Tuesday, September 27, 2022

Data Will Eventually Save The Day In Astrophysics

This short summary paper only analyzes the data about mass and light distributions in a galaxy cluster relative to the ΛCDM Standard Model of Cosmology. 

Once again, it shows that the ΛCDM model doesn't fit the data: In real life, inferred dark matter sub-halos are more compact than predicted in the ΛCDM model. 

Each new finding, on multiple fronts where the ΛCDM model is contradicted leaves less and less room to argue that this model is deeply flawed.

But even more importantly, thanks to the multiple new high quality telescopes out there collecting a veritable torrent of new data on systems like this one at high precision, we have a lot of high precision measurements of gravitationally bound systems that can be fit to different models, such as gravitationally based ones, once we've finally buried the dead horse of ΛCDM and begun to seriously consider alternatives.

I'm comfortable that because so many independent groups are collecting so much data, which is being widely shared, and because theorists are not as monolithic as they are in the high energy physics field, that eventually we will get the right answers, even if the sociology and culture of the astrophysics discipline means that it takes us longer to get to the right answer than it should.
Strong gravitational lensing (SL) has emerged as a very accurate probe of the mass distribution of cluster- and galaxy-scale dark matter (DM) haloes in the inner regions of galaxy clusters. The derived properties of DM haloes can be compared to the predictions of high-resolution cosmological simulations, providing us with a test of the Standard Cosmological Model. 
The usual choice of simple power-law scaling relations to link the total mass of members with their luminosity is one of the possible inherent systematics within SL models of galaxy clusters, and thus on the derived cluster masses. Using new information on their structural parameters (from HST imaging) and kinematics (from MUSE data), we build the Fundamental Plane (FP) for the early-type galaxies of the cluster Abell S1063. We take advantage of the calibrated FP to develop an improved SL model of the total mass of the cluster core. 
The new method allows us to obtain more accurate and complex relations between the observables describing cluster members, and to completely fix their mass from their observed magnitudes and effective radii. Compared to the power-law approach, we find a different relation between the mass and the velocity dispersion of members, which shows a significant scatter. Thanks to a new estimate of the stellar mass of the cluster members from HST data, we measure the cumulative projected mass profiles out to a radius of 350 kpc, for all baryonic and DM components of the cluster. 
Finally, we compare the physical properties of the sub-haloes in our model and those predicted by high-resolution hydrodynamical simulations. We obtain compatible results in terms of the stellar-over-total mass fraction of the members. On the other hand, we confirm the recently reported discrepancy in terms of sub-halo compactness: at a fixed total mass value, simulated sub-haloes are less compact than what our SL model predicts.
Giovanni Granata, "Improved strong lensing modelling of galaxy clusters using the Fundamental Plane: detailed mapping of the baryonic and dark matter mass distribution of Abell S1063" arXiv:2209.11776 (September 21, 2022) (published in The Hypatia Colloquium 2022 book of proceedings).

The body text explains that:
Galaxy clusters are the most massive gravitationally bound structures in the Universe, and around 85 − 90% of their total mass is under the form of dark matter (DM). As a consequence, they are excellent astrophysical laboratories to test our hypoteses on the nature of DM itself. Thanks to several dedicated photometric and spectroscopic surveys, strong gravitational lensing (SL) has become the most accurate probe of the total mass distribution in the cores (out to a few hundreds of kiloparsecs from the centre) of massive galaxy clusters. SL can be combined with baryonic mass diagnostics to disentangle the mass distribution of cluster- and galaxy-scale DM haloes from the total mass distribution of the cluster. The resulting DM mass profiles can be compared to the predictions of high-resolution cosmological simulations, based on the Λ cold dark matter (CDM) Cosmological Model. 

The remarkable improvement in the accuracy of SL models, driven by recent observational campaigns, has allowed us to map robustly the mass distribution of the DM haloes hosting the member galaxies (usually referred to as sub-haloes). On this scale, a significant discrepancy between the predictions of SL models and highresolution simulations has recently emerged: at a fixed galaxy total mass, sub-haloes extracted from SL models are more compact than their simulated counterparts [2].

[2] Meneghetti, M., et al., 2020, Science, 369, 1347 . . . 

As anticipated, comparing the physical properties of the DM sub-haloes as predicted by SL models to the most recent cosmological simulations is a test of the foundations of the Λ CDM cosmological model on which the simulations are based, and of the micro-physics of DM. 

We first compare the stellar-to-total mass fractions of the cluster members with the predictions of recent HOD studies based on DM-only N-body simulations [7]. We find a significant discrepancy: the stellar mass fraction values predicted by SL models are almost an order of magnitude higher than those predicted by the HOD procedure. This discrepancy is resolved if one considers, instead, hydrodynamical simulations, which include gas particles and stars, as well as the effects of the interaction between baryons and DM during the formation of clusters. We consider high-resolution simulations of clusters with a mass similar to that of AS1063 from [8]. We perform two-dimensional projections to simulate the lensing observational conditions. In this case, we find compatible stellar mass fraction values from the SL model and the simulation suite. 

Secondly, we examine how sub-haloes extracted from lensing models compare to their simulated counterparts in terms of maximum circular velocity, which is a proxy for their compactness. [2] recently found that hydrodynamical simulations predict high-mass sub-haloes (total mass M > 10^10 M ) to be significantly less compact than forecast by a sample of state-of-the-art SL models, including the model of Abell S1063 presented in [4]. 

The new technique we adopt significantly impacts the relation between the total mass and the maximum circular velocity of the sub-haloes, obtaining again a different slope compared to [4] and allowing for the inclusion of a scatter. However, as shown in Figure 2, our results agree with those from [4] in the mass range considered, thus confirming the reported discrepancy. Several tests to infer the origin of this discrepancy are being performed, focusing both on SL modelling and on the implementation of the cosmological simulations. However, no conclusive answer has been obtained so far. This leaves several open questions, and could point towards a new fundamental challenge for the Λ CDM paradigm.

[1] is the result from the modeling of strong gravitational lensing observations in this study (blue), [4] is the distribution predicted by the ΛCDM model (red).

See also here ("At z~2 there is ~40% less dark matter mass on average within R(e) compared to expected values based on cosmological stellar-mass halo-mass relations.")

Monday, September 26, 2022

A Rare Physics Op-Ed

The Guardian Newspaper has a rare opinion-editorial article about the value of certain kinds of theoretical and phenomenological physics research headlined:
No one in physics dares say so, but the race to invent new particles is pointless: In private, many physicists admit they do not believe the particles they are paid to search for exist – they do it because their colleagues are doing it. 

It was written by Sabine Hossenfelder who, as usual, is spot on in challenging the practice she describes not as entirely invalid, but as having minimal value. It begins playfully:

Imagine you go to a zoology conference. The first speaker talks about her 3D model of a 12-legged purple spider that lives in the Arctic. There’s no evidence it exists, she admits, but it’s a testable hypothesis, and she argues that a mission should be sent off to search the Arctic for spiders.

The second speaker has a model for a flying earthworm, but it flies only in caves. There’s no evidence for that either, but he petitions to search the world’s caves. The third one has a model for octopuses on Mars. It’s testable, he stresses.
She continues:
Almost every particle physics conference has sessions just like this, except they do it with more maths. It has become common among physicists to invent new particles for which there is no evidence, publish papers about them, write more papers about these particles’ properties, and demand the hypothesis be experimentally tested. Many of these tests have actually been done, and more are being commissioned as we speak. It is wasting time and money.

Since the 1980s, physicists have invented an entire particle zoo, whose inhabitants carry names like preons, sfermions, dyons, magnetic monopoles, simps, wimps, wimpzillas, axions, flaxions, erebons, accelerons, cornucopions, giant magnons, maximons, macros, wisps, fips, branons, skyrmions, chameleons, cuscutons, planckons and sterile neutrinos, to mention just a few. We even had a (luckily short-lived) fad of “unparticles”.

All experiments looking for those particles have come back empty-handed, in particular those that have looked for particles that make up dark matter, a type of matter that supposedly fills the universe and makes itself noticeable by its gravitational pull. However, we do not know that dark matter is indeed made of particles; and even if it is, to explain astrophysical observations one does not need to know details of the particles’ behaviour. The Large Hadron Collider (LHC) hasn’t seen any of those particles either, even though, before its launch, many theoretical physicists were confident it would see at least a few.

Talk to particle physicists in private, and many of them will admit they do not actually believe those particles exist. . . .  

[T]he biggest contributor to this trend is a misunderstanding of Karl Popper’s philosophy of science, which, to make a long story short, demands that a good scientific idea has to be falsifiable. Particle physicists seem to have misconstrued this to mean that any falsifiable idea is also good science.

In the past, predictions for new particles were correct only when adding them solved a problem with the existing theories. For example, the currently accepted theory of elementary particles – the Standard Model – doesn’t require new particles; it works just fine the way it is. The Higgs boson, on the other hand, was required to solve a problem. The antiparticles that Paul Dirac predicted were likewise necessary to solve a problem, and so were the neutrinos that were predicted by Wolfgang Pauli. The modern new particles don’t solve any problems.

In some cases, the new particles’ task is to make a theory more aesthetically appealing, but in many cases their purpose is to fit statistical anomalies. Each time an anomaly is reported, particle physicists will quickly write hundreds of papers about how new particles allegedly explain the observation. . . . 

Ambulance-chasing is a good strategy to further one’s career in particle physics. . . . since ambulance-chasers cite each other’s papers, they can each rack up hundreds of citations quickly. But it’s a bad strategy for scientific progress. . . .  
I believe there are breakthroughs waiting to be made in the foundations of physics; the world needs technological advances more than ever before, and now is not the time to idle around inventing particles, arguing that even a blind chicken sometimes finds a grain. As a former particle physicist, it saddens me to see that the field has become a factory for useless academic papers.
Also, note that criticism of physics scholarship really has two parts: 

(1) particles "to make a theory more aesthetically appealing", which aren't actually needed (like axions, see-saw mechanism neutrinos, supersymmetric particles and extra Higgs bosons), and 

(2) particles to explain statistical anomalies (often from one or two recent experiments) which have not adequately exhausted explanations that don't require new physics (like the X17 particle, leptoquarks to explain lepton universality violations in semi-leptonic B meson decays, sterile neutrinos, and new particles proposed to explain a W boson mass measurement out of step with other recent W boson mass measurements).

But, both are out of hand and a waste of time and money that would be better spent focusing on more well motivated proposals.