Nick Samaras is a Ph.D. student at the Astronomical Institute of Charles University, in the Faculty of Mathematics and Physics, in Prague, Czech Republic. He works on cosmological simulations with Milgromian Dynamics (MOND). He has obtained his M.Sc. degree in Theoretical Physics at Cergy University, in France after having completed his B.Sc. in Mathematics at the Aristotle University of Thessaloniki, in Greece. In his following guest post he writes about the cosmological principle and a recent essay titled “Heart of Darkness” by Prof. Subir Sarkar.
The Standard Model of Cosmology (SMoC) has been considered as the correct description of the Universe and its evolution for decades now. General Relativity along with the mysterious Dark Energy, embedded on the Friedmann–Lemaître–Robertson–Walker (FLRW) metric, provide the outset for the ΛCDM (Λ Cold Dark Matter cosmological) model. The FLRW metric is a formula derived from the General Relativity and corresponds to a homogeneous, isotropic and expanding universe. It is the mathematical tool with which one calculates distances on a 4-dimensional (time and the 3 dimensions of space) model. Nonetheless, according to more sophisticated investigations and the increase of observational data, the current theory faces a great number of challenges.
The Homogeneity and Isotropy hypothesis holds a convenient ground to do Cosmology. The so-called Cosmological Principle states that the Universe is very much alike anywhere over a typical scale of about 250/h Megaparsec (Mpc) (1 parsec = 1 pc is approximately equal to 3.26 light-years, unit of length). Remember that the Milky Way has a diameter of approximately 40 kpc, the Local Group of Galaxies is about 3 Mpc across, and the Virgo supercluster spans over about 30 Mpc). However, do the observations agree with this? Is there enough evidence to install the Cosmological Principle on a solid paradigm? How concrete are the cornerstones of the SMoC?
Subir Sarkar, an Emeritus Professor at the Rudolf Peierls Centre for Theoretical Physics, University of Oxford, argues that the real universe to be very different to the ΛCDM model and in particular the Cosmological Principle to be violated. Unraveling the record, the cosmological constant Λ (often being referred to as Einstein’s biggest blunder, the cosmological parameter causing the accelerating expansion) differs by many orders of magnitude when estimated from Quantum Field Theory (QFT), compared to what is inferred from Cosmology. He also emphasises an inconsistency when attempting to calculate the vacuum energy in QFT. The fact that the zero-point (vacuum) energy does not gravitate (otherwise it would have already dominated the Universe letting it evolve in a completely different way) have been kept aside even by the great Wolfgang Pauli, Prof. Sarkar points out.
Besides “the worst theoretical prediction in the history of physics” (Michael Hobson, George Efstathiou, and Anthony Lasenby), looking at the Cosmic Microwave Background (CMB – the primordial relic radiation released approximately 300,000 years after the Big Bang), its anisotropy dipole is larger than expected at high redshift (a cosmological way to calculate distances from us, based on the redshift of spectral lines). He notes that all matter in our nearby Universe has a coherent bulk flow approximately aligned with the direction of the CMB dipole. Several experiments, spanning from the 70s until these days, show that the bulk flow continues out to approximately 300 Mpc, remarkably not converging to homogeneity. The Indian theoretical astrophysicist wonders about Milne’s quote “the Universe must appear the same to all observers”, advocating historical changes in the field.
Sakar and his collaborators identified that the large dipole is not from the local universe. They have discovered that the cosmic rest frame of matter traced by quasars and the CMB don’t coincide. Thus, it is determined that the apparent acceleration is not happening because of the cosmological constant. It’s only a result of our non-Copernican position in the bulk flow. Consequently, the cosmic acceleration is not isotropic. ΛCDM begins to disintegrate …
Dark Energy, which drives the cosmos to accelerated expansion, in the form of an until-now-completely-not-understood repulsive force increasing with time, is therefore an occurrence generated by an over-interpreted conventionalised model which needs to be seriously revised. Leaving out the inflationary era a few moments after the Big Bang and the ambiguous premise of Dark Matter, the SMoC has been tested sufficiently to be replaced by a more detailed developed theory. Last, Prof. Sarkar, supporting that the Universe has different matter contents in different regions, encourages younger researchers to work out in greater depth an improved model of the real Universe .
David Levitt is a retired biophysicist from the Department of Integrative Biology and Physiology at the University of Minnesota. In the following guest blog he explains his attempt of approaching astrophysics and cosmology and why he decided to write an introduction to Milgromian Dynamics (MOND). Interested readers can download his review “MOND for Dummies” at the end of the guest blog.
I am a retired biophysicist, teaching myself astrophysics and cosmology. Approaching this subject with this fresh perspective, one is immediately struck by the remarkable drama presented by the conflict between the standard Lambda Cold Dark Matter (ΛCDM) and Modified Newtonian Dynamics (MOND) paradigms. In addition to MOND being ignored by most of the astrophysics community, there is also a nearly complete neglect of MOND in current science teaching and popular science presentations. Although there are many detailed technical reviews of MOND along with the wonderful book by David Merritt (see also blog contribution 55) for a general audience, there is a surprising lack of a simple short review of the MOND/ΛCDM issues accessible to someone with, say, a knowledge of college physics but no background in astrophysics. This lack became frustratingly clear when my attempt to convey my excitement about this subject to my scientific colleagues and grandson taking university physics failed because I could not find an appropriate reference to refer them to.
I have, presumptuously, taken it upon myself to write such a review, “MOND for Dummies”, which is linked below. Although there are obvious disadvantages of taking on this project as an amateur in this field, there are also advantages. Firstly, I know the issues that resonate with someone approaching this subject without an astrophysics background. And, secondly, I am aware of the importance of keeping the math and physics simple. I have focused this review primarily on spiral galaxy dynamics because it provides, I believe, the most dramatic confirmation of MOND predictions along with being understandable at the level of college physics. I hope that it conveys my enthusiasm for what, I believe, is the most important and exciting problem in physics today and that it provides a convincing case that MOND is a stunning theory that makes some remarkable predictions that are nearly perfectly confirmed experimentally. For many readers of this blog, the issues discussed in my review are well known and redundant. However, I am sure you also have colleagues and students that are not aware of the drama playing out in this field and to whom you might refer it.
Please find here the link to “MOND for Dummies” (updated April 27) written by David Levitt.
The observed Universe consists of a mix of various types of galaxies ranging from ellipticals, spirals, lentriculars, and irregulars. Generally speaking, elliptical and lentricular galaxies are roundish, while spiral galaxies are typically very flat rotating disks, looking round face-on but are like knife edges when seen edge-on. A galaxy morphological classification has been originally introduced by J. H. Reynolds and adapted later by Edwin Hubble in 1936 and has been further developed for example by Gérard de Vaucouleurs and Allan Sandage. Interestingly, most of the observed galaxies are very flat disk galaxies, with ellipticals making up only a small fraction out to a redshift of 0.6 (see e.g. Delgado-Serrano et al. 2010). Our own Milky Way is also a spiral galaxy. If we would be able to move away from our own galaxy sufficiently far to see its full dimension, the Milky Way would look similar to the galaxy NGC 891. This spiral galaxy is seen edged-on from Earth and has a very thin disk consisting of stars and gas as shown in Figure 1.
Figure 1: The image shows the edge-on spiral galaxy NGC 891, which has a very thin stellar disk. This galaxy has an appearance similar to our Milky Way galaxy and the faint disk extends to much larger distances than shown on this photograph. Most of the galaxies in the local Universe are such spirals and only a few are roundish ellipticals (Delgado-Serrano et al. 2010). Credits: https://en.wikipedia.org/wiki/NGC_891.
The observed thinness can be used as a test of cosmological models and of gravitational theories as follows: In a cosmological theory, in which galaxies collide and merge, the final galaxies would be crash-damaged and would appear thicker and roundish. In a cosmological theory in which galaxies form from contracting rotating gas clouds (Wittenburg, Kroupa & Famaey 2020) without later crashing into each other or merging, the vast majority of galaxies would remain thin rotating disks. This happens because, as a gas cloud collapses, it flattens under it own gravitation and spin-up due to conservation of rotational momentum thereby automatically becoming a thin disk. On the vastly smaller scale of planetary systems, the Solar system formed in just this way.
In this SMoC, galaxies begin to form in the early Universe first as very small dark matter haloes into which gas falls and where stars begin to form. As the Universe expands the small dark matter haloes merge and the galaxies become larger and more massive. The dark matter haloes are always much larger in extend than the gas and stellar parts of the galaxies, and this has important implications for the evolution of galaxies: If a galaxy with its own dark matter halo moves through a dark matter halo of another galaxy, it experiences a drag and decelerates. This effect is called “Chandrasekhar dynamical friction” (see the discussion on dynamical friction to our Blog Post 51). As a consequence, interacting galaxies merge within a short time scale of about 1-3 Gyr. Because of the huge dark matter haloes, we expect a higher merger rate of galaxies in the SMoC compared to models without cold or warm dark matter. Galaxy mergers typically decrease the angular momentum of galaxies causing a thickening of the galactic disk. This dramatic loss of angular momentum of galaxies in ΛCDM simulations has been widely discussed over the past decades. That is, it has been known for a long time that the dark-matter based models lead to galaxies that are too thick compared to their diameter. Interestingly, it has been shown that simulated galaxies with a very quiescent merger history do not suffer from an excessive loss of angular momentum and are able to form and retain fairly flat disks. This suggests that mergers need to be less frequent in the observed Universe than predicted by the ΛCDM framework – and this questions at the same time the existence of the hypothetical cold or warm dark matter particles because dynamical friction is much less efficient in nature than expected to be the case in the SMoC. However, the situation appeared to have changed in 2014: Vogelsberger et al. 2014 claimed, in their Nature paper, that the angular momentum problem has been solved in the self-consistent hydyrodynamical cosmological ΛCDM Illustris simulation:
“Simulating the formation of realistic disk galaxies, like our own Milky Way, has remained an unsolved problem for more than two decades. The culprit was an angular momentum deficit leading to too high central concentrations, overly massive bulges and unrealistic rotation curves. The fact that our calculation naturally produces a morphological mix of realistic disk galaxies coexisting with a population of ellipticals resolves this long-standing issue. It also shows that previous futile attempts to achieve this were not due to an inherent flaw of the ΛCDM paradigm, but rather due to limitations of numerical algorithms and physical modelling.“
Although these simulations form a variety of galaxy types, any viable cosmological model also has to reproduce the observed fraction of late and early type galaxies. Using the latest state-of-the-art hydrodynamical cosmological ΛCDM cosmology, we showed that the produced galaxy morphology distribution significantly (at more than the five sigma confidence level) disagrees with local observations. Galaxies formed in the ΛCDM simulations are systematically thicker than in reality as shown in Figure 2. Thus, contrary to the claim by Vogelsberger et al. 2014, the angular momentum problem has not been resolved: The high fraction of thin disk galaxies falsifies ΛCDM cosmology!
Figure 2: Sky-projected aspect ratio distribution of observed and simulated ΛCDM galaxies. A typical disk galaxy has a thickness of about 0.7kpc and a diameter of about 30kpc such that the true aspect ratio is q=0.023. But galaxies on the sky are tilted at various angles and the observer only sees the projected ellipse, such that the on-sky distribution of this ratio, qsky, shows larger values. The observed thicknesses of galaxies in the GAMA and SDSS surveys are plotted above as the solid black and dashed grey lines. Galaxies formed in the cosmological ΛCDM simulations (Illustris, IllustrisTNG, and EAGLE) are systematically far too thick compared to the observed galaxies in the SDSS and GAMA Galaxy Survey. Credits: The High Fraction of Thin Disk Galaxies Continues to Challenge ΛCDM Cosmology (Haslbauer et al. 2022)
It is often argued that the thickening of galaxies in the ΛCDM simulations is related to the implemented “baryonic feedback description”, i.e, the algorithm that defines how the gas is turned into stellar particles and how these stellar particles heat the surrounding gas through their radiation, winds and supernova explosions. The Illustris(TNG) and EAGLE simulations rely on two completely different feedback models as well as using very different computer programmes and calculation methods but both fail to reproduce the vast number of observed thin galaxies. Moreover, the same baryonic feedback description must also explain other various small-scale problems faced by the ΛCDM model: the formation of early type galaxies (the downsizing problem, e.g. Yan, Jerabkova & Kroupa 2021), bar pattern speeds of galaxies, the missing satellite problem, the core-cusp problem, the disk of satellites problem, the local Gpc-scale void, etc. It is highly implausible that all the problems can be solved by just changing the feedback description.
It is quite possible that the angular momentum problem (or the too-thick-galaxy problem) is the consequence of a failure of the hierarchical structure formation of the ΛCDM framework – this framework not being the correct one to model the real Universe. We tested this: We pulled those galaxies out of the simulation that have a quiescent merger history, and yes, these are indeed slightly thinner than galaxies which had at least one major merger in the past. However, this still cannot explain the discrepancy between the observed and simulated galaxies. In addition to the major mergers, the ΛCDM model predicts a high frequency of minor mergers, meaning that many small galaxies merge with a massive galaxy. These mergers are unavoidable in a cold- or warm-dark-matter-based model, i.e., in the SMoC, leading to a thickening of the galactic disks. In fact, in the dark matter based models, galaxies grow largely through mergers and every galaxy has its own rich history of mergers, a so-called “merger tree”.
Consequently, the fraction of thin disk galaxies is expected to be higher in a model which does not rely on cold or warm dark matter, in which galaxies form mostly through the above mentioned collapsing rotating gas clouds (Wittenburg, Kroupa & Famaey 2020) and in which mergers are rare. But this would imply that we also need a different law of gravity, for example MilgrOMiaN Dynamics (MOND). In these models dynamical friction is less efficient, resulting in a much smaller number of merger events. Simulations of interacting galaxies indeed demonstrate that the galaxies merge much less efficiently: even very strong encounters lead to mergers only after a few orbits, in contrast to the very rapid (within one to two orbits) mergers of the galaxies in the dark matter based models (Renaud, Famaey & Kroupa 2016). Self-consistent cosmological MOND simulations are underway in the Bonn-Prague research group to test if a MOND cosmology can indeed account for the observed vast number of thin disk galaxies.
Almost 50 years after the postulation that galaxies are surrounded by dark matter haloes (Ostriker & Peebles 1973), the ΛCDM simulations still cannot explain the structural properties of observed galaxies.
How does the Milky Way galaxy fit into the above conclusions?:
The usually-encountered thinking amongst the vast majority of astronomers is that the Milky formed, according to the SMoC, i.e. through mergers, and the stellar and gas streams such as the Gaia-Sausage-Enceladus structure are taken to be, essentially, the proof of this (e.g. Naidu et al. 2021). But, as discussed above, this very rich merger history (after all, the Galaxy is a major galaxy) would not allow the Milky Way to remain a thin disk galaxy, and the SMoC is falsified as a relevant model for the Universe in any case. So, can the Milky Way be understood in MOND?
The observed rotation curve is well accounted for by MOND (McGaugh 2008). The Milky Way has a thick disk component which consists of stars that are older than about 10 Gyr with a thickness of about 2 kpc, and a still forming thin disk making about 90 per cent of the mass of the whole Galactic disk which is composed of stars up to ages of 10 Gyr and a thickness of about 0.7 kpc. The diameter is about 30 kpc, and this whole disk has a warp. One could argue that the thick disk is a result of a merger and that the SMoC is therefore valid. But this is incorrect, because this line of thinking would imply that the majority of the disk, namely the thin disk, would have to grow without mergers, which is not possible in the SMoC.
But these components (the thick and thin disks and the warp and its orientation) are well explainable in MOND if the Milky Way had an encounter with the Andromeda galaxy about 10Gyr ago, i.e. near a redshift of z=2 (Bilek et al. 2018). This model starts with a young Milky Way having a thin disk, and the encounter with the young Andromeda galaxy thickened this disk and produced a warp. A new thin disk grew within the thickened ageing disk as the Milky Way accreted further gas after receding from Andromeda, fuelling its on-going star formation. While there exists no SMoC calculation which can explain these features of the Milky Way and at the same time the disk of satellites around the Milky Way and around Andromeda, the encounter calculations in MOND between the Milky Way and Andromeda nicely produces all of this for free, therewith also solving the planes-of-satellites problem completely naturally (Bilek et al. 2018; Banik, O’Ryan & Zhao 2018; Bilek et al. 2021; Banik et al., 2022, submitted).
In conclusion, it is therefore apparent that in the modern non-relativistic theory of gravitation, which MOND is, the vast majority of galaxies being thin disks as well as major properties of the Local Group of galaxies become understandable naturally, while the SMoC fails to do so entirely.
In The Dark Matter Crisis by Moritz Haslbauer, Marcel Pawlowski and Pavel Kroupa. A listing of contents of all contributions is available here.
We humans are, as any living creature, and by necessity, conservative beings. We need to be, since typically most of us prefer to sustain their comfortable arrangements. The cave person will prefer to stay near their cave if the nearby plains below are full of fodder, and they would kill off threats. The cosmologist prefers to stay with their dark matter that made them big and important. So when a comet/climate crisis is discovered to approach Earth and is calculated, with some margin or uncertainty, that it will extinguish known life, it can be easier for the majority to just ignore this and to trust in everything turning out all right in the end. Keep the high spirits up, don’t worry and keep smiling and do not frown, do not spoil the mood by doomsday blubber, don’t look up to the threat. So let us ignore that the temperature of the oceans has already increased by nearly 2 degrees centigrade and that another increase by three will kill off most plankton as shown by Sekerci & Petrovskii (2018) with Earth’s atmosphere consequently running out of oxygen.
What has this to do with modern cosmology? I would claim: everything. The modern, successful homo cosmologicus vehemently defends their dark matter against all odds, even if it means killing the scientific method (testing and falsification of hypotheses using reproducible logical methods); they resist change to their habitat as long as the vast landscape of rewards, awards, grants and riches remains abundant.
In the Golden Webinar on “Tension in the Hubble Constant – Does it mean new Physics?“, the speaker very nicely explained the measurements of the Hubble constant using different distance ladders and which role the uncertainties play. Three points struck me: (1) The speaker declared that the physical reason for the Hubble Tension remains unknown. (2) The speaker declared there to be no other known major tension between observations and the Standard Model of Cosmology (the SMoC, or LCDM model). (3) During the panel discussion, a long time was spent on Penrose’s Conformal Cyclic Cosmology hypothesis and it was speculated that fading dark matter might account for the Hubble Tension. The panel largely agreed that no one knew what dark matter was – it might have a large number of degrees of freedom, thus allowing the introduction of an arbitrary number of free parameters to fit almost anything.
Concerning the three points above, I wrote into the chat two questions (see Figure 1 below). Essentially, accepting the well-observed Gpc-scale KBC void as being a real structure of the Universe, the Hubble Tension must then arise from it logically (Haslbauer et al. 2020). This is because galaxies are accelerated gravitationally towards the sides of the void, and an observer within the void (as we are) then measures an apparent faster expansion of the local Universe (see figure 2 in 52. Solving both crisis in cosmology: the KBC-void and the Hubble-Tension). The Hubble Tension therefore has a very simple physical explanation.
In fact, a real Hubble Tension does not exist: it is merely an apparent effect caused by the observed KBC void (Haslbauer et al. 2020), and it would have been predicted if Wong, Suyu et al. (2020) and Riess et al. (2021) had not made their observations of expansion. It is the same reason, in essence, why apples fall to the Earth: replace the galaxies by apples, and they will fall to where they are attracted to, which is the side of the underdensity.
It was a wonderfull event and fascinating to see how the panel very happily discussed the entirely speculative fading dark matter concept in the context of the Hubble Tension, but no-one appeared to dare to raise the possibility that it might simply be due to the observed KBC void, as in fact it must be. I tried to help the panel by posting my question into the chat, but it appeared to me that, in the intimidating presence of highbrow scientists, discussing fading dark matter was acceptable, while raising the obvious solution was no-go. After all, who wants to ask a seemingly silly down-to-Earth question (“can the observed Gpc underdensity be responsible for the apparent Hubble Tension?”) in view of such intellectual Mt. Everests.
The second point above by the speaker I also found impressive, given that other independent falsifications of the LCDM model at more than five sigma confidence have been published, see the list A-F below. It seems that these contributions were missed in Chicago, or that Chicago Cosmologists “do not look up”. I guess they do not need to look up, since they are already on Mt. Everest.
I am still trying to digest this, which is why I wrote the above first paragraph.
Why was neither the Golden Webinar speaker nor the panel willing to delve into the true physical reason for the Hubble Tension? I think that the problem is that the KBC void, which causes the Hubble Tension, falsifies the SMoC with more than 5sigma confidence (Haslbauer et al. 2020), because the SMoC cannot grow such large and deep under densities within a Hubble time. And furthermore, the Chicago Cosmologists, as represented by the speaker and author (next), seem adamantly to refuse to discuss MOND seriously. But MOND is the only known modern non-relativistic theory of gravitation in which the Universe can grow such a large observed void and observed early very massive interacting galaxy clusters (Asencio et al. 2021). We covered this galaxy-cluster problem on a previous occasion. In MOND, there is no Hubble tension (since the voids form naturally) and very massive interacting galaxy clusters also form naturally in the earlier Universe.
I was interested, since the author is viewed by many to be an outstanding cosmologist, and I expected a fair, balanced and up-to-date review of cosmology for the community of Nuclear and Particle Physicists. This is an important review: Annual Reviews are corner stones of literature. Often they are the first entry point into a research field. Their role is thus truly important. On contemplating the review, I decided to write the following letter – let it speak for itself:
Letter sent on 17th of January 2022 to those addressed (with minute modifications for this forum):
(CC to Editors, Committee Members and Staff of the Annual Review of Nuclear and Particle Physics, and researchers working on MOND),
I kindly ask you to adjust this article to represent the modern state of affairs truthfully: As it stands, the article is not a review but a biased misrepresentation of the state-of-the art in the research field. It misrepresents the entire field of cosmology to the research community in Nuclear and Particle Physics.
If not-citing highly relevant research literature is considered to be equivalent to plagiarism, then you have provided a major example of such ill conduct: "Papers published in A&A should cite previously published papers that are directly relevant to the results being presented. Improper attribution — i.e., the deliberate refusal to cite prior, corroborating, or contradicting results — represents an ethical breach comparable to plagiarism." (citing from "Ethical issues: the A&A policy concerning plagiarism and improper attribution: https://www.aanda.org/index.php?option=com_content&view=article&id=136#Ethical_issues ).
In your article, we read "Sec. 3.1.2. False starts. In 1983, Milgrom noticed...."
This is an unacceptable representation of an entire highly successful and vibrant research field in which an increasing number of brilliant young physicists are active in. You claim in this section that MOND cannot be falsified. This is wrong. We are actively working on falsifying this theory. MOND can be falsified by, for example, finding systems that do not obey the non-linear MOND Poisson equation.
Your article is not aware of or purposefully ignores that
The LCDM standard model of cosmology is in tension with the data on many different scales with significantly more than 5 sigma confidence.
The data which are in tension with LCDM are at the same time naturally (i.e. without adjustment of any parameter) explained in a cosmological model which is based on Milgromian gravitation (MOND) without cold or warm dark matter.
Some of the relevant very recent major peer-reviewed research contributions (ignored by your article) on this are:
Your article neither cites nor discusses these, and falsely implies the LCDM model to be consistent with the data at the precision level. Further, the review appears to suggest there to be no other model (without dark matter) that can claim comparable success. Claiming today that the LCDM model is a "triumph of precision cosmology" (Sec. 4.1 in your article) is purposefully propagating outdated misinterpretations to an audience who are non-experts in this research field.
I will publish the contents of this email as an open letter, and I hope to receive a constructive reaction.
(Helmholtz-Institut for Nuclear and Radiation Physics, Bonn; Astronomy Institute, Charles University, Prague)
(Guest post by Tobias Mistele, December 1st, 2021)
Tobias Mistele is a PhD student at the Frankfurt Institute for Advanced Studies studying hybrid MOND-dark-matter models. Besides his physics research he also works on Scimeter.
Hybrid models, which combine dark matter and modified gravity, were long neglected. In this post, I explain why such models are now attracting attention as a path out of a stalemate.
There is observational evidence for missing baryonic mass on both cosmological and galactic scales. Most notably, the fluctuations in the cosmological microwave background (CMB) on cosmological scales and rotation curves on galactic scales. Traditionally, this is explained by non-relativistic dark matter particles (cold dark matter, CDM) that do not interact much except gravitationally. These CDM particles form a pressureless fluid on cosmological scales and later accumulate around galaxies, forming a dark matter halo. The pressureless fluid explains the fluctuations in the CMB. The mass of the halo around galaxies explains galactic rotation curves. An alternative paradigm is modified gravity. Instead of postulating particles that produce additional mass, modified gravity postulates a different gravitational force. Modified Newtonian Dynamics (MOND) is a modified gravity model that is quite successful on galactic scales. For example, consider the so-called Radial Acceleration Relation (RAR) shown in Figure 1. This is a relation between the standard Newtonian gravitational acceleration due to the stars and gas in a galaxy, gbar = GMb/r2, and the total acceleration gobs we infer from observed rotation curves. In a world without dark matter and without modified gravity, these two are the same, gbar = gobs. In the real world they are not. This is the missing mass problem in galaxies. More importantly, this relation between gobs and gbar has little scatter. Thus, the total acceleration gobs can be predicted just from the baryonic mass distribution, i.e. from gbar.
This is non-trivial in DM models. In principle, two galaxies with the same baryonic mass distribution (the same gbar) can have different dark matter halos and thus a different gobs, but this doesn’t happen. In contrast, MOND naturally explains this. In fact, the RAR shows precisely what MOND postulates. At Newtonian accelerations gbar larger than a0 ≈ 10-10 m/s2 nothing new happens. Newton’s gravitational force law remains. But at accelerations gbar smaller than a0, the total acceleration changes to (a0gbar)1/2.
Unfortunately, both CDM and MOND remain unsatisfactory when considered individually. MOND, for example, cannot explain all the missing mass on galaxy cluster scales. And, so far, no MOND-based models have been able to explain the fluctuations in the CMB, at least not without introducing some type of dark matter after all. CDM, on the other hand, has its own problems. For example, there is so far no convincing explanation for MOND-like scaling relations like the RAR. There’s just no reason why the dominant dark matter halo should be predictable from the visible baryonic mass in such a simple way. Another problem is that observed galactic bars tend to rotate faster than what CDM predicts. The dynamical friction of a CDM fluid slows down galactic bars. Then there’s the plane-of-satellites problem. Satellites of the Milky Way co-orbit in thin, planar structures. A natural explanation would be that these satellites were created from the tidal tails of interacting galaxies. But then they would not have their own dark matter halo which contradicts their high internal velocity dispersion. Also, CDM seems to be too slow to grow large structures. Massive clusters at high redshift like El Gordo are very unlikely to form so early in CDM.
So if not MOND or CDM – then what? One answer is both! That’s what hybrid MOND-dark-matter models are about. These are models that have both a pressureless fluid on cosmological scales (to explain the CMB) and a MOND-like force in galaxies (to explain e.g. the RAR).
Let me illustrate the general ideas behind hybrid models with an example – a model called superfluid dark matter (SFDM) proposed by Berezhiani & Khoury. This model has various problems, but it serves as a good illustration of the general features of hybrid models. SFDM postulates a specific new type of particle that behaves like standard CDM on cosmological scales and therefore explains the CMB in the standard way. But around galaxies, these particles condense to form a superfluid. The collective excitations of this superfluid, called phonons, then mediate a MOND-like force in galaxies. This MOND-like force is an emergent property of these particles in the superfluid phase. This is how this model explains MOND-like scaling relations like the RAR.
Of course, the superfluid itself has a mass. This produces a standard gravitational force that affects stars and gas. That is to say the superfluid also acts as dark matter in galaxies. So then we have both a MOND-like force and dark matter around galaxies. But does this not solve the missing mass problem twice? So that rotation velocities end up even larger than what we observe?
The answer to that is that the superfluid DM component is usually subdominant within galaxies because the superfluid halo is very cored. Its mass becomes relevant only at larger radii. This is illustrated in Figure 2 for the Milky Way rotation curve.
One might be tempted to adjust one’s models so that the DM contribution becomes even smaller. Just to reproduce the MOND-like scaling relations even more cleanly. But one must be careful with this. Some amount of dark matter is needed in hybrid models to explain the missing mass on galaxy cluster scales for which MOND cannot fully account and, in some models, also to explain gravitational lensing.
Superfluid dark matter is not the only hybrid model. For example, recently Skordis and Złosnik proposed a model that reproduces MOND in galaxies (SZ model). This is also a hybrid model and has, deservedly, received a lot of attention since it is fully-relativistic and it was demonstrated explicitly that this model fits the CMB. Like in SFDM, the MOND and DM components are related to each other in the SZ model.
Such a common origin for the cosmological and galactic phenomena is theoretically appealing. But not all hybrid models have such a common origin. For example, the so-called νHDM model does not. Moreover, such a common origin often brings about internal tensions that must be carefully avoided.
In SFDM, for example, this common origin means that the phonon field is involved both in providing the DM and the MOND components. One technical consequence is that the usual U(1) symmetry of the superfluid must be explicitly broken which has various non-technical implications. For example, the superfluid equilibrium state might not be valid on timescales longer than galactic timescales.
The common origin for the DM and MOND components complicates things also for the SZ model. In this model, there is a kind of mass term for the static gravitational field in galaxies. Mass terms generally make forces short-range. To keep the gravitational force in galaxies long-range, the mass term must be chosen small. But a smaller mass term in galaxies means a larger pressure of the DM-like fluid in cosmology. Observations indicate a very small pressure of the DM fluid. So the galactic and cosmological phenomena push the model in different directions. This has forced the authors to include certain non-linearities as a counter.
Besides these model-specific constraints, there is also a new type of phenomenon that quite generally constrains models with a common origin for the MOND and DM components. Namely, stars often lose energy just by moving through a galaxy. Let me explain.
Accelerated charges produce electromagnetic waves. Accelerated masses produce gravitational waves. In general, whenever matter is coupled to a force carrier (e.g. the electromagnetic or the gravitational field), matter that accelerates produces waves corresponding to that force carrier. But even non-accelerated matter objects can produce waves. Namely if they move faster than the speed with which these waves propagate. For example, in a medium, electromagnetic waves propagate slower than the vacuum speed of light. Charged particles in such a medium emit electromagnetic waves if they move faster than this reduced speed of light. These waves are then called Cherenkov radiation. Such charged particles lose energy and slow down. A similar phenomenon occurs frequently in modified gravity theories whenever gravitational waves propagate at less than the vacuum speed of light. This is called gravitational Cherenkov radiation. Usually, only highly relativistic matter objects emit Cherenkov radiation, both in modified gravity theories and in electromagnetism. This is because the propagation speed of waves is usually relativistic, so that only relativistic particles are fast enough.
But this is different in hybrid MOND-DM models with a common origin for the MOND and DM components. Such models usually contain a force carrier (for the MOND-like force) whose associated waves propagate with non-relativistic speed (because this force is related to the non-relativistic dark matter fluid). Thus, even non-relativistic objects like stars might move faster than the wave propagation speed associated with the MOND force. Such stars will then lose energy and slow down, because they emit a special type of gravitational Cherenkov radiation. For example, in SFDM stars that move faster than the superfluid’s speed of sound will lose energy by emitting sound waves and slow down until they are slower than the superfluid’s speed of sound. This is illustrated in Figure 3. A star may be on a standard circular orbit when it is sufficiently slow, but will otherwise lose energy and circle towards the center of a galaxy.
Figure 3: The orbit of a star in the plane Z = 0 of a galaxy with (dotted orange line) and without (straight blue line) the Cherenkov radiation typical of hybrid MOND-DM models with a common origin for the MOND and DM components. The two cases are labeled as “With friction” and “Without friction” because in the specific approximation used, the Cherenkov radiation acts like an effective friction force on the star. Credits: Tobias Mistele
This reasoning applies only to models with a common origin for the DM and MOND components. So it does apply to SFDM and the SZ model, but not to the νHDM model. When actually doing the calculation one needs to be careful because of the non-linearities that are inherent in any MOND model. Still, it is possible to rule out part of the parameter space of SFDM using the observed Milky Way rotation curve. Basically, one requires that stars that orbit around the Milky Way with the rotation curve velocity do not lose much of their energy during the Milky Way’s lifetime. The SZ model avoids such constraints due to a special property. The coupling to matter is much larger in the static limit than in dynamical situations, which suppresses the energy emitted by Cherenkov radiation. Though I should say that the calculation for this model was done in a simplified setup so that the result should be taken with a grain of salt.
To sum up, the observational evidence for both MOND-like scaling relations on galactic scales and a DM-like fluid on cosmological scales has only become more convincing in recent years. This motivates hybrid MOND-DM models. We may not yet have a completely satisfactory model and much remains to be explored. Still, this general type of model will likely become ever more relevant in the future.
In The Dark Matter Crisis by Moritz Haslbauer, Marcel Pawlowski and Pavel Kroupa. A listing of contents of all contributions is available here.
This year, Pavel Kroupa was asked to hold a Golden Webinar in Astrophysics on the dark matter problem. This contribution provides the link to the recording of this presentation which has now become available on YouTube. In this presentation, Pavel Kroupa argues that the dark matter problem has developed to become the greatest crisis in the history of science, ever. This contribution also provides links to recordings available on YouTube of previous related talks by the same speaker from 2010 (the Dark Matter Debate with Simon White in Bonn) and 2013 (in Heidelberg). This might allow some insight into how the debate and the research field have developed over the past dozen or more years.
1) Golden Webinar: “From Belief to Realism and Beauty: Given the Non-Existence of Dark Matter, how do I navigate amongst the Stars and between Galaxies?”
On April 9th, 2021, Prof. Pavel Kroupa presented a talk in the Golden Webinars in Astrophysics series on “From Belief to Realism and Beauty: Given the Non-Existence of Dark Matter, how do I navigate amongst the Stars and between Galaxies?”. The talk is now available on Youtube:
The slides to the talk without the fictitious story can be downloaded here:
If you are interested in other talks presented during The Golden Webinars in Astrophysics series, you can find the record of those already presented on their Youtube Channel, and here is a list of upcoming talks. The Golden Webinars are provided as a free public service and have no registration fees.
2) The vast polar structures around the Milky Way and Andromeda
In November 2013, Prof. Pavel Kroupa presented “The vast polar structures around the Milky Way and Andromeda” in the Heidelberg Joint Astronomical Colloquium. In the talk he discussed the failures of the Standard model of cosmology and the implications for fundamental physics.
A blog entry from 2012 on the vast polar structure (VPOS) of satellite objects around the Milky Way can be found here.
3) Bethe-Kolloquium “Dark Matter: A Debate”
In November 2010, Prof. Simon White (Max Planck Institute of Astrophysics, Garching) and Prof. Pavel Kroupa (University of Bonn) debated on the concept and existence of dark matter during the Bethe Colloquium in Bonn. Their presentations and the subsequent debate are available here:
a) Presentations by Prof. White and Prof. Kroupa
Summary of both presentations:
b) The Debate
The German-language television channel 3sat produced a TV report on the Bethe Colloquium, which can be also found on Youtube (available only in German):
In The Dark Matter Crisis by Moritz Haslbauer, Marcel Pawlowski and Pavel Kroupa. A listing of contents of all contributions is available here.
1) To obtain an introduction to MOND and MOND-cosmology, those interested might like to watch the talk below by Dr. Indranil Banik (past AvH Fellow in the SPODYR group at Bonn University, now at St.Andrews University). It was held on Sept. 30th, 2021 at the University of Southampton.
Also, the following two previous talks are relevant:
Note that “true prediction” is used throughout this text to mean a prediction of some phenomenon before observations have been performed. Today, many numerical cosmologists and an increasing number of astrophysicists appear to be using a redefinition of “prediction” as simply being an adjusted calculation. Thus, the modern scientists observes data, then calculates what the cosmological model would give, adjusts the calculation to agree with the data, and then publishes this as a model prediction.
David concludes this essay with “But I hope that scientists and educators can begin creating an environment in which the next generation of cosmologists will feel comfortable exploring alternative theories of cosmology.”
In addition to the performance of a model in terms of true predictions, a model can also be judged in terms of its capability to be consistent with data. This is a line of approach of model-testing followed by me and collaborators, and essentially applies the straight-forward concept that a model is ruled out if it is significantly falsified by data. Rigor of the falsification can be tested for using very different independent tests (e.g. as already applied in Kroupa et al. 2010). We have been covering this extensively in this blog. For example, the existence of dark matter particles is falsified by applying the Chandrasekhar dynamical friction test (as explained in Kroupa 2012 and Kroupa 2015): Satellite galaxies slow down and sink to the centre of their primary galaxy because of dynamical friction on the dark matter haloes. This test has been applied by Angus et al. (2011) demonstrating lack of evidence for the slow down. The motions of the galaxies in the nearby galaxy group M81 likewise show no evidence of dynamical friction (Oehm et al. 2017). Most recently, the detailed investigation of how rapidly galactic bars rotate again disproves their slow-down by dynamical friction on the dark matter halos of their hosting galaxies, in addition to the dark-matter based models having a completely incompatible fraction of disk galaxies with bars in comparison to the observed galaxies (Roshan et al. 2021a; Roshan et al. 2021b). All these tests show dark matter to not exist. Completely unrelated and different tests based on the larger-scale matter distribution and high-redshift galaxy clusters have been performed in great detail by, respectively, Haslbauer et al. (2020) and Asencio et al. (2021). Again, each of these individually falsify the standard dark-matter based models with more than five sigma confidence.
In summary: (a) By applying the formalisms of the philosophy of science to the problem whether the dark-matter-based models or the Milgromian models are the better theories in terms of their track record in true predictions, David Merritt demonstrates the latter to be far superior. (b) By applying the model-falsification approach by calculating the significance of how the models mismatch the data, we have come to the exact same conclusion.
As alluded to by David Merritt, the frightening aspect of our times is that the vast majority of cosmological scientists seem either not capable or willing to understand this. The lectures given by the leaders of cosmological physics, as can be witnessed in the Golden Webinars in Astrophysics series, collate an excellent documentation of the current disastrous state of affairs in this community. In my Golden Webinar in Astrophysics I describe, on April 9th 2021, this situation as
because never before have there been so many ivy-league educated researchers who en masse are so completely off the track by being convinced that a wrong theory (in this case dark matter cosmology) is correct while at the same time ignoring the success of another theory (in this case Milgromian dynamics). At next-to-all institutions, students appear to be indoctrinated by the “accepted” approach, with not few students in my lectures being surprised that the data appear to tell a different story. Many students even come to class believing that elliptical galaxies are the dominant type of galaxy, thus having an entirely wrong image of the Universe in their heads than what is truly out there. Once before there was a great clash of ideas, famously epitomised by Galileo Galilei‘s struggle with the Church. But this was very different, because traditional religious beliefs collided with modern scientific notions. Today, the Great Crisis is within the scientific community, whereby scientists ought to be following the evidence rather than belief. Belief should not even be a word used by scientists, as it implies a non-factual, not logical approach. Rather than belief, we as scientists need to objectively test hypotheses which need to be clearly stated and the results of the tests must be documented in terms of significance levels.
A large number of dwarf galaxies in the Fornax cluster (Figure 1) appear to be disturbed, most likely due to tides from the cluster gravity. In the standard cosmological model (ΛCDM) , the observable structure of the dwarfs is barely susceptible to gravitational effects of the cluster environment, as the dwarfs are surrounded by a dark matter halo. Because of this, it is very hard to explain the observations of the perturbed Fornax dwarfs in this theory. However, these observations can be easily explained in MOND, where dwarfs are much more susceptible to tides due to their lack of protective dark matter halos and the fact that they become quasi-Newtonian as they approach the cluster center due to the external field effect.
Figure 1: Fornax galaxy cluster. The yellow crosses mark all the objects identified in the Fornax deep survey (FDS) for this region of the sky, the black circles are masks for the spikes and reflection haloes, and the red crosses mark the objects that pass the selection criteria to be included in the FDS catalog. Image taken from Venhola et al. 2018.
The impact of tides on what the dwarfs look like is illustrated in Figure 2, which shows the fraction of disturbed galaxies as a function of tidal susceptibility η in ΛCDM and MOND, with η = 1 being the theoretical limit above which the dwarf would be unstable to cluster tides. Moreover, there is a lack of diffuse galaxies (large size and low mass) towards the cluster center. This is illustrated in Figure 3, which shows how at low projected separation from the cluster center, dwarfs of any given mass cannot be too large, but larger sizes are allowed further away. Figure 3 thus shows a clear tidal edge that cannot be explained by selection effects, since the survey detection limit would be a horizontal line at 1 on this plot such that dwarfs above it cannot be detected. Diffuse dwarf galaxies are clearly detectable, but are missing close to the cluster center. Another crucial detail in Figure 3 is that dwarfs close to the tidal edge are much more likely to appear disturbed, which is better quantified in Figure 2 in the rising fraction of disturbed galaxies with tidal stability η. The tidal edge is also evident in Figure 2 in that the dwarfs only go up to some maximum value of η, which should be close to the theoretical stability limit of 1. This is roughly correct in MOND, but not in ΛCDM.
Figure 2: Fraction of disturbed galaxies for each tidal susceptibility bin in MOND (red) and ΛCDM (blue). Larger error bars in a bin indicate that it has fewer dwarfs. The bin width of the tidal susceptibility η is 0.5 in MOND and 0.1 in ΛCDM (each data point is plotted at the center of the bin). Notice the rising trend and the maximum η that arises in each theory.
Figure 3: Projected distances of Fornax dwarfs to the cluster center against the ratio Re/rmax, where Re is the dwarf radius containing half of its total stellar mass, and rmax is the maximum Re at fixed stellar mass above which the dwarf would not be detectable given the survey sensitivity. The dwarfs are classified as “disturbed” (red) “undisturbed” (blue). The black dashed line shows a clear tidal edge – at any given mass, large (diffuse) dwarfs are present only far from the cluster center. This is not a selection effect, as the survey limit is a horizontal line at 1 (though e.g. some nights could be particularly clear and allow us to discover a dwarf slightly above this).
We therefore conclude that MOND and its corresponding cosmological model νHDM (see blog post “Solving both crises in cosmology: the KBC-void and the Hubble-Tension” by Moritz Haslbauer) is capable of explaining not only the appearance of dwarf galaxies in the Fornax cluster, but also other ΛCDM problems related to clusters such as the early formation of El Gordo, a massive pair of interacting galaxy clusters. νHDM also better addresses larger scale problems such as the Hubble tension and the large local supervoid (KBC void) that probably causes it by means of enhanced structure formation in the non-local universe. These larger scale successes build on the long-standing success of MOND with galaxy rotation curves (“Hypothesis testing with gas rich galaxies”). MOND also offers a natural explanation for the Local Group satellite planes as tidal dwarf galaxies (“Modified gravity in plane sight”), and has achieved many other successes too numerous to list here (see other posts). Given all these results, the MOND framework appears better suited than the current cosmological model (ΛCDM) to solve the new astrophysical challenges that keep arising with the increase and improvement of the available astronomical data, which far surpass what was known in 1983 when MOND was first proposed.
In The Dark Matter Crisis by Moritz Haslbauer, Marcel Pawlowski and Pavel Kroupa. A listing of contents of all contributions is available here.
The isolated but nearby galaxy NGC 3109 has a very high radial velocity compared to ΛCDM expectations, that is, it is moving away from the Local Group rapidly, as shown by Peebles (2017) and Banik & Zhao (2018). One of the few possible explanations within this framework is that NGC 3109 was once located within the virial radius of the Milky Way or Andromeda, before being flung out at high velocity in a three-body interaction with e.g. a massive satellite. In the new research paper “On the absence of backsplash analogues to NGC 3109 in the ΛCDM framework”, which was led by Dr. Indranil Banik, it is shown that such a backsplash galaxy is extremely unlikely within the ΛCDM framework. Basically, such galaxies cannot occur in ΛCDM because they ought to be slowed-down due to Chandrasekhar dynamical friction exerted on NGC 3109 and its own dark matter halo by the massive and extended dark matter halo of the Milky way. Making it worse, NGC 3109 is in a thin plane of five associated galaxies (the “NGC 3109 association”, rms height 53 kpc; diameter 1.2 Mpc), all of which are moving away from the Local Group (Pawlowski & McGaugh 2014), whereby the dynamical friction ought to slow down the galaxies in dependence of their dark matter halo masses. This makes its thin planar structure today unexplainable in ΛCDM.
Interestingly, the backsplash scenario is favoured by the authors (Banik et al. 2021), but in the context of MOND. In this theory, much more powerful backsplash events are possible for dwarf galaxies near the spacetime location of the past Milky Way-Andromeda flyby because in MOND galaxies do not have dark matter halos made of particles. A galaxy thus orbits through the potential of another galaxy unhindered and ballistically. The envisioned flyby could also explain the otherwise mysterious satellite galaxy planes which are found around the Milky Way and Andromeda. It now seems that the flyby may well be the only way to explain the properties of NGC 3109, since a less powerful three-body interaction is just not strong enough to affect its velocity as much as would be required. But a Milky Way-Andromeda flyby is not possible in ΛCDM as their overlapping dark matter halos would merge.