The early universe contained far fewer miniatures black holes than previously thought, making the origin of our cosmos’ missing matter an even greater mystery, a new study suggests.
Miniature or primordial black holes (PBHs) are black holes thought to have formed in the first fractions of a second after the Big Bang. According to leading theories, these small-sized singularities sprang into existence from rapidly collapsing regions of thick, hot gas.
Infinitely dense pockets of spacetime are how many physicists explain the universe’s dark matter, a mysterious entity that, despite being completely invisible, makes the universe much heavier than can be explained by the matter we see.
But even though the hypothesis is popular, it has one big problem: we have yet to directly observe any primordial black holes. Now, a new study has offered a possible explanation for why they didn’t form, opening cosmology’s dark matter problem to wider speculation.
According to the research, the modern universe could have formed with far fewer primordial black holes than previous models estimated. The researchers published their findings May 29 in the journal Physical review papers.
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“Many researchers feel this [primordial black holes] are a strong candidate for dark matter, but there would have to be a lot of them to fulfill this theory,” lead author Jason Christianoa graduate student in theoretical physics at the University of Tokyo, said in a statement. “They are also interesting for other reasons, since since the recent innovation of gravitational wave astronomy, there have been discoveries of binary black hole mergers, which can be explained if PBHs exist in large numbers. But despite these strong reasons for their expected abundance, we haven’t seen any directly, and now we have a model that should explain why this is so.”
A hole in the picture
The universe began 13.8 billion years ago with Big Bangcausing the new cosmos to explode outwards due to an invisible force known as dark energy.
As the universe grew, ordinary matter, which interacts with light, gathered around invisible clumps dark matter to create the first galaxies, linked together by a vast cosmic web. Today, cosmologists estimate that ordinary matter, dark matter, and dark energy make up about 5%, 25%, and 70% of the composition of the universe, respectively.
Originally, the universe was dark, a plasma fluid that no light could penetrate without being caught by the electromagnetic fields produced by moving charges. However, after 380,000 years of cooling and expansion, the plasma eventually recombined into neutral matter, emitting static microwaves that became the universe’s first light, the cosmic microwave background (CMB).
Cosmologists have been looking for these early black holes by studying this first picture of the baby universe. However, so far, none have been found.
Some physicists think it’s possible that they haven’t discovered the large number of primordial black holes needed to account for dark matter simply because they haven’t yet learned how to detect them.
But by applying a model built on an advanced form of quantum mechanics called quantum field theory to the problem, the researchers behind the new study came to a different conclusion — we can’t find any primordial black holes because most of them just they are not there.
Primordial black holes are believed to have emerged from the collapse of short but powerful gravitational waves that ripple throughout the universe. By applying their model to these waves, the researchers found that it may take far fewer of these waves to combine than other theories estimate in order to form larger structures throughout the universe. And the fewer waves needed to recreate the picture, the fewer primordial black holes.
“It is widely believed that the collapse of short but strong wavelengths in the early universe is what creates primordial black holes,” Christiano said. “Our study suggests that there should be far fewer PBHs than would be needed if they are indeed a strong candidate for dark matter or gravitational wave events.”
To confirm their theory, the researchers will look in the future to hyper-sensitive gravitational wave detectors such as Laser Interferometer Space Antenna (LISA) project.which will be sent into space on an Ariane 3 rocket in 2035.
