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?


2 Answers 2


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, 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 (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 is what we observe of this curvature of spacetime. Gravitational waves, 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, 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.

  • $\begingroup$ 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? $\endgroup$
    – BenjaminF
    Commented Dec 19, 2018 at 15:33
  • $\begingroup$ 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? $\endgroup$
    – Joshua
    Commented Dec 19, 2018 at 16:33
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    $\begingroup$ @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. 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. $\endgroup$ Commented Dec 19, 2018 at 22:00
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    $\begingroup$ @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! ;-) $\endgroup$ Commented Dec 19, 2018 at 23:29
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    $\begingroup$ 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. $\endgroup$ Commented Dec 20, 2018 at 3:02

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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