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Hey Crazies.
There’s this super-uniform microwave static coming from all directions.
Seriously. All directions.
It’s the cosmic microwave background, the CMB.
Tune your radio telescope to the right frequency range and you get static,
no matter which way you point it.
Here? Static.
There? Static.
Everywhere? Static!
Which, you know, is kind of weird.
But, what is it? Where did it come from? And, frankly, why should we care?
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So, we noticed pretty quickly that the static wasn’t all the same frequency.
As you turn the frequency knob, the signal gradually gets louder, hits a peak, and then gets quieter again.
We can even figure out the intensity of each of those frequencies and plot it on a graph.
When we do that, we get something that looks like this.
It’s a thermal spectrum.
What’s a thermal spectrum?
It’s light emitted because of temperature.
Say you’ve got a rock sitting on a table.
The reason you can see it is because light is reflecting off of it.
But let’s say you painted it with the blackest paint you could find.
Now we’d call it a black body, which is just an object that’s black when cold.
It might not be able to reflect light anymore,
but it would still be emitting light in infrared just because it’s at room temperature.
That light is made by the molecules inside the rock.
If we make the rock really hot though, we can get it to emit visible light.
The thermal spectrum would just peak in a different place.
We could even figure out the temperature based on where the peak is.
As it turns out, the CMB is also a thermal spectrum.
It just peaks in the microwave range instead of the visible range.
Based on where it peaks, we know whatever is emitting this light is about 2.7 kelvin.
And, let me tell you, this spectrum is as close to perfection as you can get.
Just to give you an idea of how perfect,
this is the Sun’s thermal spectrum compared to the ideal curve based on its temperature.
It might be close enough to call the Sun a black body, but it’s not perfect.
Doing the same comparison for the CMB gives us this.
Look how beautifully it matches.
Mmm! Good stuff.
It’s as close to a perfect thermal spectrum as you’re going to get.
Which leads to an obvious question:
What the heck is emitting this thing?
Could space itself be emitting it?
I mean, space is pretty black.
Seriously, check this out.
I can color this paper with a sharpie and it seems pretty black, right?
Now look what happens when I poke a hole in it and put a dark void behind it.
See how much blacker that dark void is?
Space is the blackest black!
But hold up.
If this black void started to emit light,
we’d have to assume there was some kind of matter inside it.
After all, temperature is atomic jiggles and you can’t have atomic jiggles without atoms.
Hmm, let’s see.
For something that cold, it’s a pretty strong signal.
Like seriously? 2.7 Kelvin? Just a few degrees above absolute zero?
That’s cold!
Plus, it’s coming from every single direction. What could possibly be everywhere?
Wait a minute!
Microwaves are just a type of light and the speed of light is finite.
If this thing’s coming from far enough away, it could be coming from the distant past.
To the timeline!
No no, not when it was discovered, when it was made!
If you want to learn about the discovery of the CMB, you can check out Physics Girl’s video.
It’s a good one.
You might even recognize a name or two in the end credits.
But, no, I mean the cosmic timeline. We’re talking billions of years ago.
Before there were humans, before the Earth formed, before the first stars in the galaxy lit up,
before there were even atoms in the universe, space was filled with plasma.
That’s not the stuff in my blood, right?
No, you definitely don’t want this kind of plasma in your blood.
Plasma is a soup of charged particles. It’s way too hot for neutral atoms to exist.
But, say you and one of my clones are floating around in it
with invincibility powers.
You’d both see a pale orange light coming from every direction.
Not from everywhere in the universe though.
During the plasma years, the universe at this time was about 80 million light-years across,
but you could only see a few thousand light-years in any direction.
That’s only one ten-thousandth of the way across the universe.
See, light doesn’t get very far in a plasma
It keeps bumping into charges like protons and electrons.
It was like a very violent pinball game.
The light is constantly being created and destroyed.
That light was far more likely to be coming from nearby.
If my clone were to drift away from you, he’d gradual be obscured by a fog of particles.
That all changes when neutral atoms happen.
The universe was continuously expanding and cooling off.
When it got to an age of about 380,000 years old, it dropped below a temperature of 3000 kelvin.
That plasma that was filling the universe stopped being a plasma.
Electrons and protons came together to form neutral Hydrogen.
With the charges gone, light was free to move through the universe unobstructed.
The universe became transparent.
We call that period in cosmic history “recombination”
because names are dumb sometimes.
The light emitted during that time is the same light we’re receiving as the CMB.
That static we get from all directions was created during recombination,
when the universe was only 380,000 years old.
At the time, the temperature was 3000 kelvin, so the distribution peaked in the near-infrared.
There was quite a bit of visible light in there.
Hence all the orange you’d see if you were around back then.
But a long time has passed. The universe is much bigger and much cooler.
Now, it peaks in microwave range.
That corresponds to a temperature of 2.7 kelvin.
It’s still a thermal spectrum, just a much colder one.
But, remember how I said the spectrum was as close to perfect as you can get?
Well, close to perfect is not the same as perfect.
There are some tiny temperature variations.
The CMB has spots.
The blue spots are slightly colder and the orange spots are slightly hotter.
Of course, this is the universe, so those spots are enormous.
Today, they’re about 550 million light-years across.
Like I said, enormous.
But, when those spots were made, the universe was 1100 times smaller.
They were only about half a million light-years across at the moment of recombination.
And those spots were made with sound waves.
The density of that primordial plasma wasn’t perfectly uniform.
The slightly more dense parts collapsed under gravity
and then rebounded in a shock wave that traveled over half the speed of light.
That’s how we know how big they were.
Before recombination, when everything was a plasma, matter and light were coupled.
Remember the pinball game?
Light didn’t get very far without interacting with matter.
So, any changes in the structure of matter
were accompanied by the same changes in the pattern of light.
But, during recombination, everything changed.
Light became decoupled from matter and the pattern of light was frozen.
That pattern is what we see in the cosmic microwave background.
This is a map of the structure of the universe when it was only 380,000 years old.
And that map tells us a ton about the universe.
Like, a crap-ton.
Stuff that lets us model the entire timeline of cosmic history.
Of course, the CMB is the not the only place we get that information.
It’s important to collect corroborating data from multiple sources.
Like patterns in the locations of galaxies.
This structure matches the CMB.
That simulation was created by a super computer called MIRA.
I got to see it in-person a couple years ago. It was a great experience.
There’s a vlog if you’re interested.
Anyway, modeling is important because we can only see as far back as the CMB.
It’s the oldest light in the universe.
Any physical record of the universe before that was destroyed almost immediately.
All we have for that era are the models and data we collect from particle accelerators.
So, what’s the CMB?
It’s the cosmic microwave background.
It’s cosmic because it fills the observable universe.
Probably the entire universe too.
It’s microwave because it peaks in the microwave range of the electromagnetic spectrum.
And it’s a background because it’s a very uniform signal coming from all directions.
The CMB is a thermal spectrum left over from when the universe was only 380,000 years old.
The CMB is an afterglow from the big bang.
It’s a record of the distant past.
So got any questions about the cosmic microwave background?
Please ask in the comments.
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And until next time, remember, it’s ok to be a little crazy.
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If gravity is a gradient in time, how can gravity affect photons? They don’t perceive time, right?
Right. Photons don’t personally experience time,
but they’re not taking the measurements. We are.
And, in our reference frame, they are traveling through space and time.
Anyway, thanks for watching.