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?