Cosmic Microwave Background Anisotrophy Detected: What Does It All Mean?

May 1, 1992

By Jeffery Kahn,

Since 1964, when cosmic microwave background radiation (CMB) was first discovered, scientists have searched the skies, seeking evidence that the temperature of the CMB is not exactly uniform in all directions. These CMB inhomogeneities -- if they could be detected -- could be mapped, pointing scientists toward a better understanding of how our universe has evolved over its 15-billion year history.

At a 1977 American Physical Society meeting, a team led by astrophysicist George Smoot of LBL's Physics Division and UC Berkeley's Space Sciences Lab reported the first measurements of temperature variations in the microwave sky. Obtaining its data from onboard a NASA Ames U-2 jet, the team announced detection of a "dipole anisotropy."

Slight temperature variations were measured when instruments were pointed at 180-degree opposite directions in the microwave sky. The research team attributed this difference not to early conditions in the universe, but to the motion of our own galaxy in space. Researchers said a Doppler shift was being detected, causing the sky to appear hotter in the direction the galaxy is heading, and cooler behind.

Smoot, along with many other scientists around the world, continued his quest for what he believed would be minute CMB variations across the breadth of the entire sky. Like most researchers, he believed the best way to detect these differentials would be from a satellite in space.

At that time, Smoot and his team had begun work on a new satellite instrument, the Differential Microwave Radiometers (DMR), that he had first proposed be built back in 1974. Rather than measure temperature directly, a differential radiometer uses a pair of antennae to detect the difference in temperature between two separate parts of the sky. This particular DMR measures the difference in microwave power between two regions of the sky separated by 60 degrees.

The DMR actually includes three radiometers, each designed to detect a specific wavelength of microwaves. Specific frequencies were targeted for measurement where the signal is strongest from the deepest, ancient reaches of space rather than from "local" galactic sources. In fact, at the frequencies selected, the CMB is more than 1,000 time stronger than the galactic microwave emissions.

After years of work, the DMR was launched aboard NASA's Cosmic Background Explorer (COBE) satellite on November 18, 1989. One of three instrument packages aboard COBE, the satellite was placed into a near-polar orbit. Operating 900 kilometers above the Earth, COBE is high enough that the residual effects of the Earth's atmosphere do not affect its instruments, yet low enough to minimize interference by the charged particles in the Earth's radiation belts.

The DMR is incredibly precise, able to measure temperature differences of one part in a million. Three independent techniques have been used to calibrate the instrument. Solid-state noise sources provide in-flight calibration as do measurements of the moon, which are compared to known values. Thirdly, the instrument was carefully calibrated inside a cryogenic vacuum chamber prior to launch.

Researchers devised computer models to separate the CMB signal from noise such as the magnetic effects of the Earth and microwave emissions from within the galaxy. Additionally, the three sets of data from the three radiometers provide a high degree of redundancy, helping to identify any anomalies.

For Smoot, last week's announcement of the discovery of evidence for variations in the cosmic background represents the fruit of two decades of research. Detected by their temperature differentials, these distinct regions of space are believed to be the primordial seeds produced in the Big Bang. Scientists believe that over billions of years these structures evolved into galaxies and the large structures of the present-day universe.

Says Alan Kogut, a member of the research team, "We're looking at something older and larger than anything else we know. The smallest area we can measure with the DMR would hold millions of galaxies like our Milky Way, with plenty of room between them. If the theories are right, the patterns we see in the DMR sky maps were laid down immediately after the Big Bang, 15 billion years ago. Compared to that, the dinosaurs might as well have died yesterday. It's as close to the start as we're ever likely to get."

The temperature and size of the spots detected by the NASA COBE DMR team are in agreement with theories that state that anywhere from 20 to 90 percent of the matter in the universe is "dark matter."

Dark matter remains an enigma to science. Its existence is inferred only because its gravity influences the motion of ordinary matter. Yet, dark matter is unlike ordinary matter or anything conceived even in laboratory accelerators.

Commenting on the nature of the hot and cold regions his team has discovered, Smoot says, "This is like looking at the invisible man and seeing only the footprints. In the case of these primordial structures, what we're seeing is only the gravitational effects. That's its footprint.

"Theory tells us that these vast regions cannot be composed of just ordinary matter. It must be something we've never seen in our laboratories. If it were ordinary matter, which does interact with light, then we would see much greater temperature variations. Only matter that does not interact with light except through gravity could have such slight temperature variations."

Currently, more than 100 theories exist on the nature of dark matter. These theories must deal with how dark matter evolved to make large scale structures (galaxies) as well as how it would manifest as temperature variation in the microwave sky.

"Because of the constraints imposed by these new findings," Smoot says, "we now have narrowed the field of dark matter theories to a handful of candidates. Our results, when coupled with the ultimate discovery of dark matter, will unify physics on the largest and smallest scales, fusing together the fields of particle physics and cosmology."