BioEd Online

Tiny fluctuations in the density of matter after the Big Bang are definitely reflected in the distribution of galaxies in our Universe, according to two research groups. The findings confirm theories of how the Universe grew from being almost uniformly smooth to having dense clusters of stars and galaxies.

Theorists calculated in the 1960s that galaxies must have been seeded in places where matter had slightly gathered together immediately after the Big Bang, which is thought to have created the Universe several billion years ago.

These fluctuations were seen as ripples in the cosmic background microwave radiation by NASA's Cosmic Background Explorer in 1992, and NASA's Wilkinson Microwave Anisotropy Probe in 2003. But this radiation, which is often described as the afterglow of the Big Bang, originated a mere 400,000 years after the event, long before galaxies formed.

Two sky surveys have now seen evidence of the fluctuations in the separations of galaxies that existed 10 billion years after the Big Bang. This establishes a firm link between primordial instabilities in the Universe and the graininess we see in the cosmos today.

The Two-Degree Field Galaxy Redshift Survey (2dFGRS), based at the Anglo-Australian Telescope in New South Wales, Australia, has spent ten years mapping the distribution of 221,000 galaxies. In a complementary effort, the Sloan Digital Sky Survey (SDSS) at the Apache Point Observatory in Sunspot, New Mexico, has observed 46,000 galaxies in the northern hemisphere over six years.

The two groups presented their results on 11 January at the American Astronomical Society conference in San Diego, California.

Smoking gun

The researchers looked at the distance between pairs of galaxies, and found that there was a slight excess of those that were separated by 500 million light years, just as predicted.

Seeing the ripples from the microwave background radiation amplified into the pattern of galaxies is evidence for a connection between the two, says Daniel Eisenstein, a member of the SDSS team from the University of Arizona, Tucson.

The surveys have also provided the most accurate measurement of the mass in the Universe, finding that just 18% of its mass is visible stuff made of atoms and stable subatomic particles. The rest is dark matter, which may consist of more exotic, undiscovered particles that are only detectable through their gravitational influence on the surroundings.

Perfect match

"The amazing thing about these results is that they are in perfect accord with predictions of our standard cosmological model," says Eisenstein. He says this confirms that the shape of the universe must largely be decided by dark energy, the mysterious force that is driving the Universe's expansion at a much greater rate than expected.

Preliminary results in 2001[unchecked] had tentatively suggested that galaxies showed a characteristic distribution, "but both surveys now see it very convincingly," Eisenstein adds.

Having matching evidence from the two groups is vital, says Martin Rees, England's astronomer royal. "Fundamental questions about the Universe are too important not to have a second opinion," he says.

The teams now hope to use the surveys as a yardstick to measure how the Universe's rate of expansion has changed over time. The influence of dark energy seems to have grown significantly over the last few billion years, so working out the separation of galaxies of very different ages could help to pin down exactly how the unidentified force works.