Black holes are one of the strangest environments in the universe, places where past the event
horizon the physics of Einstein that describe the universe we see so perfectly, ultimately
break down.
Even the nature of the singularity within the black hole is not well understood, especially
in regards to whether a wormhole, or even an entire other universe lies beyond or does
not.
But we do know that there are several possible origins for black holes, the most familiar
being a giant star’s core collapse into a black hole, but also the possibility of
the creation of micro black holes in particle accelerators at high enough energies, but
there is another hypothesized type.
A primordial black hole.
Primordial black holes are perhaps one of the spookiest objects in nature.
If they exist, they are unbelievably ancient, having formed shortly after the big bang when
the environmental conditions of the universe were very dense, and in principle, some areas
could have had regions dense enough to collapse into black holes, even very small ones.
Because these black holes did not form from collapsing stars, they can have surprisingly
low masses, but also the possibility of higher mass ones as well.
But there is a limit as to how tiny they can be in nature, at least today.
During this mystery shrouded early period of the universe, the big bang’s aftermath
might have produced micro black holes, very tiny ones sometimes called Planck Relics.
But these are thought to be gone from the universe today because they have had sufficient
time to evaporate via Hawking radiation which robs the black hole of mass.
So the smallest ones still existing today would be, and have actually been, suggested
to be a good candidate for at least some dark matter, providing enough of them still exist.
But that’s a big if, this is all still hypothetical, and not yet proven.
But it is possible that baseball sized black holes wander the universe that have been here
since nearly the dawn of the universe.
True artifacts of the very earliest days of all existence.
And while it’s hotly debated just how many of these there can be, the question of if
they are dangerous also looms.
Or what of the risks of even larger black holes wandering the universe?
After all, the solar system has encountered many star systems in close approaches over
the course of its history.
Could it have ever encountered a black hole?
A primordial one certainly, if they indeed exist.
In fact one of the hypotheses regarding the weird gravitational effects that some objects
in the outer solar system seem to exhibit, presumably from an unknown gravitational source,
is the infamous planet nine hypothesis.
But it actually need not be a planet, a very small primordial black hole would fit the
bill as well, but it would have to be very small, as in the size of a tennis ball with
a mass of between 5 and 15 earth masses.
And good luck finding that in the vastness of the outer solar system, whereas a full
on planet would be much easier to locate in all sky surveys, such as the coming Vera Rubin
observatory.
But failing that, it’s been recently suggested that we might actually be able to detect a
primordial black hole in the outer solar system due to flashes of radiation caused by interaction
with outer solar system rocks, should any wander close to it.
But if such a planet is never found, and the gravitational effects continue to be seen
through the gravitational perturbation of outer solar system objects, then a very small
black hole will become something to take a serious look into.
But over all, we already have some hints that primordial black holes might exist.
The idea of the primordial black hole has been around for decades, but the concept fell
out of favor for a long time because nothing like one had ever been seen.
That was until LIGO detected a black hole merger through observation of gravitational
waves.
The weird thing here is that the two black holes involved were not very big in the context
of what we know about black holes, just about 30 times the mass of the sun each.
That doesn’t make them primordial black holes per se, but it gets somewhat close,
propelling the idea back into mainstream scientific thought.
After all, nothing in nature seems to prohibit primordial black holes, and when you start
seeing low mass black holes that could be of that origin then you have to entertain
the idea.
But the obvious question here is what happens if a black hole of any size hits a planet
like earth?
Could it be detected beforehand?
Has it ever happened?
And would it cause a mass extinction event?
Well certainly a large enough black hole encounter in the past would have resulted in no present
earth at all.
We’d have been torn apart and become part of a black hole’s singularity long before
the arrival of humanity if that were the case, and since we’re here, earth clearly has
not been subject to this in its history.
And while stars and planets are eaten by supermassive black holes at the centers of galaxies, it’s
not that common in the suburbs of the galaxy where we are located, or indeed even the urban
environment of the galactic center.
It happens, but not that often, or the galaxy would have long ago been consumed by its population
of black holes.
But what of a small primordial black hole hitting earth?
Well it depends first on how numerous they are.
If primordial black holes make up the entirety of dark matter, then we’ve been hit on the
order of once every billion years, so we could expect four hits in the past.
If they do not make up all dark matter, this number drops.
And also the damage something like that depends on the speed at which it were moving, if it
were traveling fast enough it would pass through the earth, destroying anything in its path
and generating explosive heat, but it would not necessarily destroy the planet itself;
depending on the mass it would be more like an asteroid impact or even effectively a non
event.
If this has ever happened, basically earth getting shot by a cosmic black hole bullet,
there seems to be no evidence in the geologic record of it, though it seems unlikely that
such a thing would be preserved in any detectable way if it were small, though some have suggested
that evidence might still exist, it would just be very difficult to detect and would
show as a small region of heat altered rock, a needle through the haystack indeed when
you don’t know exactly where to look.
Yet, this is the more likely scenario as primordial black holes are predicted to move very fast
through the universe.
But if you happened to catch a slow one of sufficient mass, then this is an earth ender.
If it were to interact gravitationally with earth and get trapped, it would settle in
the earth’s core and it would slowly feed upon the planet before devouring it entirely
leaving a bit more massive of a black hole behind.
One wonders if entire exoplanets in the universe have met this unfortunate fate, should primordial
black holes of the proper size exist.
But regardless, an interaction with a primordial black hole could constitute an extinction
event if an inhabited world were hit by one.
Depending on its size the effects on a planet would be widespread, in the same way as an
asteroid impact of large size would be in that it might set off massive volcanism, environmental
effects, and just general havoc on a world from secondary effects before devouring the
planet entirely.
But that doesn’t mean that black holes aren’t in earth’s future, whether primordial or
artificial.
In regards to natural primordial black holes.
If one lurks in the outer solar system then it would be well worth attempting to find
it and study the physics of black holes from the comfort of our backyard.
But on the other side of the coin, the reality is that small black holes are potentially
incredibly useful.
While large black holes emit very little Hawking radiation, painfully so for very large ones
that take immense amounts of time to evaporate, the smaller the black hole the more energy
it releases and the faster it evaporates.
As an energy source, Microblack holes can emit enough power to run a space station or
a star ship, or more, yet at the same time pose little gravitational danger because of
their low mass.
If it weren’t for the massive radiation, you could actually stand relatively near a
microblack hole.
But while you wouldn’t get sucked in, the radiation would kill you in the form of gamma
rays, energetic particles and anti-particles and so on.
But that energy is useful technologically and could be harnessed, so long as you got
rid of the black hole before it evaporated enough to become a huge explosion hazard after
melting your equipment as its energy became more intense.
Interestingly, very small black holes below a certain gravitational limit, emit energy
so strongly that they can’t accrete matter, so must evaporate.
If these existed in large numbers during the early days of the universe, it would have
made for a very energetic environment during the micro black hole evaporation period, on
top of a very hot and already energetic environment post big bang.
But it also means these very tiny black holes wouldn’t have ever posed a threat to earth
given their nature, and can’t, should we ever create an artificial one, since a very
tiny black hole of just a few atoms forced together with laser beams would evaporate
almost instantly.
Another oddity about larger primordial black holes in general is that they may actually
have served as the initial seeds of galactic supermassive black holes, or even some intermediate
black holes that might still exist in the universe.
This would make the seeds of galactic supermassive black holes amongst the first objects in the
universe, some are predicted to have formed even less than a second after the big bang,
and may have played a role in the later overall formation of galaxies.
Whether primordial black holes offer a viable explanation for dark matter is hotly debated.
On the one side, some researchers have placed limits on just how many of them there could
be in the universe, but other models suggest that smaller ones may cluster around the 30
solar mass primordial black holes.
Interestingly however, the merger of the two 30 solar mass black holes detected by LIGO
in 2016, that the rate at which the two black holes merged could support the notion that
all dark matter is made up of primordial black holes, if they cluster, though it was later
argued that they could only ever account for a small portion of dark matter, all things
considered.
And there are mysteries within gamma ray and X ray background radiation that may fit models
where primordial black holes are very common.
And there are certain mysteries where small primordial black holes might fit the bill.
One is Jupiter and Saturn’s temperatures.
These planets are warmer than models predict, and no one knows why.
But it may be because small primordial black holes might lurk deep inside them interacting
and creating the excess heat.
If inside a star, it’s conceivable that it might result in a premature supernova or
other nuclear weirdness observed with some stars.
But there may be other more straightforward ways to detect primordial black holes through
studying gravitational waves.
As it stands, what we see in the study of gravitational waves so far are mergers, such
as those of black holes and neutron stars.
But large primordial black holes of dozens of solar masses are on the table here.
We actually have a probe into this one, which are gravitational lensing events.
If black holes of this size were very common and primordial in nature, we would see a lot
more gravitational lensing events in the universe than we actually do, meaning that large primordial
black holes cannot account for even ten percent of dark matter, probably much less.
But that’s still a lot of black holes, and if just a tenth of a percent of dark matter
is made up of primordial black holes, then mergers between primordial black holes should
happen at the same rate as normal black hole mergers.
The problem however is that it’s difficult to tell the difference between a primordial
black hole merger and a normal black hole merger, or a combination of the two.
Still, there are hints that we may have already observed a primordial black hole merger.
But as to black hole mass extinctions, while they are possible, it’s also very situational
and the likelihoods are very low that it’s ever happened, or ever will.
And even in environments where your planet can get eaten by a monster black hole, the
environment surrounding the black hole would have long ago caused any mass extinction,
in reality a complete wiping out of all life on a world, long before the planet got close
enough to be torn apart by tidal forces.
So the likelihood is low, but in certain situations, and this may have happened somewhere out in
the universe at some point in time, perhaps long ago, some world saw the end of its biosphere
at the hands of a small black hole.
And today, there may be nothing left, but that primordial black hole that was both present
at the dawn of the universe, and then ate an inhabited planet from inside.
A spooky scenario indeed.
Thanks for listening!
I am futurist and science fiction author John Michael Godier currently worried about the
microblack hole.
Think about it, you’re wandering the universe as you’ve done since one second after the
big bang, everything’s nice, things have cooled off, the weather is overall better
these days and all of a sudden a gigantic blue object shows up thousands of times your
diameter and into it you go, only to pop out the other side almost instantly.
What the heck was that, is about all you could say as you head back off into the universe.
Very disturbing and be sure to check out my books at your favorite online book retailer
and subscribe to my channels for regular, in depth explorations into the interesting,
weird and unknown aspects of this amazing universe in which we live.