Cosmology

(Section Not Complete)

Distance Methods Applied to the Virgo Cluster
Method Uncertainty
(Mag)		 Distance
(Mpc)		 Uncertainty
(Mpc)		 Range
Mpc
Cepheid 0.16 14.9 1.2 20
Novae 0.40 21.1 3.9 20
Planetary Nebulae 0.16 15.4 1.1 30
Globular Clusters 0.40 18.8 3.8 50
Surface Bright. Fluct. 0.16 15.9 0.9 50
Tulley-Fisher 0.28 15.8 1.5 > 100
D-Sigma 0.50 16.8 2.4 > 100
Type 1a Supernova 0.53 19.4 5.0 > 1000
Source: B. W. Carrol and D. A. Ostlie, An Introduction to Modern Astrophysics, Addison-Wesley (1996).

Cosmology

Java Applet (not finished): Galaxy sizes and the Hubble Law

Semipopular essays in cosmology

The Light Cone: An Illuminating Introduction to Relativity

(Here is an example of a thought experiment in special relativity).

Twin Paradox applet.

Our Hierarchical Universe

Hot Big Bang Model

Steps to the Hubble Constant (Distance Scale Ladder)

Hubble Time

The inverse of the Hubble constant H has the units of time because the Hubble law is

v = H d

where v is the velocity of recession, H is the Hubble constant, and d is the distance. Thus, from this equation, we have that 1/H = d/v. but d/v is distance divided by velocity, which is time (e.g., if I travel 180 miles at 60 miles/hour, the time required is t = d/v = 180/60 = 3 hours).

Thus, the Hubble time T is just the inverse of the Hubble Constant:

T = 1 / H

Taking a value of H = 20 km/s/Mly (where Mly means mega-light years),

where all the factors are necessary to convert the time units to years.

The physical interpretation of the Hubble time is that it gives the time for the Universe to run backwards to the Big Bang if the expansion rate (the Hubble "constant") were constant. The Hubble "constant" actually isn't constant, so the Hubble time is really only a rough estimate of the age of the Universe.

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