Bang when the universe rapidly expanded its dimensions. Unlike “regular” black holes, which are forged by the explosive deaths of massive stars, PBHs are thought to have been created by slightly denser regions in spacetime. As a result, these objects could be tiny, on scale with planetary masses (and much smaller).
“The early universe was so dense that even a 30-50 percent fluctuation in density would make a patch of primordial plasma into a black hole,” Kusenko said. “Furthermore, in recent years, theorists have discovered new and very generic scenarios in which black holes could form in the early universe. So, we may be approaching a big discovery from different ends.”
Kusenko and his colleagues explore one particular PBH formation scenario in their study: false vacuum bubbles, or tiny patches of spacetime that maintain a lower-energy state than their cosmic surroundings. These anomalous bubbles may have collapsed to create PBHs, a process that could have produced a population of black holes that “accounts for all dark matter,” according to the study.
Even weirder, any vacuum bubbles that grew too large to collapse into black holes may have become baby universes stemming off the early cosmos. To an outside observer, these bubbles would look like PBHs, but from the inside, they would appear to be an expanding universe, much like our own.
Naturally, this raises the astonishing question: Do we exist in one of these baby universes that looks like a PBH from the outside?
“It is not impossible,” Kusenko said, “although the baby universes we considered were devoid of matter. Some additional physics could probably make them look more like ours to an internal observer.”
It’s a trippy concept, but it will remain in the land of theory until scientists are able to procure real observational data that supports it. To that end, Kusenko’s team is optimistic that their hypothesis could be put to the test by the Subaru Hyper Suprime-Cam (HSC) atop Mauna Kea in Hawaii, or the Rubin Observatory Legacy Survey of Space and Time (LSST) in Chile, which is due to begin operations in 2022.
Every few minutes, HSC scans the full extent of the nearest galaxy, Andromeda, which has provided scientists with a precise and constantly updated picture of galactic dynamics. One goal of the project is to spot any wandering PBHs that warp and brighten starlight with their gravitational fields. The camera has already flagged a potential PBH candidate that appears to have a mass roughly equal to that of the Moon.
Once it is collecting data, the LSST would be able to conduct similar surveys of the Milky Way’s galactic center, which “would easily test the PBH scenario, thanks to its large mirror aperture, wide field of view, higher detector sensitivity, and the expected superb image quality that allow for a simultaneous monitoring observation of many stars at one time,” according to the study.
If dark matter actually does consist of bubble-formed PBHs, most of them would be too small to detect individually, even with such sophisticated astronomical surveys.
“Unfortunately, we cannot detect black holes of arbitrarily small masses, and, if primordial black holes make up dark matter, the bulk of those black holes would go undetected,” Kusenko said. “However, if black holes formed in the early universe in a variety of sizes, we can catch the black holes by the tail of their distribution.”
In other words, looking for larger PBHs can help scientists make inferences about their smaller counterparts. PBHs may also collide with neutron stars, a type of dense dead star, so “we may be able to detect primordial black holes as they destroy the neutron star,” Kusenko added.
These efforts could shed much-needed light on the nature of dark matter, baby universes, and countless other questions about the universe. PBHs, assuming they exist, have managed to evade detection from humans for decades, but a new generation of wide-field telescopes may finally reveal these hidden anomalies and all of their juicy secrets.