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For years, astronomers have been
unable to find up to half of the matter in the universe.
The missing baryon problem put into question understanding
of the physics of the Big Bang.
We may just have solved it.
Fingers crossed.
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Our astronomical surveys have revealed an observable universe
full of hundreds of billions of galaxies, each of them
with as many stars.
The shining light of these stars illuminates or is conspicuously
absorbed by gas and dust within those galaxies.
When we extrapolate observations to the entire observable
universe, we find a billion trillion suns worth of mass.
However, we've known for some time that around 95%
of the energy content of the universe
is in dark matter and dark energy.
This dark sector doesn't interact with light in any way
and so is invisible to us.
The remaining 5%, the light sector,
represents all of the regular matter in the universe.
Yet, what if I told you that all of the stars
and galaxies and galaxy clusters only comprise
10% of the light sector?
The rest has proved as elusive as the dark sector.
We think it must exist as extremely diffuse gas
in between the galaxies.
Yet, our intense searches miss up to half of it, at least
until now.
First, a quick refresher on dark matter and dark energy.
And there's plenty more detail in some previous episodes.
Dark matter is believed to be an invisible stuff that
interacts only through gravity.
It comprises 80% of the mass of the universe or around 25%
of its total energy content.
Its gravity holds galaxies together
and governed to the growth of large-scale structure
in our universe throughout cosmic time.
Now where dark matter pools, dark energy pushes.
It's anti-gravitational and causes the expansion
of the universe to accelerate.
This energy of the vacuum comprises 70% of the universe's
energy content.
The remaining 5% is regular baryonic matter.
By the way, a baryon is a 3-quark particle
like a proton or a neutron.
But really, baryonic matter refers to atomic matter.
It's the stuff of stars, planets, gas, dust, you, me.
Baryonic matter interacts with light,
so we can search for it by scanning
the electromagnetic spectrum.
Except, by doing so, we miss most of it.
This is the missing baryon problem.
There's a huge discrepancy between the amount
of baryonic matter our surveys find
and the amount that our theories say should be out there.
And those theories are pretty solid.
For example, in the first several minutes
after the Big Bang, hydrogen fused
into deuterium and helium.
The proportion of hydrogen that ended up getting fused
is very dependent on the density of that hydrogen, so
the baryonic mass.
Measuring the relative abundance of helium and deuterium today
tells us that there should have been 10 times as much hydrogen
to start with than we actually see today
in galaxies and clusters.
The second way to calculate the expected baryonic mass
is with the cosmic microwave background radiation.
Perhaps, you remember this stuff.
It's the light released at the moment
the first atoms formed nearly 400,000 years after the Big
Bang.
We still see that light today traveling to us
from distant parts.
It carries with it a map of the structure of the cosmos
from those early times.
These speckles are fluctuations in density
that would later collapse to become the galaxy clusters.
And by analyzing these fluctuations,
we can figure out the relative abundance of baryons
to dark matter.
See, before the photons of the cosmic background radiation
were released, they were trapped in the searing hot plasma
of baryonic matter.
The interplay between baryonic and photons
resulted in density oscillations.
Much like sound waves rippling outwards
from high density regions, these baryonic acoustic oscillations
helped produce a smaller family of speckles
compared to the largest blobs on the CMB map.
Those large blobs are driven by dark matter, which
doesn't interact with light, so it can't
produce density oscillations.
By analyzing the distribution and speckle sizes
in what we call the CMB power spectrum,
we can find the relative amount of baryonic versus dark matter.
Again, we calculate that they should be way more baryonic
matter than we see in galaxies.
Following the release of the cosmic background radiation,
gravity continued to do its work and collapse
these faint fluctuations into gargantuan clusters
of galaxies.
Supercomputer simulations reveal the shape
of this large-scale structure that
should result from this gravitational collapse.
It's the cosmic web.
Rivers and sheets of dark matter flow into giant dark matter
halos, dragging baryonic matter with them.
In the nexuses between filaments matter
is dense enough for galaxies to form.
Now surveys of galaxies confirm that this
is what the large-scale structure of the universe
looks like.
And yet, when we add up the mass from those galaxies,
most of the baryonic matter predicted by a theory
is missing.
So where is it?
Well, our best guess is that it's
in the form of a very diffuse plasma, atoms
stripped of their electrons in between the galaxies.
Now some of that stuff we can see.
If the plasma is hot enough, then it
emits detectable X-rays.
We typically see that stuff inside galaxy clusters
where the plasma is relatively dense
and is energized by the light of the galaxies themselves.
On the other hand, if the material is cool enough,
then nuclei can recapture their electrons
and become a gas instead of a plasma.
This cool gas then absorbs signature wavelengths
from light that passes through it.
Absorption features in the light of distant quasars
reveal this gas lurking between clusters of galaxies.
However, by looking only at the hottest
or the coolest material, we don't find nearly enough of it
to account for the missing baryons.
It seems that the missing material
must be in the intermediate temperature range.
It must be hot enough to still be a plasma.
Otherwise, it would produce absorption features.
But it can't be so hot or dense as to emit detectable X-rays.
This tells us that the best hiding place for the missing
baryons is the giant filaments that
form the cosmic web stretching in between galaxy clusters.
That material would be cooler than the clusters themselves,
but should at least be hot enough to form a plasma.
See, the vast tidal effects of nearby galaxies
create shocks that can heat those baryons
to hundreds of thousands or even millions of Kelvin.
At the same time, this stuff is expected
to be extremely low-density, only around 10 times
that of intergalactic space.
That makes it a more perfect vacuum than anything
we've created in a lab or even exists in the Milky Way.
And yet, those filaments are vast, tens of millions
of light years long.
And so those solitary baryons could add up
to more mass than all of the galaxies in the universe.
So how do we spot this stuff?
Two research groups have figured it out.
The secret is the thermal Sunyaev-Zel'Dovich effect.
We talked about the kinetic SZ effect in our episode
on dark flow.
The thermal SZ is similar.
And again, it makes use of the cosmic microwave background.
As photons from the CMB pass through a giant filament,
the hot plasma in the filament grants it a little energy
boost.
In fact, the electrons in that plasma scatter
CMB photons to higher energies.
So if there's enough of this stuff,
then the CMB map should be slightly hotter
directly in between galaxies that
are connected by filaments.
And it turns out it is hotter.
Two teams, Graaff and collaborators and Tanimura
and collaborators, just published the results
of their attempts to look for the Sunyaev-Zel'Dovich Effect.
They both used the latest Planck satellite CMB
map in the presumed locations of large-scale structure
filaments, which they assumed was
between pairs of nearby massive galaxies, the type typically
found in giant dark matter halos.
Now it wasn't an easy experiment.
The SZ effect is tiny.
And so the researchers needed to add together the results
from many, many galaxy pairs.
Graaff et al.
used a million galaxy pairs, while Tanimura et al.
used 260,000.
Both teams report detection of the thermal
Sunyaev-Zel'Dovich effect with around 5 sigma significance.
Or to translate, they found the baryons.
These filaments seem to have enough of this hot diffuse
plasma to match the amount expected from the models.
We're a very important step closer to accounting
for all of the missing baryons.
And this is actually a huge relief.
If our predictions for the relative mass in baryons
versus dark matter was so wrong, then
it would mean the our understanding
of the physics of the Big Bang was seriously off.
So it seems that most of the regular matter in our universe
is spread out in the vastness of intergalactic space,
still flowing with rivers of dark matter
into the galaxy clusters.
As those baryons fall into the dense nexuses
of the cosmic web, they'll feed galaxies with material
to form new stars.
In fact, this verifies that the epoch
of star formation in our universe is far from over.
In fact, it's only just beginning.
The stuff of countless future solar systems
is still riding the cosmic web, falling
in from the darkest reaches of space time.
Last week, we talked about virtual particles, zero point
energies and the nature of nothing.
You guys had something to say.
Michael asks whether the space containing an intrinsic energy
also means that it has intrinsic mass?
Well, the answer is, as Gareth Dean put it, sort of.
A non-zero vacuum energy would have a gravitational effect.
But if it's exactly the same everywhere,
then there's no net attractive force.
Yet, it would still push the universe
towards positive spatial curvature.
So enough vacuum energy could result
in a closed, rather than infinite, universe.
And rather differently to regular matter,
vacuum energy doesn't dilute in an expanding universe.
This leads to the unintuitive result
that it acts repulsively, accelerating expansion.
And this is, of course, what our universe is doing.
Check out our dark energy playlist for details.
And tune in next week for even more.
Jeremy Zambelli asked whether the annihilation
of virtual matter anti-matter particles
would introduce energy into the universe
and therefore violate the law of conservation of energy?
Well, no.
The energy is borrowed from the energy
of the vacuum for the minuscule time allowed by the uncertainty
principle.
On particle annihilation, it's given back
without producing a real photon.
Jeremy also asks, if virtual particles control faster
than the speed of light, can't they
escape the event horizon of black holes
via Hawking radiation?
Well, actually, it's going to take a few episodes to properly
answer that.
So just stay with us.
TS1336 was expecting last week's episode
to be about the discovery of gravitational waves
from merging neutron stars.
Don't be silly, TS1336.
We like to release our episodes or
new [INAUDIBLE] announcements at least a month
before the announcement.
Check out our September 13th episode.
Eliran Cohen suggests that, instead
of presenting lame theories, we should
travel 1,000 years into the future
and bring back exact answers.
Uh, we already do that.
Where do you think we get all of this stuff?
Unfortunately, there's still no theory of everything
in 1,000 years and still not flying cars, would you believe?
Waste of time.
Next season, we are going to try 10,000 years.
Maybe we'll at least get some evidence for the string theory
or something.