When the James Webb Space Telescope finally saw the edges of the universe,
we knew we had a problem.
Webb 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 started 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 those 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 Eddington Limit.
And it might just be the key to everything. I’m Alex McColgan, and you’re watching Astrum.
A black hole breaking physics? By now, I really shouldn’t be surprised.
It takes time to cook up a galaxy. Interstellar gas and dust need time to subtly come together
under gravity, until a critical mass is reached and stars start 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 JWST 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 Webb. 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 Webb’s resolution. However,
Scientists at the University of Texas studying Webb’s Cosmic Evolution Early Release Survey
realised 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 disks. This explains why the 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. That 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. Stellar black holes have masses a few to a hundred 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 Supermassive 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 Eddington 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.
Stellar 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 Eddington 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 Eddington limit,
and stars that brush against its boundary – such as Wolf-Rayet stars, bright stars
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 disks are a different story. As we discussed earlier, accretion disks around
Supermassive black holes can be incredibly bright, particularly around supermassive
black holes – sometimes dwarfing the brightness of the stars in the galaxies they reside in.
With brightness comes resistance to gravity, and black holes have to obey the Eddington limit too.
So even though given enough time and mass, Stellar black holes could theoretically
grow into Supermassive black holes, it doesn’t seem plausible that this actually explains all
the Supermassive 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 supermassive black
hole in that timeframe, it would require those black holes to be feeding at near the Eddington
Limit non-stop 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 Supermassive 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 sighted any – at least, none for sure.
Supermassive black holes are common at the centre 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 hypothesise that Supermassive 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 supermassive 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 this is by no means certain.
appearing directly from the interstellar dust might be the answer scientists were looking for.
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 light 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 infra-red ranges. But even its infra-red emissions were too dim to be
picked up by heavy hitters like Hubble alone. It took the Chandra-COSMOS Legacy Survey’s combined
telescopes and the incredible resolution power of the James Webb 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 disk 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 Eddington Limit. And not just by a little – it’s 40 times
over the accretion speed limit. It is well on the way to having its license revoked.
How is this possible? It turns out that breaking the Eddington limit is in fact possible, but only
for short bursts, or in sneaky ways. For example, jets can help you get around the Eddington limit:
if all your photons are being blasted off in a single, concentrated direction,
all the other directions can eat to their hearts content, with no photon feedback
getting in the way of a good meal. There are other possibilities. While
Eddington’s limit says that once the brightness of the accretion disk becomes too high all the
black hole’s 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-Eddington
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
supermassive 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 supermassive 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 accreting matter past the Eddington Limit, it proves such
a thing is possible. But what of the other strange things JWST saw? Where are the intermediate black
holes that stellar black holes ought to grow into, on their way to becoming Supermassive?
Why are there so many galaxies in the early universe, more than our models 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.
As powerful as the JWST 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. That’s a lot of data. Accrete too
much information all at once, and it might prove difficult to absorb it all. So says Eddington,
and his rule is never to be broken. Except, of course, when it is.
much information all at once, and it might prove difficult to absorb it all. So says Eddington,
and his rule is never to be broken. Except, of course, when it is.