Back in 2016, the LIGO (Laser Interferometer Gravitational-Wave Observatory) team discovered the first-ever gravitational waves, coming from a collision of two black holes that happened billions of years ago.
Along with the puzzling finding came a little surprise: the black holes were determined to have very odd masses; in fact, so peculiar that it opened up a riveting possibility: the black holes that LIGO heard collide may have merged when the Universe was less than a second old.
How to Create a Black Hole
We know how black holes are made in the modern-day Universe: you have a star – the bigger it is, the better. You then have to wait until the star burns through all its supply of hydrogen, which might take a few tens of millions of years.
At the end of the object’s life, the star will collide on itself in a torrent of energy, also known as a supernova. In the fires of the explosion, the densities of the nucleus can attain a sufficiently insane state that nothing can stand in the way of the inward pull of gravity. At the time most of the star is shattering outward, a fraction of it collapses inward on itself, bending continuously into a black hole.
The more massive the star, the bigger the black hole, which is what makes the LIGO findings rather interesting: those shattering black holes had masses of 30 and 35 times the size of the Sun. To produce a black hole that massive, you either have to have an insanely gigantic star, or you need to build them up from mergers of smaller black holes.
Back then, when the findings were revealed, both scenarios seemed possible. Stars that big simply don’t exist anymore, and mergers are too rare to build up like that. That’s why researchers took into consideration another possibility: the black holes could have a different origin.
The Big Bang Black Hole Generator
The young Universe was a rather crazy place to be in. Temperatures and pressures were extreme, radical phase changes impacted the entire cosmos, and insane transformations redesigned the laws of nature.
Back then, if settings were right, any old smudge of gas may have suddenly shrunk itself to create a black hole of any size, really. For every theoretical physicist trying to solve the problem of the so-called primordial black holes, there is at least one hypothetical system for creating them, which includes everything from inflation theory to colliding Universes.
So, in a way, it is easy for primordial black holes to justify the early LIGO finding, but if you want to fill the Universe with black holes from the Big Bang, you need more than a single discovery with LIGO.
Universe Filled With Primordial Black Holes
So, researchers wondered what would a Universe filled with primordial black holes look like. The question needs to be answered if one wants to test this theory.
For starters, the black holes may randomly collide with other things, gravitationally pull other objects, and just casually create chaos around. When it comes to probable detectability, black holes are not completely black as they might glow, even if dimly, though the quantum mechanical process known as the Hawking Radiation.
Exploding black holes may have messed with the early Universe, changing the number of elements or the appearance of the cosmic microwave background. They may also be behind some of the gamma-ray bursts telescopes sometimes detect in the skies.
However, despite all the attempts, researchers cannot settle the existence of primordial black holes with the Universe they see now. Simply put, it is incredibly difficult to explain the masses of the merging black holes that LIGO spotted, but if there were a Universe with primordial black holes, it would be detectable in other ways.