and you can learn more about them at the end of our episode.
The James Webb Space Telescope found galaxies that are too ancient-looking for our young
universe. Now you may have heard that, but JWST keeps finding them, and our recent
efforts to solve this conundrum point in wildly different directions. Have we found galaxies
older than the universe, or did we just learn something incredible about how galaxies form?
with special deals if you combine your order with previous May the 4th merch. Now on to the episode.
Telescopes are time machines. Light takes time to get to us, so we see distant objects as they were
when their light began its Earthward journey. As our telescopes become more powerful they
see further, and so more and more of the past becomes accessible to us. The James Webb Space
Telescope is one of the most powerful time machines ever built. It’s powerful enough
that it’s discovered galaxies whose light has been traveling to us since the universe was
a little over 2% of its current age. The universe back then should look different,
right? Those galaxies should look different. After all, this is when galaxies first started to grow,
when they should have been vigorously forming many of their stars. They
should look like hyperactive kids. Those types of galaxies are around back then,
yet JWST has also seen much more developed, “adult” galaxies. Some that look way too big,
and way too ancient-looking for a universe only a few hundred million years old.
Over the past year or two, this mystery has made the pop-sci rounds. That included some breathless
speculation—like the idea that the entire Big Bang model is wrong. If there are ancient
galaxies 13 billion years ago, then how can the universe be only 13 and a half billion years old?
So far we haven’t weighed in on this conundrum. Now many others have done a fantastic job laying
it out and debunking some of the foolishness. Dr Becky in particular was on top of the progress
as new data came in from JWST. So, why should we cover it now? First, a lot of you have asked for
our take and it’s time to go on record. Second, the evolution of this mystery has been quite an
emotional rollercoaster, swinging from unsolvable to solved, and now there’s new work that swings
back in the direction of WTF. So let’s talk about the current status of the early galaxy conundrum.
And before that, let’s talk a bit more about what we actually
expect the early universe galaxies to look like.
Galaxies pulled themselves together from tiny density fluctuations in the very early universe
that we see in the dappling of the cosmic microwave background. The CMB reveals regions
with tiny excesses of matter—hydrogen gas, but more importantly dark matter which outweighs the
gas by a factor of at least five. As dark matter pulled itself together by gravity it pulled the
gas in with it. And as that gas compacted, the first stars were also born. These very
early galaxies must have started small, but with pretty crazy star formation due to the enormous
abundance of gas at that time. As those galaxies continued to grow, they collided and merged,
and eventually built themselves up into the mature galaxies that we see today.
We think we understand this stuff pretty well - or at least thought we did. Between the
precise CMB measurements and our theoretical understanding of gravity, star formation,
etc., our computer simulations allow us to explore the possible growth scenarios
for galaxies. We can predict, for example, that early galaxies should be forming stars
like crazy due to the enormous amounts of raw material—-hydrogen gas—that was around back then.
One thing that should be even more robust as a prediction is the size of dark matter halos.
These are the giant pools of dark matter that encompass all galaxies and hold them together.
Because dark matter should not be strongly effected by the complicated
behavior of the gas and stars, we have high confidence in our understanding of
how those halos grew over time. Or at least we thought we knew.
One very clear prediction of this whole model is that a lot of this halo growth should have
happened in the first 10% of the universe’s age. In that first 1.5 billion years since the big bang
there shouldn’t have been essentially no very large halos, and so no very large galaxies.
And that’s what the theory and the simulations say. But to check these we need powerful time
machines— telescopes. Now as the telescopes got bigger and our cameras got more sensitive we
were finally able to probe early enough times to properly test our models of halo growth. And this
is where the problem started around 15-16 years ago when our “high redshift galaxy surveys” - aka
really far away galaxy surveys - finally reached these distances we got some answers.
And those surveys started seeing a few cases of what looked like giant halos at
earlier and earlier times. And also the hint of overly red galaxies in the early universe.
Galaxies that are actively forming new stars should be bright at short
wavelengths because they have lots of giant, hot, short-lived stars. They
are relatively blue in color compared to older galaxies. And older galaxies
that are no longer so actively forming stars look much redder because these
short-lived blue stars exploded already.
Those early surveys didn't reveal anything crazy yet—just a handful of cases where the galaxies
looked too big and/or too evolved compared to what was expected from our models. In 2018,
Charles Steinhardt and team articulated this emerging tension in their paper “the impossibly
early galaxy problem”. But impossible things are well, impossible. It’s right
there in the name. So several not-impossible explanations for these galaxies were devised.
For one, we don’t see these overly-large dark matter halos directly. We only see the starlight,
and we use that starlight to infer the mass of the stars and then the mass of
halos that contain them. But that step requires assumptions, and if any of those assumptions
are wrong then perhaps we got the wrong halo mass. And the redness that suggests an old
stellar population could be due to other things too, and there are several other
issues with these inferences besides. I’ll come back to possible solutions in a bit.
But even if the galaxies observed in these ground-based surveys were really
too old looking and too big looking, well we haven’t quite broken all of
astrophysics and cosmology yet. If we strain our models of galaxy formation,
we can potentially speed up galaxy evolution to show why these things exist
by the time the universe is 10% of its current age. What we’d really like to do
is to look back even further in time to see if we can find the time when these galaxies
were themselves growing. And that’s what we did with the James Webb Space Telescope.
JWST was designed to push this early-galaxies game to new extreme limits. It’s the largest
telescope ever deployed to space, which means the most powerful. It’s also designed to be sensitive
to very long wavelengths of light—deep into the infrared part of the electromagnetic spectrum.
The “mid-infrared” as we like to call it. That’s important for these first galaxies
because of the expansion of the universe. Their light has been traveling to us for much of the
age of the universe. Because these EM waves traveled through expanding space, they were
stretched out—their wavelengths lengthened. This is cosmological redshift, and higher redshift
means longer travel time and greater distance. For the galaxies we’re interested in, this redshift
converts visible and even ultraviolet light into mid-infrared—which is why JWST is needed.
The first efforts with JWST used similar methods to our ground-based galaxy surveys. So maybe I
give you a bit more detail on that. The first thing you do in a galaxy survey is to take
pics with different filters corresponding to different wavelength bands and then you compare
the amount of light in each filter—we call this photometric imaging. We can learn a lot of stuff
from the ratios of filter brightnesses— and so the colors. We discover candidate galaxies this way,
we make a crude estimate of the distance because the color ratios suggest a redshift,
we can constrain the stellar population—both the number of stars and the distribution of
different types—for example, the redder colors mean an older population with few massive stars.
So what did JWST find? Well first, it confirmed the presence of overly massive
and overly old-looking galaxies from the earlier studies—and remember that was from
a cosmic age of around 10%--a redshift of 4. Now a critical advantage of JWST
is that it’s sensitive enough to do proper spectroscopy on these galaxies—it can measure
their “spectrum”-- the amount of light as a function of wavelength. These spectra
confirmed that the redshifts are indeed very high. They also confirmed that the redness of
the spectra is due to highly evolved stellar populations, rather than, say, there being a
lot of dust in the galaxies, which can cause similar reddening in photometric analyses.
With these confirmations of old-looking early galaxies, researchers more broadly really
started to pay attention. The media started to pick up on it when JWST pushed to greater
distances and earlier times and kept finding these things. Currently, the earliest candidate giant,
evolved galaxy discovered by JWST is at redshift 7.3, at just 5% of the universe’s age.
Now it seems we have a real conflict with our models of galaxy formation.
Dark matter halos too large and stellar populations too evolved for the short amount
of time they had to develop. And this is about why we started to hear some hysterical claims that the
universe is twice as old as we thought, or that the big bang model is completely overturned.
It’s not. And it isn't. There’s just so much independent corroboration of our model of an
expanding 13 point something billion year old universe. You can’t point to
one admittedly intriguing discrepancy and decide to throw the baby universe out with
the bath water. There are much more parsimonious explanations for our improbably early galaxies.
Let’s look at one of the most compelling. Remember
that when we calculate the masses of these supposedly gigantic dark matter
halos, we based them on the starlight that we see, and that requires an understanding
of the relationship between these two things, the starlight and the dark matter halo mass.
And that involves assumptions. Here’s how that
works. One assumption is that halo mass is connected to the mass in
stars, and that “stellar mass” is connected to the amount of light we see in those stars.
But we don’t see the light from all of the stars—typically the light we collect is dominated
by the brightest stars in the galaxy. We then have to decide on the relative numbers of the different
types of stars so we can extrapolate from the observer starlight to the mass of all stars,
seen and unseen. And there's another assumption. And, yes, from there we can get to the halo mass.
Knowing the distribution of stellar masses in a galaxy on the other side of the universe
takes some guesswork to say the least. And maybe the biggest unknown there is something
called the initial mass function—the IMF. It tells us the relative numbers
of stars at different masses that form when a burst of star formation happens.
We typically use the IMF that has been measured for the Milky Way galaxy, sometimes with various
refinements. But we don’t know that stars formed with similar mass distributions in
the early universe. And they probably didn’t. For example, when there’s less heavy elements
around from generations of old stars, its easier to make really gigantic stars because gas clouds
don’t fragment as much when the collapse. That would give what we call a “top-heavy” IMF— so
more massive stars form in a given burst of star formation relative to what happens in the Milky
Way. And more bright, massive stars means that these galaxies would be overly bright for a given
halo mass. So if we’re determining halo mass from the light of those massive stars then we
over estimate halo mass. Such a “top-heavy” IMF is probably the leading contender for explaining
the apparent giantness of the dark matter halos. And this may even solve the conundrum.
But the mystery does stop here. A new study just found exactly the opposite
result of this. This study claims to have identified a sample of galaxies that are
what those “impossibly early” galaxies became in this part of the universe.
And because this sample is much closer, we can detect the light of much fainter,
lower-mass stars. And so figure out the initial mass function down to much lower masses.
And the result is bad. This study found that the IMF is actually bottom-heavy in
those galaxies. There are way more low mass stars—little red dwarfs and whatnot—compared
to the Milky Way for example. And definitely very bottom-heavy compared
to what we think a top-heavy IMF might look like. The top-heavy IMF that was supposed
to solve the problem of the impossible galaxies. And in fact, this bottom-heavy IMFworsens the
problem. An excess of low-mass stars means that when we convert galaxy light to stellar
mass we underestimate that stellar mass, and presumably underestimate it's dark matter halo
mass. And that makes it even harder to figure out how those things grew so fast so quick.
OK, before we join our crazy uncle on the anti-Big Bang conspiracy facebook group, some words of
caution. We don’t really know whether this new study really did successfully identify the modern
counterparts of our impossible galaxies, and the authors admit that, calling them “likely
descendents”. We also don’t know what complicating interactions these galaxies could have had in the
intervening 13 billion years to muddle the IMF. We also don’t know that a bottom-heavy
IMF rules out an IMF that’s also top-heavy. In other words, there could be an over-abundance
of low mass stars AND of high mass stars all compared to masses around that of the Sun. And
it is plausible to assume an IMF like this that would still bring down those early halo masses.
There are similar challenges in explaining the apparent redness of these early galaxies.
The key here is that we need a way to shut star formation down much
more quickly than we thought likely. Perhaps the leading contender is quite awesome—early
quasars—supermassive black holes in the hearts of these galaxies blasting out
radiation and winds that heats up and expels gas so that stars stop forming
and the population can evolve quickly. We know this sort of feedback from quasars
happens, but we now need to understand why its so extreme in the early universe. That involves
very rapid growth of supermassive black holes—yet another very real and very interesting problem.
The most likely solution to all of this is that we’re going to learn an enormous
amount about the surprising processes behind structure growth, star formation,
black hole seeding and growth, and who knows what else. And once we figure it out,
the impossible early galaxies will become more than just possible—they’ll become inevitable:
a natural part of our updated understanding of our early spacetime.
when you drag your tabs downward, Opera splits the screen up to four, resizable ways.
If you’d like to try out all the features of the Opera browser there’s a link in the description.