‘So, what happened after the Big Bang? How can the stars, planet and the human who manufactured me form? What happened to the radiation left after annihilation?’
‘To understand the formation of astronomical objects, we must have a clear concept about the legacy of the Big Bang.’
Once the electron-positron pairs had annihilated,
when the universe was about one second old, radiation became the dominant constituent
of the universe. At first, gamma rays were abundant. These photons with intense
penetrating power are ordinarily produced only in nuclear explosions and by
radioactive decay of unstable nuclei, neutrinos and antineutrinos were also
present. These elementary particles have zero rest mass but are characterized
by energy and spin. Neutrinos have an extremely weak interaction with matter
and are consequently very difficult to detect. The core of the sun is believed
to be a prolific source of energetic neutrinos, according to the theory that
nuclear reactions provide the source of solar energy.
Neutrinos are the only direct link we have to the fiery core of the sun. Because they interact so weakly with matter, we can effectively peer into the innards of the sun by studying neutrinos. Detection of solar neutrinos therefore would provide the ultimate verification that the sun is a gigantic thermonuclear fusion reactor. However, attempts to measure a sufficient number of solar neutrinos have thus far been unsuccessful.
To detect neutrinos produced by the nuclear reactions that power the sun, scientists has utilized an enormous neutrino telescope, which consists of a tank of cleaning fluid surrounded by a jacket of water deep underground inside a gold mine. A nuclear reaction occurs very rarely, when a neutrino passes through the cleaning fluid; this reaction enables the experimenters to infer the neutrino’s passage through the fluid. The method of detection is based on a reaction between an isotope of chlorine and a neutrino, which produces a nucleus of a rare radioactive argon isotope and an electron. Despite the large flux of neutrinos that we believe are emitted by the sun, very few are absorbed on their passage through the detector. Although the reaction is infrequent, the few times per month that it is expected to occur can be verified by means of a very sensitive technique for detecting tiny traces of argon. The mine was chosen as a location so that the neutrinos from the sun would be the only possible source of argon. Cosmic rays, for example, were a contaminant that had to be excluded. The experiments to detect neutrinos appear recently to have met with some success; however, relatively few neutrinos were detected as compared with the predicted value. Only about one-third of the flux of neutrinos predicted by the standard model of the solar core was observed.
Some bizarre theories have been developed to account for the discrepancy. The readjustment time for any temperature change in the solar core is about 10 million years, whereas neutrinos propagate to us in about 6 minutes. Perhaps the solar core is now slightly (by a few percent) colder than our model predicts; no change in the solar luminosity at atmospheric temperature would occur for a long time. Unfortunately for the theories, we believe that the sun is highly stable against the occurrence of any such changes.
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Super-Kamiokande neutrino decter |
Another theory of the neutrino shortfall invokes dark matter. Suppose that the halo dark matter consists of exotic relic particles from the very early universe and that these particles, which occasionally become trapped inside the sun, do not annihilate, for example. Very massive neutrinos would be a suitable candidate. Neutrinos annihilate only with antineutrinos. In this case, over the age of the sun, the concentration building up would amount to about one hundred billionth of the mass of the sun. This is a tiny fraction, but it leads to an intriguing result: these dark particles, heavier than protons, collect in the innermost core of the sun, where one consequence of their accumulation is that heat diffuses a little more readily, because these weakly interacting particles travel much further than protons before undergoing any collision. Thus, the center of the sun, is slightly cooler than it otherwise would be, which affects the rate of the nuclear reactions power the sun. There are two pathways for hydrogen to burn into helium and release nuclear energy. Because one of these, involving carbon, nitrogen, and oxygen as catalysts, requires a slightly higher temperature, it is suppressed and the number of neutrinos expected to be emitted from the sun is reduced.
In the search for the elusive solar neutrinos, the novel idea is that the trapping of dark matter in the solar core could be responsible for suppressing the flux detectable in the chlorine experiment. The next step will be to use a new detection material, gallium. Because gallium is sensitive to lower energy neutrinos than chlorine, a gallium experiment can test whether the sun is slightly cooler in the center than current models allow. The neutrinos must be produced in order for the sun to maintain its observed luminosity. To mount a gallium experiment requires a considerable amount of gallium, perhaps 50 tons, which is equivalent to the entire world production for about a year. But the gallium is returned when the experiment is completed, and at least two solar neutrino gallium experiments are being planned. If the gallium experiment were unsuccessful in detecting solar neutrinos, we would have to resort to an extreme solution indeed—for example, we might have to allow the electron neutrinos produced in the sun to change, or oscillate, into other types of neutrinos such as muons or taus neutrinos (in all, there are at least three species of neutrino). With such assumptions we could evade the detection technique, which is sensitive only to electron neutrinos.
We expect that most plausible candidates for the dark-matter particles will eventually annihilate, resulting in a negligible heat input into the solar core. Although only about 1 part in 100,000 billion of the sun now consists of dark matter, this is sufficient to have one observable consequence: the high-energy neutrinos emitted during the annihilations escape unimpeded from inside the sun and may he observable on the earth with a suitable detector. The same deep underground detectors used to search for proton decays are sensitive to these high-energy neutrinos; so far, none has been detected from the sun, but the search is continuing.
Although there are 100 million neutrinos for every atom in the universe, the neutrinos left over from the big hang have decayed in energy as the universe has expanded. Their energy is presently only one-thousandth of an electron volt, more than a billion times less energetic than the predicted solar neutrinos. Unfortunately, the only means of detecting neutrinos requires that they possess energy high enough to trigger certain nuclear reactions. Consequently, the presence of a sea of cosmological background neutrinos remains an unverified prediction of the big bang theory. No direct observations of this sea of neutrinos that pervades the universe are contemplated in the foreseeable future. (An effect would have resulted on the evolution of the universe, however, in a nonstandard Big Bang model in which the neutrinos had vastly outnumbered the antineutrinos in the early moments. In this case, the production of neutrons would have been inhibited, and subsequent nucleosynthesis would have been correspondingly affected—little or no helium could have been produced. The converse situation, an excess of antineutrinos over neutrinos, would inhibit proton production and would at first also result in little helium.) The most direct way we could measure neutrinos from the early universe would be by their gravitational effect if they were massive enough, as they are in some models of dark matter, to have an appreciable influence on cosmological evolution, or if they are unstable, and decay into observable photons. In either case, it seems possible that their presence might yet be inferred.
