TL,DR: can an object ‘beyond’ the visible edge of the universe affect us gravitationally?
Inspired by a short story from a 1940’s Weird-Tales type pulp magazine. Short story shorter: bleeding-edge astronomical measurements show that all the visible galaxies are orbiting a point in space so far away that we can not yet see it - UNTIL WE DO, when the light from a supersupersupermassive star* at the gravitational center of the observable Universe finally reaches Earth and sterilizes it in fire. So in that fictional universe, we could feel the gravity of the Uberstar even though its light had not yet reached us. It sparked a question as to whether something similar could occur in our universe.
Could there be some primordial black hole hiding out beyond the edge of the observable universe, a billion light years beyond the CMBRs ‘emitting surface’, such that a) we could not yet have felt its gravitational effects and b) there would also be no impact on the CMBR that we could see - UNTIL WE DO? At which point we would simultaneously feel the gravity of the black hole for the first time, however attenuated?
Or does cosmic inflation somehow mean that the gravitational effect of all mass in the universe, even from over the visible horizon, was baked into all of spacetime right from the Big Bang, so there can’t be any such surprises lurking in the dark?
*Big. Really big. You just won’t believe how vastly hugely mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist, but that’s just peanuts to this star.
I would say yes and if we haven’t felt it we never will due to the expansion of the universe.
My pull-out-of-the-butt theory of a non-astrophysicist is that the space beyond our vision is beyond huge. I think that the so-called ‘matter-antimatter almost but not quite equal annihilation’ doesn’t exist but just in our part of the universe random fluctuations caused a very slight matter prevalence. If you were able to go beyond our horizon you would find, in some directions, that over-representation getting less until it was about equal and if you kept going it might happen again or antimatter slightly over present…and so on and so on. Heck, physical laws are likely to change slowly as you go along as well.
We will likely never know as it is beyond our horizon and I do not think FTL travel is likely to ever be found.
Isn’t the theory that all matter was present at the big bang and then moved away from eberything else? If so then we have been experiencing all matters gravity since the beginning. But IANAAP either, so I don’t really know.
The answer to your question is so complicated that it is very simple: yes (and no).
In more depth: although in theory the gravitational field of a discrete mass can be calculated and measured by itself, nothing in the real universe actually exists on its own. The gravitational field that we experience on the surface of the Earth or in Earth orbit is dominated by the Earth with some minor but readily observable contributions by the Moon and Sun, e.g. tidal effects. Although there are many other masses larger than Luna in the Solar System (e.g. all of the planets) they are far enough away that even the largest of them have negligible effects on the Earth down to any degree of direct measurement. (Jupiter does have an impact upon the orbital dynamics of the Earth over the timeframe of millennia but not in a way that we could measure gravitationally; we only know it by inference of orbital cycles, and even that is still hotly debated.)
However, the underlying plenum of spacetime which conveys the gravitational “force” (described in General Relativity by the geometric curvature of the pseudo-Riemannian manifold, or in layman’s terms, the ‘topo map’ of spacetime) is a superposition of the influence all masses in the universe because even though a mass may currently be very far from your locus it still has an influence across all of spacetime, at least assuming that it was once in causal proximity (which the ‘Big Bang’ theory implicitly assumes). Now, masses that are currently beyond the cosmological horizon from your point of view can no longer causally impact you—that is, gravitational waves emitted by some change in mass-energy distribution cannot reach you in finite time because they can only travel at c—but the residual effects upon the universal gravitational field still remain for all time, just as the mass within the event horizon of a black hole still has an effect even though you can no longer observe it or otherwise receive information about it.
So the object beyond the ‘visible edge’ of the universe still has gravitational effects that permeate the universe, and if you could somehow distinguish its influence from that of all other masses you could trace back the path that it took through spacetime (although in practice that is impossible). However, any perturbation that mass might experience out beyond the cosmological horizon could never be observed because the expansion (‘stretching‘) of spacetime means that the path length would be infinite.
Practically speaking, the effect of large masses beyond your immediate sphere of influence can be very significant—causing the Solar System to rotate about the Milky Way, and the the peculiar motion of the Local Group of galaxies including our own toward the Great Attractor toward the Laniakea Supercluster, and so forth—this effect is distributed so evenly that it is impossible to measure directly and can only be inferred by looking at differential redshift of other far-away galaxies and superclusters. The expansion of spacetime itself—attributed without any real evidence or observation to the so-called theory of “dark energy”—may entirely be due to the influence of mass beyond the cosmological horizon or indeed outside of the ”brane” of our universe entirely in a way that we can never directly observe other than its gravitational influence.
I hope that made everything as clear as the plot of a Wachowski movie.
With the accelerating expansion of the universe, the observable universe is sort of shrinking. But objects never appear to leave the observable universe, they only appear to slow down (infinitely) due to redshifting.
There should be a limit beyond which an object travelling straight at us at the speed of light (i.e. a photon) will never reach us. If some event is hypothesized more than X light-years away, not only can we not observe it now, but we can never observe it ever. X is our cosmological event horizon, and it shrinks over time.
I am pretty sure it has been shown that gravity propagates at the speed of light. If our sun poofed out of existence it would take eight minutes(ish) for earth to go flying off into the universe.
I think it is well accepted that the universe is bigger than the visible universe. Which means, there are parts of the universe beyond our horizon and neither light nor gravity has had time to reach us yet.
I listen to a lot of Fermi Paradox/Space podcasts and I’ve heard it postulated several times that if FTL travel was possible, we would know about it because we would have seen other civilizations visiting us from across the universe.
That may not obtain. It depends on the nature of the FTL. Just off the top of my head, I can think of three forms of FTL used in SF that do not allow access to everywhere in the universe within a reasonable amount of time. “Reasonable” here meaning you don’t have to spend a couple years or more in STL travel. I’m sure there are others that haven’t occured to me.
I don’t think we will develop FTL travel, but not for that reason. It violates our current understanding of physics.
I’m sure the sci-fi stories had limitations for story plot points. But were we to find a way to go faster than light, why assume there would be an upper limit?
Faster-than-light travel ‘violates’ General Relativity (GR) in the sense of mass moving against the static background of spacetime faster than c but there are certainly ways to formulate the Einstein field equations (EFE) to realize the movement of a mass or information between two points separated by an interval greater than that light could travel. Whether these mathematical formations could be practically realized is another question (as all require enormous energy and/or regions of negative energy density) but the theory of GR does not make it impossible. Of course, we know that entangled quantum particles can influence each other over any interval with apparent simultaneity, so GR is not a complete theory anyway.
The more general argument against faster-than-light travel as a physically realizable phenomenon because it would make an utter mess of causality, and as far as we can tell the universe appears to be causal at the macroscopic level.
Which most traveling spacecraft would need to do. Never say never, of course, but the idea of folding space to hop across vast space distances seems to be forever out of the realm of human achievement. It would seem that the energy requirement alone would be prohibitive.
I think it is important to note that nothing can move faster than the speed of light. By which I mean a rocket/hypothetical pushing device pushing something through space cannot ever exceed light speed. We know that.
So, FTL travel (if at all possible) will need to warp space or move through other dimensions or something else.
It is certainly speculative in the truest sense of the term insofar as it would require physics beyond what we currently understand, but to qualify it as “forever out of the realm of human achievement” is a very limiting perspective. To someone in 1820, the idea of putting objects into orbit that could facilitate global communications or sending a “Mechanical Turk”-like machine to orbit another planet and send by high resolution color images would have been absolutely prohibitive, not only because of the limits of technology but the limitations of was then termed “natural philosophy”, and if these things are not quite routine today they are well within the means of a wealthy nation with a strong technological base. The “energy requirements” two centuries ago would have been limited to animal power and very crude steam engine; it took Michael Faraday and James Clerk Maxwell to understand what we know today as electromagnetism to even have the basic physics to convert mechanical work into concentrated electrical energy which is used in nearly every technology today, and yet this is now so commonplace the average person on the street wouldn’t even recognize these names as historical figures of any significance.
This is not to say that superluminal “faster than light” travel will ever be possible, and if it is the conveyance for it is unlikely to look like anything imagined by science fiction shows and movies today, but we should not be so arrogant to assume that everything that humanity is capable of can be predicted by the frankly very little we actually understand about the nature of the universe.
OR–
Maybe several such civilizations have visited us, but but the people who have seen their spaceships routinely get dismissed as cranks, drunks, and crazies.
