# Talk:Redshift

## My Edits

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 = anow/athen (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)

## Fritz Zwicky and Ten Bruggencate

OK, so first of all I'm only 13 years old. But I've done some research on this recently (for fun) and I think I have some suggestions. I think that the scientific articles of Fritz Zwicky and Ten Bruggencate would be extremely helpful for this article.

After Edwin Hubble discovered the redshift/distance correlation, he said that the redshift was due to the Doppler effect. However, Fritz Zwicky noted that the redshift/distance relationship wasn't really that great. If the universe was expanding, the redshift/distance correlation should be pretty much linear, but for further distances the relationship was not that good. In fact, it was out of the margin of error involved during calculations and had to be attributed to something else. So Zwicky proposed that light lost energy as it traveled through gravitational fields (or that the energy got transferred via gravitational fields to intervening matter).

Of course, that was a nice idea, but it was just a hypotheses. So here's where Ten Bruggencate comes into the scene. He wanted to test Zwicky's hypotheses, so he decided to test the redshifts of globular clusters. These globular clusters were a good testing sample since their distances would be somewhat accurate, and he could just deal with redshifts. If Hubble's idea was right, then they should all have the same redshifts, but if Zwicky's idea was right, then their redshifts should relate to the intervening matter around them. And that's just what he found - that the redshifts of the globular clusters were definitely related to the intervening matter surrounding them. Zwicky's theory was proven!

So to sum up, redshift is caused by light travelling through gravitational fields. The longer the light has traveled, the more gravitational fields it has gone through, and the redder it appears. When light does not go through many gravitational fields, it does not appear as redshifted. Hubble's constant actually varies up to 30% (you can even see that by having a look at any redshift/distance diagram).

Below are the two articles I'm talking about.

You can also see even in a traditional diagram like this how when stars are clustered together, the redshift/distance correlation is even more messed up.

Written by JasperTech, Sunday, January 20, 2013