The cosmic connection There is a very close connection between particle physics and astrophysics. I’m going to show two examples: Type II supernovas Dark matter SN1994

Particle decay Heavier particles decay into lighter ones, which is why the world is made of up and down quarks and not top quarks. As an example, free neutrons decay into protons, electrons and electron anti-neutrinos: n  p e e This example is called beta decay. Question: why don’t neutrons within an atomic nucleus decay? _

Particle decay In such decays, several quantities are conserved: Energy, linear momentum, angular momentum Electric charge The total number of leptons (anti-leptons count as -1) The total number of baryons (anti-baryons count as -1) n  p e e We have one baryon to start, one baryon to end. We have zero leptons to start and zero to end (the antineutrino counts as -1, the electron as +1) The “bar” means “antiparticle” _

Particle Decay All decays can go in reverse, too (inverse beta decay) ep  n e or e n  pe General rule: When you move a particle from one side of the decay equation to the other, it changes into it’s antiparticle Verify that lepton number and baryon number are conserved in this decay.

Beta decay and supernovas Why is this important in astrophysics? Beta decay is crucial in the generation in stars of all the chemical elements. And inverse beta decay plays a central role in Type II supernovas Supernova 1064 remnant A Type II supernova occurs due to gravitational collapse

Type II supernovas In type II supernovas (stars much more massive than our sun), when all the nuclear fuel is used up, the gravitational pressure is so great that the atoms collapse! The electrons and protons undergo inverse beta decay: ep  n e Every proton in the star becomes a neutron, forming a neutron star. The neutrinos escape(10 57 of them!) and carry away a large part of the supernova energy. Galaxy M31 with a SN on the outter edge

Type II Supernovas The neutron star resulting from a Type II supernova is about 10 km in radius and rotates with periods of seconds or less. In 1987 neutrinos from a nearby supernova were detected in two detectors on earth! Here’s a problem for your students: If a star the size of our sun with a rotation period of 10 hours collapses to a radius of 10km, how fast would it be rotating to conserve angular momentum?

Dark Matter As early as 1933 Zwicky showed that gravitational effects in galaxy clusters could not be explained by the visible matter. In the 1980’s, rotational curves of galaxies gave more evidence for the existance of unseen or dark matter. Rotational velocity vs distance from the galactic center

Gravitational Lensing Einstein predicted in 1936 that light from distant objects could be bent by massive objects between us and the distant object. Multiple, distorted images are evidence for gravitational lensing.

Dark matter Quantitative calculations of gravitational lensing give strong evidence that galaxies have much more matter than we see.

Galactic Haloes The rotational curves of galaxies and the observed gravitational lensing can be explained if galaxies are surrounded by a huge halo of dark matter. The dark matter makes up 80% of the matter in galaxies!

What is the dark matter? We don’t know, but the answer almost certainly lies in particle physics. In the 1980’s particle theorists developed what is still a leading candidate for a theory beyond the Standard Model. Supersymmetry predicts that every particle we know has a supersymmetric partner which is much more massive and interacts only weakly with regular matter. Supersymmetry solves deep problems in the mathematics of the Standard Model. It was developed for this reason, and not because it solves the dark matter problem.

Is Supersymmetry real? “Supersymmetry is an offer nature can’t refuse” Dmitri Nanopolis, theorist “There ain’t no supersymmetry”, Leo Bellantoni, experimentalist “Experiment is the sole judge of scientific truth”, Richard Feynman We are pushing hard to find supersymmetry. But as yet there is no direct experimental evidence for it.

Dark Energy It gets weirder…around 2000 it was found that the expansion of the universe is speeding up. The stuff in the universe is dominated by dark energy, which acts like anti-gravity. Dark energy is pushing the universe apart faster and faster. It’s nature is unknown.

The cosmic connection The science of the very big and the science of the very small are in a close, synergistic relationship. Aerial view of Fermilab Deployment of the Hubble Space Telescope from the shuttle