What was the universe like just 1 million years after the big bang?
Around 1960, physicists at Princeton including Robert Dicke and James Peebles were thinking about this question.
- 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
- 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.
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
- Penzias and Wilson had discovered the cosmic background radiaton.
Penzias and Wilson could measure at only one wavelength.
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
- 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.
Davison E. Soper, Institute of Theoretical Science,
University of Oregon, Eugene OR 97403 USA