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

Big Bang Theory (1940s)- universe violently exploded into being about billion years ago Big Bang Theory (1940s)- universe violently exploded into being about billion years ago –Was an expansion of space that brought matter along with it u/globalchange1/current/lectures/ universe/universe.html

Evidence for the Big Bang 1. Red Shift- Measurements of the red shifts of virtually all galaxies (except a few) show that the visible universe is expanding in all directions. (Remember Hubble’s Law: v = Hd)

2. Cosmic background radiation- -visible universe is permeated by "cosmic background" microwave radiation, coming from all directions -it’s the red-shifted remnant of the radiation left over from the original Big Bang -it’s the red-shifted remnant of the radiation left over from the original Big Bang -Initial Bang would have sent out strong shortwave radiation (up to trillions of degrees) in all directions -Initial Bang would have sent out strong shortwave radiation (up to trillions of degrees) in all directions

More on cosmic background radiation -Over time, radiation would spread out and cool; filling the universe -Would now strike Earth as microwave radiation only a few degrees above absolute zero (-270°C, or 3K) -Radiation mapped by COBE and WMAP observatories

COBE

WMAP

3. Universe is mostly H and He Prediction: A hot, dense expanding universe, should be predominantly hydrogen, helium (lighter elements) Prediction: A hot, dense expanding universe, should be predominantly hydrogen, helium (lighter elements) Observation: Universe is ~75% hydrogen and ~25% helium by mass Observation: Universe is ~75% hydrogen and ~25% helium by mass

What’s the Fate of the Universe? The outward momentum of the expanding universe is opposed by the inward pull of gravity- Which force is stronger??? 3 scenarios: 1. Open universe- continual expansion 2. Closed universe (“Big Crunch”)- expansion will turn into a contraction 3. Flat universe- expansion will slow down and eventually stop

So… What will happen? Answer depends on the total amount of matter in the universe (expressed as the average density of matter) Answer depends on the total amount of matter in the universe (expressed as the average density of matter) –Average density can be observed by astronomers –It includes light and dark matter Compare average density to critical density (the dividing point between open or closed universe; about kg/m 3 ) Compare average density to critical density (the dividing point between open or closed universe; about kg/m 3 )

If average density = critical density, then the universe is flat If average density = critical density, then the universe is flat If average density < critical density, then the universe is open If average density < critical density, then the universe is open If average density > critical density, then the universe is closed If average density > critical density, then the universe is closed SO FAR- astronomers favor the open model ALSO- data suggests that rate of expansion is speeding up

Inflationary Universe Model (1979) Proposed by Alan Guth Proposed by Alan Guth Universe expanded very rapidly during a small fraction of a second right after the BB Universe expanded very rapidly during a small fraction of a second right after the BB May explain why the universe is so large, and where the large amounts of matter in the universe came from May explain why the universe is so large, and where the large amounts of matter in the universe came from