Essay: What is the Cosmic Microwave Background?

In 1965, two young astronomers at Bell Labs discovered an annoying hiss coming from their radio telescope. It turned out to be the Cosmic Microwave Background (CMB): a vast sea of energy left over from the Big Bang, perceptible as a whisper of microwave radiation. Fourteen billion years old, the CMB is the oldest light in the Universe.

The CMB covers the whole sky, but that doesn't make it easy to observe. Since the light waves that make up the Cosmic Microwave Background have stretched so significantly over time, they are no longer visible to the eye and can only be detected by antennas and receivers specially designed to pick up light with long wavelengths, such as microwaves. These antennas are also very, very sensitive to differences in temperature. (Imagine a scale that could measure the weight of an elephant down to a millionth of a gram.) The first evidence of tiny fluctuations in this microwave radiation was found in 1991 by NASA's Cosmic Background Explorer (COBE) satellite. These are represented by hotter (redder) and cooler (green or blue) zones, or blobs, in a map of the CMB.

Experts believe that when inflation ended, it had the effect of a rock being tossed into a pond on a calm day. Waves rippled through the young plasma (a soup of matter and radiation that exists only at extraordinarily high temperatures). This cosmic jiggling created hot spots where matter clumped together and cold spots where it was less dense. Matter and radiation were coupled at that time because the photons of light couldn't travel far before getting reabsorbed by the plasma.

When the infant Universe was around 400,000 years old, it had expanded and cooled enough for the radiation to decouple from the matter and stream out into space in all directions. Although the radiation was no longer affected by the behavior of matter, it bore the imprint—the visual fingerprint—of the distribution of matter at the moment of decoupling. That radiation is what we now call the CMB, and it is a snapshot of that moment 400,000 years after the Big Bang when light was first free to travel. Because the denser, hotter areas served as gravitational seeds for future cosmic structures like clusters of galaxies, these temperature fluctuations carry information about the structure of the Universe in its infancy. In cosmological theory, the CMB's temperature fluctuations are the key to understanding today's cool Universe, with all of its spread-out galaxies, in terms of the hot dense plasma that made up the early Universe.

What do the blobs in the CMB tell us?

A key prediction of inflation theory—which posits the Universe's expansion from something smaller than a proton to something about the size of a grapefruit in roughly 0.00000000000000000000000000000001 of a second—is that temperature fluctuations in the background radiation should form a uniquely distinct pattern of warmer and cooler blobs. Inflation predicts what the CMB map should look like.

Think of a bell. When struck, it produces a tone that relates to the size and shape of the instrument. It also produces overtones, which contain additional information about the bell. Like a bell, the Universe is an object, with a certain size and shape and composition. In effect, inflation rang the bell of the Universe, sending a tone, and overtones, rippling across it. The overtones appear on the map as the smaller blobs. Theorists were relieved when this wave pattern was detected in 2001 by a team from the University of Chicago using a telescope at the South Pole called the Degree Angular Scale Interferometer, or DASI.

These observations yield a precise measurement of the total amount of matter and energy in the Universe. (The larger the clump, the more matter and energy.) Since the size and distribution of these clumps match the inflation model, the CMB provides impressive evidence for this theory of the origin of the Universe. "We believe that the Microwave Background is going to be a Rosetta Stone for cosmology," predicts Michael Turner of the University of Chicago.

Why are astrophysicists so excited about the CMB?

Cosmology, the study of the evolution of the Universe, has experienced an explosion of activity since the discovery of the CMB. The field has changed from a purely theoretical enterprise to the empirical study of what populates the physical Universe. Says Turner, "Cosmologists are right at the cusp. We have these fantastic ideas about the Universe, and we now have the technology and the instruments to test them."

Inflation predicts just how much mass and energy the whole Universe contains, and CMB evidence supports this prediction. According to our measurements of the number of stars and galaxies, ordinary matter—the substance of which planets, stars and human beings are made, and which reflects and emits light—makes up only about 5 percent of the mass and energy in the Universe. Of the remaining 95 percent, about 25 percent is now thought to consist of "dark matter," a type of matter that does not interact with light, but whose existence can be deduced by the gravity it exerts. The two kinds of matter combine to make up 30 percent of the total matter and energy of the Universe. What does the remaining 70 percent of the cosmic pie consist of? Until five years ago, we didn't have a clue.

In the late 1990s, two independent experiments involving supernovas showed that the Universe is not just expanding, as Edwin Hubble's 1929 observations suggested, but expanding at an accelerating rate. This meant that there had to be a force counteracting gravity and pushing galaxies apart at a faster and faster rate. In 1905, Einstein had shown that energy and mass are equivalent. His transformation equation (E=mc2) established that the energy needed to explain the acceleration of the Universe would be equivalent to 70 percent of its total mass-energy content. Incredibly, this percentage conforms exactly to the CMB measurements of what is missing after matter and dark matter have been taken into account. "It is the piece of the puzzle that we were looking for. It is the energy that balances the books and makes everything fit together," Turner explains. Scientists are calling this mysterious substance "dark energy," though they don't know what it actually is. "It's new physics," says astrophysicist John Carlstrom. "It's just a terribly exciting time."

More data, bigger questions

The mystery of dark energy leads to many other baffling questions, requiring cosmologists to rethink fundamental notions about the nature of the Universe. Some of the new ideas are downright bizarre, like the implication that the Universe we see is just a tiny piece of a much more vast Universe, or just one of an infinite number of bubble Universes constantly being born. Will fundamental physical laws explain what processes governed the formation and composition of our Universe, or reveal it to be the result of one of many possible patterns? Were there many Big Bangs, or just one? Did anything exist before the Big Bang? We don't know—nor do we know whether we'll ever find out. The only certainty is that more cosmic puzzles lie ahead. "If inflation is right, we will have made a giant step in our understanding of the Universe. And we will have answered almost all of the outstanding questions that the generation before us put forward," says Turner. "But inflation theory still leaves some unanswered questions, and I think that's what theorists like. We don't want to get to the end of it all at once, because then there won't be any more fun."