## Question about dark matter, dark energy, and MOND

For Modified Newtonian Dynamics proposed by Milgrom, there is a
deviation from the classical gravitational force (or a deviation from
the classical Newtonian dynamics, depending on your perspective) once
the acceleration of a particle is less than a0. Thus one can account
for the star motion on the outer fringe of galaxies without invoking
dark matter.

One interesting observation is that a0 is also the acceleration scale
where dark energy should play a significant role. My question is why
dark energy is not usually considered for intra-galactic (non-
cosmological) dynamics with less-than-a0 acceleration? Would dark
energy change the predictions of MOND?

-Max

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 On Sun, 14 Dec 2008 21:59:07 +0100 (CET), Max wrote: >For Modified Newtonian Dynamics proposed by Milgrom, there is a >deviation from the classical gravitational force (or a deviation from >the classical Newtonian dynamics, depending on your perspective) once >the acceleration of a particle is less than a0. Thus one can account >for the star motion on the outer fringe of galaxies without invoking >dark matter. But not in galaxies involved in collisions, eg: Bullet cluster, MACS J0025.4-1222. > >One interesting observation is that a0 is also the acceleration scale >where dark energy should play a significant role. My question is why >dark energy is not usually considered for intra-galactic (non- >cosmological) dynamics with less-than-a0 acceleration? Would dark >energy change the predictions of MOND? > >-Max You ask the question as if the answer matters. Putting aside the arguments about MOND's applicability in galaxies, MOND does not explain the large scale structure of the universe in either the structuring of matter [eg, results from the Millenium simulation...] or the power spectrum of the CMB. My explanation is that dark energy is not usually considered since the proponents of MOND seem to reject not just dark matter but dark energy, and it would be ... odd ... to include dark energy. Regardless, returning to dark energy. It does not appear to have any influence on galaxies as the galaxies are gravitationally bound and dark energy isn't strong enough to have a significant impact.
 On Dec 15, 12:26 pm, eric gisse wrote: > Regardless, returning to dark energy. It does not appear to have any > influence on galaxies as the galaxies are gravitationally bound and > dark energy isn't strong enough to have a significant impact. On the outer fringe of a galaxy, the negative pressure of dark energy (cosmological constant, quintessence, or whatever it's called) would be larger than the gravitational pull (MOND or Newtonian). And stars would be unbound, zipping away from the galaxy. It is certainly not what is observed. The absence of dark energy for intra-galaxy dynamics is so baffling for me. [[Mod. note -- While in widespread use, the term "dark energy" is somewhat unfortunate; I have seen several suggestions that "smooth tension" would be a more suitable term. With that in mind, the key point is that this "smooth tension" is *smooth* and is only significant on a cosmological scale -- an individual galaxy (or matter on its outskirts) isn't going to feel any significant forces from the smooth tension. -- jt]]

## Question about dark matter, dark energy, and MOND

In article <gi3s2r$3ev$1@fb07-hees.theo.physik.uni-giessen.de>, Max
<awsg02@yahoo.com> writes:

> For Modified Newtonian Dynamics proposed by Milgrom, there is a
> deviation from the classical gravitational force (or a deviation from
> the classical Newtonian dynamics, depending on your perspective) once
> the acceleration of a particle is less than a0. Thus one can account
> for the star motion on the outer fringe of galaxies without invoking
> dark matter.
>
> One interesting observation is that a0 is also the acceleration scale
> where dark energy should play a significant role. My question is why
> dark energy is not usually considered for intra-galactic (non-
> cosmological) dynamics with less-than-a0 acceleration? Would dark
> energy change the predictions of MOND?

Reason 1: The motivation for MOND was to avoid dark matter. Not
believing in dark matter but believing in dark energy is not really an
option for someone who doesn't want to believe in dark matter. Reason
2: No-one knows what dark energy is or what its behaviour is like in
detail, especially at small (intra-galactic) scales. (We see
unambiguous effects only at large (cosmological) scales.) Reason 3:
MOND was developed mainly to explain the observed flat rotation curves
of galaxies, which is not what one expects if the mass distribution
follows the light distribution. At least phenomenologically, MOND
offers a simple explanation. An explanation involving dark energy would
be contrived.

 In article , eric gisse writes: > On Sun, 14 Dec 2008 21:59:07 +0100 (CET), Max > wrote: > > Putting aside the arguments about MOND's applicability in galaxies, > MOND does not explain the large scale structure of the universe in > either the structuring of matter [eg, results from the Millenium > simulation...] or the power spectrum of the CMB. True, in a sense. On the other hand, I think it's fair to say that we don't know if that statement would still hold if the same amount of research was put into MOND as has been put into "standard" cosmology. > My explanation is that dark energy is not usually considered since the > proponents of MOND seem to reject not just dark matter but dark > energy, and it would be ... odd ... to include dark energy. Right; that's probably the main reason. > Regardless, returning to dark energy. It does not appear to have any > influence on galaxies as the galaxies are gravitationally bound and > dark energy isn't strong enough to have a significant impact. In the conventional form (and there is no evidence that this is not the case), dark energy is distributed completely smoothly. In that case, I believe (someone correct me if I am wrong) that the only effect it could have would be to change the size of a gravitationally bound structure. To notice this, we would have to compare expected and observed sizes, probably at a variety of scales. The data are too messy to note any deviations, at least for the value of the cosmological constant which we believe we know from cosmological observations.
 See also http://arxiv.org/abs/0809.1183 which explains how dark energy, why MOND acceleration ~ Hubble scale and other mysteries can be explained by backreaction, On Dec 15, 4:14 pm, hel...@astro.multiCLOTHESvax.de (Phillip Helbig--- remove CLOTHES to reply) wrote: > In article , Max > > writes: > > For Modified Newtonian Dynamics proposed by Milgrom, there is a > > deviation from the classical gravitational force (or a deviation from > > the classical Newtonian dynamics, depending on your perspective) once > > the acceleration of a particle is less than a0. Thus one can account > > for the star motion on the outer fringe of galaxies without invoking > > dark matter. > > > One interesting observation is that a0 is also the acceleration scale > > where dark energy should play a significant role. My question is why > > dark energy is not usually considered for intra-galactic (non- > > cosmological) dynamics with less-than-a0 acceleration? Would dark > > energy change the predictions of MOND? > > Reason 1: The motivation for MOND was to avoid dark matter. Not > believing in dark matter but believing in dark energy is not really an > option for someone who doesn't want to believe in dark matter. Reason > 2: No-one knows what dark energy is or what its behaviour is like in > detail, especially at small (intra-galactic) scales. (We see > unambiguous effects only at large (cosmological) scales.) Reason 3: > MOND was developed mainly to explain the observed flat rotation curves > of galaxies, which is not what one expects if the mass distribution > follows the light distribution. At least phenomenologically, MOND > offers a simple explanation. An explanation involving dark energy would > be contrived.
 Max wrote on Sun, 14 Dec 2008 21:59:07 +0100: > For Modified Newtonian Dynamics proposed by Milgrom, there is a > deviation from the classical gravitational force (or a deviation from > the classical Newtonian dynamics, depending on your perspective) The pictures are equivalent regarding usual MOND applications. > once > the acceleration of a particle is less than a0. Thus one can account > for the star motion on the outer fringe of galaxies without invoking > dark matter. MOND also is useful for some aspects of cosmology. E.g. WMAP data (second peak?) was predicted by one MOND theoretician before observation, whereas dark matter theorists were forced to change their wrong initial estimations once the data was known. You may find histories as this and lot of other useful information in http://www.astro.umd.edu/~ssm/mond/ > One interesting observation is that a0 is also the acceleration scale > where dark energy should play a significant role. Hum, not still sure about this. > My question is why > dark energy is not usually considered for intra-galactic (non- > cosmological) dynamics with less-than-a0 acceleration? Because due to small value of cosmological constant the effects can be ignored for both regimes a >> a0 and a << a0. > Would dark energy > change the predictions of MOND? Yes. Notice that MOND reduces to Newtonian theory for a >> a0. However, the presence of a cosmological constant changes the equations, for example the Poisson equation for the Newtonian potential may be modified in presence of cosmological constant. MOND was not designed for cosmology but for galactic scale. -- http://www.canonicalscience.org/ Usenet Guidelines: http://www.canonicalscience.org/en/m...uidelines.html
 In article , juanREMOVE@canonicalscience.com writes: > MOND also is useful for some aspects of cosmology. E.g. WMAP data > (second peak?) was predicted by one MOND theoretician before > observation, whereas dark matter theorists were forced to change their > wrong initial estimations once the data was known. I think this is stretching it a bit. Yes, on the surface the claim is true. However, IIRC there was no "MOND cosmology" involved in calculating the power spectrum, but rather a standard calculation was done using a value for Omega consistent with what MOND folks believe (i.e. no dark matter). Whether one NEEDS MOND cosmology to do a comparison between CMB theory and observations is a different question. My point is simply that the "predicted" power spectrum (i.e. the second peak) is a standard one---there are so many parameters that there is more than one way to get it---which happens to agree with MOND in a superficial sense. It's also too harsh to say "dark matter theorists were forced to change their wrong initial estimations once the data was known". This is almost a definition of good (perhaps not canonical :-) ) science: revise one's view of the world as more and better data come in. Yes, the WMAP data aren't compatible with cardboard models, but it wasn't like everyone (except MOND folks) believed these cardboard models, then WMAP caused "the cosmology textbooks to be rewritten". One CAN explain WMAP within the context of standard cosmology. Yes, the reason for this is that there are many parameters one can adjust (the fact that MOND just has one parameter makes it a good theory, since it makes clear predictions), but the parameter combination which agrees with WMAP also agrees with essentially everything else. > You may find histories as this and lot of other useful information in > > http://www.astro.umd.edu/~ssm/mond/ Yes, a good source of information.
 Max wrote on Sun, 14 Dec 2008 21:59:07 +0100: > For Modified Newtonian Dynamics proposed by Milgrom, there is a > deviation from the classical gravitational force (or a deviation from > the classical Newtonian dynamics, depending on your perspective) The pictures are equivalent regarding usual MOND applications. > once > the acceleration of a particle is less than a0. Thus one can account > for the star motion on the outer fringe of galaxies without invoking > dark matter. MOND also is useful for some aspects of cosmology. E.g. WMAP data (second peak?) was predicted by one MOND theoretician before observation, whereas dark matter theorists were forced to change their wrong initial estimations once the data was known. You may find histories as this and lot of other useful information in http://www.astro.umd.edu/~ssm/mond/ > One interesting observation is that a0 is also the acceleration scale > where dark energy should play a significant role. Hum, not still sure about this. > My question is why > dark energy is not usually considered for intra-galactic (non- > cosmological) dynamics with less-than-a0 acceleration? Because due to small value of cosmological constant the effects can be ignored for both regimes a >> a0 and a << a0. > Would dark energy > change the predictions of MOND? Yes. Notice that MOND reduces to Newtonian theory for a >> a0. However, the presence of a cosmological constant changes the equations, for example the Poisson equation for the Newtonian potential may be modified in presence of cosmological constant. MOND was not designed for cosmology but for galactic scale. -- http://www.canonicalscience.org/ Usenet Guidelines: http://www.canonicalscience.org/en/m...uidelines.html
 Phillip Helbig---remove CLOTHES to reply wrote on Tue, 23 Dec 2008 11:37:28 +0100: > In article , > juanREMOVE@canonicalscience.com writes: > >> MOND also is useful for some aspects of cosmology. E.g. WMAP data >> (second peak?) was predicted by one MOND theoretician before >> observation, whereas dark matter theorists were forced to change >> their wrong initial estimations once the data was known. > > I think this is stretching it a bit. Yes, on the surface the claim is > true. However, IIRC there was no "MOND cosmology" involved in > calculating the power spectrum, but rather a standard calculation was > done using a value for Omega consistent with what MOND folks believe > (i.e. no dark matter). You say is not accurate, lacking a satisfactory relativistic MOND theory, the "MOND cosmological solution" was approximated by a conventional model with no CDM. It was claimed this *approximated* MOND would fail at some level but would be enough for an estimation for the amplitude ratios of the peaks http://xxx.lanl.gov/abs/astro-ph/9907409 http://xxx.lanl.gov/abs/astro-ph/0008188 > Whether one NEEDS MOND cosmology to do a comparison between CMB theory > and observations is a different question. My point is simply that the > "predicted" power spectrum (i.e. the second peak) is a standard > one---there are so many parameters that there is more than one way to > get it---which happens to agree with MOND in a superficial sense. > > It's also too harsh to say "dark matter theorists were forced to > change their wrong initial estimations once the data was known". A beatiful aspect of science is that claims and prediction are archived. The *prediction* done by dark matter theorists using the 1999 Lambda-CDM model is reproduced in the left image at http://www.astro.umd.edu/~ssm/mond/CMB1.html At the right you can see the prediction done by a MOND theorist using a purely baryonic (MOND) model. Both predictions were done before the data was known. The data (green dots) showed excellent agreement with MOND theoreticians prediction. That is already part of history of science :-) Once the data was known, dark matter theoreticians changed the original parameters in the Lambda-CDM model to *fit* the observed data. Two essential remarks: i) MOND theoreticians predicted the right data, *before* observation. Dark matter theoreticians prediction failed and now they only fit the right data, *after* observation, using a many parameters model. ii) Why do dark matter theoreticians failed at their prediction? You seems to ignore this important point. Initially they used standard values for the parameters in the Lambda-CDM model. After this failed they assumed nonstandard values for parameters for fitting the observed data. For instance, the barion density used in Lambda-CDM models is open to objections (inconsistencies with densities given by other tests). Whereas MOND solution uses a more consistent barion density. http://xxx.lanl.gov/abs/astro-ph/0312570 Taking into account that MOND was *not* initially developed for cosmology but for galactic phenomena, its partial aplicability to cosmology is really surprising, at least for some of us. -- http://www.canonicalscience.org/ Usenet Guidelines: http://www.canonicalscience.org/en/m...uidelines.html
 In article , "Juan R. González-Álvarez" writes: > Phillip Helbig---remove CLOTHES to reply wrote on Tue, 23 Dec 2008 > 11:37:28 +0100: > > > In article , > > juanREMOVE@canonicalscience.com writes: > > > >> MOND also is useful for some aspects of cosmology. E.g. WMAP data > >> (second peak?) was predicted by one MOND theoretician before > >> observation, whereas dark matter theorists were forced to change > >> their wrong initial estimations once the data was known. > > > > I think this is stretching it a bit. Yes, on the surface the claim is > > true. However, IIRC there was no "MOND cosmology" involved in > > calculating the power spectrum, but rather a standard calculation was > > done using a value for Omega consistent with what MOND folks believe > > (i.e. no dark matter). > > You say is not accurate, lacking a satisfactory relativistic MOND > theory, the "MOND cosmological solution" was approximated by a > conventional model with no CDM. It was claimed this *approximated* MOND > would fail at some level but would be enough for an estimation for the > amplitude ratios of the peaks I don't really see the difference between our two statements. > > Whether one NEEDS MOND cosmology to do a comparison between CMB theory > > and observations is a different question. My point is simply that the > > "predicted" power spectrum (i.e. the second peak) is a standard > > one---there are so many parameters that there is more than one way to > > get it---which happens to agree with MOND in a superficial sense. > > > > It's also too harsh to say "dark matter theorists were forced to > > change their wrong initial estimations once the data was known". > > A beatiful aspect of science is that claims and prediction are archived. > > The *prediction* done by dark matter theorists using the 1999 Lambda-CDM > model is reproduced in the left image at Yes, but even in 1999 there were many other predictions, namely for different values of the parameters entering into the calculation, which one can vary within the known uncertainties. Of course, the author has an axe to grind (and would be the first to admit it), so of course a prediction from standard cosmology was chosen which highlights its shortcomings. > At the right you can see the prediction done by a MOND theorist using a > purely baryonic (MOND) model. Which differs not at all from a standard cosmological prediction with no CDM. Calling it MOND is stretching it. Yes, MOND (usually) has no (need for) CDM, but not everything with no CDM is MOND. In particular, there is no "MOND physics" involved in this "MOND prediction". I'm not saying that this is unjustified; maybe it is a good approximation to what MOND cosmology would offer, but until there is a MOND cosmology, no-one can say. > Both predictions were done before the data was known. Yes, otherwise they wouldn't be predictions. > Once the data was known, dark matter theoreticians changed the original > parameters in the Lambda-CDM model to *fit* the observed data. This is what scientists do. The whole point of fitting models to CMB data is to find out what the parameters are. If they were known for certain beforehand, we wouldn't need to do CMB observations to determine them. > i) > MOND theoreticians predicted the right data, *before* observation. Dark > matter theoreticians prediction failed and now they only fit the right > data, *after* observation, using a many parameters model. First, as already noted, the MOND prediction has little to do with MOND directly. Second, it's in a sense unfair to disadvantage a theory just because it has more free parameters. Yes, I think it is impressive that MOND, with only one free parameter, isn't ruled out, but as always one can't prove a theory, only disprove it. It's also unfair to compare the MOND prediction to a standard prediction which turned out to be wrong; after all, there were other standard predictions (and no, there aren't enough parameters that one can fit everything). As Bill Press said a few years ago, someone knows the value of the Hubble constant exactly; we just don't know who that person is. :-) > ii) > Why do dark matter theoreticians failed at their prediction? You seems > to ignore this important point. There were many predictions; some fit the data, others don't. Thus, some are eliminated, others are still around. Where is the failure? Failure would be if NO standard models could fit the data, or if ad-hoc ideas (i.e. ideas which no-one had mentioned before) had to be introduced to fit the data. Neither is the case. > Initially they used standard values for the parameters in the Lambda-CDM > model. After this failed they assumed nonstandard values for parameters > for fitting the observed data. Yes, in a sense, but the "standard values" were partly chosen because, mathematically, they were easy to work with. (Yes, a few people took them as gospel, but they were in a minority even back in the 1990s.) As long as they fit the then-known data, why use others? But the very fact that other parameter combinations were seen to fit the data shows that these standard values were just a working hypothesis (and predictions for all kinds of parameter combinations were done long before the data were in).
 On Dec 16, 7:42=A0pm, hel...@astro.multiCLOTHESvax.de (Phillip Helbig--- remove CLOTHES to reply) wrote: > In article , eric giss= e writes: > > Regardless, returning to dark energy. It does not appear to have any > > influence on galaxies as the galaxies are gravitationally bound and > > dark energy isn't strong enough to have a significant impact. > > In the conventional form (and there is no evidence that this is not the > case), dark energy is distributed completely smoothly. =A0In that case, I > believe (someone correct me if I am wrong) that the only effect it could > have would be to change the size of a gravitationally bound structure. = =A0 > To notice this, we would have to compare expected and observed sizes, > probably at a variety of scales. =A0The data are too messy to note any > deviations, at least for the value of the cosmological constant which we > believe we know from cosmological observations. Hmm, the "gravitationally bound" argument is traditionally used to explain (away) our failure to observe expansion, as opposed to acceleration in that expansion, on the scales of both the solar system and the Milky Way. This would seem to imply that the data _would_ be clean enough to detect that, if it actually happened on these scales. Since Anderson et al. used NASA radiometric ranging data to demonstrate that the Pioneer acceleration anomaly is not also experienced by planets, I would guess that this is also the preferred method for demonstrating that Hubble expansion does not actually happen on the scale of the solar system. However, I can't see that working on the scale of our local galaxy, so would appreciate clarification on what the data actually is, for the galaxy, if anyone can fill me in on this. It seems to me that there are similarities in the considerations for observing expansion and its acceleration, locally (except, of course, that the effects of Hubble expansion should be much, much easier to detect). It also seems to me that, unless the gravitationally bound argument can predict the physics in the transition region, and where that region actually is, one might be justified in viewing that argument with a modicum of healthy scepticism.
 In article , Chalky writes: > On Dec 16, 7:42 pm, hel...@astro.multiCLOTHESvax.de (Phillip Helbig--- > remove CLOTHES to reply) wrote: > > In article , > > eric gisse writes: > > > > Regardless, returning to dark energy. It does not appear to have any > > > influence on galaxies as the galaxies are gravitationally bound and > > > dark energy isn't strong enough to have a significant impact. > > > > In the conventional form (and there is no evidence that this is not the > > case), dark energy is distributed completely smoothly. In that case, I > > believe (someone correct me if I am wrong) that the only effect it could > > have would be to change the size of a gravitationally bound structure. > > > To notice this, we would have to compare expected and observed sizes, > > probably at a variety of scales. The data are too messy to note any > > deviations, at least for the value of the cosmological constant which we > > believe we know from cosmological observations. > > Hmm, the "gravitationally bound" argument is traditionally used to > explain (away) our failure to observe expansion, as opposed to > acceleration in that expansion, on the scales of both the solar system > and the Milky Way. Yes. > This would seem to imply that the data _would_ be > clean enough to detect that, if it actually happened on these scales. In principle, yes, but in practice the "expected size" mentioned above is not known precisely enough. The real universe is messy. There is a paper by Lineweaver et al. which discusses this---I don't have the reference right now, but can dig it up if necessary. They show that while the expansion of the universe doesn't cause bound objects to expand, it does change their equilibrium size (very slightly). A change in the rate of expansion would in turn change this. But the effect is too small to be observed. (Note that the cosmological redshift of an object, in general, is not constant with time. Spectrographs are now becoming so precise that the change in the cosmological redshift can be noticed on a timescale of years or decades. Not long ago, this was also an effect which was there in principle but not measurable in practice.) > Since Anderson et al. used NASA radiometric ranging data to > demonstrate that the Pioneer acceleration anomaly is not also > experienced by planets, I would guess that this is also the preferred > method for demonstrating that Hubble expansion does not actually > happen on the scale of the solar system. However, I can't see that > working on the scale of our local galaxy, so would appreciate > clarification on what the data actually is, for the galaxy, if anyone > can fill me in on this. Again, the real universe is messy. Assuming the galaxy DID expand with the Hubble flow, this would mean a typical difference in stellar positions of the order of one-tenth the size of the Earth's orbit over the course of a year. Thus, a direct measurement doesn't seem possible, since the effect would be swamped by other effects. > It also seems to me that, unless the gravitationally bound argument > can predict the physics in the transition region, and where that > region actually is, one might be justified in viewing that argument > with a modicum of healthy scepticism. Why do you think there is any confusion about the transition region?
 On Dec 28, 7:19 pm, hel...@astro.multiCLOTHESvax.de (Phillip Helbig--- remove CLOTHES to reply) wrote: > (Note that the cosmological redshift of an object, in general, is not > constant with time. It is certainly nothing like constant within the context of GR, that's for sure. That is one reason why the relevant calculations are sufficiently tedious and long winded, to need a computer. > Spectrographs are now becoming so precise that the > change in the cosmological redshift can be noticed on a timescale of > years or decades. Are you talking here about changes in the cosmological redshift of the same object, when viewed years or decades later? If so, this development sounds particularly interesting. > > It also seems to me that, unless the gravitationally bound argument > > can predict the physics in the transition region, and where that > > region actually is, one might be justified in viewing that argument > > with a modicum of healthy scepticism. > Why do you think there is any confusion about the transition region? 1) Because it wasn't that long ago, relative to the lifetime of GR (and of myself), when many physicists thought the entire universe might be gravitationally bound, in the sense of debating whether it would eventually collapse on itself, under the influence of gravity. 2) Because I have never seen or heard of such an analysis, and am myself confused about how one would approach that particular problem in dynamics.
 In article <76a8cd14-7687-478f-a8a9-c502c261e48e@o40g2000prn.googlegroups.com>, Chalky writes: > On Dec 28, 7:19 pm, hel...@astro.multiCLOTHESvax.de (Phillip Helbig--- > remove CLOTHES to reply) wrote: > > > (Note that the cosmological redshift of an object, in general, is not > > constant with time. > > It is certainly nothing like constant within the context of GR, that's > for sure. That is one reason why the relevant calculations are > sufficiently tedious and long winded, to need a computer. Actually, almost all calculations assume it is constant in time. It is calculated for one time, NOW. I'm not referring to objects at different distances. > > Spectrographs are now becoming so precise that the > > change in the cosmological redshift can be noticed on a timescale of > > years or decades. > > Are you talking here about changes in the cosmological redshift of the > same object, when viewed years or decades later? If so, this > development sounds particularly interesting. Yes. > > > It also seems to me that, unless the gravitationally bound argument > > > can predict the physics in the transition region, and where that > > > region actually is, one might be justified in viewing that argument > > > with a modicum of healthy scepticism. > > > Why do you think there is any confusion about the transition region? > > 1) Because it wasn't that long ago, relative to the lifetime of GR > (and of myself), when many physicists thought the entire universe > might be gravitationally bound, in the sense of debating whether it > would eventually collapse on itself, under the influence of gravity. This is not really the same as being gravitationally bound in the traditional sense. Is the universe a black hole if a sufficient density is reached? Not in any meaningful sense. If you put in the numbers, it might look like that, but that's more a consequence of dimensional analysis.
 On Dec 30, 10:00=A0am, hel...@astro.multiCLOTHESvax.de (Phillip Helbig--- remove CLOTHES to reply) wrote: > > > Why do you think there is any confusion about the transition region? > > > 1) Because it wasn't =A0that long ago, relative to the lifetime of GR > > (and of myself), when many physicists thought the entire universe > > might be gravitationally bound, in the sense of debating whether it > > would eventually collapse on itself, under the influence of gravity. > > This is not really the same as being gravitationally bound in the > traditional sense. Then, perhaps, you should explain what this "traditional sense" is. To be precise, what is the traditional time limit for deciding that, if a receding object has not returned in that time, it will never return /and/or is not gravitationally bound?
 Phillip Helbig---remove CLOTHES to reply wrote: [[bound systems don't participate in the overall Hubble expansion]] > In principle, yes, but in practice the "expected size" mentioned above > is not known precisely enough. The real universe is messy. There is a > paper by Lineweaver et al. which discusses this---I don't have the > reference right now, but can dig it up if necessary. They show that > while the expansion of the universe doesn't cause bound objects to > expand, it does change their equilibrium size (very slightly). A change > in the rate of expansion would in turn change this. But the effect is > too small to be observed. You may be referring to the (excellent) paper Tamara M. Davis, Charles H. Lineweaver, John K. Webb "Solutions to the tethered galaxy problem in an expanding universe and the observation of receding blueshifted objects" http://arxiv.org/abs/astro-ph/0104349 American J. Physics 71 (2003) 358-364 My personal favorite paper along these lines is Richard H. Price "In an expanding universe, what doesn't expand?" http://arxiv.org/abs/gr-qc/0508052 I think this was also published in American J. Physics, but I don't have the reference. Finally, I should also point out Tamara M. Davis, Charles H. Lineweaver "Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe" http://arxiv.org/abs/astro-ph/0310808 -- -- From: "Jonathan Thornburg [remove -animal to reply]" Dept of Astronomy, Indiana University, Bloomington, Indiana, USA "Washing one's hands of the conflict between the powerful and the powerless means to side with the powerful, not to be neutral." -- quote by Freire / poster by Oxfam

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