[ad_1]
The universe could be filled with tiny ancient black holes. And researchers may be able to prove it.
These mini black holes since the beginning of time, or primordial black holes (PBHs), were first invented decades ago. The researchers proposed them as an explanation dark matter, an invisible substance that exerts a gravitational attraction in space. Most explanations for dark matter involve hypothetical particles with special properties that help them evade detection. But some researchers think swarms of tiny black holes moving like clouds in space offer a clearer explanation. Now, a new study explains where these PBHs might have come from and how astronomers could detect the aftershocks of their birth.
Where did the little black holes come from?
A black hole is a singularity, an infinitely dense point in space filled with matter. It forms when that matter becomes so thick that the force of severity overwhelms everything else and the matter collapses. It deforms space time and it surrounds itself with an “event horizon”, a spherical border region beyond which no light can escape.
The general laws relativity allow black holes to exist at any scale; crush an ant strong enough and it will collapse into a black hole just like a star; it will just be incredibly small.
Most PBH theories assume that these objects have masses like small planets, with event horizons as small as grapefruits. It’s a whimsical idea, still on the fringes of black hole and dark matter physics, said Joey Neilsen, a Villanova University physicist who wasn’t involved in the new study. But recently, as other dark matter theories have turned out to be empty, some researchers have taken a second look at the notion of PBH.
If PBHs are out there, they must be very old. In the modern universe, there are only two known methods for creating new black holes from normal matter: stars much heavier than the sun that collide or explode. So each known black hole weighs more than the whole solar system (sometimes much more).
Related: Is the mysterious “Planet 9” of our solar system really a black hole the size of a grapefruit?
Creating tiny black holes requires a whole other set of mechanisms and ingredients.
Those ingredients would be “the Big Bang stuff, the same stuff that creates the stars and galaxies,” Neilsen told LiveScience.
Immediately after the big Bang, the expanding new universe was filled with hot, dense, largely undifferentiated matter that expanded in all directions. There were small pockets of turbulence in this quagmire – still visible as fluctuations in the Cosmic Microwave Background (CMB), the afterglow from the Big Bang – and those fluctuations gave the structure of the universe.
“If it’s a bit denser at point A, then the stuff is gravitationally attracted to point A,” Neilsen said. “And throughout the history of the universe, that attraction causes gas and dust to fall inward, join together, collapse and form stars, galaxies and all the structures in the universe that we know of.”
Most PBH theories involve very intense fluctuations in the early universe, stronger than those that formed galaxies.
In this new paper, the researchers place those intense fluctuations during a period known as “inflation”. In the first trillion-trillion-billionths of a second after the Big Bang, the universe expanded exponentially fast. That early rapid expansion gave spacetime its current “flat” shape, the researchers believe, and likely prevented space from ending up curved, like Live Science previously reported.
In a new article published on November 20 on arXiv database, the researchers propose that during inflation, there may have been times when all of spacetime was strongly curved, before eventually flattening out. Those short bends, however, would have produced fluctuations in the expanding universe intense enough to eventually form a large population of Earth-mass black holes.
How to find tiny black holes
The simplest way to prove this theory correct is to look for “secondary gravitational waves” (SGWs) that echo in the universe, the researchers wrote.
These waves, much weaker than the gravitational waves produced by the collision of black holes, would resonate from the same perturbations that formed the PBHs. They would be subtle vibrations in the universe, inaudible to current detectors. But two future methods may be able to find them.
One approach: pulsar timing arrays. Space is filled with swirling neutron stars known as pulsars that send bursts of energy towards Earth as they rotate. Pulsars are like precise and predictable ticking clocks in the sky, but their signals can be distorted by gravitational waves. A secondary gravitational wave passing between the Earth and a pulsar would warp space-time, causing the pulsar’s beat to arrive somewhat sooner or later in ways that a pulsar timing matrix could detect.
There is a problem with this plan though: Pulsar timing arrays would rely on the precise beat detection of the pulsars they emit. radio waves. And one of the most important radio detectors in the world, the giant Arecibo telescope in Puerto Rico, has been substantially destroyed, as reported by the sister site of Live Science Space.com.
But even if a high-quality pulsar timing experiment doesn’t work in the next 15 years, the next generation of gravitational wave detectors should be sensitive enough to pick up these secondary gravitational waves, the authors wrote.
Right now, gravitational wave detectors are buried underground, looking for fluctuations in space-time by measuring changes in the travel time of light over long distances. But other effects – small earthquakes, waves crashing against distant beaches, and even rabbits hopping overhead – can confuse the signal. In 2034, the European Space Agency plans to launch the Laser Interferometer Space Antenna (LISA), a much more sensitive space gravitational wave detector that avoids these pitfalls. And LISA, the authors wrote, should be able to pick up secondary gravitational waves.
Such a detection, they wrote, would prove that PBHs account for most (if not all) of the dark matter in the universe.
Originally published on LiveScience.
Source link
