# What’s the temperature of space?

## A STAFF REPORT FROM THE STRAIGHT DOPE SCIENCE ADVISORY BOARD

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Dear Straight Dope: What is the temperature in space? I can imagine that it changes in our solar system relative to the distance from the sun but what about outside our solar system or galaxy, in the middle of nowhere? Does the universe have an average temperature? Does it get colder as the universe expands? Do solar systems keep heat in? Just a thought. Henk

Karen replies:

The temperature of the universe is 2.725 +/- 0.002 degrees Kelvin. The Kelvin scale, for those who slept through high school physics, is the same as the Celsius scale, only it starts at absolute zero. In short, the universe is pretty cold. Fortunately, our Milky Way galaxy is about 0.0001 K toastier.

Measuring the temperature of the universe isn’t as easy as you might think–you don’t just stick a thermometer into space and see the mercury shrink to the 3 K tick mark. We measure the temperature of the universe by using Planck’s famous black body radiation law, one of the first results of quantum physics. This law states that every object radiates electromagnetic energy according to its temperature. Planck’s law gives the spectral radiance as:

R(l,T)2phc2l5(exp(hc/lkT) – 1)

The shape is a lopsided bell curve, and the peak is given by Wein’s displacement law, lmax = C/T, where C is the constant 2.898 x 10-3 Kelvin-meters. Black body radiation explains how night vision goggles work. Humans have a temperature of about 98.6 degrees F; if you plug that into the equations, you find that humans give off peak radiation in the infrared. SARS detectors in Asian airports look for humans with temperatures greater than 99.5 degrees F. Black body radiation is also how we measure the temperature of stars: their temperature is directly related to their color.

(A note about radiated vs. reflected light: your shirt isn’t yellow because it is 5000 degrees, but because the dyes in the shirt selectively reflect yellow wavelengths.)

So now that we know how to measure temperature by looking at the emitted radiation spectrum, you can look at the radiation coming from empty space. The measured spectrum peaks in the microwave range and turns out to be a black body spectrum for an object of 2.725 K temperature. In fact, the universe is the most perfect black body radiator ever measured. (Most physical objects absorb or reflect light in certain wavelengths, which distorts their black body spectra.)

The universe’s radiation was discovered by accident in 1965 by Arno Penzias and Robert Wilson. They were investigating communications equipment, and thought there would be less noise at microwave wavelengths. Alas, they found all kinds of noise. Naturally they thought the problem was with their equipment and tried to eliminate the noise by cooling the equipment in liquid nitrogen. Still noisy! Finally in frustration they aimed their antenna into space and found . . . the same noise. Clearly the radiation “noise” they found was coming from the universe, and was named the cosmic microwave background (CMB).

The CMB had been predicted many years earlier in the 1940s when George Gamow and his students realized that the radiation emitted by the universe when it was young and hot should still be around, red-shifted to a few degrees above absolute zero. The discovery of the CMB was compelling evidence of the Big Bang, and won Penzias and Wilson the 1978 Nobel Prize.

In 1989 the COBE (COsmic Background Explorer) mission was launched. One of its missions was to measure the temperature of the universe accurately. COBE found that the temperature of the universe is 2.725 K. This temperature agrees remarkably well with that predicted by the Big Bang theory.

But COBE scientists didn’t stop there. They noticed that the universe looked a little cooler in one direction, and a little warmer in the other direction. This is called the dipole variation and is caused by the Doppler effect of our solar system’s motion relative to the cosmic background radiation, enabling the COBE scientists to measure the speed of the solar system: 371 km/sec.

COBE didn’t stop there either. After correcting for the Doppler effect, scientists found that the temperature of the galactic plane (coming from our galaxy) is slightly warmer than the rest of the universe. That’s right–we live in the tropics of the universe.

Still more: in 1992 COBE found that the CMB from outside our galaxy is not uniform! There are hotter spots and colder spots (we’re talking temperature swings of 0.0002 K) in the universe. While the temperature of the universe and the galaxy and the speed of the solar system were interesting results, the discovery of the anisotropy (variation) of the CMB was huge, HUGE! The variation in the universe’s temperature shows how the matter and energy of the very early universe (300,000 years of age) were distributed. In order for the mass of the universe to be clumped together nowadays in galaxies and galaxy clusters, theory requires that the early universe be non-uniform. The COBE discovery revolutionized cosmology. The data give us rich information about the initial conditions of the universe, which is almost better than being present at the Big Bang, because it was, you know, really really hot.

A more recent CMB experiment, the Wilkinson Microwave Anisotropy Probe (WMAP), was launched in 2001 to map the CMB more precisely. It’s a little over halfway finished with its four-year mission. WMAP can detect temperature differences of a millionth of a degree. The WMAP results have been used to find the age of the universe (13.7 billion years) and the time when the first stars formed (200 million years after the Big Bang), much earlier than scientists thought. WMAP also found that the rate of expansion of the universe, in case anyone is interested, is 71 kilometers per second per megaparsec. Because the WMAP measures the non-uniformity details of the universe very well, the data have also been used recently to see if the universe is finite but "wrapped." A “wrapped” universe is one that is constructed like a video game: it looks infinite, but when you move off the edge of the left side, you reappear on the right side. (You’ve probably seen Star Trek episodes like this.) If the universe were “wrapped” then you would see the same temperature patterns in two different places in the universe, indicating they are actually the same place. So far no evidence has been found that the universe is "wrapped." You definitely want to keep your eyes open for more wonderful results from WMAP in the next few years.

Karen

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