Lecture 39 – Cosmology III – what proof do we have that the Big Bang Occurred? Big Bang = highly compressed, hot stage of the universe Old alternative…”Steady State” model of the universe, galaxies move apart, new ones form in the holes

The Cosmic Background Radiation For first 400,00 years, everywhere hot gas, conditions like solar photosphere Matter radiated and absorbed light Light was emitted and absorbed by matter

As the universe expanded, the gas cooled Question: what would have happened to the radiation as the universe expanded? Redder or bluer?

When the gas cooled to about 3000K, recombination occurred When the gas cooled to about 3000K, recombination occurred. E+p H, neutral gas. Absorption of light was now negligible, universe became transparent BIG REALIZATION was that this radiation should still be here, redshifted by z=1000. There should be a glow everywhere in the sky, describable by a blackbody with a temperature of 3K.

The Cosmic Background Radiation The “surface of last scattering” The afterglow of the Big Bang

Cosmic Microwave Background was discovered in 1965 This really is a map of the sky, black zero emission, green that of a blackbody of 2.7K!

To see ripples in the CMB, you really have to go down in sensitivity Results from spacecraft WMAP, released on February 11, 2003

From these tiny fluctuations grew the structure we see in the universe today Clusters of galaxies, galaxies, and stars

So what kind of universe do we live in? open, closed…? Recall that it depends on the mean density Via the parameter

Determining the mean density of the universe

The answer is…. For “bright matter” omega = 2.2E-03 All “baryonic matter” (from abundance of light elements), omega = 0.02 – 0.05 Taking account of dark matter in clusters of galaxies, omega = 0.33 Best estimate is that omega < 1 Abstract of Turner article

The truly weird feature of this is that most of the matter in the universe is not even Baryonic, and thus is of an unknown form. Taken at face value, this result indicates that omega is < 1, and we live in an open universe. This was the status a few years ago. Then things got even stranger