<p>You're right that in principle we cannot know if the distant galaxies are still receding from us. When, however, we think that they indeed are, it hinges on the <em>cosmological principle</em>, viz. that the Universe is homogeneous and isotropic, i.e. "looks the same everywhere and in all directions". This is a philosophically motivated statement, which is continuously tested observationally. So far, there has been no significant experimental evidence that this is not correct.</p>

<p>When we observe the velocities of galaxies throughout the Universe, we see a smooth velocity as a function of time, or distance, with some intrinsic scatter which is due to the gaalxies' <em>peculiar motion</em>, i.e. their velocities <em>through</em> space, which is usually of the order of 100–1000 km/s. That is, distant galaxies move away from us <em>fast</em>, and nearby galaxies move away from us <em>slowly</em>. As you say, the expansion rate decreased with time, but only until a certain point, after which it started increasing again (due to dark energy).</p>

<p>The fact that we see that nearby galaxies are <em>still</em> receding, together with the cosmological principle, implies that the distant galaxies should <em>also</em> still be receding. We know of no reason that they shouldn't. Of course this is no proof, but in physics, there <em>are</em> no proofs, only verifications and falsifications. To postulate that the distant galaxies no longer recede, and to be taken seriously, you would need to come up with an experiment that could verify this hypothesis, preferably along with a mechanism that would be able to cause this phenomenon (this looks a bit condescending when I type it; this is not my intention).</p>

<p>The expansion rate is currently — and hence locally — around $70\,\mathrm{km}\,\mathrm{s}^{-1}\,\mathrm{Mpc}^{-1}$. Due to the aforementioned velocity scatter, we cannot probe expansion below ~1 Mpc, but at this scale, space doesn't really expand anyway due to the mutual gravitational attraction of the Local Group. This means that in principle, the expansion of the Universe <em>could</em> have come to a halt within the last few million years, and still be in accord with the cosmological priciple. However, this would mean that the evolution of expansion velocity as a function of time would have a sharp kink. Again, this is not impossible, but we know of no physical mechanism which would be able to cause this.</p>

CC BY-SA 3.0
2016-01-04T22:31:57.637Z
Thank you @pela. I found the third paragraph of the answer very instructive. So, no matter we doubt in something, if it complies with the general observations and there is no anomalies, it is the most probable true.
Miroslav Popov
CC BY-SA 3.0
2016-01-05T08:59:48.403Z
@MiroslavPopov: Yes, you can put it that way. We try not to come up with more complicated hypotheses than what is needed to match the observations. This is the essence of [Occam's razor](https://en.wikipedia.org/wiki/Occam's_razor).
pela
CC BY-SA 3.0
2016-01-04T10:30:19.320Z
2016-01-04T10:30:19.320Z
pela
<p>Yes, I know that most galaxies have a red shift and that means they are moving away from us. </p>

<p>The problem is that the farthest galaxies are 13.8 billion light years away. That means that the info is 13.8 billion years old.</p>

<p>What has been puzzling me for years is that the red shift observation shows that the galaxies were receding 13.8 billion years ago. We don't have direct data on what they are doing now. It is natural that the universe was expanding with a tremendous speed at that time because it was near to the big bang, but how do we know that it still is?</p>

<p>If the furthest galaxies are moving with higher speed than the nearer, that means to me that their speed is reducing with the time.</p>

CC BY-SA 3.0
2016-01-02T12:14:14.677Z
you probably meant 13.7
Vojta Klimes
CC BY-SA 3.0
2016-01-02T16:05:24.827Z
Now is a tricky concept when there's a speed limit for information.
Wayfaring Stranger
CC BY-SA 3.0
2016-01-02T16:06:35.440Z
@VojtaKlimes: Do you really think we know the distances to three significant figures (relative distances maybe, but absolute distances?) Anyway we are talking about galaxies, the most distant of which has a nominal distance of [~13.1 Gly](http://www.space.com/29319-farthest-galaxy-ever-found.html) as of May 2015.
Conrad Turner
CC BY-SA 3.0
2016-01-02T16:10:35.707Z
Cosmological models based on our best data and physical theories are used to try and understand what distant galaxies will be observed doing Gyears hence, assuming there will be anyone here to do the observing. and that there will be a here.
Conrad Turner
CC BY-SA 3.0
2016-01-02T16:39:14.240Z
@ConradTurner I did not say anything about what I think I only wrote that OP meant 13.7 (actually 13.8 is right..) because there was 14.8 billion years old before OP corrected it
Vojta Klimes
CC BY-SA 3.0
2016-01-02T16:50:54.607Z
Related: https://physics.stackexchange.com/questions/172903/why-do-we-form-cosmological-theories-based-on-old-data
HDE 226868
CC BY-SA 3.0
2016-01-03T06:41:40.080Z
In addition to @HDE226868's well documented answer: the "advantage" of looking back in time is that we can reconstruct what happened x years ago by looking at stuff at x lightyears distance. That way we could reconstruct a lot of things. And that's all we have. If "something" happens beyond the event horizon, we won't know/see it. We can just theorize on it, based on historical data.
agtoever
CC BY-SA 3.0
2016-01-03T06:44:40.217Z
I think this post answer to my question very well: http://physics.stackexchange.com/a/172915/102796 by @userltk
Miroslav Popov
CC BY-SA 3.0
2016-01-04T00:23:49.260Z
@ConradTurner That Space.com article gets the distance measure horribly wrong in an all too common way. From the redshift in the paper, z=7.73, they calculate that the *light* has  travelled 13.1 gigalightyears, which means the current distance to the object is much larger (around 29.6 gigalightyears by Planck 2015 cosmology), and that the distance to the object at the time of emission was much smaller (only about 1 gigalightyear).
Thriveth
CC BY-SA 3.0
2016-01-04T01:19:48.890Z
Is there any way to observe expansion of the universe within, say, a local radius of 50 light years?
Howard Miller
<universe><big-bang-theory>
How do we know that the Universe is still expanding now?
Miroslav Popov
# How do we know that the Universe is still expanding now?

(Asked by: username_0 on 2016-01-02)

Yes, I know that most galaxies have a red shift and that means they are moving away from us.

The problem is that the farthest galaxies are 13.8 billion light years away. That means that the info is 13.8 billion years old.

What has been puzzling me for years is that the red shift observation shows that the galaxies were receding 13.8 billion years ago. We don't have direct data on what they are doing now. It is natural that the universe was expanding with a tremendous speed at that time because it was near to the big bang, but how do we know that it still is?

If the furthest galaxies are moving with higher speed than the nearer, that means to me that their speed is reducing with the time.

**Tags:** universe, big-bang-theory

> **Comments:**
> **username_1**: you probably meant 13.7  
> **username_2**: Now is a tricky concept when there's a speed limit for information.  
> **username_3**: @VojtaKlimes: Do you really think we know the distances to three significant figures (relative distances maybe, but absolute distances?) Anyway we are talking about galaxies, the most distant of which has a nominal distance of [~13.1 Gly](http://www.space.com/29319-farthest-galaxy-ever-found.html) as of May 2015.  
> **username_3**: Cosmological models based on our best data and physical theories are used to try and understand what distant galaxies will be observed doing Gyears hence, assuming there will be anyone here to do the observing. and that there will be a here.  
> **username_1**: @ConradTurner I did not say anything about what I think I only wrote that OP meant 13.7 (actually 13.8 is right..) because there was 14.8 billion years old before OP corrected it  
> **username_4**: Related: https://physics.stackexchange.com/questions/172903/why-do-we-form-cosmological-theories-based-on-old-data  
> **username_5**: In addition to @HDE226868's well documented answer: the "advantage" of looking back in time is that we can reconstruct what happened x years ago by looking at stuff at x lightyears distance. That way we could reconstruct a lot of things. And that's all we have. If "something" happens beyond the event horizon, we won't know/see it. We can just theorize on it, based on historical data.  
> **username_0**: I think this post answer to my question very well: http://physics.stackexchange.com/a/172915/102796 by @userltk  
> **username_6**: @ConradTurner That Space.com article gets the distance measure horribly wrong in an all too common way. From the redshift in the paper, z=7.73, they calculate that the *light* has  travelled 13.1 gigalightyears, which means the current distance to the object is much larger (around 29.6 gigalightyears by Planck 2015 cosmology), and that the distance to the object at the time of emission was much smaller (only about 1 gigalightyear).  
> **username_7**: Is there any way to observe expansion of the universe within, say, a local radius of 50 light years?  

---

## 1 answers

### Answer by username_8 on 2016-01-04. Score: 1 | Accepted answer

You're right that in principle we cannot know if the distant galaxies are still receding from us. When, however, we think that they indeed are, it hinges on the *cosmological principle*, viz. that the Universe is homogeneous and isotropic, i.e. "looks the same everywhere and in all directions". This is a philosophically motivated statement, which is continuously tested observationally. So far, there has been no significant experimental evidence that this is not correct.

When we observe the velocities of galaxies throughout the Universe, we see a smooth velocity as a function of time, or distance, with some intrinsic scatter which is due to the gaalxies' *peculiar motion*, i.e. their velocities *through* space, which is usually of the order of 100–1000 km/s. That is, distant galaxies move away from us *fast*, and nearby galaxies move away from us *slowly*. As you say, the expansion rate decreased with time, but only until a certain point, after which it started increasing again (due to dark energy).

The fact that we see that nearby galaxies are *still* receding, together with the cosmological principle, implies that the distant galaxies should *also* still be receding. We know of no reason that they shouldn't. Of course this is no proof, but in physics, there *are* no proofs, only verifications and falsifications. To postulate that the distant galaxies no longer recede, and to be taken seriously, you would need to come up with an experiment that could verify this hypothesis, preferably along with a mechanism that would be able to cause this phenomenon (this looks a bit condescending when I type it; this is not my intention).

The expansion rate is currently — and hence locally — around $70\,\mathrm{km}\,\mathrm{s}^{-1}\,\mathrm{Mpc}^{-1}$. Due to the aforementioned velocity scatter, we cannot probe expansion below ~1 Mpc, but at this scale, space doesn't really expand anyway due to the mutual gravitational attraction of the Local Group. This means that in principle, the expansion of the Universe *could* have come to a halt within the last few million years, and still be in accord with the cosmological priciple. However, this would mean that the evolution of expansion velocity as a function of time would have a sharp kink. Again, this is not impossible, but we know of no physical mechanism which would be able to cause this.

> **Comments:**
> **username_0**: Thank you @username_8. I found the third paragraph of the answer very instructive. So, no matter we doubt in something, if it complies with the general observations and there is no anomalies, it is the most probable true.  
> **username_8**: @MiroslavPopov: Yes, you can put it that way. We try not to come up with more complicated hypotheses than what is needed to match the observations. This is the essence of [Occam's razor](https://en.wikipedia.org/wiki/Occam's_razor).  

---

