What was the universe like just 1 million years after the big bang?

• At very early times, the hydrogen and helium in the universe would have been a gas spreading throughout space.
• The gas was expanding.
• Expanding gasses cool.
• So the early gas would have been hot.
• At an early time, say 1000 years after the big bang, it would have been an ionized gas.
• The ionized gas would have been full of photons.
• The photons would exhibit a black body spectrum.
• But it would have been opaque.
  • Each photon could go only a short distance before being absorbed by an electron or proton.
• At some time (~1 million years) the temperature would drop below about 3000 K.
• Then atoms would form.
• Then the gas would be transparent.
• From then on, the photons would move through the universe without being absorbed.
• They are still here.
• But, the spectrum is redshifted.
• A redshifted blackbody spectrum is still a blackbody spectrum, but at a lower temperature.
• Dicke and Peebles estimated that the temperature would be just a few K.

Thinking about the microwave background

Microwaves to make your phone work better

• In the early 1960s, Arno Penzias and Robert Wilson were working on a large horn shaped microwave antenna at Bell Telephone Laboratories.
• They were interested in communications with satellites.
• In 1964, they found radio noise that wouldn't go away.
• It was coming from everywhere.
• Eventually, they learned that the Princeton people had a cosmological explanation.
• Penzias and Wilson had discovered the cosmic background radiaton.

COBE

Was it really blackbody radiation as expected for the radiation left over from the big bang?

Further experiments over the years indicated ``yes'' but they were difficult and not very precise.

In 1989 a sattelite called the Cosmic Background Explorer (COBE) was placed into orbit. Its purpose was to take a detailed look at the radiation.

Results:

• The radiation is very accurately blackbody radiation with a temperature of 2.73 K.
• The radiation is a little warmer in one direction and a little cooler in the opposite direction.
  • This indicates that we are moving through the radiation with a velocity of 390 km/s.
  • Taking into account our motion about the center of the Galaxy, the Galaxy is moving with a velocity of about 600 km/s.
• Except for this directional difference, the radiation is very isotropic.
  • That is, it looks the same in all directions.
  • That means that the gas from which it came was very homogeneous.