Lesson 58: Evidence for the Big Bang
In this lesson, we will look at the two main predictions made by the Big Bang
Theory which have been confirmed, including the existence of the cosmic
microwave background and the observed composition of matter in the universe.
The Big Bang Theory has been widely accepted because of two major predictions
that it makes that have been verified.
As you recall, at the end of the era of nuclei, most of the electrons in
the universe joined with nuclei to form neutral atoms. When this happened, the
photons that had been trapped and were bouncing between electrons were now free
to travel unobstructed through the universe.
If that is really what happened,
scientists predicted that
those photons should still be traveling randomly through space even today. This
prediction was verified in 1965 when two physicists working at Bell Labs
discovered the cosmic microwave background.
Using a satellite that
could detect microwaves, they detected microwave photons coming from all
directions in the sky, and the thermal radiation spectrum of the microwaves
matched the expected characteristics of the radiation released at the end of
the era of nuclei.
In addition to the expected thermal radiation spectrum showed by the cosmic
microwave background, another key feature is how the background temperature is
not perfectly uniform. As shown in the map above, there are areas that are very
slightly warmer, meaning that the early universe was not perfectly uniform.
This supports the idea of the Big Bang Theory that there were areas of the
universe with slightly enhanced density that could have allowed gravity to
collect matter into clumps which later became the seeds for star and galaxy
formation.
Another prediction of the Big Bang Theory involves the proportion of the
elements that are present in the universe. Everywhere in the universe, ordinary
matter is made up of about three-quarters hydrogen and one-quarter helium by
mass.
When scientists take the current microwave background temperature of the
universe, almost three Kelvin, and combine that with the number of protons
observed, they are able to calculate how hot the universe was during the era of
nucleosynthesis and figure out how much helium should have resulted from the
nuclear fusion that occurred. Their calculations match up with the observed
composition of the universe.