How does the CMB tell us what’s in the Universe?
The Big Bang’s leftover glow tells us a lot more than just where we came from.
“Cosmology is the study of the origin, evolution, and fate of objects in the observable universe. […] The key to the birth and evolution of such objects lies in the primordial ripples observed through light shining through from the early universe.” -Wayne Hu
The hot Big Bang might have started our Universe as we know it some 13.8 billion years ago, but there’s a piece of it still visible to us today. Because the “bang” happened everywhere at once, there’s light that’s been traveling in all directions for 13.8 billion years, and some of it is just arriving at our eyes today. Because the Universe has been expanding this entire time, the wavelength of that initially hot light has gotten stretched, all the way from gamma rays through visible light and into the microwave portion of the spectrum. This leftover glow from the Big Bang shows up today as the Cosmic Microwave Background, or CMB. Today, it’s perhaps the best piece of evidence we have for what the Universe is made of.
