True ZITs: can such things be?

True ZITs: Can such things be?

When observers discovered the cosmicbackground of microwave radio radiation 20 years ago, they were happy with its apparent smooth isotropy. In any direction they looked, they saw radiation of the same temperature, and that was good because they believed that the universe was homogenous and isotropic, and so the microwave background, which comes down to undisturbed from early times, should have been smooth.

Now, observers would like to find minutefluctuations in the temperature of the background radiation. If they don't eventually find them, a good deal of present cosmological theory will go overboard.

The good news is that scientists havereported three measurements that could be one sort of the desired anisotropies. More work is necessary to find out if they are the real things, but one of the experts in the field, David Wilkinson of Princeton (N.J.) University, comments: "It smells as though we're getting close to anisotropy, close to a real detection."

In general, observers distinguish threesize scales of temperature fluctuation in the background radiation. One refers to the present time, and two refer to distant epochs in the past. The large scale covers an area in the sky 90[deg.] across of greater, with the background slightly warmer over the area of that size and slightly cooler over another. This scale refers to the present time, and an anisotropy on this scale, the "dipole anisotropy," has been known for several years. Astronomers take it as the result of a motion of our galaxy toward the galaxies in the Virgo cluster.

The smallest scale, fluctuationsaround a minute of arc in extent, would result from fluctuations in the density of matter in the universe that occured at the very beginning and might have been the seed around which galaxies and clusters of galaxies formed. No hint of these has been seen.

The present tentative results concernthe middle range, areas a few degrees across. At the recent 13th Texas Symposium on Relativistic Astrophysics held in Chicago, three groups reported apparent fluctuations of temperature in areas 5[deg.] or 8[deg.] across. They are Edward Fomalont and Kenneth I. Kellermann of the National Radio Astronomy Observatory in Charlottesville, Va., who worked with the Very Large Array (VLA) of radiotelescopes near Socorro, N.M.; Bruce Partridge of Haverford (Pa.) College, who also worked with the VLA; and Rod Davies of the Nuffield Radio Astronomy Laboratories in Jodrell Bank, England, and Anthony Lasonby of Cambridge University, both working with an antenna in the Canary Islands.

Typical of what these groups find is atemperature variation of 5 parts in 100,000 over an area of 8[deg.] in the sky. Mapped with false colors to indicate temperature differences, such a pattern looks like acne, and so some of the scientists involved call this the ZIT model.

The ZITs refer to an interesting time, ascosmologists describe it, back at a red-shift of 1,000. (For comparison, the most distant and oldest quasars we can see have redshifts around 4.) Wilkinson translates a redshift of 1,000 as when the universe was 100,000 years old (compared with a present age between 10 billion and 20 billion years). The ZITs thus refer to fluctuations in the famous cold, dark matter that seems to pervade the universe and may be enough to close it. They also refer to regions that in earlier times had developed out of contact with each other and were then beginning to overlap each other.

In the earliest days of the universe, twodifferent expansions were concurrently going on. These were the expansion of space -- that is, of the universe itself -- and the expansion of our (or any observer's) horizon. Astrophysicist David Schramm of the University of Chicago says that people may deceive themselves if they imagine the Big Bang as a kind of explosion. He prefers to use the analogy of raisin-bread dough. As the dough rises, it expands everywhere fairly evenly. As a result, the raisins are carried farther and farther from each other, even though they do not move with respect to the dough right around them. Galaxies, like the raisins, are carried farther and farther from each other as the space between them (like the dough) expands, but they do not necessarily move with respect to the space right around them.

Meanwhile our horizon is expanding,too. At any time we can see only the objects from which light has had time to get to us since the beginning of the universe. If we could have been around when the cosmos was 1 second old, we would have seen only objects less than 1 light-second away, not as far as the moon now is. As time goes on, each observer sees objects farther and farther away. Horizons expand at the speed of light; space expands presumably at a different speed. Expanding horizons may gradually gain on the expansion of space, or they may not.

It is not clear how big the universe wasat time zero, though it seems to have been extremely smaller than it is now. However, in the earliest moments, a number of regions could have developed independently of one another, because their horizons did not overlap and they could not communicate and so could not affect events in each other. Eventually horizons began to overlap. Each observer began to see regions that had developed independently of his or her own immediate surroundings. The differences between them should show up as minute variations in the equilibrium temperature of the cosmic microwave background, on the order of 1 part in 1,000 or less. Such phenomena should produce the ZITs.

Are the reported observations reallyZITs? Wilkinson cautions that they could be galactic bremsstrahlung, radiation produced by galaxies moving through the intergalactic gas. Spectra will tell the difference: True ZITs will have the blackbody spectrum characteristic of the cosmic microwave background. The present observations are all at single frequencies, because radiotelescopes observe one frequency at a time. Astronomers are now planning to look at other frequencies to see whether they can fill in the proper spectra.