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.