Neutrinos, the lightest of the known elementary particles, weigh a billionth
(one part in a thousand million) of a hydrogen atom at most, and can account
for no more than one-fifth of the dark matter in the Universe, according to
findings by astronomers in Cambridge, who used data from the
Anglo-Australian telescope 2dF Galaxy Redshift Survey (2dFGRS). The results
will be presented by Dr Ofer Lahav of Cambridge University at the UK
National Astronomy Meeting in Bristol on Wednesday 10 April.

The findings come from detailed study of the 2dF (two-degree field) Galaxy
Redshift Survey, compiled using the Anglo-Australian telescope in New South
Wales, Australia. The telescope has created the world’s largest
three-dimensional catalogue of galaxies so far, currently consisting of
220,000 galaxies. A team of 30 researchers is analysing the survey to answer
fundamental questions about the Universe.

Neutrinos come in three different varieties, and were long thought to have
no mass at all, but observations of neutrinos emitted from the Sun and
created by cosmic rays in the Earth’s atmosphere have in the last few years
revealed that this cannot be the case. A determination of the masses of
neutrinos would provide clues about the physics of processes occurring under
conditions beyond the reach of current particle physics experiments.

It has long been known that there is more to our Universe than we can see in
the starry sky. Indeed, astronomers now know that the visible parts of the
Universe, such as stars and galaxies, only constitute a small fraction of
its total mass. Neutrinos do not interact with light, and are therefore a
candidate for the mysterious invisible dark matter in the Universe. The mass
of the neutrinos affects the growth of clumps that evolve into the large
structures we observe in the Universe at the present epoch. Since neutrinos
are very light, they move at nearly the speed of light over vast regions,
smoothing out the clumpiness of the matter.

To study this effect of the tiny neutrinos on the universe, Dr Oystein
Elgaroy and Dr Ofer Lahav (both from the Institute of Astronomy, University
of Cambridge, UK) together with other 2dFGRS team members, compared the
distribution of galaxies mapped out by the 2dFGRS with theoretical
calculations of how the matter would be distributed in model universes with
different values for the neutrino mass. From this confrontation of theory
with observation, they were able to conclude that the neutrinos must have a
mass smaller than a billionth of a hydrogen atom. They also concluded that
the neutrinos make up less than 20 % of the dark matter in the Universe, and
that the rest therefore has to be in some as yet unknown form.

“It is fascinating that we can use enormous structures like galaxies to
learn about the properties of the lightest of all the particles in the
Universe,” says Oystein Elgaroy.

“The dark matter problem has bothered astronomers for over 70 years. If
indeed neutrinos have mass, the composition of matter and energy in the
universe is even more complicated than the astronomers have so far
imagined,” says Ofer Lahav.

Recently a group of particle and nuclear physicists announced that they had
observed a new type of nuclear decay process involving neutrinos. Their
result is still being debated by scientists around the world but, as it
stands, it implies that the three neutrinos have very nearly the same mass,
and that its value is roughly a few parts in ten billion of the mass of a
hydrogen atom.

“Our result from the galaxy survey does not rule out a neutrino mass as
deduced from the particle physics experiment,” says Oystein Elgaroy. The
redshift surveys of millions of galaxies that will be completed in the next
few years will set even tighter limits on the mass of the neutrino”.

Ofer Lahav adds: “The latest cosmological data suggest that the universe is
a mysteriously dark place. It is probably made of four entities, three of
them rather exotic: ordinary matter, neutrinos, another form of dark matter
which is ‘cold’ and energy (so-called ‘dark energy’ or vacuum energy)
represented by the cosmological constant, suggested originally by Einstein”.


Dr. Oystein Elgaroy, Institute of Astronomy, University of Cambridge,
Madingley Road, Cambridge CB3 0HA. Tel. (+44) (0) 1223 337517.

Dr. Ofer Lahav, Institute of Astronomy, University of Cambridge, Madingley
Road, Cambridge CB3 0HA. Tel. (+44) (0) 1223 337540.

For additional comments on the 2dF survey contact 2dFGRS coordinators:
Dr Matthew Colless, Research School of Astronomy and Astrophysics,
National University. Tel. (+61) 2 6125 8030.

Prof John Peacock, University of Edinburgh Tel. (+44) (0)131 668 8390.
Fax (+44) (0) 131 668 8416. email


1. Designed and built by the Anglo-Australian Observatory, the 2dF
instrument is one of the world’s most complex astronomical instruments, able
to capture 400 spectra simultaneously. A robot arm positions up to 400
optical fibres on a field plate, each to within an accuracy of 20
micrometres. Light from up to 400 objects is collected and fed into two
spectrographs for analysis. The expansion of the Universe shifts galaxy
spectra to longer wavelengths. By measuring this ‘redshift’ in a galaxy’s
spectrum, the galaxy’s distance can be determined.

The 2dF survey covers a total area of about 2,000 square degrees, selected
from both northern and southern skies.

The 2dF galaxy redshift survey website, including a fly-through movie of the
survey, is at

2. UK National Astronomy Meeting Web site:

3. RAS Web site:

“It’s remarkable when you realise that the energy that created the chemical
signature of a clump arose in a star a considerable distance from its cloud,
and travelled in a collimated beam, perhaps some light years through
interstellar space, to create the HH object light source,” says Dr Viti.
“That’s some searchlight!”

RAS Web site:

UK National Astronomy Meeting Web site: