Universe put on the scales
Posted: March 10, 2001

Only 35 percent of the Universe's contents is in the form of matter, according to findings published in the journal Nature by astronomers using the Anglo-Australian Telescope near Coonabarabran in eastern Australia. The rest is believed to be in the form of 'dark energy'. This measurement, the most accurate to date, is based on data from 141,000 galaxies. It confirms other studies indicating that the Universe will expand forever because there is too little mass to provide gravity to rein it in.

The team has also gathered the best existing evidence that large-scale structures in the Universe - giant superclusters of galaxies - evolve over time by collapsing under the influence of gravity. "This has allowed us to weigh the universe," said the paper's lead author, Professor John Peacock of the Royal Observatory Edinburgh.

The findings are the first major piece of science to arise from the 2dF (two-degree field) galaxy survey, which leads the world in mapping galaxies. It has now mapped more than 150,000 and will reach its target of 250,000 by the end of the year, making it ten times larger than the largest previous survey.

"The matter density of the Universe is extremely low," said Dr Matthew Colless of the Australian National University, one of the survey team leaders. "On average there might be one atom per cubic metre of space."

"The major constituent of the Universe is believed to be some kind 'dark energy', which is pushing the Universe apart."

The 2dF survey shows clearly that ninety percent of galaxies are distributed on the surfaces of big 'bubbles' in space, with the rest falling into dense clusters.

"We use the galaxies as a tracer of mass in the Universe," explained survey team member Professor Richard Ellis of Caltech.

"Of the total matter in the universe, most is in the form of 'dark matter', which gives off no radiation," he said. "But it does seem that the visible matter is distributed much like the dark matter. They know about each other."

As the universe expands, the galaxies recede from us. The recession velocity (speed) of a galaxy is proportional its distance from us, so the velocities can be used to determine the positions of the galaxies in space.

The 2dF team used their map of the galaxy distribution to measure the total mass density of the universe - what proportion of the Universe's content is mass - in two ways.

In the first method, the astronomers compared the measured clumping of galaxies into superclusters with the size of small temperature fluctuations in the cosmic microwave background, which measure density fluctuations at early times. The amount of growth in structure required to match the clumping today requires the universe to have a 'flat' geometry (without spatial curvature), with about 35 percent of its energy in the form of matter and about 65 percent in the form of 'vacuum energy', also known as 'dark energy'.

The astronomers also measured the mass density by looking at how galaxies move under the influence of gravity.

As well as its recession velocity, any galaxy has a velocity that it has acquired by falling towards other concentrations of mass - visible galaxies and/or dark matter.

These extra velocities distort the structure of the galaxy survey map in the direction looking out from Earth - that is, along our line of sight to the galaxies.

A statistical analysis of these galaxy motions shows that on small scales the galaxies are typically orbiting each other very rapidly in dense groups and clusters, but that at larger scales the galaxies are all falling in towards mass concentrations. The size of this infall is related directly to the amount of matter in the Universe. This method too gives a figure for the mass density that agrees well with the standard cosmological model. It also provides the first detailed confirmation of the gravitational instability paradigm for the formation of large-scale structure.