Massive neutrinos solve a cosmological conundrum

Scientists have solved a major problem with the current standard model of cosmology identified by combining results from the Planck spacecraft and measurements of gravitational lensing in order to deduce the mass of ghostly sub-atomic particles called neutrinos.

Map of the cosmic microwave background
Map of the cosmic microwave background made by the European Space Agency's Planck spacecraft. The results of this work on neutrinos were based in part on these observations. Credit: ESA, LFI and HFI Consortia (2013). Click for a full-size image.

The team, from the Universities of Manchester and Nottingham, used observations of the Big Bang and the curvature of space-time to accurately measure the mass of these elementary particles for the first time.

The recent Planck spacecraft observations of the Cosmic Microwave Background (CMB) - the fading glow of the Big Bang - highlighted a discrepancy between these cosmological results and the predictions from other types of observations. The CMB is the oldest light in the Universe, and its study has allowed scientists to accurately measure cosmological parameters, such as the amount of matter in the Universe and its age. But an inconsistency arises when large-scale structures of the Universe, such as the distribution of galaxies, are observed.

Professor Richard Battye, from The University of Manchester School of Physics and Astronomy, said: "We observe fewer galaxy clusters than we would expect from the Planck results and there is a weaker signal from gravitational lensing of galaxies than the CMB would suggest.

"A possible way of resolving this discrepancy is for neutrinos to have mass. The effect of these massive neutrinos would be to suppress the growth of dense structures that lead to the formation of clusters of galaxies."

Neutrinos interact very weakly with matter and so are extremely hard to study. They were originally thought to be massless but particle physics experiments have shown that neutrinos do indeed have mass and that there are several types, known as flavours by particle physicists. The sum of the masses of these different types has previously been suggested to lie above 0.06 eV (much less than a billionth of the mass of a proton).

In this paper (published in Physical Review Letters on 7th February 2014), Professor Battye and co-author Dr Adam Moss, from the University of Nottingham, have combined the data from Planck with gravitational lensing observations in which images of galaxies are warped by the curvature of space-time. They conclude that the current discrepancies can be resolved if massive neutrinos are included in the standard cosmological model. They estimate that the sum of masses of neutrinos is 0.320 +/- 0.081 eV (assuming active neutrinos with three flavours).

Dr Moss said: "If this result is borne out by further analysis, it not only adds significantly to our understanding of the sub-atomic world studied by particle physicists, but it would also be an important extension to the standard model of cosmology which has been developed over the last decade."

The paper is published in Physical Review Letters on 7th February and has been selected as an Editor's choice.

Background notes

A copy of the paper is available from or

Jodrell Bank's role in Planck

This paper makes use of CMB data from the European Space Agency's Planck spacecraft and from the South Pole Telescope.

Jodrell Bank Centre for Astrophysics is directly involved with the two lowest frequencies of the Low Frequency Instrument on board Planck, the 30 and 44 GHz radiometers. These have four and six detectors respectively, operating at 20 Kelvin (-253.15 degrees Celsius). The resolution on the sky is 33 and 27 arc minutes, and the sensitivity 1.6 and 2.4 micro K (over 12 months). The cryogenic low noise amplifiers which are the heart of the radiometers were developed at Jodrell Bank, with help from the National Radio Astronomy Observatory in Virginia, USA.

Dr B Maffei and Dr G Pisano are involved in the other focal instrument, the HFI. First at Cardiff University and then at The University of Manchester, they have played a major role in the design, development and calibration of the Focal Plane Unit, in particular the cold optics, in collaboration with the Institut d'Astrophysique Spatiale, France, Maynooth University, Ireland and JPL/Caltech, USA.

The work to understand the Galactic emission seen by Planck is being co-led from Jodrell Bank by Emeritus Professor Rod Davies and Dr Clive Dickinson. A number of projects are led by Jodrell Bank scientists, including Professor Richard Davis and Dr Clive Dickinson. Each of the 14 projects focuses on one aspect of the Galaxy as seen by Planck, including the electrons that gyrate in the Galactic magnetic field, the ionized gas that pervades the interstellar medium and the dust grains that emit across the entire frequency range that Planck is sensitive to. Jodrell Bank is also leading the calibration and identifying systematics in the LFI data.

Jodrell Bank and Gravitational Lensing

This work uses gravitational lensing data from the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS).

The first gravitational lens to be discovered, the Double Quasar, was found by Dr Denis Walsh of The University of Manchester during a radio survey of the northern sky using telescopes at Jodrell Bank. Follow-up work by Walsh and collaborators using an optical telescope at Kitt Peak in the USA, led to its identification as the first gravitational lens (see Walsh et al 1979, Nature 279, 381). The quasar is at a redshift of 1.41 (a co-moving distance of about 13.7 billion light years) and is lensed by a bright galaxy (and associated cluster of galaxies) at a redshift of 0.355 (a co-moving distance of about 4.54 billion light years) forming a second image of the quasar core and inner jet.

One of the major current e-MERLIN legacy programmes (led by Neal Jackson of The University of Manchester and Stephen Serjeant of The Open University) will take advantage of its unique combination of sharpness of view and sensitivity to faint emission to map the multiple images in gravitational lenses and thereby study the evolution of the distribution of mass in distant galaxies.

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