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Universe Weighed and 'Found Wanting'

Only 35% of the Universe's contents is in the form of matter, according to findings published in
the journal Nature today [8 March] 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% of its energy in the form of matter and about 65%
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.

The findings are published in Peacock et al., "A measurement of the cosmological mass density
from clustering in the 2dF Galaxy Redshift Survey," Nature Volume 410 Number 6825 Page
169 - 173 (2001).


Professor John Peacock, Royal Observatory Edinburgh

Professor Richard Ellis, Caltech

Dr Matthew Colless, Research School of Astronomy and Astrophysics, ANU

Dr Joss Bland-Hawthorn, Anglo-Australian Observatory

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