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).