Talk:Redshift

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I edited for grammar and added the "Support for Big Bang" section, a stub. It should be added to immensely, as my knowledge of physics only goes into a basic understanding of electromagnetism and relativity. --Pastafarian

Doppler Effect

Isn't this an example of the Doppler Effect? Not merely analogous to it? KingOfNothing 20:43, 26 May 2008 (EDT)

I'm not sure that the article expresses it all that well, but the Doppler effect is produced by the source moving relative to the observer, whereas some if not most of the redshift is not due to the source moving through space, but due to the space itself between the source and observer expanding (as part of the expansion of the universe). Philip J. Rayment 08:08, 27 May 2008 (EDT)

Thanks for the explanation. --KingOfNothing 18:07, 4 June 2008 (EDT)

There are four sources of redshift.

Doppler effect

The Newtonian space z = v/c.

Relativistic Doppler

Doppler effect when time dilation is taken into account gives z + 1 = (1+v/c)γ. This can be observed with in the Ives–Stilwell experiment.

Cosmic expansion

z = a_now/a_then (a is the scale factor of the universe). As the space that light travels through expands, the wavelength of the light becomes longer. This requires light to be traveling over a region of empty space (not within the galaxy - space isn't expanding faster than gravity is holding it together). If z is less than 0.01, then the redshift from space expansion is minimal compared to the other sources. For comparison, the Andromeda galaxy has a blueshift of 0.0001 [1] (warning VERY large page)

This source of redshift does not apply to any objects that are gravitationally bound to the observer (you can't say that Andromeda's light is from the future). It doesn't work within the galaxy, or within the local group of galaxies, or even the cluster of galaxies that we are part of. The gravity of those various objects cancels the expansion of space between them.

Assuming cosmological redshift being the most significant part of the redshift you can then use f = 1/((1+z)^3/2) to approximate the age of the universe at when the light was sent on its way. The exact value of depends on the shape of the universe (flat, open, or closed). The 3/2 value is for a flat universe. 'f' is the fraction of the age of the universe when the light was emitted. For z = 0.2 gives you 1/(1.2)^3/2 = 1/1.3145341... = 0.7607.... Though this is only good to one significant digit (z was only one sig digit) so lets call it 0.8. The universe was 80% of its current age when that light was emitted.

Gravity

As light climbs out of a gravity well, it becomes redshifted. As it falls into the gravity well, it becomes blueshifted. This can be observed with the frequency of GPS satellites and binary pulsars. How significant this is depends on how massive the light source is.

Not sure how to work this into the article itself though. --Rutm 19:04, 4 June 2008 (EDT)