Neutrinos star in the disaster flick "2012," where they penetrate and melt Earth's core. Their role however has been debunked by Scientific American's Philip Yam.
Nevertheless, Yam reports that since neutrinos are high-speed and thus one of the first particles to reach us from a stellar explosion or other stellar disaster, radiation from neutrinos might nevertheless be dangerous on Earth.
Yam cites, "[p]hysicist Juan Collar, now at the University of Chicago," who "theorized in 1996 that the death of certain kinds of stars could produce so many high-energy neutrinos that they would interact with atoms in organic tissue and lead to mass deaths from cancer." The cancer deaths, says Yam, might take years however.
Yam continues quoting Collar: "[t]he frequency of such stellar deaths, though exceedingly rare, is consistent with mass extinctions in Earth’s history." Whether or not such a disaster will occur, it's generally believed that neutrino radiation, like cosmic microwave background radiation, permeates the universe. In part because of neutrino abundance, physicists today consider neutrinos candidates for both "dark matter" and "dark energy."
Dark matter is different from ordinary matter in the universe in that it does not burn or radiate, and hence can only be detected by its effects on ordinary matter.
In 1970 Kenneth Freeman realized that the amount of hot gases detected spinning rapidly at the Milky Way's periphery must be held together by more than the visible matter. Some of that invisible matter of course must have mass to attract the particles of galaxies.
Dark matter is classified as either hot (radiating), or cold. Dark matter is believed to be the "web" that glues the galaxies together (BBC), a web that's apparently just been revealed by WMAP (Wilkinson Microwave Anistropy Probe), by analysis of photons that have traveled through space, in particular by the gravitational effects of dark matter on the photons.
Dark energy on the other hand is the anti-gravity force believed to cause the universe's rate of expansion to accelerate over time. To get data about changes in the universe expansion rates over time, expansion rates of different areas of the universe at different distances from Earth are compared. (Note that: the light that reaches Earth from these various areas has traveled different lengths of times; that is, we see each area at a different point in time because of its distance.)
About 72 percent of the universe must be dark energy and 23 percent dark matter, according to the latest WMAP data about how matter in the universe affects photons that travel through it (matter that does not radiate, does not emit light we can see, is measured through its effects on visible matter). Only four or five percent of the universe is ordinary matter, composed of electrons, protons, neutrons, and atoms.
Filling the Universe?
According to "Big Bang" theory, neutrinos, as noted, may permeate the universe almost to the extent of photons, and thus may be among the most common particles.
One reason that neutrinos supposedly fill all corners of the universe is because of their high speed. Neutrinos have supposedly traveled at light speed (and thus been "relativistic" according to quantum theory) till very recently. Many neutrinos today thus formed at the birth of the universe. Others are produced in the interiors of stars. Recent research (CERN, June 8th, 2012) suggests that neutrinos today may travel at near light speed but not faster.
According to theory, less than a second into the expansion of the early universe, anti-neutrino and neutrino pairs annihilated one another to create radiation as more pairs formed. The neutrinos traveled however as noted at light speed for some time, and thus supposedly contributed with photons to form "the energy density of the universe."
Supposedly the neutrino radiation is slightly cooler than the photon radiation. It's believed to be about 1.9 degrees Kelvin or about -271.2 degrees celsius.
Neutrinos and Dark Energy
One reason the neutrino is considered a candidate for dark energy is because its mass seems to be close to dark energy density. If the neutrino turns out not to be a good candidate, and no other candidate is found, a new theory of gravity may be needed to explain dark energy.
Neutrinos and Dark Matter
Dark matter, as noted above, may explain how the galaxies are held together (the opposite of dark energy, which seems to "push" galaxies away from one another as the universe expands), and also how, after the "Big Bang," the beginning of the universe, the universe's matter clustered less than uniformly (viewed on a large scale, however, the universe's matter is in fact somewhat uniform). The early spread of a particle with mass is used to explain the galaxies' clustering, since masses do tend to cluster.
The neutrino is considered a candidate for dark matter because there are as noted many neutrinos in the universe and likewise much dark matter. With only four percent of the universe ordinary matter, that is, composed of neutrons, protons, electrons, atoms (dwarf stars, stars that no longer emit sufficient light to be easily observed, once thought to be candidates for dark matter, contain such ordinary matter), other (non-ordinary matter) candidates for dark matter must of course be considered.
Physicists are looking for particles that interact only via gravity and the "weak force." (The "weak force" refers to nuclear decay; protons and neutrons in ordinary matter interact via "the strong force;" protons and electrons also interact electromagnetically.) Neutrinos are one such particle. Various particles are actually possible candidates for dark matter, but the neutrino is a candidate whose existence has been verified in repeated measurements.
Because of neutrinos' near-zero or zero rest masses, they were believed to be candidates only for the universe's cool dark matter, but new ideas about their spins and possible masses (based on evidence that they oscillate, that is change from one type of neutrino to another) makes them also candidates for hot dark matter.
Words of Caution
Neutrinos as noted are one of the fastest-moving of the known particles (in a vacuum). NASA however argues that much dark matter should not be fast moving at all, as fast-moving matter would be less likely to coalesce, to glue matter together into galaxies.
No one has seen a neutrino, not the way protons and electrons have been "seen." What has been seen is beta radiation deep inside the earth, attributed to neutrinos, which because they interact weakly with other particles, pass right through matter like the Earth (and thus made their way to its core in the disaster movie).
Although believed to be many times the Planck length in diameter, the neutrino's size may be only about a fifty-thousandth that of a quark, and quarks too have yet to be actually "seen." Neutrino mass is believed to be close to zero, the mass of a photon. Neutrinos as noted interact with matter via gravity (which exerts a bit of pull, if neutrinos do in fact have mass) and the "weak force" (the force involved in nuclear decay).
Elusiveness: Plus or Minus?
Because neutrinos interact only very weakly with other matter, they are difficult to observe. Conversely, because of this, they arrive at Earth perhaps little mutated and thus give clues about the interactions that produced them (they may however change "flavor," that is "oscillate" into another variety, something they are believed to do rather frequently).
Dark Matter and Dark Energy: Research Priorities
By exploring background radiation such as that of neutrinos, and dark matter and energy, scientists hope to learn more about the origin of the universe, and its rate of expansion. Dark energy and dark matter are as noted mapped today using the WMAP microwave probe. NASA is also getting retired "spy telescopes" to replace the Hubble which itself has been retired (UPI, 2012). One goal apparently is to look at dark energy, when the telescope is finally outfitted, which alas will not happen probably before 2020.
In 2007, NASA and the Department of Energy jointly funded a National Research Council study by the "Beyond Einstein" program, according to Science Daily. In September 2007, the committee released its report and noted that the Joint Dark Energy Mission (JDEM) is its top priority for research in its "Beyond Einstein" program, and that the JDEM will be its first "Beyond Einstein" mission. That mission will involve a "space-based observatory" designed to measure dark energy. Still another telescope used to study both dark energy and dark matter is the LOFAR, which is now multinational in Europe. Its array is the largest in world.
Measuring Cosmic Neutrino Activity: A New Technique?
Physicist Lily Schrempp proposes one way that the behavior of the universe's neutrinos might be observed, that might lead to a better understanding of the universe's evolution. Shrempp thinks that fluctuations in absorption dips in spectra from "extremely high energy" cosmic (EHEC) neutrinos measured on Earth will reveal something called "neutrinoless double beta decay." Neutrinoless double beta decay occurs when two photons change into neutrons, with the two neutrinos annihilating one another. Such decay, with neutrinos annihilating one another has yet to be observed.
Cosmic radiation, Schrempp believes, will allow us to measure the interactions of neutrinos more effectively than man-made accelerators can allow us to do, and may help to confirm whether the neutrino is a "Majorana particle." Majorana particles are their own anti-particles and thus two Majorana particles of the same type annihilate when they meet.
Although many particles "acquire" a "Dirac"mass through a perhaps "molasses-like" field vector known as the "Higgs Boson," particles that are their own anti-particles may have "Majorana" masses. Some physicists conjecture that neutrino mass involves "seesawing" between Dirac and Majorana masses. Neutrino mass must figure in any theory of neutrinos as "dark matter" of course.
According to Shrempp, energies reached in man-made accelerators "are easily surpassed by the cosmic laboratory from which the existence of extremely high-energy cosmic neutrinos . . . is predicted." Schrempp expects "the diffuse EHEC flux arriving" at Earth "to exhibit sizeable absorption dips" from neutrino annihilation. Schrempp believes that the locations in the spectrum of such dips in radiation will be determined by the "energies of the annihilation processes." Says Schrempp, providing "that the dips can be resolved on earth, they could produce the most direct evidence for the existence . . . so far" of the cosmic neutrino background, and also even "possibly reveal the nature of Dark Energy."