### Assignments:

Unfinished Assignment Study Questions for Lesson 58

### Lesson Objectives:

- The cosmic microwave background
- Abundance of elements

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.