Universal Time?

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Is the time since the Big Bang the same for every point in our universe?
If so, does that imply there really is a "present" moment everywhere and
not just in my immediate locality?
 

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  • #2
Orodruin
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This is a deeper question than you may have intended. There is no unique way of defining what "now" is in the Universe (not even locally!) and hence there is no way of uniquely answering your question. Even if you do take a definition of now, you then have to define what you mean by such a thing as "the time since Big Bang". The time corresponding to an observer arriving to the "now" will generally depend both on your definition of "now" and the history of how the observer has moved.

With those caveats, in cosmology there are some standard assumptions. One of them is that the Universe is isotropic and homogeneous. This in itself actually singles out a particular definition of "now", namely the "now" that makes the Universe isotropic and homogeneous. It also singles out a particular type of observer, namely observers that actually observe the Universe as isotropic and homogeneous, e.g., do not move relative to the CMB. Such observers are called comoving observers because they do not move relative to the background. Now, when cosmologists talk about the "age of the Universe", what they are referring to is the time elapsed since the Big Bang that would be measured by a comoving observer. By definition, this time is the same everywhere in the same "now" (again, with the definition of "now" from above).
 
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kimbyd
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This is a deeper question than you may have intended. There is no unique way of defining what "now" is in the Universe (not even locally!) and hence there is no way of uniquely answering your question. Even if you do take a definition of now, you then have to define what you mean by such a thing as "the time since Big Bang". The time corresponding to an observer arriving to the "now" will generally depend both on your definition of "now" and the history of how the observer has moved.

With those caveats, in cosmology there are some standard assumptions. One of them is that the Universe is isotropic and homogeneous. This in itself actually singles out a particular definition of "now", namely the "now" that makes the Universe isotropic and homogeneous. It also singles out a particular type of observer, namely observers that actually observe the Universe as isotropic and homogeneous, e.g., do not move relative to the CMB. Such observers are called comoving observers because they do not move relative to the background. Now, when cosmologists talk about the "age of the Universe", what they are referring to is the time elapsed since the Big Bang that would be measured by a comoving observer. By definition, this time is the same everywhere in the same "now" (again, with the definition of "now" from above).
Small quibble: it's observers who view the universe as isotropic alone. Homogeneity is not directly observed. It can only be inferred by making certain assumptions.
 
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Just wanted to add (or perhaps merely restate) a couple of things to what @Orodruin said. The co-moving observers who pick out a definition of "now" are analogous to people standing still on the surface of the Earth. It makes a lot of sense to treat their view as special in some sense, since they're doing the same thing as most of the mass in the neighbourhood ("neighbourhood" is a rather larger concept in cosmology, of course). But in terms of physical laws, it's an arbitrary choice.

Another issue is that Earth-bound clocks don't show the same times as co-moving clocks. We're moving with respect to them and are in an over-dense region, so our clocks tick slightly slower than a co-moving observer's would. This is the point that Orodruin was making in his first paragraph - even at the same event, two clocks will not necessarily agree how long it's been since the Big Bang. And in general they will have different notions of simultaneity.

So the answer to your question is pretty much "no". We can justifiably pick a sensible definition of "the universe at the same time since the Big Bang" in terms of vo-moving clocks, and it's convenient and standard practice to do so. But we still picked it. Physical law doesn't care what definition we pick.
 
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Orodruin
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Small quibble: it's observers who view the universe as isotropic alone. Homogeneity is not directly observed. It can only be inferred by making certain assumptions.
Fair point. Observing homogeneity would be equivalent to observing spatially separated regions of the Universe. However, I think assuming that we are not situated in a special point which just happens to be the point around which the Universe is isotropic is a very strong leap of faith.
 
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kimbyd
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Fair point. Observing homogeneity would be equivalent to observing spatially separated regions of the Universe. However, I think assuming that we are not situated in a special point which just happens to be the point around which the Universe is isotropic is a very strong leap of faith.
There are physicists who make that very argument with respect to the accelerated expansion, though. One potential alternative to dark energy that has been proposed is that we live near the center of a very large void. One argument I heard during a scientific talk was that sure, the void model is fine-tuned, but a cosmological constant is many orders of magnitude more fine-tuned, so it shouldn't be discarded.

This kind of model turns out to be difficult to distinguish because General Relativity provides precious little for us to distinguish a spherically-symmetric universe from a homogeneous one. Still, detailed observations of large scale structure can be used here to demonstrate that void models just don't fit the data:
https://arxiv.org/abs/1007.3725
 
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No, because of what was said above. Time changed based on relative motion and mass.

However, MOST of the macroscopic universe exists in about the same time scale. If you polled all of civilizations out there who’ve pondered the question of how old the universe is, they’d all have about the same answer.
 
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kimbyd
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No, because of what was said above. Time changed based on relative motion and mass.

However, MOST of the macroscopic universe exists in about the same time scale. If you polled all of civilizations out there who’ve pondered the question of how old the universe is, they’d all have about the same answer.
Not enough to really matter. If we imagine an observer stationary with respect to the Hubble flow, and one who moves at a constant 1000km/s relative to the Hubble flow, the moving observer would perceive about one month less time passing over 13 billion years.
 
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Is the time since the Big Bang the same for every point in our universe?
The answer is that there exists a time of "August 16 2018 0:05 GMT" at every point in space. We can call that "now" if you like. If that's not really what you are asking, I think you need to be more specific.
 

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