Cosmic Microwave Background Radiation

Scientific discovery rarely unfolds smoothly or predictably. Eight years before the theoretical prediction of relic radiation, Andrew McKellar measured the spectra of stars and discovered interstel­lar material that was excited to a temperature of 2.3 K.17 He had no explanation for the excitation, which is caused by the radia­tion from the big bang.18 While Gamow’s prediction of a universe bathed in cold, microwave radiation sat in the literature, several experimenters had the detection of the radiation within their grasp but either did not control systematic errors well enough or were not aware of the importance of the observation. Robert Dicke at Princeton was, and by 1964 he and his team were hot on the trail of the big bang signature. But as they were preparing a radiometer on the roof of the physics building they got a call from Bell Labs. “Boys, we’ve been scooped,” was Dicke’s memorable response.19 The Nobel Prize was awarded in 1978 to Arno Penzias and Robert Wilson of Bell Labs for their discovery.

What was the nature of the radiation that Penzias and Wilson detected? To understand this involves rewinding the history of the

universe to its very early epochs. The Hubble expansion is a lin­ear relationship between distance and recession velocity: more dis­tant galaxies are moving away from us quicker. Although at first glance this seems to imply that we have a privileged location in the universe, a hypothetical observer in another galaxy would see exactly the same relationship that Hubble saw. In a uniform three­dimensional expansion each observer thinks they are the center of the universe. Since all cannot be, none are. Nor can we see an edge to the universe, so we can’t place ourselves with respect to a boundary. The Copernican principle holds. There’s no discernable center to the universe.

Reversing the expansion projects to a time when all galaxies were on top of each other: the big bang. But a simple backward extrapolation overestimates the age of the universe because matter tugs on other matter, so the expansion rate has slowed since the earliest epochs. In an expanding universe model the major observ­able is redshift, a stretching of the radiation from distant galax­ies due to the expansion of space-time itself. Redshift is simply related to the factor by which the universe has expanded since the radiation was emitted. One plus the redshift is the expansion fac­tor. Ironically, the universe is easier to understand in early times. Before structure forms, the universe behaves just like a simple gas, where the temperature and the average density both increase going back toward the big bang. Once gas starts to collapse by grav­ity, the physics is very complex. Stars and galaxies started form­ing when the universe was about ten times smaller than it is now, about 13 billion years ago.20

Extrapolating further backward, there was a time when the uni­verse was much denser than it is now and hot enough that atoms were ionized. Electrons liberated from atomic nuclei interacted with radiation and stopped the photons from traveling freely. It was as if the universe was shrouded in an impenetrable fog. As the universe expanded and thinned out and cooled, it became trans­parent and radiation could travel without interruption. This spe­cial epoch is the earliest time we can “see” into the universe. In a big bang model, the background radiation comes from a time when the universe was a thousand times smaller than it is today, and a thousand times hotter. Infrared photons from 380,000 years after the big bang, when the temperature was about 3000 K, have been stretched a thousand-fold to become microwave photons in a vast and frigid universe with a temperature a little below 3 K.

The picture of the sky in microwaves is an extraordinary baby picture of the universe. Imagine as an adult that you were shown a picture of yourself a few hours old. Since those waves are from the universe as a whole, they permeate space and they travel in every direction through expanding space. There are trillions of relic photons from the big bang in any volume like that of one breath. However, their radiant intensity coming from any direction in space is only 0.00001 Watt or a ten-millionth of a light bulb.21 If you can find an old-fashioned image tube TV and tune it between stations so you see only static, about 1 percent of the white specks on the screen are interactions of the dots of phosphor with those microwaves.22 The big bang is all around us.