10 April 2002
Embargoed to 1700 AEST Wednesday 10 April (0700 British standard time)
Neutrinos, the lightest of the known elementary particles, can make up no more than a fifth of the Universe's 'dark matter', according to a new finding made with the Anglo-Australian Telescope near Coonabarabran, NSW.
Dr Ofer Lahav of Cambridge University will present the result to the UK National Astronomy Meeting in Bristol, UK, on Wednesday 10 April (UK time).
It's the latest in a string of cosmology findings flowing from the world's largest galaxy survey, the 2dF (two-degree field) Galaxy Redshift Survey, which has mapped the 3-D positions in space of more than 220 000 galaxies.
The survey, which started in 1997, winds up on the night of Thursday 11 April (AEST) when astronomers Dr Scott Croom and Dr Simon Driver make the last observations.
"The observing may be done, but there's a lot of science still to come," says Dr Matthew Colless of the Australian National University, one of the coordinators of the 30-strong survey team.
The sheer size of the survey has allowed astronomers to pit theory against observation in ways not possible before.
The neutrino finding is an example.
Dr Oystein Elgaroy and Dr Ofer Lahav, both of Cambridge University in the UK, used the galaxy survey data to show that neutrinos must weigh less than a billionth (one part in a thousand million) of a hydrogen atom.
Neutrinos come in three different 'flavours'. It was long thought that they had no mass at all, but in recent years that idea was overturned.
The Universe is awash with neutrinos, most left over from the Big Bang. So with even a tiny mass they could make up the Universe's unseen 'dark matter' the dominant form of matter in the Universe.
The value of the mass of the neutrino affects how tightly clustered galaxies are. So Drs Elgaroy and Lahav calculated how galaxies would be clustered for different values of the neutrino mass, then compared their predictions with what the galaxy survey actually mapped.
But their finding that neutrinos are extremely light means the dark matter problem is still unsolved.
In a second result, also being presented by Dr Lahav at the UK meeting, researchers based at the Institute for Computational Cosmology (ICC) in Durham and at Caltech in California used the 2dFGRS results to show the that their computer simulation of the Universe was 'spot on'.
The simulation predicted that the brightest galaxies are associated with the most massive concentrations of dark matter, and so they should be clustered more tightly than average galaxies.
Very bright galaxies are extremely rare. The 2dFGRS has been the first survey to map enough of them for the astronomers to test the prediction.
Because the prediction turned out to be right, astronomers can now have
confidence that they have a good idea of how the invisible 'dark matter'
is distributed throughout the Universe.
Australian 2dFGRS coordinator
Dr Matthew Colless, Research School of Astronomy and Astrophysics,
Last 2dFGRS observing run
Dr Scott Croom, Anglo-Australian Observatory
Dr. Ofer Lahav, Institute of Astronomy, University of Cambridge, Madingley
National Astronomy Meeting Press Room phones (9-12 April only):
Dr Terry Bridges, Anglo-Australian Observatory, Sydney
Modelling of the Universe
Dr Carlton Baugh, Institute of Computational Cosmology, Dept of Physics,
Dr Peder Norberg, Institute of Computational Cosmology, Dept of Physics,
The paper on which this finding is based is available from the astro-ph
preprint server at http://xxx.lanl.gov/abs/astro-ph/0204152. It
Modelling of the Universe
1. The 2dF Galaxy Redshift Survey: luminosity dependence of galaxy clustering
by P. Norberg et al. (the 2dFGRS team). Monthly
2. The 2dF Galaxy Redshift Survey: The dependence of galaxy clustering
on luminosity and spectral type by P. Norberg et al. (the 2dFGRS team).
Monthly Notices of the Royal Astronomical Society, in press.
1. Designed and built by the Anglo-Australian Observatory, the 2dF instrument
is one of the world's most complex astronomical
2. The 2dF galaxy redshift survey website, including a fly-through movie
tel: +61 2 9372 4251 (bh), 0419-635-905 (mob)