As a physician, taking temperatures is something very familiar to me. Temperature has long been considered one of the “vital signs” used in conjunction with a constellation of other pieces of information to determine patient’s state of health.
At Amundsen-Scott South Pole Station much effort is directed to taking the temperature of the universe. This is done not to determine its health, but rather to attempt to peer back through the mists of time to its birth. There are several different theories regarding the creation of the universe, but the one that currently best fits with observed scientific data is called The Big Bang. This theory postulates that long ago the universe was a small sphere of white-hot fog composed of hydrogen plasma that rapidly expanded and cooled. When it had cooled enough to allow electrons and protons to form neutral atoms, the previously opaque fog became transparent. Photons of light could now travel through the vacuum of space at 186,000 miles (300,000 kilometers) per second for literally eons and then eventually fall upon the retina of an inquisitive astronomer.
A traditional optical telescope is designed to gather light from distant objects and focus it at a point where the image can be viewed with an eyepiece or a camera. This works fine for glowing objects (stars) or those that reflect light (planets or moons), but not so well for dark objects. It is the dark areas of the sky that are best examined by a radio telescope. Radio telescopes look at the some parts of the electromagnetic spectrum that are outside the range of visible light. The South Pole is especially well suited to explore the microwave segment of the electromagnetic spectrum since the extremely cold air freezes out water vapor that would otherwise absorb energy, much in the same way water in food absorbs energy in a microwave oven and produces heat. As the driest place on Earth, the South Pole provides a clear (microwave) window to the heavens.
As the universe continued to expand it cooled to a very low temperature…just a few (a little less than 3) degrees above absolute zero, which is as cold as anything can possibly become. Even at these low temperatures, the remnants of the original plasma cloud continue to “glow” in the microwave region of the electromagnetic spectrum, well outside the visual range our eyes can detect. This microwave glow was first detected by two physicists at Bell Laboratories in New Jersey in 1964. They were doing some experiments with a large microwave antenna and noticed they were getting a uniform signal wherever they pointed the antenna at the sky. Robert Wilson and Arno Penzias had just serendipitously discovered the Cosmic Microwave Background or CMB, for which they were later awarded the Nobel Prize. The CMB is thought to represent the universe near the time of its creation and appears as a uniform background temperature across the heavens to all but the most sensitive thermometers.
Since the universe is so cold, to take its temperature requires a very special thermometer called a bolometer. Invented by an American astronomer, Samuel Langley in 1878, today the ones in use at the South Pole have a superconducting (essentially no internal resistance to electricity) absorptive element that operates at temperatures near absolute zero. These temperature sensors are able to measure the infinitesimally small differences in temperature that exist in the CMB. Just as you feel warmth from the Sun’s rays falling on your skin, so the bolometer is able to “feel” the warmth of microwaves that have traveled from the edge of the universe. To measure temperatures this low, the thermometer has to be cooled to an even a lower temperature by using helium and special cooling pumps able to chill the sensors to just a few hundredths of a degree above absolute zero.
A map of the CMB is color coded to show the slight differences in temperature observed by instruments at the South Pole and in earth orbit. The CMB of the universe is best represented by a hollow sphere with the Earth at its center and the outer edges of the universe over 14 billion light-years away.
Next time we will take a closer look at some of the instruments measuring the vital signs of the universe.