<p>The cosmic "now" is well-defined: It is the time for an observer that has always been at rest in the Universe's <em>comoving</em> coordinates, i.e. the coordinates that expand along with the Universe. Although this reference frame is no more special than any other frame, it makes sense to use this. For instance, it is the only frame in which the cosmic microwave background is (statistically) isotropic, and it is the frame in which you'll see the same number density of galaxies in all directions.</p>

<p>You could (rightfully) define "now" in another frame, but it wouldn't be of much use. For instance, for a neutrino released 1 s after Big Bang, the age of the Universe is some ten thousand years, but it doesn't really tell you anything about the Universe.</p>

CC BY-SA 4.0
2018-09-20T16:31:45.803Z
2018-09-20T16:31:45.803Z
pela
<p>There is no cosmic definition of now because of special relativity, because of rate at which time flows for a particular object depends upon your point of view and your velocity. For a rocket traveling at speed of light, journey of alpha centauri might take a week. For us it would seem like 4 years. As a result of special relativity even the meaning of ‘simultaneous’ is related to your point of view</p>

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2018-09-20T21:07:23.183Z
Is there a question in there somewhere?
Jim421616
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2018-09-20T23:37:14.493Z
@Jim421616 Yes, it's in the title. Usually you're expected to ask/repeat a question in the text, I suppose, but I think it's pretty clear what the intention is: How is cosmological time/rest frame defined. At least that's how I interpreted the question, and I don't see a reason for downvoting it (I'm not implying that _you_ downvoted, but someone did).
pela
CC BY-SA 4.0
2018-09-20T23:39:26.583Z
Hmm ok. I do see your point, but I wasn't the one who downvoted it. Usually, I'd give a short description of the problem in the title, then expand on it and restate it in the body.
Jim421616
<cosmology><space-time><special-relativity>
What exactly is the cosmic definition of ‘now’ is?
Udbhav 
# What exactly is the cosmic definition of ‘now’ is?

(Asked by: username_0 on 2018-09-20)

There is no cosmic definition of now because of special relativity, because of rate at which time flows for a particular object depends upon your point of view and your velocity. For a rocket traveling at speed of light, journey of alpha centauri might take a week. For us it would seem like 4 years. As a result of special relativity even the meaning of ‘simultaneous’ is related to your point of view

**Tags:** cosmology, space-time, special-relativity

> **Comments:**
> **username_1**: Is there a question in there somewhere?  
> **username_2**: @username_1 Yes, it's in the title. Usually you're expected to ask/repeat a question in the text, I suppose, but I think it's pretty clear what the intention is: How is cosmological time/rest frame defined. At least that's how I interpreted the question, and I don't see a reason for downvoting it (I'm not implying that _you_ downvoted, but someone did).  
> **username_1**: Hmm ok. I do see your point, but I wasn't the one who downvoted it. Usually, I'd give a short description of the problem in the title, then expand on it and restate it in the body.  

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## 1 answers

### Answer by username_2 on 2018-09-20. Score: 6 | Accepted answer

The cosmic "now" is well-defined: It is the time for an observer that has always been at rest in the Universe's *comoving* coordinates, i.e. the coordinates that expand along with the Universe. Although this reference frame is no more special than any other frame, it makes sense to use this. For instance, it is the only frame in which the cosmic microwave background is (statistically) isotropic, and it is the frame in which you'll see the same number density of galaxies in all directions.

You could (rightfully) define "now" in another frame, but it wouldn't be of much use. For instance, for a neutrino released 1 s after Big Bang, the age of the Universe is some ten thousand years, but it doesn't really tell you anything about the Universe.


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