Time Since Big Bang: One Moment Everywhere?

In summary, the concept of "now" in the Universe is not uniquely defined and can vary based on different definitions and the history of an observer's movement. However, in cosmology, there is a standard assumption of isotropy and homogeneity which allows for a particular definition of "now" to be used by comoving observers. This definition also applies to the measurement of time since the Big Bang, which is considered to be the same everywhere in the same "now". There are some objections to this assumption, such as the possibility of living in a special point in the Universe, but current observations do not support this idea.
  • #1
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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
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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  • #3
Orodruin said:
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.
 
  • #4
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.
 
  • #5
kimbyd said:
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.
 
  • #6
Orodruin said:
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
 
  • #7
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.
 
  • #8
newjerseyrunner said:
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.
 
  • #9
clinden said:
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.
 

1. What is the Big Bang theory?

The Big Bang theory is the scientific explanation for the origin of the universe. It proposes that the universe began as a single point of infinite density and has been expanding ever since.

2. How long ago did the Big Bang occur?

The Big Bang is estimated to have occurred approximately 13.8 billion years ago.

3. What evidence supports the Big Bang theory?

Some of the key pieces of evidence that support the Big Bang theory include the cosmic microwave background radiation, the abundance of light elements in the universe, and the observational data showing that galaxies are moving away from each other.

4. How does time since the Big Bang relate to the age of the universe?

Time since the Big Bang is essentially the same as the age of the universe, as the Big Bang is considered to be the starting point of the universe.

5. Can we measure time since the Big Bang?

Yes, scientists use various methods such as studying the expansion rate of the universe and the distance of distant objects to estimate the time since the Big Bang. However, due to the extreme conditions at the very early stages of the universe, it is difficult to determine an exact measurement.

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