Neutrinos and SN1987A

1. Neutrinos and SN1987A Brent Tunis

2. What exactly are Neutrinos? • Neutrinos were formally discovered by Enrico Fermi in 1934 when he realized that, in order to maintain the conservation of momentum, there had to be another particle that was involved in the transformation of a neutron into either a proton or an electron • “Neutrinos are very weakly interacting, electrically neutral particles that are involved in nuclear interactions where protons are changed into neutrons or vice versa, and in other reactions as well”

3. The Larger Significance of Neutrinos • Neutrinos were once thought to be massless, but they are now known to have a very small mass of only about 0.5 to 5 billionths that of a proton • Even though small in size, it is believed that neutrinos were produced and released into the universe, as a result of the Big Bang, in numbers comparable to the amount of photons released • Such a large amount of neutrinos means that they could contribute significantly to the overall matter of the universe

4. Neutrino Experiments • Neutrinos are extremely hard to find as they are so small and can travel through almost everything without interaction. It was not until the 60’s that scientists first to began to detect them. • The Homestake experiment was one of the first that successfully studied neutrinos that were emitted from the fusion process in the core of the Sun and subsequently made their way through Earth. • It was scientifically proven, however, that this experiment was only observing about a third of the neutrinos that it should be recognizing. This led scientists to conclude that the electron neutrinos that were being emitted from the Sun were transforming into tau and muon neutrinos (less abundant known versions that the Homestake experiment was not designed to detect). This was a phenomenon dubbed neutrino oscillation and it meant that neutrinos could not be massless (complicated equations prove that oscillation is not possible without some mass).

6. Supernova 1987 A • On February 23, 1987, a large and unusual burst of neutrinos was detected over a 13 second period. They came from an explosion in the Large Magellanic Cloud, a nearby dwarf irregular galaxy. They sped through the Universe and passed through Earth as a result of a supernova that was spotted 20 hours later by telescope (because although photons and neutrinos travel at relatively similar speeds, photons are not released until a shock wave reaches the surface of a star). The detection of a large amount of neutrinos from the core collapse of a star confirmed theories both about neutrinos and supernovae. • If neutrinos were massless they would necessarily all travel at the speed of light and arrive at the same time; this was not the case with the observed 13 second interval. It was estimated from the blast that neutrinos may have a mass less than 17 billionths that of a proton and that although they may be numerous in the universe, they are less massive than the mass required to close the universe for Hubble constants greater than 50kms-1Mpc-1.

7. More Information about Supernova 1987A • It was specifically in the nearby Tarantula Nebula • It was discovered by Ian Shelton of the University of Toronto at Las Campanas Observatory, Chile. • It was originally thought that supernovae type II only came from red supergiants but it was a blue supergiant. This meant it was roughly 20 times smaller than a red supergiant but had a higher surface temperature. This helps explain why it was not as bright as other supernovae. • It was the closest supernova since SN 1604 (which was observed in the year 1604 and took place in the Milky Way itself) and was about 50 kiloparsecs or 164,000 light-years away.

9. Dark Matter • Since the discovery of Neutrinos and the suggestion that they in fact have small, but overall significant masses, it has been suggested that they are in fact one of the elements of the Universe’s dark matter (called so because it makes up most of the matter in the universe, but emits little or no light and is near impossible to see). Given the extremely small recorded size of neutrinos it is now believed that they cannot alone make up the bulk of the Universe’s dark matter. • Neutrinos are classified as hot dark matter because the term hot refers to the high speeds at which they move throughout the Universe. It is this speed that helps scientists determine how neutrinos create structure in the Universe as they begin to clump gravitationally.

10. Works Cited • Hawley Holcomb. The Foundations of Modern Cosmology • http://www.astro.ucla.edu/~wright/neutrinos.html • http://zebu.uoregon.edu/~soper/StarDeath/sn1987a.html • http://en.wikipedia.org/wiki/Supernova_1987a • http://en.wikipedia.org/wiki/Neutrinos