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Posted 20 June 2008

Particle Physics, Astronomy
and Cosmology

by Frikkie de Bruyn


1. Introduction

In the study of astronomy and cosmology both the very large and the very small objects are important.The particle most used by astronomers and cosmologists is probably the photon. Without the photon life would be impossible, observation of objects could not be done and the spectroscopic analysis of stars, interstellar gas clouds, etc. would be impossible. In short, without the photon (light) there would be no science. Literally hundreds of particles have been discovered which are unstable and decay in a very short time or exist fleetingly under conditions of extreme temperatures and/or densities. This is very important for both the astronomer and the cosmologist since it tells a story of the contents of very dense stars, such as neutron stars and the birth of our universe. Einstein’s revolutionary equation, E = mc2, unveiled more than a century ago, tells us that the mass of a particle is equivalent to an enormous amount of energy. A particle is also wave and its energy contents are equivalent to its wavelength. This wave/particle duality has been proved experimentally hundreds of times. Let us look at the importance of the standard model of particle physics for both the astronomer and the cosmologist.

2. Astroparticle Physics

Astroparticle physics is an excellent example of the importance of particle physics for astronomy. It refers to astronomers (astrophysicists) using the interactions of particles to study ultradense objects such as white dwarfs and neutron stars. Particle physics is also used to study the functioning of stars such as the sun. Currently the contents of a group of radio quiet neutron stars are being studied using satellite observation such as the American Rossi X-Ray Timing Explorer and the Chandra X-Ray Observatory to study the interiors of neutron stars. It is very interesting to note that “star quakes” called glitches occur on neutron stars. The question is of course, what cause these glitches? Astrophysicists have been studying the neutron star PSR J0537-6910 called the Big Glitcher to find answers to the nature of the contents of the interiors of these stars.

The theory is that the matter inside the core of the star might become so compressed that the neutrons in the star break down into its constituent quarks and gluons. Gluons are the carriers of the strong force holding the quarks together inside mesons (Pion, Kaon and Eta) and hadrons (Proton, Neutron, Lambda, Sigma and Omega) and the protons and neutrons inside the nucleus. Quarks have never been observed unconfined, but it is very strongly hinted that they exist unconfined in a state called asymptotic freedom at the core of a group of neutron stars. The glitch’s size is a reflection of the mass, density and other characteristics of the neutron star. The glitches or quakes speed up the spin of the neutron star while the star’s magnetic field slows down the spin of the crust. Scientists believe that the crust of a neutron star consist of iron. The superfluid interior of the star speeds up the spin of the star. As the difference in the two speeds widens the superfluid reaches a critical point like a flywheel. The built up of tension is transferred to the exterior of the star to spin faster. The result is a crack in the iron surface of the star causing the glitch. It is believed that some of the quarks will escape through the crack in the crust probably to form a meson such as a Pion.

For the technically inclined reader, quark/gluon fluid (QGF) is a phase transition from a quark confined stage to a quark/gluon state in asymptotic freedom. The study is called Quantum Chromo Dynamics (QCD), the study of strongly interacting matter as opposed to QED (Quantum Electro Dynamics) the study of weekly interacting matter. In QGF the colour charge of quarks and gluons is screened and the colour charge is non-abelian or non-commutative. A remarkable property of QGF is its very low viscosity. QCD is one part of the modern theory of the Standard Model of Particle Physics. 

3. Cosmology and Particle Physics

It is almost certain that the Quark Gluon Fluid referred to above existed in the very early universe. It appears that the QGF has already been spotted at CERN near Geneva in Switzerland and the hadron collider (RHIC) at Long Island, New York. Cosmologists make use of the collision of particles at velocities very close to the speed of light to create conditions close to those presumably existed in the very early universe. From the collisions scientists create a fireball hot and dense enough to set quarks and gluons free in a state of asymptotic freedom. It must be emphasized that the fireball of free quarks is transient of nature and existed only for a fleeting moment. Quarks and gluons cannot be observed to exist freely but scientists can conclude from the outfall of particles that formed from the fireball that quarks and gluons have indeed existed unconfined. From the results of these experiments it is concluded that, since conditions similar to the very early universe have been created, the QGF existed in the very early universe.

The underlying principle of the interactions of particles in the Standard Model of Particle Physics is the idea of symmetries. The various elementary particles that have been discovered can interact via one or more of the fundamental forces. This means that the four fundamental forces, gravity, the strong nuclear force, the weak force and the electromagnetic force must have existed as one super force in the very early universe also known as the Grand Unifying Theory or GUT. Based on the experimental and theoretical research in particle physics scientists have proposed an evolutionary sequence of events known as the standard cosmological model. In terms of this model the temperature of the universe immediately following the Big Bang was incredibly high, about 10 x power of 32  Kelvin. During this period the three Fundamental forces (gravity, the strong and the electroweak force) behaved as a single unified force. It should be mentioned that the weak and electromagnetic forces have already been unified as a single force. At about 10 to the power of -43 seconds after the Big Bang (the Planck time) the gravitational force separated from the other from the remaining super force with its own identity. Meanwhile the electroweak force still continued to act as a unified force called the GUT force.  At about 10 to the power of -35 seconds after the Big Bang the GUT force separated into the strong and the electroweak force. The electroweak force still manifested itself as one force until approximately 10 to the power of -10 seconds after the Big Bang when the weak nuclear force and the electromagnetic force  separated as the two forces we know today.

Until 10 to the power of -35 seconds after the Big Bang when the strong nuclear force separated from the GUT force, all particles were similar, and there was no distinction between quarks and leptons (that is the electron, muon, tau, electron neutrino, muon neutrino and the tau neutrino). After this quarks and leptons became distinguishable and eventual quarks and antiquarks formed hadrons such as protons and neutrons and their antiparticles. At approximately 10 to the power of -4 seconds after the Big Bang the temperature of the universe had cooled to about 10 to the power of 12 K hadrons mostly disappeared. A very small fraction of protons and neutrons survived and the majority of particles were leptons and neutrinos.. When the universe cooled to about 10 to the power of 9 K small nuclei such as helium began forming and about 500 000 years after the Big Bang and a temperature of about 3000 K hydrogen and helium atoms began forming. Today we find the relic of the Big Bang, the Cosmic Microwave Background Radiation at a temperature of about 2.7 K. (1)

1) Statistics taken from Cutnell & Johnson. Fifth Edition. 2001, Physics. Von Hoffman Press, New York.

Frikkie de Bruyn            




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