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Cosmology Basic Assumptions of Cosmology

Cosmology Basic Assumptions of Cosmology

Cosmology AST2002 Prof. Voss Page 1 the study of the 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 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 ! 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) - that part that we can see

Basic Assumptions of Cosmology

Homogeneity uniformly spread through at sufficiently large scales not actually observed looks the same in all directions at sufficiently large scales Cosmological any observer in any sees the same general features - Earth is not a "special" place

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no edge or center Universality - physical laws are the same everywhere Newton - gravity the same for apples on Earth and the

Fundamental Observations of Cosmology

1. It gets dark at night. 2. The universe is expanding light from distant 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 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 to find fate

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WIMPs exotic weakly interacting unknown in laboratories

MACHOs Massive Compact Halo Objects low mass stars or 18-13 have been observed but too few


Early History no distinction different forces dominated nearly equal

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matter - gamma ray ⇔ +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 25% but little heavier no stable isotopes of atomic mass 5 or 8

6 10 years - 3000 K 18-11 neutral 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

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+ 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 show a Great Wall largest known structure plus filaments and voids also found in simulations of gravitational attraction