The primary characteristic that defines  black holes is in the name.

Black holes   are black.

The gravitational pull at the  event horizon is so powerful that not even   light can escape.

In this case, black  means absence of light.

But we also think   of black as indicating absence of colour.

But it turns out there is a way to make a   coloured black hole—as long as by colour  you mean quantum chromodynamic charge.

There are two main types of black holes that  we definitely know exist.

There are the stellar   black holes that might be 10-50 times the  mass of the Sun.

These are the ones left   behind when the most massive stars end their  lives in supernova explosions— and these things are   physically tiny on astronomical scales, with  event horizons only tens of kilometers across.

Then we have the supermassive black holes in the  centers of galaxies, weighing in at millions to   billions of times the Sun’s mass with radii on  the scale of solar systems.

Mass seems to be the   most important property distinguishing one black  hole from another because it determines the size   of their event horizon.

The rotation of a black  hole—its spin—is the next most important because   it squishes that otherwise spherical event horizon  and drags the fabric of space around a black hole.

But according to the no-hair theorem of  Jacob Bekenstein, black holes can   have only three properties and only three—mass, spin, and charge.

Now we covered mass and spin,   so what about charge?

Normally when we  think about black hole charge we think   about electric charge—the stuff that electrons  and protons have and that shocks you when you   drag your feet on the carpet then touch  a doorknob.

Black holes have a hard time   holding onto electric charge because they quickly  attract the opposite charge from matter in the   surrounding universe and neutralize.

They are in  contact with the cosmic doorknob, if you like.

But the charge in the no-hair theorem  doesn’t just apply to the positive and   negative electric charge that feels the  electromagnetic force.

It refers to any   of the charges corresponding to one  of the fundamental quantum forces.

So   could there be black holes that hold  onto these weirder types of charge?

The most likely candidate is for  a black hole to be charged with   the so-called colour charge of the strong  force.

We have episodes on colour charge,   the strong force, and the  theory describing it—quantum   chromodynamics.

But in short, the strong force  holds quarks together in protons and neutrons,   and holds those together in atomic nuclei.

There is one type of charge associated with   the electromagnetic interaction - electric charge,  which can be positive or negative.

But there are   three types of charge for the strong force.

They’re named after colours: red, green, blue,   with their “negatives” being anti-red, anti-green  and anti-blue.

They aren’t actually these colours,   but the naming is not an entirely a poetic choice.

There’s a deeper correspondence to how visible   colours and strong-force colours are related  to each other, which we explain in this video.

OK, so the no-hair theorem tells us that black  holes can exhibit any of the charges of the   fundamental forces, so we should be able  to have a red or an anti-blue black hole,   from a quantum chromodynamic perspective.

But could these really exist out there in   the universe?

And if so, might they have some  real influence on the universe that we could   detect?

According to a study published this  May by PhD student Elba Alonso Monsalve and   professor David I. Kaiser at MIT.

, the answer  is yes and yes.

Or at least maybe and maybe.

The reason you probably haven’t heard about  colour-charged black holes before is because   at first glance they shouldn’t exist.

I mentioned that electrically charged   black holes aren’t really a thing because  they neutralize themselves.

Colour-charged   black holes should be even harder to make.

That’s  because of something called colour confinement.

In almost all environments in the modern universe  it's impossible to observe an isolated color   charge, like a single quark.

All color charges  need to coalesce together to form color-neutral   objects, particles called hadrons.

So you’ll  never see a single green quark—you’ll see a green,   a red and a blue quark bound together into a  proton or neutron.

Or maybe a red and anti-red   quark in a meson.

The colour combination  of these quark composites, or hadrons,   will cancel out to zero colour charge.

As  real black holes grow from consuming these   bound particles, they never have a  chance to build up colour charge.

But that doesn’t mean it’s impossible to  separate colour charge.

Colour confinement   works at temperatures below 10 trillion Kelvin.

But above that temperature, individual quarks   and gluons are no longer bound and can fly around  freely in a quark-gluon plasma.

In such a plasma   it’s possible to have little patches of more  green or anti-red or whatever.

If we could   create a black hole within such a patch we could  presumably create a colour-charged black hole.

And, as it happens, there was a time when the  universe was filled with a quark-gluon plasma,   and by chance we expect that in that time  many, many black holes may have formed.

It   may stand to reason that colourful black holes  were once the most natural thing in the world.

So, right after the big bang, the universe  consisted of an extremely hot and dense soup of   matter and radiation.

It was also very slightly  clumpy—there was a little more matter here,   a little less there—fluctuations that  pulled on surrounding material due to   their slightly higher gravity, causing them to  eventually grow into the bumps of the cosmic   background radiation, and eventually into  galaxies.

But here and there the size of   the density fluctuations were large enough to  grow even more quickly.

Quickly enough to form   black holes.

These first black holes are called  primordial black holes.

We’ve never found one,   but they could be everywhere.

Of  course we did episodes on PBHs.

Because these things didn’t form from the cores  of dead stars, PBHs could potentially have any   mass.

From as small as a millionth of a gram to as big as  hundreds of thousands times the sun's mass.

If they   formed at the massive end they may have been  the first seeds of supermassive black holes   and may help explaining how those things got  so big so fast.

If at the lighter end they may   be a good candidate for dark matter.

The mass  these things formed at depends on exactly when   they formed in the first second-ish after  the big bang.

If early then there should   be lots of smaller PBHs.

If later then  there should be fewer but larger ones.

OK, so in the extreme conditions of that  first second the universe may have made   countless black holes.

If that black hole  generation happened on the early side—in   the first one-100,000th of a second—then we have  the conditions we need for colorful black holes.

Before 10^-5 seconds the temperature of the  universe everywhere was higher than 10^13 Kelvin,   which is the temperature at which bound  states of quarks can’t exist.

So back then the   entire universe was filled with free quarks and  gluons—what we call a quark-gluon plasma.

Although   the net colour charge of this plasma was zero,  if you zoom in close enough on little patches   you would find regions that are randomly a bit  more of one colour than the others.

And if such   regions were dense enough to collapse into a PBH,  that would give us our colour-charged black hole.

The scientists behind this new study  wanted to figure out whether these   colour-charged PBHs might exist, and  in what quantities.

To determine that   they first needed to know when these things  formed—remember, it’s the formation time that   determines the size of the PBHs, and the  size of the PBHs determines whether they   are small enough to capture one of these tiny  colour-charged regions of the quark-gluon plasma.

These researchers use another assumption  to choose that formation time.

I mentioned   that some people think PBHs could  explain dark matter.

But that only   works for a narrow range of  primordial black hole masses,   because our observations have ruled out there  being lots of black holes outside this range.

That range is roughly between that of a medium  asteroid and a small moon, or 10^14-10^19 kg.

If   PBHs are dark matter then they have to be in this mass range, and they must have formed between 10^-21   and 10^-16 seconds after the big bang, and this  is the formation range the researchers consider.

The universe at that age is well  within the quark-gluon plasma period,   so PBHs formed then could potentially have  colour charge if they were small enough to   land in one of these random fluctuations of  colour.

Unfortunately, most of the PBHs that   would have formed at this time, if they formed  at this time, would be too big to capture any   considerable amount of net color charge.

Such PBHs would have formed color neutral.

But there’s still hope for coloured black  holes.

One of the main scenarios for PBH   formation does not form all of the black holes  with the same mass.

Rather there’s a peak mass,   and that’s the mass that would contribute  the most to dark matter.

But there should   also be a long tail of smaller and smaller  black hole masses that form alongside these bigger ones.

Some of these would actually be small enough  to capture significant colour charge.

In the   scenarios explored by the researchers an enormous  number of tiny, colorful PBHs should have been   formed alongside the larger, neutral PBHs  that would go on to become dark matter.

In order to capture significant colour  charge, a PBH would need to have a mass   smaller than around 20 tons, which would make  its event horizon ten thousand times smaller   than a proton.

But these things could hold  a lot of colour change—more than any other   colored object previously studied.

In fact,  a subset of these color charged black holes would   form with near maximum possible amount of color  charge that black holes can theoretically have:   these are called extremal, or near-extremal  black holes, in which the internal colour charge   counteracts the internal gravitational field to  the point of almost nullifying the event horizon.

Okay, we do have one more problem before we can start building our coloured black hole   detectors.

All black holes leak away their  mass over time due to Hawking radiation.

The minimum mass black hole that could still  be around today is around 100 billion kg.

The   smaller the black hole, the faster they leak.

Any  black hole small enough to have significant colour   charge would be long gone.

In fact, a 20 ton black  hole evaporates in one-ten-thousandth of a second.

That doesn’t sound like very long, but compared to  the rapidly changing state of the early universe,   this was an aeon.

It’s something like 100  trillion times longer than the age of the   universe at which those PBHs formed.

In  fact, that 10^-4 second evaporation time   would allow those coloured PBHs to exist well  beyond the end of the quark-gluon plasma epoch   and enter into the era of color confinement  when hadrons start to form.

That potentially   makes them the only colour-charged  objects in the universe at that time.

But do they keep that colour charge?

I mentioned  that electrically charged black holes quickly lose   that charge.

But for those early colour-charged  PBHs we have reason to hope that they maintain   their charge.

Remember that these things are  tiny—less than one ten thousandth the size of   a proton.

That means they absorb material  very slowly.

Also, once the colored black   hole becomes surrounded by color neutral  hadrons after the quark-gluon epoch is over,   a cloud of virtual colored particles could form  around it from the quantum vacuum and provide   screening, and make it behave like an almost  color neutral hadron.

For one thing it would   stop attracting opposite charges, allowing it  to maintain its own internal charge for longer.

Also, this screening process could have left an  imprint in the form of gravitational waves:   although these would probably be  too faint and high-frequency to   detect with current detectors.

Some of  these black holes could have also lasted   much longer into the era of big bang  nucleosynthesis, the next chapter in   the history of the universe when the first  sight nuclei began forming from protons and neutrons.

The surviving colored black   holes would be the near-extremal ones, the ones  with most color, which happen to have the lowest   Hawking temperature and thus evaporate slower.

These might have changed the distribution of   hadrons and disturbed the formation of nuclei  in such a way that we may be able to find a   discrepancy in the ratio of the numbers of light  elements generated in that period.

This would be   the first hint and evidence of this kind that  points to any of this having actually happened.

There is one possible way that colour-charged  black holes might exist in the modern universe.

Some physicists believe that black  holes don’t ever fully evaporate,   but instead stop when their size reaches  the smallest definable length—the   Planck length.

These are called Planck relics.

Primordial black holes that started out smaller   than a trillion kg would now be Planck relics,  and if there are enough of them they could also   make up dark matter.

And, another in a chain of  if’s—if everything I just described is the case,   then these still-existent Planck relics could  potentially have colour charge.

That would be   great, because there’s almost no way to detect  neutral Planck relics.

Colour charge may give   them an interaction with atomic nuclei,  which means we could build a detector.

As cool as all of this sounds, are these  colored black holes actually thing?

We   don’t have direct observational access to that  chaotic first fraction of a second.

However,   the theoretical basis for their formation  is solid, and if this is right, it could lead to real,   observable effects that help us refine our  understanding of the early universe.

They   may even help solve the problem  of dark matter.

We just need the   universe to have started out with a sprinkling of multicoloured holes in its fabric of spacetime.