Gravitational waves LIGO (Laser Interferometer Gravitational-Wave Observatory ) in Louisiana. A laser beam is split, the light directed down two long arms at 90 degrees to one another and bounced back and recombined. Their interference pattern is monitored using a photodetector which locks on a specific dark fringe.

Gravitational waves As gravitational wave passes the strain of space is different in the two arms as the wave propagates and so the path length changes and the interference fringes shift. North leg West leg

Gravitational waves Expected variations in strain are less than General relativity predicts frequency of gravitational waves to vary from 100 Hz for pulsars to Hz for binary stars. Currently several ground-based detectors LIGO (USA), VIRGO (Italy/France), GEO (Germany/Great Britain), and TAMA (Japan). Detectors on Earth however limited by seismic vibrations, curvature of Earth, and the requirement to operate the laser in a vacuum to reduce power loss. Operating in space may be a good way to avoid these problems. The Laser Interferometer Space Antenna (LISA) will feature 3 spacecraft accurately positioned 5,000,000 km apart providing 3 interferometer signals. It is scheduled for launch in the next decade.

Cosmic Rays What are cosmic ray particles? Energetic particles from outer space that impinge on Earth's atmosphere seemingly from all directions.

Cosmic Rays What are cosmic ray particles? This is a similar composition to that of the solar system. If we believe that most cosmic ray particles originate from outside the solar system why is this interesting? Of note are the relatively high proportions of lithium, beryllium, and boron, all believed to originate from the breakup of carbon, nitrogen and oxygen by fast moving protons in a process called spallation.

Cosmic Rays What are cosmic ray particles? The variety of particle energies reflects the wide variety of sources up to extreme energies of around eV. (Particle accelerators can only produce eV). The spectrum is not thermal (M-B distribution) but is power law shaped at high energies.

Cosmic Rays Where do cosmic ray particles come from? Radius of curvature of charged particle in a magnetic field is Magnetic field in solar system is T and so a 100 GeV proton will have a radius of curvature of 3×10 11 m. This is about 1 A.U. What does this mean? For low energies, trajectory bears no relation to location of source and that these cosmic ray particles could have been created in the solar system.

Cosmic Rays Where do cosmic ray particles come from? Amongst other ideas we find that the low energy cosmic ray flux does vary with the solar cycle. Based on the data how could we test where cosmic rays produced in the solar system could originate from? Galactic magnetic fields are typically of order 3× T but direction varies. The radius of curvature for 100 TeV protons is then around m. So cosmic ray particles follow a random walk through the galaxy and their origin cannot be ascertained. This is why the flux is isotropic.

Cosmic Rays Where do cosmic ray particles come from? eV protons and below originate from the solar system eV protons perform random walks throughout the galaxy eV protons may originate from outside the galaxy. As to where ‘outside the galaxy’ means exactly we are not sure. The favoured model is that they are produced in the accelerating shockwave of a supernova but who knows…

Cosmic Rays What is the total flux of cosmic ray particles? The maximum in the proton spectrum is at about 1 GeV and the flux at this energy is 2 m -2 s -1 sr -1 (MeV) -1. Integrating flux over all energies gives total flux as 1 MeV/m 3. (energy density of starlight is 0.3 MeV/m 3 ).

Cosmic Rays How old are cosmic ray particles? Beryllium, half life of 3.9×10 6 years, is produced via spallation and decays to boron. We model the process of beryllium spallation to predict the ratio of beryllium to boron produced and then compare this to the measured ratio to see how much beryllium has decayed. From this we can extract the mean age giving value of 10 7 years indicating galactic origin.

Cosmic Rays Interaction with Earth’s atmosphere C.R.P.s collide with nuclei of oxygen and nitrogen. These collisions, in a process known as a shower, result in the production of billions of pions and kaons, unstable mesons which quickly decay into muons. Muons reach Earth’s surface as they dont interact strongly with the atmosphere. Muons are ionizing radiation, and may easily be detected by many types of particle detectors such as spark chambers or scintillation detectors. All of the produced particles stay within about one degree of the primary particle's path.

Cosmic Rays Interaction with Earth’s atmosphere The sun is a source of cosmic rays: the solar wind consists of protons and electrons ejected from the sun's corona and from solar flares. They have enough energy to ionize the various gasses in the upper atmosphere (oxygen and nitrogen), which then causes Aurorae. Oxygen emissions: green or brown, depending on the amount of energy absorbed. Nitrogen emissions: blue if the atom regains an electron after it has been ionized. Red if returning to ground state from an excited state.

Cosmic Rays Interaction with Earth’s atmosphere: why only Aurorae at poles?

High energy observational astrophysics The end.