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Has anyone done any work to see if the extra gravity we have in galaxies is balanced by the current theoretical requirement for Dark Energy. I am just wondering if expansion is borrowing something like energy but paying it back with increased gravity locally?
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Yes, here.
Order of Kilopi
I'm not sure what you mean by "balanced." I will assume you mean "explained" in this context. I expect the gravitational effect of the dark energy within the galaxy is much less than 1% of the "extra gravity" observed, which leads to the conclusion that dark matter must be something else.Originally Posted by sirius0
Everyone is entitled to his own opinion, but not his own facts.
Established Member
Yes i don't think I wrote that well. I meant to say that perhaps the expansion of the universe as an énergy' is balanced by the apparent excess of énergy' in a galaxy's gravitation. Is dark energy the universe's conservation payback for galaxies having more gravity than they should. Has anyone investigated this?
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What do you mean with "the expansion of the universe as an energy"? What has the expansion of the universe to do with an energy?
What do you mean with "the apparenty excess of energy in a galaxy's gravitation"? Do you perhaps refer to the observed fact that the gravitational force of a galaxy is greater in its outer rims than expected from the observed matter? If yes, what has that to do with an "excess of energy"?
I take this to mean that the "excess of energy in a galaxy's gravitation" is compensated by the dark energy, i. e. that the two add up to zero. Did you mean that?
If yes: why should that be the case? Why should the universe somehow be "compelled" to pay back that?
BTW: are you aware that gravitational potential energy is
1) negative in Newtonian mechanics, and
2) not even well-defined in General Relativity?
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The negative assignment of gravitational potential energy (gpe) in Newtonian mechanics is a choice of "convenience" and does not mean it is negative. The absolute gpe is given by E=mc^2. Since this yields fantasticly large numbers, this "choice of convenience" has stuck, but it has nothing to do with reality.
"By convention," gpe=0 at infinity, and is "negative" at distances of less than infinity. But in reality, gpe=mc^2, and is always positive.
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How exactly do you use E=mc^2 to calculate the gpe, and why do you think the result is more "real" than the one obtained by using the usual convention?
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Are you aware of the fact that for a force for which a potential exists, the force is the negative gradient of the potential energy? (by *definition* of the term "potential energy" in general - this has nothing to do with the "negative" convention for gpe you mentioned).
So, according to your term for gpe, we would have for the gravitational force F_g:
F_g = - grad (gpe) = - grad (m c^2) = 0
No gravitational force at all there when one uses your expression for the gpe. Something wrong here, don't you think?
In contrast, when one uses the usual expression (gpe = - G m M/r + const., where the constant is usual chosen to be zero, G is the gravitational constant, m is the mass of the body for which one wants to have the gpe, M is the mass of the body to which m is attracted, and r is the distance between the centers of mass), then one gets:
F_g = - grad (- G m M/r + const.) = - G m M/r^2 e_r, where e_r is a unit vector in radial direction. Which is just the well-known Newtonian law of gravity. I rest my case.
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Energy is like temperature: no such thing as negative.
We have negative temperature scales, but these are "by convenience."
Picture 1 kg of material. It has m=1 kg, and gpe equal to mc^2 = 1 kg x (3E8 m/s)^2 = 9E16 joules.
Now, lower that 1 kg to the event horizon of a black-hole, extracting gravitational energy from it as you go. Although it will have the same number of baryons, it will have a rest-mass of something like 0.9 kg. Its gpe is: 0.9 kg x (3E8 m/s)^2 = 8E16 joules. In other words, it has lost 10,000,000,000,000,000 joules.
In the mixed-up convential sense, we would say it has "negative 1E16 joules." But it does not have "negative energy" any more than ice at -10 C has "negative energy." It has 10,000,000,000,000,000 joules less energy than when you started with it, but it still has a large, positive energy, reflected in its rest-mass energy.
This does not violate the force/gradient convention.
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My thoughts behind this question is along the lines of virtual particles. Apparently particles can appear spontaeniously in a vacuum but then they pay their energy back. Some like to say that vacuum energy and dark energy are related. If so and if dark energy is what causes space to expand I was wondering where the payback occurs. Its just that once people start saying energy I think there should be conservation.
I wanted to see how clean this direction of thought is (the concept of dark energy). My thoughts were along the line of "What if the black holes in the centre of a galaxy are 'borrowing'more gravity than they should."Then "the pay back for this could be the expansion."
You will notice too that I wrote énergy' with the apostrophies as Idon't see how this can be energy; I am doubtful of the whole dark energy thing. I was doing some background thinking to my ATM; asking a question to see how developed DE theory is.
it is probably hard to see exactly what my agenda is as I appear even now to be espousing DE theory. I am not very linear.
If you want to know what I am up to then look at my posts or follow this link
http://www.bautforum.com/showthread.php?t=49339
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It's often described that way, yes - but in fact, such vacuum fluctuations don't violate energy conservation. The particles appear not one by one, but at least in pairs - and in such a pair, one particle has positive, the other has negative energy; the total energy is zero. Hence there is no energy to be "paid back".
Indeed.
Space expands on its own. Dark energy does not cause the expansion - it causes the expansion to accelerate.
Well, energy isn't in general conserved in GR. See
http://math.ucr.edu/home/baez/physic...energy_gr.html
What is "borrowing gravity" supposed to mean, and what has that to do with energy?
How could the expansion "pay back" that? What has the expansion to do with energy?
Then what was your reason for bringing up the conservation of energy?
Why? What do you know about it?
What is "dark energy theory" supposed to mean, exactly?
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I'd like to stay on in this thread first - one becomes entangled in too many discussions much too quickly.
Established Member
Wrong. Energy isn't defined up to a constant - hence by choosing that constant appropriately, arriving at a negative energy is no problem at all.
The comparison to temperature makes little sense.
Why on earth should its rest mass decrease??? And how did you arrive at the number 0.9 kg?
Its total energy (rest energy plus kinetic energy plus potential energy) is positive, right. But its potential energy is negative.
You haven't addressed my argument that your gpe = m c^2 would lead to a gravitational force F_g = 0.
Established Member
You're not letting it make sense.
Volume is another example: no such thing as negative volume. You can subtract one volume from another--for example, if you you wall a room in half, then the volume of one side is the starting volume minus the volume of the other side--but volume in-and-of-itself cannot be negattive. No sphere exists with negative volume; no matter exists with negative potential energy.
Because rest mass equals potential energy divided by c-squared, and at the event horizon, the material has much less potential energy, so its mass decreases, in proportion to lost potential energy.
The 0.9 kg comes from knowing that about 10% of the rest mass energy of material is radiated away before crossing the event horizon. (Read this somewhere)
You haven't understood what I am saying. Between here and the event horizon, 1 kg. of matter loses about 10% of its potential energy. Since it lost all this energy in going from here to there, the force field is not zero.
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Neither (absolute) temperature nor volume has an arbitrary additive constant in its definition, so again: comparing this to energy makes no sense.
Wrong. Rest mass equals rest energy divided by c-squared. Where on earth did you get the idea from that the E in the formula is potential energy?
Reference, please.
Well, so in your formula E=mc^2, m depends on the distance to the event horizon? If yes: please tell me how exactly it depends on this distance. Provide a formula. And, hint: that formula should lead to the known gravitational force F_g = G m M/r^2.
Order of Kilopi
Dark matter.
Dark energy.
You forgot about Dark Force!
Scientific American Blog: First Dark Matter, Then Dark Energy, Now a Dark Force?
The main annual conference of the American Astronomical Society began this morning, and it didn't take long to roll into action. In one of the very first sessions, Glennys Farrar of New York University described some startling hints of a fifth force of nature, on top of the Fab Four: electromagnetism, gravity, and the two forces that govern atomic nuclei. The idea of a fifth force has a checkered history, and experiments seem to rule it out. But those experiments apply only to ordinary matter. They say nothing about dark matter.
[...]
Farrar concluded that some new force must be helping to accelerate it. Such a force might also explain a number of other discrepancies that astronomers have found. For instance, the universe contains more galaxy superclusters than standard dark-matter models predict. A new force would grease the wheels of supercluster formation.
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HUBBLE MAPS THE COSMIC WEB OF "CLUMPY" DARK MATTER IN 3-D
SEATTLE - An international team of astronomers using NASA's Hubble Space Telescope has created the first three-dimensional map of the large-scale distribution of dark matter in the universe.
Dark matter is an invisible form of matter whose total mass in the universe is more than five times that of "normal" matter (i.e., atoms). The nature of dark matter is still unknown. Its presence in the universe is inferred from its current influence within galaxies and clusters of galaxies, and the gravitational effect it has had on the evolution of structure in the universe. The first direct detection of dark matter was made this past year through observations of the Bullet Cluster of galaxies.
This new map provides the best evidence to date that normal matter, largely in the form of galaxies, accumulates along the densest concentrations of dark matter. The map reveals a loose network of filaments that grew over time and intersect in massive structures at the locations of clusters of galaxies.
The map stretches halfway back to the beginning of the universe and shows how dark matter has grown increasingly "clumpy" as it collapses under gravity.
The dark matter map was constructed by measuring the shapes of half a million faraway galaxies. To reach Hubble, the light of the galaxies traveled through intervening dark matter. The dark matter deflected the light slightly as it traveled through space. Researchers used the observed, subtle distortion of the galaxies' shapes to reconstruct the distribution of intervening mass along Hubble's line of sight, a method called "weak gravitational lensing."
For astronomers, the challenge of mapping dark matter in the universe has been similar to mapping a city from nighttime aerial snapshots showing only streetlights. Dark matter is invisible, so only the luminous galaxies can be seen directly. These new map images are equivalent to seeing a city, its suburbs and country roads in daylight for the first time. Major arteries and intersections become evident, and a variety of neighborhoods are visible.
Mapping dark matter's distribution in space and time is fundamental to understanding how galaxies grew and clustered over billions of years. Tracing the growth of clustering in dark matter may eventually also shed light on dark energy, a repulsive form of gravity that would have influenced how dark matter clumps.
The research results appeared online today in the journal Nature and were presented at the 209th meeting of the American Astronomical Society in Seattle, Wash., by Richard Massey and Nick Scoville. Both researchers are from the California Institute of Technology, Pasadena, Calif.
"It's reassuring how well our map confirms the standard theories for structure formation," said Massey. He calls dark matter the "scaffolding" inside of which stars and galaxies have been assembled over billions of years.
Researchers created the map using the Hubble's largest survey to date of the universe, the Cosmic Evolution Survey, otherwise known as COSMOS. The survey covers an area of sky nine times the area of the Earth's moon. This allows for the large-scale filamentary structure of dark matter to be evident. To add 3-D distance information, the Hubble observations were combined with multicolor data from powerful ground-based telescopes, Europe's Very Large Telescope in Chile, Japan's Subaru telescope in Hawaii, the U.S.'s Very Large Array radio telescope, New Mexico, as well as the European Space Agency's orbiting XMM-Newton X-ray telescope.
The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. The Space Telescope Science Institute, Baltimore, conducts Hubble science operations. The Institute is operated for NASA by the Association of Universities for Research in Astronomy, Inc., Washington.
For more information and images about this research, visit:
http://www.nasa.gov/hubble
http://hubblesite.org/news/2007/01
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Does DE has any link with vacuum energy ( Casimir ).??
If this topic has been discussed, then any link will help me.
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Yes, there are indeed strong connections. For a thorough discussion, see e. g. these webpages:
http://math.ucr.edu/home/baez/vacuum.html
http://www.astro.ucla.edu/~wright/cosmo_constant.html
Order of Kilopi
This runs the risk of introducing too many unconstrained variables to be considered real science. If you see acceleration of visible matter that you cannot understand, you might say it is due to gravity from invisible matter, but that only has meaning if you can then use your physics to predict how the invisible matter should behave. If that also requires new physics, then you can fit anything, just with the appropriate combination of known and unknown forces on known and unknown matter. It seems to me you are still making progress if you have known forces on unknown matter (CDM), or unknown forces on known matter (MOND), but not when you have unknown forces on unknown matter. It sounds like we're back to epicycles and deferents. Then again, if there really is a fifth force on dark matter, I guess we're stuck with it, by I fear it will make the science pretty questionable until we can actually study dark matter directly.
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What I asked for is for what reason you need this separation force to exist.
It is said by all the people that the gravity has the same intensity regardless of the direction of measurement and that it is isotropic. But I think that the gravity will work not only three dimensionally but also two or one dimensionally. In the two or one dimensional gravity, the gravity will be concentrated in one plane or line and will have a stronger effect than three dimensional gravity has. Then the separation forces are necessary in order to smash the gravity into two or one dimension.
What is Energy and how do you measure it?
"Energy" is the emission of light. In the beginning there was only the light with the energy ( = Eo ). At present equilibrium, the total matter in the space is (1/2)Eo/c^2 and the total energy has decreased to (1/2)Eo, so I think.
Your proposal would at least increase the dark matter estimate.
Galaxies are arranged into sheets and filaments surrounding vast empty voids. Galaxies have "two or one dimensional" gravitational spreads which will work as "surface tension" for forming large scale bubble, or void, structures.
Unlike Pontius Pilate, you cannot wash your hands of this matter. You brought up this complex mathematics, so you are responsible for the disposition of the imaginary part.
I should not be presented as Pilate. So I would like to answer it. Im( F ) might be the value which should be minimized in order that the definite space be stabilized.
http://www.geocities.jp/imyfujita/galaxy/galaxy01 [...]
Iori Fujita
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