The news is all over the physics blogs today: NASA's John Mather and George Smoot from Lawrence Berkeley Laboratory will share the 2006 Nobel Prize in Physics (and the accompanying $1.4 million), for "their discovery of the blackbody form and anisotropy of the cosmic microwave background radiation (CMB)." Mather and Smoot were the key players on the COsmic Background Explorer (COBE) project, an orbiting spacecraft designed to detect faint temperature variations in the CMB. We here at Cocktail Party Physics join everyone else in raising a glass in honor of Smoot and Mather. And even though it seems a bit redundant, given the abundance of quality blog posts from, you know, actual working scientists, we can't resist offering our own take on the subject.
That's partly because the "map" of the early universe produced by COBE was announced at the 1992 APS April Meeting in Washington, DC -- one of the first physics conferences I attended as a budding young science writer. The press conference was standing room only, with TV cameras from all the major news networks, as well as radio and print reporters, and it was easy to get swept up in the excitement, especially since the gist of what they found could be easily grasped: a snapshot of the universe in its infancy, and an explanation for the origin of galaxies and stars.
A bit less easy for a nonscientist to grasp is why the finding was so significant in terms of evidence for the Big Bang. The critical vocabulary Word of the Day is "Anisotropy": small variations in different directions, in this case, variations of temperature in the CMB in different directions. But lots of things can be anisotropic, even something as basic as the polarizing lenses in sunglasses: if you hold the lens in one direction, polarized light streams through, but hold it in a different direction, and that light is blocked. Since the lens behaves differently depending on direction, it can be said to be anisotropic. Plasmas can have anisotropic properties, too: for example, they may have a magnetic field oriented in a preferred direction. (Jen-Luc Piquant points out that this concept is still a lot easier than trying to explain asymptotic freedom -- or the color interaction of quarks -- which was the subject of the 2004 Nobel Prize in Physics.)
When COBE measured faint fluctuations in the CMB's temperature, it lent considerable support for the Big Bang model of the universe's birth. It could also be argued -- as the Nobel Foundation's press release phrases it -- that "these measurements marked the inception of cosmology as a precise science." That's why Stephen Hawking called the COBE results "the greatest discovery of the century, if not of all time."
Still, like all the best science stories, this one doesn't begin with COBE; if you want to get technical about it, the story begins with the birth of the universe. A few non-scientists might have been wondering, as they watched the news today: What is this thing called the cosmic microwave background radiation? It's literally the remnant of the massive numbers of particle/antiparticle collisions physicists believe occurred in the immediate aftermath of the Big Bang. When opposite particles collide, they annihilate into radiation -- I like to think of that early cosmic environment as a kind of Thunderdome for subatomic particles, matter facing off against antimatter in a great cosmic war of attrition.
Matter won, although why that should be the case is one of the many remaining questions in physics. There should have been equal amounts of matter and antimatter, but for some reason, in the first few fractions of a second of our universe's life, a tiny surplus of matter appeared. And that tiny imbalance was sufficient to wipe out all the antimatter in the universe in about a second. That's because the universe began to cool as it expanded, until the temperature was too low to create new pairs of particles to replenish the "armies". The smattering of leftover matter eventually formed into the planets, stars and other celestial bodies that make up the observable universe. And the CMB is the echo of that great cosmic battle.
This isn't the first Nobel Prize associated with the CMB: Arno Penzias and Robert Wilson shared the 1978 Nobel Prize when they first discovered the CMB in the 1960s, quite by accident. At the time, the physics community was divided into two camps on the question of the universe -- which might come as a surprise to younger non-scientists, for whom the notions of a Big Bang and cosmic inflation are pretty much a given. But back then, the dominant view was that the universe was unchanging and would remain in a steady state forever. A few mavericks argued in favor of the Big Bang model, based on Edwin Hubble's 1929 discovery that the galaxies are moving away from each other. In this view, the universe was once infinitely dense, with all matter emerging in a single cataclysmic explosion. But in order for that model to be correct, there should be the equivalent of a cosmic afterglow of about 3 degrees K, per the prediction of Princeton physicist Robert Dicke. At the time, no one had been able to detect it experimentally.
It's a classic example of scientific serendipity. Penzias and Wilson weren't looking for the CMB; they were using a 20-foot horn-shaped antenna (salvaged form an obsolete satellite transmission system) as a radio telescope to amplify and measure radio signals from the Milky Way and other galaxies. The problem is that they couldn't get rid of all the interference in order to make precise measurements: there was an irritating hissing noise in the background, like static. It was a uniform signal, in the microwave frequency range, and it seemed to be coming from everywhere at once. They tried everything, even installing a pigeon trap to oust roosting birds and removing the accumulated droppings, but they couldn't get rid of the hissing. So they consulted with Dicke, who confirmed the "discovery": "We've been scooped," he told his Princeton colleagues. (The lowly pigeon trap is now part of the permanent collection at the Smithsonian Institute's National Air and Space Museum.)
(Interesting side note: the presence of the CMB is the reason why an official "third law" of thermodynamics is that one can't ever completely reach Absolute Zero, at least until the CMB does. No matter how cold atoms become -- and scientists have cooled them to within fractions of degrees above Absolute Zero, as in Bose-Einstein condensates -- there is still the slightest movement, and therefore heat. The atoms can't become any cooler than the surrounding universe, which is roughly 2.7 degrees K.) [UPDATE: astute readers, like commenter Mary, will notice that my late-night attempt to explain this isn't quite right. BECs are colder than the CMB -- mere millikelvins above Absolute Zero, in fact, although in order to accomplish this feat, scientists must perform all kinds of "tricks" like evaporative cooling and laser traps, in isolated, tightly controlled lab conditions. This is what happens when you over-simplify. I thought a basic thermodynamic explanation would be easier for non-scientists to grasp, but as Mary notes, the more complex, and more accurate, explanation, has to do with uncertainty, i.e., quantum mechanics. So, duly noted. And mea culpa.]
The CMB's discovery made the Big Bang the dominant model for the early universe but scientists were still a bit fuzzy about why there would be stars and clusters of galaxies instead of an evenly distributed dust cloud. They figured there had to be minute temperature fluctuations in the CMB, variations in the density of matter in the early universe -- bringing us back to the Word of the Day: anisotropy. COBE was the first experiment sensitive enough to detect those fluctuations, even though the variations were at the level of parts per hundred thousand. COBE also provided the most precise average temperature of the universe to date: 2.726 degrees K.
Like the radio interference that plagued Penzias and Wilson, the COBE scientists had to weed out a foreground cloud of microwave radiation emitted by our solar system, our galaxy, and most other celestial objects, before they could precisely measure those very faint variations in the CMB temperature. They also had to account for all the motion in the universe: the earth moves around the sun, the sun meanders about the Milky Way, etc.
This brings us to the second key element in the COBE story: the notion of blackbody radiation, which enabled them to make the measurements in the first place. Basically, emitted radiation by the early universe (for our purposes, the "body") is distributed between the various wavelengths of the electromagnetic spectrum, and the shape of that spectrum depends entirely on temperature. So if we know the temperature of such a "blackbody" (technically, a perfect emitter and absorber of radiation, not literally "black"), we can precisely predict what the resulting spectrum should look like. NASA launched COBE on November 18, 1989, and got the first results after a mere nine minutes of observations. The accumulated data points formed a perfect blackbody spectrum -- the universe is a perfect emitter and absorber of radiation. It was such a perfect match with theory that, when the resulting curve was first shown at the 1990 American Astronomical Society meeting, there were audible gasps in the assembled scientists, followed by a standing ovation. From this, the team was able to measure the minute temperature fluctuations in the CMB, and therefore where matter in the universe began to aggregate.
The story doesn't end with COBE, either. The Boomerang and DASI detectors have added even more detail to the microwave background, and most recently the WMAP project supplied the best values known thus far for such critical cosmological parameters as the actual age of the universe (Young Earth Creationists take note: it's a lot older than 6000 years); the curvature of spacetime; and when the first atoms, starts, etc. began to form. Who knows what scientists will find when they begin analyzing the data collected by the forthcoming European Planck satellite?
In honor of Mather, Smoot, and all the fine folks who worked with COBE, we spent the day wearing our spiffy T-shirt depicting this classic cartoon from xkcd: