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We spent the last decade listening to the vanishingly faint vibrations in the fabric of
spacetime from black holes colliding across the universe. Now, hundreds of detections later, we
finally thought we understood the story they were telling. But then we spotted something that should
be impossible. A black hole smaller than known stellar astrophysics allows. A black hole that may
have formed in the Big Bang itself. It may tell us something incredible about our universe—but
also equally importantly about what this young field of gravitational wave astronomy has become.
much as it's inspired us, check it out at the Space Time merch store. Now on to the episode.
Ten years ago, LIGO detected gravitational waves for the first time. It was one of
those discoveries that immediately felt like the beginning of something much bigger. Yes,
we’d proved Einstein right. Again. But we'd also gained an entirely new way of observing
the universe. What else would that lead to? Entirely new types of collapsed object? Dancing
cosmic strings? Echoes from the Big Bang? Even cracks in the deeper depths of Einstein’s theory?
Revolutionary observations are exciting because we don’t know what the new field might evolve
into. Now fast-forward ten years. We're finally far enough away from that first detection to ask
what gravitational-wave astronomy actually has become. Has it lived up to those expectations?
Has it changed astrophysics? Or has it mostly confirmed what we already thought we knew?
That question has been on my mind because LIGO, Virgo and KAGRA released their latest
gravitational-wave catalog—now nearly 400 detections of gravitational monsters
colliding across the cosmos. But just as we started to think we understood what we were
seeing—building and refining our theories of how they must have come to be… something new
appeared. It's only a candidate for now, but if it survives further analysis, it may contain
a black hole that’s lighter than anyone thought possible. A black hole that could NOT have formed
from a star. It’s not confirmed, but people are paying attention. We'll come back to it.
Now, the first gravitational-wave detection was a huge headline, and shortly thereafter a Nobel
prize. Fair enough. The Laser Interferometer Gravitational Wave Observatory and its cousins
are, after all, are miracles of human ingenuity and chutzpah. The kilometer-scale vacuum systems;
the mirrors isolated from vibrations smaller than the width of an atomic nucleus;
sensitivity improving one observing run at a time until esoteric tricks of quantum optics
pushed the fundamental limits of measurement. Thousands of scientists and engineers working
together for decades to measure distortions of spacetime of only about one part in 10²¹.
Virgo and then KAGRA joined the two LIGO detectors to improve our ability to localize
sources on the sky. An ecosystem of fast-response "traditional" observatories made ready to follow
up gravitational wave hits. So much work by so many people, even before the first detection—of
black holes colliding a billion light years away. But then something more interesting happened.
LIGO and its partners got to work. We’ve since watched spacetime wobbles many hundreds of times
and how have nearly confirmed 400 mergers of black holes, with a few neutron stars in the mix. We
quietly built something far more powerful than any one measurement of black hole mergers. We built a
catalog. And in that catalog lies a pattern. And in that pattern we can infer a universe.
An interesting thing about every successful observatory is that it eventually fades away.
And I mean that nobody asks anymore whether the James Webb Space Telescope can see distant
galaxies—we shifted to wondering at the galaxies themselves—and then wondering what this tells us
about structure formation in our universe. We marveled that the Large Hadron Collider
could make Higgs Bosons— but then we dug into measuring the particle's properties—then asked
what its existence tells us about the quantum structure of reality. The instrument disappears
and the universe takes center stage. The same thing has happened with
gravitational-wave astronomy. Signals that once would have been buried in the
noise became routine detections. The catalog grew—10 mergers, 50, 100—eventually there were
too many to think about one at a time. And I think that's the moment this field really
crossed a threshold. The “discoveries” were no longer individual events; the discovery
was the population of events. And it’s in this catalog that the real science happens.
There's actually a name for the transition that gravitational wave astronomy is going through. The
philosopher of science Thomas Kuhn argued that scientific revolutions eventually settle into
what he called normal science. It’s a slightly unfortunate name, because the most exciting
science happens in the “normal” phase. In the case of a revolutionary observational technique,
Kuhnian normality is the moment a new instrument becomes dependable enough that scientists stop
asking whether it works, and start using it to answer questions they couldn't even
ask before. That's where gravitational-wave astronomy is today. With a single detection,
we can say that gravitational waves are real and that black holes sometimes merge. And we
can measure things about the masses and the spins and their distance and whatnot.
But with a catalog of detections we can infer a huge hidden population of black
hole pairs out there that spend billions of years orbiting one another in complete
darkness. And from the spacetime ripples that wobble though the Earth from the
last fraction of a second of their inspiral, we can learn the properties of that whole
hidden population. We learned that many of these black holes are much heavier than
we thought likely. That taught us that there are unexpected growth mechanisms
after black holes initial formation for the collapse of massive stellar cores.
From the black hole spins we inferred some really
surprising things about the way these merging black holes
find each other.
A little while ago, when we were talking about the Dark Energy Survey, I said that reality
often hides in the relationships between data. That's what's happening here. The
universe isn't revealing itself one merger at a time. It's revealing itself through
the patterns those mergers form together. The inference chain is pretty incredible
if you think about it. Remember that LIGO and co never measure any of the stuff we figure out. No
ancient binary progenitor stars for example, not even the binary black holes themselves.
We measure a miniscule ripple in spacetime lasting a fraction of a second. We get to binary
black holes by asking “what sort of phenomenon could have produced this?” And once we build the
catalog—once we know the population properties—we ask a different question: “What sort of universe
could have given rise to such a population?” The ripple remembers the merger and the merger
remembers the parent stars. And with many ripples we can infer all the way back to the universe that
produced the stars that produced the merging black holes and the faint signal that LIGO catches.
All of astronomy works this way. It’s a particularly awesome example of “reverse
inference”, where we’re only given the after-the-fact traces and need to infer
the processes that led to them. Like a detective figuring out a crime from a room full of subtle
clues. And astronomy is especially dependent on reverse inference because there’s no re-running
the experiment. Other fields have the luxury of regrowing the culture in a petri dish or
re-smashing protons in the collider. Astronomy gets a handful of ancient photons, neutrinos,
gravitational waves, and has to figure out the universe that created them. But before
we can do that, we need this intermediate step of the catalog. Of the population. Of
what our universe normally makes. The work then of normal science is to understand what
is normal. So maybe it’s a good name after all. And the reason we care so much about the ordinary
is that it allows us to prove ourselves wrong. Attempting to falsify our models—to disprove
our ideas about the universe is the highest priority of science. And so a hallmark of a
mature field of science is that it’s no longer trying to construct the basic explanations—it’s
trying to break them. Instead of asking “can I explain this?”, a mature science asks "Does my
explanation still survive the next observation?" Let’s take the example of the correlations between
the spins of merging black holes. If we assume that all merging black holes come
from massive binary stars that formed from the same cloud of interstellar dust then we
have a prediction. The spins of those black holes should both carry the signature of the
original rotation of the birth cloud. The binary black holes should have correlated spin. But when
we look at hundreds of black hole mergers, we find that indeed many have correlated spin,
indicating formation by the expected process. But there’s a second population in which the
spins of the merging pair are uncorrelated with each other—pointing in random directions. That
falsifies the hypothesis that all mergers come from binary stars. We now have a second
population of black hole pairs that came together after their progenitor stars died. In that case,
they probably found each other after falling towards the centers of dense star clusters.
No individual merger points confidently to one formation scenario or the other, but
every detection nudges the weight of evidence in a different direction. After hundreds of events,
we start to be more confident that, across the universe and back through its history,
both of these processes played out. And that we live in a universe in which both are possible.
The growing catalog also hints that many black holes are a bit too massive and spinning a bit
too rapidly for either of the above scenarios, allowing us to motivate another formation
scenario. Some black holes probably sink into the dense gas disks surrounding active
supermassive black holes in galaxy centers. There, the gas can shepherd them together,
allowing repeated mergers that naturally produce unusually massive, rapidly rotating black holes.
This is still just a hypothesis—but one that's increasingly consistent with the
growing population of LIGO detections. But again, the evidence is only in the
population, not in any one observation. This is why gravitational wave astronomy
is rapidly evolving into a mature science, The individual discoveries are less important than the
broader, statistical picture. The universe leaves the most telling traces of its past in statistics,
not in objects. A mature science interrogates those statistics to interrogate its models
of the universe that might explain them. And to eliminate the models that fail. A mature science
is in the business of eliminating explanations until, gradually, the picture that remains starts
looking less like one possible universe and more like the universe we actually live in.
Which brings us back to that one curious signal of the impossibly light black hole. We don't directly
measure the masses of each black hole—instead we measure a particular combination of the two
masses—the chirp mass. So on November 12th last year, both LIGO detectors and Virgo quivered under
an exceptionally faint spacetime ripple that suggests an extraordinarily low chirp mass.
If confirmed, it’s very likely that at least one of the objects has less than the mass of the Sun.
Such objects should be impossible by the established modes of black hole formation.
A dense stellar core of one solar mass does not collapse into a black hole—it forms a white dwarf.
But above the Chandrasekhar mass of 1.4 solar masses it becomes a neutron star,
and only above 3 or so times the sun’s mass can a dead stellar core collapse into a black
hole. Now white dwarfs just aren’t compact enough to produce a LIGO signal on merger,
but neutron stars and black holes simply can’t form at such a low mass—at least not
via stellar evolution. But why did the field have to be mature in order to spot this thing?
Well imagine that the very first LIGO signal had indicated a sub-solar mass black hole merger.
What would we have concluded? Well probably that our instrument or our analysis method was flawed.
We would have spent months or years tweaking and recalibrating and hoping to catch another.
Instead, this apparently-tiny black hole merger came a decade in, when we know that
LIGO reliably finds and measures appropriately large masses for its black hole mergers.
We know the instrument and analysis works. So now, rather than stressing that we messed
up somehow, we get to move forward with science. LIGO has told us what is ordinary—and black
hole pairs of 3 to 50 solar masses are routine. Those are consistent with black holes formed in
the collapse of the dead cores of massive stars. But a sub-solar mass black hole is NOT consistent
with this picture. If confirmed, it will falsify the hypothesis that all merging black holes come
from stars. We’d need yet another formation channel. If this thing is real, the best
explanation seems to be that those black holes are primordial. That they formed in the unimaginably
dense conditions of the early universe, before the first stars had even begun to shine. Finding
primordial black holes with LIGO would allow us to extend our reverse inference process all
the way back to the moments after the Big Bang. That's an extraordinary possibility, and so also
requires extraordinary evidence. More detailed analysis is needed to ensure it isn’t a fluke of
detector noise and that the inferred masses are correct. This candidate may still disappear or
never rise to the level of a confident detection. That sounds disappointing, but it means the
process of this now-mature science is working. Of course, if the sub-solar masses prove to
be real then we have another mini-scientific revolution. It starts with one detection—the
first primordial black hole. Then we find another, and another, and build the catalog and measure the
population. From things like the number density and the mass distribution we’ll learn how and when
these PBHs formed, just as we did with the regular black hole mergers. In the case of PBHs, we’d
learn whether these things can explain dark matter and derive powerful constraints on the conditions
of the extremely early universe. Finding multiple primordial black holes with LIGO would allow us to
extend our reverse inference process all the way back to the moments after the Big Bang,
and PBH detection would become normal science.. Gravitational wave astronomy is right now
transitioning into a mature field, at least with respect to “normal” black hole mergers.
Be excited when it uncovers extraordinary new things, like this possible PBH. But be satisfied
when it doesn’t, because this meticulous mapping of the normal is the real work,
and the key to an iteratively better understanding of the whole of space time.