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Homework for today was WORKBOOK EXERCISE: “Expansion of the Universe” (pg. 87-88 in workbook) Mapping the Universe Chapter 16 The Local Group • The Local Group – group of galaxies – Milky Way is member – at least 30 galaxies • 3 large spirals – Milky Way, Andromeda, M33 • remainder mostly dwarf ellipticals QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Galaxy Clusters • Most galaxies found in “galaxy clusters” QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. – Examples: Virgo cluster, Coma cluster • Superclusters – extremely rich galaxy clusters QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Structure in the Universe • Galaxy surveys show structure – voids – filaments/sheets • Galaxy clusters – found within the filaments/sheets • Superclusters – found at intersections of several filaments/sheets Evolution of Galaxies • Galaxy formation – Universe ~14 billion years old – star formation more active in past • Galaxies change, as a result of: QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. – star formation – stellar evolution – galaxy collisions & cannibalism Dark Matter • Mass of Galaxy Clusters – determined by galaxy orbital motion – M/L ~ 300 – Universe is 99% dark matter The Big Bang ncompressed) decompressor are needed to see this picture. Chapter 17 Cosmological Principle • Cosmological principle – on large scales, Universe is: • isotropic – looks the same in all directions • homogeneous – any region of space is about the same as any other Age of the Universe • Hubble: Universe is expanding • “Run movie backward” – at beginning • all matter within small region (single point) • very, very hot • Subsequent expansion – – – – called Big Bang not an explosion galaxies not moving (flying apart) space is expanding • Age of the Universe To = 1/H (Hubble time) ~ 14 billion years Big Bang Model • Early universe very hot, very dense (mostly radiation) • As it expanded, it cooled • Eventually, atomic nuclei form – when universe about 3 minutes old – only simple atoms (hydrogen [deuterium], helium, lithium) • Later, matter becomes transparent – light escapes – produces continuous spectrum (blackbody) • cosmic microwave background radiation (CMB) • Much later, stars and galaxies form Evidence for the Big Bang QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. 1. 2. Expanding universe Cosmic Background Radiation – – – 3. discovered by Penzias & Wilson in 1960’s (won Nobel Prize 1978) light released when matter became transparent Observed background matches predicted temperature (3 K) Nucleosynthesis – – – Model predicts Universe is 75% hydrogen, 25% helium. Matches observed abundances Details provide strong evidence Fate of the Universe • Big Bang: 3 possible fates – depends on mass (=gravity) of Universe – open universe (the Big Chill) • not enough mass to stop the expansion • universe expands forever – flat universe • exactly enough mass to stop the expansion, but in infinite time – closed universe (the Big Crunch) • enough mass to stop expansion • followed by contraction back to single point • Latest results (Supernova Cosmology) – Indicate that the Universe is ACCELERATING! QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Current Status • Careful tests via CMB observations – Universe is flat – 4% normal matter • 0.3% is luminous – 23% dark, exotic matter • what is it? – 73% dark energy • produces anti-gravity • expansion rate is increasing • what is it? QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.