A new paper tries to make sense of the DESI results that suggest a sum of the three neutrino masses that is equal to or smaller than the minimum value suggested by neutrino oscillation experiments (which is roughly 0.06 eV).

An outlier data point in the DESI data and a known methodological issue in the Planck 18 data that a later version of the analysis of the Planck data has corrected largely resolve this issue. A non-constant "cosmological constant" would also help resolve the discrepancy and is increasingly favored by observation.

Even with relaxed bounds, however, the cosmological constraints on the sum of the three neutrino masses are still considerably tighter than those derived from direct measurements of the lightest neutrino mass. And, cosmological bounds, along with other data sources, continue to favor a "normal hierarchy" of neutrino masses over an "inverted hierarchy."

Cosmological neutrino mass bounds are becoming increasingly stringent.The latest limit within ΛCDM from Planck 2018+ACT lensing+DESI is ∑mν < 0.072eV at 95% CL, very close to the minimum possible sum of neutrino masses (∑mν > 0.06eV), hinting at vanishing or even ''negative'' cosmological neutrino masses.

In this context, it is urgent to carefully evaluate the origin of these cosmological constraints. In this paper, we investigate the robustness of these results in three ways: i) we check the role of potential anomalies in Planck CMB and DESI BAO data; ii) we compare the results for frequentist and Bayesian techniques, as very close to physical boundaries subtleties in the derivation and interpretation of constraints can arise; iii) we investigate how deviations from ΛCDM, potentially alleviating these anomalies, can alter the constraints.

From a profile likelihood analysis, we derive constraints in agreement at the ∼10% level with Bayesian posteriors. We find that the weak preference for negative neutrino masses is mostly present for Planck 18 data, affected by the well-known "lensing anomaly".It disappears when the new Planck 2020 HiLLiPoP is used, leading to significantly weaker constraints.Additionally, the pull towards negative masses in DESI data stems from the z=0.7 bin, which is in ∼3σ tension with Planck expectations.Without these outliers, and in combination with HiLLiPoP, the bound relaxes to ∑mν<0.11eV at 95% CL. The recent preference for dynamical dark energy alleviates this tension and further weakens the bound.As we are at the dawn of a neutrino mass discovery from cosmology, it will be very exciting to see if this trend is confirmed by future data.

Daniel Naredo-Tuero, et al., "Living at the Edge: A Critical Look at the Cosmological Neutrino Mass Bound" arXiv:2407.13831 (July 18, 2024).

## 8 comments:

Wikipedia gives for the value of the cosmological constant 0.7. What are the error bars on this? Or are error bars meaningless due to uncertainlies in the cosmological model? Maybe 0.0 is appropriate. Consistent with Deur? A limit from positive gamma or negative gammw. What is String theory like for gamma equal to 0? DeSitter in the Swampland but how does a limit to 0 fit in?

According to the Particle Data Group, the value of the cosmological constant is:

1.088*10^-56 ± 3*10^-58 m^-2. https://pdg.lbl.gov/2024/reviews/contents_sports.html

The relative uncertainty is one part per 36.3 (which is about ± 3%).

Wikipedia, however, states that the cosmological constant is measured to be on the order of 10^−52 m^−2. I would trust the Particle Data Group over Wikipedia in this case.

A value of 0.7 doesn't make sense because the cosmological constant is not dimensionless. You are probably confusing it with Omega sub lambda, i.e. Ω(Λ) ≈ 0.7, which means that at the current moment of time, dark energy makes up about 70% of the mass-energy of the observable universe. This will get larger in the future and was smaller in the past according to standard models of cosmology.

"Consistent with Deur?"

Deur is based on a framework of GR without a cosmological constant. He sees it as a corollary of the dark matter phenomena arising from gravitational field self-interaction. In other words, gravitational fields end up doing more than par amount of work in the seemingly dark matter bound galaxy which less weaker gravitational fields between galaxies than you would expect. He has a cosmological paper that fits this analysis successfully to the data.

In Deur's analysis the cosmic coincidence (i.e. the dark matter phenomena and dark energy phenomena have a similar order of magnitude at the current time in the tally of the stuff in the universe in a LambdaCDM cosmology) is causally related and not a coincidece.

"A limit from positive gamma or negative gammw. What is String theory like for gamma equal to 0? DeSitter in the Swampland but how does a limit to 0 fit in?"

Not sure that I follow where you are going with this. It is a bit too abbreviated for me to track your thinking.

The Particle Data Group value of Ω(Λ) = 0.685(7), which is about a 1% relative uncertainty. It also notes that this is a derived parameter in 6-parameter ΛCDM fit.

I suspect you are saying gamma when you mean lambda which is the cosmological constant.

"The cases of spacetime of constant curvature are de Sitter space (positive), Minkowski space (zero), and anti-de Sitter space (negative). As such, they are exact solutions of the Einstein field equations for an empty universe with a positive, zero, or negative cosmological constant, respectively." https://en.wikipedia.org/wiki/Anti-de_Sitter_space

We appear to live in a DeSitter space which has a small positive cosmological constant. There are lots of really neat theoretical properties of Anti-Desitter space.

"In theoretical physics, the anti-de Sitter/conformal field theory correspondence (frequently abbreviated as AdS/CFT) is a conjectured relationship between two kinds of physical theories. On one side are anti-de Sitter spaces (AdS) that are used in theories of quantum gravity, formulated in terms of string theory or M-theory. On the other side of the correspondence are conformal field theories (CFT) that are quantum field theories, including theories similar to the Yang–Mills theories that describe elementary particles.

The duality represents a major advance in the understanding of string theory and quantum gravity. This is because it provides a non-perturbative formulation of string theory with certain boundary conditions and because it is the most successful realization of the holographic principle, an idea in quantum gravity originally proposed by Gerard 't Hooft and promoted by Leonard Susskind.

It also provides a powerful toolkit for studying strongly coupled quantum field theories. Much of the usefulness of the duality results from the fact that it is a strong–weak duality: when the fields of the quantum field theory are strongly interacting, the ones in the gravitational theory are weakly interacting and thus more mathematically tractable. This fact has been used to study many aspects of nuclear and condensed matter physics by translating problems in those subjects into more mathematically tractable problems in string theory." https://en.wikipedia.org/wiki/AdS/CFT_correspondence

This is cool except for that fact that it has no applicability to our universe and doesn't provide a useful analogy or starting point to understand our universe.

When the cosmological constant is zero then we have a "flat" universe which our universe almost is or seems to be, but not exactly, because the cosmological constant is measured to be almost, but not quite, zero and positive.

"What is String theory like for gamma equal to 0?"

String theory is always an intractable mess from which no meaningful predictions can be made.

Re Deur, see:

"One of the most important problems vexing the ΛCDM cosmological model is the Hubble tension. It arises from the fact that measurements of the present value of the Hubble parameter performed with low-redshift quantities, e.g., the Type IA supernova, tend to yield larger values than measurements from quantities originating at high-redshift, e.g., fits of cosmic microwave background radiation. It is becoming likely that the discrepancy, currently standing at 5σ, is not due to systematic errors in the measurements.

Here we explore whether the self-interaction of gravitational fields in General Relativity, which are traditionally neglected when studying the evolution of the universe, can explain the tension. We find that with field self-interaction accounted for, both low- and high-redshift data are simultaneously well-fitted, thereby showing that gravitational self-interaction could explain the Hubble tension. Crucially, this is achieved without introducing additional parameters."

Corey Sargent, Alexandre Deur, Balsa Terzic, "Hubble Tension and Gravitational Self-Interaction" arXiv:2301.10861 (January 25, 2023).

"Field self-interactions are at the origin of the non-linearities inherent to General Relativity. We study their effects on the Cosmic Microwave Background anisotropies. We find that they can reduce or alleviate the need for dark matter and dark energy in the description of the Cosmic Microwave Background power spectrum."

A. Deur, "Effect of the field self-interaction of General Relativity on the Cosmic Microwave Background Anisotropies" arXiv:2203.02350 (March 4, 2022).

"We check whether General Relativity's field self-interaction alleviates the need for dark matter to explain the universe's large structure formation. We found that self-interaction accelerates sufficiently the growth of structures so that they can reach their presently observed density. No free parameters, dark components or modifications of the known laws of nature were required. This result adds to the other natural explanations provided by the same approach to the, inter alia, flat rotation curves of galaxies, supernovae observations suggestive of dark energy, and dynamics of galaxy clusters, thereby reinforcing its credibility as an alternative to the dark universe model."

Alexandre Deur, "Effect of gravitational field self-interaction on large structure formation" arXiv: 2018:04649 (July 9, 2021) (Accepted for publication in Phys. Lett. B) DOI: 10.1016/j.physletb.2021.136510

"Numerical calculations have shown that the increase of binding energy in massive systems due to gravity's self-interaction can account for galaxy and cluster dynamics without dark matter. Such approach is consistent with General Relativity and the Standard Model of particle physics. The increased binding implies an effective weakening of gravity outside the bound system. In this article, this suppression is modeled in the Universe's evolution equations and its consequence for dark energy is explored. Observations are well reproduced without need for dark energy. The cosmic coincidence appears naturally and the problem of having a de Sitter Universe as the final state of the Universe is eliminated."

A. Deur, “A possible explanation for dark matter and dark energy consistent with the Standard Model of particle physics and General Relativity” (August 14, 2018) (Proceeding for a presentation given at Duke University, Apr. 2014. Based on A. D. PLB B676, 21 (2009); A.D, MNRAS, 438, 1535 (2014). The published version is https://link.springer.com/article/10.1140/epjc/s10052-019-7393-0).

in case Deur is proven wrong

there is this

arXiv:2407.13820 [pdf, html, other]

Emergence of phantom cold dark matter from spacetime diffusion

Jonathan Oppenheim, Emanuele Panella, Andrew Pontzen

Comments: 21 pages + appendix

Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)

A way to reconcile general relativity and quantum field theory without quantising the geometry is to demand the metric evolve stochastically. In this article, we explore the consequences of such a proposal at early cosmological times. We find the stochastic evolution results in the spatial metric diffusing away from its deterministic value, generating phantom cold dark matter (CDM). It is produced primarily at the end of the inflationary phase of the Universe's evolution, with a statistical distribution that depends on the specifics of the early-times cosmological model. We find the energy density of this phantom cold dark matter is positive on average, a necessary condition to reproduce the cosmological phenomenology of CDM, although further work is required to calculate its mean density and spatial distribution. If the density is cosmologically significant, phantom dark matter acts on the geometry in a way that is indistinguishable from conventional CDM. As such, it has the potential to reproduce phenomenology such as structure formation, lensing, and galactic rotation curves. We conclude by discussing the possibility of testing hybrid theories of gravity by combining measurements of the Cosmic Microwave Background with tabletop experiments.

Sorry, I am late in getting back. I have been troubleshooting a lawnmower. I think the ignition is dead. Anyway. Yes I confused gamma and lambda, I meant lambda. I have thought some more. If you have two models with the same physical parameter, then in fitting to data, the systematic error must be big enough to cover the fitted values from both models. So lambda equal to 0 is consistent with data. I am interested in this in connection with string theory and the Swampland. (I in no way support either.) It is said that a positive lambda cannot be obtained from string theory. But what about 0, perhaps as a limit from negative lambda? Is lambda calculated from string theory or is it just input? Maybe other stuff is input and that determines lambda? Maybe noone knows? Maybe one just selects a background metric with whatever lambda you want, then how is positive lambda excluded?

"So lambda equal to 0 is consistent with data."

It is not, at a five sigma level, relative to a GR without a cosmological constant model. Of course, you could devise a non-GR gravity model (or one that controversially applies GR in a manner other than the way it is conventionally operationalized, like Deur's approach). There is strong evidence of some effect, although it isn't clear with the latest observations if it is really a constant, or exactly what source or mechanism creates it.

"It is said that a positive lambda cannot be obtained from string theory. But what about 0, perhaps as a limit from negative lambda? Is lambda calculated from string theory or is it just input? Maybe other stuff is input and that determines lambda? Maybe no one knows? Maybe one just selects a background metric with whatever lambda you want, then how is positive lambda excluded?"

I don't know. Somebody, indeed, probably at least hundreds of somebodies, if not thousands of them, do know what string theory says on these points. I just don't happen to be sufficiently knowledgable about the details of string theory math to be one of them. Maybe I'll look into it and find out.

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