Traditionally, we think of black holes as stellar black holes, black holes formed when
a star with sufficient mass collapses in on itself because the outwards radiation pressure
can no longer withstand the inward force of gravity and then they inevitably form a singularity
and event horizon.
Today, however, we’re going to be discussing primordial black holes, black holes that formed
at the very early stages of our universe, immediately after the big bang; emerging from
high density regions of matter in which a big fluctuation occurs, such that you’ve
got a region of increased amount of matter.
Now if the matter falls within its Schwarzschild radius, the speed needed to escape equals
the speed of light, or in other words it has become a black hole.
I think that one of the most common misconceptions about black holes is that folks tend to think
that every black hole is a stellar black hole.
This isn’t true.
A primordial black hole is an example of a type of black hole that didn’t form from
a star.
Another type of black hole is a supermassive black hole.
Supermassive black holes are occasionally considered to be different from stellar black
holes, but they’re the same thing.
The only difference being that supermassive black holes have millions of more solar masses
than the common stellar black hole and tend to be located in the centre of galaxies.
For all we know, the supermassive black holes in the centre of galaxies or any black holes
we’ve seen this entire time could be primordial black holes, we just can’t rule them out,
yet.
The supermassive black hole in the centre of the Milky Way, Sagittarius A*, has four
million times the mass of our sun for instance.
However, primordial black holes (or PBHs for short), are very different.
They’re sizes can range from a few grams to possibly even to match the mass of supermassive
black holes, depending on which formation model you look at.
Or it’s very possible that they don’t exist at all.
At the moment we can’t prove or disprove PBH’s but that isn’t to say that black
holes haven’t been proven.
That’s another common misconception about black holes.
Black holes do exist out in the universe and are proven, and have been proven for a long
time.
As long as a region of space contains a high enough density of mass or energy, general
relativity tells us that a black hole will form.
So most folks just assumed they exist.
And with the first discovery of gravitational waves back in September 14th 2015, the evidence
for black holes is building everyday.
Actually very recently LIGO announced the fifth detection of gravitational waves.
It was detected on August 17th and lasted for 100 seconds, which is way longer than
ever before.
But what’s amazing about this detection is that it was from two neutron stars merging,
whereas all the previous detections have been from black hole mergers.130 million light
years away, and 130 million years ago two neutron stars were orbiting each other and
due to their loss of energy through the emission of gravitational waves, they spiralled inwards
and merged.
Telescopes all around the world detected this in all areas of the electromagnetic spectrum.
And around 1.7 seconds after the main gravitational wave detection, NASA’s Fermi gamma ray telescope
identified a burst of gamma rays.
This is fantastic news, because for several decades gamma ray bursts have been thought
to come from neutron star mergers, but we lacked any evidence to be sure.
This new area of astronomy, has been a very fascinating one and continues to be so.
Before any detections made by LIGO, it was always thought that the main source of gravitational
waves would be neutron star mergers and up until this detection it has only been black
hole mergers, which is surprising because the universe, overall, has more neutron stars
than black holes.
But that doesn’t necessarily mean that there are more black hole mergers than neutron star
mergers, it could suggest that black hole mergers tend to produce more prominent gravitational
waves and only now have we had a neutron star merger close enough and strong enough to detect.
And as our detecting methods improve, we may develop ways to detect primordial black holes.
One possible way to detect PBHs is by their Hawking radiation.
Stephen Hawking realised that black holes evaporate thermal radiation, due to virtual
particles popping into existence at the event horizon.
Everywhere, virtual particles are constantly popping into existence.
They come in pairs, a particle and an antiparticle.
After existing for an extremely short amount of time, the two particles recombine and annihilate
each other.
So if you have one of these particle pairs pop into existence at the event horizon, one
particle will fall into the black hole but the other particle is able to escape into
space.
And when this happens, the black hole loses an extremely tiny amount of its mass.
Hawking showed that black holes emit this Hawking radiation at a rate inversely proportional
to their mass.
So a light black hole evaporates very rapidly and has an associated temperature that is
very high.
A massive black hole evaporates much slower and has an associated temperature that is
very low.
And, of course, primordial black holes, if they exist, will also be evaporating their
mass away, although the process is exceptionally slow.
Some PBHs that formed straight after the big bang will be evaporating into nothing right
this second, and some smaller ones will have already evaporated into nothing.
A particularly interesting one is a black hole with the mass of 1015 grams, which is
about the mass of Mount Everest.
Anything formed 10-23 seconds after the big bang with a mass below 1015 grams, or a billion
tons, will have all evaporated by today, due to Hawking radiation.
So as these 1015 gram black holes are coming to the end of their lives, they're emitting
Hawking radiation faster than they’ve ever done before as their mass is very low.
When these black holes reach the end of their lives and are emitting Hawking radiation at
an exceptionally fast rate, we don’t know what happens to them.
Although we can’t prove or disprove PBHs at the moment, we can answer another question:
are primordial black holes dark matter?
Despite us detecting dark matter only very recently, dark matter is estimated to take
up 27% of all matter in the universe while dark energy takes up a whopping 68% and normal
matter, that we’re made of and is all around us, surprisingly only takes up 5%.
So why did we think it was there, before we had detected it recently?
Well, the regular matter in the universe that seemingly makes up all the galaxies that we
observe, is not strong enough to hold these galaxies together.
They are rotating with such speed that the gravity generated by the regular matter could
not possibly hold them together — all the stars should be flying off into space, but
they’re not!
So we suggested that there is an invisible matter, which we called dark matter, which
accounts for this mysterious extra gravity holding the galaxies together.
And our theory was correct!
Very recently two separate teams found this missing matter, which was found to be made
of particles called baryons, linking galaxies together through filaments of hot, diffuse
gas.
How does this link to PBHs?
Well it’s possible that primordial black holes are dark matter and is responsible for
this extra gravity.
If this is the case, then the PBHs should create obvious gravitational lensing.
They would regularly pass in front of celestial bodies, such as distant stars, galaxies and
so on, and we would be able to see this.
Depending on PBH mass, this would cause an effect known as microlensing.
And well, we don’t see enough of this.
Now that doesn’t mean that it rules out all PBHs for being dark matter, but it does
rule out a lot of possible masses.
As well as this, we can also rule out all the masses that have already evaporated by
now due to Hawking radiation.
This includes all PBHs lighter than 1015 grams, or about a billion tons.
We should also be able to see the effects of these PBHs when they’re travelling the
cosmos.
One such effect occurs as a PBH goes close enough and pulls apart loosely bound binary
systems.
And we see loosely bound binary systems.
These observations are helping us constrain the possible masses of PBHs into a very narrow
set.
We’re left with two options, either there are lots of PBHs with masses of around 1021
kilograms, so about the mass of a large asteroid like Ceres, or a much smaller number of really
big PBHs (so around 20 to 100 solar masses).
The latter is a lot less likely than the former option.
So what does it look like for the future of PBHs?
Well, soon enough, we’ll either see evidence of primordial black holes at these masses
or discover that PBHs are actually very rare and that they aren’t an explanation for
dark matter.
Or they might not exist at all.
Comment down below your thoughts on PBHs.
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