Michelle Thaller
Cosmic Microwaves: Warm and Fuzzy All Over
All throughout history, it seems like the most important, epochal scientific discoveries have been made completely by accident. Modern astronomy is no exception.
Take the discovery of the microwave background radiation in 1965. Two astronomers, Arno Penzias and Robert Wilson, constructed a telescope in New Jersey (that looked for all the world like a giant ear-horn) to look for natural sources of microwaves in space. Things were going fairly well, but everywhere they looked, there seemed to be some background noise, a kind of microwave static.
The two researchers came to the natural conclusion that if this noise was the same in all directions, then it must not be coming from the sky at all, but from a flaw in their instrument. As they poked around the interior of the telescope, they immediately found the problem -- pigeon poop. I'm totally serious here. A flock of pigeons had taken roost in the telescope, covering the interior with the usual residue associated with uninvited feathered guests. And nitrogen-rich pigeon droppings do contain a molecule whose natural vibrations can create microwaves.
So, after chasing away the pigeons and completing a fairly odious scrubbing-out job, the astronomers sat back down to their experiments. Lo and behold, the pesky microwave noise was still there, still the same in all directions.
And thus, in 1978, Penzias and Wilson received the Nobel Prize in physics for discovering that the mysterious microwaves did not, in fact, come from pigeon poop. Many scientists believe that what they had discovered remains one of the defining achievements of the 20th century: the observational relic of the Big Bang, what we now call the microwave background radiation. The Big Bang is the humorous name for our current scientific theory of the beginning of the universe. And while astronomers and physicists argue endlessly about the details of this cosmic origin, most of them agree on the essence of the Big Bang: fifteen billion years ago, the universe exploded.
Nobody pretends to know why this explosion happened, or what the universe was like before. But Penzias and Wilson's discovery transformed the Big Bang from an esoteric theory in a real, observable event. So what are these microwaves, exactly?
Moments after the Big Bang, the universe was extremely hot. At first we don't even have words for how hot it was (we get tired of writing out all those zeroes, too), but after a few seconds, it had cooled down to a manageable few billion degrees. The reason the universe was cooling off so fast was that it was expanding.
And here's where things get a little unintuitive. When you think of a normal explosion, you think of stuff flying apart in all directions, into empty space. That's not what happened during the Big Bang. The universe is everything there is -- all the space, time, and matter that exists. There's no "empty space" outside the universe for it to expand into.
Instead, space itself expanded. Every part of the universe got farther away from every other part, with no edges to the explosion, and no empty center either. It's difficult to think about, and in fact, astronomers have had to invent some extra dimensions of space to explain it.
So, as the universe expanded, the intense heat and energy of the Big Bang spread out with the increasing space, cooling off as it did. But even after 15 billion years or so, the universe hasn't cooled off completely. And that's what the microwave background radiation is: the residual heat of the Big Bang, cooled off over the lifetime of our universe.
The cubic centimeter of space right at the end of your nose glows faintly, in microwaves, from the aftermath of the Big Bang. So do the vast light-years of space between the galaxies. That's why the microwaves are the same in all directions; every bit of space was present at the moment of the Big Bang, and every bit still has a faint glow from that unimaginably powerful explosion.
You can actually detect this background radiation yourself, in the comfort of your own home. By turning your television to an empty channel, you can find that fuzzy, gray static that used to be common before the advent of cable television (you should disconnect the cable first). About one fourth of the static-ey blips of light that swarm around on your television screen are caused by the cosmic microwave background, and come from the Big Bang.
While it does make me feel kind of warm and fuzzy (as microwaves would, I suppose) to think about this marvelous glow that connects the entire universe together, scientists didn't stop there. If this radiation really is a direct relic of the Big Bang, then might there be some information hidden in it that describes the origin of our universe? For a long time, nobody could wring any more clues out of the microwaves. In every direction, their temperature was constant and smooth, to any accuracy astronomers could measure.
In 1989, things changed dramatically. Despite its enigmatic uniformity, the microwave background was deemed important enough that NASA launched a dedicated spacecraft to study it. The Cosmic Background Explorer (COBE) was sent up in the hope that above the interference of our atmosphere, in the cold vacuum of space, the background would yield up its secrets. The accuracy of COBE's detectors was staggering; it was able to detect variations in the background on the order of millionths of degrees. And at that level of scrutiny, a whole new universe opened up.
COBE was designed to create a map of the entire sky, detecting areas where the microwave background (and the early universe) was slightly warmer or cooler than average. And it turned out that the microwave background did have variations, although they were tiny. But tiny temperature variations in the early universe have, well, astronomical repercussions.
We have no idea why or how, but the explosion of the Big Bang seems not to have gone off entirely smoothly. Some regions of the young universe were just slightly more dense than others, and as the universe expanded, these denser regions became areas where matter could clump together, forming the first building blocks of galaxies. Without these variations, the universe would have evolved into a dull, diffuse haze of disconnected hydrogen and helium atoms.
When astronomers looked at these temperature variations, what they were really seeing was the very first footholds of life in the universe. Without these miniscule flaws in the structure of the early universe, matter could not collected together, forming galaxies, stars, planets, and human bodies.
Further studies of the microwave background have been even more intriguing. A series of instruments flown on high-altitude balloons out of Antarctica has revealed the large-scale shape of the variations, and an amazing pattern has emerged. The tiny temperature variations look exactly like what you get when you ring a bell.
When you hit the side of a metal bell, waves of compression run through its body. Some areas of the bell get denser as the metal is compressed, while other areas get stretched. The compressed bits of the bell heat up, just a tiny amount, while the stretched bits cool off. These vibrations in the metal bump up against the air surrounding the bell, and the characteristic "ringing" sound waves are created. The variations in the microwave background look just like this. It's as if a giant sound wave moved through the entire early universe, making it ring.
Back when the microwave background was first created, the universe was very, very much denser than it is today -- it hadn't had much time to expand yet. The density of the entire universe might have been comparable to water, although it's hard to imagine what a fluid would behave like at the super-heated temperatures after the Big Bang.
Still, what we seem to be seeing in the microwave background variations are sound waves traveling through the entire universe, compressing some areas together, while stretching others apart. In essence, some vast, cosmic sound (voice, if you don't mind the anthropomorphizing) created the first structures in the universe, which somehow became us. And this voice is still audible, just, in the millionth-degree variations in the microwave background. It's singing in every tiny bit of space, including that bit at the tip of your nose.