Cosmology-Cosmology- the study the study of the origin, evolution of the origin, evolution and composition of the and composition of the universeuniverse

Big Bang Theory (1940s)-Big Bang Theory (1940s)- universe violently exploded into universe violently exploded into being about 13-15 billion years being about 13-15 billion years agoago– Was an expansion of Was an expansion of

space that brought space that brought matter along matter along with itwith it

http://www.globalchange.umich.edu/globalchange1/current/lectures/universe/universe.html

Evidence for the Big BangEvidence for the Big Bang1.1. Red Shift-Red Shift- Measurements of the Measurements of the

red shifts of virtually all galaxies red shifts of virtually all galaxies (except a few) show that the visible (except a few) show that the visible universe is expanding in all universe is expanding in all directions. (Remember Hubble’s directions. (Remember Hubble’s Law: v = Hd)Law: v = Hd)

http://www.globalchange.umich.edu/globalchange1/current/lectures/universe/universe.html

2. Cosmic background radiation- 2. Cosmic background radiation- -visible universe is permeated by -visible universe is permeated by "cosmic background" microwave "cosmic background" microwave radiation, coming from all directionsradiation, coming from all directions

-it’s the red-shifted remnant of the -it’s the red-shifted remnant of the radiation left over from the original radiation left over from the original Big BangBig Bang

-Initial Bang would have sent out -Initial Bang would have sent out strong shortwave radiation (up to strong shortwave radiation (up to trillions of degrees) in all directionstrillions of degrees) in all directions

More on cosmic background More on cosmic background radiationradiation

-Over time, radiation would spread out -Over time, radiation would spread out and cool; filling the universeand cool; filling the universe

-Would now strike Earth as microwave -Would now strike Earth as microwave radiation only a few degrees above radiation only a few degrees above absolute zero (-270absolute zero (-270°C, or 3K)°C, or 3K)

-Radiation mapped by COBE and -Radiation mapped by COBE and WMAP observatoriesWMAP observatories

COBECOBE

WMAPWMAP

3. Universe is mostly H and 3. Universe is mostly H and HeHe

PredictionPrediction: A hot, dense expanding : A hot, dense expanding universe, should be predominantly universe, should be predominantly hydrogen, helium (lighter elements)hydrogen, helium (lighter elements)

ObservationObservation: Universe is : Universe is ~75% hydrogen and ~75% hydrogen and ~25% helium by mass ~25% helium by mass

What’s the Fate of the What’s the Fate of the Universe?Universe?

The outward momentum of the expanding The outward momentum of the expanding universe is opposed by the inward pull universe is opposed by the inward pull of gravity- Which force is stronger???of gravity- Which force is stronger???

3 scenarios:3 scenarios:1.1. Open universe-Open universe- continual expansion continual expansion2.2. Closed universeClosed universe (“Big Crunch”)- (“Big Crunch”)-

expansion will turn into a contractionexpansion will turn into a contraction3.3. Flat universe-Flat universe- expansion will slow expansion will slow

down and eventually stopdown and eventually stop

So… What will happen?So… What will happen? Answer depends on the Answer depends on the totaltotal amount amount

of of mattermatter in the universe (expressed in the universe (expressed as the as the averageaverage densitydensity of matter) of matter)– Average density can be observed by Average density can be observed by

astronomersastronomers– It includes light and dark matterIt includes light and dark matter

Compare average density to Compare average density to criticalcritical density density (the dividing point between (the dividing point between open or closed universe; about 10open or closed universe; about 10-26-26 kg/mkg/m33))

If average density = critical density, If average density = critical density, then the universe is flatthen the universe is flat

If average density < critical density, If average density < critical density, then the universe is openthen the universe is open

If average density > critical density, If average density > critical density, then the universe is closedthen the universe is closed

SO FAR- astronomers favor the open SO FAR- astronomers favor the open modelmodel

ALSO- data suggests that rate of ALSO- data suggests that rate of expansion is speeding upexpansion is speeding up

Inflationary Universe Model (1979)Inflationary Universe Model (1979) Proposed by Alan GuthProposed by Alan Guth Universe expanded very rapidly Universe expanded very rapidly

during a small fraction of a second during a small fraction of a second right after the BBright after the BB

May explain why the universe is so May explain why the universe is so large, and where the large amounts large, and where the large amounts of matter in the universe came fromof matter in the universe came from