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Cosmology Basic Assumptions of Cosmology

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Cosmology Basic Assumptions of Cosmology Cosmology AST2002 Prof. Voss Page 1 Cosmology the study of the universe as a whole Geometry finite or infinite? center? edge? static or dynamic? Olbers' Paradox Why does it get dark at night? If the universe is static, infinite, eternal, uniformly filled with stars, then every line of sight will eventually hit a star. Although light from each star decreases as 1/R2, the number of stars in a given direction increases as 1/R2, so each direction will be as bright as the Sun! Sagittarius star field Olbers assumed dust blocks distant stars but eventually heats to same T, glows like sun. universe not infinitely old Edgar Allen Poe (1848) light from distant stars has not reached us yet universe is expanding 18-01b light from distant stars red-shifted to very low freq. (cold) observable universe - that part that we can see Basic Assumptions of Cosmology Homogeneity matter uniformly spread through space at sufficiently large scales not actually observed Isotropy looks the same in all directions at sufficiently large scales Cosmological Principle any observer in any galaxy sees the same general features Copernican Principle - Earth is not a "special" place 12/3/2001 Cosmology AST2002 Prof. Voss Page 2 no edge or center Universality - physical laws are the same everywhere Newton - gravity the same for apples on Earth and the Moon Fundamental Observations of Cosmology 1. It gets dark at night. 2. The universe is expanding light from distant galaxies has red shifts ∝ distance 18-03a 18-03b no center Hubble constant 70 km/s/Mpc ⇒ age 14 billion year Geometry of Space-Time Einstein's General Relativity matter ⇒ local distortion Black Holes Large Scales ultimate fate determined by closed flat open density or universe universe universe total mass positive zero negative curvature curvature curvature Critical Density 4×10-30gm/cm3 if density is greater ⇒ closed expand then contract equal ⇒ flat expansion slows less than ⇒ open expansion continues Search for Dark Matter to find fate 12/3/2001 Cosmology AST2002 Prof. Voss Page 3 WIMPs exotic weakly interacting unknown in laboratories MACHOs Massive Compact Halo Objects low mass stars or planets 18-13 gravitational lens have been observed but too few The Big Bang Early History no distinction different forces radiation dominated nearly equal 12/3/2001 Cosmology AST2002 Prof. Voss Page 4 matter - antimatter gamma ray ⇔ particle+antiparticle as it cooled the composition changed differentiated segregated 18-16 initially gamma rays in 4 sec cooled: neutrons, protons, electrons complete 2 minutes more cooling produced 18-12 hydrogen 75% deuterium helium 25% but little heavier no stable isotopes of atomic mass 5 or 8 6 10 years - 3000 K 18-11 neutral atoms formed universe became transparent Cosmic Background Radiation red shift remnant of 3000 K blackbody radiation from universe at age 1 million corresponds to cooler 18-09a 18-09b 18-09c temperature 2.7 K COsmic Background Explorer COBE satellite 1992 showed 2.7 K radiation (from age 1 million) uniform with slight variations inflationary universe theory 12/3/2001 Cosmology AST2002 Prof. Voss Page 5 quantum mechanics + general relativity + cosmology ⇒ very rapid expansion around 10-35 sec uniformity of cosmic background radiation slight variations in COBE ⇒ flat universe Gravity causes matter to clump together amplifies small variations over time giving Large-Scale structure in the universe maps of most distant galaxies N and S from Milky Way show a Great Wall largest known structure plus filaments and voids also found in simulations of gravitational attraction 12/3/2001.
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