Talk:Big Bang/Archive 18

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Archive 18


Contents

Factual review (per FA bid)

This article is very close to my day job, so I may be a very picky commentator. For sure it gives a good overview of the Big Bang and obviously represents a lot of hard work. Still, it seems to me to contain numerous half-truths, ambiguities and minor factual errors. Here is an edited list:

  • Lead description of the "initial state" is weaseling and misleading; a natural reading would suggest a finite density at the singularity. As a "key premise" this needs a better explanation. "Alpha-beta-gamma theory": it's not so called: Big bang nucleosynthesis (BBN) is current. A-B-G was a famous first attempt, but its answer is radically wrong.
  • History Give more dates; name Vesto Slipher, he deserves credit. Use "static" not "steady-state" for Einstein's model. Per footnote, vs. text: Lemaitre's primeval atom was 1931, his 1927 theory didn't start with a big bang. Hubble's 1929 contribution is described twice; it should emphasise that Hubble contributed the distances and used Slipher's redshifts. BB is contrasted twice with Einstein's model. Milne & Tolman appear out of sequence, they published in the 1930s, cf Steady state proposed 1950. Mention radio source & quasar count controversy (convinced many of BB prior to CMB discovery). In fact WP needs an article on evolution of the universe.
  • Overview needs to point out that the earliest phases are speculative (see comment on inflation below). Sentence on production of Quark-Gluon plasma not needed, breaks up story. Needs more dates (not just "later.."): eg age of universe at hadron formation, e-/e+ annihilation. Discussion of Dark energy suggests wrongly we'd expect linear v/z relation otherwise; also signature is slower velocities at large distances than expected by extrapolation from now.
  • Underpinning Copernican principle is implied by cosmological principle, not independent. Can't fathom in what way S-Z "tests" the cop. principle. Tests of parameter variability can't be naively extrapolated back to t=0, they may vary faster in the early universe. The FRW metric is kinematic, i.e. based on Reimann geometry but not Einstein's dynamic theory. I wouldn't say the metric "relies" on a coord chart, that's just one way of expressing the metric. The idea that "space expands" is perpetually debated; its not clear that there is a real difference from true motion. It's far from clear that the Hubble expansion tries to "pull apart" bound systems like solar systems, especially if embedded in a larger bound system like the local group. I don't understand the comment that the BB corresponds to a finite interval of conformal time... possible confusion with finite interval of comoving space on a conformal diagram?
  • Evidence The "three pillars" is very 1970's... nowadays there are many more, e.g. cluster masses, ages of oldest stars, etc. Hubble's law: why do descriptions of this always go on about the easy bit (getting v from z) and never discuss Hubble's actual contribution (getting D)? The formula is misleading as it is true for present-day velocities, not for velocities as measured on the light cone (anyway at high redshift (1+z) is expansion factor, not directly related to v). CMB photons are produced later than baryogenesis on any definition; individual photons retain their identity from a few days after the BB, when true emission mechanisms freeze out (see 3K by Partridge), subsequent Compton scattering is not an emission process. I'd like to see recombination distinguished from decoupling: photons remain well coupled until almost all electrons recombine. Somehow COBE's most famous result (detecting fluctuations) is omitted, as is the Nobel to Smoot & Mather. Discussion of Omega=0.3 is ahistorical as clear evidence for flatness didn't turn up until the Boomerang & Maxima results in 2000. Discussion of nucleosynthesis brushes a mess under the carpet, of 4 tests, D agrees with WMAP, He-3 has lousy errors, He-4 formally disagrees, and Li-7 radically disagrees. These can be explained away but it's not a big success.
  • Issues & problems Inflation is not regarded as firmly established by most cosmologists; it is just the most plausible hypothesis. Otherwise why are NASA & ESA contemplating hugely expensive space probes (see Beyond Einstein program) to see if it's true? Description of inflation implies it violates Hubble's law; not so! Also description involving horizon is too compressed to avoid misunderstandings (e.g. there are several different horizon concepts). Flatness comment uses "geometry" instead of "curvature", confusing local geometry with topology (see shape of the universe). The flatness tuning is bogus since infinitely good tuning at t=0 is required by the FRW assumption; much better to emphasise the oldness problem. AFAIK the monopole problem is outdated since GUTs consistent with modern particle physics limits don't produce monopoles in worrying quantities. Discussion of lab CPT test is out of place; this is not a serious candidate for baryogenesis. Dark matter: the universe (now) is far more lumpy than expected w/o DM. Or, the early universe (CMB fluctuations) should have been lumpier than observed to get to present state without DM. The implications of the bullet cluster for alternatives to DM (e.g. TeVeS) are still being assessed. Give it a couple of years. Dark energy the wrong idea that CMB obs. established flatness in the 1990's resurfaces. It's worth emphasising that the Omega values change radically with time & are quoted for present day.
  • Future Star formation is not suppressed by cosmic expansion but by running out of interstellar gas within each galaxy. On the other hand further galaxy formation has already been suppressed, pretty much. For almost all theories, protons decay long before astrophysical black holes evaporate.
  • Beyond the big bang Might discuss before the big bang, if latter is defined as state of very high density and temperature in thermal equilibrium. Several theories discussed here do imply that the BB was an event in the history of a larger universe, not the beginning; this should be made clear.
  • Religion Worth saying that Fred Hoyle frequently claimed that the big bang was religiously motivated. It didn't help that Georges Lemaitre was a Jesuit priest.

PaddyLeahy 20:54, 30 April 2007 (UTC)

Hey, an interesting list. I agree with much of what you're saying, but not all (more on this below). But, um, maybe you could consider actually doing the edits? Otherwise it just seems a little inefficient to list things that you feel should be changed here, and then wait for other people to implement the suggestions that you've made (be bold).
As I said, I agree with the bulk of what you're saying. I'm sorry to focus on the negatives, but here are some examples where I don't think you're on the mark;
  • Lead "...a natural reading would suggest a finite density at the singularity" not sure what you mean here, but maybe you meant infinite density?
Nope, lead asserts a finite ("tremendous") density for the initial state... begs the question, what is the initial state (if not the singularity)?
  • Underpinning
    • "The FRW metric is kinematic, i.e. based on Reimann geometry but not Einstein's dynamic theory." (btw, I think you mean Riemannian geometry). Perhaps what you're trying to get at is that you only need GR to specify the differential equation satisfied by the scale factor a(t) in the FRW metric, but not the FRW metric itself. Fair enough, I don't agree that is an important point to make here. One of the assumptions of GR (which follows essentially from the equivalence principle) is that gravity should be described by a metric on spacetime, so I'm quite comfortable regarding the fact that spacetime should be described by a metric as part of Einstein's theory.
Judgement call on importance; I don't like to quote unnecessary assumptions.
    • "The idea that "space expands" is perpetually debated; its not clear that there is a real difference from true motion." Debated by whom? It's pretty well accepted. And what do you mean by "true motion" in this context?
dx/dt? There's an interesting diatribe about this on John Peacock's web site, for a start. Arguably the difference between "galaxies move" and "space expands" is just the coordinate choice.
    • "It's far from clear that the Hubble expansion tries to "pull apart" bound systems like solar systems, especially if embedded in a larger bound system like the local group." See geodesic deviation. This is a natural way to define the "force" from Hubble expansion, but it's just much smaller than the binding forces in those systems.
But this analysis ignores the space-time curvature associated with the bound system. (Which may be embedded several layers deep in systems which have already dropped out of the Hubble flow... see space-time diagrams on Ned Wright's site).
    • The conformal time issue. Whether the singularity (or whatever) is a finite conformal time to the past does depend a little bit on the assumptions you make about what goes on in the very early universe. I think the point that section's trying to make is that there are horizons in the universe (according to the model) that can be understood using conformal time.
Mea culpa, my misreading... I thought the "finite interval of conformal time" referred to the big bang itself rather than to the gap between then and now. Might be re-phrased to make it more transparent.
  • Evidence
    • "...not for velocities as measured on the light cone..." do you mean high redshift here? Or what? I'm a little confused as to your meaning. For low redshifts (z<1) the Hubble's Law formula works pretty well. Also, when Hubble proposed his law, that's the formula he used. Like other laws, it can get modified as people probe more deeply (in this case to larger z).
Yes, I mean it would be good to distinguish v(t) = HD(t) (always true) from the more messy redshift-distance formula. Lots of redshifts at z > 1 nowadays!
    • "...Discussion of nucleosynthesis brushes a mess under the carpet..." What mess? BBN is incredibly successful. There are no other explanations for the abundances of primordial elements that even come close. Also, your criticism of BBN doesn't take account of any observational systematics that could be involved in these "discrepancies."
Seems like special pleading to me: 1 success, 2 failures. Obviously there are systematics but I could think up plenty if it was D wrong and Li right. The Spite plateau (constant Li-7 in oldest stars) looks so convincing... (I'm not claiming this disproves the BB, but it is not a nice clean proof).
  • Issues and Problems I do agree that "Hubble's Law" is misused here.
    • "...confusing local geometry with topology..." Not sure I see it. Right now nothing is said about topology in the article...
To a naive reader geometry implies the full structure including topology, as the picture suggests. elliptical refers to a specific topology (in fact the one with poles of spherical geometry identified). Referring to curvature (esp with a suitable link) reduces scope for confusion.
    • "...The flatness tuning is bogus since infinitely good tuning at t=0 is required by the FRW assumption..." Nope, FRW accommodates arbitrary spatial curvature thanks to the k parameter that appears in the metric.
For any FRW universe with a big bang, however close you want the density to be to the critical density, it will be that close for some sufficiently early time. i.e. just saying Omega is very close to unity at (unspecified) early times doesn't add anything to the basic FRW assumption. OTOH saying the universe is still expanding after 10^61 Planck times does indicate fine tuning.
    • You're right about the lumpiness issue. But...
    • "...The implications of the bullet cluster for alternatives to DM (e.g. TeVeS) are still being assessed..." Yes... But for the majority of workers in the field, TeVeS is not a viable or attractive alternative to dark matter, although some people do (and of course should) work on it. I think TeVeS falls under undue weight here, though maybe a debate on alternatives could be in the dark matter article.
Respectfully disagree. MOND was attracting increasing support (e.g. from those working on galactic structure) even before Bekenstein relativised it. For instance Rees nods in the MOND direction when discussing dark matter in Just Six Numbers. Scientists like to have alternative ideas they can test against the standard picture. And the history of MOND is full of attempts to rule it out that proved to be just misunderstandings of the theory.
  • "Dark Energy"
    • "...the wrong idea that CMB obs. established flatness in the 1990's resurfaces." CMB observations were instrumental in establishing flatness. Are you taking issue with the decade? The first measurements of the location of the first peak in the CMB, which is an excellent indicator of curvature, took place in the '90s.
Yes I'm taking issue. Despite partizan claims by supporters of experiments like QMAP and TOCO, I don't think the community (I'm in it) was convinced about the first peak until the Boomerang results (early 2000, I remember it well).
    • "...that the Omega values change radically with time..." Sure, they'll be different in the future, but it'll take tens of millions of years to notice. I see this as kind of like saying that one should note that the age of the earth is quoted as the present day value.
But the context is a discussion spanning the whole history of the universe. It's obvious to the casual reader that the age of the earth changes with time, not so for Omegas.
Like I said, sorry to focus on the negatives -- I think the other things that you mentioned would make great contributions to the article and could really improve it, if you chose to edit them in. Wesino 22:16, 30 April 2007 (UTC)
So of course that's one reason I didn't edit the article directly, the other being that I don't have time to enter into revert wars on all these points esp. to dig out supporting references. PaddyLeahy 00:03, 1 May 2007 (UTC)
You obviously have some fair amount of scientific knowledge, but I suspect you haven't really thought deeply about some of the responses above (e.g., dx/dt is "true motion"??). Without getting drawn into further arguments on these points, let me just summarize what I see as the problem with some of your proposals. You're saying that we should insert claims that
  • One shouldn't assume Einstein gravity to do cosmology,
  • The Hubble flow is not due to the expansion of space,
  • BBN is wrong,
  • The universe has nontrivial topology,
  • The flatness tuning argument accepted for at least three decades (proposed by Dicke and Peebles, and used by Guth to argue for inflation) is wrong,
  • MOND is an acceptable replacement for dark matter.
These are all fringe-y positions. As someone "in the community," you should know this. It's misleading to assert that these represent consensus positions in the field, and in what's essentially an introductory article on the big bang, no less. These points are worth debating, perhaps, but these are all issues where the vast majority of the community has already come down on one side, and it isn't the side you're advocating.
It's too bad that you have the time to trash the article (as well as a lot of the standard cosmological model along the way) but not enough to try to fix things! Some of the points that you originally made were pretty good, and dealt with widely accepted physics/astronomy. I was also intrigued by some of the historical points you made, as right now the article is a little weak in that area.
But there's just no point in adding references to all kinds of fringe topics, when the basic science of the consensus model isn't well represented in the article. I'm sure you'd have to agree with that. Wesino 08:10, 1 May 2007 (UTC)

(unindent) Ho, hum. First, I'm glad you agree with a lot in my original post. I'd be very pleased if all the editors also agreed on these points but it seems a bit hopeful. It wasn't obvious to me that the points you highlight above would turn out to be particularly controversial, mainly because I was not suggesting that these should be stated as facts, just that the article should be more nuanced. But from your reaction I think it would have generated a lot of heat if I'd just boldly edited the article, so I'm glad I didn't. My take on this article (and actually many WP cosmology articles) is that there is a tendency to take the simplest model that (just about) fits the data and claim that this is the community consensus position. Working scientists like to keep a bunch of different theories in the air at once, are acutely aware of the points in "standard models" where empirical support is weak or alternative models are available, and would like these to be made clear in encyclopedia articles.

On your points above, I wasn't objecting to the use of GR, just to the statement that GR was a necessary assumption for the FRW metric. I'd rather GR was brought in as an additional assumption needed to find a(t), but I'm happy to bow to consensus if this is thought too picky. I've cited a discussion by the author of a leading postgrad textbook, Cosmological Physics which argues that the phrase "expansion of space" is misleading (and he is not a one-man band); I just asked that this phrase not be stated as gospel. For the record, I don't think that BBN is wrong, but in it's current state it is incorrect to say that the observations (except of deuterium) unambiguously agree with the BBN predictions: this disrespects the observers who came up with values that as the cited review shows, are several/many sigma off (For He-3 there is apparent agreement but huge potential systematics). In a sense this "pillar" has become less persuasive recently, because the errors are getting smaller but discrepancies are not going away (presumably due to systematics, but that's my biased position). I don't think the article should claim this as a triumphant vindication, although I totally agree with the point that even qualitative agreement is quite impressive. Non-trivial topology has been the subject of numerous observational tests and even claims that it may explain some anomalies in WMAP; no part of standard cosmology relies on the topology being simple. So, I think the article should allow interesting topology as a possibility. Dicke & Peeble's discussion of flatness/oldness revolves around how long the universe has lasted, i.e. it emphasises oldness as much as flatness. Later presentations have often cheated and the version in the current article follows those, unfortunately. On MOND/TeVeS, I was objecting to the claim that this was definitively ruled out by the bullet cluster. I do think it is sufficiently notable to deserve a mention: Bekenstein (2004) had 138 citations last time I looked, which is pretty good going for a fringe theory. It would be hard to find a working scientist who would say we shouldn't thoroughly investigate it before concluding that non-baryonic DM is the only possible explanation. PaddyLeahy 11:08, 1 May 2007 (UTC)

I think that this is too much discussion and not enough action. I agree with most, if not all, of Paddy's and Wesino's commentary: including the bits which seem to be in contradiction. I encourage Paddy to go through and make edits and the rest of us will modify as we see fit.
Two comments:
  • The SZ effect indeed tests the Copernican Principle by confirming the T_0/a relation for the CMB. If we lived in a universe which violated the Copernican Principle, we could explain our observations as being due to a Milne-type "explosion" and the CMB as being a radiation signature of it. However, if everybody sees the CMB at the same temperature for the same space-like interval, we must not be at such a preferred position.
  • The BBN gives the right abundance for hydrogen and deuterium is extremely important. This confirmation is qualitative only in the sense that it is less well-determined than other evidences (e.g. the position of the acoustic peaks in the CMB power-spectrum). Scientists tend to be overly-skeptical when talking shop for good reason, but when the day is done it is a good idea to put all your cards on the table. Paddy is right, we spend a lot of time worrying about the holes: but this has the unintended effect of causing us to dwell on them unnecessarily in summative conversations. Wikipedia is not a conference talk, it is supposed to be an encyclopedia. That BBN works at all is amazing and while a brief mention can be made of systematics issues, dwelling on this idea opens up cans of worms that are generally unintended. Do you know how much Eric Lerner drones on-and-on about these BBN "problems"?
--ScienceApologist 13:41, 1 May 2007 (UTC)
OK, 3 editors is a consensus. Made a start; I'll do more when I have some books handy. I'd be happy for other motivated parties to do some of the work... PaddyLeahy 19:35, 1 May 2007 (UTC)
I see SA has already started. I deliberately avoided mentioning the Planck epoch in the overview, since that is definitely in the "speculative" phase, not in the "good reasons to believe" phase. Again, I was deliberately vague when this started but for me it's when kT = 1 TeV or so, i.e. the experimentally tested range. PaddyLeahy 19:49, 1 May 2007 (UTC)
I think this is a matter of taste. The Planck epoch is when the models give contradictory predictions. After that time, the models are pretty much consistent. They may yet be proved wrong, but they are inductively sound predictions that are made. Before the Planck epoch is anybody's guess, for the most part. --ScienceApologist 20:32, 1 May 2007 (UTC)
Yes, SA is right. More editing, less arguing! I hope I haven't offended with my comments, but sometimes you feel like someone's got to make the argument, you know? In any case, I think the flurry of edits that have been going on over the last few hours have definitely improved things. Wesino 20:57, 1 May 2007 (UTC)
On the Planck epoch, I've tried to steer a middle course here, so it's mentioned but in a kind of "terra incognito" way. I wonder if there's a way to denote the lines that separate physics everyone agrees on (kT < 1 TeV) from physics people kind-of sort-of agree on but is untested (1 TeV < kT < 1016 GeV) to physics where it seems like nobody has a clue (kT > 1016 GeV)? Wesino 20:57, 1 May 2007 (UTC)

I'm glad for the attention to detail and nuance being discussed above, but I hope it doesn't get in the way of the big picture in this article. Don't forget the audience. Viva la balance. Cheers, Gnixon 22:51, 2 May 2007 (UTC)

No limits

while the traditional approaches and theory includes ideas of limitations as the speed of light's being limited, in reality there are no limits.

entrainment STUDIES SHOW ENTRAINMENT occurs at speed & range of 66 - 125 light years / sec (quite large distances....) (500 light years divided by 8-15 seconds elapsed time)

a traditional approach would be matt breathing where one breathes in expanding to the outer edge limits (of all reality) and then exhales ... the anhk (or outer - to sept tillion light years "edge" and then collapses to the point / heart center)

/s/capt queeg sr

fiducial singularity, Planck epoch

In response to some questions raised by Art LaPella I've tried to fix the sentence in question (I don't know who the original author was), which before my edits ran something like:

Nonetheless, the physical models are consistent to within a tiny fraction of a second in advance of the fiducial singularity.

Part of the problem was that in advance of the singularity would have been before the Big Bang (which isn't generally discussed in the BB models, and certainly isn't a point of agreement amongst physical models!). Also I'm not sure that it's a "fiducial" singularity. True, my Merriam-Webster defines this word as "(esp. of a point or line) assumed as a fixed basis of comparison." but my understanding is that usually this word applies when you have many versions of something, but want to single out one as the basis against which others are compared. I could be wrong.

It's a tricky paragraph, though, since what you really want to say is that nobody knows what's going on in the planck epoch, and after that we make a whole lot of assumptions/guesses (though founded on QFT/GR etc) basically until BBN when you have some idea what's going on. Definitely, that paragraph can be improved. Cheers, Wesino 22:41, 2 May 2007 (UTC)

As I mentioned above in a discussion that had died off when I got to it, I think we should be very careful about discussing singularities too prominently---could be misleading to the novice reader. Gnixon 22:52, 2 May 2007 (UTC)
Hmm... Yeah, I see where you're coming from. Definitely (and especially given how looong the article is now) there's a case to be made for excising the singularity stuff. I wouldn't object myself, I think there tends to be a lot of duplication of information on Wikipedia so brevity is always a good idea (my POV). I guess I can kind of see pros and cons to discussing the singularities, something like --
Con: The singularity concept is confusing to novice readers, and it's not clear exactly how it fits in to the whole picture. We could just keep things simple and say something like "the bb theory assumes the universe started in a hot dense state, and then X happened," which would save space and make things easier to read. Interested readers could be referred to more detailed articles.
Pro: It's a nice, "hic sunt dracones" reminder that it isn't all worked out yet. It is an interesting (albeit almost... philosophical) point that when you run the equations of physics in one direction they eventually break down.
How do others feel about this? Wesino 23:09, 2 May 2007 (UTC)
Pro: most readers will have already heard of the idea that the Big Bang is an explosion "from a point" or some such, so it's worth mentioning to say that it ain't necessarily so. NB I think User:ScienceApologist introduced the "fiducial" word to mean that whether the singularity exists or not, we're using the nominal time of the singularity as a reference point. Personally I think this introduces more confusion than it solves. PaddyLeahy 23:14, 2 May 2007 (UTC)
I liked the way one part of the article (intro?) referred to some post-singularity time as the "initial state," just characterizing it as "hot and dense" or something, but I noticed from somewhere on this talk page that it caused confusion. It would probably be worth saying explicitly in the intro that we expect known physics to break down before that point (or before a "singularity".) I'm down with mentioning singularities if we can find an elegant way to make clear that they're considered unphysical. It would be great if this article could help dispel the common conception of an explosion from a point. Gnixon 17:37, 3 May 2007 (UTC)
Yikes! Just noticed "singularity" in the diagram at the top. That really makes me wince. Gnixon 17:40, 3 May 2007 (UTC)

NPOV, MOND and TeVeS

I'd like to open a discussion about our presentation of some alternative cosmological theories in this article. It's clear from this talk page that the case I'm going to focus on is controversial, so instead of just doing the edits, I want to put the argument out in the open.

I think we are vastly overstating the case for alternatives to dark matter in the article, which causes a conflict with NPOV and more importantly undue weight policies on Wikipedia.

First, an estimate of the importance of DM alternatives in the scientific community. I obtained this through searches in two databases of scholarly articles, the NASA Astrophysics Data System and the Stanford Linear Accelerator Center (SLAC) SPIRES database, both publicly available. These databases index some of the same papers, but do not overlap completely. Any paper that you publish on astronomy, astrophysics, general relativity, high-energy physics, string theory, quantum gravity, or cosmology (even if you just post to the arXiv) it will appear on one (and probably both) of these databases.

On NASA/ADS, a search for abstracts with the phrase "dark matter" returned 23,457 articles. Searching for abstracts containing "modified," "newtonian," and "dynamics" gave 550, searching for "MOND" without "modified" or "newtonian" (to avoid overcounting) gives 353, searching for "TeVeS" yields 39.

Ideally one could search for abstracts containing "tensor" "vector" and "scalar", but this gives way too much noise with articles about cosmological perturbation theory. Here are the search strings I used on the search page and the results:

23457 "dark matter"
550 +modified +newtonian +dynamics
353 MOND -modified -newtonian
39 teves

Now assuming ALL of the articles that mention MOND/TeVeS are pro (thus including titles like "Bullet Cluster Disproves MOND" in the pro camp) I get a ratio of (23457)/(550+353+39) = 24.9 dark matter articles per pro-MOND/TeVeS articles in the scholarly literature.

One can do the same thing on SPIRES with the search terms dark matter (5040) MOND (103) Modified Newtonian Dynamics (99) and TeVeS (8). This gives a weighting factor (again assuming all articles that mention MOND/TeVeS are pro) of (5040)/(103+99+8) = 24.0, similar to the number obtained above.

Now, how much weight do we give each point of view in the article? In the present revision of this article, a quick word count reveals that in the dark matter section, the explanation of the dark matter model is 178 words long, and that for MOND/TeVeS 121 words. Thus in the article the weighting ratio is something like 178/121 = 1.5.

This means that in the Wikipedia Big Bang article, we are giving alternatives to dark matter roughly 17 times more weight than does the scholarly literature. To bring the ratio in the article more in line with the 25-to-1 ratio in the existing scholarly literature, alternatives would warrant seven words. I suggest:

"Some scientists advocate alternatives to dark matter."

In place of the paragraph on MOND/TeVeS in the dark matter section.

Before you flame me -- Am I saying that no-one should research MOND/TeVeS/whatever? No. Am I saying that people who do are fools? Certainly not. Do I think that one shouldn't discuss alternative ideas? Not at all. Should science have an intellectual monoculture, where establishment ideas rule the roost and cannot be questioned? Definitely not.

In an introductory article, let's give similar weights to ideas as would the experts in the field. We don't have to say that scientists agree on everything, and it doesn't have to come down to word counts, but right now we are vastly overstating the case for some of these "alternative" models. Wesino 11:44, 5 May 2007 (UTC)

You are right, Wesino. Such advocacy does not belong in this particular article. Alternatives to dark matter are not even related to the subject of the Big Bang. I'm eliminating the entire discussion. --ScienceApologist 14:15, 5 May 2007 (UTC)
I concede Wesino's point about the unbalance, though his numerical data is exaggerated: TeVeS has existed for less than 3 years and the software tools to compare it to observations are far more primitive than for GR, which gives a sociological bias to the conventional theory in the current literature; moreover, the literature on Dark matter is substantial from the late 1970s on. (I would not be making this fuss if it was still just MOND, which cannot really be reconciled with the rest of physics, despite its empirical success). Contra SA, the issue is pretty important for the Big Bang since if you replace the theory of gravity the whole theory has to be re-calculated from scratch, which is exactly what a number of people (not including me) are currently doing with TeVeS. I think you'll find that most recent popular accounts of dark matter will at least mention the modified gravity option. So I have restored the mention of MOND/TeVeS but in a much briefer form. I think the current version is more or less what Wesino was hoping for. PaddyLeahy 15:15, 5 May 2007 (UTC)
I don't buy this as an argument for inclusion. While I submit that you would need to recalculate the Friedman equations and try to reconcile WMAP results if GR is wrong, this is true regardless of whether the model is MOND/TeVeS or any other outfit. What makes MOND/TeVeS notable for the dark matter page is that it is the most popular "alternative" explanation. However, this is one level of specificity removed from the Big Bang and doesn't add any information about this cosmological model to our article. If there actually was an alternative explanation that could modify Lambda-CDM to account for MOND, then we would have something to report. As it is, all we have are arguments from another (though plainly related) field of study. Therefore mention of this idea rightly belongs on the dark matter page, not on this page. --ScienceApologist 22:24, 5 May 2007 (UTC)
OK then, replaced specific refs to MOND/TeVeS with alternatives to general relativity. I'm resisting characterizing this as a set of mavericks challenging the establishment, because in fact perfectly establishment scientists are investigating TeVeS, and indeed mostly conclude that it does seem to do the generic business (lensing, structure formation, BBN, CMB). Specific cases like the bullet cluster are stronger tests because MOND has slightly fewer knobs to adjust than GR+DM. But I'm not trying push MOND (on the main article space, anyway), just explain why DM isn't 100% certain, and at the same time indicate why this would have a big impact on the Big Bang model. PaddyLeahy 00:06, 6 May 2007 (UTC)
Still not convinced. CDM is used as a matter of course by theoretical cosmologists in N-body simulations, for example. To build, from the ground-up, a new theory of gravity will require an explanation for why N-body simulations have been successful. There will have to be some sort of accounting for why the supposed "epicycles" of non-baryonic dark matter work so well today when and if an explanation of the "heliocentric" paradigm shift of the alternative GR formulation is offered. I'm not arguing that respectable scientists don't propose alternatives, only that these proposals are too initial to warrant comment on this general, summary page.
To further describe my point, please consider the section of the article on dark energy. It is rather blythely assumed there that dark energy is all but certainly a negative-pressure term associated with a cosmological-constant like variable in the Einstein Equations. Only don't tell Rocky Kolb that (wink, wink)! I'm of the opinion that we handle dark energy properly on this page: the alternatives are a level of specificity removed and lack the detail development and incorporation into neostandard theory. As an interesting aside I might add that the theoretical work with dark energy is minimal when compared to the theoretical work that takes place with dark matter (mostly dark energy allows for the right amount of dark matter in order that the Friedman Equations yield a flat universe). This is why I'm of the opinion that alternatives to GR should be excluded from our discussion of dark matter. If people are really interested to learn more about the subject, let them read about them in the dark matter article itself. To include them here is to document the (sidelined) debate in the wrong place.
--ScienceApologist 03:05, 6 May 2007 (UTC)

(un-indent) I think ScienceApologist makes some very good points here. I think the central point is that it's just too far removed from Big Bang topic itself (though possibly germane for the dark matter article, which already has a section on alternative explanations). There are separate objections that I have for TeVeS and such, but since it's not on the article page I won't bring them up. I believe that the off-topic nature of the paragraph/sentence/whatever is by itself sufficient reason to remove.

Another problem is that the premise for inclusion is that "if GR is wrong, then we have to recalculate everything." For one thing, this is evident. For another, it would be absurd to insert similar statements elsewhere in the article. (In the CMB section, caveats that electromagnetism or atomic physics is wrong, in BBN nuclear physics or thermodynamics, then predictions must be recalculated). I am making a valid slippery slope argument because there is exactly as much evidence that GR is wrong as there is that electrodynamics, thermodynamics, atomic physics, etc are wrong -- none.

One final point. The argument is repeatedly made that "respectable scientists work on TeVeS." No one has ever argued the opposite here. The bone of contention is whether cosmologists in general feel that things like TeVeS are reasonable replacements for GR+CDM in the contexts of lensing, structure formation, CMB, BBN, and the Bullet cluster. A careful reading of your previous post reveals that claim that scientists who work on TeVeS mostly conclude this is true. I would say there is a strong selection bias there -- people who work on theories generally believe they're true. But are these only five people out of 1,000? Or 900 out of 1,000? If you were to strengthen your claim to say that most cosmologists think it's a comparable or better option than CDM, that's just false. Most cosmologists do not think that TeVeS is a serious contender to CDM (maybe it will be, but right now it's not).

So based on the reasoning in the first two paragraphs, and SA's previous post, I'm going to remove the alternative gravity stuff. It's off-topic, and the interested reader will find an entire section in the dark matter article and a whole article on alternatives to general relativity. Wesino 10:20, 6 May 2007 (UTC)

Actually this discussion highlights a fundamental difference in philosophy about NPOV that I mentioned earlier. For Wesino and SA, the "default theory" should be presented as fact unless there is an alternative theory that a significant fraction of the community believe is true. My interpretation of NPOV is that if the majority of the community is not convinced "beyond reasonable doubt" that the default theory is true, then the general reader deserves to know this and have a pointer to some of the other ideas kicking around. Your attitude gives the misleading impression that scientists are much more sure than they really are, it denies the reader relevant links (it took me a while to find the alternatives to GR article... a naive reader would not think of looking), and your conditions for mentioning rival theories will almost never arise in a mature science: I can't imagine that it will ever happen that say 30% of cosmologists are full convinced by MOND and the rest reject the idea out of hand. Instead, there will be rough agreement on the (subjective) probability of MOND being right. (I'd give odds of 5-10% at this point, not that out of line with Wesino's paper count). Compare Martin Rees's comment (in Just Six Numbers) that the COBE results allowed him to increase his personal odds on the big bang scenario as a whole from 90% to 99%... can we be more certain today about DM than we were about the big bang in 1990?
To SA: why are people doing all these CDM simulations? Answer: to calculate the implications of the theory and compare the results with observations, i.e. to see if the theory is right. They wouldn't be doing this if the theory was regarded as fact. (They'll do the same with TeVeS, given time). In fact around 30 papers have been published recently on "f(R) gravity", a theory which purports to dispose of dark energy, so if I was consistent I'd advocate a mention there too. OTOH the Kolb dispute is essentially on who can do the GR maths correctly, Kolb et al or everyone else; it's not an empirical point and Kolb et al are definitely a "tiny minority" there.
To Wesino: Unlike gravity on galactic & larger scales, we do have compelling experimental and observational evidence the EM, thermodynamics, and nucleosynthesis "work" pretty much as currently understood in the conditions relevant for big bang theory. The article even mentions one caveat, about fundamental constants changing with time. It also mentions that n-body simulations actually show a couple of significant discrepancies with observations (cuspy halos & dwarf galaxies), so all is not hunky-dory with GR+DM... we must modify one or both. My point about "mainstream" scientists is that experts on cosmology and galaxy dynamics (e.g. Binney, Silk), who had nothing to do with proposing MOND or TeVeS, are now publishing papers containing detailed calculations using the theory in order to assess it (in some cases clearly with the intention of ruling it out, though they have not yet been successful in this), i.e. work is not just being done by its inventors. PaddyLeahy 21:53, 6 May 2007 (UTC)
Would not a theory, TeVeS, which adds vector and scalar fields to the tensor field of GTR be equivalent to GTR plus new matter fields (the vector field and the scalar field)? Should not the quanta of these new fields be observable particles? JRSpriggs 05:14, 7 May 2007 (UTC)
I don't claim to fully understand TeVeS, but as far as I do understand it, it seems to be a more radical revision of GR than just adding a couple of fields... and a quantized version would be at least as far off as a quantum gravity theory with GR as its low-energy limit. OTOH Saunders' biscalar version of relativistic MOND apparently does predict a class of DM particles as you suggest. PaddyLeahy 12:46, 7 May 2007 (UTC)
PaddyLeahy, you are spouting factual inaccuracies. SPIRES says that Joe Silk has published precisely two papers that cite Bekenstein's TeVeS paper (Phys. Rev. D70, (2004) 083509). Here is a link to the abstract of one, and the other. The first one concludes that MOND is not a CDM replacement. From the abstract of the second one:
... we find that the cold dark matter is strongly favoured with Bayesian probability ratio of about one in two hundred.
Hardly resounding support for anything other than CDM. And as far as I can tell, despite your claim to the contrary, Binney has never written a paper citing Bekenstein's PRD70 article. Here's the SPIRES link if you care to check for yourself. Wesino 21:18, 7 May 2007 (UTC)
What I said was that Silk & Binney have published papers applying the theory, with a critical point of view, but had failed to rule it out. Your quotes support that for Silk: "strongly favoured" is not the same as a definitive proof. Silk's other paper says that MOND works for clusters if and only if heavy neutrinos are invoked, which is agreed by MOND partisans. The papers by Binney I referred to were on MOND rather than TeVeS since they addressed problems in the non-rel regime: [1] and [2]. Again these highlight problems but not fatal objections. PaddyLeahy 23:05, 7 May 2007 (UTC)
MOND's agreement with galaxy rotation curves has never really been in doubt. The trouble has always been its extension to other regimes, especially to relativistic regimes and cosmological scales where it never really worked well, if at all.
Keep in mind that this was the paper cited in the context of serious people taking alternative gravity seriously, and it still clearly talks about them being disfavored.
Also, in science there are no definitive proofs, just degrees of probability. (Unless you're a sound science proponent) Anything happens with finite probability, you just wouldn't be advised to bet on it. C'mon. Wesino | t | 00:45, 8 May 2007 (UTC)

Thanks to Wesino for his sober analysis of this problem. I concur with his conclusions. --Pjacobi 12:03, 8 May 2007 (UTC)

dark matter edit

I just changed this sentence in the dark matter section --

The detection of dark matter is sensitive only to its gravitational signature...

since I wasn't clear if the assertion was that DM interacts only gravitationally, or if that's just how we've detected it thus far. Since it's free to interact in other ways besides gravity (eg, WIMPs) I thought the new version might be more accurate. Wesino 19:21, 6 May 2007 (UTC)