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Neutron stars aren't dark matter--we figured that out a while ago. But new
research is telling us that they may be dark matter factories. They may
produce the exotic axion, one of the most popular dark matter candidates.
To understand reality we need to understand its building blocks. The
standard model of particle physics does a great job of describing all
known particles—the three generations of quarks and leptons plus the bosons that
carry the four fundamental forces plus the mass-granting Higgs particle. But there are
a few holes in the standard model that we need to fill if we want the full picture.
One of those holes is the so-called strong CP problem. It’s been called one of the most
underrated problem in all of physics. Probably because it’s such an abstract problem that it’s
really hard to describe in non-math terms. We gave it a shot in this video—but the long-story-short
is that the strong force is expected to violate charge-parity symmetry—flipping charge and then
doing a mirror reflection of, say, a proton or neutron—should not get you back where you
started—but in quantum chromodynamics it seems to do exactly that. The most popular way to fix the
problem is to introduce a new quantum field that eliminates the CP-violating terms in QCD. And with
every new quantum field you get new particles—and in this case the particle is called the axion.
Axions are especially popular because they’re a great dark matter candidate—and
that’s because they’re extremely elusive and they interact electromagnetic field
only very weakly. We discuss the prospects of axionic dark matter in that same previous vid.
But axions can only be dark matter if they make up around 80% of the mass of the universe,
and that’s only possible if they were produced in pretty ridiculous numbers right after the big
bang. Even if these things don’t solve the dark matter problem, we really want to know if they
exist because of the less famous but still vexing strong CP problem. Oh, and axions are a pretty
generic prediction of string theory, so if you want the latter you probably also want the former.
Now we’ve trying looking for axions in various ways, but it’s tricky because of that very weak
electromagnetic interaction. But axions do have an unusual EM interaction that might give them away.
Axions sometimes do this thing where they convert into photons,
and photons sometimes convert into axions—in both cases in the presence of strong magnetic
fields. This could have a number of effects, like allowing us to shine a light through a
solid wall by using magnets to flip photons into axions and back again.
Various experiments to detect this phenomenon haven’t found anything,
but that could be because we can’t generate magnetic fields strong enough. We expect the
rate of conversion between axions and photons to be very sensitive to magnetic field strength—and
more magnetic field is better. But we don’t know the lower limit of that field strength to get
a detectable signal. The strength of the fields used in modern axion detection experiments maxes
out at about 10 Tesla, and the current world record magnetic field is 45 Tesla—not bad,
but piddling compared to what certain space magnets are able to generate. Take magnetars—a
type of neutron star that has cranked its magnetic fields up to the vicinity of 10 billion Tesla.
If axions were not produced prodigiously in the Big Bang, neutron stars may be our universe’s most
efficient axion factories. But that’s only true if they meet certain fairly specific conditions.
Let’s start with the conditions needed to produce axions. First is the strong magnetic
field—in certain neutron stars we definitely have that—it’s why we chose them in the first
place. Second is a prodigious source of photons to convert into axions. Actually
the requirement is a bit more stringent than that. Axions can be produced when we
have an time-varying electric field that’s parallel to a magnetic field. Now a photon
is a single quantum of oscillating electric field, with a perpendicular magnetic field
that moves with it. If you send photons into an external magnetic field, in some cases the
photon’s electric and the external magnetic fields line up and you should produce axions.
If we want a LOT of axions we need a lot of photons. The most promising way to do
that with neutron stars is with a large-scale electric field those stars can generate rather
than individual photons. Early neutron star models suggested that a very strong electric
field should be generated due to the rotation of the neutron star relative to its magnetic
field. This is related to the effect in a standard electrical generator, where
rotation of conducting material in a magnetic field generates current. In a neutron star, a
component of the electric field generated this way should be in the direction as the magnetic field.
Now, we’ll come back to the requirement that this electric field be time-varying.
First we should take a closer look at this requirement for parallel electric
and magnetic field. Why? Because nature hates that. Nature does its very best to
resist the electric field becoming parallel to the magnetic—which means nature seems to
want to shut down axion production. And it does this in a couple of ways.
First, this electric field we just generated tends
to rip electric charges from the surface of the neutron star, upon which those changes
rearrange themselves to neutralize the parallel component of the electric field.
‘ Secondly, when the strength of electric field gets
high enough, we expect a phenomenon similar to the formation of lightning. Under extreme voltages,
insulating materials can suddenly become conductors in a process known as electrical
breakdown. When charge builds up in storm clouds, electrical breakdown in the air results in the
flow of massive currents—aka lightning. Well, the vacuum is a pretty good insulator, but it can
also break down until extreme voltates—at least theoretically. The result of this vacuum breakdown
is the production of many pairs of electrons and positrons. And in a neutron star, these
would work to short out the electric field, and so could potentially put an end to axion production.
Now nature may want to shut down our axion factory, but neutron stars are so extreme
they may be able to manage it anyway. The region of extreme magnetic field around the
neutron star is called magnetosphere. There the super-intense magnetic field
co-rotate with the star—up to several hundred times per second in some cases.
The further we get from the neutron star, the faster the magnetic field lines have to sweep
through space in order to keep up with the rapidly spinning surface of the star. At a certain point,
those magnetic field lines would have to move faster than the speed of light to keep up,
and so they don’t. The field lines lag behind the surface and the field lines in the magnetosphere,
resulting in these helical magnetic fields trailing off into space,
all of which arise from regions around the neutron star’s magnetic poles—the polar caps.
Magnetic fields that twist like this will induce the flow of currents—that’s Ampere’s
law. So charged particles get lifted from the magnetosphere at the polar cap region
and travel along these helical field lines and are sprayed into space. Now remember that
an excess of charged particles can shut down our electric field—whether they are
particles lifted from the surface, or generated in the vacuum breakdown. So now we have a way to
drain those particles into space, which means we make be able to rescue our axion factory.
So, do we get axions or don’t we? Well, bringing together everything I described,
we end up with a hellishly complex process that is very difficult to fully explore with
just pen and paper. The only way to properly figure this out is to build neutron stars in
our computers with something called plasma particle-in-cell simulations.
And the answer seems to be that nature fails to thwart our axion production. In particular, the
vacuum breakdown is extremely bursty—an avalanche of pair production shorts the parallel electric
field temporarily—but then the field switches back on. This actually leads to exactly what you want
for axion production—a rapidly fluctuating electric field. Another really interesting
thing is that the electric field oscillations are associated with the emission of powerful
electromagnetic waves. And indeed neutron stars do emit such waves in the form of the intense radio
jets of pulsars. We’ve never fully understood the source of these jets, and so these new simulations
may do two things: explain the observed radio jets and give us our axion source.
However, once generated, the radio photons and axions follow markedly different paths.
Radio waves are strongly beamed—they swivel with the magnetic poles in narrow cones,
leading to the pulsations we observe when the beam sweeps across our line of sight. Axions,
by contrast, can shoot off in all direction. They can convert
back into radio waves in directions way off the standard pulsar beam.
This actually gets us to a prediction that might allow us to test for axion production. If axions
can lead to radio waves being launched in random directions, not just with the standard pulsar
beam, then we should expect neutron stars to glow over a wide spectrum of radio frequencies at all
times. And they don't. Some of the most sensitive radio telescopes in the world, including Arecibo
in Puerto Rico, Parkes in Australia, and the Green Bank Telescope in West Virginia have been
staring at pulsars for the last fifty years and those pulsars are pretty radio-dark besides the
regular blips of the radio beams. But this darkness has allowed scientists to set some
of the strongest constraints on axions to date, using observations from 27 nearby neutron stars.
Even if the axions produced by a neutron star aren’t immediately detectable through
conversion into radio waves, there is one more way we can spot them.
Around 90% of axions created by a neutron star are expected to escape into space—with
some of them being converted into photons on the way. But the other 10% are created
with velocities below the escape velocity of the neutron star, which means they fall
back and end up orbiting the star. And by the way, the escape velocity of a neutron star is
around half the speed of light, so all even the captured axions can be traveling fast.
These trapped axions build up in this increasingly dense cloud around the star
over thousands or millions of years, and that axion cloud might have a few observable affects.
1) They might explain another mysterious phenomenon called
nulling, where the pulsar’s radio emission is temporarily suppressed.
2) they also might result in bright radio emission—which I already told you we don’t see
in bright neutron stars. But the radio light from these captured axion clouds would have
a much narrower frequency range that may have rendered it undetectable in previous searches.
3) when a neutron star’s magnetic field eventually fades out over 10 to 100s of millions of years,
these axion clouds may evaporate in a burst of radio emission that may last millions of years,
or may last only a fraction of a second. If the latter,
models show that these would bear a striking resemblance to the mysterious Fast Radio
Bursts—distant and stupendously bright radio flashes have perplexed astronomers for years.
So there are a few radio signatures that might reveal this axion cloud,
and radio astronomers are already working to pin these down, while pulsar experts are trying to
model these processes in more sophisticated ways, to see if they really do match the observations,
for example of nulling phenomenon or fast radio bursts. And future radio facilities
like the Square Kilometer Array may reveal axionic radio signatures that are currently undetectable.
I should add here that if neutron stars do produce axions they don’t produce enough to
explain all of dark matter. We’d still need a lot of axions produced in the early universe
to achieve that. But if axions are even one component of dark matter, then having dark
matter factories in the modern universe could be pretty helpful for finding and studying the stuff.
To summarize—axions may or may not exist, but if they do then there’s a very good chance
that neutron stars pump them out in enormous quantities and that neutron stars are also
cloaked in clouds of these strange particles, and that axions may even explain pulsar radio
jets and fast radio bursts. That’s a lot of ifs, but if these ifs work out then the payoff
is huge—it’ll be a step beyond the standard model and perhaps will give us at least one
of the components of dark matter. Definitely worth keeping our eye on them neutron stars
to see what strange things are being formed in those tiny patches of curved spacetime.
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