Models must change to explain new and better data. The geocentric model failed because of new data (galileo). The history of cosmology is a gradual letting go of the idea that the earth, sun, & milky way o(cid:272)(cid:272)up(cid:455) a spe(cid:272)ial pla(cid:272)e . Cosmological principle: we do not occupy a special place, there are no special places. The universe is clearly (cid:862)(cid:272)lu(cid:373)p(cid:455)(cid:863) (cid:271)ut o(cid:374) la(cid:396)ge s(cid:272)ales it is: In a small forest, you can see open space between trees. In a big forest, every sightline hits a tree. 1) there was evolution (fewer stars in the past). We can only see out the distance that light traveled since the big bang (13. 7 billion light years) 2) the universe is expanding, so distant objects are redshifted. Evidence for big bang: expansion of the universe. Observational fact: all galaxies (and distant quasars) are receding from us. The universe is expanding and there was a beginning=big bang.