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Geometry of Look-Back
Not much work seems to be done nowadays on open (aka hyperbolic) or
closed (aka spherical) manifolds, but it's instructive to consider what they would look like as a nightsky. The answer is they would look just the same as flat space, but if one were to hop in a rocket and travel out there, one would find that objects are closer than their angular size indicates in an open manifold (i.e., "foreshortened"), and in a closed manifold they would be further away than expected. So nowadays we model that we can't visually distinguish at all between these alternative curvatures. But spectroscopy illustrates how nature finds ways to convey information -- astronomers of 100 years ago would be astonished at how much signal there is in mere light. My supposition is that there is indeed a visual way to distinguish between open, flat, and closed manifolds, and that the cosmological redshift shows us the way. Regardless of all the complex constructs of standard cosmology, the simple anchor is that cosmological redshift results from recession. No recession, no big bang. So alternative viewpoints of the redshift are not welcome to some -- which is no reason not to try, of course. Lopez-Corredoira gave a useful review of static models in his paper "Angular Size Test on the Expansion of the Universe" (2010 IJMP,19,245; arxiv:1002.0525) and observed (as have others) that 1/z is well-fit to angular size across all redshifts -- without need of evolution, dark matter, dark energy, whatever. Occam is calling. So these are threads for me to follow, hopefully to assemble into a coherent whole, after the holidays. cheers, Eric. |
#2
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Geometry of Look-Back
In article , Eric Flesch
writes: Not much work seems to be done nowadays on open (aka hyperbolic) or closed (aka spherical) manifolds, What sort of work should be done? Observations indicate that the universe is at least very close to spatial flatness, which is probably why these models aren't mentioned very much these days. but it's instructive to consider what they would look like as a nightsky. The answer is they would look just the same as flat space, but if one were to hop in a rocket and travel out there, one would find that objects are closer than their angular size indicates in an open manifold (i.e., "foreshortened"), and in a closed manifold they would be further away than expected. Not just geometry but also expansion history determines the relation between the angular and physical size of an object, for a given redshift. So nowadays we model that we can't visually distinguish at all between these alternative curvatures. The whole point of classical cosmology is to distinguish between such models. But spectroscopy illustrates how nature finds ways to convey information -- astronomers of 100 years ago would be astonished at how much signal there is in mere light. My supposition is that there is indeed a visual way to distinguish between open, flat, and closed manifolds, and that the cosmological redshift shows us the way. OK, classical cosmology also makes use of the redshift. Regardless of all the complex constructs of standard cosmology, the simple anchor is that cosmological redshift results from recession. No recession, no big bang. OK. So alternative viewpoints of the redshift are not welcome to some -- which is no reason not to try, of course. You make it sound like the big bang is an assumption, but actually it is a conclusion. Lopez-Corredoira gave a useful review of static models in his paper "Angular Size Test on the Expansion of the Universe" (2010 IJMP,19,245; arxiv:1002.0525) and observed (as have others) that 1/z is well-fit to angular size across all redshifts -- without need of evolution, dark matter, dark energy, whatever. Occam is calling. The question is whether, within the observational errors, one can show that 1/z is a better fit than, say, the current standard cosmological model. Measuring angular size is easy. The hard part is determining what physical size it corresponds to. This classical test has, due to observational (not theoretical) difficulties not produced anything useful up until now. (In some sense, CMB measurements are an angular-size test, though.) So these are threads for me to follow, hopefully to assemble into a coherent whole, after the holidays. cheers, Eric. We'll be looking for some testable predictions. |
#3
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Geometry of Look-Back
In article ,
Phillip Helbig---undress to reply writes: The question is whether, within the observational errors, one can show that 1/z is a better fit than, say, the current standard cosmological model. Exactly so. Substantial data sets of supernova distance moduli are published, so at least a first test shouldn't be hard. The DMs are luminosity distances, so one has to use some theory to convert to whatever distance 1/z is supposed to represent. Measuring angular size is easy. In principle, at least. Not always so in practice. The hard part is determining what physical size it corresponds to. This classical test has, due to observational (not theoretical) difficulties not produced anything useful up until now. (In some sense, CMB measurements are an angular-size test, though.) Why only "in some sense?" I thought they were exactly an angular size test. Also baryon acoustic oscillations (BAO). So far, both are consistent with standard cosmology. Aren't CMB fluctuations one of the reasons the standard cosmology is standard? In the standard cosmology, angular size distance _decreases_ as redshift increases beyond a certain value (around z=1.9 or so). If 1/z is the angular size distance, I wouldn't expect it to come anywhere close to fitting beyond that even if it fits OK at smaller redshifts. But there's no substitute for comparison with data. -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
#4
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Geometry of Look-Back
On Tue, 18 Dec 12, Steve Willner wrote:
Exactly so. Substantial data sets of supernova distance moduli are published, so at least a first test shouldn't be hard. The DMs are luminosity distances, so one has to use some theory to convert to whatever distance 1/z is supposed to represent. A difficulty is that SN DMs are presented with the assumption that redshift represents time dilation of the SNe. That this yields that the most distant SNe have sub-par luminosities seems to ring no alarm bells amongst the researchers. One of the models that I'm juggling treats time dilation as the square root of the redshift, which would restore the expected luminosities of the farthest SNe. If raw SNe data were published instead of the redshift-processed stuff, that would be a boon to testing alternative models. Measuring angular size is easy. In principle, at least. Not always so in practice. Indeed, just today there's a new preprint http://arxiv.org/abs/1212.3869 , "Evolution of the Sizes of Galaxies over 7z12 Revealed by HUDF", in which the highest-z galaxies look unresolved. They jump through hoops to resolve them, but it reminds me of the "elliptical hosts" of many hi-z quasars which also look unresolved. And in that paper they don't publish the raw angular sizes, it's all presented as kPc after processing using the FRW model. Oh, for just one graph of raw angular size vs z !! Eric |
#5
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Geometry of Look-Back
In article , Steve Willner
writes: The hard part is determining what physical size it corresponds to. This classical test has, due to observational (not theoretical) difficulties not produced anything useful up until now. (In some sense, CMB measurements are an angular-size test, though.) Why only "in some sense?" I thought they were exactly an angular size test. Also baryon acoustic oscillations (BAO). So far, both are consistent with standard cosmology. Aren't CMB fluctuations one of the reasons the standard cosmology is standard? Yes. By "in some sense" I mean that there are other parameters involved. In other words, the physical length of the "standard rod" is not known in advance, but depends on some other parameters. |
#6
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Geometry of Look-Back
In article ,
Eric Flesch writes: A difficulty is that SN DMs are presented with the assumption that redshift represents time dilation of the SNe. Assumption? Light curves are consistent with time scales multiplied by 1+z. Anything very different is inconsistent with the data. That this yields that the most distant SNe have sub-par luminosities seems to ring no alarm bells amongst the researchers. Introducing an non-zero cosmological constant, when nearly everyone up to then was convinced it was zero, isn't an "alarm bell?" One of the models that I'm juggling treats time dilation as the square root of the redshift, Easily ruled out by the data. If raw SNe data were published instead of the redshift-processed stuff, Raw data are published. For example, Astier et al. (2006 A&A 447, 31) give references to lots of light curves for nearby SNe. Reiss et al. (2004 ApJ 607, 665; 2007 ApJ 659, 98) give light curves for z1 SNe. There are lots of others, though nowadays many groups publish only distance moduli, not full light curves. It would take some searching around to find more light curves, but quite a few are available. You could probably get more if you asked for them. -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
#7
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Geometry of Look-Back
In article , Steve Willner
writes: That this yields that the most distant SNe have sub-par luminosities seems to ring no alarm bells amongst the researchers. Introducing an non-zero cosmological constant, when nearly everyone up to then was convinced it was zero, isn't an "alarm bell?" Indeed! Note also that the currently favoured value for the cosmological constant predicts not just a dimming with redshift but, at large redshift, a brightening. This is a very specific prediction, and hard to get from other models of the dimming. One of the models that I'm juggling treats time dilation as the square root of the redshift, Is there any physical motivation for this? There is a very clear physical motivation for multiplying by (1+z). It would take some searching around to find more light curves, but quite a few are available. You could probably get more if you asked for them. These days, it isn't possible to publish all data in a paper journal. However, probably most of the stuff is available, even back to the raw data, either by asking or due to some observatory policy which makes all data public after a certain time. Since only a minority need such data, and when they do, probably in electronic form, I think it is OK not to publish it conventionally. |
#8
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Geometry of Look-Back
On Sat, 22 Dec 12, Phillip Helbig wrote:
I wrote: One of the models that I'm juggling treats time dilation as the square root of the redshift, Is there any physical motivation for this? Yes, one model of "geometry of look-back" is that we see the past as smaller and slower than the present, because of the drift of a (new) cosmological factor. This maps into seeing the nightsky as an open-manifold universe with a redshift. So this proposes to swap this one new cosmological factor for all of yours (dark energy, etc, you know what they all are). But I have to well-fit this to all observations, which is daunting for me, since so much observational data is published only as post-FRW-processed data, which is hard for me to decode backwards. I may indeed have to do as you and Steve Willner kindly suggest, which is to request the original data from the authors. I remind all that the usual riposte to the "many worlds" advocates is that we prefer to economize on universes. Similarly, I wish to economize on all the magic tropes of modern cosmology, and remind all that things flying apart at high speed is no way to model a universe. And happy holidays! :-) |
#9
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Geometry of Look-Back
In article , Eric Flesch
writes: One of the models that I'm juggling treats time dilation as the square root of the redshift, Is there any physical motivation for this? Yes, one model of "geometry of look-back" is that we see the past as smaller and slower than the present, because of the drift of a (new) cosmological factor. Unless there is some physical motivation for this OTHER THAN explaining the observations, this seems a rather ad-hoc solution. This maps into seeing the nightsky as an open-manifold universe with a redshift. So this proposes to swap this one new cosmological factor for all of yours (dark energy, etc, you know what they all are). All? The only thing remotely strange is dark energy, better known as the cosmological constant, and mathematically that has been around for 100 years. Interesting that when observations indicated a slightly more complicated universe, it turned out that 1920s cosmology already had a solution. Dark matter? If that is strange, then that means that the default assumption is that all matter glows, which seems strange to say the least. Non-baryonic matter, meaning most of the universe is made out of something we are not? Is that strange? Most of the matter we know about is in stars, but we ourselves are not stars, and no-one finds that strange. But I have to well-fit this to all observations, which is daunting for me, since so much observational data is published only as post-FRW-processed data, which is hard for me to decode backwards. Not only that, but often interpreted in the light of a certain FRW model. I may indeed have to do as you and Steve Willner kindly suggest, which is to request the original data from the authors. I don't think that will be a problem. Many data are available today even without asking---usually not directly in publications, but in online resources mentioned there. Similarly, I wish to economize on all the magic tropes of modern cosmology, Again, modern cosmology is surprisingly boring. Recently, the 9-year WMAP papers appeared on arXiv. A huge amount of data, and no indications that we need to revise our cosmological model. In particular, the large-scale model is, again, 1920s cosmology. and remind all that things flying apart at high speed is no way to model a universe. Unless you have a really, really, really different theory of gravity, you have to explain the stability of the universe if it is not flying apart. |
#10
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Geometry of Look-Back
In article ,
Phillip Helbig---undress to reply writes: Unless you have a really, really, really different theory of gravity, you have to explain the stability of the universe if it is not flying apart. Collapsing would be OK, too. Phillip knows that but didn't mention it because it's contrary to observations. The point is that a static universe might be in equilibrium, but it is unstable unless one puts in new physics. Of course we've seen weird physics turn out to be right in some cases (QM comes to mind!), but GR works so well on small scales that new physics in that realm looks unlikely. Nevertheless, if a new model fits the data (and isn't grossly contrived with a huge number of free parameters), I'd expect people to consider it. -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
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