# Gravitational potential and the past

1. Apr 27, 2013

### h_cat

The medium gravitational potential must have been higher in the past than today (assuming constant mass/energy and an expanding universe). That said it would mean that the effective speed of light in any place of the past would have been slower than today (assuming a constant c at constant gravitational potential). Every process would have been slower, ever length would appear longer. Part of the redshift would result from this phenomenon. It would further mean that the age of the universe must be infinite if the universe started from a point or at least (if we assume a very small but not infinitely small space) much older that told.

oy_cat

2. Apr 27, 2013

### jambaugh

Every process slower than what, every length appear longer that what? Speed of light slower than today as measured how? You are using comparatives with implicit norms which, when you make them explicit in an empirical way renders your blanket statements meaningless in terms of observable phenomena.

If I assert that every object in the universe doubled in scale last week, you can't test it because you use one object to measure another (it's called a ruler). Similarly with time and clocks. We relate the two by establishing a standardized norm "the speed of light in vacuum". We define length in terms of how far light travels in a given clock tick so in current usage the speed of light is by definition a constant, a non-empirical unit conversion constant.

We do this because we recognize an ambiguity (gauge degree of freedom) in the comparison of lengths and durations across different spatial and time separations. Picking c as a constant (and so able to be set to 1 in normalized units) is what amounts to fixing the gauge.

It is the same procedure as when you recognize only voltage differences are observable and so arbitrarily pick a point in a circuit as "the ground" and set its voltage to zero. Your statements in part equate to saying "the voltage of the ground in circuits is really 10volts!" You can adopt that view but doing so changes nothing about predictions of behavior of observable phenomena and only adds an additional mathematical complication to the calculations.

3. Apr 27, 2013

### Mordred

This is definetely one of the best descriptives on the constant of c as a constant I have read in a long time. Thank you for that nice comparision to the voltmeter. It definetely describes it beautifully.

For the purpose of the OP. Cosmologists have studied and considered various changes in any parameter used in distance measures. Each parameter used in the LQCM or other paradigm models such as MOND and LQC. In many ways the manner that alternative models compete with each other provides a qreat check of any given model.
If LQC shows better accuracy of this process, The manner used becomes a standard methodology. For one example MOND does better than LCDM on small dwarf galaxy rotations. So that portion of MOND application becomes a guideline of what we would expect to see in our results. If a model improves upon the results. Then that model becomes the standard to improve upon.
This also applies to any variable used in Cosmology. No matter how unlikely variations are thought to occur.

4. Apr 28, 2013

### h_cat

So two galaxies in the very past when they where much closer together than now would not have a higher redshift that similar galaxies at the same distance now? (of course we would need another distance measurement that redshift). Their gravitational potential would be higher and so should the redshift be. (higher that the redshift of two galaxys at a distance in the past like those would have now)

fh_cat

5. May 1, 2013

### timmdeeg

The observed redshift of galaxies in the very past is due to the expansion of the universe, i.e. due to the ever increasing distances. As the light travels, it becomes increasingly redshifted.

We will never see the light of similar galaxies at the same distance now, because these recede faster than light. But people living close enough to see them will see their light redshifted, again, according to the expansion of the universe during the time, the light was travelling.

In the expanding empty (means no gravitational potential) universe models one can think of massless galaxies. Observers will see them redshifted depending on distance and rate of expansion.

Last edited: May 1, 2013
6. May 1, 2013

### Mordred

To add to this the average density of the universe in the past is not sufficiently dense enough at any point in time that we can possibly see to cause a gravitational time dilation. The only dilation we see is as the others mentioned due to expansion. This dilation is already factored in our calculations.

this on going thread is on the same subject

7. May 1, 2013

### timmdeeg

To my understanding even much higher matter density of the universe in the past wouldn't cause gravitational time dilation, because in contrast to Schwarzschild metric the gravitational potential in the FRW model is isotropic. May be this wording isn't correct. What I mean is that as long as we keep the picture that the redshift is due to expansion the density plays no role.

8. May 1, 2013

### Mordred

That makes perfect sense, Its also provides one of the best explanations of why a Universal overall density cannot have a time dilation effect. Think of the analogy of the overall gravity being a flat sheet in a particular time slice. There are no depressions in that sheet during that time slice. So no gravitational redshift.

Given that, the only redshift viable from a future observer vs overall density would be due to expansion. Gravitational redshift would only apply to individual bodies.

Last edited: May 1, 2013