Picture this. It's the year 2500. The
first probe to leave the Milky Way is
finally passing our galaxy's outermost
star. A historic moment is being
broadcast to every settled planet. The
probe's name is Wonderlust One. Its
destination, our closest neighboring
galaxy, Andromeda.
But just as onlookers celebrate,
something strange happens.
Impact.
Worlds. Watch in shock as the middle of
the probe crumples in on itself, causing
its metal to distort, buckle, tear, some
vanishing entirely. The remains of the
probe start spinning in space. Its
systems dead. And then the culprit
becomes clear.
Wonderust one was hit by a primordial
black hole.
Of course, what I just told you is
fiction. But tiny black holes with
masses so small they are comparable to
an adult rhino might not be. They could
be out there circling our galaxy's
edges. and we may have just found proof
that they actually exist.
I'm Alex Mccoan and you're watching
Astramm. Join with me as we delve into
the darkness to solve the mysteries of
primordial black holes. What are they?
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
Web 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. 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 QSO1
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.
Have a look at this beautiful image
captured by the James Webb Space
Telescope. Immediately, you'll notice
two types of objects. First, you'll see
the nearby stars within our own Milky
Way galaxy, which look like points of
light surrounded by defraction spikes.
But further in the background, you'll
see entirely new galaxies, each
containing billions of stars smeared out
into elliptical or spiraling blurs. What
you probably won't see are distinct
individual objects within those other
galaxies as they are far too distant and
blend in with all the other sources of
light. But there is one exception to
that rule. One type of object that is so
bright you can make it out from billions
of light years away. So bright that it
stands out from all the surrounding
stars in its galaxy. A quazar.
But what if I told you that a whole
bunch of tiny quazars were hiding in
plain sight right within our own Milky
Way? How can that be when they are some
of the brightest objects in the
universe?
In this video, we'll get to see how
that's possible and how a team of
scientists used images of just one
nearby micro quazar to gain a deeper
perspective into the inner workings of
quazars all over the universe.
I'm Alex Mccoan and you're watching
Astramm. Join me today as we unravel the
story of these astronomical lighouses,
one cosmic photon at a time.
Astronomers have been pointing their
telescopes up at the night sky for
centuries. Yet, it was only in the 1950s
that they set out on a quest to map out
what the universe around us looks like
in radio waves rather than visible
light. Think of this as the astronomers
version of putting on night vision
goggles. At first, they didn't see
anything out of the ordinary. Many of
the same galaxies that emitted visible
light also emitted radio waves. So, they
could be seen with or without the
goggles. But then, among the stars and
galaxies that were known to populate the
night sky, astronomers saw something
new. something that shone very brightly
in radio waves, but had never before
been seen in visible light. And there
wasn't just one of these objects. By
1960, observations quickly grew until
hundreds of mysterious radio sources had
been recorded across our sky.
At the time, astronomers didn't have a
clue what the origin of these radio
waves could be. For all they knew, the
waves could come from a giant
intergalactic network of alien radios.
But what they did learn from follow-up
observations using more precise
telescopes like the Hail telescope is
that you could see these objects in the
visible part of the spectrum. They were
just extremely small and faint. They
were far too small to be galaxies. They
appeared as points, not smeared out
blobs in the sky. But they also couldn't
be regular stars because regular stars
don't typically emit much energy at all
in radio waves. For this reason, these
mysterious objects came to be known as
quasi stellar radio sources, which quite
literally translates to something like a
star, but not a star. It emits a lot of
radio waves. That name eventually got
shortened to something that was a little
bit more palatable, quazars.
Even with the catchy name, it took
astronomers and physicists nearly 30
years to really pin down what these
quazars were. And unfortunately, it's
probably not alien radios. Although the
real answer is still extremely
fascinating. It turns out that the
explanation centers around black holes
and not just ordinary black holes, but
super massive black holes that can be
millions or even billions of times the
mass of the sun. Such super massive
black holes are frequently found at the
center of large galaxies. And although
black holes can't emit any light
themselves, they can make quite the
spectacle out of the accretion discs
orbiting around them. If you want all
the details, you can cue up our older
video on quazars here, but for now, let
me just give you a quick summary. Matter
orbiting a black hole can spiral inward
as it loses energy due to internal
friction. Much of that energy ultimately
gets converted into light that streams
outward from the black hole. But in some
cases, the accretion disc also splits
out relativistic jets of ionized matter
cumated by powerful magnetic fields
along with the spin of the black hole
and its surroundings. The particles in
these jets can propagate through the
universe from galaxy to galaxy as cosmic
rays, sending out radio signals as
they're pushed around by intergalactic
magnetic fields. Overall, quazars emit
so much energy that a single quazar in a
distant galaxy will outshine the other
hundreds of billions of stars in that
galaxy combined.
That's why when quazars were first
observed, they appeared as small as a
star, but far brighter than any star
outside of the Milky Way could have ever
been. Now, the Milky Way has its own
super massive black hole right in its
center, too, called Sagittarius, a star.
But for better or for worse, the
accretion disc around our black hole is
too thin and faint to call it anything
close to a quazar. And until something
dramatic happens, like the collision
with the Andromeda galaxy that's
scheduled for a few billion years in the
future, Sagittarius A star will likely
remain calm and mellow.
On one hand, we're kind of lucky because
a quazar that close to Earth would emit
enough high energy radiation to be
potentially fatal to terrestrial life,
which makes you wonder about the kinds
of life that could arise in galaxies
that do contain quazars.
But on the other hand, we're missing out
on a second source of light in the sky,
illuminating our planet almost as
brightly as the sun. And it would be so
cool to get to study a quazar from up
close.
So, can you imagine my surprise when I
learned that we might actually have the
best of both worlds? Quazars, alive and
active in the Milky Way, and yet not so
powerful as to wipe out our entire
biosphere.
How could this be possible? If the one
super massive black hole at the center
of our galaxy isn't a quazar, then where
could they be?
It took a couple of decades, but
scientists eventually discovered that
quazars can form around ordinary black
holes, too, not just super massive ones.
These ordinary black holes, just a few
times more massive than the sun,
typically form when giant stars
collapse. So, there are loads of them
spread throughout practically any
galaxy.
You probably wouldn't expect one of
these black holes to have its own
accretion disc, but once in a while, a
nearby orbiting star might get sucked
into its gravitational pull and act as a
source for the smaller, less deadly
version of a quazar, a microquazar.
Even though the jets of a microquazar
are much weaker, they can be easier to
study because of their proximity to
Earth, allowing us to learn more details
about the properties and dynamics of
both microquazars and their larger full
scale counterparts.
The very first micro quazar was
discovered in 1979 and was given the
obvious name SS433
as it was originally documented as the
433rd
entry in a catalog of stars compiled by
astronomers Nicholas Sandelik and Bruce
Stevenson 2 years prior. To date, only a
handful of micro quazars have ever been
found. And SS433
in particular has been a subject of
widespread wonder and research. Despite
its name, SS433 is more than just some
ordinary star in a catalog. It is a
typea super giant orbiting around a
stellar mass black hole in a binary
system. As the black hole accretes
material from the super giant star, it
produces a pair of jets perpendicular to
the line of sight from Earth, meaning
that the jets themselves will never
actually hit us or reach our detectors.
But as the jets propagate outward, they
emit light in all sorts of directions,
allowing us to image them at various
wavelengths and learn more about how
they are produced.
One of the first things we saw was that
there are actually two types of jets
emanating from SS 433. The inner jets
extend out just a couple of light years
from the black hole before fading away,
while the outer jets appear 75 light
years out and span an additional 300
lightyear or so. We still don't know how
exactly these outer jets form or why
they terminate after 300 lighty years.
and we only have a rough idea of what
they're even made of. But a recent study
from the Hes collaboration gave us some
new clues to answer these questions.
One such clue is the presence and
distribution of high energy photons
called gamma rays being emitted by SS
433.
Ordinarily, stars or accretion discs
could not be expected to produce any
gamma rays because their temperatures
are far too low. But the outer jets
coming out of SS433 seem to emit them in
spades.
What's even more surprising is that even
though the outer jets are extremely hot,
the spectrum of light that they're
emitting isn't consistent with an
ordinary thermal distribution of
photons. So what's generating all of
those gamma rays? The current theory is
that the outer jets contain high energy
electrons that can bump into lower
energy photons passing through the
system and they transfer their energies
to the photons during those collisions.
This process goes by the name of inverse
Compton scattering.
The scientists at Hess realized that
measuring the energy in these emitted
gamma rays can give us a lot of
information about the energy of the
electrons in the jets that produced them
in the first place. And what they found
was in full support of the inverse
Compton scattering theory. The highest
energy gamma rays carrying over 10 terra
electron volts of energy per photon or
about 10 trillion times more energy than
a photon of visible light were primarily
coming from the innermost region of the
outer jets. Gamma rays with lower
energies, on the other hand, came from
correspondingly farther distances down
the jets of S433.
This distribution is a clear indication
that electrons farther down the jets
have less energy to transfer to photons
than electrons right at the beginning.
Or in other words, as the electrons
stream along the outer jets, they tend
to lose energy over time, presumably due
to their collisions with photons. But
how did the electrons acquire such large
energies to begin with? Models suggest
that their initial launch velocity from
the black hole was just about 1 quarter
of the speed of light. But by the time
they reach the outer jets, these high
energy electrons appear to be moving at
speeds greater than 99% the speed of
light, multiplying their initial
energies by a factor of 1 billion.
The Hess collaboration believes that
this wild acceleration occurs due to a
shock wave at the base of the outer jets
caused by complicated configurations of
magnetic fields. Each time an electron
interacts with the shock front, it gains
a boost of speed as if it crossed a
boost pad in Mario Kart. And due to the
diffusion of electrons back and forth
within the jet, some electrons can cross
over this boost pad over and over and
over again like an infinite speed
glitch. This process is called diffusive
shock acceleration and it may be
responsible for amping up electrons to
energies in the hundreds of terror
electron volts just enough to be able to
produce the high energy gamma rays
observed by Hess. But shedding light on
the production of photons within the
outer jets of SS 433 is just the
beginning. Micro quazars scattered
throughout the Milky Way can hold
treasure troves of information about the
inner workings of even the most massive
quazars in the center of distant
galaxies. And now that we know to look
for them, micro quazars are less like
hidden gems and more like beacons in the
sky, posing for our astronomers to
capture their beauty in an entire
spectrum of photographs and revealing
ever more shocking surprises.
So, what surprised you from today's
video? Was it the possibility of shock
waves in deep space near black holes? or
was it the fact that the Milky Way is
home to dozens of little quazars of its
own? Let us know your thoughts in the
comments below.
When the James Web Space Telescope
finally saw the edges of the universe,
we knew we had a problem.
Web was able to resolve light emitted
from stars 13 billion years ago, helping
us to peer back in time to some of the
universe's earliest moments. But what we
saw was not a sparsely populated proto
universe where matter was only just
starting to coalesce into the first tiny
intermittent galaxies here and there.
The early universe was a bustling place.
It had galaxies,
too many of them. They were too bright.
And the black holes we start to spot in
their hearts had grown too big too
quickly.
Some began to proclaim that our models
were wrong and cosmology was in crisis.
And while some of these problems have
begun to alleviate as better data came
in, other problems simply became more
prominent. But in a strange twist, one
of the most resilient mysteries in all
of this might be about to unravel thanks
to a black hole with an impossibly big
appetite.
Its name is lid 568
and we've just seen it breaking the
Edington limit consuming matter faster
than it should be able to and it might
just be the key to everything.
I'm Alex Mccoan and you're watching
Astramm. Join me today as we explore lid
568, the Edington limit and its
groundbreaking implications on
cosmology.
A black hole breaking physics. By now, I
really shouldn't be surprised.
It takes time to up a galaxy.
Interstellar gas and dust need time to
subtly come together under gravity until
a critical mass is reached and stars
begin to ignite. These stars live and
die and from their deaths, new stars are
formed. This too takes time.
Cosmologists have observed our universe
and based on what they saw created
models for how old our universe is and
how quickly galaxies form. which is why
the James Web Space Telescope data
caused such a crisis. Things were not as
the models predicted. Fortunately, some
of those problems proved solvable in the
months after the data was released. For
example, the brightness of the galaxies
we could see through web. This
brightness implied that there were far
too many stars present in those
galaxies. So many stars should have
taken much longer to form. And yet there
they were, fuzzy red dots at the edge of
Web's resolution. However, scientists at
the University of Texas studying Web's
cosmic evolution early release survey
realized there could be another
explanation for all that excess light.
And counterintuitively,
that explanation was black holes.
If we work under the assumption that
there were massive black holes in these
galaxies rapidly consuming cosmic gas,
then the intense friction given off by
these hungry leviathans as they ate
created an excess of light in their
accretion discs. This explains why
galaxies overall seemed brighter and
were throwing off our estimates. Once
you add these shining black holes, you
don't need so many stars. The mass of
each problematic galaxy dropped and
everything fell back into line with the
cosmological model. Problem solved.
This reinforced how important black
holes are to our understanding of the
early universe, which was in and of
itself a problem because the black holes
themselves broke our models, too. In
particular, cosmologists struggled with
the thorny question of how they'd come
to be.
For small black holes known as stellar
black holes, there was no issue. Stella
black holes have masses a few to 100
times that of our sun and we understand
very well how they are formed.
They are the collapsed remnants of a
sufficiently massive star and there
would have been time for such black
holes to form in the early universe. But
scientists were struggling with the
super massive black holes with masses
tens of thousands to billions of times
that of our sun which tend to lurk at
the central point of galaxies and due to
something called the Edington limit.
There just shouldn't have been time for
these kinds of black holes to have
formed where and when webb saw them. And
yet there they were and they were
numerous.
Stella black holes can grow as time goes
on, provided you funnel more mass into
them. But how quickly?
In 1920, an English astronomer and
physicist called Arthur Edington
formulated the idea that there was a
limit to how quickly either a star or a
black hole could grow. This was because
photons carry momentum. A tiny amount,
true, but enough to exert a push. This
is what pushes solar sails on certain
hypothetical spaceship designs. That
tiny amount of momentum imparted by
photons. For mass to enter into a star,
it has to push against a constant stream
of photons that are radiating outward.
And at a certain level of brightness,
not even gravity is strong enough to
pull against the flow. This is called
the Edington limit. and stars that brush
against its boundaries, such as Volraier
stars. Bright stars at least 20 times
more massive than the sun emanating
powerful stellar winds are just the
slightest nudge away from blowing
themselves apart. For black holes, you
might think this would be less of a
problem. Isn't the whole point of black
holes that they don't radiate any light?
But their accretion discs are a
different story. As we discussed
earlier, accretion discs around super
massive black holes can be incredibly
bright, particularly around super
massive black holes, sometimes dwarfing
the brightness of the stars in the
galaxy they reside in. With brightness
comes resistance to gravity, and black
holes have to obey the Edington limit,
too.
So even though given enough time and
mass, stellar black holes could
theoretically grow into super massive
black holes, it doesn't seem plausible
that this actually explains all the
super massive black holes we see in the
early universe. simulations have been
run and although it is technically
possible to grow a stellar black hole
into a super massive black hole in that
time frame, it would require those black
holes to be feeding at near the Edington
limit nonstop since their birth, which
just doesn't happen. Black holes in real
life often run out of mass nearby and
need to wait to run into more or for
more to come to them. To further
complicate the matter, we're not
completely sure that super massive black
holes are the grown-up version of
stellar black holes in the first place.
Although it seems like common sense to
assume so, scientists have been confused
at the lack of the intermediate stage of
black holes observable in our universe.
To be frank, they've not cited any, at
least none for sure.
Super massive black holes are common at
the center of galaxies. And there are
thought to be 100 million stellar black
holes in our Milky Way alone based on
the number we've seen. But intermediate
black holes are suspiciously lacking
with only a handful of potential
candidates. You would think we'd see a
lot more.
Struggling for certainty, scientists
began to hypothesize that super massive
black holes were instead born in some
other way. Many cosmologists have been
exploring the idea that because
everything was much closer together in
the early universe, things might have
been dense enough that interstellar dust
itself could conceivably have collapsed
to form a black hole directly, skipping
the star step altogether. If this is
true, and there is some evidence to
support the theory, then perhaps super
massive black holes were once capable of
simply being born that size or near it
right from the offset, even if such a
thing is no longer possible in our more
spread out universe today. But this is
by no means certain. But then lid 568
came onto the scene and the pendulum
swung the other way again.
Lid 568 is a very distant black hole
between 12.1 and 12.3 billion lighty
years away. It's so far away from us
that we can't see it at all using
visible light. The expansion of the
universe has redshifted it all into
infrared ranges. But even its infrared
emissions were too dim to be picked up
by heavy hitters like Hubble alone. It
took the Chandra Cosmos Legacy Surveys
combined telescopes and the incredible
resolution of the James Web Space
Telescope to see it at all. Even then,
LID 568's whole galaxy is little more
than a faint red and compact dot. But
the light emissions from this red dot
are revealing.
X-rays given off by lid 568's accretion
disc reveal that it was actively
consuming matter in its galaxy's heart
in a way that no one expected.
You see, LID 568 crucially breaks the
Edington limit. And not just by a
little, it's 40 times over the accretion
speed limit. It's well on its way to
having its license revoked. How is this
possible?
It turns out that breaking the Edington
limit is in fact possible, but only for
short bursts or in sneaky ways. For
example, jets can help you get around
the Edington limit. If all your photons
are being blasted off in a single
concentrated direction, all the other
directions can eat to their heart's
content with no photon feedback getting
in the way of a good meal. There are
other possibilities. While Edington's
limit says that once the brightness of
the accretion disc becomes too high, all
the black holes food will be blown away.
There is a period of time before this
happens where a greedy black hole can
snatch at the escaping matter and
potentially enter super Edington
territory. like an overeager diner, it
might pay for it later. But for a short
burst, that level of accretion can
occur.
If this is true, it might just explain
how super massive black holes in the
early universe came to exist. And
certainly, LID 568 exists and is the
clearest example to date of a black hole
accreting this quickly. That demands our
consideration. Much like the impossible
super massive black holes themselves,
they are there. It's now our job to
attempt to understand how that's
possible.
So mysteries remain. If lid 568 is
accreasing matter past the Edington
limit, it proves such a thing is
possible. But what of the other strange
things Web saw? Where are the
intermediate black holes that stellar
black holes ought to grow into on their
way to becoming super massive? Why are
there so many galaxies in the early
universe more than our model should
allow? Do direct collapse black holes,
ones formed from the cosmic dust itself
with no stellar intervening step, really
exist?
Our models might be on the right track,
but something is missing or incomplete.
Black holes exert a phenomenal influence
on our universe, and there are secrets
surrounding them that they guard
jealously.
But as powerful as the James Web Space
Telescope is, it has not allowed us to
crack this mystery. Not yet. But then
perhaps we ought to be pacing ourselves.
Thanks to our telescopes, we have seen
billions of trillions of stars and
that's a lot of data. Accrete too much
information all at once and it might
prove difficult to absorb it all. So
says Edington and his rule is never to
be broken except of course when it is.
On the 14th of September 2015,
scientists at the Laser Interpherometer
Gravitational Wave Observatory detected
gravitational waves directly for the
first time. A stunning achievement that
led to the 2017 Nobel Prize in Physics.
Why was this significant? Well, here's
an analogy. Let's imagine that human
beings evolved without the ability to
see light. For thousands of years, we'd
fumble in the dark, relying on our other
senses, until one day, someone invented
a machine that could perceive light for
us. In time, we'd see everything from
the tips of our noses to the farthest
flung galaxies. This analogy captures
the magnificence of LIGO. It's about
much more than proving a scientific
prediction. LIGO enables us to perceive
the physical universe and understand
reality on a new level. Like photons,
gravitational waves travel at the speed
of light as they ripple across spaceime.
Their signals are all around us. By
listening for gravitational waves with
some of the most sensitive instruments
ever built, scientists are recording
tremors of distant violent events. the
formation of black holes, supernova
explosions, and potentially exotic
phenomena we haven't discovered yet. So,
what are gravitational waves? What
causes them? And why is LIGO's ability
to detect them already transforming our
understanding of the universe?
I'm Alex Mccoan and you're watching
Astramm. Join me today as we learn about
gravitational waves, unpack the
groundbreaking technology behind LIGO,
and anticipate some of the stunning
developments that lie around the corner.
Gravitational waves are one of the
stranger implications of Albert
Einstein's general theory of relativity.
As we've covered previously, spacetime
is a model that combines the three
dimensions of space and the fourth
dimension of time into a single
manifold.
All objects with mass create curvature
in spacetime and objects with a lot of
mass create a lot of curvature which we
experience as gravity.
A simple way to visualize this is to
think of a pool ball resting on an
elastic surface and a bowling ball
resting on that same surface. The more
massive bowling ball will create more
curvature. As objects move across
spacetime, that curvature changes
position with them. One of the amazing
consequences is that when objects of a
certain mass accelerate, they can send
ripples across spaceime as gravitational
energy. While this requires a special
set of conditions, namely a very massive
object undergoing acceleration, such a
cataclysmic event would send ripples or
gravitational waves outward at the speed
of light. Think of them like ripples on
a pond, but instead of water, they
travel through the fabric of spaceime in
all directions.
As in the pond analogy, these
disturbances become weaker as they
radiate outward. To an observer, the
distance between objects would appear to
expand and shrink as the gravitational
wave passes. Mind-boggling to imagine.
Yet, although Einstein predicted the
existence of gravitational waves, he was
pessimistic about our chances of ever
detecting them. He thought that these
disturbances would be so small as to
escape our ability to measure them. And
who could blame him? Many of the changes
in distance that LIGO seeks to measure
are 1 10,000th the length of a proton.
Yes, you heard that correctly. 10,000
times smaller than a single proton.
And yet these signals would come encoded
with all kinds of information about
their origins, when they originated, how
far they traveled, and what kind of
event produced them.
This is where LIGO comes in. It consists
of two observatories funded by the
United States National Science
Foundation and operated by MIT and
Caltech.
Among its driving forces are renowned
physicists Kip Thorne, Raina Weiss, and
Barry Barish. All of whom shared the
2017 Nobel Prize for their decisive
contributions to the detection of
gravitational waves.
LIGO is essentially a large scale and
very sensitive interpherometer, an
invention that's been around since the
1880s. An interpherometer essentially
measures what happens when light waves
are combined from two or more sources.
For example, you could use an
interpherometer to test whether light
travels at different speeds through
different substances, such as through
air or water. Even a subtle difference
in speed will produce an interference
pattern when the light waves combine.
Much like what happens when two ripples
on a pond intersect. If the peak of one
ripple hits the valley of a second
ripple, they will subtract from each
other, producing a flat surface.
However, if the peaks line up exactly,
it means the waves are in phase and add
to each other. This is essentially what
the interpherometer measures with light.
By seeing how in or out of phase two
light waves are, an observer can infer
the relative speed of the waves. And the
larger and more powerful the
interpherometer, the more sensitive it
is. Here's how it works. LIGO has two
observatories located in Hanford,
Washington, and Livingston, Louisiana.
Why two? Well, you need at least two
detection sites to triangulate where the
signals are coming from. Each
observatory continuously fires a
powerful laser at a beam splitter
positioned at a 45° angle. The laser
beam has to operate at around 750 kW.
Powerful enough to vaporize you
completely if you've got in its path.
The splitter then splits the laser beam
perpendicularly.
The light in each arm travels down a 4
km vacuum cavity with a mirror at the
end of it. The beams then bounce between
this mirror and a recycling mirror at
the other end nearly 300 times,
increasing the distance from 4 to 1,200
km.
Remember what we said with
interferometers, bigger is better. After
completing nearly 300 trips, the laser
beams combine at the beam splitter and
head to a photo diode, which is a light
sensitive semiconductor.
If undisturbed, the beams will be in
phase, meaning their frequencies will
subtract each other and no light will
arrive at the photo diode. But if
there's a gravitational wave, the
distance each beam travels will be
slightly different and they'll be out of
phase. The photo diode will pick up a
signal indicating the presence of a
gravitational wave.
Now, this is how it works in a perfect
world. But in reality, the
interferometer is constantly picking up
noise. To minimize this, LIGO uses
incredibly smooth 40 kg mirrors
suspended by silica threads. Any
particles in the interferometer's arms
are also a problem, which is why LIGO
pumps the air from its vacuum chambers
to 1 trillionth of atmospheric pressure.
But there's another problem. At these
minuscule levels, even quantum mechanics
are a nuisance because they introduce
randomness into photon behavior. LIGO
mitigates this with an optical cavity
which squeezes the light. This squeezing
minimizes the light phase's noise and
squeezes it into amplitude noise which
the interferometer doesn't measure. In
other words, the quantum randomness will
show up more in the height of the waves.
Quantum randomness is a fact of life. It
can't be eliminated, but it can be
shifted. Much as you might move clutter
from your bedroom floor to your closet,
the chaos isn't gone, just out of sight
for the moment. Plus, the goal isn't to
eliminate noise completely, but to get
the best signal to noise ratio possible.
That's a pretty good overview of how
LIGO works. So, what has it discovered?
As I mentioned earlier, LIGO detected
its first signal in 2015.
Named GW150914,
scientists studied the data and learned
that it was caused by the merger of two
black holes about 1.6 6 billion lighty
years away. These black holes, which
were 29 and 36 solar masses, became a
binary and spiraled around each other
until they merged and released a blast
in the final 20 milliseconds that was so
powerful. Now, get ready for this number
because this is what the scientists
actually think. It contained 50 times
the combined light power of every star
in the observable universe.
At the risk of sounding crude, that is
nuts. I've read this fact many times
over and I still cannot comprehend what
it means. Yet, after traveling for 1.6 6
billion years and finally reaching LIGO.
The disturbance was so faint it moved
LIGO's 4 km arm 1,000th of the width of
a proton.
To visualize this, imagine the distance
between us and Proximus Centuri and
changing it the width of a human hair.
That is the level of precision LIGO was
able to detect. If that's not one of the
most astonishing feats in human history,
I don't know what is. And this was just
the first gravitational wave LIGO
detected. The second detection occurred
3 months later in December 2015.
That signal also came from a black hole
merger which took place 1.4 billion
lighty years away. Over its initial
three runs, LIGO recorded more than 80
black hole merges. And in August 2017,
it detected the merger of two neutron
stars named GW170817.
This signal was notable for being the
first gravitational wave to be
cooperated by electromagnetic
observations from 70 observatories
across the planet. This was a
breakthrough not only in gravitational
wave detection, but in multi- messenger
astronomy.
It turns out LIGO was just warming up
during these three runs. As of May 2023,
LIGO has begun its fourth run with
better sensitivity than ever. After its
latest round of upgrades, which kept
LIGO offline for 3 years, the
observatories now have more reflective
mirrors, better mirror suspension, and
improved light squeezing with lower
quantum uncertainty.
And this time, LIGO also has the support
of Kagura, a new interpherometer
observatory in Hida, Japan.
Kagura is located underground, making it
the world's first subterranean
gravitational wave observatory and also
the first to use cryogenic mirrors.
During an engineering run on the 18th of
May, LIGO scientists say they already
received a signal that was possibly
caused by a neutron star being swallowed
by a black hole. We'll have to wait a
while for confirmation, but if these
early results are any indication, LIGO
is about to blow the doors of our
understanding of gravitational wave
generating phenomena.
So, what other developments lie ahead?
India is preparing a collaborative
project called LIGO India or Indigo
which will help LIGO triangulate better
location data. In 2027 to 2028, LIGO
will implement its LIGO Voyager upgrade
which will achieve higher sensitivity
with four times heavier mirrors and
higher frequency lasers. And in the more
distant future, a third generation
facility has been proposed called Cosmic
Explorer.
This facility would feature two new
observatories with arms spanning 40 km
and 20 km respectively. Remember with
interpherometers bigger is better. But
the proposal that really excites me is
the laser interferometer space antenna
or LISA.
This would be the first space-based
gravitational wave observatory which
would utilize three spacecraft in a 2.5
million km long configuration.
This interferometer would be so big and
so precise scientists hope it would be
adept at uncovering exotic and
theoretical sources of gravitational
waves such as cosmic strings and other
speculative phenomena.
In theory, it could help us stare
directly into the fabric of reality.
With a planned launch date of 2037,
we're still over a decade away, but it's
never too early to start counting the
years.
So, there you have it. An overview of
LIGO and how scientists are using
gravitational waves to better understand
the universe. They give us evidence of
extremely remote and ancient phenomena
that cannot be measured by other means.
And they can be a secondary way to
measure observations made by other
instruments like the web telescope or
Hubble.
In time, this revolutionary field should
allow us to understand the nature of our
universe, its history, and even its
future. I hope you found this episode as
fascinating as I have.
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