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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.

SOURCE: SciDAC || SciDAC is a program within the Office of Science of the U.S. Department of Energy seeking to enable Scientific Discovery through Advanced Computing.

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].

Analysis of the Planck cosmic microwave background data is available at JPL / Caltech and via the European Space Agency. For a more visual approach, check out Planck Cosmology Results 2013.

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