Antigravity.

[Copernicus]

Could antigravity cause the universe to expand or would it be the shell theorem all over again.

[grant hutchison]

Well, "antigravity" is either a science fiction term, or a catch-all term for a whole variety of expansive forces that oppose gravity. Anything that opposes gravity helps the Universe expand, or hinders a tendency to collapse.

The shell theorem applies only to inverse-square forces, and the interior of uniform spherical shells. So inverse-square antigravity inside a spherical shell would cancel itself out - there would be no net force pushing you away from the inside surface of the shell. But that doesn't seem particularly relevant.

Grant Hutchison

[Copernicus]

Well, "antigravity" is either a science fiction term, or a catch-all term for a whole variety of expansive forces that oppose gravity. Anything that opposes gravity helps the Universe expand, or hinders a tendency to collapse.

The shell theorem applies only to inverse-square forces, and the interior of uniform spherical shells. So inverse-square antigravity inside a spherical shell would cancel itself out - there would be no net force pushing you away from the inside surface of the shell. But that doesn't seem particularly relevant.

Grant Hutchison

Thanks.

[Copernicus]

I'm looking for a force that could push all mass from the center of a hypothetical universe. Obviously it would have to look like every point was expanding away from every other point. As was discussed earlier in a separate thread, at the following location, https://forum.cosmoquest.org/showthr...-of-the-proton . Currently, if we use gravity or charge there is no way for something to push everything away from the center. If there was such a thing as a type of gravity, how would that antigravity have to work, if it pushed all mass from the center of a universe, it would have to look like everything was accelerating away from everything. What would the antigravitational field lines look like. Would anti gravity be proportional to distance or inversely proportional distance etc. I will work on this as well.

[Delvo]

First, you'd need a universe with a center. We don't have one of those sitting around, so there's not much basis for saying what would happen in one.

Second, you'd need to decide whether you're looking for "everything moving away from the center" or "everything moving away from everything else", because those are different processes which would result in different movements.

[DaveC426913]

You don't necessarily need to move everything away from the centre, or even move everything away from each other.
If space itself were expanding - literally the space between everything was growing - then distances would simply get larger.

[Ken G]

Antigravity is often the way the effects of dark energy are described. It's not just sources of mass that produce gravity, pressure produces it also-- it's just usually very weak because pressure depends on the internal kinetic energy and the mass-energy usually comes much more from rest mass than from kinetic energy (we live in a highly nonrelativistic universe for reasons that are not well understood and might be anthropic in nature). But a "cosmological constant" acts like a negative pressure that produces a very significant gravity-- but since the pressure is negative, it's antigravity. The negative pressure is spread evenly through all space, so the antigravity is also, and the expansion of space accelerates uniformly as a result. It is inverse-square gravity, and the shell theorem does apply, you are simply free to pick any point you like as the "center" and go from there-- it works just fine (remarkably, and that it works is related to the flatness of space on large scales).

[Copernicus]

Antigravity is often the way the effects of dark energy are described. It's not just sources of mass that produce gravity, pressure produces it also-- it's just usually very weak because pressure depends on the internal kinetic energy and the mass-energy usually comes much more from rest mass than from kinetic energy (we live in a highly nonrelativistic universe for reasons that are not well understood and might be anthropic in nature). But a "cosmological constant" acts like a negative pressure that produces a very significant gravity-- but since the pressure is negative, it's antigravity. The negative pressure is spread evenly through all space, so the antigravity is also, and the expansion of space accelerates uniformly as a result. It is inverse-square gravity, and the shell theorem does apply, you are simply free to pick any point you like as the "center" and go from there-- it works just fine (remarkably, and that it works is related to the flatness of space on large scales).

Would the redshift of light be the opposite of gravity. So would

[Reality Check]

Would the redshift of light be the opposite of gravity.

They are 2 different things. Redshift is a measured shift in spectra. Gravity is a force. Redshift is caused by the motion of the source (SR), gravity (GR) and an expanding universe (GR again). Your equation looks like an attempt at gravitational redshift.

[Copernicus]

They are 2 different things. Redshift is a measured shift in spectra. Gravity is a force. Redshift is caused by the motion of the source (SR), gravity (GR) and an expanding universe (GR again). Your equation looks like an attempt at gravitational redshift.

Argh! Would the equations for gravitational and antigravitational redshift be opposite.

[Ken G]

Would the redshift of light be the opposite of gravity. So would

It does seem like it might do that, but that's not how it works out. Gravity in cosmology simplifies such that the 1+z factor ends up being exactly the factor by which "space itself" has expanded since the light was emitted. The antigravity effect of dark energy is producing acceleration of the expansion, so it makes 1+z even larger than it would have been without dark energy (antigravity). But this does not mean regular gravity itself produces blueshift, regular gravity decelerates the expansion so produces less redshift than you would have had without the regular gravity.

[Copernicus]

Would the redshift of light be the opposite of gravity. So would

I was open to the idea that gravity may be a local effect and anti gravity would be a non local effect.

[Reality Check]

Argh! Would the equations for gravitational and antigravitational redshift be opposite.

Gravitational redshift is a textbook derivation from GR. The speculative concept of negative mass suggests that the same textbook derivation can be done by replacing M with -M. So the equations would be opposite in mass sign.

[Copernicus]

Is there anything that precludes gravity working in 50 million light year radius and anything over 50 million light years being antigravity. So only stuff that is less than 50 million light years can be gravitationally bound.

[Grey]

Is there anything that precludes gravity working in 50 million light year radius and anything over 50 million light years being antigravity. So only stuff that is less than 50 million light years can be gravitationally bound.

Well, as I mentioned over here, various nearby superclusters that are components of Laniakea appear to be bound, even though Laniakea itself does not appear to be. The Virgo Supercluster is the largest of these, at about 110 million light years, and since it appears to be gravitationally bound, that would seem to be a problem with a 50 million light year limit.

But in general, the problem would be that when we run simulations of the universe developing including gravity that does not suddenly reverse at a specific distance, they seem to reproduce something like the large scale structure we see today quite well. If you wanted to test out a model with gravity behaving differently, you'd presumably have to run comparable simulations and see how it works out.

From a different perspective, though, those models do already include a dark energy term, which as Ken G notes acts very much like an antigravity effect, that results in the expansion accelerating. There are a number of different models for how dark energy could work (from a simple static "cosmological constant" to models that include parameters that could change over space or time, like "quintessence"). It might make sense for you to look at the dark energy models that cosmologists are already working on, and see if there are any similar to what you have in mind. Here's a paper that provides a brief overview, although it's a few years out of date.

[Copernicus]

Well, as I mentioned over here, various nearby superclusters that are components of Laniakea appear to be bound, even though Laniakea itself does not appear to be. The Virgo Supercluster is the largest of these, at about 110 million light years, and since it appears to be gravitationally bound, that would seem to be a problem with a 50 million light year limit.

But in general, the problem would be that when we run simulations of the universe developing including gravity that does not suddenly reverse at a specific distance, they seem to reproduce something like the large scale structure we see today quite well. If you wanted to test out a model with gravity behaving differently, you'd presumably have to run comparable simulations and see how it works out.

From a different perspective, though, those models do already include a dark energy term, which as Ken G notes acts very much like an antigravity effect, that results in the expansion accelerating. There are a number of different models for how dark energy could work (from a simple static "cosmological constant" to models that include parameters that could change over space or time, like "quintessence"). It might make sense for you to look at the dark energy models that cosmologists are already working on, and see if there are any similar to what you have in mind. Here's a paper that provides a brief overview, although it's a few years out of date.

Thank you grey, I will look at it. I should have said radius of 50 million light years, but right now it is a pretty arbitrary number anyways, perhaps. I liken the concept of the idea to the strong force and weak force, which are pretty limited in scope, but on galactic cluster scales. I was just wondering if there was physics that definitely said no, this is not a workable idea.

[Grey]

Thank you grey, I will look at it. I should have said radius of 50 million light years, but right now it is a pretty arbitrary number anyways, perhaps. I liken the concept of the idea to the strong force and weak force, which are pretty limited in scope, but on galactic cluster scales. I was just wondering if there was physics that definitely said no, this is not a workable idea.

There are certainly a fair number of people who have tried to avoid the need for dark matter by instead introducing a modification to the way gravity works, but so far, none have really held up well. As for dark energy, there's a real sense in which it is itself a modification to the way gravity works; it shows up as a term right in Einstein's field equations. It's the details that matter, though. If you want to propose that some specific modification of gravity works well to explain the universe we see, or that a different version of dark energy works better than some of the current proposals, you have to show that it really does, quantitatively.

[Copernicus]

There are certainly a fair number of people who have tried to avoid the need for dark matter by instead introducing a modification to the way gravity works, but so far, none have really held up well. As for dark energy, there's a real sense in which it is itself a modification to the way gravity works; it shows up as a term right in Einstein's field equations. It's the details that matter, though. If you want to propose that some specific modification of gravity works well to explain the universe we see, or that a different version of dark energy works better than some of the current proposals, you have to show that it really does, quantitatively.

Hi Grey, I have lots of ideas. Some of them are obviously wrong. I appreciate you helping me know whether my idea was obviously wrong or not. I will have to find some way to quantitatively prove the idea.

[Ken G]

I will have to find some way to quantitatively prove the idea.

But what you should realize is the path that you are talking about treading is a well-worn one. Many physicists have attempted essentially the same idea under the heading of "MOND", modifications to Newtonian dynamics. So you should start by looking at what has already failed, and I think by the time you do that, you will see there is no "low-hanging" fruit on this one.

[Copernicus]

But what you should realize is the path that you are talking about treading is a well-worn one. Many physicists have attempted essentially the same idea under the heading of "MOND", modifications to Newtonian dynamics. So you should start by looking at what has already failed, and I think by the time you do that, you will see there is no "low-hanging" fruit on this one.

I don't disagree with dark matter. I also don't disagree with dark energy, although I have reservations about accelerated expansion. I have to wonder if there are other arguments for dark energy beside accelerated expansion.

[Shaula]

I have to wonder if there are other arguments for dark energy beside accelerated expansion.

Curvature and large scale structure evolution both imply that there is something out there other than matter and dark matter contributing to the energy density of the universe. The dark energy hypothesis is that the same thing that is responsible for the acceleration of expansion accounts for these factors too. It's all indirect evidence so far but the fact that these three independent factors can be explained by one consistent mechanism strengthens, but doesn't prove, the case.