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Natural Science Forum / Physics / Research / November 2007Dark Energy and MOND
If MOND turns out to be correct, and obviates the need for Dark Matter, is that likely to have an impact on the need for Dark Energy to explain the presumed accelerating expansion of the universe?  SignatureDirk http://www.transcendence.me.uk/ - Transcendence UK Remote Viewing classes in London > If MOND turns out to be correct, and obviates the need for Dark Matter, > is that likely to have an impact on the need for Dark Energy to explain > the presumed accelerating expansion of the universe? How does MOND survive the empirical necessities of General Relativity? There is only one observable universe. It only admits to one set of rules at all scales. SignatureUncle Al http://www.mazepath.com/uncleal/ (Toxic URL! Unsafe for children and most mammals) http://www.mazepath.com/uncleal/lajos.htm#a2 > > If MOND turns out to be correct, and obviates the need for Dark Matter, > > is that likely to have an impact on the need for Dark Energy to explain [quoted text clipped - 3 lines] > Relativity? There is only one observable universe. It only admits to > one set of rules at all scales. That is an usual misunderstanding of MOND. MOND holds for MOdified Newtonian Dynamics. MOND is non relativistic, therefore MOND does not apply to high velocities. MOND never was designed to be a substitute of GR but a model for galactic dynamics (because GR cannot account for data there). Your "empirical necessities of General Relativity" are covered by Relativistic MOND. The scheme in a (c, a_0) formal space of available theories is R-MOND ----> MOND | | | | v v GR ----> Newton That is, R-MOND reduces to GR results when a >> a_0. The more popular R-MOND theory I know is TeVeS. It gives same results than GR in the corresponding limit. >> > If MOND turns out to be correct, and obviates the need for Dark Matter, >> > is that likely to have an impact on the need for Dark Energy to explain [quoted text clipped - 27 lines] >The more popular R-MOND theory I know is TeVeS. It gives same results >than GR in the corresponding limit. Nobody has even solved the TeVeS field equations for even the simplest possible case - spherical symmetry. On Nov 11, 7:50 am, Eric Gisse <jowr.pi.nos...@gmail-nospam.com> wrote: > On Sat, 3 Nov 2007 15:23:13 +0000 (UTC), "Juan R." > >The more popular R-MOND theory I know is TeVeS. It gives same results > >than GR in the corresponding limit. > > Nobody has even solved the TeVeS field equations for even the simplest > possible case - spherical symmetry. I may confess I am really perplexed with your wrong thought! Regarding the General Relativity limit, see section III C "General relativity limit" of TeVeS on [1]. {BLOCKQUOTE [1] In Sec. III C we demonstrate that TeVeS has a GR limit for a range of small k and K } K and k are two parameters on TeVeS (see [1] for details). Of interest is also the section D "Generic general relativity limit". Once the TeVeS field equations have been reduced to GR field equations, it is a trivial fact that the computation of spherical symmetric solutions can be done using standard techniques from GR literature. You would consult some standard textbook for this part if you do not know how obtain solutions of that kind. E.g. see [2] for a detailed discussion on the computation of spherically symmetric static solutions to GR field equations. Since TeVeS reduces to GR in a well-defined limit, TeVeS gives the same predictions than GR for the classical Solar System tests, for instance. That is standard stuff. You may ignore this aspect of science but new proposed theories are obligated to give predictions compatible with the old theories in some specific limit. It is obvious that astronomers & astrophysicists would not take TeVeS so seriously on the contrary [#]. One can also study TeVeS corrections to GR spherical solutions. This is a more interesting point. For instance on section V, "The post- Newtonian corrections" [1], the author computes the O(k) + O(K) corrections to a Schwarzschild metric. Of interest are also spherical symmetry solutions on a pure MOND dynamical regime. See, for instance, section IV B "The MOND limit: spherical symmetry" [1]. [1] Phys. Rev. D 2004, 70, 083509. [2] General Relativity. University of Chicago Press; 1984. Section 6.1. Wald R. M. [#] I am not supporting TeVeS. I cited TeVeS in an early reply like being the more popular relativistic MOND theory i know. ======== I follow http://canonicalscience.com/guidelines.txt In article <5osmq6Fo6ijeU1@mid.individual.net>, Dirk Bruere at NeoPax <dirk.bruere@gmail.com> writes: > If MOND turns out to be correct, and obviates the need for Dark Matter, First, the latter does not necessarily follow from the former. MOND is primarily concerned with explaining the rotation curves of galaxies. It is entirely possible that MOND might turn out to be the correct explanation there, but that (somewhat less) dark matter is needed for other reasons. A few years ago, people considered alternatives such as massive neutrinos are the dark matter, or something else is, if neutrinos are massless. We now know that neutrinos do have mass, but too little to make up most of the dark matter. > is that likely to have an impact on the need for Dark Energy to explain > the presumed accelerating expansion of the universe? No. "Dark Energy" is a bad name. As Sean Carroll pointed out, essentially everything has energy, and many things are dark. A better name would be "smooth tension", since this refers to the defining characteristics: tension, i.e. negative pressure, which essentially nothing else has, and smooth, i.e. not clumpily distributed. The "dark" in both names suggests an affinity which is probably not there. (If black holes were called dark holes, I'm sure many people would be asking what the connection is between dark holes and dark energy, whether dark holes are the same as dark matter etc.) I prefer the name "cosmological constant" for dark energy. Yes, some people define dark energy to be more general, perhaps with a different equation of state and/or changing with time, but as long as there is no evidence that a traditional cosmological constant cannot explain the data, and up until now there is none (though of course one should continue to check and not rule out something more complicated), I'll stick with "cosmological constant". A cosmological constant has essentially the same effect as matter with respect to the curvature of the universe, i.e. it is the sum of the two which determines whether the universe is spatially open, flat or closed. With respect to the expansion of the universe, however, it has an opposite effect, i.e. more mass leads to more deceleration, and more cosmological constant leads to more acceleration. (Theoretically, although the evidence points in the other direction, the cosmological constant could be negative, in which case it would cause more deceleration but also make the universe less closed.) If we look at the expansion of the universe itself, we essentially determine the values of the cosmological constant (lambda) and the matter density (Omega), independent of other estimates of the matter density. As long as the expansion doesn't indicate LESS matter than is determined otherwise to exist, we are OK, and things look fine in this respect. So, to zeroth order, MOND has no effect here. Of course, if MOND is correct, then the method of determining the cosmological parameters from observations such as the m-z diagram (the main evidence for acceleration and a cosmological constant) probably needs to be revised, but this requires a cosmological theory of MOND, which doesn't yet exist. On smaller scales, where more direct evidence for dark matter exists (i.e. the matter density seems to be higher than matter which can be accounted for with traditional methods), then the value of the cosmological constant plays no role. Although there are some arguments for MOND, I think one of the arguments against it is that Omega determined globally is just a bit more than Omega determined more directly on smaller scales (and it is natural that it is slightly larger, since more local determinations necessarily miss any smoothly distributed component). If we reduce the dark matter if we believe in MOND, then we would have to increase the value of the cosmological constant to keep the universe at the (nearly) flat value determined from CMB observations. However, that would imply more acceleration than is observed in the m-z diagram. In other words, MOND would imply increasing the value of the cosmological constant in one area, but this would be inconsistent with another area. So, again, would could construe this as an argument against MOND, though, again, one really can't say much until one has a complete cosmological theory of MOND and re-analyses all observations in that context, which is a HUGE undertaking. While in some sense it is worth doing, as long as traditional cosmological models can explain all the data---which seems to be the case---then the motivation for developing a detailed alternative theory is small. Dans l'article <fgeinp$6gf$1@online.de>, helbig@astro.multiCLOTHESvax.de (Phillip Helbig---remove CLOTHES to reply) écrit: ] A better name would be "smooth tension", since this refers to the defining ] characteristics: tension, i.e. negative pressure, which essentially ] nothing else has, and smooth, i.e. not clumpily distributed. if we admit a "dynamical" cosmological constant (e.g. quintessence), do you know the limits on its clumpiness placed by observations ? Signature| ~~~~~~~~ Martin Ouwehand ~ Swiss Federal Institute of Technology ~ Lausanne __|___________ Email/PGP: http://personnes.epfl.ch/martin.ouwehand ____________ To understand God's thoughts we must study statistics, for these are the measures of His purpose [Florence Nightingale] >> is that likely to have an impact on the need for Dark Energy to explain >> the presumed accelerating expansion of the universe? > >No. [...] >A cosmological constant has essentially the same effect as matter with >respect to the curvature of the universe, i.e. it is the sum of the two [quoted text clipped - 5 lines] >constant could be negative, in which case it would cause more >deceleration but also make the universe less closed.) All of these statements are based the Friedmann equation, which is a consequence of general relativity. But if MOND is correct, then general relativity is not. So I don't really see how to justify your "No" above. I'd answer the question with an "It depends." To be specific, it depends on what MOND predicts for the expansion of the Universe, i.e., on what the MONDian version of the Friedmann equation is. Back when I paid closer attention to this subject, which was a number of years ago, MOND was not a sufficiently precise theory to give an answer to that question. Has that changed? -Ted Signature[E-mail me at name@domain.edu, as opposed to name@machine.domain.edu.] In article <fgffm3$fko$1@bigbang.richmond.edu>, ebunn@lfa221051.richmond.edu writes: > >> is that likely to have an impact on the need for Dark Energy to explain > >> the presumed accelerating expansion of the universe? [quoted text clipped - 6 lines] > consequence of general relativity. But if MOND is correct, then > general relativity is not. Indeed; see the caveats in my post. One extreme would be "we don't have a fully fledged theory of MOND, so we can't say anything". The other would be "in the appropriate limits, a fully-fledged MOND will correspond to traditional physics" (as alluded to by Juan R in his post). The truth is probably somewhere in-between. > If MOND turns out to be correct, and obviates the need for Dark Matter, > is that likely to have an impact on the need for Dark Energy to explain > the presumed accelerating expansion of the universe? See the good discussion at Cosmic Variance: http://cosmicvariance.com/2007/11/01/dark-matter-still-existing/#comments. The near consensus is that only dark matter can explain the spatial variations in attraction (i.e., indicating gravity coming from distinct regions, not just anything about how it varies with distance from known matter.) Pioneer 10 anomalies still unexplained, but one commenter thinks its from oddities of the space craft, not weird gravity. |
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