The issue here is the evolution of structure in the universe. We see that the CMB is very smooth, and that implies that the matter and radiation in the universe had almost exactly the same density and temperature everywhere at the epoch of recombination (300,000 years A.B.E.). But we see that the universe today is highly structured, with superclusters of galaxies defining the surfaces of giant voids, typically a few hundred million light years in diameter (see Galaxy Clusters and Large Scale Structure). How did the distribution of matter in the universe change from almost perfectly smooth to highly structured?

Astrophysicists believe that the variations in the CMB are manifestations of very slight fluctuations in density of matter in the early universe. After recombination, gravity attracts the matter toward the regions of slightly elevated density, and this process becomes amplified as these regions become denser. We call this process gravitational instability. (In 1684 AD, Isaac Newton pointed out that this would happen.) We suspect that this instability will amplify the almost imperceptible fluctuations that we see in the early universe into the giant structures (voids and superclusters) that we see in the universe today.

Because the universe has expanded by a factor of roughly 1,300 since the epoch of recombination, the giant voids seen in today's universe would have been roughly 1,300 times smaller at recombination than they are today. In fact, such voids come from fluctuations in the Cosmic Background Radiation having typical angular diameters of about 1^o. Note that this angular diameter is nearly the same as the angular diameter of the cosmic horizon at the recombination epoch (see HORIZON). That means: the mass within the cosmic horizon at the epoch of recombination is roughly equal to the mass of a supercluster of galaxies today. Cosmologists think that this coincidence is no accident, because that the density fluctuations at the epoch of recombination will naturally have their greatest amplitude at a scale size corresponding to the cosmic horizon.

The trick is to show that gravitational instability can cause these tiny initial fluctuations to develop into clusters and superclusters of galaxies with a spatial and size distribution similar to that seen in the universe today. To demonstrate that, cosmologists simulate the development of these instabilities with supercomputers. You can find several spectacular MPEG movies illustrating the results of such simulations at the Grand Challenge Cosmology Consortium. The short answer is: they do!

According to these simulations, galaxies and clusters of galaxies began forming at redshifts in the range 3 - 4, when the universe was 2 - 3 billion years old. Observations appear to support this result. If galaxies and quasars were common at earlier times (say, with redshifts greater than 4), we should have been able to find many of them with the Hubble Space telescope and large ground-based telescopes; but we have not. We think that we are now looking deep enough that we can see the universe as it was before most galaxies were born. Moreover, the galaxies that we do see with redshifts in the range 2 - 4 have different shapes than the ones we see today. They may be newborn galaxies -- see Galaxies: snapshots in time.

First Light: However, astronomers have found a few galaxies with redshifts greater than 5, and astronomers suspect that some galaxies must have formed at even earlier times (i.e., at greater redshifts). We believe that the very first galaxies formed when the universe was about one billion years old, and we would expect such galaxies to have redshifts of about 10. We call this the epoch of "first light" -- the time when the very first stars and galaxies in the universe began to shine.

But astronomers probably won't be able to detect such sources with the telescopes we have today. The reason is that hydrogen gas in intergalactic space blocks radiation at wavelengths less than 1216 Angstrom units. But radiation with wavelengths greater than 1216 Angstroms emitted by galaxies or quasars having redshifts greater than 6 will arrive at Earth with wavelengths greater than 8512 Angstroms (7 x 1216 = 8512), i.e., in the infrared band of the spectrum. Even the Hubble Space telescope and the great 10-meter ground-based telescopes are not sensitive enough to detect very distant galaxies at infrared wavelengths. So, to see them, we'll need a powerful new infrared telescope that operates in space, beyond the infrared glow of the Earth's atmosphere. That is one of the main goals of the Next Generation Space Telescope, which NASA hopes to launch before 2010.

The cosmic microwave background revisited: Thanks to such calculations, this story for the origin of structure in the universe is beginning to look very plausible. Indeed, the results from the Boomerang experiment have confirmed that the density fluctuations at the epoch of recombination really do have their greatest amplitude at angular sizes of about 1^o (the horizon size). See MICROWAVE BACKGROUND.

In Summer 2001, NASA launched the Microwave Anisotropy Probe (MAP) satellite. MAP will make an all-sky map of the sky with angular resolution of 0.3^o, sufficiently fine to measure precisely the distribution of scale sizes of fluctuations of the CMB radiation. (Click here to see a comparison of what we expect to see with MAP and what we have seen with COBE.)

In fact, cosmologists think that the results from MAP will tell us all the fundamental numbers that determine the evolution of the universe: the closure parameter, W₀; the densities of ordinary (baryonic) and dark matter; and the cosmological constant, L*. (*You might say: "Wait a minute! I thought you said that the cosmological constant was Einstein's biggest blunder. Why bring it back into the picture?" Well, we may have no choice. See Mysteries.)

• Microwave Anisotropy Probe; and
• An Introduction to the Cosmic Microwave Background by Wayne Hu.

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Last modified April 15, 2001
Copyright by Richard McCray