Ripples in the cosmic pond: astronomers have detected a long-sought relic from the early universe

Natural History, April, 2005 by Charles Liu

The next time you're walking beside a quiet body of water, toss in a handful of pebbles. Each pebble will create ripples that spread outward. Soon the ripples will collide, overlap, and form wavy, cross-hatched patterns that will expand outward to the water's edge.

Ripples permeate the cosmos as well. According to the big bang theory, matter and energy in the universe were originally spread almost uniformly throughout space. But within moments after the big bang, as the universe rapidly expanded, tiny fluctuations--peaks and valleys of varying density--developed in the expanding matter and energy. The densest regions, where gravity was strongest, acted like pebbles in a pond--except that the "pond" they disturbed was the universe itself, and the "water" they rippled was the stuff of the cosmos.

Those gravity-induced ripples, imprinted into the early, expanding universe, should still be visible in the overall distribution of galaxies in space--albeit very faintly, and on vast cosmic scales. Until recently, however, the existence of the ripples could not be confirmed. Now, two groups of astronomers--one led by Shaun Cole of the University of Durham in England, the other led by Daniel Eisenstein of the University of Arizona in Tucson--have announced that the ripples have at last been found.

To find a pattern in nature, you have to know its scale. Think about the bricks in a wall. From a distance, it's easy to see the regular pattern of the brick wall. If you zoom in too close to the wall, though, all you'll see are the random pits of an individual brick. Similarly, if you look at the universe closely enough to see, say, our solar system, you'll never recognize the grand texture of the universe, or even our galaxy.

So what's the scale for seeing ripples left over from the early universe? Theorists calculate that such ripples should now trace regions some 500 million light-years across (they have expanded a great deal since they were formed in the early universe). Such regions would be outlined by visible matter--galaxies; so to map out the ripples, just map out galaxies. The only trouble is, detecting faint differences in galaxy densities across such a large volume would require pinpointing the positions of many millions of galaxies--a daunting task.

Amazingly, several huge galaxy surveys have recently made such studies feasible. None of the surveys is perfect--survey-makers still cannot measure the precise position of every galaxy in the sky. Instead, they must resort to some form of sampling--much the same way pollsters here on Earth carefully sample people's opinions--that will limit the potential for systematic errors. Then, when the astronomers analyze the galaxy data to search for matter ripples, they must try to work with the strengths of the survey data, if plausible results are to be extrapolated for the universe as a whole.

With those caveats in mind, Cole's group worked with the 2dF Galaxy Redshift Survey, a data set of 221,414 galaxies out to a distance of 2 billion light-years from Earth. From the positions of the galaxies the group computed the so-called power spectrum of the survey--a measure of the overall texture of the galaxies' distribution in space.

Eisenstein's group used a very different data set, from the Sloan Digital Sky Survey, and a subtly different strategy. Rather than basing their analysis on every galaxy in a given volume of space, they chose a sparser sample from a much larger volume that would optimally trace out large-scale ripples of matter. Then they made a statistical tally of how far apart these galaxies are from one another.

In spite of the two groups' dissimilar techniques, their results were remarkably similar. Cole's analysis showed peaks and valleys consistent with the existence of a broad pattern of ripples 500 million light-years across. Eisenstein's analysis showed that galaxies are statistically much more likely to be 500 million light-years apart than say, 400 million or 600 million light-years apart.

By studying the ripples in a pond an astute observer can deduce the size of the pebbles that made them and perhaps even the depth of the water. Just so, working backward from the scale of the cosmic ripples, the two teams were able to calculate some basic characteristics of the universe Matter makes up between 20 and 3C percent of the contents of the universe; all the rest is "dark energy," whose true nature is entirely unknown. Moreover, only about a fifth of the matter in the universe is made up of ordinary matter--the protons neutrons, and electrons we all learned about in science class. The composition of the remaining four-fifths is "dark matter"--whose nature is also a complete mystery.

Those calculations of the relative abundances of dark energy and dark matter in the universe are consistent with other recent findings, including the results from the Wilkinson Microwave Anisotropy Probe [see "Sharper Focus," by Charles Liu, May 2003]. That's great news, because it sets the foundation of modern cosmology on even firmer footing. These first scientific results are only the first step, however. The origin of the enormous matter ripples actually predates the formation of the cosmic microwave background, the oldest directly observable relic of the early universe. Now, nearly 14 billion years after they first began to spread, the ripples could open up a new view on the infant cosmos.