‘Ghost Particle’ Sized Up by Cosmologists

Cosmologists at University College London (UCL), England, have recently reported a new calculation that led them one step closer to determining the mass of the elusive neutrino particle. While most of the studies made until now used particle detectors, the new research accomplished its objective by gazing into celestial galaxies.

Cosmologists at UCL were able to estimate distances between galaxies and work out the upper limits of a neutrino mass. (Source: UCL)
Cosmologists at UCL were able to estimate distances between galaxies and work out the upper limits of a neutrino mass. (Source: UCL)

Most cosmologists agree that a neutrino has a mass, although it is extremely small and extremely hard to measure. For instance, a neutrino particle is capable of passing through a whole light year (about ten trillion miles) of lead without hitting a single atom.

Neutrinos are created as a result of certain types of radioactive decay or nuclear reactions such as those that take place in the sun, in nuclear reactors, or when cosmic rays hit atoms. Most neutrinos passing through the earth emanate from the sun, and more than 50 trillion solar neutrinos pass through the human body every second.

This latest work, published in the journal Physical Review Letters, is based on the principle that the huge abundance of neutrinos (there are trillions passing through each person in any given moment) has a cumulative effect on the matter of the cosmos. Naturally, this effect forms into “clumps” of groups and clusters of galaxies. “Of all the hypothetical candidates for the mysterious Dark Matter, so far neutrinos provide the only example of dark matter that actually exists in nature,” says Professor Ofer Lahav, head of UCL’s Astrophysics Group.

Since neutrinos are extremely light, they move across the universe at great speeds, resulting in this “clumpiness” of matter. By analyzing the distribution of galaxies across the universe, the team of scientists was able to work out the upper limits of neutrino mass. “It is remarkable that the distribution of galaxies on huge scales can tell us about the mass of the tiny neutrinos,” explainedProfessor Lahav. According to his paper, the existence of the largest ever 3D map of galaxies, called Mega Z, was central to this new calculation. The map covers over 700,000 galaxies, and it was recorded by the Sloan Digital Sky Survey.

Another crucial element of this research was a new method, which measures the color of each of the galaxies. Thanks to this practice, the cosmologists at UCL were able to estimate distances between galaxies. By combining the map retrieved from the Sloan Digital Sky Survey with the measurement of colors, the team was able to chart temperature fluctuations in the after-glow of the Big Bang. Dr. Filipe Abadlla, one of the team members, said: “This is one of the most effective techniques available for measuring the neutrino masses. This puts great hopes to finally obtain a measurement of the mass of the neutrino in years to come.”

The new results – based on the largest ever survey of galaxies in the universe – evaluate the total neutrino mass at no larger than 0.28 electron volts. In comparison, it is less than a billionth of the mass of a single hydrogen atom (the smallest atom known to man). This is one of the most accurate measurements of the mass of a neutrino to date.

Dr. Shaun Thomas, whose PhD thesis was a major part of this research, commented: “Although neutrinos make up less than 1% of all matter they form an important part of the cosmological model. It’s fascinating that the most elusive and tiny particles can have such an effect on the universe.”

The authors are confident that a larger survey of the universe, such as the one they are working on (called the international Dark Energy Survey), will yield an even more accurate weight for the neutrino, potentially at an upper limit of just 0.1 electron volts.

TFOT has also covered a research concerning the neutrino’s nature, conducted at Queen Mary’s Particle Physics Research Centre. Another research study that TFOT covered concerns a theory about the universe’s inflation; the research was conducted at the National Institute of Standards and Technology.

For more information about the neutrino’s latest mass measurement, see UCL’s press release.

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