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Unformatted text preview: Chapter 10 Cosmology Cosmology is the study of the universe, or cosmos, regarded as a whole. Some questions addressed by cosmologists are What is the universe made of? Is it finite or infinite in spatial extent? Did it have a beginning at some time in the past? Will it come to an end at some time in the future? In addition ot dealing with Very Big Things, cosmology also deals with very small things. Early in its history, as well see later on, the universe was very hot in addition to being very dense, and interesting particle physics phe- nomena were occurring. Thus, a brief review of elementary particle physics will be useful as a preface to this chapter. Particle physicists tend to measure energy in units of electron volts (eV). 1 The conversion between electron volts and joules is 1 eV = 1 . 60 10- 19 J. The most cosmologically important particles are listed in Table 10.1. The 1 And in multiples thereof, such as keV (10 3 eV), MeV (10 6 eV), GeV (10 9 eV), and TeV (10 12 eV). Table 10.1: Particle Properties particle symbol rest energy (MeV) charge proton p 938.3 +1 neutron n 939.6 electron e- 0.511-1 neutrino e , , < 2 10- 6 photon dark matter ? ? 224 225 objects that surround us in everyday life are made of protons, neutrons, and electrons. Protons and neutrons are both examples of baryons , where a baryon is defined as a particle made of three quarks. 2 A proton ( p ) contains two up quarks, each with a charge q = +2 / 3, and a down quark, with a charge of q =- 1 / 3. A neutron ( n ) contains one up quark and two down quarks. A proton has a mass (or equivalently, a rest energy) that is 0.1% less than that of a neutron. A free neutron is is unstable, decaying into a proton with a decay time of n = 940 s, about a quarter of an hour. Electrons ( e- ) are examples of leptons , a class of elementary particles that are not made of quarks. 3 The mass of an electron is small compared to that of a proton or neutron; the electric charge of an electron is equal in magnitude, but opposite in sign, to that of a proton. On large scales, the universe seems to be electrically neutral, with equal numbers of protons and electrons. The component of the universe made of atoms, molecules, and ions is called baryonic matter , since only the baryons contribute significantly to the mass density. Neutrinos ( ) are also leptons. Neutrinos have no electric charge, and interact with other particles only through the weak nuclear force or grav- ity. There are three types, or flavors, of neutrinos: electron neutrinos ( e ), muon neutrinos ( ), and tau neutrinos ( ). Although recent experiments indicate that the different neutrino types have different masses, those masses must be small compared to the electron mass, with m c 2 < 2 eV being the approximate upper limit on the rest energy....
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