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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Doh, I'l try to dumb this down as much as possible.
In newest theories, expansion of universe is considered to be accelerating. Acceleration is allegedly driven by mysterious dark energy, which interacts by no interaction except gravity but constitutes vast majority (74%) of universe.
There is now some information around which seems contradictory to me. Negative energy would be required in order for such a repulsive gravitational effect. Existence of negative energy poses problems with things like naked singularities, and dark energy is considered to be 'normal' energy like we have around.
Rather, on several sites dark energy is said to posses ''negative pressure''. My question is, what interaction is responsible for this negative pressure, if not gravity? What is it anyway?
Wikipedia apparently tried to give some clue on this:
One might wonder, how can pushing cause attraction? How can pulling cause repulsion? Naively, this sounds like a contradiction. The solution is:
* The pushing of positive pressure (and the pulling of negative pressure) are non-gravitational forces which just move substances around within space without changing space itself. * But the gravitational attraction (or repulsion) they cause operates on space itself, decreasing (or increasing) the amount of space between things. It is this which determines the size of the universe. * There is no necessity that these two effects should act in the same direction. In fact, they act in opposite directions.
But it just confuses more. How can something ''operate on space'' in order to produce such an effect? And if those are ''non-gravitational'' forces, what are they?
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
My best guess is sort of like the charged pith ball experiments we see in Physics so much. Like charges repel, so now we not only have like and dislike ordinary matter, but like and dislike 'dark matter' (whatever that is).
Now if dark mater only interacts with itself, much like what we consider as matter around us, then it undergoes attraction and repulsion without interacting with our matter, with the exception of gravitational forces (from what Sulaiman is saying).
So What we could assume is that if the dark matter is repelling from itself its causing a gravitational 'wake' that could pull in ordinary matter. If we think of two bodies being pulled apart in a bathtub it causes a low pressure zone in the middle. And similarly if two 'dark' bodies are attracting each other it causes a gravitational wake that disperses ordinary matter. You can see that in the tub too, the water will flow to lowest pressure.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Firkragg: When you start messing with the fabric of space-time itself, the results can be very confusing and counter-intuitive, since your intuition only knows how to deal with ordinary objects moving within space-time. The expansion of the universe is not at all like inflating a black balloon with white dots painted on it, but that is probably the best metaphor you can use in only 3 dimensions.
It doesn't really matter anyway, since cosmologists and particle physicists just make this stuff up when they're writing their grant proposals. They used negative pressure because one of their equations that describes the behaviour of dark energy has a term for pressure in it, and if they made that negative, the answers fitted the observations. A *real* cynicist would probably say that 74% of the mass of the universe is made of "fudge factor" :D
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi
Sulaiman and Hazmat, I see you are trying to say pretty much the same thing. If dark energy had some kind of 'dark charge' which causes repulsion only among dark energy, it would, naively said, be able to drag ordinary matter along with through gravity.
But what is now ''dark charge'', anyway? If such a thing exists then it would have to be interaction currently unknown to physics. And everyone cites interaction by no force except gravity as basic property of dark energy.
The exact nature of this dark energy is a matter of speculation. It is known to be very homogeneous, not very dense and is not known to interact through any of the fundamental forces other than gravity.
Question is just that simple, which interaction is doing it?
Steve: LOL. As I think ''messing with the fabric of space-time itself'' is cool because it's confusing and counter-intuitive.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
I'm doing my best to fix my posting but still vast majority of my threads get ignored.
I think you missed the vital third ingredient for my demonstration
Sulaiman, I have no idea what are you trying to say.. There is no such a thing as a dark charge, negative pressure or whatever. There is no (to me) apparent reason for alleged cosmic acceleration.
I just hoped somebody of very knowledgeable guys over here will shed some light on this, and possibly help me understand lots of other things along.
Registered Member #187
Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140
Has anyone ever discussed the possibility that we may be naive to assume that space is a "perfect" conductor of photons? Space is dirty and we have painfully limited knowledge of what kind of dirt it contains. It doesn't seem too crazy to assume that the energy a photon carries could be altered or lost by some kind of interaction with space dirt, or equally valid, with space itself, resulting in a sort of "drag" effect for lack of a better term.
For example, if a photon were to lose energy, likely due to its dependence on space as it's medium for travel, it is reasonable to expect the wavelength to become stretched in an amount that is very small and at least somewhat constant (this effect is feasible to postulate since it is analogous to a photon that is slowing), the effect would accumulate over very long distances, at which point the effect has grown to observable proportions and would be observed on earth as red-shifted, and then it is assumed that this must be a visual version of the dopplar effect, which means it is moving away from us.
Conclusion of naive astrophysicist: This looks exactly like the dopplar effect so that it must be. Everything must be moving away from us!
If we consider the possibility of "space-drag", the farther away something is, the more red-shifted it will appear, which does not require that it be moving away from us at all! Conversely, if we really like our dopplar assumption, we will conclude that we have indeed found that some are moving away faster than others. And coincidentally, the one's that are farther away are moving faster!
Now consider the very different results of each assumption if the cosmic object were moving away from us, and it is not known if any acceleration is occuring. The dopplar scientist first observe a redshift and conclude that it is moving away. Many years later it is observed again and the red-shift has intensified. The conclusion is that it's velocity is somehow increasing. If we are believers that photon energy is slowly lost to space interactions, the cosmic object would simply be moving away from us with constant velocity. Two reasonable and valid hypotheses with profoundly different results.
I don't claim to be more knowledgeable than anyone else on this subject, but I do enjoy entertaining alternative possibilities when I think that conclusions like this one (accelerating expansion) are drawn and assumed to be true without question. I personally don't feel comfortable with the apparent eagerness to invent incredibly abstract existances of bizarre types of matter, in an attempt to hasten our understanding of the universe. I don't feel the same sense of urgency to generate these colassal fudge factors like I suspect "dark matter" to be. To me it says, "I need to conjure up something with a description that will render it completely impermeable to any known means of detection and therefore cannot be disproved".
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
The expansion of the universe is a theory backed by several independent types of observational proofs. It has stood the test of time for 80 years. So far everyone that has had other ideas have been left looking silly.
A hypothesis that the universe does not expand is not going anywhere, you would need a complete theory that can explain everything we observe. In addition you need a testable prediction that the current theory will fail.
If not the new theory is worse than the old one and there is no reason to consider a switch.
Step 1 would be to prove that there is red shift over distance or time independently of velocity.
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Universal acceleration
If such a thing exists then it would have to be interaction currently unknown to physics. And everyone cites interaction by no force except gravity as basic property of dark energy.
