# Accelerating universe question

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1. Aug 4, 2015

### Vidar T

Can we really say that the universe is accelerating its expansion because we find that galaxies further away have more redshift? We know that we are looking back in time when we look at the stars, and when we look at the galaxies furthest away from us, we are seeing them as they were a long time ago. Accually, the older the galaxy the more redshift it has. Dont this mean that things are slowing down and that everything was moveing faster in the beginning?

2. Aug 4, 2015

### phinds

This question is asked here with great regularity. Yes, we can be sure. I suggest a forum search for discussions.

3. Aug 4, 2015

### Vidar T

Thanks, i will check.

4. Aug 4, 2015

### bapowell

Say you observe a supernova at a redshift of z=1, i.e. a supernova that occurred when the universe was half its current size. In an accelerating universe, expansion in the past was slower than today and so, in comparison with a decelerating universe, the universe took longer to expand from half its size to its present size. Because it takes longer, light travels farther and hence supernovae appear fainter at a given redshift in an accelerating universe than in a non-accelerating universe.

Last edited: Aug 4, 2015
5. Aug 4, 2015

### Vidar T

Thanx:-) I would also like to ask if we are sure that the speed of light has allways been the same, or if it is somehow affected by the expansion of the universe? I will search the treads for an answer, but i apritiate any good explination to this:-)

6. Aug 5, 2015

### Staff: Mentor

We are as sure as we can be at this time that the speed of light is constant. That's not to say that we are 100%, absolutely sure with no possibility that we could be wrong, only that the available evidence is overwhelmingly in favor of the speed of light being a constant.

Last edited: Aug 5, 2015
7. Aug 5, 2015

### Gaz

Well considering the numbers of redshift is going into the 8,s The galaxies would have had to be moving 8 times the speed of light.
I've been wrapping my head around this all day lol.

8. Aug 5, 2015

### marcus

There's a nice formula for the relativistic doppler effect. It is not as simple as just z itself or z+1 but you can look it up.

Google "relativistic doppler" and see what you get.

I get
https://en.wikipedia.org/wiki/Relativistic_Doppler_effect#Motion_along_the_line_of_sight

Last edited: Aug 5, 2015
9. Aug 5, 2015

### marcus

So if z = 8 (e.g. the example you mentioned) then 1+z = 9
that z+1 is usually a more useful number than z itself, it is the actual ratio of wavelengths, or frequencies

And then you can solve for β which is the radial speed expressed as a fraction of the speed of light---namely β = v/c

(1+β)/(1-β) = 81

80/82

But nobody is saying that the galaxies are moving that fast thru the flat non-expanding Minkowski space of special relativity . The cosmo redshift is not a doppler effect of some relative speed at this or that time.

It is the cumulative effect of how much the distance grew while the light was traveling. A GR thing not an SR thing.
z+1 = distance now/distance then
distances grow at changing speeds throughout the whole time so there is no simple way to analyze in doppler terms based on this or that speed. Wavelengths are stretched by the same factor as distances.

I think you know that, but say it anyway in case anyone reading does not.

Last edited: Aug 5, 2015