Gravity can be repulsive!

Never heard that before! Could you elaborate, just a link will do?

In short: in general relativity, gravity can be made repulsive if the things generating it (matter and energy) violates the “Energy conditions”. These energy conditions are known to be violate-able in various conditions. For example, if you believe in the cosmological constant interpretation of the accelerating expansion of the Universe, then gravity in our Universe is “mostly” repulsive instead of attractive.

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Thanks. Though would I be right in saying, noting the “can” and “if”, that repulsive gravity is currently a theoretical concept?

I repeat: if you believe in mainstream cosmology, the reason the universe expansion is accelerating is because of gravity being repulsive. How “theoretical” this is is up to you. This mainstream interpretation is the belief of >90% of physicists.

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So “dark energy” is repulsive gravity? Is it associated with mass? Is it directly proportional to some high-degree polynomial function of distance?

I thought the meaning of Dark Energy was an open question…is Dark Energy really agreed (90% of @physicists ) to be some variant of gravity? Aren’t other explanations in the mix?

I will answer both these questions together, as they are really the same question. Dark energy in mainstream LambdaCDM cosmology is NOT repulsive gravity (or some variant of gravity). However, as I said,

In LambdaCDM (mainstream) cosmology, dark energy is thought to be an energy called Lambda, that violates the “Energy conditions”, thus generating repulsive gravity.

The analogy is: Earth is not gravity, but Earth generates attractive gravity as to Dark energy is not gravity, but dark energy generates repulsive gravity.

Edit: now to the other questions:

Mass is always related to energy by E=mc^2.

This question is not well defined enough for me to answer. What exactly are you asking is directly proportional to some high-degree polynomial function of distance? Distance from where to where? You have to define your coordinates. Why high-degree polynomial function of distance?

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I’d appreciate a comment on my threads regarding how dark matter fluid appears to have negative mass / repulsive gravity

How is what you stated here connected to specific theories of antigravity, e.g. this theory by M. Villata?

It seems that Villata’s proposal is a specific (but unproven) theory where antimatter and matter generate antigravity, and its viability is dependent upon experimental tests of CPT (such as seeing how antimatter reacts to gravity). But what you said seems to be coming out of a more general framework, namely that dark energy must generate antigravity of some kind. Can you clarify this? Does your statement (which you claim is mainstream) necessarily posit antigravity between matter and antimatter?

I am a layman. My question was whether repulsive gravity is solely theoretical or is it an observational fact. I have done a little googling and I didn’t find anything to suggest RG is a real phenomenon. Hence my question. If 90% physicists think RG is real, they surely have evidence that supports their conclusion. Just wondering what that evidence is.

Are you sure? You won’t like what I have to say.

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No. Dark energy in the form of Lambda violates the strong energy conditions, and generates repulsive gravity that causes the accelerating expansion of the Universe – a repulsion.

Evidence is LambdaCDM: the Universe is being “stretched” by the effects of gravity generated by dark energy. If all gravity does is attractive, the Universe will collapse back into a point at some point in the future – a scenario that has been largely ruled out (this re-collapse scenario is the mainstream thought ~70 years ago, before the evidence for an accelerating expansion has been found).

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How does the strong energy condition apply to actual observations? What I mean is that in the vast majority of experimental tests we see that gravity is attractive, not repulsive. Yet we also know that dark energy exists. Thus, in what regimes or conditions does gravity change from being attractive to repulsive? (i.e. can this be parameterized using some length scale? Is this something that can only be observed in the universe as a whole, but not in individual things? I’m just trying to reconcile lab tests of gravity and what you said which seems to flow out of cosmological observations.)

The strength of repulsive gravity.

As with normal gravity, the distance from one mass to another.

It can’t be the square of distance, or it would just uniformly lesses the apparent gravitational constant. It must be a higher power of distance, because it predominates at higher distances. And it must apparently grow stronger as distance increases.

It’s not about the length scales but about the objects that generates the gravitational field. This effect only happens for objects that violates the energy conditions. The only objects we know of that violates energy conditions are exotic things like the Lambda (dark energy of LambdaCDM). Alas, no lab on Earth has them on stock to do experiments on!

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Write me the equations. It seems to me that you’re thinking as if gravity is

F = \frac{GMm}{r^2} \; ,

which is true only for Newtonian gravity, and only for point masses, but I don’t want to presume.

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What differentiates dark energy from ordinary matter such that the former violates the energy conditions but not the latter?

But how do we know that dark energy is the “effects of gravity”? That seems to be the sticking point.

When you figure it out, please send the answer to PRL. The only answer we have at the moment is just that the observations + assumptions from LambdaCDM demand it to be so.

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Dark energy is not the effects of gravity, as I mentioned,

The expansion of spacetime is the effect of gravity, as gravity under general relativity is just spacetime.

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