Homework for today wasWORKBOOK 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
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Galaxy Clusters
• Most galaxies found in “galaxy clusters”– Examples: Virgo cluster,
Coma cluster• Superclusters
– extremely richgalaxy clusters
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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:– star formation– stellar evolution– galaxy collisions &
cannibalism
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Dark Matter
• Mass of Galaxy Clusters– determined by galaxy orbital motion– M/L ~ 300– Universe is 99% dark matter
The Big Bang
Chapter 17
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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 UniverseTo = 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
1. Expanding universe2. Cosmic Background Radiation
– discovered by Penzias & Wilson in 1960’s (won Nobel Prize 1978)
– light released when matter became transparent
– Observed background matches predicted temperature (3 K)
3. Nucleosynthesis– Model predicts Universe is 75%
hydrogen, 25% helium.– Matches observed abundances – Details provide strong evidence
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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!
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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?
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