<p>There is no name for this.</p>
<p>As we look further in space, we also look further back in time. The &quot;edge of the observable universe&quot; is in the early universe. At an early time in the evolution of the universe, the whole of space was so hot and dense that it was opaque.  In whichever direction we look, we can't look back further than this. The is called the cosmic microwave background. There is no direction in which we can look in which we don't see the cosmic microwave background. There is no direction that contains no visible matter.</p>
<p>There are directions that contain no galaxies or other more nearby objects, but there is no name for these directions.</p>
<p>There is a name for the region of space (or rather the period of time) that is beyond the furthest galaxies, but closer (or more recent) than the cosmic microwave background.  This is the &quot;Dark ages&quot;.</p>

CC BY-SA 4.0
2019-03-07T15:29:27.997Z
@Paul Sure, the CMB has been redshifted to microwaves, but it was originally visible light, and like most of the light in the universe, it was emitted by hot hydrogen & helium. FWIW, it was originally an orange colour, as shown at the end of [this answer](https://physics.stackexchange.com/a/133943/123208).
PM 2Ring
CC BY-SA 4.0
2019-03-06T21:01:26.493Z
2022-08-02T23:04:02.033Z
2022-08-02T23:04:02.033Z
James K
<p>I think you're asking two (related) questions: First you're asking about the &quot;boundary&quot; of the observable Universe, i.e. the points in space separating the observable Universe from the rest of the Universe. Second, you're asking about a line going from us to this boundary.</p>
<h4>The particle horizon</h4>
<p>The observable Universe is bounded by the <a href="https://en.wikipedia.org/wiki/Particle_horizon" rel="nofollow noreferrer">particle horizon</a>, which is defined as the spherical, non-physical shell centered on our position, and with a radius equal to the distance light has had the time to travel since the Big Bang.</p>
<h4>The radius of the Universe</h4>
<p>This distance, which is your second question, is simply &quot;the radius <span class="math-container">$R$</span> of the Universe&quot;, and may be calculated by integrating the <a href="https://en.wikipedia.org/wiki/Scale_factor_(cosmology)" rel="nofollow noreferrer">scale factor</a> <span class="math-container">$a(t)$</span> from a time <span class="math-container">$t=0$</span> to <span class="math-container">$t=[\mathrm{today}]$</span>:
<span class="math-container">$$
R = \int_0^{13.8\,\mathrm{Gyr}}\!\!\!\!\!\!\frac{c}{a(t')}dt'.
$$</span>
The scale factor depends on the expansion rate <span class="math-container">$H_0$</span> and the density parameters <span class="math-container">$\Omega_{\mathrm{m},\Lambda,\gamma,k}$</span> of the various components of the Universe. For the observed values of <a href="https://ui.adsabs.harvard.edu/abs/2020A%26A...641A...1P/abstract" rel="nofollow noreferrer">Planck Collaboration et al. (2020)</a>, this integral amounts to <span class="math-container">$46.2$</span> billion lightyears.</p>
<p><a href="https://i.sstatic.net/ExApR.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/ExApR.png" alt="uni" /></a></p>

CC BY-SA 4.0
2022-08-03T08:57:20.313Z
2022-08-03T08:57:20.313Z
pela
<p>Wouldn't the edge of the universe just be the location where there is no matter or energy beyond. Assuming light has reached the farthest from the big bang point, wouldn't the edge just be at the farthest locations where light has traveled. Beyond the edge would just be the same as the empty space all around us (locations where there is no matter or energy) except that there would be no matter or energy within an agreed upon distance. Can empty space have a distance, yes, but that doesn't mean it is part of the universe.</p>

CC BY-SA 4.0
2022-08-02T22:13:26.367Z
There's no Big Bang point. Please see https://physics.stackexchange.com/q/136860/123208
PM 2Ring
CC BY-SA 4.0
2022-08-04T02:35:27.360Z
This does not provide an answer to the question. Once you have sufficient [reputation](https://astronomy.stackexchange.com/help/whats-reputation) you will be able to [comment on any post](https://astronomy.stackexchange.com/help/privileges/comment); instead, [provide answers that don't require clarification from the asker](https://meta.stackexchange.com/questions/214173/why-do-i-need-50-reputation-to-comment-what-can-i-do-instead). - [From Review](/review/late-answers/29804)
WarpPrime
CC BY-SA 4.0
2022-08-02T21:56:06.187Z
2022-08-02T21:58:45.993Z
2022-08-02T21:58:45.993Z
Dustin Williams
<p>The observable universe is merely how far we can see with current telescope technology.</p>

<p>Space itself is endless. Oh, there's no end of people here who will aggressively argue this. But empty space, by definition, cannot have an end. It can only be infinite. Literally infinite.</p>

<p>There could very well be an edge in the sense that there's no more matter, where stars, planets and galaxies etc simply tapers off. After that, it would be endless black space.</p>

CC BY-SA 4.0
2019-03-07T22:59:49.607Z
2019-03-07T22:59:49.607Z
White Prime
<p>Is there a name for a point at the edge of the observable universe at which there is no visible matter, that appears completely dark?</p>

<p>I'm thinking something like "empty space", but that could equally apply to an empty void between galaxies, which isn't quite what I'm looking for.</p>

<p>So it would refer to a theoretical line in space from the observer to the edge of the universe which contains no visible matter. Is there a term for this?</p>

CC BY-SA 4.0
2019-03-06T19:55:40.583Z
Maybe I don't quite understand your question, but why would we need a special name for voids near the edge of the observable universe?
PM 2Ring
CC BY-SA 4.0
2019-03-07T13:42:46.880Z
I think the reason is indirectly implied from your comment, because "voids near the edge of the observable universe" isn't precisely what I'm looking for either. Cosmic voids can be obscured behind galaxies, so that wouldn't work as a description either. I just want to be able to describe the "blank" areas without having to write a full sentence each time. Maybe something like "empty cosmic sky", or "transparent cosmic background"?
Paul
<observable-universe><terminology>
Terminology for empty space at the edge of the observable universe?
Paul
# Terminology for empty space at the edge of the observable universe?

(Asked by: username_0 on 2019-03-06)

Is there a name for a point at the edge of the observable universe at which there is no visible matter, that appears completely dark?

I'm thinking something like "empty space", but that could equally apply to an empty void between galaxies, which isn't quite what I'm looking for.

So it would refer to a theoretical line in space from the observer to the edge of the universe which contains no visible matter. Is there a term for this?

**Tags:** observable-universe, terminology
> **Comments:**
> **username_1**: Maybe I don't quite understand your question, but why would we need a special name for voids near the edge of the observable universe?  
> **username_0**: I think the reason is indirectly implied from your comment, because "voids near the edge of the observable universe" isn't precisely what I'm looking for either. Cosmic voids can be obscured behind galaxies, so that wouldn't work as a description either. I just want to be able to describe the "blank" areas without having to write a full sentence each time. Maybe something like "empty cosmic sky", or "transparent cosmic background"?  

---

## 4 answers

### Answer by username_2 on 2019-03-06. Score: 6

There is no name for this.

As we look further in space, we also look further back in time. The "edge of the observable universe" is in the early universe. At an early time in the evolution of the universe, the whole of space was so hot and dense that it was opaque. In whichever direction we look, we can't look back further than this. The is called the cosmic microwave background. There is no direction in which we can look in which we don't see the cosmic microwave background. There is no direction that contains no visible matter.

There are directions that contain no galaxies or other more nearby objects, but there is no name for these directions.

There is a name for the region of space (or rather the period of time) that is beyond the furthest galaxies, but closer (or more recent) than the cosmic microwave background. This is the "Dark ages".

> **Comments:**
> **username_1**: @Paul Sure, the CMB has been redshifted to microwaves, but it was originally visible light, and like most of the light in the universe, it was emitted by hot hydrogen & helium. FWIW, it was originally an orange colour, as shown at the end of [this answer](https://physics.stackexchange.com/a/133943/123208).  

---

### Answer by username_3 on 2022-08-03. Score: 3

I think you're asking two (related) questions: First you're asking about the "boundary" of the observable Universe, i.e. the points in space separating the observable Universe from the rest of the Universe. Second, you're asking about a line going from us to this boundary.

#### The particle horizon

The observable Universe is bounded by the [particle horizon](https://en.wikipedia.org/wiki/Particle_horizon), which is defined as the spherical, non-physical shell centered on our position, and with a radius equal to the distance light has had the time to travel since the Big Bang.

#### The radius of the Universe

This distance, which is your second question, is simply "the radius $R$ of the Universe", and may be calculated by integrating the [scale factor](https://en.wikipedia.org/wiki/Scale_factor_(cosmology)) $a(t)$ from a time $t=0$ to $t=[\mathrm{today}]$:
$$
R = \int\_0^{13.8\,\mathrm{Gyr}}\!\!\!\!\!\!\frac{c}{a(t')}dt'.
$$
The scale factor depends on the expansion rate $H\_0$ and the density parameters $\Omega\_{\mathrm{m},\Lambda,\gamma,k}$ of the various components of the Universe. For the observed values of [Planck Collaboration et al. (2020)](https://ui.adsabs.harvard.edu/abs/2020A%26A...641A...1P/abstract), this integral amounts to $46.2$ billion lightyears.

[![uni](https://i.sstatic.net/ExApR.png)](https://i.sstatic.net/ExApR.png)


---

### Answer by username_4 on 2022-08-02. Score: -3

Wouldn't the edge of the universe just be the location where there is no matter or energy beyond. Assuming light has reached the farthest from the big bang point, wouldn't the edge just be at the farthest locations where light has traveled. Beyond the edge would just be the same as the empty space all around us (locations where there is no matter or energy) except that there would be no matter or energy within an agreed upon distance. Can empty space have a distance, yes, but that doesn't mean it is part of the universe.

> **Comments:**
> **username_1**: There's no Big Bang point. Please see https://physics.stackexchange.com/q/136860/123208  
> **username_5**: This does not provide an answer to the question. Once you have sufficient [reputation](https://astronomy.stackexchange.com/help/whats-reputation) you will be able to [comment on any post](https://astronomy.stackexchange.com/help/privileges/comment); instead, [provide answers that don't require clarification from the asker](https://meta.stackexchange.com/questions/214173/why-do-i-need-50-reputation-to-comment-what-can-i-do-instead). - [From Review](/review/late-answers/29804)  

---

### Answer by username_6 on 2019-03-07. Score: -4

The observable universe is merely how far we can see with current telescope technology.

Space itself is endless. Oh, there's no end of people here who will aggressively argue this. But empty space, by definition, cannot have an end. It can only be infinite. Literally infinite.

There could very well be an edge in the sense that there's no more matter, where stars, planets and galaxies etc simply tapers off. After that, it would be endless black space.


---

