Dark matter = modified gravity? Scrutinising the spacetime–matter distinction through the modified gravity/ dark matter lens

We investigate the interaction between dark energy and dark matter in the framework of irreversible thermodynamics of open systems with matter creation/annihilation. We consider dark energy and dark matter as an interacting two-component (scalar field and ``ordinary'' dark matter) cosmological fluid in a homogeneous spatially flat and isotropic Friedmann-Robertson-Walker Universe. The thermodynamics of open systems as applied together with the gravitational field equations to the two-component cosmological fluid leads to a generalization of the elementary dark energy-dark matter interaction theory, in which the decay (creation) pressures are explicitly considered as parts of the cosmological fluid stress-energy tensor. Specific models describing coherently oscillating scalar waves, leading to a high particle production at the beginning of the oscillatory period, and models with a constant potential energy scalar field are considered. Furthermore, exact and numerical solutions of the gravitational field equations with dark energy-dark matter interactions are also obtained.

Playing dice with Einstein: M. Jammer, Einstein and Religion: Physics and Theology. (268 pp.) Princeton University Press, Princeton, 1999, hardback, US $26.95, UK £18.95, ISBN 0-691-00699-7. [Translation and revision of Einstein und die Religion published by Universitätsverlag Konstanz.

The intra-mercurial planet

Cartography of the space of theories: An interpretational chart for fields that are both (dark) matter and spacetime

It is accepted in modern cosmology that the scalar field responsible for the inflationary stage of the early Universe is completely transformed into matter. It is assumed that the accelerated expansion is currently driven by dark energy (DE), which is likely determined by Einstein’s cosmological constant, unrelated to the scalar field responsible for inflation. We consider a cosmological model in which DE can currently have two components, one of which is Einstein’s constant ( $$\Lambda $$ ) and the other, smaller dark energy variable component DEV ( $${{\Lambda }_{V}}$$ ), is associated with the remnant of the scalar field that caused inflation after the main part of the scalar field has turned into matter. We consider only the stages of evolution of the Universe after recombination ( $$z \lesssim 1100$$ ), where dark matter (DM) is the predominant component of matter. It is assumed that the transformation of the scalar field into matter continues at the present time and is accompanied by the reverse process of the transformation of DM into a scalar field. The interconnection between DM and DEV, which leads to a linear relationship between the energy densities of these components after recombination $${{\rho }_{{{\text{DM}}}}} = \alpha {\kern 1pt} {{\rho }_{{{\text{DEV}}}}}$$ , is considered. Variants with a dependence of the coefficient $$\alpha (z)$$ on the redshift $$z$$ are also considered. One of the problems that have arisen in modern cosmology, called Hubble Tension (HT), is the discrepancy between the present values of the Hubble constant ( $${{H}_{0}}$$ ) measured from observations of the Universe at small redshifts ( $$z \lesssim 1$$ ) and the values found from fluctuations of the cosmic microwave background in the Universe at large redshifts ( $$z \approx 1100$$ ). In the model under consideration, this discrepancy can be explained by the deviation of the existing cosmological model from the conventional $$\Lambda $$ cold dark matter (CDM) model of the flat Universe by the action of the additional dark energy component DEV at the stages after recombination. Within this extended model, we consider various $$\alpha {\kern 1pt} (z)$$ functions that can eliminate the HT. To maintain the ratio of DEV and DM energy densities close to constant over the interval $$0 \leqslant z \lesssim 1100$$ , it is necessary to assume the existence of a wide spectrum of dark matter particle masses.

The dark side of the universe: from Zwicky to accelerated expansion

It is accepted in modern cosmology that the scalar field responsible for the inflationary stage of the early Universe is completely transformed into matter. It is assumed that the accelerated expansion is currently driven by dark energy (DE), which is likely determined by Einstein’s cosmological constant, unrelated to the scalar field responsible for inflation. We consider a cosmological model in which DE can currently have two components, one of which is Einstein’s constant (Λ) and the other, smaller dark energy variable component DEV (ΛV), is associated with the remnant of the scalar field that caused inflation after the main part of the scalar field has turned into matter. We consider only the stages of evolution of the Universe after recombination (z=1100), where dark matter (DM) is the predominant component of matter. It is assumed that the transformation of the scalar field into matter continues at the present time and is accompanied by the reverse process of the transformation of DM into a scalar field. The interconnection between DM and DEV, which leads to a linear relationship between the energy densities of these components after recombination ρDM=αρDEV, is considered. Variants with a dependence of the coefficient α(z) on the redshift z are also considered. One of the problems that have arisen in modern cosmology, called Hubble Tension (HT), is the discrepancy between the present values of the Hubble constant (H0) measured from observations of the Universe at small redshifts (z≲1) and the values found from fluctuations of the cosmic microwave background in the Universe at large redshifts (z≈1100). In the model under consideration, this discrepancy can be explained by the deviation of the existing cosmological model from the conventional Λ cold dark matter (CDM) model of the flat Universe by the action of the additional dark energy component DEV at the stages after recombination. Within this extended model, we consider various α(z) functions that can eliminate the HT. To maintain the ratio of DEV and DM energy densities close to constant over the interval 0⩽z≲1100, it is necessary to assume the existence of a wide spectrum of dark matter particle masses.

虽然暗物质的存在已经得到了大量的天文观测的支持, 但暗物质的属性是什么仍然是个未解之谜.近期 暗物质探测的实验和理论研究均取得了长足的进展. 本文从暗物质问题的提出讲起, 介绍了暗物质的基本特点和 可能的粒子物理候选者, 之后详细介绍了暗物质研究的最新进展.(1) 暗物质研究的早期历史.从星系旋转曲线、 引 力透镜、 微波背景辐射等方面介绍了暗物质的观测证据, 特别是暗物质丰度起源的标准热退耦理论机制和典型的暗 物质粒子候选者, 如弱相互作用有质量粒子等.(2) 暗物质粒子的实验探测的基本原理和手段, 如地下直接探测和 空间间接探测等.重点综述了近期实验研究的进展.在地下直接探测方面综述了10 GeV以下轻质量暗物质的探测 实验: SuperCDMS(super cryogenic dark matter search), CDEX(China dark matter experiment)等, 以及大质量暗物质探 测中的液氩探测器DarkSide等.(3) 暗物质未来的碰撞方向性探测实验, 如DRIFT(directional recoil identification from tracks), MIMAC(MIcro-tpc MAtrix of Chambers)等.在空间间接探测方面介绍暗物质湮灭到宇宙线粒子中涉及 到的宇宙线粒子产生和传播的基本理论.(4) 已有的实验, 如Fermi-LAT(Fermi large area telescope)和AMS(Alpha magnetic spectrometer)-02在高能宇宙线电子和核子方面已经取得的成果, 特别是近期DAMPE(dark matter particle explorer)卫星实验的首个结果中看到的正负电子总流强中的新现象和疑似反常现象以及AMS-02的反质子结果对暗 物质搜寻的影响.展望了未来在反核子, 如反氘和反氦方面可能取得的结果及其对暗物质研究的重要性.

It is accepted in the present cosmology model that the scalar field, which is responsible for the inflation stage in the early universe, transforms completely into matter, and the accelerated universe expansion is presently governed by dark energy (DE), whose origin is not connected with the inflationary scalar field. We suppose here that dark matter (DM) has a common origin with a small variable component of dark energy (DEV). We suggest that DE may presently have two components, one of which is the Einstein constant Λ, and another, smaller component DEV (ΛV) comes from the remnants of the scalar field responsible for inflation, which gave birth to the origin of presently existing matter. In this note we consider only the stages of the universe expansion after recombination, z≃1100, when DM was the most abundant component of the matter, therefore we suggest for simplicity that a connection exists between DM and DEV so that the ratio of their densities remains constant over all the stages after recombination, ρDM=αρDEV, with a constant α. One of the problems revealed recently in cosmology is a so-called Hubble tension (HT), which is the difference between values of the present Hubble constant, measured by observation of the universe at redshift z≲1, and by observations of a distant universe with CMB fluctuations originated at z∼1100. In this paper we suggest that this discrepancy may be explained by deviation of the cosmological expansion from a standard Lambda-CDM model of a flat universe, due to the action of an additional variable component DEV. Taking into account the influence of DEV on the universe’s expansion, we find the value of α that could remove the HT problem. In order to maintain the almost constant DEV/DM energy density ratio during the time interval at z<1100, we suggest the existence of a wide mass DM particle distribution.

In the background dynamics of a spatially flat FLRW model of the universe, we investigate an interacting dark energy (DE) model in the context of Lyra’s geometry. Pressure-less dust is considered as dark matter, mass of which varies with time via scalar field in the sense that decaying of dark matter particles reproduces the scalar field. Here, quintessence scalar field is adopted as DE candidate which evolves in exponential potential. Mass of the dark matter particles is also considered to be evolved in exponential function of the scalar field. Cosmological evolution equations are studied in the framework of dynamical systems analysis. Dimension-less variables are chosen properly so that the cosmological evolution equations are converted into an autonomous system of ordinary differential equations. Linear stability is performed to find the nature of critical points by perturbing the system around the critical points in the phase-space. Classical stability is also executed by finding out the speed of sound. Dynamical systems explore several viable results which are physically interested in some parameter regions. Late-time scalar field-dominated attractors are found by critical points, corresponding to the accelerating universe. Scalar field-displacement vector field scaling solutions are realized that represent late-time decelerated universe. Dark energy-dark matter scaling solutions are also exhibited by critical points which correspond to accelerated attractors possessing similar order of energy densities of dark energy and dark matter, that provides the possible solutions of coincidence problem.

The predicted size of dark matter substructures in kilo-parsec scales is model-dependent. Therefore, if the correlations between dark matter mass densities as a function of the distances between them are measured via observations, we can scrutinize dark matter scenarios. In this paper, we present an assessment procedure of dark matter scenarios. First, we use Gaia’s data to infer the single-body phase-space density of the stars in the Fornax dwarf spheroidal galaxy. The latter, together with the Jeans equation, after eliminating the gravitational potential using the Poisson equation, reveals the mass density of dark matter as a function of its position in the galaxy. We derive the correlations between dark matter mass densities as a function of distances between them. No statistically significant correlation is observed. Second, for the sake of comparison with the standard cold dark matter, we also compute the correlations between dark matter mass densities in a small halo of the Eagle hydrodynamics simulation. We show that the correlations from the simulation and from Gaia are in agreement. Third, we show that Gaia observations can be used to limit the parameter space of the Ginzburg–Landau statistical field theory of dark matter mass densities and subsequently shrink the parameter space of any dark matter model. As two examples, we show how to leave limitations on (i) a classic gas dark matter and (ii) a superfluid dark matter.

More than sixty years ago Zwicky made the case that the great clusters of galaxies are held together by the gravitational force of unseen (dark) matter. Today, the case is stronger and more precise: Dark, nonbaryonic matter accounts for 30% +/- 7% of the critical mass density, with baryons (most of which are dark) contributing only 4.5% +/- 0.5% of the critical density. The large-scale structure that exists in the Universe indicates that the bulk of the nonbaryonic dark matter must be cold (slowly moving particles). The SuperKamiokande detection of neutrino oscillations shows that particle dark matter exists, crossing an important threshold. Over the past few years a case has developed for a dark-energy problem. This dark component contributes about 80% +/- 20% of the critical density and is characterized by very negative pressure (p_X < -0.6 rho_X). Consistent with this picture of dark energy and dark matter are measurements of CMB anisotropy that indicate that total contribution of matter and energy is within 10% of the critical density. Fundamental physics beyond the standard model is implicated in both the dark matter and dark energy puzzles: new fundamental particles (e.g., axion or neutralino) and new forms of relativistic energy (e.g., vacuum energy or a light scalar field). A flood of observations will shed light on the dark side of the Universe over the next two decades; as it does it will advance our understanding of the Universe and the laws of physics that govern it.

The existence of an early matter-dominated epoch prior to the Big Bang Nucleosynthesis (BBN) may lead to a scenario where the thermal dark matter cools faster than plasma before the radiation-dominated era begins. In the radiation-dominated epoch, dark matter free-streams after it decouples both chemically and kinetically from the plasma. In the presence of an early matter-dominated era, chemical decoupling of the dark matter may succeed by a partial kinetic decoupling before reheating ends, depending upon the contributions of different partial wave amplitudes in the elastic scattering rate of the dark matter. We show that the s-wave scattering is sufficient to partially decouple the dark matter from the plasma, if the entropy injection during the reheating era depends on the bath temperature, while p-wave scattering leads to full decoupling in such cosmological backdrop. The decoupling of dark matter before the end of reheating causes an additional amount of cooling, reducing its free-streaming horizon compared to the usual radiation-dominated cosmology. The enhanced matter perturbations for scales entering the horizon prior to the end of reheating, combined with the reduced free-steaming horizon, increase the number density of sub-earth mass halos. The resulting boost in the dark matter annihilation signatures could offer an intriguing probe to differentiate pre-BBN non-standard cosmological epochs. We show that the free-streaming horizon of the dark matter requires to be smaller than a cut-off to ensure a boost in the sub-earth halo populations. As case studies, we present two examples: one for a scalar dark matter with s-wave elastic scattering and the other one featuring a fermionic dark matter with p-wave elastic scattering. We identify regions of parameter space in both models where the dark matter kinetically decouples during reheating, amplifying small-scale structure formation.

This work is a review of the last results of research on the Scalar Field Dark Matter model of the Universe at cosmological and at galactic level. We present the complete solution to the scalar field cosmological scenario in which the dark matter is modeled by a scalar field $\\Phi$ with the scalar potential $V(\\Phi)=V_{0}(cosh {(\\lambda \\sqrt{\\kappa_{0}}\\Phi)}-1)$ and the dark energy is modeled by a scalar field $\\Psi$, endowed with the scalar potential $\\tilde{V}(\\Psi)= \\tilde{V_{0}}(\\sinh{(\\alpha \\sqrt{\\kappa_{0}}\\Psi)})^{\\beta}$, which together compose the 95% of the total matter energy in the Universe. The model presents successfully deals with the up to date cosmological observations, and is a good candidate to treat the dark matter problem at the galactic level.

This paper modifies the Farnes’ unifying theory of dark energy and dark matter which are negative-mass, created continuously from the negative-mass universe in the positive-negative mass universe pair. The first modification explains that observed dark energy is 68.6%, greater than 50% for the symmetrical positive-negative mass universe pair. This paper starts with the proposed positive-negative-mass 11D universe pair (without kinetic energy) which is transformed into the positive-negative mass 10D universe pair and the external dual gravities as in the Randall-Sundrum model, resulting in the four equal and separate universes consisting of the positive-mass 10D universe, the positive-mass massive external gravity, the negative-mass 10D universe and the negative-mass massive external gravity. The positive-mass 10D universe is transformed into 4D universe (home universe) with kinetic energy through the inflation and the Big Bang to create positive-mass dark matter which is five times of positive-mass baryonic matter. The other three universes without kinetic energy oscillate between 10D and 10D through 4D, resulting in the hidden universes when D > 4 and dark energy when D = 4, which is created continuously to our 4D home universe with the maximum dark energy = 3/4 = 75%. In the second modification to explain dark matter in the CMB, dark matter initially is not repulsive. The condensed baryonic gas at the critical surface density induces dark matter repulsive force to transform dark matter in the region into repulsive dark matter repulsing one another. The calculated percentages of dark energy, dark matter, and baryonic matter are 68.6 (as an input from the observation), 26 and 5.2, respectively, in agreement with observed 68.6, 26.5 and 4.9, respectively, and dark energy started in 4.33 billion years ago in agreement with the observed 4.71 ± 0.98 billion years ago. In conclusion, the modified Farnes’ unifying theory reinterprets the Farnes’ equations, and is a unifying theory of dark energy, dark matter, and baryonic matter in the positive-negative mass universe pair. The unifying theory explains protogalaxy and galaxy evolutions in agreement with the observations.