The age of the universe: easy question - hard answer

[Garth]

Has the universe expanded over time? It depends on what ruler you use to measure it.
An 'atomic' ruler using the size of an atom as the standard of length would measure an expanding universe.
A 'photonic' ruler using the wavelength of a 'cosmological' photon (e.g. sampled from the CMB) as the standard of length would measure the universe to be static. Photons 'expand' with the universe even in GR.

But how do you explain cosmological redshift without expansion? The measurement of gravitational and cosmological redshift is a comparison of the energy of the photon with the mass of the atom it interacts with. If the wavelength of a photon is defined to be the standard of length then its energy is conserved. (Why should photons on null-geodesics crossing curved space-time with no other interactions lose energy?) If so then the mass of the atom it interacts with must have increased. Now we are no longer in GR but some other gravitational theory, such as the Jordan conformal frame of http://www.kluweronline.com/oasis.htm/5092775.

Garth

 

[turbo]

Dark matter and dark energy are required to comply with GR, not BB, and inflation is neither required or forbidden.

Gotta go to work, but quickly, inflation is absolutely required in the standard model to provide the isotropy in your [2].

The BB theory did not predict inflation. Inflation was required to rescussitate the BB model, and is a patch (epicycle), not a predictive success.

The BB model did not predict that the universe has expanded over time. That's putting the cart before the horse. The BB/expanding universe was proposed after Hubble and others observed that generally more distant objects have higher redshifts. Hubble was not very comfortable with the redshift=expansion concept, but the BB was a whiz-bang idea that captured popular sentiment, and it is presently being promoted as "true", "proven", etc. BB theory began as an extrapolation from the redshift=expansion idea, not the other way around.

 

[selfAdjoint]

The BB theory did not predict inflation. Inflation was required to rescussitate the BB model, and is a patch (epicycle), not a predictive success.

There is a subtle but important difference between something like inflation and an epicycle. An epicycle closes out discussion, it does not exist in "explanation space" but in "calculation space". Epicycles, and deferents, and the whole ptolemaic megillah, were never proposed as explanations or required to sustain any. Ptolemy specifically said the "causes"were the province of phlosophers and he was just proposing a calculation system to "save the appearances", i.e compute the positions of the heavenly bodies.

On the other hand inflation came originally with a physical explanation; that turned out to have gaps and so there has been a lot of physical research to find the correct explanation. Nobody (except maybe you and your friends) regards it as just a toss-away. And now it has become an important test for theories of quantum gravity. Those which can account for inflation have crowing rights over thse which cannot.

In general, a coarse view could regard almost any scientific innovation as "an epicycle", since they are typically a response to some failing of existing theory. If there weren't any failing, there wouldn't be room for any new physics!

 

[Garth]

On the other hand inflation came originally with a physical explanation.....In general, a coarse view could regard almost any scientific innovation as "an epicycle", since they are typically a response to some failing of existing theory. If there weren't any failing, there wouldn't be room for any new physics!

So where is the Higgs boson?

Garth

 

[turbo]

On the other hand inflation came originally with a physical explanation; that turned out to have gaps and so there has been a lot of physical research to find the correct explanation. Nobody (except maybe you and your friends) regards it as just a toss-away. And now it has become an important test for theories of quantum gravity. Those which can account for inflation have crowing rights over thse which cannot.

In general, a coarse view could regard almost any scientific innovation as "an epicycle", since they are typically a response to some failing of existing theory. If there weren't any failing, there wouldn't be room for any new physics!

I don't regard inflation as a toss-away, but as a "patch" required to keep the BB universe homogeneous and isotropic. If someone working on quantum gravity can explain a mechanism by which inflation starts (and stops, by the way) at the appropriate times to explain these observed qualities without having to invent additional entities and events, he or she will have accomplished something wonderful.

One reason why I find the BB model so suspect is that it requires many mysterious entities and events in order to stay viable. Paraphrasing Richard Feynman: Nature is complex, but the underlying rules are simple. In the standard model, complexity, unexplained behavior, and unobserved entities abound. This is not an encouraging sign.

 

[Chronos]

I don't regard inflation as a toss-away, but as a "patch" required to keep the BB universe homogeneous and isotropic. If someone working on quantum gravity can explain a mechanism by which inflation starts (and stops, by the way) at the appropriate times to explain these observed qualities without having to invent additional entities and events, he or she will have accomplished something wonderful.

One reason why I find the BB model so suspect is that it requires many mysterious entities and events in order to stay viable. Paraphrasing Richard Feynman: Nature is complex, but the underlying rules are simple. In the standard model, complexity, unexplained behavior, and unobserved entities abound. This is not an encouraging sign.

It is fair to characterize inflation as a patch, but I empathize with SA's discomfort in terming it an epicycle. It suggests adding complexity to rescue a flawed theory. Inflation simplifies the model by explaining flatness, homogeneity, high energy relic particles, and smoothly leads to big bang nucleosynthesis. Inflation has plenty of theoretical support. It occurs in many models, depending on initial conditions - and therein lies the rub. It emerges in models that are mutually exclusive and the permissible range of initial conditions is too broad to distinguish which models are viable and which are not. It is logical to continue gathering clues to narrow those ranges.

I do not understand your assertion the big bang model "requires many mysterious entities and events in order to stay viable". Please be specific. As previously noted, you cannot undermine the big bang model using dark matter, dark energy or Higgs bosons without turning GR and particle physics models on their heads as well. That's a lot to put on the same plate.

 

[turbo]

I do not understand your assertion the big bang model "requires many mysterious entities and events in order to stay viable". Please be specific. As previously noted, you cannot undermine the big bang model using dark matter, dark energy or Higgs bosons without turning GR and particle physics models on their heads as well. That's a lot to put on the same plate.

Perhaps I should have said "standard model" instead, to be very inclusive, but the BB is inextricably intertwined with GR. It is a model of an emergent universe in GR space-time. You could just as well posit an emergent universe in Euclidean space, or perhaps a steady state universe in either of those two geometries.

As for the requirements of the BB model, the putative present-day accelerated expansion of the BB universe requires dark energy of some kind regardless of the geometry (GR, Euclidean, Klingon ). GR does not demand this acceleration, nor does particle physics, QED or any other field of science - just BB cosmology.

 

[moving finger]

I don't regard inflation as a toss-away, but as a "patch" required to keep the BB universe homogeneous and isotropic. If someone working on quantum gravity can explain a mechanism by which inflation starts (and stops, by the way) at the appropriate times to explain these observed qualities without having to invent additional entities and events, he or she will have accomplished something wonderful

I guess you regard inflation as a "patch" because inflation does not follow as a necessary consequence of other more accepted fundamental physical theories? This is as it may be, but the fact remains that inflation does neatly explain a number of features of our universe which cannot be easily explained in other ways. Not only the observed homogeneity and isotropy of the universe, but also why the actual mass density appears so close to the critical mass density, and why the universe apparently started in a very unusual entropy state (the "unreasonable aim of the creator" which Penrose cannot explain in any other way - though Penrose himself seems to be an inflationary sceptic).

Is there perhaps an alternative to inflation which explains homogeneity, isotropy, critical mass and initial entropy as neatly and simply as inflation does?

 

[Garth]

Is there perhaps an alternative to inflation which explains homogeneity, isotropy, critical mass and initial entropy as neatly and simply as inflation does?

Yes freely coasting cosmolgy , which requires a mechanism to deliver the strictly linear cosmological expansion (Kolb's K-matter in A coasting cosmology ) that is provided by Self Creation Cosmology.

Inflation explains critical mass only if combined with Dark Energy. The WMAP data is consistent not only with flat geometry but more generally with conformally flat geometry, it being angular in nature, but a conformally flat model, such as the freely coasting model, would not need DE. It also identifies DM as well as being baryonic. Occam's razor anyone?

 

[moving finger]

Inflation explains critical mass only if combined with Dark Energy. The WMAP data is consistent not only with flat geometry but more generally with conformally flat geometry, it being angular in nature, but a conformally flat model, such as the freely coasting model, would not need DE. It also identifies DM as well as being baryonic. Occam's razor anyone?

Thanks for the leads. I'll check them out.
But I'm not sure that Dark Energy has anything to do with critical mass? I think you mean Dark Matter? In fact inflation predicts that the mass density of the universe should be close to the critical mass density and therefore it "requires" Dark Matter (or something similar) to exist.

 

[Garth]

But I'm not sure that Dark Energy has anything to do with critical mass? I think you mean Dark Matter? In fact inflation predicts that the mass density of the universe should be close to the critical mass density and therefore it "requires" Dark Matter (or something similar) to exist.

Inflation predicts exact critical density.

However observations of gravitational lensing and cluster dynamics can provide only 1/3 and 1/5 critical density respectively. But the WMAP data is consistent with a flat geometry i.e. exact critical density. Furthermore the recent distant S/N Ia observations indicated cosmological acceleration.

So the shortfall of ~2/3 critical density was made up by a new concept, DE, which might also then be conscripted to explain the acceleration as well.

However nobody knows what DE is, although there are plenty of ideas, (leaky gravity anyone?) and therefore it might be considered a 'patch' or 'epicycle' introduced to save the standard model.

Observed density is only about 0.4% critical density max, and the maximum baryonic density coming out of the standard BBN is only 4% critical density so DM is required to make up the difference to 27% as required by modelling of the WMAP data.

That there are published alternatives that do not require such patches (although they may have their own individual problems) ought to be more readily acknowledged.

Garth

 

[SpaceTiger]

Inflation explains critical mass only if combined with Dark Energy.

Though it's also worth noting that dark energy potentially explains the supernovae measurements as well. I share your skepticism of the standard model, but to be fair, I think it contains the fewest epicycles/patches/tooth fairies of all currently available theories.

 

[moving finger]

Inflation predicts exact critical density.

not wishing to seem pedantic, the critical density is a real number. inflation forces the mass/energy density to be asymptotically close to this critical density by multiple (but limited) number of doublings of space. The actually mass/energy density would only become exactly equal (in a strictly mathematical sense where real numbers are concerned) to the critical density if those doublings continued indefinitely, which they did not. We must therefore accept the possibility (even with inflation) that the actual mass/energy density is not "exactly" equal to the critical mass density

However observations of gravitational lensing and cluster dynamics can provide only 1/3 and 1/5 critical density respectively. But the WMAP data is consistent with a flat geometry i.e. exact critical density.

again i take issue with your use of the word "exact" in this context. Do you know the error bars involved? (the error bars need to be literally of zero length for an exact match)

However nobody knows what DE is, although there are plenty of ideas, (leaky gravity anyone?) and therefore it might be considered a 'patch' or 'epicycle' introduced to save the standard model.

Your original post regarding "patches" on this thread referred to Inflation as a patch, which is where I joined the discussion. Any concept which is not predicted or required by accepted basic theory can be considered a patch - so both Dark Matter and Dark Energy are also "patches" in this sense.

Dark Energy can be considered to be the vacuum energy (the energy contained within the vacuum) with an equation of state given by pressure = - energy density (for matter the equation of state tells us that pressure is independent of energy density, and for radiation pressure = 1/3 x energy density). Negative pressure leads to repulsion (this was the source of the original cosmological constant in Einstein's equations), hence vacuum energy causes the universe to expand.

 

[Chronos]

One thing I find striking about inflation: It suggests the universe has and will always be within a quantum fluctuation of perfectly flat. That is compelling. It is too much a coincidence that the universe appears to be almost perfectly flat now, unless it was always flat.

 

[SpaceTiger]

One thing I find striking about inflation: It suggests the universe has and will always be within a quantum fluctuation of perfectly flat.

Well, that depends on the cosmological parameters of the universe. In a simple open (omega_tot<1) matter-dominated model, the comoving Hubble radius increases and eventually encompasses the whole of the pre-inflation universe. The geometry pre-inflation is completely unknown, so in this case, the universe couldn't be guaranteed to always be flat. If, however, we enter another era of accelerated expansion (as would be the case with a cosmological constant), then the universe would actually get more flat with time because the comoving Hubble radius would decrease.

 

[moving finger]

One thing I find striking about inflation: It suggests the universe has and will always be within a quantum fluctuation of perfectly flat. That is compelling. It is too much a coincidence that the universe appears to be almost perfectly flat now, unless it was always flat.

I don't think so. The energy/mass density may have been driven extremely close to the critical density (ie Omega was almost exactly equal to 1) during inflation (ie the universe was almost perfectly flat at the end of inflation, with a value of Omega equal to 1 within 15 decimal places according to Alan Guth), but what evidence do we have that it remains perfectly flat today, or will continue to be flat in the future? The experimental evidence is that the CMB is isotropic only to one part in 10^5, and the latest WMAP observations have an uncertainty in Omega of plus or minus 0.02 (see http://www-ctp.mit.edu/~guth/iccs/iccs-guth.pdf ). All we can conclude from this is that the universe is "almost" flat now, but certainly not "within a quantum fluctuation of perfectly flat".

It is too much a coincidence that the universe appears to be almost perfectly flat now, unless it was always flat.

No need to invoke coincidences. The explanation is inflation, which drove the universe to almost perfect flatness at the time of the Big Bang; but that does not rule out subsequent deviations from flatness.

 

[SpaceTiger]

No need to invoke coincidences. The explanation is inflation...

Well, let's be a little careful there. Inflation is an extremely nice solution to a lot of problems, but it has yet to make any really convincing predictions. There may be other nice solutions to the flatness and horizon problems that we simply haven't thought of yet. The trouble with inflationary theory is that it can produce virtually any final result if you tweak the parameters enough.

Besides, we're still struggling to explain how inflation stopped.

 

[moving finger]

Well, let's be a little careful there. Inflation is an extremely nice solution to a lot of problems, but it has yet to make any really convincing predictions.

Incorrect. Inflation theory was first developed around 1980, when many cosmologists did not accept the universe is necessarily flat and isotropic, and there certainly was little experimental evidence to suggest that it was either flat or isotropic. Inflation was developed precisely because the conventional Big Bang model was inadequate to properly explain the universe - that's exactly why new theories are developed in the first place. Inflation predicts that the universe will be flat and isotropic, and the really good experimental evidence confirming this is the case has come in only in the last couple of years (WMAP data), long after the theory was established.

There may be other nice solutions to the flatness and horizon problems that we simply haven't thought of yet. The trouble with inflationary theory is that it can produce virtually any final result if you tweak the parameters enough.

There may be, and there may not be, other solutions. Until someone comes up with a better explanation, which fits the facts as well or better than inflation, then we can either say "I have no idea" or "inflation provides a good explanation". I know which answer I prefer.

Besides, we're still struggling to explain how inflation stopped.

And your point is? Just because a theory does not provide all of the answers is not a reason to reject it as a good explanation for the answers it does provide.

I can imagine there were sceptics around in Newton's time who said " let's be a little careful, there may be other solutions to the gravity problem we haven't thought of yet"... of course Newton was not entirely correct, and he could not explain where gravity "came from", but his ideas and insight advanced science and mechanics for the next 200 years.

 

[SpaceTiger]

Incorrect. Inflation theory was first developed around 1980, when many cosmologists did not accept the universe is necessarily flat and isotropic

You're joking, right?

His original paper was entitled: Inflationary universe: A possible solution to the horizon and flatness problems

By the way, isotropy is not a prediction of inflation, it's a generic property of universes that obey the cosmological principle.

Inflation was developed precisely because the conventional Big Bang model was inadequate to properly explain the universe - that's exactly why new theories are developed in the first place. Inflation predicts that the universe will be flat and isotropic, and the really good experimental evidence confirming this is the case has come in only in the last couple of years (WMAP data), long after the theory was established.

I think you need to brush up on your cosmology history a bit. WMAP gave us very precise measures of the cosmological parameters, but their orders of magnitude (all that's needed to determine that the problems existed) have been known for decades.

There may be, and there may not be, other solutions. Until someone comes up with a better explanation, which fits the facts as well or better than inflation, then we can either say "I have no idea" or "inflation provides a good explanation". I know which answer I prefer.

I didn't say there was anything wrong with inflation, I just said that it's not yet on an entirely solid foundation. You shouldn't simply assume theories are right simply because you haven't thought of an alternative. There's a long history of such mistakes in physics.

Besides, I'm not saying "I have no idea", I'm saying that inflation is not a sure thing, as you seemed to suggest.

And your point is? Just because a theory does not provide all of the answers is not a reason to reject it as a good explanation for the answers it does provide.

I'm not rejecting it, I'm saying that we need to converge on a stopping mechanism before we can say that it's true beyond reasonable doubt. Besides, the lack of a stopping mechanism is not a failure to explain something, it's an inherent problem with the model.

I can imagine there were sceptics around in Newton's time who said " let's be a little careful, there may be other solutions to the gravity problem we haven't thought of yet"...

I certainly hope that there were. Such skepticism is extremely helpful for the scientific process.

 

[moving finger]

WMAP gave us very precise measures of the cosmological parameters, but their orders of magnitude (all that's needed to determine that the problems existed) have been known for decades.

At the time Guth first published there were certainly those who "believed" the universe was flat and isotropic, but there was very little real evidence that this was indeed the case. Inflation predicted that the universe should be very flat, not just within "orders of magnitude" - and the observational verification of this has only been made very recently. Inflation was developed as the explanation of a perceived problem, and since its development the magnitude of the "problem" has been confirmed to be in perfect agreement with the predictions of inflation.

By the way, isotropy is not a prediction of inflation, it's a generic property of universes that obey the cosmological principle.

The Cosmological Principle is a principle, not a theory, and since it is a principle it makes no predictions. Any cosmological theory must obey the Cosmological Principle otherwise it is doomed to failure. The predictions of inflation obey the Cosmological Principle. Isotropy is one prediction of the theory of inflation. Isotropy may also be a prediction of other theories, but that doesn't invalidate the claim that it is predicted by inflation.

I didn't say there was anything wrong with inflation, I just said that it's not yet on an entirely solid foundation. You shouldn't simply assume theories are right simply because you haven't thought of an alternative. There's a long history of such mistakes in physics.

Science proceeds by people advancing hypotheses to explain the facts, and those hypotheses then being (possibly) disproven. No hypothesis can ever be proven to be 100% correct, the best any hypothesis can hope for is to stand the test of time by agreeing with observations and not being proved incorrect. This is where inflation is right now. Its standing up there waiting to be shot down, and so far nobody has offered up any viable contenders or shot it down.

I'm saying that inflation is not a sure thing, as you seemed to suggest.

You're misreading me. I never said inflation is a "sure thing" - what I am saying is its the best candidate we have right now. If you know of a better candidate then I'm sure we would all love to hear about it.

 

[SpaceTiger]

At the time Guth first published there were certainly those who "believed" the universe was flat and isotropic, but there was very little real evidence that this was indeed the case. Inflation predicted that the universe should be very flat

Actually, the flatness problem is not just about the flatness of the universe now, but also in the past. The reason is that an omega~1 universe is extremely unstable and if it's within a factor of 100 of flat now, that means that it was very, VERY close to flat at the time of recombination (must fine-tune H_0 to one part in 10^55). In 1980, they knew that the density was within a factor of 100 of the critical density, so there was definitely a problem, as his paper suggests. I suggest you read it for more details.

The Cosmological Principle is a principle, not a theory, and since it is a principle it makes no predictions. Any cosmological theory must obey the Cosmological Principle otherwise it is doomed to failure. The predictions of inflation obey the Cosmological Principle. Isotropy is one prediction of the theory of inflation. Isotropy may also be a prediction of other theories, but that doesn't invalidate the claim that it is predicted by inflation.

Ok, you're right in the sense that isotropy does arise from inflation, but the point I was trying to make was that it's not even close to being a generic prediction of inflation and certainly can't be used to defend its veracity. It's also not true that they didn't know about it at the time, as the horizon problem can be seen from even the crudest observations of the microwave background.

Science proceeds by people advancing hypotheses to explain the facts, and those hypotheses then being (possibly) disproven. No hypothesis can ever be proven to be 100% correct, the best any hypothesis can hope for is to stand the test of time by agreeing with observations and not being proved incorrect. This is where inflation is right now. Its standing up there waiting to be shot down, and so far nobody has offered up any viable contenders or shot it down.

Agreed and I think it has every right to be the preferred theory. I'm just not confident beyond a reasonable doubt, as I said. It simply doesn't have the predictive power of a theory like relativity or Newtonian gravity.

You're misreading me. I never said inflation is a "sure thing"

"No need to invoke coincidences. The explanation is inflation, which drove the universe to almost perfect flatness at the time of the Big Bang..."

I think I can be forgiven for misreading that as certainty.

- what I am saying is its the best candidate we have right now. If you know of a better candidate then I'm sure we would all love to hear about it.

I don't need an alternative to be skeptical. If someone tells you that they saw aliens, do you automatically believe them if you can't immediately come up with another explanation? Besides, in case you didn't know, a large fraction of the astronomical community shares my skepticism. I can understand that you would react this way, as I'll bet you get a lot of crackpots on this message board, but I can assure you that my position on this issue is not fringe.

 

[Garth]

First WMAP has not shown that the universe is flat, only that it is conformally flat, its measurements are angular in nature and conformal transformations preserve angular relationships. When will this glaring fact be recognised?? If the universe were truly flat then there should be no shortage of low l anisotropies in the data. Their deficit exists not only in the WMAP data but also the BALLOON and COBE as well and cannot be ignored as experimental error.

Secondly inflation simply resolves, or fixes a problem of GR cosmology. If the theory did not need to be fixed there would be no reason to invoke inflation.

Thirdly alternatives do exist A Concordant “Freely Coasting” Cosmology, which do not have the problem in the first place. Why is not more significance given to them? Occam's razor would normally guarantee their significance.

Yes, reference comments on my previous post I was using the word 'exact' in an observational and not mathematical sense. And I do consider DM and DE together with inflation to be 'patches' or 'epicycles' invoked to save the standard theory (search for my previous posts on the subject).

Garth

 

[turbo]

... of course Newton was not entirely correct, and he could not explain where gravity "came from", but his ideas and insight advanced science and mechanics for the next 200 years.

Einstein could not explain where gravity "came from" either. He modeled gravitational effects mathematically, and expressed those effects as though masses curve space-time. That approximation works very well (as does the Newtonian approximation) in simple systems, like planets circling a star. It does not work well on galactic and calactic-cluster scales. It cannot model galactic rotation accurately, and does not predict the observed strong cluster lensing or cluster binding. Dark matter (distributed "just so" in each relevant application) is required to fix GR gravity. Instead of inventing dark matter, physicists could have saved a couple of decades, and revisited the curved space-time model of gravitation. That model cannot be reconciled with quantum field theory (which appears to be VERY accurate) - when several conflicts occur with a basic model, especially on such a huge span of scales (nuclear to galactic), we should suspect a problem with the model, and not resort to patches or epicycles.

 

[SpaceTiger]

That model cannot be reconciled with quantum field theory (which appears to be VERY accurate) - when several conflicts occur with a basic model, especially on such a huge span of scales (nuclear to galactic), we should suspect a problem with the model, and not resort to patches or epicycles.

They tried a model that altered gravity at weak accelerations (MOND) and it has performed very poorly. Dark matter is more than just a fix, we have indepedent evidence for it. See, for example:


Clowe et al. 2004

I know dark matter is not an elegant solution to the rotation curve problem, but it works. The discrepancies with QFT at small scales are, more than likely, only relevant at small scales.

 

[turbo]

They tried a model that altered gravity at weak accelerations (MOND) and it has performed very poorly. Dark matter is more than just a fix, we have indepedent evidence for it. See, for example:


Clowe et al. 2004

I know dark matter is not an elegant solution to the rotation curve problem, but it works. The discrepancies with QFT at small scales are, more than likely, only relevant at small scales.

The paper is an interesting and compelling refutation of MOND at cluster scales. The underlying observations are not a confirmation of the existence of dark matter, however. The existence of dark matter is inferred from the failure of GR to properly predict the cluster lensing and cluster member velocities. Further observations of this type cannot be regarded as independent evidence for the existence of dark matter. We know that GR cannot properly predict galactic rotation curves, cluster binding, or cluster lensing, given the luminous matter observed. Some of us are so certain that GR must be right that we are unwilling to challenge it, and so we have to invent entities to "balance the books".

Let's pretend that non-baryonic dark matter does not exist, and we have to observe the universe with the radiation, particles, matter, of the known classical universe. In that case, GR's gravitation model works extremely well on most scales. GR fails to be predictive on galactic scales and larger, however. If we resist the temptation to enshrine GR as "perfect" (and Einstein's made his views on epistemology very well known), we must admit the possibility that GR's gravitational model is flawed - a better approximation perhaps than the Newtonian model, but still not correct.

 

[SpaceTiger]

The paper is an interesting and compelling refutation of MOND at cluster scales. The underlying observations are not a confirmation of the existence of dark matter, however. The existence of dark matter is inferred from the failure of GR to properly predict the cluster lensing and cluster member velocities.

The paper is more than just a refutation of MOND. This is a case of a lensing event separate from the center of light in the cluster. In order to reproduce that in an alternative gravitational model, you'd have to alter the geometry of spacetime (or the non-GR equivalent) asymmetrically from the mass center. Although I'm sure one could concoct a model like that (as one could concoct an arbitrarily complicated one to explain anything), it would have to be extremely ad hoc.

Anyway, you haven't really given a reason for why you think that the author's arguments are wrong, so you'll have to elaborate. Your arguments seem practiced to the more traditional problems of rotation curves and velocity dispersions.

Some of us are so certain that GR must be right that we are unwilling to challenge it, and so we have to invent entities to "balance the books".

Actually, the general feeling of the astronomical community is that we wished we didn't have to invoke dark matter, as its extremely inelegant. However, it has done a far better job of explaining the observations than any alternative model of gravity.

In that case, GR's gravitation model works extremely well on most scales. GR fails to be predictive on galactic scales and larger, however. If we resist the temptation to enshrine GR as "perfect" (and Einstein's made his views on epistemology very well known), we must admit the possibility that GR's gravitational model is flawed - a better approximation perhaps than the Newtonian model, but still not correct.

We know that it's flawed because it's inconsistent with quantum field theory, but alternative models at the scales you're talking about have, so far, all failed. Dark matter is the best fit to observations at this point.

 

[selfAdjoint]

We know that it's flawed because it's inconsistent with quantum field theory, but alternative models at the scales you're talking about have, so far, all failed. Dark matter is the best fit to observations at this point.

AFAIK, QFT can't be well-defined in curved spacetime, any more than GR can be perturbatively quantized. Is that what you mean by saying GR is inconsistent with QFT? Perhaps QFT is inconsistent with GR?

Added: Plus, QFT has consistency problems of its own, see Haag's theorem for example, or Landau pole. Mere accuracy at low orders of perturbation expansion does not a consistent theory make.

 

[turbo]

The paper is more than just a refutation of MOND. This is a case of a lensing event separate from the center of light in the cluster.

That's not what the authors said - emphasis mine:

The detected mass peak is located between the X-ray peak and galaxy concentration, although the position is consistent with the galaxy centroid within the errors of the mass reconstruction.

The authors say that the lensing is consistent with the center of mass center of the clusters, which shows the presence of dark matter in the clusters and refutes the expectation (in MOND) that ram-stripped X-ray gas between the clusters should be expected to exhibit lensing effects.

Anyway, you haven't really given a reason for why you think that the author's arguments are wrong, so you'll have to elaborate. Your arguments seem practiced to the more traditional problems of rotation curves and velocity dispersions.

I am not refuting the lensing effects that they measured, nor the fact that the lensing effects are consistent with the centers of the clusters. I am saying that this is entirely expected, but is not a demonstration of the reality of dark matter. It is instead a measure of the failure of GR to properly predict the strength of gravity in very massive domains. MOND does not perform well in galactic-cluster environments, nor would I expect it to in the model that I have been working on.

We know that it's flawed because it's inconsistent with quantum field theory, but alternative models at the scales you're talking about have, so far, all failed. Dark matter is the best fit to observations at this point.

I have a compelling candidate for dark matter (although it is really more than that). It is almost impossible to detect by any means (save the Casimir Effect), and it suffuses all of space. It's the electromagnetic field of the vacuum - the Zero Point Energy. All we need is a mechanism by which these virtual particle/antiparticle pairs can be polarized. Polarization and densification of the field and the resultant self-attraction will provide the "extra" gravitational attraction in massive domains that perplex GR now. How can these virtual pairs be aligned? By a differential in the gravitational infall rate of matter vs antimatter. The Athena project at CERN is designed to produce experimentally-useful quantities of anti-hydrogen. If my model is correct, the inertial mass of anti-hydrogen and hydrogen will be identical, but their gravitational masses will differ due to matter-antimatter attraction. If the Athena project shows no difference in gravitational masses, the ZPE gravitation model is falsified. The good news is that my model can be falsified quite readily, unlike standard cosmology, which apparently cannot be falsified by any discordant observations. The ZPE gravitation model is simple, and it is based entirely on logic and on things we already know to exist (no additional entities need apply), and it may soon be proven viable or dead by Athena.

If the ZPE EM fields can be polarized by the presence of mass, Sakharof and others who suggested decades ago that the ZPE might be the source of gravitation and inertia will be taken seriously. Of course, to express forces in a field, the field will have to be capable of polarization. To my knowledge, these people never proposed a mechanism by which the ZPE fields might be polarized or densified. If they had pursued this, cosmology might look very different today.

Once it is shown that mass polarizes the ZPE EM field, inertia will be seen as a resistance to acceleration relative to the LOCAL ZPE field, and not the mysterious Machian effect where the acceleration is relative to EVERYTHING in the universe. Gravitation will be seen as force acting through local fields in Euclidean space, not as space-time curvature. It is this type of simplification of cosmology (with an actual mechanism to explain gravitation, this time) that will be required before cosmology (large-scale physics) can be reconciled with QFT. Occam's Razor leads me to believe that this simplification (get rid of curved space-time, dark matter, dark energy, etc) and its likely resolution of GR problems at extreme scales (cluster down to sub-atomic) is far more likely to be true than the repeated piling-on of mysterious entities that we have seen in recent decades. We know that the ZPE fields exist. Why posit the existence of dark matter and dark energy without first exploring the behavior of these fields with respect to embedded masses, gravity, lensing, etc?

I have been working on the ZPE gravity model for about a year, and will be willing to email a non-technical summary of it (my math skills are sorely inadequate, so it really is non-technical by necessity) to anyone who would like to look it over. Beat it up, bash it, etc - I welcome your comments, and I have thick skin as long as you only beat up the model and not me.

Apologies to Nereid and others who are sick of hearing this. I hate to annoy nice well-meaning people who believe in the infallibility of GR, but I am firmly convinced that GR's model of gravitation is wrong. A good approximation with wide applicability, but fundamentally wrong.

 

[Chronos]

A couple of thoughts. Inflation can be explained [including how it stops] using scalar fields. There is pretty good selection of papers on the concept. Here is an interesting recent entry:
http://arxiv.org/abs/astro-ph/0409071
I also share SelfAdjoints question of the source of conflict between GR and QFT. I personally feel GR is more nearly complete than QFT. GR is not the only theory that has compatibility issues with QFT, as SA noted.

 

[SpaceTiger]

The authors say that the lensing is consistent with the center of mass center of the clusters, which shows the presence of dark matter in the clusters and refutes the expectation (in MOND) that ram-stripped X-ray gas between the clusters should be expected to exhibit lensing effects.

Dark matter implies that there's mass that we can't see - i.e. that the center of mass would be in a different location from the light...which is exactly what you just said. This is not MOND-specific, it applies to any alternative gravity model. He just talks about MOND because it's the favored alternative model.

I am not refuting the lensing effects that they measured, nor the fact that the lensing effects are consistent with the centers of the clusters.

The whole point is that the lensing effects are inconsistent with the center of light in the clusters:

"The observed offsets of the lensing mass peaks from the peaks of the dominant visible mass component (the X-ray gas) directly demonstrate the presence, and dominance, of dark matter in this cluster."

Your model is very interesting and I look forward to see if its predictions are borne out, but you'll have to take into consideration evidence like that given above.