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!