Sonoma State University J.S. Tenn

Cosmology Since 1900


Einstein publishes General Relativity: Gravity is geometry.

Assumptions of homogeneity, isotropy lead to expanding or contracting models. Einstein introduces cosmological constant Λ to allow static universe.

Slipher: Spiral nebulae have large blueshifts and redshifts. This supports idea that they are external galaxies.


Friedmann: With GR and homogeneity and isotropy and no cosmological constant, there are 3 possible universes. With nonzero Λ there are many. All evolve with time.

Hubble: Except for a few nearby nebulae, velocities of recession are proportional to distances. v = H0d    H0 = 550 (km/s)/Mpc    1/H0 = 1.8 Gyr


Lemaître expands on his 1927 proposal that the Universe began a finite time ago with an explosion, but he omits "Hubble Law" from his 1931 translation.


Alpher & Gamow: Only H, D, He, Li can be made in the Big Bang.

Alpher & Herman: the universe should be filled with radiation from the Big Bang.

Bondi, Gold, Hoyle: Steady State model

1950s Baade revises distance scale: H0 = 275      1/H0 = 3.6 Gyr

Humason, Mayall, Sandage: H0 = 180     1/H0 = 5.5 Gyr

Burbidge, Burbidge, Fowler & Hoyle: All but the lightest elements can be made in stars.

1960s Quasars are found; they are more abundant at high redshifts than at low.

Penzias & Wilson find cosmic microwave background radiation (CMB).

Most cosmologists agree that the universe is evolving.

1970s Sandage & Tammann: H0 = 50     1/H0 = 20 Gyr

de Vaucouleurs, others: H0 = 100    1/H0 = 10 Gyr

“Standard Model” explains most observations.

Abundances of He, D support standard model.

Average density = 0.1 critical density.

Particle physics and cosmology interact.

1980s Guth, others: Inflationary model solves many problems, requires “flat” universe.

Simulations of early universe require cold dark matter. What is it? Is there enough to make the universe “flat?”

Huge structures are found. Is the universe really homogeneous?


COBE finds some structure in CMB.

Evidence is found that gamma ray bursters are at cosmological distances.

Deuterium abundance measured. Agreement with big bang nucleosynthesis requires baryon density ≅ 0.05 critical density.

Motions of galaxies require total matter density (baryon + ?) ≅ 0.3 critical density. To get “flat” universe cosmologists assume cosmological constant, now thought of as pressure of vacuum.

HST observations of supernovae: H0 = 67 ± 10, expansion is accelerating, Λ ≠ 0.

SuperKamiokande experiment: muon neutrino has a small mass.


BOOMERANG and MAXIMA find more structure in CMB. More evidence that matter density ≅ 0.3 critical density, but the Universe is “flat.” The other 0.7 is said to come from “dark energy.”

SNO confirms that neutrinos have mass. The total mass of neutrinos may equal that of stars.

DASI: CMB is polarized.

WMAP measures fluctuations in CMB over entire sky. Best fit to WMAP data combined with measurements of type Ia SN and Baryon Acoustic Oscillations in the galaxy distribution: Universe is “flat,” baryon density = 0.0462 ± 0.0015, dark matter density = 0.233 ± 0.013 (and it’s cold), remaining 0.721 ± 0.015 is ascribed to “dark energy.” Also H0 = 70.1 ± 1.3, age of universe is 13.73 ± 0.12 Gyr, and first stars formed between 100 and 400 million years after the expansion began. [These numbers, announced in 2008, represent five years of WMAP observations.]

MiniBooNE experiment at Fermilab confirms that there are only three kinds of neutrinos.

Preliminary (2007) results from Gravity Probe B are consistent with General Relativitistic prediction of geodetic warping to better than 1%.


Final (2011) results from Gravity Probe B interpreted as supporting General Relativitistic predictions of geodetic warping and frame dragging.

NOTE: Gyr = billion years = 109 years = Ga (giga-annum), all values of the Hubble parameter H0 are in (km/s)/Mpc, and densities are fractions of the critical density, i.e., values of omega. “Flat” means Euclidean.

Another history of cosmology, by Prof. Richard McCray of the Univ. of Colorado

and another, from the American Institute of Physics Center for History of Physics

Astronomy 350 Cosmology Cosmology Books and Links
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