Majestyk

Big Bang Afterglow Reveals Energy Repulsion
I wasn't aware that repulsive gravity was possible. Is there any theory behind this or is it purely observational?

Shadowy Man

It is observational in origin, and there are different theories out there to explain dark energy.

deano

i read in an article in new scientist that the reason dark matter/energy is repulsive is because it is under very high tension,which results in an anti gravatational force.

couldnt tell you any more cos i was reading it in a shop!

Majestyk

I did a little checking on it. The repulsive gravity is what struck me.

If, attractive gravity is the warping of spacetime then, assuming repulsive gravity is also a warping of spacetime and their effects are diametrically opposed, would this indicate that their respective effects extend into time in diametric opposition?

If, light matter consists of wave functions which come into existence then, cease to exists, does dark matter consist of wave funtions that cease to exist then, come into existence?

I'm not framing these questions properly. Thinking about it is making my head hurt.

Friar Bellows

Repulsive gravity is entirely consistent with Einstein's general theory of relativity (GR). Consider a small volume of comoving particles, and denote V(t) as the volume of this ball after a (proper) time, t. Then GR predicts the following (at t=0):

V''/V = -1/2 (flow of t-momentum in the t direction + flow of x-momentum in the x direction + flow of y-momentum in the y direction + flow of z-momentum in the z direction)

where we are working in 4-dimensional spacetime (t,x,y,z), the momentum is the four-momentum, and V'' is the second derivative of the volume of the ball with respect to time. The above equation may be written more compactly as:

V''/V = -1/2(p + Px + Py + Pz)

where p is the energy density of the volume, Px is the pressure in the x-direction, Py is the pressure in the y-direction and so on. As long as the term in the brackets is positive, the ball will shrink -- in other words gravity attracts. But if it's negative, then the ball will grow -- in other words gravity repels. I can't imagine the energy density, p, being negative, but perhaps the pressure can be negative.

Let's apply this equation to the universe, where we assume homogeneity and isotropy. Then any ball of matter (of galaxies or galaxy clusters) in this universe will remain round as time passes, and so the volume of the ball remains proportional to the cube of the radius, R. Also, the pressure components will be equal to each other (due to isotropy), Px = Py = Pz = P, and so:

3R''/R = -1/2(p + 3P) [eqn A]

Now while this equation only holds for t=0 as I have written it, it turns out to hold for all subsequent times as well (complicated to prove). If we then make the assumption that the pressure is very small compared to the energy density then we can neglect P, assume conservation of matter, i.e. pR^3 = k (where k is a constant), and obtain:

R'' = -k/6R^2 [eqn B]

which should remind you of Newton's old law of gravity. Pretty cool, huh? Now the solution of equation B gives us the usual, open closed and flat universe scenarios. If the initial expansion rate of the universe is high enough (compared to the energy density), then the universe will expand forever; otherwise, it will collapse back in on itself.

Now what about dark energy or vacuum energy? The theory goes that this dark energy or the vacuum itself may have a small amount of energy density and pressure associated with it. The properties of it are such that the pressure is negative (hello!), and is of precisely the same magnitude as the energy density. These extra pressure and energy density terms have to be added to our equations. Equation A is then modified to:

3R''/R = -1/2(p + 3P - 2/\)

where /\ is the so-called "cosmological constant" (the energy density of the vacuum or dark energy is just /\ while the pressure is equal to -/\, so that p+3P for the vacuum or dark energy is equal to -2/\),

while equation B becomes:

R'' = -k/6R^2 + /\

So assuming that /\ is positive, then the universe will never collapse back in on itself, but will continue to expand, eventually at an exponential rate.

(Ref: http://arxiv.org/abs/gr-qc/0103044 )

wordsmyth

If true, it might explain why the expansion of the universe appears to be speeding up rather than slowing down.

What exactly is the difference between Dark Matter and Anti-Matter? Could Dark Matter be Anti-Matter or is Anti-Matter too unstable to exist for any length of time?

Curtis

Dark matter is matter that is known to exist because of mass detection (ie : our galaxy has a lot more mass in it than can be accounted for by visible matter), but can't be detected by normal means. Could be very small non-radiant particles like dust, or weakly-interacting particles that are hard to define.

Antimatter is the reverse of normal matter, in that it's positive particle/waves around a negative nucleus, as opposed to "normal" matter, with a positive nucleus, and negative electrons around it.

The bit about antimatter is off the top of my head, so someone feel free to correct me if I'm wrong.

Shadowy Man

Sorry, but dust does radiate. Dark matter, whatever it is, isn't dust.

Majestyk

Thanks for the reference, Friar Bellows. That's a very informative file.