Do voids in the Universe influence the orientation of galaxies?

[turbo]

CDM was first published in 1984. You didn't see observations of voids until 1989.

Please show me how CDM predicted voids a billion light-years across, much less the 3.5 Gly void turned up by the 6dF Galaxy Survey (Very current science as surveys go.) Fitting models to observations retroactively is SOP for cosmologists, and it is not a sign that the field is well-developed, much less mature. I won't go as far as Michael Disney in deriding the field, but I agree with him that there are some serious weaknesses regarding cosmology as a hard science.

I do have a dog in this fight, since my collaborators and I are studying and publishing about redshift distributions in interacting galaxies. It is enlightening to find out how strongly theory and politics can trump observations, and even inhibit observations to the point where decent statistical analyses are difficult to perform due to a paucity of observations. It would shock you to find out how many relatively bright galaxies have no published redshift data simply because they are part of an interacting system. Arp's banishment has had a chilling effect for decades.

 

[twofish-quant]

Please show me how CDM predicted voids a billion light-years across, much less the 3.5 Gly void turned up by the 6dF Galaxy Survey (Very current science as surveys go.)

CDM is a framework. You can use cmbfast to set the parameters to get you those voids. You might complain that you can adjust the parameters to get realistic looking power spectrum, but I don't know of any other framework that gets you even that far.

Fitting models to observations retroactively is SOP for cosmologists, and it is not a sign that the field is well-developed, much less mature.

1) I thought science was all about fitting models to observations retroactively.

2) Well cosmology *isn't* a particularly well-developed, mature field. It's not like say classical mechanics or electromagnetism where we think we pretty much understand everything, or even particle physics were it's been at least a decade since anyone has observed anything that doesn't fit in our models.

The fact that we are invoking inflation, dark matter, and dark energy to get models that approach observations is a clear sign that we really don't know what's going on. And this is a problem because?

I do have a dog in this fight, since my collaborators and I are studying and publishing about redshift distributions in interacting galaxies. It is enlightening to find out how strongly theory and politics can trump observations, and even inhibit observations to the point where decent statistical analyses are difficult to perform due to a paucity of observations.

You seem to be inconsistent here. You were just complaining that people are modifying their theories to fit observations, and now you are talking about how theory trumps observations.

I don't understand this talk of lack of statistical analysis. Most of the evidence in favor of CDM involves coorelation functions and power spectrum, and I don't see a lack of observations. I'd be interested in hearing what objections you have to the statistics that is used for galactic observations, but I don't think that you can plausibly argue that that don't exist or that there is a lack of data. I'd like to see you try...

It would shock you to find out how many relatively bright galaxies have no published redshift data simply because they are part of an interacting system. Arp's banishment has had a chilling effect for decades.

The trouble is that if you take small numbers of bright galaxies, you just don't end up with enough data to get decent coorelation functions and power spectrum, and then you end up wondering about selection effects. The trend over the last decade or two has been to take massive number of measurements so that any selection effects are overwhelmed by sheer numbers. If you don't have a human being in the loop deciding which galaxies to measure and which one's not to, then it's much, much easier to model statistical bias and get good statistics. Once you put a human being in the loop to decide what to measure and what not to measure, it becomes more difficult.

But the idea that we don't have enough statistics and observations to do cosmology is one that I find very, very odd.

The other thing is that if your argument is that we ought to be interested in interacting galactic systems because the redshifts suggest that something weird is happening there, that that suggests to me that when doing large scale cosmological surveys that one should try to *exclude* those galaxies, since whatever is causes weird stuff in those system is going to bias the thing that people are interested in studying.

Also, if you do have reason to believe that "weird stuff" is happening in an interacting cluster, then at that point I think that it's a bad idea to use statistics because since you don't know what the "weird stuff" is or if it is the same "weird stuff" and you'll end up mixing apples and oranges and not figuring out what is going on.

 

[Astronuc]

Perhaps it would be worthwhile to look at James (Jim) Peebles text, Large-Scale Structure of the Universe, Princeton University Press, 1980 to see where the understanding (models) was then (ca 1970's) and where we are now.

https://www.amazon.com/gp/product/0691082405/?tag=pfamazon01-20

 

[turbo]

Perhaps it would be worthwhile to look at James (Jim) Peebles text, Large-Scale Structure of the Universe, Princeton University Press, 1980 to see where the understanding (models) was then (ca 1970's) and where we are now.

https://www.amazon.com/gp/product/0691082405/?tag=pfamazon01-20

Thanks for that link. The first chapter lays out the history of the study of large-scale structure up to the 1970's, which is handy. It seems that cosmologists were expecting homogeneity at some large scale, but kept discovering clumping at larger and larger scales as time went by. Things haven't changed much, since we keep finding structures.

6dF turned up a void 3.5 billion light-years across, so there is currently evidence of large-scale structure at least that large, and there may be much larger structures that we can observe, given improvements in instrumentation, including better optics and greater detector sensitivity. With voids framed by filaments and walls of galaxy clusters, the distribution of matter in space takes on a foamy appearance. A challenge for modern cosmology could arise from the discovery of structures on larger and larger scales, because the formation of structure through gravitational accretion takes time, and the Big Bang theory contains a self-imposed limit on the time available in which structure can form.

 

[twofish-quant]

A challenge for modern cosmology could arise from the discovery of structures on larger and larger scales, because the formation of structure through gravitational accretion takes time, and the Big Bang theory contains a self-imposed limit on the time available in which structure can form.

True, and one reason that we have "warm fuzzies" about the big bang is that there is this observed massive cutoff in power spectra at very large angles. One reason why we think that "dark matter" exists is that if it didn't, then then cutoff for structure happens way, way below where we observationally observe it. If you don't have dark matter, the universe expands quickly, and you don't have time for structure to form. The more dark matter there is the universe, the slower the universe expands and the more time there is for structures to form.

From the location of the cutoff, you can calculate where to set the parameters for the LCDM model, and you then get "warm fuzzy feelings" because where you set those parameters from structure happens to be where you set them up from completely different calculations (such as big bang nucleosynthesis). Basically by looking at the distribution of galaxies, you can figure out how much dark matter there is in the universe, and if that number matches the other estimates of dark matter, then you think you have some clue about what's going on. If you have different estimates of dark matter that give wildly different numbers, then there is a good chance that your basic model is wrong.

Also, one big advance is that we now have cheap computers in which we can use with LCDM to make very, very detailed predictions about what the universe looks like.

One big problem with popular descriptions about what is going on is that without numbers these sorts of arguments are impossible. Yes there is a cutoff in the big bang. If you talk about power spectra, you can figure out where that cutoff is, and if it is consistent with observations.

 

[twofish-quant]

6dF turned up a void 3.5 billion light-years across, so there is currently evidence of large-scale structure at least that large, and there may be much larger structures that we can observe, given improvements in instrumentation, including better optics and greater detector sensitivity.

It's fairly unlikely. You can do angular correlations, and if you do that, you'll find that there is a huge dropoff in the power spectrum once you exceed a certain angle. One problem with doing distance correlations is that you don't know whether or not you are seeing a real correlation or some galaxy evolution or observational selection effect.

With voids framed by filaments and walls of galaxy clusters, the distribution of matter in space takes on a foamy appearance.

Which is pretty inconsistent with anything that has been suggested that is wildly different from the big bang. Foams aren't fractals. If the distribution of the galaxies were determined by something like plasma physics, you'd get a cascade at all scales in which you have something that looks nothing like a foam.

A challenge for modern cosmology could arise from the discovery of structures on larger and larger scales, because the formation of structure through gravitational accretion takes time, and the Big Bang theory contains a self-imposed limit on the time available in which structure can form.

Yes, but remember that all this was written in the 1970's. Things have progressed since then.

So people in the 1980's and 1990's were specifically looking for structures that could challenge the big bang. The stuff that they did find required some model tweaking, killed a few scenarios, but ended up not challenging the BB in a major way. Now it is true that you have to assume three "tooth fairies" to get everything to work (inflation, dark matter, and dark energy), but that's a lot better than any alternative theory. Once you assume these three tooth fairies, you get massively detailed predictions about how the galaxies are distributed, abundances of elements, and the characteristics of the cosmic microwave background. The fact that people outside the field aren't aware of this, is more a failing of science journalism than anything else.

The problem is that you just can't wave your hands and say "plasma physics" and get people to take you seriously, because we have enough data that you really have a huge job trying to explain how something really weird explains the data better than any of the theories that you have.

One big problem that you have if you start invoking plasma and magnetic fields is that what tends to happen when you have plasma and magnetic fields is that structures start breaking up. You get what is known as a Kormogarov cascade in which big eddies start breaking up and forming smaller eddies, and you end up with a hell of a time trouble to explain how you get big voids from that. If you take things that we know are due to plasma and magnetic fields (which you can do in the laboratory), you come up with a specific power spectrum that looks totally different from what we see in the galaxies.

 

[turbo]

Yes, but remember that all this was written in the 1970's. Things have progressed since then.

That observation regarding the time-budget for formation of structure was my own, not from Peebles, and was made in light of some fairly recent (6dF) observation.

So people in the 1980's and 1990's were specifically looking for structures that could challenge the big bang. The stuff that they did find required some model tweaking, killed a few scenarios, but ended up not challenging the BB in a major way. Now it is true that you have to assume three "tooth fairies" to get everything to work (inflation, dark matter, and dark energy), but that's a lot better than any alternative theory. Once you assume these three tooth fairies, you get massively detailed predictions about how the galaxies are distributed, abundances of elements, and the characteristics of the cosmic microwave background. The fact that people outside the field aren't aware of this, is more a failing of science journalism than anything else.

If we need to invoke DM, we have to do so in ways that allow it to "fix" observations in various scenarios, including the too-flat rotation curves of spiral galaxies, and the too-large binding energy of clusters. So we not only have to believe in that tooth fairy, but also believe that it is distributed "just so" to do its magic. It's not "clean".

The problem is that you just can't wave your hands and say "plasma physics" and get people to take you seriously, because we have enough data that you really have a huge job trying to explain how something really weird explains the data better than any of the theories that you have.

One big problem that you have if you start invoking plasma and magnetic fields is that what tends to happen when you have plasma and magnetic fields is that structures start breaking up. You get what is known as a Kormogarov cascade in which big eddies start breaking up and forming smaller eddies, and you end up with a hell of a time trouble to explain how you get big voids from that. If you take things that we know are due to plasma and magnetic fields (which you can do in the laboratory), you come up with a specific power spectrum that looks totally different from what we see in the galaxies.

I think you're confusing me with someone else. I never invoked any plasma cosmology. Anywhere, any time. Nor do I think that is a productive line of research.

 

[twofish-quant]

Here is a link that talks about the power spectrum

http://www.astro.caltech.edu/~george/ay21/eaa/eaa-powspec.pdf

The reason people believed in the 1970's that there wouldn't be any large scale structure was that the general belief was that the universe consistent only of baryons. If you assume that the universe consists only of baryons then what happens is that the baryons collide and clump pretty quickly, so you don't get any large structures.

If you assume CDM the you get a closer match to what you actually see. It's not an exact match, but it's close enough so that you can argue that your basic picture is right and the differences are due to details. Now if galaxies formation was the *only* reason to believe that CDM exists, then it would smell fishy, but there are other reasons to think that it's there. So it kind of makes sense.

 

[twofish-quant]

If we need to invoke DM, we have to do so in ways that allow it to "fix" observations in various scenarios, including the too-flat rotation curves of spiral galaxies, and the too-large binding energy of clusters. So we not only have to believe in that tooth fairy, but also believe that it is distributed "just so" to do its magic. It's not "clean".

Reality is messy. If you have any better ideas, let's hear them. Whenever someone comes up with a "clean model" it usually means that they don't have much useful data come in.

The fact that dark matter is a pretty large tooth fairy is why people are interested in modified gravitation models. The good thing about dark matter is that you just have to involve one tooth fairy with really one parameter, and it fixes a lot of things. LCDM has basically nine or ten parameters, and you get an incredible amount of stuff out if you have those parameters.

I think you're confusing me with someone else. I never invoked any plasma cosmology. Anywhere, any time. Nor do I think that is a productive line of research.

Curiously I think that plasma cosmology *is* a productive line of theoretical research. The good thing about plasmas is that 1) we know they exist and 2) there are all sorts of weird effects associated with plasmas, so I wouldn't be that surprised at all of there was some weird effect that explains a lot. I wouldn't write my Ph.D. dissertation around plasma cosmology, but to use a financial analogy, weird stuff like that is like a lottery ticket that might make you a millionaire if you are lucky.

When I say "here are the problems with plasma cosmology" it's not to end discussion, it's to further it.

 

[turbo]

Reality is messy. If you have any better ideas, let's hear them. Whenever someone comes up with a "clean model" it usually means that they don't have much useful data come in.

The fact that dark matter is a pretty large tooth fairy is why people are interested in modified gravitation models. The good thing about dark matter is that you just have to involve one tooth fairy with really one parameter, and it fixes a lot of things. LCDM has basically nine or ten parameters, and you get an incredible amount of stuff out if you have those parameters.

Modified gravity would be "clean" in the sense that no additional entities (tooth fairies) would be needed. Consider that G is not a constant, but is variable, increasing with the average matter-density of the space in which G is to be calculated between interacting masses. Scaled appropriately, variable G could eliminate the "missing mass" problems observed on galactic and cluster scales. We already have lots of estimates of mass densities of galaxies and clusters, and those are used to compute an apparent deficit in the mass required to produce the observed gravitational effects, which is then used as a quantification of the DM required. What would we get if we instead scaled up G in every instance until the observed gravitational effects are reproduced and the mass-deficit is erased? Variable G is probably not going to become popular anytime soon, since there is so much momentum in LCDM, but it is simple and clean with no additional entities or parameters.

 

[turbo]

I followed your link to the power-spectrum paper and from there to another on structure formation.

Galaxy formation stutters into action around z ∼ 5. Only a tiny fraction of the stars present today would have formed prior to that time. By z ∼ 3, the epoch when galaxies isolated by the ‘Lyman-break’ technique1 are observed, galaxy formation has started in earnest, even though only 10 per cent of the final population of stars has emerged. The midway point is not reached until about a redshift of 1–1.5, when the universe was approximately half of its present age.

This model is already ruled out by SDSS observations of quasars from z~5.7-6.5. Since luminosity falls off as a square of the distance, if these high-z quasars are at the distances implied by their redshifts, they would have to be the most luminous objects in the universe, powered by accreting BHs of billions of solar masses, feasting on host galaxies of perhaps a trillion solar masses or more. How did such beasts form only a few hundred million years after recombination? Another puzzle is that all of these quasars feature Solar or super-Solar metallicities with no observed evolution in either absolute nor relative metallicities. Moreover, despite the huge column densities to such distant objects not a single one was found to be lensed. These observations are explained in detail in Michael Strauss' presentation to the Space Telescope Science Institute on 11/02/2005. You will enjoy the talk.

http://www.stsci.edu/institute/itsd/information/streaming/archive/STScIScienceColloquiaFall2005/

 

[twofish-quant]

Consider that G is not a constant, but is variable, increasing with the average matter-density of the space in which G is to be calculated between interacting masses. Scaled appropriately, variable G could eliminate the "missing mass" problems observed on galactic and cluster scales.

The trouble with that is that that means that you have increased gravity in the early universe which means that all of the things that seem to match get blown to hell. BBN nucleosynthesis breaks. Structure formation breaks. You end up with too little deterium, you end up with galaxies that are too clumpy, etc. etc.

What you really want (and you get that with some modified Newtonian and f(R) models) is gravity that increases when you have large distances. The thing about f(R) models is that they make some concrete predictions about structure formation which last I've heard, don't match.

What would we get if we instead scaled up G in every instance until the observed gravitational effects are reproduced and the mass-deficit is erased?

Because in order to match observations you end up having to have a different gravity law for every single galaxy and cluster you are trying to measure.

Variable G is probably not going to become popular anytime soon, since there is so much momentum in LCDM, but it is simple and clean with no additional entities or parameters.

It's not social momentum, but fitting things to observations. Once you try to fit modified gravity to observation it becomes you end up with something that's more complex and with a lot more parameters than LCDM. People have tried. People have tried very hard.

If someone wrote a paper that says we assume this one crazy thing about G and everything works, then that would turn heads (which is what happened with inflation). There has been a lot of effort in getting this to work, and no one has been able to come up with anything. The "problem" (and I use that term ironically since it's a good thing) is that we have just too much damn data.

 

[twofish-quant]

Since luminosity falls off as a square of the distance, if these high-z quasars are at the distances implied by their redshifts, they would have to be the most luminous objects in the universe, powered by accreting BHs of billions of solar masses, feasting on host galaxies of perhaps a trillion solar masses or more. How did such beasts form only a few hundred million years after recombination?

Since CDM implies top-down formation of galaxies, I don't see the problem.

Another puzzle is that all of these quasars feature Solar or super-Solar metallicities with no observed evolution in either absolute nor relative metallicities. Moreover, despite the huge column densities to such distant objects not a single one was found to be lensed.

We are getting into the details of galaxy formation which is a process that no one quite understands, so yes, there are huge numbers of things that make no sense right now. And this means?

 

[turbo]

Because in order to match observations you end up having to have a different gravity law for every single galaxy and cluster you are trying to measure.

Same law, but with a variable G instead of a constant. We already quantify the mass-deficit to adjust the projected DM to fit observations. Leave DM out of it and scale G until the observed gravitational effects are fit. Do this for galaxies, loose groups, clusters, etc, rinse and repeat. If the relationship doesn't hold for clusters of similar masses and velocity dispersions, then it's back to the drawing board, but I'd like to see the consistency of the fit, and whether it can be used to predict the behavior of structures that have not previously been modeled.

 

[turbo]

Since CDM implies top-down formation of galaxies, I don't see the problem.

the author that you quoted on power-spectra had linked to the paper that I quoted, citing bottom-up formation, with galaxy formation only getting underway by z~5 or so. The apparent existence of trillion-solar-mass galaxies at z~6.5 seems a bit problematic, doesn't it? Anyway, if you have the bandwidth and the time, I highly recommend Strauss' presentation. He is the science spokesperson for SDSS and his presentation is clear and entertaining.

Edit: Here is the link to that 2006 paper citing bottom-up hierarchical matter formation.

http://www.astro.caltech.edu/~george/ay21/eaa/eaa-strucformsims.pdf

 

[Chronos]

My point is that cherry-picking some models that happen to fit observations after the fact is not equivalent to "prediction", since there was no consensus on viability of the models prior to the observations. With enough freely adjustable parameters, you can make almost any model fit observations, though that is a shaky way to conduct science.

That sounds suspiciously similar to your tenants. Do you have a ground breaking revelation in mind, or are satisfied by sniping at mainstream interpretations?

 

[twofish-quant]

Leave DM out of it and scale G until the observed gravitational effects are fit.

Been tried. Look up "modified Newtonian dynamics" (MOND), there are problem several dozens (if not hundreds) of papers that are trying to do that. The problem is that what you end up with looks (at this point) to have more tooth fairy than LCDM.

One problem is trouble is that you can't get a galaxy rotation curve for even one galaxy to work right without assuming a custom gravitational law. Now if you try multiple galaxies and clusters then it turns out that you have to put in a custom power law for each galaxy and cluster and what's worse, no one has been able to come up with a rule that says what law of gravity works with what galaxy and cluster.

Maybe someone will, but it's pretty clear that it's not as simple as "just change G." No one has been able to get anything simple to work.

If the relationship doesn't hold for clusters of similar mases and velocity dispersions, then it's back to the drawing board, but I'd like to see the consistency of the fit, and whether it can be used to predict the behavior of structures that have not previously been modeled.

Do a literature search for MOND. It's pretty clear that anything really simple won't work. Now you can make any model work by making it sufficiently complicated, but a complicated theory has no predictive power. Right now (and things could change in the next few years), LCDM ends up being far, far simplier and more predictive than any modified gravity model that anyone has come up with.

One problem is that MOND is right now at the level of just trying to come up with a coherent gravity rule. As far as I know, no one has even thought about the implications of MOND to CMB anisotropy and galactic structure formation.

 

[twofish-quant]

the author that you quoted on power-spectra had linked to the paper that I quoted, citing bottom-up formation, with galaxy formation only getting underway by z~5 or so. The apparent existence of trillion-solar-mass galaxies at z~6.5 seems a bit problematic, doesn't it?

So the writer of the paper is wrong and you have incredibly enormous galaxies at z=6.5. What's the problem? Galaxy formation is something that no one has any clue about, so if you argue that some one's scenario of galaxy formation is bogus, that's not going to bother me or anyone else right now.

You'll only run into problems if you end up arguing that whatever caused galaxy formation had structures that were wildly different from the initial distribution of matter.

 

[twofish-quant]

Right now the really big focus of interest in cosmology is in the "dark ages". The period between CMB and the first observable galaxies. Since you can't see anything, you don't have any data, since you have no data, you can make up lots of stuff most of which is wrong.

Data is coming in, so we'll have a much better idea of what's going on in about a year or two.

About how bad science journalism is in relation to cosmology, there is this very unhealthy obsession over "what happened before the big bang" and barring something really weird, I doubt there is going to be much progress on that over the next two years. Also journalists have this other obsession with "young maverick overturns deep established truths" (which I think is a product of the Vietnam War era). This causes science journalists to tend to ignore parts of science where there are no established truths. Right now, you can make up any galaxy formation scenario you want, and people will nod their heads because there is a huge lack of data. Once more data comes in, most of those scenarios will end up in the trash can.

 

[turbo]

Been tried. Look up "modified Newtonian dynamics" (MOND), there are problem several dozens (if not hundreds) of papers that are trying to do that. The problem is that what you end up with looks (at this point) to have more tooth fairy than LCDM.

In MOND, the constant G still is a constant as far as I know (there may be other flavors), though it is assumed that the forces derived from Newtonian dynamics do not hold in low-acceleration regimes. Of course MOND doesn't work on cluster scales, so the range of applicability is limited.

 

[turbo]

About how bad science journalism is in relation to cosmology, there is this very unhealthy obsession over "what happened before the big bang" and barring something really weird, I doubt there is going to be much progress on that over the next two years.

I tend to ignore most of that stuff, like collapse, bounce, multiverse, etc. It makes for lively reading, I guess, judging from the number of articles that get cranked out.

 

[twofish-quant]

In MOND, the constant G still is a constant as far as I know (there may be other flavors), though it is assumed that the forces derived from Newtonian dynamics do not hold in low-acceleration regimes.

There have been tons of different versions of MOND. If you assume that things are an inverse square law, and that you are just changing G by a constant everywhere, then you end up with something that clearly won't work, because if you change G by a constant everywhere, then stellar evolution models go bonkers. Even very small changes in G means that stars burn brighter and dimmer.

So just rescaling G everywhere by a constant won't work, so you have to have variable G that depends on different things, and at that point any changes in G can be folded into the gravity law. Take whatever rule you come up with divide it by a r^2, and that's your variable G.

One thing that about modified gravity models is that they are all at the "curve fitting" stage. People are just trying to get some consistent rule that works, and they've had not much success at this. No one really knows or really cares right now what causes gravity to behave diferently. That's one big strike against modified gravity models.

It's reminiscent of the old saying that democracy is the worst system of government except for anything else. You can say the same with LCDM. If you look at it in isolation is looks ugly, ad-hoc, with lots of weird unjustified assumptions. Point taken. It's just that no one has come up with anything better, and it's not for lack of trying.

 

[turbo]

One thing that about modified gravity models is that they are all at the "curve fitting" stage. People are just trying to get some consistent rule that works, and they've had not much success at this. No one really knows or really cares right now what causes gravity to behave diferently. That's one big strike against modified gravity models.

At least MOND had a predictive success, with the galaxy rotation curves of LSBs falling nicely in line. That is encouragement for the modified-gravity crowd because it hints that some phenomena can be modeled without DM and that perhaps a more general law could be derived with a greater range of application than individual galaxies.

 

[Astronuc]

From the OP, the new structure or part of the structure is at ~7 billion ly or 2 Gpcs. Is this the furthest discovery of the web structure? Or does this just fill in a hole in the currently known web structure?

A&A 505, L9-L12 (2009)
DOI: 10.1051/0004-6361/200912929
Letter
The spectroscopically confirmed huge cosmic structure at z = 0.55
http://www.aanda.org/index.php?option=article&access=doi&doi=10.1051/0004-6361/200912929

 

[twofish-quant]

At least MOND had a predictive success, with the galaxy rotation curves of LSBs falling nicely in line. That is encouragement for the modified-gravity crowd because it hints that some phenomena can be modeled without DM and that perhaps a more general law could be derived with a greater range of application than individual galaxies.

MOND has one predictive success. LCDM has about three that I can think of off hand. (CMB anisotropy, helium abundances, and the existence and location of acoustic peaks). Also with CMB and galaxy structures, LCDM has much more detailed predictions than MOND. Also, if the LHC comes up with evidence for a particle that matches what LCDM requires, that will be a pretty big predictive success. LCDM requires a pretty big tooth fairy, but if you start seeing wings flapping and pixie dust, the fact that someone made such as outrageous prediction that seems to have evidence to support it, it quite an achievement.

As far as MOND, the fact that you got *something* is why people are working on it. Once you pull a rabbit out of the hat, people start taking notice. Again a lot of this involves, just keep plugging away and well see if it works or not. It's also not either/or. One thing that you have to be careful as a theorist is not to fall too much in love with your own ideas, since you don't determine what is right or not, nature does, so it's pretty common to spend five years or so working on a theoretical framework that turns out to just not work out in the end. But getting things to the point where you've managed to convince yourself that what you've been working on for five years is *NOT* the answer is quite a major and difficult accomplishment, and usually you end up with a lot of stuff that you can salvage for something else.

There is the possibility that you need *both* dark matter and MOND to make everything work out.

The problem with MOND is that it's pretty useless right now for the problems I'm interested in. I'm interested in gas dynamics, and LCDM gives people a framework to do gas dynamic calculations whereas MOND does not. However, it's not a waste to use LCDM as a paradigm even if it turns out to be wrong. If it turns out that modified gravity is correct then at some point someone is going to have to come up with an explanation for why the world *seems* like it has dark matter, and at that point it should be possible to take all of the theoretical work that has been done with LCDM and "translate" it into modified gravity.

 

[turbo]

However, it's not a waste to use LCDM as a paradigm even if it turns out to be wrong. If it turns out that modified gravity is correct then at some point someone is going to have to come up with an explanation for why the world *seems* like it has dark matter, and at that point it should be possible to take all of the theoretical work that has been done with LCDM and "translate" it into modified gravity.

Exactly! There has already been a lot of work done estimating cluster masses and peculiar motions so that the "missing mass" can be quantified. If modified gravitation shows promise, that work is directly transferable.

 

[turbo]

From the OP, the new structure or part of the structure is at ~7 billion ly or 2 Gpcs. Is this the furthest discovery of the web structure? Or does this just fill in a hole in the currently known web structure?

A&A 505, L9-L12 (2009)
DOI: 10.1051/0004-6361/200912929
Letter
The spectroscopically confirmed huge cosmic structure at z = 0.55
http://www.aanda.org/index.php?option=article&access=doi&doi=10.1051/0004-6361/200912929

That structure is at z~0.55, but there have been other examples of LSS at higher redshifts. It's really old news, in astronomical-research terms, but in 2002, Venemans et al reported the discovery of a protocluster (excess of lyman alpha emitters) surrounding a radio galaxy at z~4.1.

http://www.iop.org/EJ/article/1538-4357/569/1/L11/16078.web.pdf?request-id=0828b6d3-39ca-4d17-b4e2-ec9b6f9ad5a2

 

[lopkiol]

This? http://engweb.swan.ac.uk/~gabbriellir/javaview/wp-foam.html"

 

[matt.o]

6dF turned up a void 3.5 billion light-years across, so there is currently evidence of large-scale structure at least that large, and there may be much larger structures that we can observe, given improvements in instrumentation, including better optics and greater detector sensitivity. With voids framed by filaments and walls of galaxy clusters, the distribution of matter in space takes on a foamy appearance. A challenge for modern cosmology could arise from the discovery of structures on larger and larger scales, because the formation of structure through gravitational accretion takes time, and the Big Bang theory contains a self-imposed limit on the time available in which structure can form.

Do you have a link to a published paper for this ~1Gpc scale void found in the 6dF survey?

 

[turbo]

Not a peer-reviewed paper, though I'd be very surprised if John Huchra hasn't already published on or has on in the works. I'll take a look.

http://www.newscientist.com/article/dn16903-new-cosmic-map-reveals-colossal-structures.html

Scientists are still analysing the new map, but a few features stand out immediately. The biggest concentration of matter seen by the survey is a previously known giant pileup of galaxies called the Shapley supercluster, which lies about 600 million light years from Earth.

The survey also found some enormous voids – regions of space that are relatively empty, including one that is about 3.5 billion light years across.

"This is as big as I've ever seen," survey team member John Huchra of the Harvard-Smithsonian Center for Astrophysics told New Scientist.

Another large void about 1 billion light years across was discovered previously.