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 or the interior of the sun...
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 degrees 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 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, 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
universe that we see today, 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!