<p>We don't know. This is a depressing thing that makes me, at least, feel so small. The only thing we might assume, that if they once were accelerating that fast, they probably will do it still (even faster), but of course we can't be sure if anything have happened.</p>

<p>It's like this with all the long distances in the universe. How do we know if Proxima Centauri (4.2 light years away) have died/exploded? We wait, 4.2 years.</p>

CC BY-SA 4.0
2018-11-05T16:28:56.743Z
2018-11-05T16:28:56.743Z
Casimir Rönnlöf
<p>I think you raise an interesting point that is usually glossed over, and it's really a cosmology question (since it concerns the large-scale doings of the universe). And I think the best answer is that we assume the universe to be 1) isotropic and 2) homogeneous, meaning that if the universe itself has some property somewhere, then it has it essentially <strong>everywhere</strong>. These cosmological assumptions, known as the <a href="https://en.wikipedia.org/wiki/Cosmological_principle" rel="nofollow noreferrer">Cosmological</a> Principle, are very well motivated: see for instance <a href="http://www.ir.isas.ac.jp/~cpp/teaching/cosmology/documents/cosmology01-02.pdf" rel="nofollow noreferrer">here</a>, which explains that so far large scale experiments justify the assumptions.</p>

<p>So in truth, we don't <strong>know</strong> in the sense that I know what time it is on my wrist watch; rather, we <strong>know</strong> it in the sense that I know the sun will rise again tomorrow - it is a well verified assumption about nature's doings.</p>

CC BY-SA 4.0
2018-11-05T16:56:47.083Z
2018-11-05T16:56:47.083Z
Daddy Kropotkin
<p>According to Hubble more distant galaxies are moving away faster but since the light took billions of years to reach us we should say they WERE moving away faster billions of years ago. How do we know that those galaxies where light is just reaching us know are STILL moving away at all.... let alone faster</p>

CC BY-SA 4.0
2018-11-06T09:38:22.923Z
We don't even *really* know that, say, 2 + 2 is 4, in other places.
Fattie
CC BY-SA 4.0
2023-05-24T05:17:33.640Z
Does this answer your question? [How do we know that the Universe is still expanding now?](https://astronomy.stackexchange.com/questions/13055/how-do-we-know-that-the-universe-is-still-expanding-now)
ProfRob
<expansion><observable-universe><hubble-constant>
how do we know the universe is STILL expanding?
Paul Wagner
# how do we know the universe is STILL expanding?

(Asked by: username_0 on 2018-11-05)

According to Hubble more distant galaxies are moving away faster but since the light took billions of years to reach us we should say they WERE moving away faster billions of years ago. How do we know that those galaxies where light is just reaching us know are STILL moving away at all.... let alone faster

**Tags:** expansion, observable-universe, hubble-constant

> **Comments:**
> **username_1**: We don't even *really* know that, say, 2 + 2 is 4, in other places.  
> **username_2**: Does this answer your question? [How do we know that the Universe is still expanding now?](https://astronomy.stackexchange.com/questions/13055/how-do-we-know-that-the-universe-is-still-expanding-now)  

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## 2 answers

### Answer by username_3 on 2018-11-05. Score: 2

We don't know. This is a depressing thing that makes me, at least, feel so small. The only thing we might assume, that if they once were accelerating that fast, they probably will do it still (even faster), but of course we can't be sure if anything have happened.

It's like this with all the long distances in the universe. How do we know if Proxima Centauri (4.2 light years away) have died/exploded? We wait, 4.2 years.


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### Answer by username_4 on 2018-11-05. Score: 0

I think you raise an interesting point that is usually glossed over, and it's really a cosmology question (since it concerns the large-scale doings of the universe). And I think the best answer is that we assume the universe to be 1) isotropic and 2) homogeneous, meaning that if the universe itself has some property somewhere, then it has it essentially **everywhere**. These cosmological assumptions, known as the [Cosmological](https://en.wikipedia.org/wiki/Cosmological_principle) Principle, are very well motivated: see for instance [here](http://www.ir.isas.ac.jp/~cpp/teaching/cosmology/documents/cosmology01-02.pdf), which explains that so far large scale experiments justify the assumptions.

So in truth, we don't **know** in the sense that I know what time it is on my wrist watch; rather, we **know** it in the sense that I know the sun will rise again tomorrow - it is a well verified assumption about nature's doings.


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