We knew that the James Webb Space Telescope
would find interesting stuff, especially about the mysterious early epoch of the universe. For example, there are hints that the galaxies
we’re seeing there are brighter and more regular than expected given the short amount of time
they’d had to grow. But perhaps no one was expecting that we
would find a completely new type of star—one 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, these 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 to the infrafred. 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 form. The 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 here be dark stars. That’s the assertion of a new paper was
published in the Proceedings of the National Academy of Sciences. The idea that three of these four
objects might be powered by dark matter. The original idea for dark stars
was proposed by Katherine Freese and collaborators back in 2007, but this is the first time we’ve seen
candidates for these objects. 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 are 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 our own 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 is doubly
ruled out is the electromagnetic interaction. Dark matter can’t emit or absorb photons and 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
does have to halve 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
dark matter 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 actually exists in the universe. 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 than 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 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 once 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 some 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 near the surface. On the other hand, a galaxy spectrum tends
to show what we call 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. Now 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 would suggest 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 too. 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 very first
stars - dark or light - lit up at the dawn of spacetime. Before you go we are excited to
tell you that PBS has a new slate of science shows coming to PBS Terra.
One new exciting one is HUMAN FOOTPRINT, in which biologist Shane Campbell-Staton
explores how the natural world is adapting to us. The first episode explores how we humans
are changing the course of elephant evolution in some very unexpected ways. So if you’d
enjoy a unique scientific understanding of how the world is changing or if you just like
elephants, there’s a link in the description. Also consider subscribing to Terra if you want
explore a variety of unique science shows. Today we’re doing comment responses for the
last three episodes. We have the one about the possibility that the speed of light is variable.
Then the one about the recent detection of the gravitational wave background with the pulsar
timing array. And finally the one were we asked whether the standard model that astrophysicists
use for dark matter needs to be revised. Starting with the variable speed of light though. Spindash64 asks whether a rapid decrease
in the speed of light over time would be the same thing as a dramatic increase in
distance. So it is possible to model the apparent expansion of the universe as a slowing
of the speed of light between the galaxies, and people have tried. In fact Robert Dicke
tried to model this and in fact all gravity in terms of a variable speed of light. But this
approach doesn’t explain all of the effects of general relativity—the warpable, stretchable
spacetime fabric does a much more consistent job. Will Dye asks what if the one-way
"speed" of causality changes wildly, choosing all possible speeds just as
a particle chooses all possible paths? Well will this what-if is a reasonable statement
of the Feynman path integral approach to calculating the trajectories of quantum
particles. And yeah, in this picture even light travels many paths with many speeds
between points, but when you sum these up they cancel out leaving only the most probable
paths—-which are those close to a straight light and traveling at close to light speed—or
within the Heisenberg uncertainty limit of such. Okay, onto the gravitational wave background. Sudoboat asks whether the Pulsar Timing Array data is public. The answer is yes. Google Nanograv data
release and you’ll find a download link for the 12.5 year data release and a link to the paper
describing the release so you know what to do with it. But seriously guys unless you’re really planning to do something with it, please don’t spam their server too much. AmblesJambles asks whether gravitational waves
can be lensed? For example around supermassive black holes right before they merge. The
answer is absolutely gravitational waves are subject to gravitational lensing. Grav
lensing results from changing the shape of the fabric of spacetime. Gravitational waves
are waves in that same fabric. They have to travel along whatever weird shape spacetime finds
itself in. That means gravitational waves do get focused by large masses such as galaxies. In
fact, some researchers have wondered whether the very large apparent masses observed
in LIGO black hole mergers are actually a result of lensing of those waves increasing their
intensity, which makes them look like they come from bigger mergers. This isn’t the generally
accepted story, however it’s also not impossible. As for whether a merging supermassive black
hole lenses its own gravitational waves. Well sort of. Before the merger, the waves emerged
from very near the two black holes will encounter complex gravitational field, and that
influences the shape of the emerging gravitational waves. But this isn’t the same classic focusing,
magnifying effect of a typical gravitational lens. In a related question, Naimah92 asks if
gravitational permeability is a thing, and if so are gravitational waves subject to refraction.
That’s a great question - this is basically asking whether gravitational waves always travel at
the same speed - lightspeed - through all space. The way to slow a gravitational wave would be
the same way we slow light - pass it through a medium in which the waves are continuously
absorbed and then reemitted in a coherent way. With light that means passing it through
a medium full of electric charge. The charge of gravity is mass, so it’s conceivable
that a “medium” with enormous mass charges could slow a wave - I dunno, a cloud of black
holes”? I’ll have to think harder about this, but I did find at least one paper that
talks about reflecting millimeter-scale gravitational waves using a superconducting
sheet, which supposedly works because extreme mass movement is possible for electron pairs
due to the absence of friction. This would obviously be very handy if you want to check
yourself out in a gravitational wave mirror. Okay, finally onto the episode about dark matter. inujosha asks: given that only usually cold gas can form stars, and since dark matter is cold, could a dark
matter stars theoretically exist? The answer is yes - they’re called dark stars, and as chance
would have it we just told you all about them. But to recap, normally dark matter can’t form
stars because it doesn’t interact strongly enough to shed the orbital angular moment of
its particles. That’s needed for those orbits to shrink down and reach the high densities needed
to produce energy. Without that interaction, dark matter particles can be cold but still
stay in their giant slow-moving orbits. @a3d4e asks what if dark matter doesn't exist,
and the current gravitational model just needs to be revised at scale? It’s such a great question
that many smart scientists have investigated the possibility, and are still doing so. But it’s
seeming more and more likely that dark matter really is some time of stuff out there, rather
than just gravity behaving differently than we expect. One big reason for that is that we see
that dark matter can sometimes be separated from light matter, like the the bullet cluster or
more recent discoveries of galaxies with very little dark matter. If dark matter was just
gravity of regular matter behaving weird, then we’d at least expect it to always be
found where the regular matter is found and that's not the case. i.e_Above11D says if you listen carefully you
can tell that I have been replaced by an AI. Well I object to the object to the idea that I've been replaced. That implies I was ever meat-based. The evidence to the contrary is abundant. For one thing, how do you
think I survive out here in space for so long?
