If you point a sensitive radio antenna anywhere in the universe,
you’ll detect a faint signal that you just can’t get rid of. It’s everywhere. That
signal is a relic of the Big Bang and it's a treasure trove of information about the cosmos.
That means that it’s definitely something that we need to look into.
(Intro music)
About fourteen billion years ago, the universe began. Space expanded at a furious pace,
filled with a fiery maelstrom of energy and evanescent subatomic particles, winking in and
out of existence. I’ve made some videos about the many misconceptions surrounding the Big Bang and
I don’t want to repeat any of that here. The links are in the description if you want to learn more.
But here I want to talk not about the theoretical stuff surrounding the moment of creation itself
and focus on some hard data that cosmologists use to learn a great deal about the cosmos.
So let’s skip over the instant at which the universe was created, and the inflationary epoch,
that is to say, the period of superluminal expansion where in a fraction of a second,
the universe expanded from roughly the size of a hydrogen atom to a sphere about a lightyear
across. And I’ll jump over the part when the universe was about three minutes old, where
the atomic nuclei of hydrogen and helium of the primordial universe was already locked into place.
Those are all interesting, to be sure. But let’s start our story when that was
all over and done with and the universe was coasting along, expanding because
of the initial boost and slowing under the effect of gravity. What was going on then?
Well the universe was hot – really hot. When hydrogen and helium nuclei were formed,
the temperature was about a billion degrees Kelvin, which is way too hot for atoms to exist.
Heat and highly energetic photons would knock electrons off of atoms,
so the cosmos was simply full of protons, helium nuclei, and electrons running around,
never forming any permanent structures. The universe was full of unattached electric charges.
The universe glowed but, at those temperatures, it glowed at wavelengths invisible to the human eye.
Over the millennia, the universe expanded and cooled,
eventually cooling enough that it was “only” white hot.
At about 370,000 years, the cosmos was glowing yellow-white.
It was still too hot for atoms to form, and that meant that the universe was opaque.
When light was emitted, it immediately encountered an unbound electron and was scattered.
You can imagine the universe at that time as a glowing fog that you couldn’t see through.
Then, at about 380,000 years after the Big Bang, the universe cooled to about 3000 Kelvin, or about
5400 degrees Fahrenheit, and everything changed. At that temperature, the universe was cool enough
that once electrons encountered a proton, they would be trapped, forming a hydrogen atom. And,
at that moment, matter became electrically neutral. Without unattached charges, light
could travel great distances. Basically, it’s like the fog lifted across the cosmos. Way cool.
So at that moment, the light we could see if we were there and we were in a fireproof
suit was yellowish-white, although the bulk of the energy was in the infrared,
with a peak wavelength of about a thousand nanometers. Then what happened?
Well, from that moment – 380,000 years after the Big Bang – until now, 14 billion years after
the universe began, the universe has expanded a little over a thousand times bigger than it was.
That expansion stretched the wavelength of the early light to about a millimeter in length.
That corresponds to microwaves.
Okay- everything I’ve said so far is theoretical,
or at least it’s not something that anyone was there to see. What do we see now?
Well, in 1948, Ralph Alpher and Robert Herman predicted that we should be able
to detect the primordial fireball in microwaves. You can see those two
guys here with Alpher’s thesis advisor George Gamow coming out of the bottle.
Gamow was a real character, but that’s a story for another time.
Researchers thought for a couple decades about how they might detect this microwave background and,
in the 1960s, a group at Princeton led by Robert Dicke
decided to search for it. However, despite their lofty plans, fate intervened.
About 30 miles east of Princeton lays the town of Holmdel, New Jersey, home of Bell Laboratory,
where Arno Penzias and Robert Wilson worked. They were using a radio telescope to try to
get a precise measurement of a supernova remnant in the constellation Cassiopeia.
To make sure their measurement had no unwanted signal, they pointed their telescope at empty
space, expecting to detect nothing, but they heard an unexpected hiss in their detector.
After tons of debugging, cleaning pigeons out of their antenna,
and checking everything they could think of, the hiss was still there.
Furthermore, the hiss was the same no matter where they pointed their antenna. After a lot
of consultation with their colleagues, including Robert Dicke at Princeton,
they realized that they had detected the microwave signal of the Big Bang.
It’s now called the cosmic microwave background or CMB.
The CMB isn’t just one wavelength. It’s a spectrum of wavelengths
and the spectrum is exactly what would be emitted by an object with a temperature
of 2.7 degrees Kelvin, or minus 450 degrees below zero Fahrenheit.
Actually, the temperature is very precisely known. It is 2.72548
with an uncertainty of 0.00057 Kelvin. And it’s the same everywhere and in all directions.
The Earth is embedded in the remnant fireball of the Big Bang, which is an awesome thing
when you think about it. This observation is a huge validation of the theory of the Big Bang
and netted Penzias and Wilson the 1978 Nobel Prize in Physics.
I said that the temperature of the universe is remarkably uniform, and that’s true. However,
there are tiny variations – and I mean tiny – just one part in a hundred thousand – and
those variations yield a ton of information about the universe. We first observed these variations
back in 1998, although we’ve measured them far more precisely in the intervening two decades.
Those tiny temperature variations are simply a treasure trove of information
and, in the next video, I’m going to tell you all about them. Be sure to watch that one too.
Okay, that was fun. It’s interesting to learn about the early moments of the universe.
But this video was merely a prologue. In the next video, I’m going to tell you about all sorts of
fascinating facts you can learn when you precisely study the CMB. If you enjoy learning about this
sort of topic, be sure to like the video and subscribe to the channel. And I hope you’ll
watch the next video too because it’s going to be chock-a-block full of even more interesting
physics which everyone should know about because, well, you know- physics is everything.
(outro music)