Observational Foundations of Modern Cosmology 

1. 1929: Hubble measures the expansion of the Universe

(a modern version, using Type Ia supernovae)

(from Hubble 1936)


2. 1948:  The abundances of the light elements

  • Alpher, Bethe, and Gamow: Conditions in the early universe could explain the elemental abundances of the light elements (particularly helium).

  • Alpher and Herman also predict that the radiation field of the big bang should have "cooled" as the universe expanded, yielding a universal 5K (microwave) background.

3. 1965: Penzias and Wilson discover the microwave background

  • The CMB is amazingly isotropic - the same temperature (down to 1 part in 105) is observed in every direction in the sky.

Those three observations are consider the "classical foundations" of modern Big Bang cosmology. To them, I will add three more:

4. 1980s and 1990s: Redshift surveys show large scale structure

Significant structure exists, but on the largest scales the universe is homogeneous.

These last two observations, in particular, lead us to the cosmological principle:

The Universe is Homogeneous and Isotropic

(The Las Campanas Redshift Survey)

5. 1990-2000+ Anisotropies in the Microwave Background

Microwave anisotropy map from the WMAP satellite (2003).

6. 2000+ Curvature in the Hubble Diagram

The use of Type Ia supernovae as distance indicators became both well-understood physically, and observationally tractable at large redshift. Hubble diagrams (distance-redshift) became more accurate and began to probe expansion history as a function of redshift:

Figure courtesy Ned Wright's Cosmology Tutorial