Cosmology = the study of the Universe as a whole, including its history
and origin.

What evidence is there that the Big Bang really happened?

1) The observed expansion of the Universe: 
	all but the nearest galaxies have redshifts in their spectra.
	Hubble's law: the farther the galaxy, the greater the redshift (so 
        v = H0 r).


2) The expansion was predicted by Einstein's General Theory of Relativity
(1915). Einstein thought it was too fantastic, until Edwin Hubble
discovered it (1929).


3) The cosmic background radiation: is exactly the thermal spectrum
predicted for a Universe that was once hot and dense.
	
This was discovered in 1964 by Arno Penzias and Bob Wilson (Nobel prize,
1978).


4) The abundances of helium and other light elements (deuterium, lithium):  
agree with predictions by nuclear physics.

During the first three minutes, the whole Universe was as dense and as hot
as the center of a star. We understand stars (and H-bombs) well enough to
extend these predictions to the early Universe. Measured since the 1960s
by many people.


5) As we look deep into the Universe, we see evolution, or change over
time.

Recall the concept of look-back time: as we look deep into space, we look
back in time, because light has a finite speed, and took time to get here.

Looking deep into the Universe, we see:

a) Spiral galaxies are much more common in the distant past. Elliptical
galaxies are more common today, and in the centers of galaxy clusters,
because of collisions between galaxies.

b) Galaxies of all kinds were much bluer, with many short-lived, hot
stars, in the distant past.

c) Galaxies are now more often found in clusters than they were in the
past. The clustering happened recently, because of gravity.

d) Quasars (active galaxies that are the most luminous objects that are
steady sources [not explosions] in the Universe) were much more common in
the past than they are today.