9. State-of-the-art cosmology: the current status

Last week’s contribution “Is LambdaCDM or standard cosmology a 4th order speculation, and ought it be further researched?”  was concerned with the recent suggestion by Prof. Abraham Loeb that alternative approaches should be followed to advance science. But at the same time he proposes the alternative approach “MOND plus netrinos” to be a second order speculation not worth the effort.

Following this logic of Loeb, it becomes immediately apparent that LCDM, or standard/concordance cosmology, is at least a 4th order speculation, with the corresponding implications.

Dr. Garry Angus is a very talented young cosmologist currently at the University of Torino, Italy, but shortly moving to the University of Cape Town, South Africa, who has been working in the field of “MOND plus neutrinos”. Dr. Angus is the recipient of the Cormack Bequest Prize for his 2007 publication on the topic of the Bullet Cluster, Neutrino Dark Matter and Alternative Gravity. This prize is awarded annually to the most outstanding postgraduate student contribution to astronomical research in Scotland.

Below he directly addresses Abraham Loeb’s assertion concerning his field.

It should be noted, before reading Garry’s text, that in the LCDM field (4th order speculative science) whole armies of researchers (hundreds?) have been toiling over the past decade to improve the computations and observations. It is the accepted model of cosmology, and over the past 10-15 years the very major professorships in cosmology or extragalactic astrophysics have been filled with experts in this one specific field. In contrast,  “MOND plus neutrinos” (2nd order speculative science according to Abraham) has been worked on by not more than about 2 researchers, while the other alternative, Modified Gravity (MOG), has been worked on by not many more researchers than that as well.

It nevertheless turns out that LCDM sort of works on large scales, and MOND plus neutrinos does at least as well as far as the existing work allows us to judge. Indeed, as Dr. Garry Angus shows below, the cosmic microwave background (CMB) power-spectrum is fitted perfectly well in MOND + neutrinos. On  scales smaller than about ten million light years LCDM fails however, while non-Newtonian/Einsteinian gravity works brilliantly (Kroupa et al. 2010).

Thus, MOND plus neurinos seems to be the astrophysically most modern and successful cosmological description we have currently. 

Dr. Garry Angus writes:

I’d like to just make a comment on why MOND+neutrinos is not a 2nd order speculation. I  don’t know how familiar you are with the literature on MOND+neutrinos, but no one, to my knowledge, has ever suggested that the CMB can be fit by MOND plus the active neutrinos – be they 2.2eV or 0.1eV. Skordis et al. (2006) clearly showed the apparently high 3rd peak is not compatible with even 3×2.75eV in TeVeS, even if the critical MOND acceleration (a_0) is boosted by a factor of 4.

It should be a well known fact to all cosmologists that replacing  Omega_CDM x h2  with the same energy density in sterile neutrinos gives as good a fit to the CMB. The proviso is that this energy density comes in the form of a single, thermal sterile neutrino species. Given a reasonable mixing angle, it is perfectly possible for these sterile neutrinos to be thermalised in the very early Universe. This means the neutrino has a mass of ~11eV. A figure of the MOND + neutrino CMB calculation can be seen here where we fit both WMAP 7 and ACBAR data. As can be seen, the theortical fit is near to perfect to the CMB data.

This has nothing to do with MOND. In fact, it requires MOND to have no influence at redshift z>1000 and a cosmological constant is still required. It just so happens that an 11eV sterile neutrino would resolve all problems MOND has in clusters of galaxies. At 100~kpc (about 300 thousand light years) in basically all clusters there is a 10:1 ratio of DM:baryons (after accounting for MOND), it is only at distances like 1Mpc that there is a 2:1 ratio. Angus, Famaey & Diaferio (2010) looked at 30+ groups and clusters and made the intriguing observation that the Tremaine-Gunn limit for the 11eV neutrinos is reached in every system, but never need be exceeded for very sensible values of the brightest cluster galaxy’s mass-to-light ratio.

These neutrinos would free stream out of Milky Way type galaxies, so all the successes of MOND at galaxy scales would be unaltered. The ramification of this is that the galaxies must collapse under their own gravity (enhanced by MOND) without the aid of a cold dark matter halo (see Sanders 2008). Linked to this, we have run preliminary cosmological simulations that incorporate MOND and 11eV sterile neutrinos and the conclusion is that they form roughly the correct number of clusters of galaxies as a function of cluster mass. It could just as  easily have ended up in a big black hole or with no structure forming at all.  If we run numerical simulations with the 11eV neutrino and no MOND, then no structures form i.e. MOND is essential for massive neutrinos to work.

Whether the correct number of galaxies form is an incredibly difficult question to answer and the numerical tools are nowhere near ready – basically because the 2 or 3 people with the necessary expertise to develop the codes can’t get jobs for love nor money. However, based on the successes of MOND at galaxy scales, we do expect that MOND+sterile neutrinos can reproduce that observed properties of galaxies with no effort, unlike CDM. For example, as long as a galaxy forms, we know trivially that it will conform to the baryonic Tully-Fisher relation. Furthermore, the highly organised distribution of satellite galaxies surrounding the Milky Way (see Kroupa, Theis & Boily 2005; Metz et al. (2009); Kroupa et al. 2010)  will immediately be explained to be tidal dwarf galaxies. Currently they have no explanation in LCDM.

For these reasons I don’t see how MOND+neutrinos is a second order speculation. If one has MOND, then there is an incredibly high chance that 11eV sterile neutrinos must exist and by the same token, if sterile neutrinos at 11eV exist then MOND is needed. In the former case it is possible that a deeper theory of MOND will spring a surprise which conspires at cluster scales, for the expansion history, during the formation of the acoustic peaks of the CMB and during structure formation to resemble an 11eV sterile neutrino. Abraham Loeb mentioned briefly that the baryonic acoustic oscillations appear at the scale predicted by LCDM. I don’t believe this is fully accurate. They appear at the scale defined by a scale factor that evolves as if it has a dominant dark matter component, hot, cold or warm. The peaks themselves do not require the dark matter to be cold.

A lot of people talk about neutrinos being against the design, spirit or original intention of MOND. I feel this is never very helpful. As Professor Milgrom clearly states above, the state of observational astronomy was very different in the early 80s. That MOND works at all highly disfavours the need for any type of Warm or Cold dark matter, but if that dark matter is hot and hot enough to free stream from galaxies then MOND (in the 80s) made no predictions about its presence. Nowadays, MOND without sterile neutrinos and MOND with sterile neutrinos are two different models with very different predictions for cosmology.

In addition to the cosmological evidence for 11eV sterile neutrinos, there also exists tentative particle physics evidence from Miniboone (see Giunti & Laveder 2008). And, more experiments are in the pipeline, for instance the T2K experiment will be able to put excellent constraints on 11eV sterile neutrinos, with results probably released in early 2012. Unfortunately, Planck will not offer any evidence for the specific mass of sterile neutrinos because the CDM model with a very low mass (say <0.1eV) thermal sterile neutrino would generate an identical power spectrum. It can, however, rule out the existence of further thermalised neutrino species (i.e. if N_eff=3).

I should add that although 11eV sterile neutrinos is my preferred solution that extends MOND to cosmology, there are others. HongSheng Zhao & Baojiu Li are working very hard testing a model that combines the MOND effect, the cosmological dark matter and dark energy into the same field that behaves differently depending on environment, which is a very nice idea. It boils down to the same essential ingredient for cosmology, however, and that is hot dark matter plus MOND.

by Anton Ippendorf, Pavel Kroupa and Marcel Pawlowski (25.08.2010, “State-of-the-art cosmology: the current statusin “The Dark Matter Crisis – the rise and fall of a cosmological hypothesis” on SciLog. See the overview of topics in  The Dark Matter Crisis.


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