# A question on the rate of time in the Light we observe

1. Sep 24, 2006

### popfuzz

Hello everyone,
this is my first post, so if this question has been asked before please point me to the post to read an answer.

I was wondering if there is an effect on observed time that we observe from the light that we see on the earth. Also if there is an effect on time, is there a distance from
the earth in the universe in which the universe is expanding at exactly the speed of light. And if this is the case are all observations beyond this point moving backwards in time?

Also if this is true does this shift in observed time have an effect on calculations that are made on the age of the universe and the rate at which the universe expands?

2. Sep 24, 2006

### popfuzz

Ok I was researching my own question a bit, i read about the Twin Paradox on wikapedia

Based on that, I should assume that light from farther distances is stretched (red shift) and that time is viewed at a slower rate. That is if I view an object that is near the edge of the viewable universe for a few years that I may be only viewing a few seconds in the life of that object. Am I correct in thinking that?

Also what is the effect of viewing objects that are past the distance in which the viewable universe is expanding faster then the speed of light? Has light been stretched to a point where time freezes? Or is the viewable light moving backwards in time?

3. Sep 25, 2006

### hellfire

Yes, this is called cosmological time dilation. This is a general relativistic effect for frames that are comoving with the expansion of space (frames that have no peculiar speeds). Special relativity is not applicable for cosmology, but only for situations where you can assume space-time to be static and flat. The cosmological time dilation factor is (1 + z) for a redshift z. You can read in Ned Wrights Cosmology FAQ:

This is the Hubble distance d = c / H. It is a consequence of the Hubble law v = H d that provides the recession speed v at distance d, with H the Hubble parameter. If you put c the speed of light and H = 71 km/s Mpc and convert units, you obtain d = 13,700 million light-years. Objects that are located today farther away are receding at speeds greater than c today. The redshift of the objects located at the Hubble distance is about z ~ 1.4.

You can see that there is no way to get negative time intervals with the time dilation factor I gave you above (the redshift z is always a positive value) and that there is no different behaviour between z < 1.4 and z > 1.4.

Last edited: Sep 25, 2006