There are two types of leptons, the electron and the neutrino. The neutrino is a strange particle, not discovered directly, but by inference from the decay of other particles by Wolfgang Pauli in 1930. It has no charge and a very small mass. It interacts with other particles only through the weak force (i.e. it is immune to the strong and electromagnetic forces). The weak force is so weak, that a neutrino can pass through several Earth's of lead with only a 50/50 chance of interacting with an atom, i.e. they are effectively transparent to matter.

The weakly interacting nature of neutrinos makes them very difficult to detect, and therefore measure, in experiments. Plus, the only sources of large amounts of neutrinos are high energy events such as supernova, relics from the early Universe and nuclear power plants. However, they are extremely important to our understanding of nuclear reactions since practically every fusion reaction produces a neutrino. In fact, a majority of the energy produced by stars and supernova are carried away in the form of neutrinos (the Sun produces 100 trillion trillion trillion neutrinos every second).

Detecting neutrinos from the Sun was an obvious first experiment to measure neutrinos. The pioneering experiment was Ray Davis's 600 tonne chlorine tank (actually dry cleaning fluid) in the Homestake mine, South Dakota. His experiment, begun in 1967, found evidence for only one third of the expected number of neutrino events. A light water Cherenkov experiment at Kamioka, Japan, upgraded to detect solar neutrinos in 1986, finds one half of the expected events for the part of the solar neutrino spectrum for which they are sensitive. Two recent gallium detectors (SAGE and GALLEX), which have lower energy thresholds, find about 60-70% of the expected rate.

The clear trend is that the measured flux is found to be dramatically less than is possible for our present understanding of the reaction processes in stars. There are two possible answers to this problem: 1) The structure and constitution of stars, and hence the reaction mechanisms are not correctly understood (this would be a real blow for models that have otherwise been very successful), or 2) something happens to the neutrinos in transit to earth; in particular, they might change into another type of neutrino, called oscillation (this idea is not as crazy as it sounds, as a similar phenomenon is well known to occur with the meson particles). An important consequence to oscillation is that the neutrino must have mass (unlike the photon which has zero mass).

By the late 1990s, the oscillation hypothesis is shown to be correct. In addition, analysis of the neutrino events from the supernova 1987A indicates that the neutrinos traveled at slightly less than the speed of light. This is an important result since the neutrino is so light that it was unclear if its mass was very small or exact zero. Zero mass particles (like the photon) must travel exactly the speed of light (no faster, no slower). But objects with mass must travel at less than the speed of light as stated by special relativity.

Since neutrino's interact very weakly, they are the first particles to decouple from other particles, at about 1 sec after the Big Bang. The early Universe is so dense that even neutrinos are trapped in their interactions. But as the Universe expands, its density drops to the point where the neutrinos are free to travel. This happens when the rate at which neutrinos are absorbed and emitted (the weak interaction rate) becomes slower than the expansion rate of the Universe. At this point the Universe expands faster than the neutrinos are absorbed and they take off into space (the expanding space).

Now that neutrinos have been found to have mass, they also are important to our cosmology as a component of the cosmic density parameter. Even though each individual neutrino is much less massive than an electron, trillions of them are produced for every electron in the early Universe. Thus, neutrinos must make up some fraction of the non-baryonic matter in the Universe (although not alot of it, see lecture on the large scale structure of the Universe).

Cosmic Background Radiation :

One of the foremost cosmological discoveries was the detection of the cosmic background radiation. The discovery of an expanding Universe by Hubble was critical to our understanding of the origin of the Universe, known as the Big Bang. However, a dynamic Universe can also be explained by the steady state theory.

The steady state theory avoids the idea of Creation by assuming that the Universe has been expanding forever. Since this would mean that the density of the Universe would get smaller and smaller with each passing year (and surveys of galaxies out to distant volumes shows this is not the case), the steady-state theory requires that new matter be produced to keep the density constant.

The creation of new matter would violate the conservation of matter principle, but the amount needed would only be one atom per cubic meter per 100 years to match the expansion rate given by Hubble's constant.

The discovery of the cosmic microwave background (CMB) confirmed the explosive nature to the origin of our Universe. For every matter particle in the Universe there are 10 billion more photons. This is the baryon number that reflects the asymmetry between matter and anti-matter in the early Universe. Looking around the Universe its obvious that there is a great deal of matter. By the same token, there are even many, many more photons from the initial annihilation of matter and anti-matter.

Most of the photons that you see with your naked eye at night come from the centers of stars. Photons created by nuclear fusion at the cores of stars then scatter their way out from a star's center to its surface, to shine in the night sky. But these photons only make up a very small fraction of the total number of photons in the Universe. Most photons in the Universe are cosmic background radiation, invisible to the eye.

Cosmic background photons have their origin at the matter/anti-matter annihilation era and, thus, were formed as gamma-rays. But, since then, they have found themselves scattering off particles during the radiation era. At recombination, these cosmic background photons escaped from the interaction with matter to travel freely through the Universe.

As the Universe continued to expanded over the last 15 billion years, these cosmic background photons also `expanded', meaning their wavelengths increased. The original gamma-ray energies of cosmic background photons has since cooled to microwave wavelengths. Thus, this microwave radiation that we see today is an `echo' of the Big Bang.

The discovery of the cosmic microwave background (CMB) in the early 1960's was powerful confirmation of the Big Bang theory. Since the time of recombination, cosmic background photons have been free to travel uninhibited by interactions with matter. Thus, we expect their distribution of energy to be a perfect blackbody curve. A blackbody is the curve expected from a thermal distribution of photons, in this case from the thermalization era before recombination.

Today, based on space-based observations because the microwave region of the spectrum is blocked by the Earth's atmosphere, we have an accurate map of the CMB's energy curve. The peak of the curve represents the mean temperature of the CMB, 2.7 degrees about absolute zero, the temperature the Universe has dropped to 15 billion years after the Big Bang.

Where are the CMB photons at the moment? The answer is `all around you'. CMB photons fill the Universe, and this lecture hall, but their energies are so weak after 15 billion years that they are difficult to detect without very sensitive microwave antennas.

CMB Fluctuations :

The CMB is highly isotropy, uniform to better than 1 part in 100,000. Any deviations from uniformity are measuring the fluctuations that grew by gravitational instability into galaxies and clusters of galaxies.

Images of the CMB are a full sky image, meaning that it looks like a map of the Earth unfolded from a globe. In this case, the globe is the celestial sphere and we are looking at a flat map of the sphere.

Maps of the CMB have to go through three stages of analysis to reveal the fluctuations associated with the early Universe. The raw image of the sky looks like the following, where red is hotter and blue is cooler:

The clumpiness of the CMB image is due to fluctuations in temperature of the CMB photons. Changes in temperature are due to changes in density of the gas at the moment of recombination (higher densities equal higher temperatures). Since these photons are coming to us from the last scattering epoch, they represent fluctuations in density at that time.

The origin of these fluctuations are primordial quantum fluctuations from the very earliest moments of are echo'ed in the CMB at recombination. Currently, we believe that these quantum fluctuations grew to greater than galaxy-size during the inflation epoch, and are the source of structure in the Universe.

Fluctuations and the Origin of Galaxies :

The density fluctuations at recombination, as measured in the CMB, are too large and too low in amplitude to form galaxy sized clumps. Instead, they are the seeds for galaxy cluster-sized clouds that will then later break up into galaxies. However, in order to form cluster-sized lumps, they must grow in amplitude (and therefore mass) by gravitational instability, where the self-gravity of the fluctuation overcomes the gas pressure.