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Invisible to the naked eye,
Our night sky is scattered with the hundreds of
billions of galaxies that fill the known universe.
Like the stars, these galaxies form constellations,
Hidden patterns that echo the
reverberations of matter and light
From an epoch long before galaxies ever formed.
These are the Baryon Acoustic Oscillations (BAO),
and they may hold the key to
understanding the nature of dark energy.
[PBS Space Time intro]
The field of cosmology and the study
of the universe on its larger scales
was once the least precise
in all of astrophysics.
The impossibly vast distances made
accurate measurements near impossible.
But as our telescopes and our techniques
improved over the last few decades,
Things are now very different.
We live in the era of precision cosmology,
in which we know the stunning detail,
the properties that govern the very birth,
evolution, and the end of our universe.
We talked about one of those
properties in a recent episode,
The Hubble constant, and about a growing conflict in its measured value, which hints at strange new physics.
Today, we'll talk about another measurement
that may help resolve this crisis:
The baryon acoustic oscillations.
They are the fossils of the first
sound waves in the universe,
imprinted on the distribution
of galaxies on the sky.
And in these patterns, we could read
the expansion history of the universe.
Let's start at the beginning.
For the first few hundred thousand
years in the life of our universe,
All of the space was filled with
hydrogen and helium in plasma form.
Protons and the lightest of nuclei forged
in the first minutes after the Big Bang,
and still so hot that no atoms could form,
and electrons buzzed free of their nuclei.
These particles of matter are our baryons.
There's also light, in fact, around
a billion photons for every electron.
but no photon is safe
from a free electron.
Unbound electrons present a huge target
to scatter any wavelength of light.
And scatter they did, constantly.
A photon could barely travel any distance
before colliding with an electron.
The electrons in turn exerted their
electromagnetic pool on the nuclei.
We say that in this state, light was coupled with matter,
And baryons and photons formed a single strange fluid:
A Baryon-Photon plasma.
There are three profound differences between
the behaviour of matter in this state
Compared to the gentle
gas nebula of the modern universe.
First: The Baryon-Photon plasma was opaque.
There exists no lines of sight to anything 
during that time, and we'll come back to that later.
Second: Light was able to exert an
enormous pressure on this plasma,
as we'll see that it'd lead to the production
of colossal sound waves.
And third: Those sound waves travelled fast.
Rapid interaction between the
charged particles of the plasma
via the trapped photons meant that ripples
in the plasma travelled at over half the speed of light.
Mixed in this soup of baryons and
photons was dark matter.
In fact, dark matter outweighs
baryons by a factor of five,
Which means it was, by far, the dominant
gravitational influence in the early universe as it still is.
But unlike baryons, dark matter
does not interact with light at all.
Light exerts no pressure on dark matter.
Okay, so, the universe is filled with this hot
ocean of baryons, photons, and dark matter.
Now, in order for it to do anything interesting,
We need one final ingredient:
Density fluctuations.
A teensy bit more matter here,
or a teensy bit less there,
These fluctuations probably were the
remnants of random quantum fluctuations
From when the universe was subatomic in size.
And now that they've expanded enormously by cosmic inflation in the beginning instant of the Big Bang.
Immediately after that,
two competing forces began to work.
Each over-dense region pulled
gravitationally on its surroundings,
gravitationally on its surroundings,
drawing matter towards it.
In particular, the dark matter flowed
inwards towards this density peak.
But also at that density peak, the imprisoned photons exerted an enormous outward pressure.
To equalise this pressure, radiation pushed
outwards and carried the baryons with it.
This resulted in an acoustic wave,
a true sound wave in the form of an
expanding shell of increased density.
And remember, sound travelled at over half the speed of light back then, so the shell expanded fast.
But as it expanded, so did the universe.
As matter became more diffused, and the photons
themselves were stretched, redshifted to ower energies,
themselves were stretched, redshifted to ower energies,
the universe cooled.
At 380,000 years, the plasma hit
a critical temperature of 3000 Kelvin,
around the surface temperature
of the coolest red dwarf stars.
At this temperature, electrons could finally be
captured by nuclei and the first true atoms formed.
The baryons transitioned in phase
from a plasma to a gas.
We call this phase transition event:
We call this phase transition event:
Recombination.
While free electrons were able
to interact with any frequency of light,
electrons bound into atoms are restricted
to only those specific frequencies
corresponding to the energy level
transitions of that atom.
As a result, light and matter were no longer coupled.
The universe went from opaque to transparent
over a period of several thousand years.
As the wave of plasma
and photons decoupled,
light began to stream freely through the universe
as the cosmic background radiation.
But the plasma, now hydrogen and helium gas, stalled.
The speed of sound dropped from half the speed of light
to only hundreds of meters per second.
The wave of plasma-turned-gas
essentially froze in its current state.
The radius of that shell became fixed to the rate of expansion of the universe.
And its size?
Well, the exact distance that sound could travel
over the age of the universe at that time.
We call this the sound horizon. And at recombination,
it was around five hundred thousand light years.
While all of this was happening,
dark matter was doing its own thing.
Immune to the radiation pressure, the central dark matter overdensity had continued to grow
It pulled on the expanding shell, and was pulled by it.
When the expanding wave froze,
both dark matter and baryons
flowed together and consolidated the new structure.
Once more in the gravitational grip of dark matter,
hydrogen and helium could begin the long
work of collapsing into stars and galaxies
work of collapsing into stars and galaxies
as the universe continued to expand.
And now over thirteen and a half billion years later,
the universe is expanded by a factor of 1,100.
So those rings should be 150 mega parsecs across,
500 million light years.
So how does this primordial history lesson help us understand the subsequent expansion of the universe?
Well, crazily, we can still see those rings,
not made of plasma or gas, but made of galaxies.
Before I show you what they look like,
a note of caution:
The story I just told you about the early development of these density fluctuations is far from the whole picture.
In reality, the density waves
sloshed inwards and outwards.
Gravity pitted against radiation pressure.
Density everywhere oscillated with a rate of oscillation depending on the size of the initial fluctuation.
Those are the oscillations in the baryon acoustic oscillations, and the complex signature of that sloshing
is imprinted in detail on the temperature map of the cosmic microwave background radiation.
That something will explore in a follow-up episode,
because it is perhaps the most powerful
tool we have in cosmology.
But today, we're gonna focus on this
first outbound density pulse.
Then, what it looks like now
in the arrangement of galaxies.
So, the acoustic shells at recombination
overlapped in a complex web.
Those rings were further smeared out
over the thousands of years it took
for the universe to fully transition
 from plasma to gas.
Collapse that web into galaxies
over the age of the universe,
and at first glance, it looks like a random
smattering of galaxies on the sky.
But the pattern is there.
Detecting it required the most detailed 
surveys of the heavens ever conducted:
It required galaxy redshift surveys.
Redshift is just the amount by which
a galaxy's light has been stretched
as it travelled through
the expanding universe.
The more stretching, the longer that light has travelled,
and so the more distant that galaxy must be.
Redshift gives distance. And with careful measurements of galaxies' positions on the sky,
a redshift survey can produce a three-dimensional atlas of the universe.
With a galaxy redshift survey in hand, how do you find
patterns in what looks like a random splattering
of tens to hundreds of
thousands of galaxies?
Well, you expect that galaxies should mostly form in the centers of those primordial density fluctuations,
where the dark matter
was the most concentrated?
But you should also have a slight
overabundance of galaxies
at exactly 150 megaparsecs
from those clusters.
Corresponding to the acoustic rings,
everything is so hopelessly overlapping.
But here's what you do.
In your atlas of the universe, take slices of the universe, each slice a certain distance from us.
Within that slice, tally up the distances
on the sky between every pair of galaxies.
You should see a lot of galaxies
that are close together.
That's the clustering from
the giant dark matter density peaks.
They should also see a slight excess of galaxy pairs
with separations of 150 megaparsecs.
These are the galaxy pairs where one is at the center of the dark matter peak, and one is on the surrounding ring.
That bump seems small,
but it's statistically very significant.
It's also exactly where we expected it to be.
This baryon acoustic oscillations signal
was first spotted in 2005
by the Sloan Digital Sky Survey
in the northern hemisphere galaxies,
and the 2dF survey in the south.
Since then, the WiggleZ BAOs and 6dF surveys
have improved the measurements/
Now, there was a powerful driving motivation
to make this measurement and to improve it.
It was to confirm dark energy.
Dark energy was first discovered by using distant supernovae as distance measurements-
to track the rate of expansion of the universe.
Those observations revealed that
the expansion rate is accelerating
due to an unknown influence
what we call dark energy.
But such an incredible claim
requires independent checking.
It requires an independent test of the expansion history.
Enter the baryon acoustic oscillations.
See, we know exactly how big
those acoustic rings should be.
Our understanding of the behaviour of the
original baryon photon plasma is excellent.
So, we know how far the acoustic wave should have travelled before being frozen by recombination.
And we can confirm that prediction by looking at the patterns in the cosmic background radiation.
That means we know how big
those rings were when they formed,
and we can see how big they are at different points
in the modern universe from our redshift surveys.
These rings give us a standard ruler
on the sky, spanning all of cosmic time.
They allow us to track the 
expansion rate of the universe.
The baryon acoustic oscillations agree with and confirm
what we measure using supernovae distances-
The expansion of the universe is accelerating.
They confirm the existence of dark energy.
They also confirm the dark energy behaves just as is
predicted by Einsteins cosmological constant.
It appears unchanging over time, just as we'd expect if dark energy is the energy of the vacuum itself.
Frankly, dark energy aside, just being able to
see these patterns is cool enough for me.
I mean think about it. There are rings
in the sky inscribed in galaxies,
frozen echoes of the very first sound waves
to reverberate across space-time.
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Okay, so once again, we're covering
comments from two episodes.
Today, it's the 'Crisis in cosmology' and
'Negative mass perpetual motion' episodes.
That Chess Guy has a fun one. To paraphrase:
As the universe expands towards the infinite future,
does that expansion outpace the probability
of collapsing into a big crunch
due to everything quantum tunneling
towards a single point?
Well, the answer is- it depends on what size you consider. For the entire observable universe?
Almost certainly, there could be no
quantum tunneling big crunch,
but there is some smaller size
 that can eventually collapse that way.
Now, you need to wait 10^(10^25) years for the first quantum tunneling to turn iron stars into black holes,
and way, way longer than that for anything larger.
Netist comments that, "It can't be real because it would break our current understanding of physics"
...is hardly a good argument. Now, while I agree that this isn't a hard rule, it's a good guiding principle
to be dubious of new theories that
contradict extremely well tested old theories.
Remember that things like general relativity and much of
quantum field theory are verified to stunning precision.
If a new theory says, "Nuh-uh, that all wrong",
it better be able to also explain how those
theories appear to be so right while being wrong.
Luis Aldamiz recalls the appropriate quote, "Extraordinary claims require extraordinary evidence."
Again, this is a guiding principle,
and it helps scientists sort through the
overwhelming flood of poorly thought-out
"theories" that they get
bombarded with all the time.
Seriously, I should show you guys
my inbox one of these days.
Mr. Miller advises us to remember that the
number zero didn't exist for thousands of years.
Ergo, time will prove negative mass' existence.
Well, first I'd say that the number zero existed,
at least in a sense before we discovered it.
It's a common fallacy to draw
analogies between revolutionary ideas
that took a long time to prove or discover and
fringe ideas that may or may not ever be proved.
Why? Because for every revolutionary
 idea that was proved, right?
There are countless of fringe ideas that were wrong.
You've never heard of those ones.
Not everything that we can imagine exists.
In fact, we can imagine
infinitely more things than a real.
Still, it's worth looking into the wacky ideas for
the tiny fraction that do spark revolutions.
But don't get too attached to any one of them.
flux_capacitor notes an alternate model
for how negative mass might behave.
In so-called bimetric gravity, you can
have positive and negative masses,
but each is described by its own
set of Einstein field equations.
That's kind of like having parallel spacetimes,
one with positive and one with negative masses,
which can still interact gravitationally.
In these models, like masses attract and opposite masses repel,
but you don't get the
crazy runaway motion
that occurs if you put both positive and
negative masses in the same spacetime.
So, no perpetual motion machines.
It can also be used to explain
dark energy and dark matter.
An example is the Janus model of Jean-Pierre Petit.
This is a much more sophisticated model
than the one by Jamie Farnes.
It is, however, just as speculative.
Okay, this is my favorite comment ever.
Adam Houlett asks whether these explanations
might be any more understandable
if the "actor" was actually 
knowledgeable, something-something,
translation lost because the actor
is more a pretty face than an expert.
Adam, you made me blush!
Normally, it's "He looks like a
regular-sized Tyrion Lannister".
Seriously, every astrophysicists, like me, craves being mistaken for an actor and I feel like I've finally made it.
[PBS Space Time outro]
