The Beginning of the Universe

The expansion of the universe, when played in reverse, points back to a time when the entire universe was compressed to a very small region of space. And so, Hubble’s Law provided the first observational evidence that our universe had a beginning. Up until this time, most astronomers assumed that the universe had always existed in its current form - a so-called “steady-state” model of the universe.
Read 29.3 The Beginning of the Universe to understand the origins of the Big Bang theory and how the universe evolved from a hot, dense state to its current form.
We understand a surprising amount about the first moments of the universe, thanks largely to the field of particular physics. In fact, as a young astronomy student, I read the famous book “The First Three Minutes” which is 168 pages describing the evolution of the universe during the first 3 minutes after the big bang. The book includes a chapter called “The First One-Hundredth Second.” (My memory of the book was that it was confusing and boring.)
In the decades after Hubble’s discovery, our understanding of the universe’s origins continued to advance. The most important discovery, by far, was made in 1964 when two engineers accidentally found the cosmic microwave background (CMB), direct evidence of the Big Bang. Watch the video Evidence for the Big Bang which explains how this accidental discovery led to a Nobel Prize.
Read 29.4 The Cosmic Microwave Background to learn more about the discovery and significance of the cosmic microwave background radiation, including its role in revealing the composition and age of the universe.
So, why is it the cosmic microwave background? Why not the cosmic ultraviolet background or the cosmic infrared background? Here are two ways of understanding this:
Redshift. We know that the light from distant galaxies is redshifted because of the Doppler Effect as the galaxies are moving away from us. And we know that the more distant the galaxy is, the more red-shifted it is - this is Hubble’s Law. The light in the CMB is the most distant light we can see and it is the oldest light we can see. In fact, it is the very first light that ever shined in the universe. Given its great age and distance, it is redshifted so much that the light has shifted past the visible range and through the infrared all the way to the microwave portion of the spectrum. Alternatively, we can think of this in terms of
Thermal Temperature. In the past, we learned that stars give off specific colors of light depending on their temperature. Shortly after the Big Bang, the universe had a temperature of a few hundred million degrees. As the universe expanded, it cooled, until today when the temperature of the universe has dropped down to about 3 degrees above absolute zero. This whole time, the universe continued to shine, but at such a cold temperature, the peak wavelength of light given off by the universe is in the microwave part of the spectrum.