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Sonoma State University |
Astronomy 350 J.S. Tenn |
The Hubble Constant H0 = current rate of expansion
The Density Parameter Ωm = ave. density*/critical density
The (reduced) Cosmological Constant ΩΛ
[Some authors call it λ, which was the symbol originally used by Einstein.]For several decades during the middle of the twentieth century it was almost universally assumed among cosmologists that the cosmological constant must be zero. Then cosmology was "A Search for Two Numbers," the title of a famous article by Allan Sandage which appeared in Physics Today in February 1970.
If all three are known, then the age can be calculated.
If the geometry of the Universe is Euclidean (aka “flat”), then Ωm + ΩΛ = 1.
In many models, the age of the universe is close to the Hubble time, where the Hubble time TH = 1/H0.
If ΩΛ = 0 and Ωm = 1, then T0 = (2/3)TH, or the age of the Universe = 2/3 the Hubble time. (This is the critical, or just barely open Friedmann model, also called the Einstein-de Sitter model.)
If three parameters are measured then we can calculate the remaining two, the deceleration parameter q0 and the age of the Universe T0.
* This is the total density of all matter, including dark matter.
| Please send comments, additions, corrections, and questions to joe.tenn@sonoma.edu |
JST 2008-03-19 |