Where do they come from? And should we be worried?
For a long time, tiny black holes, what
I'm going to call black hole rhinos,
seeing as they could have around the
same mass, were thought to be
impossible. Until the 1960s, we knew of
exactly one way to make a black hole,
and that was through stellar collapse. A
star 20 times the mass of our sun or
greater running out of fuel and
exploding in a supernova.
All that exploded matter collapses,
leaving behind an object so dense, not
even light could escape from its
crushing gravity. This is a black hole.
And because it formed from a star, it's
a stellar black hole. But this isn't how
all stars end. If their mass is less
than 20 times that of the sun, it still
goes supernova. But the collapse isn't
quite powerful enough to create a black
hole. Instead, we see a neutron star, a
tiny celestial object that's still
incredibly dense and made up almost
entirely of neutrons. And for stars
smaller than eight stellar masses, they
don't go supernova at all. So, there is
a limit to how big stellar black holes
can be at birth. No larger than the
largest stars. maybe a few hundred times
the mass of our sun and no smaller than
a few times more massive. So why do we
think that smaller than possible black
holes could actually exist? Strangely,
the answer is that we discovered bigger
than possible ones.
Today we know that at the heart of
almost every large galaxy lies a
gargantuan monster, a super massive
black hole. Back in the 60s, we were
just beginning to discover these
enormous beasts. The first quazar, a
highly luminous and redshifted galactic
center, was found in 1963, kickstarting
a roughly decadel long golden age of
black hole research. The first
predictions of a massive black hole at
the center of a galaxy came in 1971.
As the years passed and more evidence of
these titans came to light, we realized
they were mindbogglingly massive,
hundreds of thousands to billions of
times the mass of our sun. But they are
something of an enigma and left
scientists with one question in
particular.
How do they form?
While it's technically possible that a
stellar black hole could consume mass
and eventually grow this big, this
explanation started to break down as
examples of super massive black holes
from earlier and earlier in the
universe's history started showing up in
our increasingly distant images of the
cosmos. But for now, at least there
wasn't a better explanation.
Then came the James Webb Space
Telescope.
Launched on the 25th of December 2021
and switched on in 2022. It promised to
see further and in better detail than
any telescope had ever seen before. It
would look for stars so distant their
light had started traveling at almost
the dawn of the universe. And James Webb
did not disappoint.
These little red dots represent galaxies
with super massive black holes from just
500 million years after the Big Bang.
Scientists realized this was not enough
time for stellar black holes to grow to
that size. Not without devouring matter
faster than the laws of physics should
allow. Now, I've made a video on this
idea before, and there is some evidence
that this can happen, that black holes
can disregard our speed limits on how
fast they can eat to grow at rates we
wouldn't initially have thought
possible. That is, if we hadn't seen
them doing it. But there are plenty of
scientists who are skeptical of this
answer. And there's an alternative
theory, one that another recent James
Webb discovery may have just confirmed
as true.
When it comes to forming a black hole,
the key is density. When density is high
enough, mass becomes so compact that its
localized field of gravity passes a
tipping point and the speed at which you
would have to move to get away from it
becomes impossibly fast. I.e., you'd
have to travel faster than the speed of
light. Defying such gravity is
black holes. As I mentioned, black holes are densely packed.
The question is, how does matter get
that dense in the first place?
For that, you need a huge amount of
energy, enough to overcome the
electromagnetic force that stops
electrons or protons from getting too
close to each other. Then there's the
neutron degeneracy pressure, a quantum
mechanical force that says that two
subatomic particles, two neutrons, can't
occupy the same space at once. Neutron
stars are not dense enough for their
gravity to overcome this pressure. But
they are still so dense that a spoonful
of this matter would weigh as much as
Mount Everest, over 900 billion kg.
To make a black hole, you need forces
able to crush the whole of Mount Everest
into that teaspoon and then some. In
fact, Mount Everest would have to be
squashed into a space more than 1,000
times smaller than an atom to become a
black hole. In the universe, as it
exists today, such forces only exist in
very specific circumstances inside
collapsing stars as they turn supernova.
But go back in time and that wasn't
always so.
Let's rewind to the earliest moments of
the universe within the first second
after the big bang. There those
pressures did exist albeit just for a
moment. In this hot dense particle soup,
the distribution of matter was not
completely homogeneous or evenly
distributed. There were areas of high
particle density and others where it was
lower. Scientists believe it's possible
that in particularly dense patches of
space, black holes could have formed
from primordial matter itself, skipping
the star phase entirely.
The result is called primordial black
holes. And surprisingly, they aren't
actually a new idea. They were first
predicted in 1966 by Yakov Zeldovich and
Igor Novikov. What's interesting about
them is that they could have formed with
masses 100,000 times greater than that
of our sun. This would be more than
enough to account for the super massive
black holes that we see today,
especially given the billions of years
they've had to grow.
Curiously, they didn't have to form as
giants. They could also have been much
smaller. A black hole the size of a
single atom with mass comparable to an
asteroid or a so-called black hole rhino
that would be no bigger than a proton
but the mass of a rhinoceros.
However, this was 13.8 billion years
ago. And there are two problems with the
theory that such tiny black holes exist
today. The first is Hawking radiation.
An extremely long wavelength of
radiation that Steven Hawking
hypothesized black holes emit gradually
shrinking them over vast time frames. It
essentially tells us that by now all
tiny black holes should have evaporated
away. The second problem is that we
haven't proven that primordial black
holes actually existed in the first
place. We've never seen a black hole
arise out of the interstellar dust, or
at least we hadn't until recently.
In July 2025, NASA reported that the
James Web Space Telescope had seen two
early disc galaxies, likely in the
process of crashing into each other. But
strangely between them and not in the
center of a galaxy of its own was a
super massive black hole. It had not
somehow been ejected from either galaxy.
They also have their own super massive
black holes which left this third one
completely inexplicable.
The team who found it proposed that the
black hole formed in situ via the direct
collapse of a gas cloud. But this isn't
the only evidence that black holes can
simply appear given the right
conditions. QS01
is a super massive black hole that was
also spotted by web. It inhabits a
surprisingly small galaxy. Scientists
were able to do spectral analysis and
found that it's incredibly low in heavy
metals, elements other than hydrogen and
helium. This galaxy had less than 1% of
the oxygen that we see in our own and
researchers called it one of the most
chemically unevolved systems found in
the early universe which is telling.
Stars usually produce these elements in
just their first few generations. So the
fact that they are absent in QSO1's
galaxy suggests that very little stellar
formation has taken place yet.
compelling evidence that wherever QS01
had come from, it had likely not been
birthed by a star.
Now, although not smoking guns, these
two examples lend weight to the idea
that the early universe was capable of
producing primordial black holes. In
fact, this finding may explain where all
super massive black holes came from. And
if that's true, then it's almost certain
that super tiny black holes used to
exist, too. But they're all gone, right?
By now, they should have dissolved.
Well, according to one theory, perhaps
not.
Astronomy has a problem on its hands. As
we track the amount of gravitational
pole in the universe, it is much higher
than it should be. Scientists conclude
that there is additional matter inside
galaxies or circling them in large
halos. Something they've dubbed dark
matter because we can't see it. But you
know what else carries serious mass and
is quite hard to see? Black holes,
particularly ones with no accretion
discs because they formed directly out
of interstellar matter. Of course,
scientists have investigated this idea,
and there's no evidence of large black
holes surrounding the Milky Way.
Thankfully, if there were, stars would
go flying like bowling pins every time
one fell into our galaxy. We'd also see
the light from stars behind them
behaving strangely through the power of
gravitational lensing, as the intense
gravity of these black holes distorts
the space around them. And we simply
don't see anything that matches up to
this. But tiny black holes, our black
hole rhinos, would be very hard to
detect through either means. There's no
proof they are not there. The only
argument is to say that they would have
all dissolved by now, but this might not
be the case.
In April 2024, a study published in the
monthly notices of the Royal
Astronomical Society found that at tiny
levels, Hawking radiation might slow
down considerably, almost stopping
entirely, which means these black holes
might shrink and shrink until they
eventually stop. which if true means
that black hole rhinos might be out
there.
Many believe dark matter circles our
galaxy in a massive halo. What if
instead of all of that it was tiny black
hole rhinos?
There could be millions and millions of
them out there. A black hole minefield
lying invisible and deadly. a swarm we
could someday encounter if we ever
attempted to leave the galaxy to explore
another. We might be caged here without
knowing it.
Of course, space exploration has plenty
of dangers all of its own, but it's more
than unsettling to consider that as we
voyage out into the dark, we could be on
a collision course with a tiny
microscopic black hole with the mass of
a rhino. or that one could be hurtling
through space on its way towards us.
Let's hope we find some evidence that
this particular theory isn't true or at
least forget the idea of leaving the
galaxy. I think I'd rather stay at home.