The Universe’s Baby Photo 

The space between stars and galaxies looks completely black, but if you were to point a satellite dish towards any dark point in space, you would actually see a staticky glow a bit like the white noise that sometimes crackles across your TV screen. But this static is coming from all across the universe: no matter where you pointed your satellite dish, you’d record the same signal. But there are no astronomical objects that could create such a uniform signal - so what causes it? 

Let’s quickly review a fundamental idea: everything in the universe emits radiation. You, me, trees, zebras, toasters, tacos, Uranus, stars - they all emit a continuous spectrum of thermal radiation with a peak that depends on their temperatures. Cool things mainly emit radiation with long wavelengths and low energies, while hot things mainly emit radiation with short wavelengths and high energy. 

Image Credit

The uniform signal we observe everywhere in the sky is radiation with a wavelength of about 1 millimetre, which corresponds to a temperature of 2.7 Kelvin - very close to absolute zero. It’s called the cosmic microwave background (CMB) because it’s actually the leftover radiation from the Big Bang. 

When the Universe was born over 14 billion years ago, it was so hot and pressurised that atoms couldn’t form yet - instead, it spent 300,000 years as a soup of photons and hot plasma, which was made up of protons, neutrons, and electrons: the building blocks of atoms. The photons couldn’t get very far because they kept interacting with the pesky electrons floating around everywhere, and so the early Universe was trapped in a perpetual fog. But as the Universe expanded and cooled, atoms could finally form, and the photons escaped. 

The cosmic microwave background is a record of these early photons at the moment of their jailbreak. The Universe has since expanded massively, so these photons don’t appear as hot and energetic as they once were: they’ve been stretched out or red-shifted to much longer wavelengths and lower temperatures, to almost exactly 2.7 Kelvin (-270 degrees Celsius). They’re everywhere in the universe today - 400 of them in every cubic centimetre - and they’re like the Universe’s baby photos. 

By studying them and the tiny fluctuations within them, we can learn about the Universe’s infancy and how stars and galaxies began to form.