Mini Teaching practice session

Cosmic microwave background -Origin of universe

The Universe began with a bang, but that discovery was a complete accident. In the 1940s, George Gamow and his collaborators put forth a radical idea: that the Universe that was expanding and cooling today was not only hotter and denser in the past, but arbitrarily so. If you extrapolated back far enough, you'd have a Universe hot enough to ionize all the matter in it, while even farther back you'd break apart atomic nuclei. The idea became known as the Big Bang, with two major predictions arising:

1. The Universe we began with wouldn't have only matter made of mere protons and electrons, but would consist of a mix of the light elements, fused together in the high-energy, early Universe.
2. When the Universe cooled enough to form neutral atoms, that high-energy radiation would be released, and would travel in a straight line for all eternity until it collided with something, red shifting and losing energy as the Universe expanded.

This "cosmic microwave background" was predicted to be just a few degrees above absolute zero.

In 1964, Arno Penzias and Bob Wilson accidentally discovered the Big Bang's leftover glow. Working with a radio antenna at Bell Labs to study radar, they found uniform noise everywhere they looked on the sky. It wasn't the Sun, or the galaxy, or Earth's atmosphere... but they didn't know what it was. So they cleaned out the inside of the antenna with mops, removing pigeons in the process, but still the noise persisted. It was only when the results were shown to a physicist familiar with the Princeton group's (Dicke, Peebles, Wilkinson, etc.) detailed predictions, and with the radiometer they were building to detect exactly this type of signal, that they recognized the significance of what they found. For the first time, the origin of our Universe was known.

Minuscle - the nearly-invisible particle

'Missing energy' leads to the discovery of a minuscule, nearly-invisible particle. In all the interactions we've ever seen between particles, energy is always conserved. It can be transformed from one type into another — potential, kinetic, rest mass, chemical, atomic, electrical, etc. — but it can never be created nor destroyed. Which is why it was so puzzling, nearly a century ago, when it was found that some radioactive decays have slightly less total energy in their products than in the initial reactants. It led Bohr to postulate that energy was always conserved... except for when it was lost. But Bohr was mistaken, and it was Pauli who had other ideas.

Pauli contended that energy must be conserved, and so way back in 1930, he proposed a new particle: the neutrino. This "little neutral one" would not interact electromagnetically, but would instead have a minuscule mass and carry kinetic energy away. While many were skeptical, experiments from the products of nuclear reactions eventually detected both neutrinos and antineutrinos in the 1950s and 1960s, which helped lead physicists to both the Standard Model and the model of the weak nuclear interactions. It's a stunning example of how theoretical predictions can sometimes lead to a spectacular advance, once the proper experimental techniques are developed.

High energy particles - the science behind

All the particles we interact with have high-energy, unstable cousins. It's often said that advances in science aren't met with "eureka!" but rather with "that's funny," but this actually happened in fundamental physics! If you charge up an electroscope — where two conducting metal leaves are connected to another conductor — both leaves will gain the same electric charge, and repel one another as a result. If you place that electroscope in a vacuum, the leaves shouldn't discharge, but over time, they do. The best idea we had for this discharge was that there were high-energy particles hitting Earth from outer space, cosmic rays, and the products of these collisions were discharging the electroscope.

In 1912, Victor Hess conducted balloon-borne experiments to search for these high-energy cosmic particles, discovering them immediately in great abundance and becoming the father of cosmic rays. By constructing a detection chamber with a magnetic field in them, you could measure both the velocity and charge-to-mass ratio based on how the particle’s track curves. Protons, electrons, and even the first particles of antimatter were detected via this method, but the biggest surprise came in 1933, when Paul Kunze, working with cosmic rays, discovered a track from a particle that was just like the electron... except hundreds of times heavier!

The muon, with a lifetime of just 2.2 microseconds, was later experimentally confirmed and detected by Carl Anderson and his student, Seth Neddermeyer, using a cloud chamber on the ground. When the physicist I.I. Rabi, himself a Nobel Laureate for the discovery of nuclear magnetic resonance, learned of the muon's existence, he famously quipped, "Who ordered that?" It was later discovered that both composite particles (like the proton and neutron) and fundamental ones (quarks, electrons, and neutrinos) all have multiple generations of heavier relatives, with the muon being the first "generation 2" particle ever discovered.