.05 04.06
Is energy anti-gravity?
The expansion of the universe seems to be accelerating. A leading theory is that the universe is filled with a “dark” energy that has the effect of anti-gravity.
Does all energy share this same property?
Have fun, but please be kind.
Peace
rwj
And therefore…
No. Energy has mass (E=MC2*) and since it has mass gravity affects it. In fact light bends in gravitational fields. In fact, IIRC, light bending in the suns gravitational fields was one of the first experiments that proved that E=MC2 was correct.
The dark energy/dark matter issue, as far as I can tell, coud be a non-issue. The observed facts don’t match the theory so people are proposing dark energy and dark matter to get the theory to match what they see. There could be dark energy, or there could be a whole bunch of matter that is not yet accounted for. At least that is my understanding. IANAPhysicist and haven’t read up on the subject in a while so things might have changed or I could be wrong from the start. At the same time the last time I spoke with my friend Dick, who has a PHD in physics, he seemed to think that answer to the problem is going to be pretty mundane.
Slee
That sounds about right to me. The “dark” of dark energy is a metaphor, it just means we don’t know what the cause of the expansion is.
Hmm. For some reason this didn’t register when I posted it several hours ago. In any case, here it is again:
No; the field that is hypothesized to make up “dark energy” has the extremely unusual property of having a negative pressure. This is necessary for the repulsive effect, as discussed in the Usenet Physics FAQ.
I note that that page was written in 2001. It mentions a subsequent experiment, MAP (later renamed WMAP) which was expected to confirm or disprove the nonzero cosmological constant hypothesis. It found evidence confirming it.
That’s one way of looking at things. Yet it’s difficult to see how any amount of ordinary matter could cause the expansion of the universe to accelerate. We do have evidence that it’s expanding faster today than it did at one time in the past, and we also have evidence that all the matter we know of—including all the dark matter whose presence is inferred from its gravitational effect—is insufficient by about a factor of three to explain the observed (lack of) curvature of the universe. Admittedly the introduction of dark energy has some messy elements. There’s no shortage of hypotheses for what such energy might actually be, but no one has discovered a good way to test them.
As for the dark matter itself, there are multiple independent lines of evidence supporting its existence and approximate distribution. That’s on a much sounder footing. It’s true we can’t directly see it, but that problem didn’t stop 19th century astronomers from correctly inferring the existence of Neptune based on its gravitational effect on Uranus.
I guess I wasn’t clear on dark matter. It’s well known that something else is out there. The question is what that something else is. IIRC some people are putting forward the idea(1) that the dark matter is some new particle that we don’t know about yet. I seem to remember that someone put forth the idea(2) that if that amount of matter in ‘empty’ space was a factor of two or three larger it would account for the missing matter. It seems to me that option 2 is more likely than option 1.
But I could be wrong. I haven’t been paying as much attention to physics as I would like. That whole work thing keeps getting in the way of my interests. 
Slee
If “dark energy” exhibits a repulsive effect, wouldn’t stars tend to exhibit a sort of halo or sphere of captured particles at the point where the repulsion and the star’s gravitational attraction cancel each other out?
Dark matter and dark energy are not related, in fact they are only called dark because they cannot be seen.
Dark energy is a misnomer, because as stated earlier there is an equivalence between mass and energy, which is not the case with dark energy. It is not the energy equivalent of dark matter, its closest analogy in theory is the cosmological constant in Einsteins general relativity equations. It is also sometimes called quintessence.
We did a thread recently which covered dark matter and energy, hope this helps.
From my own personal, non-PHD in Physics perspective, it seems like history shows us it’s probably the opposite of mundane. For example, entanglement, uncertainty, etc., our universe keeps getting weirder.
I was taught in my physic class that a photon had NO mass. I am not proposing that e=mc^2 is incorrect. I am proposing that energy warps spacetime in the opposite direction as mass.
rwj
My question is what makes them think it is an unusual property? Maybe no one ever considered that anti-gravity was right there under our noses the whole time.
rwj
You shouldn’t think of dark energy as made of particles, the way dark matter probably is. It’s more of a property of spacetime itself. If you describe dark energy using Einstein’s cosmological constant, then it is spread out perfectly uniformly throughout the universe: no clumping at all.
Energy can have a positive or negative density. The former curves paths toward it, the latter away from it. It’s not so much an issue of “anti-gravity” as the kind of “pressure” positive and negative energy densities exert on space-time, and which way space-time is warped as a result of that pressure.
The proposed cosmological constant, oddly, has negative pressure but positive energy.
This one always gets me confused and even the explanation below doesn’t clear it up entirely for me but perhaps it’ll make more sense to you. That said I do not see why energy would warp spacetime in the opposite direction as still at the root of it all is the equivalence of E=mc[sup]2[/sup]. Since you can bounce back and forth between the two why would energy bend space one way and mass the other?
The anti-mass of a single photon would exceedingly small. For all intents and purposes, it would behave as if it had no mass. It would take a black hole to show this effect.
I believe the first cite talks about photons orbiting black holes.
rwj
It has no REST mass. There’s a difference.
Maybe that effect is in the c[sup]2[/sup].
Maybe mass-energy behaves a mole. First it’s on one side of the hole, and then it is whacked to the other. 
rw
Consider how energy having “anti-gravity” properties fits with our current understanding of the cosmological timeline.
t0 - t1: energy predominates; inflationary expansion. Energy condenses into mass.
t1 - t2: mass and energy reach equilibrium; inflation ends.
t2 -t3: mass predominates; the time of least energy and greatest mass. Expansion slows to minimum. Energy evaporation out of mass equals and then exceeds energy condensation into mass.
t3 -t4: energy again exceeds mass: expansion again accelerates.
t4+: energy predominates: inflation returns.
Just wondering.
rwj
Thanks for the response (you too rwjefferson). I apologize for phrasing my question badly. When I referred to “particles,” I was talking about plain ordinary space debris, interstellar dust and the like. It seems to me like such material ought to accumulate around large gravitational sources such as stars at a distance where the repulsion of “dark energy” and the attraction of gravity cancel each other. Too close to the star, and its gravity would tend to pull such debris in; too far away, and the dark energy force would tend to shove it further out and away from the star; but there ought to be a happy medium where these two forces cancel each other out, and particles of matter would tend to gather at that radius. Over time I would think that this would create a fairly conspicuous “halo” or spherical shell of dust around the star, and a similar effect ought to apply to nebula formation. If the repulsive property of dark energy exists, shouldn’t we be able to observe it at work in this manner?