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Hook up an old analog antenna to your TV and scan between the channels.
That static buzz your hear
is mostly due to the ambient radio
produced by our noisy pre-galactic Civilization
But
Around 1% of that buzz is something very different
It's the Cosmic Microwave Background radiation
The remnants of the heat glow released
when the hot dense early Universe became transparent for the first time
It sounds like random static
But that buzz contains an incredible wealth of hidden information
It holds the secrets to the composition of the Universe
and allows us to peer inside its fiery beginnings
[Intro music]
It's not that scientists have spent
half a century and built multiple satellites
to unlock the mysteries of the cosmic microwave background
We've delved into its nature before
from its formation 38,000 years after the Big Bang
to its 1964 discovery by Penzias and Wilson
with the Holmdel Horn Antenna
to its incredibly accurate mapping across the sky
with ever-better satellites.
It all culminated in this.
The Planck Satellite's map of the CMB.
Those blotches are tiny differences in temperature.
deviations one part in 10,000
from the average temperature of only 2.7 kelvin.
Those differences result from tiny variations in the density of matter right after the big bang
which evolved as colossal sound waves reverberated through the first few hundred thousand years of the Universe's life.
We explored these baryon acoustic oscillations in the last week's episode
and that's really worth watching first if you haven't yet
That episode painted a simplistic picture
A quick review
In the very beginning
Dark matter flowed towards tiny regions of increased density.
drawn by gravity
Regular matter, what we call baryons was in plasma form
with the simple atomic nuclei stripped of their electrons
in that extreme heat
in this plasma state, light and matter were locked together
As the baryons condensed into over-dense regions
this led to a massive buildup of pressure
Collapsing baryons rebounded
producing an expanding sound wave
That expanding shell was eventually frozen in place
380,000 years later when light decoupled from matter
at the formation of the first atoms
the moment of Recombination
As the universe evolved
those frozen shells collapsed into galaxies
We still see them today
Interwoven patterns of rings
drawn in galaxies on the sky
but those rings are not the whole story
Today we're going to explore the intricate patterning
Not of the galaxies but of the cosmic microwave background map
The image of the universe at the moment of recombination
We'll see that the complex dynamics of the early universe
are frozen into its spots
That pattern will tell us exactly what the universe is made of
As I said this picture of a single expanding shell of plasma is simplistic
In reality these acoustic waves pulse in and out of their over-dense regions
They oscillated
And the number of oscillations
depended on how large that over-density was
In some places the over-densities were so large
that matter only just had time to float at the centre
before being frozen in place by Recombination
no rebounding happened
And in other places the over-densities were smaller
The density wave had time to flow in
reverberate out and then get captured by the gravitational field once more
falling back to the centre
and that could happen multiple times
Everywhere in the universe the pull of gravity fought against the outward push of radiation pressure
causing density oscillations of all sizes
The state of those oscillations was then frozen
at the moment of Recombination
This messy overlapping network of oscillations resulted in
the spotty mess that is the map of the CMB
But crazily, we can untangle that mess
We can do that by thinking of these complex oscillations
as just a bunch of very simple oscillations
of all different sizes stacked on top of each other
This only works because the differences between the highest and
lowest density regions are so small
so in our calculations
we model the early Universe as many overlapping layers of
simple density fluctuations
Each layer has fluctuations of a certain size
defined by the mathematics
of spherical harmonics
Sort of like sine waves but different wavelengths
but on a 2D surface of a sphere
Fluctuations in each of these layers oscillate independently
But by adding them together you can calculate the
complex fluctuations of the early universe
Thinking about the oscillations this way
leads to a really powerful prediction
Over the 380,000 years between the Big Bang and Recombination
each of our simple oscillations did its thing
Sound waves moved inwards outwards inwards outwards
When Recombination hit
most oscillations were caught in the middle of an in or an out flow
Those are not the interesting ones
but some oscillators
those were just the right size
were caught at maximum density
matter concentrated in the middle of the fluctuation
or at minimum density
with matter at its most spread out
we call that second one maximum rarefaction
These particular oscillations defined the most obvious spots on the CMB map
because they were frozen in their most extreme states
so the most prominent spots on the CMB
will have exactly the right size
to get a single collapse
or one collapse and then an expansion
or two complete collapses
et cetera
The oscillations happened at the same speed
the speed of sound for our baryon-photon plasma
which was over half the speed of light
Okay, so
We multiply the speed of sound
by the age of the universe at Recombination
That's how far these density waves could travel
divide that by the radius of a given density fluctuation
and we get how many half oscillations it could execute
if the result is a whole number
then that fluctuation size will be at an extreme state
either all in or all out
at the moment of Recombination
So the sizes of these special spots
should follow a harmonic series
and that's exactly what we see
The best way to show this is with what we call a power spectrum
It's really just like a histogram
that plots the number of spots of every possible size
and we definitely see that some sizes are more common than others
This peak here
those are spots in which the plasma only just had time to collapse
once before Recombination
The second peak corresponds to a full compression and a full expansion
The third peak is for compression expansion and compression
and so on
Okay now so we can explain the spot sizes
But what does this actually tell us about the Universe?
Well kind of everything
In fact each of these peaks
tells us something unique
Let's go through this
The main value of that first peak
Is as a measuring tape
Seriously, a standard ruler
Spots of this size
represent fluctuations that had time to collapse
exactly once
which means their size
had to be equal to the speed of sound
times the amount of time that they had to collapse
Factoring in the expansion of the universe over that time
that size should be about half a million light years
at Recombination
theoretically
And that gives us our ruler
Now when we try to measure the size of those spots
on the sky
we actually measure an angle
We need to convert to a distance using trigonmetry
and to do that we have to assume a flat universe
By flat, I mean geometrically regular
Parallel lines stay parallel
the angles of a triangle add up to 180 degrees
However
the presence of matter and energy
as well as cosmic expansion
cause geometry to be curved
which would mess up our basic trig
So here we have a test
if we measure the angular size of a spot
and use simple geometry
and that gives us the exact
physical size that we expect
from our theory from our standard ruler
then that's a good indication that our universe is geometrically flat
And apparently it is
The spots are about one degree on the sky
which corresponds to that half a million light years
that we got from the prediction
to the limits of our abilities to measure those sizes
the universe is flat
That tells us something very important
It tells us the total amount of energy in the Universe
Energy results in positive curvature
due to its positive gravitational effect
On the other hand
an expanding universe with no energy
would have negative curvature
So a flat universe
must have exactly the right amount of energy
to flatten the geometry of the Universe
So that first peak
tells us the Universe is flat
Therefore it tells us
the total amount of energy
It tells us the sum total of baryons,
dark matter, and dark energy
We'll see how useful that is
when we look at the rest of the peaks
Okay onto the second peak
That peak represents maximum rarefaction
Fluctuations where matter had bounced back once after its initial collapse
To understand the use of the second peak
we need to use an analogy
We can think of these oscillations as being like heavy masses attached to a spring
Release the mass then it falls and then bounces up again
always back to the original position
if it's a perfect spring
But the heavier the mass
the further it will fall
before bouncing up
Think of the baryons as this mass
They're heavy and they want to fall towards our over-dense spots
But the baryons are locked with light
which act like our spring
The more baryons, the deeper matter will fall
into that over-density
Having more baryons should enhance the odd-numbered peaks
which represent the compressed state
On the other hand
the even numbered peaks
aren't directly affected by the baryons
They represent the top of the spring's rise
which is just determined
by its starting position
What does this have to do with the second peak?
Well
The more baryons
the higher the odd-numbered peaks
are compared to the even-numbered peaks
In practice, we can just use the height of the second peak
relative to the first peak
to measure the baryon content of the Universe
And those measurements tell use that that baryons constitute
only about 5% of the total energy of the Universe
And finally we get to the other peaks
which represent the smallest fluctuations
These tell us about the dark matter
Well more accurately
they tell us about the relative amount of dark matter
compared to radiation, or light
Now this is a bit too much of a rabbit hole for right now
But in short
In the first few tens of thousands of years
the Universe was in the
radiation-dominated epoch
Basically photons produced more gravity than matter
Fluctuations that were small enough
to oscillate at least once during this brief time
should be enhanced
Their peaks on the power spectrum should be raised
compared to the larger fluctuations
So by looking at the height of those smaller peaks
You can actually figure out when the radiation epoch gave way
to the Dark Matter-dominated Epoch
That in turn tells you how much dark matter there is
Spoiler: There's a lot.
OK, Let's bring all this together
The size of the spots in the first peak
tells you the total amount of dark energy
dark energy, and baryons
The second peak gives you just the amount of baryons
and the higher peaks give you the amount of Dark Matter
Combining this, we can separate the
relative contents of all three components
extrapolate that to the modern Universe
and we get the baryons constitute
only about 5% of the mass and energy
that's all of the atoms in all of the stars in all of the galaxies
basically everything you can see
The remaining 95% is the so-called Dark Sector
Dark matter is 26.5%
and dark energy is a whopping 68.5%
This is a super-important verification
because we get approximately the same numbers
when we look at the dark matter content in modern galaxies and clusters
and the dark energy based on measuring
the accelerating expansion rate of the Universe
So that's how you lay bare the secrets of the cosmic microwave background
It's an insane wealth of information
f rom what looks like random minuscule fluctuations
in this faint noisy buzz
So next time you hear the static of an untuned TV or radio
remember that in that noise can be found
the secrets of the earliest epochs of Spacetime
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In our previous episode we started
our discussion of the baryon-acoustic oscillation
the actual source or our CMB power spectrum
Let's see what you guys had to say
flux_capacitor and Marcus Kunrath
point out that the baryon-acoustic oscillations
should have produced bubbles, spheres
rather than rings on the sky
That's right, they definitely would have produced
3D spherical shells
not rings
Sorry if I wasn't clear about that
So you should actually have
overlapping bubbles of galaxies
throughout the Universe
But we only see a 2D projection
of that Universe
When we look
through the edges of these bubbles
we see many more galaxies
compared to when we look through the thin layer of the middle of the bubble
It's just like when you look at soap bubbles
the edge is darker than the centre
aspuzling noticed that
the graph of the CMB power spectrum
has a first peak at 105Mpc
rather than 150 Mpc
which is the size I actually stated
Now the reason for this is that the authors of that paper
use a different value for the Hubble constant
See, the quoted distance is the co-moving distance
It's the size the ring would have
if it existed in the modern Universe
given the rate of expansion of the Universe
The surveys that measure these rings
look so far in distance
that we're actually looking into a younger smaller Universe
In order to consistently consider ring sizes
we convert to co-moving size
So the size they would have in the modern Universe
But to do that you have to assume an expansion rate for the Universe
the Hubble constant
The power spectrum I showed
is from the Sloane Digital Sky Survey
Eisenstein et al. 2005
and they use a smaller Hubble constant
giving a smaller current ring size
than the currently accepted 150Mpc
Juki who is 13 and wants to become a theoretical physicist
wanted to thank us for our content
Juki, thank YOU for letting us know your awesome dreams!
This is exactly why we do this!
I remember that when I realised I wanted to become a physicist
It seemed like that I'd glimpsed a secret layer of reality
that I just had to figure out
It was kind of like getting my letter from Hogwarts
Seriously
And so now you've got yours too
So what does that mean?
Well it means that magic is real
and you're in for a lot of hard but fascinating work
And when you're done you'll be a Wizard.
[music]