<p>Without some reference to compare to, time passing more slowly makes no sense at all. But every clock has its own <em>proper time</em> that measures time along its own worldline. </p>

<p>In cosmological models, the <em>cosmological time</em> is the proper time of a certain kind of ideal observer: one comoving with the Hubble flow. In other words, imagine space filled with observers for whom the universe looks as close to the same in every direction as possible (zero <a href="http://en.wikipedia.org/wiki/Cosmic_microwave_background#CMBR_dipole_anisotropy" rel="nofollow">CMBR dipole anisotropy</a>). The cosmological time is what's measured by their clocks.</p>

<p>As such, there is no time dilation... because we <em>defined</em> our time coordinate as that which is measured by the comoving clocks no matter how small or dense the universe is. You can see this in the standard form of the FLRW metric:
$$\mathrm{d}s^2 = -\mathrm{d}t^2 + a^2(t)\left[\frac{\mathrm{d}r^2}{1-kr^2} + r^2\,\mathrm{d}\Omega^2\right]\text{,}$$
where nothing happens to the time component no matter what the scale factor $a(t)$ is.</p>

<p>How could we compare the rate of clocks <em>now</em> with rate of clocks <em>then</em>? We could have the past clock emit some pattern, say a light at a specific frequency, and then later catch and measure it. If we find a different frequency, we could interpret this as evidence of time dilation. But we could also fix the clocks as our time standard and interpret the frequency shift as the result of a changing scale factor.</p>

<p>We could also arbitrarily rescale the time coordinate any way we like, but there's no point in doing so because it wouldn't change the time experienced by comoving clocks.</p>

<hr>

<blockquote>
  <p>are you saying that we've decided to look at it as an increasing scale factor where clocks are fixed, instead of time dilation - but in fact both are just different point of views?</p>
</blockquote>

<p>That was phrased too poorly, so let me clarify. If you're watching a far-away supernova, the collapse and explode process will take a different amount of time in the supernova's own local rest frame than is seen by you. Taken alone, it's up to you whether you interpret this fact as time dilation of your clock relative to the supernova or a stretching of wavelengths due to cosmic expansion, or any combination of both.</p>

<p>However, what we have decided is to use a frame in which the homogeneity and isotropy of the universe is directly manifest, in which case we're not "looking" at a single supernova but all around us. For a general FRW spacetime, we cannot re-interpret cosmological redshift as due entirely to time dilation without spoiling those conditions.</p>

<p>The reason is that homogeneity and isotropy picks out a sequence of spatial hypersurfaces, i.e. a sequence of "nows", snapshots of the universe at instants of time. Then generally the distances between different galaxies are different at different times.</p>

CC BY-SA 3.0
2014-01-14T12:58:11.617Z
I am aware of that; nobody would notice their clock going slowly. Still; looking at distant stars, you are looking at light emitted billions of years ago. That light will have been emitted under different circumstances. And an important part of my question is the velocity of light; measured by ceasium 133 radiation for time and a unit of distance that does not change with the expansion of the universe, and is not recursively depending on the speed of light. Then comes the final part of my question; are we looking at a "tiny" but magnified universe when looking at distant stars?
frodeborli
CC BY-SA 3.0
2014-01-14T13:02:02.803Z
Reading your updated post; are you saying that we've decided to look at it as an increasing scale factor where clocks are fixed, instead of time dilation - but in fact both are just different point of views? That is an explanation that I probably would accept, and which takes away much of my curiosity on this subject. I've been wondering if we see redshift due to expansion, and additional redshift due to time dilation and wondered how those are differentiated.
frodeborli
CC BY-SA 3.0
2014-01-14T11:44:02.640Z
2014-01-15T00:40:20.143Z
2014-01-15T00:40:20.143Z
Stan Liou
<p>13 billion years ago, the universe was about 600 million years old according to many scientists. At that time, all matter in the universe would have to be closer, or in other words denser. </p>

<p>Did time pass more slowly, at that time, relative to now? I ask, because increased gravity causes time to pass more slowly.</p>

<p>If we'd been there, and measured the speed of light as we measure it today, would the speed of light be different?</p>

<p>Finally, does distant stars look magnified, given that a "tiny" universe is stretched on our entire sky?</p>

<p>I assume then that we'd measure time based on caesium 133 and distance not as a function of the speed of light, but as a theoretical physical "device" that would be the same physical length then and now.</p>

<light><general-relativity><special-relativity>
Did time pass more slowly in the past?
frodeborli
# Did time pass more slowly in the past?

(Asked by: username_0 on 2014-01-14)

13 billion years ago, the universe was about 600 million years old according to many scientists. At that time, all matter in the universe would have to be closer, or in other words denser.

Did time pass more slowly, at that time, relative to now? I ask, because increased gravity causes time to pass more slowly.

If we'd been there, and measured the speed of light as we measure it today, would the speed of light be different?

Finally, does distant stars look magnified, given that a "tiny" universe is stretched on our entire sky?

I assume then that we'd measure time based on caesium 133 and distance not as a function of the speed of light, but as a theoretical physical "device" that would be the same physical length then and now.

**Tags:** light, general-relativity, special-relativity


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## 1 answers

### Answer by username_1 on 2014-01-14. Score: 3 | Accepted answer

Without some reference to compare to, time passing more slowly makes no sense at all. But every clock has its own *proper time* that measures time along its own worldline.

In cosmological models, the *cosmological time* is the proper time of a certain kind of ideal observer: one comoving with the Hubble flow. In other words, imagine space filled with observers for whom the universe looks as close to the same in every direction as possible (zero [CMBR dipole anisotropy](http://en.wikipedia.org/wiki/Cosmic_microwave_background#CMBR_dipole_anisotropy)). The cosmological time is what's measured by their clocks.

As such, there is no time dilation... because we *defined* our time coordinate as that which is measured by the comoving clocks no matter how small or dense the universe is. You can see this in the standard form of the FLRW metric:
$$\mathrm{d}s^2 = -\mathrm{d}t^2 + a^2(t)\left[\frac{\mathrm{d}r^2}{1-kr^2} + r^2\,\mathrm{d}\Omega^2\right]\text{,}$$
where nothing happens to the time component no matter what the scale factor $a(t)$ is.

How could we compare the rate of clocks *now* with rate of clocks *then*? We could have the past clock emit some pattern, say a light at a specific frequency, and then later catch and measure it. If we find a different frequency, we could interpret this as evidence of time dilation. But we could also fix the clocks as our time standard and interpret the frequency shift as the result of a changing scale factor.

We could also arbitrarily rescale the time coordinate any way we like, but there's no point in doing so because it wouldn't change the time experienced by comoving clocks.

---

> are you saying that we've decided to look at it as an increasing scale factor where clocks are fixed, instead of time dilation - but in fact both are just different point of views?

That was phrased too poorly, so let me clarify. If you're watching a far-away supernova, the collapse and explode process will take a different amount of time in the supernova's own local rest frame than is seen by you. Taken alone, it's up to you whether you interpret this fact as time dilation of your clock relative to the supernova or a stretching of wavelengths due to cosmic expansion, or any combination of both.

However, what we have decided is to use a frame in which the homogeneity and isotropy of the universe is directly manifest, in which case we're not "looking" at a single supernova but all around us. For a general FRW spacetime, we cannot re-interpret cosmological redshift as due entirely to time dilation without spoiling those conditions.

The reason is that homogeneity and isotropy picks out a sequence of spatial hypersurfaces, i.e. a sequence of "nows", snapshots of the universe at instants of time. Then generally the distances between different galaxies are different at different times.

> **Comments:**
> **username_0**: I am aware of that; nobody would notice their clock going slowly. Still; looking at distant stars, you are looking at light emitted billions of years ago. That light will have been emitted under different circumstances. And an important part of my question is the velocity of light; measured by ceasium 133 radiation for time and a unit of distance that does not change with the expansion of the universe, and is not recursively depending on the speed of light. Then comes the final part of my question; are we looking at a "tiny" but magnified universe when looking at distant stars?  
> **username_0**: Reading your updated post; are you saying that we've decided to look at it as an increasing scale factor where clocks are fixed, instead of time dilation - but in fact both are just different point of views? That is an explanation that I probably would accept, and which takes away much of my curiosity on this subject. I've been wondering if we see redshift due to expansion, and additional redshift due to time dilation and wondered how those are differentiated.  

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