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Dark matter has eluded us for many decades. Even our most advanced particle colliders
and sophisticated underground detectors have come up short. But it may be that
we can finally solve this mystery with a much simpler experiment,
involving a ray of light, a good clock, and the planet Mars.
Over 80% of the mass in the universe is in the form of some mysterious, invisible, unseeable
substance that, for want of any idea whatsoever what it is, we call dark matter. At first,
many of us thought that it was just some sort of regular stuff that wasn’t easy to see—maybe
lots of small, dense, non-shining bodies like brown dwarfs or neutron stars or even black
holes. But as we ruled out most of the possible mass ranges for these, more and more of the
dark matter hunting has been about finding some kind of exotic particle—Neutrinos, axions, etc.
But, again, after years and years of looking for the dark matter particle,
we’re in the same place with this hypothesis—there’s just no credible
evidence in favor of any hypothesis for what dark matter really is.
It’s like when you look for your keys in the usual spots, don’t find them so check under the couch,
under the cat, inside the refrigerator. But when you still don’t find them you
double check your pockets. Maybe it was in one of the obvious places all along.
And there are indeed a couple of pockets still to check, where “pockets” means range of masses for
compact bodies. The most promising under-explored range is 10^17 to 10^23 g, roughly the mass range
of asteroids. Now, dark matter can’t be asteroids because there’s no conceivable way to make 5 times
the mass in asteroids than there is in stars—after all, asteroid material is made inside stars. There
aren’t that many stars, and also dark matter had to be around before the first stars.
But there is an option for a truly invisible object in that mass range, and one that would
have been around since near the beginning and that's the primordial black hole (PBH).
These days, black holes are formed in the deaths of massive stars and their masses
start at around 3 times the Sun’s mass. But, as we’ve seen before on this show, there is a
way to produce asteroid-mass black holes, and that’s right after the Big Bang. Back then,
there were tiny variations in the density of the hyper-dense sea of matter and energy that filled
the universe. Some of these would later collapse under gravity into galaxies and galaxy clusters,
but right at the beginning the most dense such regions could have collapsed directly into black
holes. These primordial black holes could have a range of masses depending on yet-unknown details
about the very early universe. But if we tune our Big Bang parameters just right,
we can get enough PHBs in the right mass range to account for all of dark matter.
Not only have we talked about PBHs before, we’ve talked about PBHs as dark matter,
AND we’ve talked about the possibility of detecting PBHs from the scars they might
leave as they pass through the Earth, or even the Moon. But such a detection is going to take quite
a bit of luck—the Earth and Moon make pretty small targets on the scale of the solar system,
and distinguishing the scars from old PBH impacts isn’t necessarily so easy.
But it turns out that we may not even need PBHs to hit planets in order to
detect them. Recent studies have shown that our entire solar system may serve
as a gigantic PBH detector, with these little black holes leaving their faint
but very possibly detectable gravitational signature on the orbits of the planets.
Let’s run some numbers to see if this can really make this
work. And by “we” I mean the scientists who are actively figuring this out as we speak.
One paper I’ll focus on in particular Tung Tran, Sarah Geller, Benjamin Lehmann and
David Kaiser and it was published just this September and I think does the best job at
assessing the plausibility of turning the solar system into a black hole detector.
Let’s start with dark matter. Based on the orbits of stars in the Milky Way, we have a pretty good
idea of the distribution of dark matter in our galaxy. The density of dark matter at our solar
system’s position in the galaxy is about 7x10^-25 g/cm^-3 or about the equivalent of the mass of a
proton in every sugar cube of volume. So, if dark matter is a proton-mass particle, something like
10 million of them pass through you every second. Experiments trying to detect dark matter particles
assume that they stream through our detectors in enormous numbers, so that even if there’s a very
low probability of interaction, we’ll eventually catch some. Although apparently not yet.
The more massive the dark matter particle,
the lower the number density. If dark matter is a compact object like a black hole,
then they’d be pretty spread out. For example, if all dark matter consists of Sun-mass black holes,
there’d need to be one per 15 cubic light year volume to do the job.
So what mass range might these PBHs have if they really do account for all of dark matter?
Astronomer’s favourite way to constrain has been with gravitational lensing. Although black holes
are invisible, their gravitational fields warp the paths of light rays passing close by. If a
black hole passes between us an a more distant star, that star will briefly brighten as its
light gets focused towards us in an effect called microlensing. The OGLE survey recently published
new results from their decades-long effort to count compact objects in the Magellanic clouds
this way. They rule out huge mass ranges of primordial black holes as dark matter,
everything from half the mass of the moon up to about 1% of the mass of the sun, roughly
the boundary between brown dwarfs and proper stars- that’s a factor of a million in mass.
And in fact the only remaining mass window that’s still wide open to potentially account for all
of dark matter is the so-called “asteroid mass range”, from around 10^17 grams to 10^23 grams, or
less than 0.1% of the moon's mass. If such small black holes make up all dark matter we’re going
to need a lot of them. If PBHs are at the bottom of range—10^17 grams—then we’d expect there to be
one somewhere in the inner solar system right now. At the upper end of this range of asteroid-mass
black holes, we might expect one to pass through the solar system every century. In between these
extremes we have them passing through the solar system on timescales of months to decades.
And that is frequent enough that we should think about how to detect one when it comes.
So we’re not going to see a primordial black hole with a telescope. In the asteroid mass range,
our PBHs have event horizons that range from the subatomic to the microscopic. So even if they were
visible they wouldn’t be visible. Instead, we’d spot a PBH passing through our solar system in
the same way we discovered dark matter in the first place—from its gravitational influence.
When an interstellar object enters the solar system it almost always
exits again—the speed it gathers falling into the Sun’s gravitational field is,
by definition, exactly what it needs to escape. Add to this the fact that
an interstellar object is on a different orbit to the solar system, so it’s already
coming in hot—with some appreciable fraction of the galactic orbital speed of 220 km/s.
The result is that a black hole fly-through is going to be pretty quick, with a passage
through the inner solar system lasting a few months at most. If this is the sort of black
hole produced by the death of a star—a stellar black hole—then its effect on the planetary
system would be devastating. Orbits may be significantly altered, planets ejected,
and the black hole may even capture planets into its own orbit. However,
the effect of an asteroid-mass primordial black hole would be no more significant than an actual
interstellar asteroid passing through the solar system—which means almost no effect.
Planets are many millions to trillions of times more massive than any such PBH,
and so the acceleration a planet experiences due to that passage will be commensurately tiny.
But tiny isn’t zero. Think about it this way: imagine you and another car are side by side on
the highway both going exactly 100.00 km/hr. Your cruise control glitches and you speed up to 100.01
km/hr. You don't notice that right away, but an hour later your car will be 10 meters ahead,
that’s an entire car length, which you’ll definitely notice. In the same way, a tiny,
parts-per-trillion change in a planet’s position will be visible over years.
Let’s say a PBH in the upper-middlish of our asteroid-mass range—10^21 grams—passes
through our solar system a little way outside the orbit of Mars. The red planet’s change in
speed would be pretty much unmeasurable at the time. But after a decade it would add
up to a difference in position of about 1m compared to the expected location.
That also sounds pretty hard to measure, given the 10s of millions of km distance
between Earth and Mars. But there are two factors that make this actually possible.
The first is that the motion of the planets is predictable. With clockwork precision,
the planets are slaves to the law of gravity—whether Newton’s or Einstein’s.
If we know the positions and velocities of a system of gravitating bodies at one point in time,
we can trace their motion far into the future. Neptune was discovered based on a
small discrepancy in the motion of Uranus, and that was calculated with pen and paper back in
the middle 1800s. Modern computers can compute the future motion of the planets
with incredible precision, although including the gravitational influence of the many smaller
residents of the solar system starts to make it complicated, and I’ll come back to that.
Knowing precisely where a planet is, even down to the centimeter, isn’t helpful if we can’t actually
measure the distance to the planet with that same precision. But fortunately, that’s also within
humanity’s skill set. We don’t measure distances to planets directly. Instead, we measure the
travel time of light. Conveniently, the speed of light in the near vacuum of interplanetary space
is constant and extremely well known—so timing its round trip from Earth and back gives us a
distance that’s as precise as the stopwatch we use to time that trip. And we do have some
exceedingly precise atomic stopwatches these days. One of the most precisely measured distances in
the solar system is the Earth-Moon separation. It’s known to about 1 mm
precision thanks to the retroreflectors—fancy mirrors—placed on the lunar surface by the US,
the Soviets, and now India. The precision of our measurement of the Earth-Moon distance
makes the moon sound like a good option for a good black hole detector. But it’s actually not our
best option. The fact that the moon is so close means we can't really treat it as a single point.
But the Earth and Moon are big, slightly irregular
balls that squish and stretch in each others gravity. Although we can measure
the distance to those retroreflectors with amazing
precision it's hard to convert that to a well defined distance to the moon.
The trick then is to look farther, to Mars.
While Mars doesn’t yet have a retroreflector big enough this, we have something that may
be even better--our own satellites orbiting Mars. At any one time over the past 20 years,
we have had at least three different functioning spacecraft in Martian orbit. It's straightforward
enough to get an exact distance to any one of these based on the signal travel time, and the
satellites can use complex triangulation to get their own altitude above the Martian surface.
By collecting these distances over time and for multiple spacecraft, we can now define and keep
track of a robust "distance to Mars", and have been able to do so for 20 years. We can likely
spot the 1 meter offset caused by the 10^21g PBH we saw earlier, and perhaps even the 10cm-ish
offset caused by a PBH 10 times smaller. That covers half of the possible range for PBH masses.
OK, so apparently we can both predict and measure the location of Mars with
the precision needed to tell if it strayed by a foot or two from where
it was supposed to be. How do we know that was caused by a passing black hole?
The solar system is a busy place, packed with planets, moons, dwarf planets,
asteroids, and comets. The mini-tweaks that the Earth and Mars might get from a PBH is
one of millions, all of which must be accounted for if the signal from the black hole is to be
separated from the noise. In fact, any asteroid in the inner solar system with a mass comparable
to our PBH would need to be included in calculations, otherwise its gravitational
effects will completely wash out the effect we’re searching for. The good news is that, while 10^20
grams or so makes a black hole as small as an atom, it makes an asteroid of that mass
about a 100 km across, and all asteroids that big are all pretty well tracked.
Also, a PBH flyby will have a very different trajectory to a solar system asteroid. Objects
in the solar system orbit pretty close to the same plane that the planets orbit in,
and all at speeds low enough to keep them bound in the Sun’s gravitational
field. A passing PBH will be moving quite a bit faster, and could come in at any angle relative
to the plane of the solar system. In-plane flybys will tend to tweak a planet’s speed,
but keep it in the same plane. Steeper flybys will tend to tug a planet slightly out of that plane,
altering its orbital inclination. Because of this, the change in a planet’s position due
to an extrasolar object versus a solar system object may be quite distinct.
Let’s talk about the two experiments we can do. One we can do right now,
and one is for the future.
We currently have around 20 years of precision data on the position of Mars,
and in principle we could find the signs of past PBH passes in these data. Some preliminary efforts
have been made to analyze this, but not with conclusive results because it’s going to require
some very sophisticated simulations to analyze the data properly. The proper way to do this
is by running many, many computer simulations to map the space of possible ways in which the
orbit of Mars could evolve in response to the gravitational influence of known and unknown
solar system objects. That will tell us whether, at some time in the past 30 years,
Mars strayed from this space of probable orbits in a way consistent with an interaction with
a massive flyby. Then we add potential extrasolar visitors to our simulation and
see how well Mars’ trajectory is explained by different flyby masses and trajectories.
There’s no direct way to tell if such a flyby was really a PBH or just an extra-solar
asteroid of the same mass. If we find it in this old data then by now it’ll be long gone,
so we can’t look for it with our telescopes. We do know that there are such asteroids living between
the stars—for example, Oumuamua, although it’s way below our desired mass range. However, if
dark matter is made of primordial black holes at these masses then those PBHs need to be way, way
more abundant that extra-solar asteroids. So if we find way too many such kicks to the Martian orbit
in those 20 years of data, it’s good evidence that they’re coming from PHBs and not asteroids.
OK, so that’s the experiment we can do now, and astronomers are indeed working on this. But
what we’d really want is to get more direct evidence. Monitoring future data from Mars
can allow us to notice a change in its motion within a few years if we’re lucky,
and then it might be possible to track the offending source of gravity as it coasts away from
the solar system. If it’s an asteroid then it’ll be visible to a sufficiently giant telescope. If
it’s a black hole it will not be. So maybe we’ll discover what dark matter is by observing nothing.
It may be that to find the next level of action in particle physics we'll need to
build a particle collider the size of the solar system. Well, we already have a solar system
sized particle detector that could discover dark matter. Assuming dark matter is made
of countless subatomic-sized black holes with the masses of asteroids. In which case we can
solve dark matter from miniscule shifts they cause in planetary paths through spacetime.
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On February 18th, 1975, a patent for the future was born. Patent US3866863A became the blueprint
for NASA’s Space Shuttle—the first reusable spacecraft. Over 30 years and 135 missions,
it shaped modern spaceflight. Now, 50 years later, we’re celebrating this legacy with
special edition PBS Space Time merch. Featuring the original patent blueprint,
we’re launching the first Space Time Hardcover Notebook in black, white, blue,
and red—perfect for tracking your 2025 ideas, goals, or even your own future patents. We’re
also re-releasing the Space Time multifunctional pen in black, white, silver, red, and blue,
plus a special edition hoodie and t-shirt in multiple colors with the patent number on the
sleeve. And by popular demand, all Space Time t-shirts—including past designs—will now be
available in a female cut. Check it all out at pbsspacetime.com/shop—link in the description.