<p>Joshua, your question combines a number of complex but related astrophysics issues, so let me try to untangle them as best I can in layperson's terms, with links where you can find more detailed information.</p>

<p>What you're describing is known as the <a href="https://en.wikipedia.org/wiki/Cosmological_horizon" rel="nofollow noreferrer">cosmological horizon</a>, which defines the limit beyond which a particle's light can never reach us because of the expansion of the Universe. While gravitational waves ("GWs") are unrelated to electromagnetic waves (what we call <em>light</em>), it's believed GWs are propagated at the <a href="https://en.wikipedia.org/wiki/Speed_of_light" rel="nofollow noreferrer">speed of light</a> (the universal limit for how fast information can travel), so the cosmological horizon would apply to gravitational waves as well.</p>

<p>However, <em>gravitational waves</em> are different from <em>gravity</em>. </p>

<p>To put it overly simply, the general theory of relativity says that mass distorts both space and time, and <a href="https://en.wikipedia.org/wiki/Gravity" rel="nofollow noreferrer">gravity</a> is what we observe of this curvature of spacetime. <a href="https://en.wikipedia.org/wiki/Gravitational_wave#Properties_and_behaviour" rel="nofollow noreferrer">Gravitational waves</a>, on the other hand, are "ripples" in that spacetime curvature caused by non-symmetric <em>acceleration</em> of mass. </p>

<p>GWs are indeed "all around us", but they are extraordinarily tiny - Albert Einstein himself doubted they could ever be detected. Our most sensitive equipment, <a href="https://en.wikipedia.org/wiki/LIGO" rel="nofollow noreferrer">LIGO</a>, looked for the very largest GWs for eight years (2002-2010) and found nothing. It wasn't until a significant enhancement came on line just three years ago (Sept 2015) that the first GW was detected - from the collision of two black holes with a combined mass equivalent to 65 Suns! - and since then we've only detected 11. GWs are <em>really</em> hard to detect!</p>

<p>Since gravity is an <em>intrinsic geometric property</em> of spacetime, it exists even beyond the cosmological horizon: it's inseparable from the Universe as a whole. All parts of the Universe contribute to the curvature of the Universe, no matter how far away one point is from another. There's no "threshold" for gravity.</p>

CC BY-SA 4.0
2018-12-19T15:33:36.703Z
Can you elaborate on that last part? How do all parts of the Universe contribute regardless their distances? If some event occurred beyond the cosmological horizon inducing gravitational waves, would it not be safe to say that those waves will have no effect on our region of the universe as they travel too slowly to surpass the horizon?
BenjaminF
CC BY-SA 4.0
2018-12-19T16:33:49.083Z
Thanks for the information, I think i can fine tune my question with the information provided. Does mass beyond the cosmological horizon contribute to the warping of the space around us? If so then wouldn't gravity transcend the speed of light or somehow be a 5th dimension?
Joshua
CC BY-SA 4.0
2018-12-19T22:00:02.433Z
@BraydenFox *Information* originating from beyond the cosmological horizon can never reach us, whether it's propagated as GW or light. But GW is a "ripple" in the fabric of spacetime, whereas gravity is involved in the [*shape* of the Universe](https://en.wikipedia.org/wiki/Shape_of_the_universe). Just because the information can never reach us doesn't mean the shape doesn't exist, nor that we can't develop a mathematical model of that shape.
Chappo Hasn't Forgotten
CC BY-SA 4.0
2018-12-19T22:49:07.543Z
@Chappo I may have misinterpreted the question and thus your answer to the question.
BenjaminF
CC BY-SA 4.0
2018-12-19T23:29:56.370Z
@BraydenFox :-). The question was difficult to begin with, given it confuses GW and gravity. Add to that, gravity being hard to explain in non-expert non-Newtonian terms, toss in a particle horizon, and it deserves a four-page article in *Nature* rather than a brief answer here! ;-)
Chappo Hasn't Forgotten
CC BY-SA 4.0
2018-12-20T02:15:40.360Z
But if gravity is information and information is limited by the speed of light then all the matter beyond the cosmological threshold can't warp our spacetime without us knowing. Otherwise wouldn't gravity transcend time? Doesn't entanglement transcend time? Stuff that exists beyond the cosmological threshold doesn't exist... yet?
Joshua
CC BY-SA 4.0
2018-12-20T03:02:38.613Z
No, gravity is *not* information, it's **geometry**. We can only *model* the curvature of spacetime beyond the observable universe, since logically we can't test by observation what we can't observe. Maybe the observable universe is part of a weird bubble of structure in a larger anisotropic universe - how could we tell? All we have is models based on mathematics, principles (eg isotropic and homogenous) and observation.
Chappo Hasn't Forgotten
CC BY-SA 4.0
2018-12-20T18:36:42.967Z
Gravity is both a fundamental force and geometry? I have to get my head around that. Do quantum fluctuations cause empty space to expand? If i have a volume of nothing and a particle poofs into existence then my volume of nothing increases... when the particle poofs out of existence the volume of nothing can't decrease... so the universe expands?
Joshua
CC BY-SA 4.0
2018-12-20T18:48:44.967Z
Perhaps a better way to express this idea is to say that a particle leaves new empty space behind when it vanishes?
Joshua
CC BY-SA 4.0
2018-12-20T19:14:46.090Z
Do quantum fluctuations occur at a given rate per volume of nothing? If so then couldn't we multiply the expected number of fluctuations by volume of particle to get a ballpark estimate of expansion rate to compare against what we think is the observed expansion rate of the universe as a sanity check?
Joshua
CC BY-SA 4.0
2019-02-13T14:48:56.060Z
@Chappo .. This is only partially related to the topic, but I am confused as to how gravity could slow down inflation during the period of super-luminal expansion.
Jack R. Woods
CC BY-SA 4.0
2019-02-13T23:39:17.550Z
@JackR.Woods "super-luminal expansion" describes the incredibly brief "pop" of inflation, but it also describes the current universe at distances beyond the Hubble radius. Do you mean the former only? I'm not aware that *gravity* "slowed down" cosmic inflation – my understanding is that inflation ended when the inflaton field's energy density hit the "valley" and the field's potential energy converted to the matter/energy that fills the universe now. But I'm no physicist; to explain this in more depth is way beyond my tools.
Chappo Hasn't Forgotten
CC BY-SA 4.0
2018-12-19T04:00:15.427Z
2018-12-20T21:00:06.983Z
2018-12-20T21:00:06.983Z
Chappo Hasn't Forgotten
<p>In extremely layman terms and I can be wrong.</p>

<p>Actually such a far away matter has no direct newtonian gravitational influence on us and as such, its departure (crossing the horizon) has not effect on us.
Moreover, on the event "crossing the horizon" nothing  change and oscillates the texture of spacetime and so we should not expect GW.</p>

<p>After that moment on, it will disappear (to us) also for, eventually, GW created by it (in case it undergos event such a merger etc.). Those GW won't be able to reach us anymore. </p>

<p>Perhaps my answer is a duplicate of the one above. I just offer this very simple picture - I would not able to do more, by the way. </p>

CC BY-SA 4.0
2018-12-20T09:51:54.107Z
2018-12-21T07:28:17.260Z
2018-12-21T07:28:17.260Z
Alchimista
<p>If space is undergoing accelerated expansion then regions of space are becoming inaccessible to us even at light speed. If gravity is bounded by light speed then inaccessible regions of space no longer contribute gravitational influence on us. If true wouldn't gravity waves... tiny variations in gravity given sensitive enough equipment... be all around us as matter crosses the threshold and becomes inaccessible to us? Does this violate conservation?</p>

<gravitational-waves>
Expansion and Gravity
Joshua
# Expansion and Gravity

(Asked by: username_0 on 2018-12-18)

If space is undergoing accelerated expansion then regions of space are becoming inaccessible to us even at light speed. If gravity is bounded by light speed then inaccessible regions of space no longer contribute gravitational influence on us. If true wouldn't gravity waves... tiny variations in gravity given sensitive enough equipment... be all around us as matter crosses the threshold and becomes inaccessible to us? Does this violate conservation?

**Tags:** gravitational-waves

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## 2 answers

### Answer by username_1 on 2018-12-19. Score: 5

Joshua, your question combines a number of complex but related astrophysics issues, so let me try to untangle them as best I can in layperson's terms, with links where you can find more detailed information.

What you're describing is known as the [cosmological horizon](https://en.wikipedia.org/wiki/Cosmological_horizon), which defines the limit beyond which a particle's light can never reach us because of the expansion of the Universe. While gravitational waves ("GWs") are unrelated to electromagnetic waves (what we call *light*), it's believed GWs are propagated at the [speed of light](https://en.wikipedia.org/wiki/Speed_of_light) (the universal limit for how fast information can travel), so the cosmological horizon would apply to gravitational waves as well.

However, *gravitational waves* are different from *gravity*.

To put it overly simply, the general theory of relativity says that mass distorts both space and time, and [gravity](https://en.wikipedia.org/wiki/Gravity) is what we observe of this curvature of spacetime. [Gravitational waves](https://en.wikipedia.org/wiki/Gravitational_wave#Properties_and_behaviour), on the other hand, are "ripples" in that spacetime curvature caused by non-symmetric *acceleration* of mass.

GWs are indeed "all around us", but they are extraordinarily tiny - Albert Einstein himself doubted they could ever be detected. Our most sensitive equipment, [LIGO](https://en.wikipedia.org/wiki/LIGO), looked for the very largest GWs for eight years (2002-2010) and found nothing. It wasn't until a significant enhancement came on line just three years ago (Sept 2015) that the first GW was detected - from the collision of two black holes with a combined mass equivalent to 65 Suns! - and since then we've only detected 11. GWs are *really* hard to detect!

Since gravity is an *intrinsic geometric property* of spacetime, it exists even beyond the cosmological horizon: it's inseparable from the Universe as a whole. All parts of the Universe contribute to the curvature of the Universe, no matter how far away one point is from another. There's no "threshold" for gravity.

> **Comments:**
> **username_2**: Can you elaborate on that last part? How do all parts of the Universe contribute regardless their distances? If some event occurred beyond the cosmological horizon inducing gravitational waves, would it not be safe to say that those waves will have no effect on our region of the universe as they travel too slowly to surpass the horizon?  
> **username_0**: Thanks for the information, I think i can fine tune my question with the information provided. Does mass beyond the cosmological horizon contribute to the warping of the space around us? If so then wouldn't gravity transcend the speed of light or somehow be a 5th dimension?  
> **username_1**: @BraydenFox *Information* originating from beyond the cosmological horizon can never reach us, whether it's propagated as GW or light. But GW is a "ripple" in the fabric of spacetime, whereas gravity is involved in the [*shape* of the Universe](https://en.wikipedia.org/wiki/Shape_of_the_universe). Just because the information can never reach us doesn't mean the shape doesn't exist, nor that we can't develop a mathematical model of that shape.  
> **username_2**: @Chappo I may have misinterpreted the question and thus your answer to the question.  
> **username_1**: @BraydenFox :-). The question was difficult to begin with, given it confuses GW and gravity. Add to that, gravity being hard to explain in non-expert non-Newtonian terms, toss in a particle horizon, and it deserves a four-page article in *Nature* rather than a brief answer here! ;-)  
> **username_0**: But if gravity is information and information is limited by the speed of light then all the matter beyond the cosmological threshold can't warp our spacetime without us knowing. Otherwise wouldn't gravity transcend time? Doesn't entanglement transcend time? Stuff that exists beyond the cosmological threshold doesn't exist... yet?  
> **username_1**: No, gravity is *not* information, it's **geometry**. We can only *model* the curvature of spacetime beyond the observable universe, since logically we can't test by observation what we can't observe. Maybe the observable universe is part of a weird bubble of structure in a larger anisotropic universe - how could we tell? All we have is models based on mathematics, principles (eg isotropic and homogenous) and observation.  
> **username_0**: Gravity is both a fundamental force and geometry? I have to get my head around that. Do quantum fluctuations cause empty space to expand? If i have a volume of nothing and a particle poofs into existence then my volume of nothing increases... when the particle poofs out of existence the volume of nothing can't decrease... so the universe expands?  
> **username_0**: Perhaps a better way to express this idea is to say that a particle leaves new empty space behind when it vanishes?  
> **username_0**: Do quantum fluctuations occur at a given rate per volume of nothing? If so then couldn't we multiply the expected number of fluctuations by volume of particle to get a ballpark estimate of expansion rate to compare against what we think is the observed expansion rate of the universe as a sanity check?  
> **username_3**: @Chappo .. This is only partially related to the topic, but I am confused as to how gravity could slow down inflation during the period of super-luminal expansion.  
> **username_1**: @JackR.Woods "super-luminal expansion" describes the incredibly brief "pop" of inflation, but it also describes the current universe at distances beyond the Hubble radius. Do you mean the former only? I'm not aware that *gravity* "slowed down" cosmic inflation – my understanding is that inflation ended when the inflaton field's energy density hit the "valley" and the field's potential energy converted to the matter/energy that fills the universe now. But I'm no physicist; to explain this in more depth is way beyond my tools.  

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### Answer by username_4 on 2018-12-20. Score: 2

In extremely layman terms and I can be wrong.

Actually such a far away matter has no direct newtonian gravitational influence on us and as such, its departure (crossing the horizon) has not effect on us.
Moreover, on the event "crossing the horizon" nothing change and oscillates the texture of spacetime and so we should not expect GW.

After that moment on, it will disappear (to us) also for, eventually, GW created by it (in case it undergos event such a merger etc.). Those GW won't be able to reach us anymore.

Perhaps my answer is a duplicate of the one above. I just offer this very simple picture - I would not able to do more, by the way.


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