If you look up at the night sky in any direction
past all the stars and more stars and galaxies and superclusters of galaxies
you will see light that has been traveling for 13.7 billion years to reach earth.
It’s the oldest and most primeval light in the universe
a picture of our cosmos in its hot younger years
and it’s called the “Cosmic Background Radiation”.
Of course you can’t really see this light with your naked eyes
because it’s in the microwave band of the electromagnetic spectrum.
But it is visible to radios and radio telescopes
and even makes up a small portion of the ‘salt and pepper’ on an analog TV.
Where does this luminescent background come from?
Well, just after the big bang, the entire universe was still so small.
It would have been very dense, scorchingly hot
and, because it hadn’t yet had time to get rough and uneven
it would also have been scrumptiously smooth.
For a while, things would have been so sweltering
that electrons didn’t settle down as parts of atoms or molecules
but instead roamed freely in a kind of red-hot cosmic soup.
That soup would have had lots of light bouncing around it, too
scattering off of electrons and protons, like a hall of mirrors.
However, as the universe expanded
there was less and less energy to be had in any one place.
And when things had cooled to just below the temperature of the sun
pairs of electrons and protons no longer had the energy to resist each other
and they fell into the electromagnetic embrace-we call the hydrogen atom.
These electrons were so enamored by their new proton love interests
that they effectively began to ignore all the light bouncing around them.
So, with fewer free electrons for light to interact with
the universe suddenly became transparent and all the pent-up light
was sent forth in whatever direction it had been headed after its last scattering
doomed to travel alone and unnoticed through the cosmos.
That is, until it bumps into something solid
When we finally see it here on earth
this light has been stretched so much by the thirteen-billion-year expansion of space
that, like a record slowing down, its frequency and color
have shifted from the original sunlight white all the way to cool microwaves
Thus, it’s often called the ‘cosmic microwave background radiation’, or CMB.
And just as we can tell the temperature of
a red or white-hot iron from its glow
this light tells us the temperature of empty space:
currently around 2.725 Kelvin, or minus 270 degrees Celsius.
However, the universe isn’t exactly 2.725 Kelvin in every direction.
If we look closely, there are small and seemingly random but noticeable bumps
all over the place, kind of like milk that’s starting to curdle.
Our best understanding is that these cosmic curds formed as quantum fluctuations
in the otherwise creamy infant universe
and then began to coagulate as the universe cooled and expanded.
It’s hard to overstate just how small, or un-bumpy
these fluctuations of temperature and density were to begin with.
The hot or cold spots were only hotter or colder than their surroundings
by a factor of about one in a hundred thousand
-that’s like noticing that a bacteria makes a beach ball bigger.
But, while this clumping of the universe initially resulted in
small variations like the ones we see in the CMB
随后 在原始的宇宙中 这些物块由于引力相互吸引
later on, the chunky curds of primordial soup attracted each other gravitationally
and they ultimately coagulated and coalesced
to form all of the massive structures in the universethat we see today
如行星 恒星 星系以及超级星团
like planets, stars, galaxies, and superclusters of galaxies.
So when we look up at the night sky past those galaxies
and see the ancient light of the cosmic microwave background radiation
we’re literally seeing the starting point, the proverbial cream, if you will
from which the starry curds of the universe congealed.
Or quite simply, proof that the moon really is made of cheese.
To give you a more complete experience of how awesome the cosmic background radiation is
we’ve made an adventure map showing it
as if it were the out-of-this-world geography of your favorite fantasy series
– except it’s really the first picture taken of our universe as a baby
We’ve included pictures of constellations and galaxies with the map
plus an overlay of what the sky looks like in infrared
And if you’re feeling imperial, you can help name the oceans and mountains on our map.
Head over to bigbangregistry.com to start exploring!