The Cosmic Microwave Background
When you look up into the night sky, between the light of stars and galaxies, it seems completely dark. However, if you were to observe the sky with a telescope sensitive to microwaves, a faint glow would be noticeable. In 1964, Arno Penzias and Robert Wilson were experimenting with the Holmdel Horn Antenna in New Jersey, when they detected low energy noise that they could not explain. After eliminating a range of possibilities (including pigeons that were nesting in the antenna!), they concluded that the noise originated from outside our galaxy (although they were unsure of the source). In fact, they had observed the cosmic microwave background (CMB), which provides compelling evidence for the Big Bang. This was arguably one of the most important discoveries in cosmology since Edwin Hubble had concluded that the universe was expanding, and Penzias and Wilson would go on to win the Nobel Prize for Physics in 1978 for their discovery. The CMB is almost uniform in all directions, with it peaking at a wavelength of 1.873mm, corresponding to a temperature of 2.725 Kelvin (so about 3 degrees above absolute zero). There are fluctuations in the CMB of about ± 200 microKelvin, as shown in the image above from the WMAP satellite. The fluctuations are thought to be because of irregularities in the density of matter about 300,000 years after the Big Bang. At this time, the universe consisted of hot, glowing gas that was about 3000 Kelvin. As the universe expanded, the light waves from this gas were stretched, increasing their wavelength by a factor of about 1000, resulting in the less energetic microwaves we observe today.
Guest article written by Emma Alexander (astronemma.tumblr.com)