Did JWST Discover Dark Matter Stars? (Script) (Patreon)

We knew that the James Webb Space Telescope would find interesting stuff, especially about the mysterious early times.  For example, there are hints that the galaxies we’re seeing are brighter and more regular than expected given the short amount of time they’d had to grow. Well, perhaps no one was expecting that we’d find a completely new type of star—one mostly made of and powered by dark matter and shining as bright as an entire galaxy. Which, by the way, might help us explain those pesky giant galaxies.

In April, a paper was published in Nature Astronomy reporting on the observation of four objects - JADES-GS-z10-0 through JADES-GS-z13-0. We’ll call these z10 through z13 for short. Not very poetic, but space is so big we ran out of cool names decades ago.

Admittedly, they don’t look all that impressive. They’re basically just dots, but that’s true of anything too far away to properly resolve. So you’ll have to trust me and be impressed anyway. Why? Because these are among the oldest and most distant objects ever seen. Z13 and z12 actually are the current second and third place holders for the most distant objects whose distances are actually confirmed, with z11 in a respectable fifth place.

They were discovered as part of the JWST Advanced Deep Extragalactic Survey - or “JADES” - whose mission is to study the very first galaxies that formed in our universe. This is only possible with JWST because of its incredible sensitivity. It can see stars whose energetic ultraviolet and visible light has been stretched far into infrared wavelengths as it traveled to us through an expanding universe. Some of these things are over 30 billion light years away, meaning their light has been traveling to us for most of the age of the universe. We’re looking at the cosmos as it was only around 400  million years after the Big Bang, or around 3% or less of its current age.

Based on the brightness of these blobs, at first glance it seems they must be entire galaxies. But enormous ones, at some hundreds of millions of times the mass of the Sun. Such big galaxies are common enough in the modern universe, but no one expected to find galaxies so big back then, when they’d had so little time to grow.

This issue with the big early galaxies discovered by JWST has caused a lot of consternation and some pretty out-there speculation. That said, let’s at least entertain the hypothesis that we’re looking at something truly weird here. After all, these are the cosmic dark ages we’re peering into - a time when the ocean of pristine hydrogen forged in the Big Bang shrouded our vision across much of the electromagnetic spectrum. It’s a time when that same pristine hydrogen was able to form stars many thousands of times more massive than today. Who knows what else might be lurking there at the edge of our map? Perhaps, here be dragons. Or perhaps dark stars.

That’s the assertion of a new paper was published in the Proceedings of the National Academy of Sciences. That three of these four objects might be powered by dark matter. The original idea was proposed by Katherine Freese and collaborators back in 2007, but this is the first time we’ve seen candidate dark stars. I should point out that the term dark star was originally used by John Michell back in 1783 to describe a very early incarnation of the black hole. But these new dark stars aren’t that. These aren’t even dark. They’re super bright. But how can dark matter, which is meant to not interact with light at all, produce so much of the stuff?

Before we build a dark star, we’re going to need a particular type of dark matter. For example it doesn’t work if dark matter is made up of mini black holes or failed stars. It has to be some kind of new undiscovered particle, and even then there are a few rules the particle has to follow to be able to make a dark star.

First, it has to obey the main rule of all dark matter: it can’t interact strongly with itself. That means one dark matter particle can’t easily bounce off another one without getting super close. That enables dark matter to avoid collapsing easily under its own gravity, which is needed to explain how it remains as a giant puffy cloud surrounding nearly all galaxies. On the other hand, particles in a cloud of gas interact strongly with each other, generating a kind of internal friction that saps kinetic energy from the gas and allows the cloud to shrink to form stars. But with very little self-interaction, dark matter stays puffy. One type of strong interaction that’s doubly ruled out is the electromagnetic interaction. Dark matter can’t emit or absorb photons. We need that for dark matter to remain dark.

The second rule for making dark stars is that, while the dark matter particle can’t interact strongly with itself, it has to have at least some weak self-interaction. This interaction strength has to be tuned so that when the cloud is large it experiences almost no interaction due to the particles being so far apart. But if we can get those dark matter particles close enough together then they will interact. This allows the dark matter cloud to collapse, just like a gas cloud, but only under certain extreme circumstances. But more importantly, it allows it to annihilate.

And that’s the last requirement. For our dark matter particles to form dark stars they have to be able to annihilate each other. This could mean the particle is its own antiparticle, allowing any two particles to self-annihilate if they get too close. That’s the assumption in the recent paper. But this could also work if antimatter comes in particle-antiparticle pairs. The key is that the annihilation happens when the particles get close enough together.

Now that we know what type of dark matter we need, let’s assume that’s the type that exists. This isn’t unreasonable—popular candidates for dark matter, like WIMPS for example, should fit these requirements. So we’re going to fill our early universe with lots of dark matter, smeared out pretty evenly across all of space. We’ll also add some hydrogen and helium at about a fifth the quantity of dark matter. And finally we have small fluctuations in the density of all that matter—regions of higher density that would pull on the surrounding material and so seed the first structures in the universe.

The seeds of the first giant stars would have been so-called mini-halos with masses of millions to hundreds of millions of times the Sun’s mass. The dark matter part would have a hard time collapsing due to being weakly interacting. However the gas in that halo would fall towards the center, perhaps en route to building a star, depending on how large this halo was. Now, as we discussed in our recent episode, dark matter can be strongly influenced by the gravitational pull of regular matter. And that’s what happens here. The growing density of gas in the center of the halo would drag some of the dark matter inwards. The density of dark matter inside the central ball of gas would rise to much higher levels that we see in the modern universe.

And this is where our dark star is born. The amount of dark matter inside the star is relatively low—only a fraction of a percent the mass of the gas. But its concentration is trillions of times higher than the dark matter in a typical galaxy. That means its particles can find each other … and remember what happens then. They annihilate, releasing an enormous amount of energy in the process—mostly in the form of very fast moving decay products. That energy release heats up the surrounding gas and actually stops it from contracting any further. The gas ball never gets dense enough for form a true star. But that doesn’t matter, because the dark matter annihilation is producing far more energy than a regular star ever could.

Our newborn dark star starts out as a bloated ball a few times Earth’s orbital radius, and several times the mass of the Sun. But then it grows, pulling in more gas and dark matter from the rich material of the surrounding young universe. It grows in size several times, perhaps to the size of Saturn’s orbit, and it may grow to millions of times the mass of the Sun. At that size, it’s glowing billions of times brighter than the Sun. Bright enough to be seen by JWST at the ends of the universe.

OK. let’s assume for a moment that dark stars existed. What happened to them? Eventually the dark matter would annihilate enough of itself that it could no longer support the gas cloud from collapsing. However it’s also possible that a dark star could replenish its supply of fuel by pulling in dark matter from the surrounding halo. Either way, when it’s done - be it millions or even billions of years, once the gas is no longer supported it would probably collapse pretty quickly into a black hole. This is actually a nice feature of this hypothesis. We know there are giant black holes in the centers of most galaxies, and those black holes seem to have grown very quickly in the early universe. Perhaps dark stars give us a way to produce the seed black holes with a million Suns worth of mass that could then grow into the billion solar mass monsters that we see in the very first quasars shining out of the early universe.

So are dark stars one of those speculations that will be almost impossible to verify or refute? Fortunately not. There are pretty straightforward observations that could distinguish a dark star from a massive early galaxy.

We can do that from the signature wavelengths of light that are absorbed or emitted by gas in whatever these objects are. When we split the light of a star into component wavelengths—what we call a spectrum—we see that there are chunks taken out of the spectrum at specific wavelengths. These are absorption line, and happen because the bright light from deep in the star gets absorbed at very particular wavelengths by atoms nearer the surface.

On the other hand, a galaxy spectrum tends to show emission lines. This is light from cold gas in between the stars that’s been illuminated by the surrounding starlight. It gives us the colours you might be familiar with in the beautiful images of nebulae.

If we can get high quality spectra of these objects, the presence of particular emission lines indicates a galaxy, but if there are no emission lines and we see absorption lines that suggests it’s a single star. And with that crazy luminosity, the dark star would be a strong contender for our explanation. Of course, it could be a mix —  a small galaxy with a dark star in its center—but hopefully the spectra will help identify that case also.

Of course those distant blobs might just be galaxies—albeit very weird ones. For a very clear take on these supposedly giant galaxies, as well as a take-down of some of the hype, check out Dr. Becky’s episodes on the subject. OK, so what do we have? Dark stars or galaxies? Well, galaxies seem more likely if only because we know galaxies exist, but dark stars are still so speculative. But this is an exciting case where we have a new and kind of strange observation and there are clear observations we can make that’ll teach us more. Whether or not these things turn out to be dark stars, we’re guaranteed to learn more about that mysterious epoch when the first stars - dark or light - lit up at the dawn of spacetime.