Untangling the Cosmic Microwave Background
WHY CAN’T WE SEE LIGHT FROM THE BIG BANG?
WHY CAN’T WE SEE ANYTHING BEFORE 400,000 YEARS AFTER THE BIG BANG?
TOP: Primordial photons are scattered in the ionized very early Universe, and free-stream once the Universe becomes neutral. An observer looking out in space, and hence back in time, sees an apparent “surface” of last scattering in all directions.
- In the aftermath of the Big Bang, the Universe was extremely hot and dense.
- Its photons were constantly being scattered by the ubiquitous free electrons.
- As it expanded, the Universe also cooled until, after about 400,000 years, it dropped below 3,000 K, the ionization temperature of hydrogen.
- At this time the electrons rapidly combined with protons to form neutral hydrogen atoms, after which each photon simply continued propagating in whatever direction it was last scattered.
- Known as the cosmic microwave background (CMB), these photons fill the cosmos.
If a TV set is tuned between channels then about 1% of the white noise seen is this remnant of the Big Bang.
However, the continued expansion of space has red-shifted so that the spectrum of these photons peaks at microwave wavelengths today.
As the earliest possible photon-image of the Universe, this is also the background against which all other observations are made.
BOTTOM: The Planck space probe’s picture of the full sky, the CMB — which it took in different bands of the microwave spectrum [published March 2013].