Astronomers weigh neutrinos with the universe

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”.

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