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Dark matter, dark energy,
there are all kinds of things in space
that we have yet to figure out.
But your real hipster astronomers don’t
worry about famous mysteries like that.
They worry about stuff like the missing baryon
problem: the fact that around a third of the
regular, ordinary matter in the universe has
refused to show up on any telescopes.
At least, until now.
In last week’s issue of the journal Nature,
an international team of astronomers reported
that they’ve finally found evidence of the
missing matter.
Honestly, it was right where
everyone expected it.
But it took decades of new techniques,
new telescopes,
and new knowledge to finally find it.
We have lots of ways of figuring out how much
stuff is in the universe.
For example, we can look at the Cosmic Microwave
Background, the echo of the Big Bang whose
colorful pattern depends on the universe’s
early makeup.
Or we can see how quickly the universe expands,
which depends partially on the gravitational
pull of all of its matter on
all of the other matter.
We can also look at how quickly
early structures formed.
The list goes on from there, but one of the
greatest achievements of modern astronomy
is that all our independent measurements
using different techniques
tell pretty much the same story.
Most of the universe is dark energy, a poorly-understood,
nonstop pressure spreading apart space.
Then, most of the rest is dark matter, a kind
of something that exerts a lot of gravity
but doesn’t emit or absorb any light.
Finally, a measly 5% of the universe is ordinary
matter, or baryonic matter, it’s the kind
that we and the Earth and the stars are made of.
Everything we’ve ever seen or touched, all
the countless stars and planets, all of that
is just 5% of what’s out there, with the
rest being stuff
that we fundamentally don’t understand.
Except, there’s also a problem:
We can’t actually find a big chunk
of that ordinary matter.
When we count up the matter in all the stars,
galaxies, and gas clouds we can see and then
extrapolate, we find that ordinary matter
makes up only about 3% or so of the universe.
That means around a third of it is missing
from our observations: We know it’s there
from all those other measurements, but we
haven’t seen it with our telescopes.
And this is just normal matter!
This is the stuff that should be relatively
easy to find!
Astronomers have dubbed this “the missing
baryon problem”, where baryonic matter is
stuff made of protons, neutrons, and electrons.
Most physicists have a slightly different
definition of baryon,
but this is the one many astronomers use.
So far, researchers have mostly assumed that
the missing baryons are hidden in certain
immense filaments of gas
that sit between galaxies,
generally known as the
Intergalactic Medium, or IGM.
The IGM can be millions of degrees Celsius,
but the gas in it is so sparse that it would
feel cold if you were in the middle of it.
Astronomers think that a huge fraction of
the universe’s mass is tied up in these
filaments, but the IGM is hard to investigate
directly.
See, it’s mostly ionized hydrogen, or hydrogen
that has lost its one and only electron.
But atoms give off and absorb light when the
arrangement of their electrons changes.
So if the hydrogen in the IGM has no electrons,
it won’t absorb or emit any light.
Which makes it really difficult to study.
Thankfully, research over the last few decades
has also indicated that there should be a
tiny amount of ionized oxygen in the gas,
too.
Oxygen starts out with more electrons than
hydrogen, so when it loses some and becomes
ionized, it still has others left over.
So we can try to see it.
And from there, we can extrapolate how much
other stuff is in those filaments of the IGM.
To find evidence of that miniscule amount
of oxygen, the astronomers in this new paper
used the European Space Agency’s XMM-Newton,
an orbiting X-ray telescope that can see the
kind of light that interacts with ionized
oxygen.
But there’s so little oxygen in the IGM
that it doesn’t just shine like a star;
you need a huge flashlight lighting it up.
For this team, that flashlight was a distant
type of quasar sitting conveniently
far behind the gas.
These are objects powered by black holes that
emit tons of radiation, more than many galaxies.
They looked at the quasar for about 18 days
total over the course of two years, and all
their observations told them that some of
the its light was getting absorbed by the
IGM’s oxygen on its way to Earth.
Based on how much light was absorbed, and
how much oxygen we think is in the IGM compared
with other elements, the team concluded that
there’s exactly enough matter in the IGM
to account for the missing baryons.
Even with two years of observations, though,
this isn’t the end of the mystery.
The team admits that there’s still a lot
of uncertainty, and that we need to look at
more filaments of IGM before the problem will
truly be solved,
just in case this gas is an anomaly.
But this paper does show that there’s a
light at the end of the tunnel and that our
hypotheses are on the right track.
And once the missing baryon problem is solved,
we’ll just need to figure out
the other 95% of the universe,
and we’ll be all set!
Thanks for watching this episode of
SciShow Space!
If you would like to keep learning more about
the universe and how weird and cool it is,
and stay up-to-date with astronomers’ newest
findings, we’ve got you covered.
You can go to youtube.com/scishowspace for
more videos and to subscribe.
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