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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