Following the recent incident, we and the SciLogs team decided to invite a renown colleague to write a guest blog post. Thinking about possible guest bloggers who are experts in the field of cosmology and approach theories such as MOND with the necessary scientific skepticism, we arrived at Scott Dodelson as one candidate.
Scott is a very well-respected cosmologist. He is a scientist at Fermilab and a professor in the Department of Astronomy and Astrophysics and the Kavli Institute for Cosmological Physics at the University of Chicago. His research focuses on the largest and smallest scales of the universe: the interplay of cosmology and particle physics. He investigates the nature of dark matter and dark energy, works on the cosmic microwave background and is also interested in modified gravity theories. In addition to his many papers, he has written the textbook “Modern Cosmology”.
We are very pleased that Scott Dodelson has accepted to write this guest post. Thank you, Scott!
Is modified gravity a viable alternative to dark matter? Or is dark matter so compelling that pursuits of modified gravity should be abandoned?
There are good reasons to believe in dark matter and to be optimistic about our chances of detecting it in the coming decade. Dark matter explains the flat rotation curves in galaxies; it accounts for the deflection of light far from the centers of galaxies and by galaxy clusters. Many aspects of galaxy clusters make sense only if dark matter is present. Perhaps most importantly, it is the key component in our modern story of how we got here: the standard cosmological model is called CDM or “Cold Dark Matter”. The small inhomogeneities captured in maps of the cosmic microwave background (CMB) grew to be the vast structure we see today via gravitational instability, but the story holds together only if dark matter is also present. The story works and it has been tested by observing the spectra of both the CMB and the distribution of matter on large scales. It is true that dark matter does not easily explain some phenomena on small scales, but there is a ready explanation for this: predictions on small scales are hard. Apart from the non-linearity of gravity, baryons play an important role on small scales, and incorporating these effects into numerical simulations is challenging. It is easiest to make predictions on large scales and those easy predictions have been confirmed with exquisite precision. Beyond all this lies the suite of experiments poised to detect dark matter. Thousands of scientists are now hunting for the particles that comprise dark matter by studying collisions at the LHC; by manning underground laboratories designed to detect it; and by launching satellites to observe the debris created when two dark matter particles in space collide and annihilate. We have reason to be optimistic.
Why then pursue modified gravity?
First, the people who study modified gravity (MG) tend to focus on small scale data rather than large scale data. They are serious, smart scientists who make observations and fit MG models to the data. These fits tend to be pretty good, often with very few free parameters and therefore the scientists gain confidence in their models. This focus on different data or different slices through the data presents a challenge to the dark matter model. Eventually, dark matter will have to explain these data sets as well. Slicing and combining things in different ways leads to different challenges than might otherwise arise. Even if you believe in dark matter, you want to confront the data in all forms. The simple (slightly condescending) way of saying this is to say that CDM must ultimately reduce to MONDian phenomenology on small scales.
More importantly, dark matter has not yet been detected. This is not the time to raise the barriers and decree that only those who accept dark matter are serious scientists. We are optimistic, but we have to accept the possibility that dark matter will not be detected in the next decade. Our initial feedback from the LHC shows no hint for the simplest model that contains dark matter, supersymmetry (although these early data are certainly not conclusive). There have been hints in direct and indirect detection experiments, but certainly nothing definitive. It is possible that we will need to think of something completely new. In so doing we are going to have to drop some assumptions, weight evidence differently than we do now. The MG community does this now by downweighting large scale data and focusing more on small scales. This may end up being the correct approach, or we may need to think of something even more radical. I do not know how to do this (How do we encourage a revolution?) but I am pretty sure suppressing alternatives is moving in the wrong direction.
The communities now are quite disparate and find it difficult to engage one another. Is the MG vs. dark matter dispute identical to the disagreements between people from different religions, say, virtually impossible to resolve because the two sides cannot communicate? Certainly not. We are scientists, and facts will change our minds. Some examples of things the vast majority of the MG community accepts or will accept:
- MG is not theoretically favored over dark matter because “dark matter is something new”. Both approaches are changing the fundamental lagrangian of nature by adding new terms and new degrees of freedom.
- The fact that Xenon100 or Fermi (or perhaps AMS in a few days) has not seen dark matter does not mean the theory is excluded. There is plenty of room in theories like supersymmetry and even more in other more generic models.
- If dark matter is detected unambiguously via direct and/or indirect detection, then MG would indeed fall outside the realm of reasonable scientific investigation.
On the other hand, our dispute does share similarities with those that divide adherents of religion. We are passionate, we come at things from different directions with different preconceptions, so it is sometimes difficult to speak the same language, to focus on a single question. At the end of the day, just like the devout in different religious traditions, we are all after the same goal, in our case, trying to understand nature. It is premature to state that our way is the only way.
The Dark Matter Crisis 
Very nice blog post. As I have myself written a invited review on MOND for Living Reviews in Relativity (http://relativity.livingreviews.org/Articles/lrr-2012-10), I also feel I should comment on the issue.
The main scientific controversy here revolves around whether particle (cold or warm) dark matter, made of yet-unknown stable elementary particles, is the ultimate explanation for the observations of mass discrepancies in galaxies. The various observational results presented in this blog are challenging this interpretation, and go far beyond the classical missing satellites and cusp/core problems. In view of this, investigating slightly or completely different explanations from plain simple particle dark matter in galaxies is sound. That surely does not mean it will be succesful, it might very well not, but this is how science works. And I of course agree with Scott Dodelson that if particle dark matter is detected unambiguously in the Milky Way via direct and/or indirect detection, then alternatives would indeed fall outside the realm of reasonable scientific investigation. But for the time being, this is not the case. Such an alternative explanation could be linked to a novel property of the fields composing the dark sector, a novel property of gravity and space-time, a novel property of inertia, or some combination of the three. The crucial point is that these novel properties should lead to a unique relation between the distribution of ‘normal’ matter (baryons) and the gravitational field in galaxies. Such a one-to-one relation is in contradiction with the a priori expectations from the standard cosmological model, because the different histories and environments of individual galaxies should a priori not lead to such a unique relation between the dark matter and baryon distributions. On the other hand, such a one-to-one matching is at the core of the MOND paradigm, which actually defines such a universal relation (Milgrom’s relation) between the distribution of baryons and the gravitational field in galaxies. This relation is indeed observed, and has not been falsified in galaxies for the last 30 years: since this is an empirical fact, calling it ‘bad science’ or pseudoscience can only be a misinformed statement.
Indeed, in my view, the observational success of Milgrom’s relation in galaxies is very interesting because the history of physics has taught us that the devil was often hidden in such ‘details’. In the present case, the detail will for sure either:
1) Teach us something fundamental about the galaxy formation process (this is the ‘CDM must reduce to MONDian phenomenology’ argument), or
2) Teach us something fundamental about the very nature of the dark sector, or
3) Teach us something new and fundamental about gravity and dynamics.
Either way, what makes it really cool is that it makes a lot of succesful predictions that cannot be made from LambdaCDM: just as examples, it predicted the shape of rotation curves of Low Surface Brightness galaxies before these objects had even been detected, and more recently predictions on the internal velocity dispersions of two satellite galaxies of Andromeda by McGaugh and Milgrom were subsequently confirmed by Tollerud et al. (http://arxiv.org/abs/1302.0848). On the other hand, a lot of also really cool theoretical properties can be studied (e.g. http://arxiv.org/abs/1202.1723), which allows us to slice and combine observational data in different ways than in the standard picture.
Now, IMO, this MOND relation cannot, by itself, really be called a theory: MOND is a paradigm based on a general (and observationally succesful) relation to which different actual MOND-theories (TeVeS, BIMOND, dipolar dark fluid, entropic gravity, etc.) must conform. As they all boil down to the same metric as General Relativity in the static weak-field limit, but with a boosted weak-field potential, gravitational lensing by galaxies is not a problem (http://arxiv.org/abs/0804.2668). As also pointed by Scott Dodelson, these theories mostly modify the fundamental Lagrangian of nature by adding new terms and new degrees of freedom, which can be thought of as parts of the ‘dark sector’ of the Universe, akin to dark energy fields: this makes Options 2 and 3 hereabove somewhat entangled with each other.
But the main weak point of MOND is that when Milgrom’s relation is applied to galaxy clusters, it fails. What is more, there is currently no viable alternative to the existence of something behaving like unseen collisionless particles to explain the lensing of interacting galaxy clusters. These are also most helpful to erase the large baryonic acoustic oscillations in the matter power spectrum, and to explain the high 3rd acoustic peak of the Cosmic Microwave Background (CMB).
Given the uncanny success of Milgrom’s relation in galaxies and the other problems for LambdaCDM pointed out in this blog, if we take the road of Options 2 or 3 hereabove, what are then the solutions? The first one is to consider some modification of gravity explaining the MOND relation in galaxies AND the existence of cosmological dark matter that would not condense on galaxy scales: these dark matter particles should thus be ‘hot’ (rather than cold or warm), for instance composed of light sterile neutrinos. The opposition to this hypothesis in the scientific community is often a philosophical one, because it would supposedly make the most complicated possible Universe, in which we would have BOTH modified gravity and particle dark matter. To me, that aesthetical argument would hold if there was not already something even more mysterious than dark matter around us, something which we dub ‘dark energy’ and which might also be related to a modification of gravity.
Another possibility is that the new fields and degrees of freedom in MOND-theories are playing themselves the role of cosmological dark matter. In the case of Bekenstein’s TeVeS, the vector field was hoped to be able to play this role. But detailed investigations have shown that it does not work.
In BIMOND, on the other hand, a framework which involves two metrics and a hypothetic ‘twin matter’ sector (as stated hereabove, MOND-theories do not necessarily have to erase any form of ‘dark sector’), Milgrom has shown that inhomogeneity differences between the matter and twin matter sector lead to effects similar to cosmological particle dark matter. It remains to be seen whether the CMB or the matter power spectrum could be reproduced in this framework, but it is not necessarily hopeless. What is more, the theory naturally incorporates a cosmological constant which naturally explains the coincidence of scale with the acceleration constant of MOND, so while CDM would have to be traded for twin matter, the cosmological constant would appear more naturally than in LambdaCDM, and the MOND phenomenology would automatically be recovered in galaxies.
Another interesting MONDian cosmological approach is the dipolar dark fluid proposed by Blanchet and Le Tiec. In their MONDian theory, the dark sector is described as a fluid endowed with a gravitational dipole moment vector. From there, in order to reproduce MOND in galaxies, they appeal to a ‘weak-clustering hypothesis’, namely the fact that, in galaxies, the dark fluid does not cluster much and is essentially at rest because the internal force of the fluid precisely balances the gravitational force, in such a way that the polarization field is precisely aligned with the gravitational one. At the cosmological level, the monopolar density of the dipolar fluid would play the role of cold dark matter, making the theory precisely equivalent to LambdaCDM at linear order for the expansion, for large scale structure formation, and for the CMB (naturally explaining the high 3rd peak). There are even predictions recently made on the CMB and large-scale structure probes of non-gaussianities (http://arxiv.org/abs/1210.4106). This is of course speculative and might turn out to be unsuccesful in the end, but it deserves to be investigated because it would naturally reproduce MOND in galaxies and solve most problems of the standard model on small scales.
Given all these recent developments, it is grossly exaggerated for people on twitter or anywhere to state that there has been no recent progress at all concerning addressing large-scale structure or CMB-related issues in MOND-theories. All these theories might fail in the end, but given the observed phenomenology on small scales, they are definitely worth investigating, and if anyone has a new idea based on entropic gravity (http://arxiv.org/abs/1106.4108) or any other framework (see http://arxiv.org/abs/1106.4984), to explain the MOND relation, such new theories are worth developping and investigating too. It is however very clear that, at the end of the road, such a theory would necessarily have to naturally reproduce the successes of LambdaCDM on large scales. This is not easy and it is absolutely fair to say that there is currently no alternative which does as well on large scales as LambdaCDM. Note however that, given currently existing theories, this could perhaps be due to lack of manpower.
To my mind, it would in any case be a mistake to persistently ignore the fine-tuning problems for LambdaCDM on small scales and the related uncanny successes of the MOND relation, as these could plausibly point at a hypothetical better new theory adressing both the large-scale and small-scalle issues: the cherry on the cake would be if such a theory could somehow unify the dark matter and dark energy phenomenologies. But again, let me be clear, if particle (cold or warm) dark matter is unambiguously detected in the Milky Way, then there is no doubt that all the people currently investigating MOND will admit that the apparent MONDian behavior of galaxies must be explained within the realm of particle galactic dark matter: this is indeed what distinguishes Science from religion. I guess the same would happen the other way around if a modification of gravity was unambiguously detected in a solar system experiment (see http://arxiv.org/abs/1212.3687).
But until this hypothetical detection of particle dark matter, given the observed galactic phenomenology and the challenging issues raised by Pavel Kroupa, Marcel Pawlowski, and many others, trying to find an explanation for these observations by thinking slightly ‘outside-of-the-box’ is very much in the realm of sound science. Whatever the final answer to these exciting questions, the very existence of such current observational challenges, and trying to address them in a standard or less standard way, is what actually makes it exciting to be a researcher in this field right now. We should all be happy and excited about it.
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The critical question of our age is: Does non-baryonic cold dark matter exist?
We’ve invoked it for good reasons, but as Benoit says, there are also good reasons to be skeptical about it. If we detect it in the laboratory, great: then we know. If we don’t, how long do we keep after it?
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What I absolutely do not get is the vitriol and personal animosity I see from some physicists about anything that questions the validity of LCDM. Don’t agree, that’s fine. Make scientific judgements, that’s fine. But I’ve heard the strangest things come out of otherwise-lucid scientists’ mouths when discussing this issue.
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What’s often forgotten is that LCDM was the “theory nobody ordered” a mere 15 years ago, overthrowing a simpler view of the Universe – yet the evidence from the outset (two completely independent yet matching SN Ia studies, both honored with the Nobel Prize since) was so strong that it became the ‘concordance model’ within just a few years. Each new test since, based on large scale properties of the Universe, has confirmed LCDM or at least failed to damage it, and there have been a lot of complementary methods tried out. Or take its predictive power: e.g. in 2000 the relative heights of the baryonic peaks in the CMB power spectrum looked strange in early balloon data (which elated MOND supporters as I recall very well) – so LCDM theorists boldly predicted that the error was rather in the data, and d’oh, it was indeed. Concordance cosmology has been called one of the truly great achievements of humankind (e.g. by C. Frenk in a review talk in Bonn in 2006): no wonder there’s backlash when someone states, ah, it’s all wrong, because we don’t understand this or that in the local Universe yet, and I rather change the laws of gravity to make it fit. This is at the core of the animosities, as I see it (who actually wrote probably one of the first-ever news stories on MOND in German, in – gasp – 1985).
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Some of the history recounted by Fischer is not how I remember it happening. The SN results were embraced in 1998 because we had by then convinced ourselves that our cosmology needed Lambda for all sorts of reasons: the age problem, the power spectrum, the low matter density, etc. It was already being called the concordance model at least since the 1995 Nature paper of Ostriker & Steinhardt. (Minor side point: I wrote a reply to that in which I said Gee, if they are correct about Lambda, the expansion rate of the Universe would be accelerating! That’s in 1996, Nature, 381, 483. The reply of the referee, already in 1996, was “wait for the SN data!”) So it is not like the SN results were announced and we all suddenly said Oh, OK, hadn’t thought of Lambda! and just immediately accepted it. Indeed, the first time I heard Lambda advocated in modern times was by Yoshii in 1993. At the time he was shouted down – we, as a community, were not yet ready to go there. In retrospect, he looks to have been right, so maybe the Nobel committee should also recognize him? I can think of others as well…
The history of the CMB peaks recounted above is simply wrong. It is true that the simplest ansatz for a MOND prediction of the CMB acoustic power spectrum was consistent with the original Boomerang data. It is also true that the data improved over time. The part relevant to MOND – the first-to-second peak amplitude ratio – was not wrong (d’oh!). Indeed, part of the no-CDM prediction was that the second peak would appear out of the noise at a very particular value. That is exactly what happened. The 1:2 peak amplitude ratio observed by WMAP is EXACTLY as predicted by McGaugh (1999 http://adsabs.harvard.edu/abs/1999ApJ…523L..99M). Where the simple no-CDM ansatz that gets the 1:2 ratio right fails is in the 2:3 peak height ratio. I have never said otherwise. No one can honestly imply that I have ignored these issues, or myself been dishonest about them. Indeed, I take the issue of the third peak very seriously, and have commented on it (long ago!) on my own website (http://astroweb.case.edu/ssm/mond/). I would be more impressed by the story Fischer tells if there were some indication that he was aware of the fact that the 1:2 prediction still stands – remarkable coincidence, that. Instead, he seems to have bought into a rather selective version of history without bothering to check the salient facts. This is a classic behavior of the closed-minded: we’re sure we’re right, so there is no need to check for contradictory facts!
I do think Fischer is correct about the emotional nature of the backlash. Scientists really hate to revisit important issues that we thought were settled. I know that I personally had the same negative emotional reaction when I first encountered MOND in my own work. If we are to be true to the standard of objectivity we set for ourselves as scientists, we have to get past that. I at least have tried to look at it from both sides.
In the meantime, can we really be so sure of a cosmology that fills the universe with invisible mass in the form of particles that have no basis in the standard model of particle physics? There was a time, give the physics we Knew, that space absolutely had to be filled with Aether. If you think we are really so much smarter than 19th century physicists that we can’t make an analogous mistake now, I refer you to the history of the field.
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Does anyone have an opinion on the following ideas?
According to the work of Milgrom, McGaugh, and Kroupa, there are 2 possibilities:
(1) Newton-Einstein gravitational theory is 100% correct but appears to be slightly wrong for some unknown reason.
(2) Newton-Einstein gravitational theory really is significantly wrong.
In case (1) I think there might be dark matter fermions which exhibit bizarre Fermi pairings across alternate universes.
In case (2) what is the simplest way that general relativity theory can fail? In the standard form of Einstein’s field equations replace the -1/2 by -1/2 + dark-matter-compensation-constant.
http://vixra.org/abs/1312.0193 “Is the space roar an empirical proof that the inflaton field exists?”
http://quantumfrontiers.com/2013/11/05/fundamental-physics-prize-prediction-green-and-schwarz/#comments (refs. 5, 6, 7)
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