THE FIRST BLACK HOLES
BBC Focus - Science & Technology|November 2021
Since just after the Big Bang, ancient black holes may have been shaping the Universe as we know it. Now, scientists are tantalisingly close to glimpsing these mysterious objects
COLIN STUART

Out there among the stars lies a hidden swarm of black holes. They have been around ever since the dawn of time, quietly and not so subtly influencing the evolution of the Universe.

Without them there would be no stars, no planets and no life to marvel at the Universe’s wonders. Now, for the very first time, we may finally have the tools to find them.

Black holes are one of astronomy’s most famous objects. Their gravity is so extreme that escape is impossible if you venture too close. There are different sizes of black holes, but they are normally gargantuan monsters considerably more massive than our own Sun. They usually remain hidden from view because no light can escape to reveal them to us. Yet we have seen them, thanks to gravitational wave detectors like LIGO and VIRGO that have detected signals from colliding black holes. We’ve never been more confident that these cosmic trapdoors exist.

However, there is one type of black hole that still remains theoretical, one that could solve a long-standing cosmological conundrum to boot.

Immediately after the Big Bang there were small fluctuations in the new Universe’s density – regions that had slightly more or less mass than the average. Where the mass was above the norm, material could have collapsed to form mini black holes. As they’ve been around for pretty much as long as the Universe itself, these black holes are called ‘primordial’ black holes.

According to theoretical models, primordial black holes can have a wide range of masses. They could be lighter than an eyelash or heavier than a star. So far we’ve been able to rule out some masses through observations, opening up two possible mass ranges or ‘windows’ for primordial black holes.

“The two windows are less than one lunar mass [one lunar mass = mass of our Moon] and a few tens of solar masses [one solar mass = mass of our Sun],” says Prof Sohrab Rahvar, an astrophysicist and cosmologist from Sharif University of Technology in Iran.

If this hidden population of miniature black holes exists, then they could account for some or all of the dark matter – the invisible glue that astronomers think helps hold galaxies like our own Milky Way together. It’s an idea that fell out of favour, but is now gaining traction again – particularly as traditional notions of what dark matter is made of continue to draw a blank.

SMALL BUT MIGHTY

With one eye on dark matter’s true identity, PhD student Gabriele Franciolini, from the University of Geneva, Switzerland, has been re-modelling the production of primordial black holes after the Big Bang in more detail. The upshot? “There could be hundreds of times more primordial black holes out there,” he says. “You can explain all of the dark matter through primordial black holes.” That’s if they have a mass in the lower window, below that of the Moon, which would make them less than one-tenth of a millimetre in diameter – about the width of a human hair.

In this case, each primordial black hole would be tiny, but together they could provide enough gravity to keep a galaxy from flying apart. If galaxies like our Milky Way are chock full of teeny adhesive black holes then they should be everywhere. Amir Siraj, a theoretical astrophysicist from Harvard University, believes there’s even a chance that one is lurking in the outskirts of our Solar System.

“YOU CAN EXPLAIN ALL OF THE DARK MATTER THROUGH PRIMORDIAL BLACK HOLES”

THE FOUR TYPES OF BLACK HOLE

PRIMORDIAL

(A RANGE OF DIFFERENT MASSES)

These are as-yet-unconfirmed relics from the formation of the Universe in the Big Bang. So far we’ve been able to rule out their existence in certain mass ranges, but the door is still open to the possibility that the Universe is littered with tiny black holes. Together, they could explain the dark matter mystery – the fact that there appears to be an invisible glue binding galaxies together.

STELLAR MASS

(5 TO 100 SOLAR MASSES)

These are your so-called ‘ordinary’ black holes. When the most massive stars die – those more than 30 times the mass of the Sun – they explode into a supernova. The star’s core collapses into an infinitely small, dense point called a singularity. The gravity is so strong that anything venturing within a certain distance of the singularity – a region called the event horizon – cannot escape its clutches.

INTERMEDIATE

(100 TO 1,000,000 SOLAR MASSES)

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