- #1

- 18

- 1

If so, does that imply there really is a "present" moment everywhere and

not just in my immediate locality?

- I
- Thread starter clinden
- Start date

- #1

- 18

- 1

If so, does that imply there really is a "present" moment everywhere and

not just in my immediate locality?

- #2

- 16,829

- 6,652

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).

- #3

kimbyd

Science Advisor

Gold Member

- 1,181

- 630

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.

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).

- #4

- 7,397

- 6,483

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

- 16,829

- 6,652

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.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.

- #6

kimbyd

Science Advisor

Gold Member

- 1,181

- 630

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.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.

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

- 1,524

- 624

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

kimbyd

Science Advisor

Gold Member

- 1,181

- 630

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.

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.

- #9

Vanadium 50

Staff Emeritus

Science Advisor

Education Advisor

- 26,413

- 9,926

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.Is the time since the Big Bang the same for every point in our universe?

- Last Post

- Replies
- 1

- Views
- 601

- Replies
- 1

- Views
- 5K

- Last Post

- Replies
- 13

- Views
- 4K

- Last Post

- Replies
- 2

- Views
- 2K

- Last Post

- Replies
- 5

- Views
- 2K

- Replies
- 7

- Views
- 2K

- Last Post

- Replies
- 10

- Views
- 5K

- Last Post

- Replies
- 5

- Views
- 2K

- Last Post

- Replies
- 7

- Views
- 2K

- Replies
- 8

- Views
- 2K