Expert: Philip Stahl - 8/21/2008

Question
I am the student of engineering but i intersted about the study of universe.My question is about the expansion of the universe.at what rate the galaxies away from eachothe.send me answer i sall be very thankful to you.

Answer
Hello,

The expansion of the universe (actually manifest in the outward recession of the galaxy CLUSTERS) all follow a defined law we call the "Hubble law". This law allows the rate of expansion (in terms of velocity of recession, v) to be obtained for a given galaxy cluster if one knows the red shift (displacement of specific spectral lines) in its spectrum. The latter is found by comparing the spectrum to the known spectrum or standard spectrum for the lines shown (e.g. Hydrogen Balmer series).

The relation between distance and recessional velocity can be expressed on a graph:

v
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!-------------------------------->  D


With the graph going from the origin up at an angle to the upper right

It may be expressed:

v  = c z  = HD

where c is the speed of light and z is the red shift. H is the Hubble constant and D is the distance.

For example, if z = 100 nm =  10^-7 m (say the Hydrogen alpha line is displaced from 656 nm to 756 nm in some galaxy cluster), then:


v  =  (3 x 10^8 m/s) (10^-7 m) = 30 m/s

or v = 108,000 kilometers/hour recessional velocity

In other words, that particular cluster is moving away from us ("expanding") at that speed.

As yoiu can see, each cluster's recessional speed will vary according to what its value of 'z' is.