Showing papers on "Structure formation published in 2023"

Cosmological structure formation and soliton phase transition in Fuzzy Dark Matter with axion self-interactions

Abstract: We investigate cosmological structure formation in Fuzzy Dark Matter (FDM) with an attractive self-interaction (SI) with numerical simulations. Such a SI would arise if the FDM boson were an ultra-light axion, which has a strong CP symmetry-breaking scale (decay constant). Although weak, the attractive SI may be strong enough to counteract the quantum ‘pressure’ and alter structure formation. We find in our simulations that the SI can enhance small-scale structure formation, and soliton cores above a critical mass undergo a phase transition, transforming from dilute to dense solitons.

Primordial Black Hole Formation in Non-Standard Post-Inflationary Epochs

Abstract: When large overdensities gravitationally collapse in the early universe, they lead to primordial black holes (PBH). Depending on the exact model of inflation leading to necessary large perturbations at scales much smaller than scales probed at the Cosmic Microwave Background (CMB) surveys, PBHs of masses ≲103M⊙ are formed sometime between the end of inflation and nucleosynthesis. However, the lack of a direct probe for the exact expansion history of the universe in this duration introduces uncertainties in the PBH formation process. The presence of alternate cosmological evolution for some duration after inflation affects the relation between (i) PBH mass and the scale of the collapsing overdensity; and (ii) PBH abundance and amplitude of the overdensities. In this review, the non-standard cosmological epochs relevant for a difference in PBH production are motivated and discussed. The importance of developing the framework of PBH formation in non-standard epochs is discussed from a phenomenological point of view, with particular emphasis on the advances in gravitational wave (GW) phenomenology, since abundant PBHs are always accompanied by large induced GWs. PBH formation in general non-standard epochs is also reviewed including the mathematical formalism. Specific examples, such as PBH formation in a kinetic energy dominated epoch and an early matter dominated epoch, are discussed with figures showing higher PBH abundances as compared to the production in standard radiation domination.

Polarized accretion shocks from the cosmic web

Abstract: On the largest scales, galaxies are pulled together by gravity to form clusters, which are connected by filaments making a web-like pattern. Radio emission is predicted from this cosmic web, which should originate from the strong accretion shocks around the cosmic structures. We present the first observational evidence that Fermi-type acceleration from strong shocks surrounding the filaments of the cosmic web, as well as in peripherals of low-mass clusters, is at work in the Universe. Using all-sky radio maps and stacking on clusters and filaments, we have detected the polarization signature of the synchrotron emission with polarization fractions ≥20%, which is best explained by the organization of local magnetic fields by strong shock waves both at the cluster peripheries and between clusters. Our interpretation is well supported by a detailed comparison with state-of-the-art cosmological simulations.

Hydrodynamical structure formation in Milgromian cosmology

Abstract: We present the first hydrodynamical cosmological simulations in the νHDM framework based on Milgromian dynamics (MOND) with light (11 eV) sterile neutrinos. νHDM can explain the expansion history, CMB anisotropies, and galaxy cluster dynamics similarly to standard cosmology while preserving MOND’s successes on galaxy scales, making this the most conservative Milgromian framework. We generate initial conditions including sterile neutrinos using camb and music and modify the publicly available code phantom of ramses to run νHDM models. The simulations start at redshift ze = 199, when the gravitational fields are stronger than $a_{_0}$ provided this does not vary. We analyse the growth of structure and investigate the impact of resolution and box size, which is at most 600 comoving Mpc. Large density contrasts arise at late times, which may explain the KBC void and Hubble tension. We quantify the mass function of formed structures at different redshifts. We show that the sterile neutrino mass fraction in these structures is similar to the cosmic fraction at high masses (consistent with MOND dynamical analyses) but approaches zero at lower masses, as expected for galaxies. We also identify structures with a low peculiar velocity comparable to the Local Group, but these are rare. The onset of group/cluster scale structure formation at ze ≈ 4 appears to be in tension with observations of high redshift galaxies, which we discuss in comparison to prior analytical work in a MONDian framework. The formation of a cosmic web of filaments and voids demonstrates that this is not unique to standard Einstein/Newton-based cosmology.

The Concentration–Mass Relation of Massive, Dynamically Relaxed Galaxy Clusters: Agreement Between Observations and ΛCDM Simulations

Abstract: The relationship linking a galaxy cluster’s total mass with the concentration of its mass profile and its redshift is a fundamental prediction of the Cold Dark Matter (CDM) paradigm of cosmic structure formation. However, confronting those predictions with observations is complicated by the fact that simulated clusters are not representative of observed samples where detailed mass profile constraints are possible. In this work, we calculate the Symmetry-Peakiness-Alignment (SPA) morphology metrics for maps of X-ray emissivity from The Three Hundred project hydrodynamical simulations of galaxy clusters at four redshifts, and thereby select a sample of morphologically relaxed, simulated clusters, using observational criteria. These clusters have on average earlier formation times than the full sample, confirming that they are both morphologically and dynamically more relaxed than typical. We constrain the concentration–mass–redshift relation of both the relaxed and complete sample of simulated clusters, assuming power-law dependences on mass (κm) and 1 + z (κζ), finding κm = −0.12 ± 0.07 and κζ = −0.27 ± 0.19 for the relaxed subsample. From an equivalently selected sample of massive, relaxed clusters observed with Chandra, we find κm = −0.12 ± 0.08 and κζ = −0.48 ± 0.19, in good agreement with the simulation predictions. The simulated and observed samples also agree well on the average concentration at a pivot mass and redshift providing further validation of the ΛCDM paradigm in the properties of the largest gravitationally collapsed structures observed. This also represents the first clear detection of decreasing concentration with redshift, a longstanding prediction of simulations, in data.

Magnetic field amplification and structure formation by the Rayleigh-Taylor instability

Abstract: We report our results from a set of high-resolution, two-fluid, non-linear simulations of the magnetized Rayleigh Taylor instability (RTI) at the interface between a solar prominence and the corona. These data follow results reported earlier on linear and early non-linear RTI dynamics in this environment. This paper is focused on the generation and amplification of magnetic structures by RTI. The simulations use a two-fluid model that includes collisions between neutrals and charges, including ionization and recombination, energy and momentum transfer, and frictional heating. The 2.5D magnetized RTI simulations demonstrate that in a fully developed state of RTI, a large fraction of the gravitational energy of a prominence thread can be converted into quasi-turbulent energy of the magnetic field. The RTI magnetic energy generation is further accompanied by magnetic and plasma density structure formation, including dynamic formation, break-up, and merging of current sheets and plasmoid sub-structures. The flow decoupling between neutrals and charges, as well as ionization and recombination reactions, are shown to have significant impact on the structure formation in a magnetized RTI.

Evidence for suppression of structure growth in the concordance cosmological model

Abstract: We present evidence for a suppressed growth rate of large-scale structure during the dark-energy dominated era. Modeling the growth rate of perturbations with the ``growth index'' $\gamma$, we find that current cosmological data strongly prefer a higher growth index than the value $\gamma=0.55$ predicted by general relativity in a flat $\Lambda$CDM cosmology. Both the cosmic microwave background data from Planck and the large-scale structure data from weak lensing, galaxy clustering, and cosmic velocities separately favor growth suppression. When combined, they yield $\gamma=0.633^{+0.025}_{-0.024}$, excluding $\gamma=0.55$ at a statistical significance of 3.7$\sigma$. The combination of $f\sigma_8$ and Planck measurements prefers an even higher growth index of $\gamma=0.639^{+0.024}_{-0.025}$, corresponding to a 4.2$\sigma$-tension with the concordance model. In Planck data, the suppressed growth rate offsets the preference for nonzero curvature and fits the data equally well as the latter model. A higher $\gamma$ leads to a higher matter fluctuation amplitude $S_8$ inferred from galaxy clustering and weak lensing measurements, and a lower $S_8$ from Planck data, effectively resolving the $S_8$ tension.

Structure formation in non-local bouncing models

Abstract: In this study, we investigate the growth of structures within the Deser-Woodard nonlocal theory and extend it to various bouncing cosmology scenarios. Our findings show that the observable structure growth rate, fσ 8, in a vacuum-dominated universe is finite within the redshift range of 0 < z < 2, contrary to previous literature. Although fσ 8 exhibits no divergences, we observe a slight difference between the evolution of the ΛCDM and the non-local DW II models. Regarding structure formation in bouncing cosmologies, we evaluate the evolution of fσ 8 near the bouncing point. Among the different bouncing cases we explore, the oscillatory bounce and pre-inflationary asymmetrical bounce demonstrate a physical profile where the growth rate begins as a small perturbation in the early epoch and increases with inflation, which can be regarded as the seeds of large-scale structures. These findings are significant because they shed light on the growth of seed fluctuations into cosmic structures resulting from non-local effects.

Cosmological Structure Formation and Soliton Phase Transition in Fuzzy Dark Matter with Axion Self-Interactions

Abstract: We investigate cosmological structure formation in Fuzzy Dark Matter (FDM) with an attractive self-interaction (SI) with numerical simulations. Such a SI would arise if the FDM boson were an ultra-light axion, which has a strong CP symmetry-breaking scale (decay constant). Although weak, the attractive SI may be strong enough to counteract the quantum 'pressure' and alter structure formation. We find in our simulations that the SI can enhance small-scale structure formation, and soliton cores above a critical mass undergo a phase transition, transforming from dilute to dense solitons.

A three-dimensional stochastic structure model derived from high-resolution isolated equatorial plasma bubble simulations

Abstract: Abstract Ionospheric structure is characterized by the space–time variation of electron density. However, our understanding of the physical processes that initiate and sustain intermediate-scale structure development does not relate directly to statistical measures that characterize the structure. Consequently, high-resolution physics-based equatorial plasma bubble simulations are essential for identifying systematic relations between statistical structure measures and the underlying physics that initiates and sustains the structure evolution. An earlier paper summarized the analysis of simulated equatorial plasma bubble (EPB) structure initiated with a quasi-periodic bottom-side perturbation that generated five plasma bubbles. The results are representative of real environments. However, the association of the structure development with individual EPBs was difficult to ascertain. This paper summarizes the analysis of new results from single isolated EPB realizations with varying parameters that affect the structure development. The evolution of the single isolated EPB realizations reveal what we have identified as a canonical structure evolution pattern manifest in the space–time development of four quantitative spectral parameters. The onset of structure occurs when the plasma bubble penetrates the F-region peak. The parameter evolution from the initiation point have a fish-like appearance. The three-dimensional structure model can be used to interpret in situ and remote diagnostic measurements as well as predicting the deleterious effects of propagation disturbances on satellite communication, navigation, and surveillance systems. Graphical Abstract

Structure formation by electrostatic interactions in strongly coupled medium

Abstract: The formation of correlated structures is of importance in many diverse contexts such as strongly coupled plasmas, soft matter, and even biological mediums. In all these contexts the dynamics are mainly governed by electrostatic interactions and result in the formation of a variety of structures. In this study, the process of formation of structures is investigated with the help of molecular dynamics (MD) simulations in two and three dimensions. The overall medium has been modeled with an equal number of positive and negatively charged particles interacting via long-range pair Coulomb potential. A repulsive short-range Lennard-Jones (LJ) potential is added to take care of the blowing up of attractive Coulomb interaction between unlike charges. In the strongly coupled regime, a variety of classical bound states form. However, complete crystallization of the system, as typically observed in the context of one-component strongly coupled plasmas, does not occur. The influence of localized perturbation in the system has also been studied. The formation of a crystalline pattern of shielding clouds around this disturbance is observed. The spatial properties of the shielding structure have been analyzed using the radial distribution function and Voronoi diagram. The process of accumulation of oppositely charged particles around the disturbance triggers a lot of dynamic activity in the bulk of the medium. As a result of this, close encounters are possible even between those particles/clusters which were initially and/or at some point of time widely separated. This leads to the formation of a larger number of bigger clusters. There are, however, also instances when bound pairs break up and the electrons from bound pairs contribute to the shielding cloud, whereas ions bounce back into the bulk. A detailed discussion of these features has been provided in the manuscript.

Consistent cosmological structure formation on all scales in relativistic extensions of MOND

Abstract: General relativity manifests very similar equations in different regimes, notably in large scale cosmological perturbation theory, non-linear cosmological structure formation, and in weak field galactic dynamics. The same is not necessarily true in alternative gravity theories, in particular those that possess MONDian behaviour (“relativistic extensions” of MOND). In these theories different regimes are typically studied quite separately, sometimes even with the freedom in the theories chosen differently in different regimes. If we wish to properly and fully test complete cosmologies containing MOND against the ΛCDM paradigm then we need to understand cosmological structure formation on all scales, and do so in a coherent and consistent manner. We propose a method for doing so and apply it to generalised Einstein-Aether theories as a case study. We derive the equations that govern cosmological structure formation on all scales in these theories and show that the same free function (which may contain both Newtonian and MONDian branches) appears in the cosmological background, linear perturbations, and non-linear cosmological structure formation. We show that MONDian behaviour on galactic scales does not necessarily result in MONDian behaviour on cosmological scales, and for MONDian behaviour to arise cosmologically, there will be no modification to the Friedmann equations governing the evolution of the homogeneous cosmological background. We comment on how existing N-body simulations relate to complete and consistent generalised Einstein-Aether cosmologies. The equations derived in this work allow consistent cosmological N-body simulations to be run in these theories whether or not MONDian behaviour manifests on cosmological scales.

Structure Formation in Non-local Bouncing Models

Abstract: In this study, we investigate the growth of structures within the Deser-Woodard nonlocal theory and extend it to various bouncing cosmology scenarios. Our findings show that the observable structure growth rate, $f\sigma_8$, in a vacuum-dominated universe is finite within the redshift range of $0

Structure formation and quasi-spherical collapse from initial curvature perturbations with numerical relativity simulations

Abstract: We use numerical relativity simulations to describe the spacetime evolution during nonlinear structure formation in $\Lambda$CDM cosmology. Fully nonlinear initial conditions are set at an initial redshift $z\approx 300$, based directly on the gauge invariant comoving curvature perturbation $\mathcal{R}_c$ commonly used to model early-universe fluctuations. Assigning a simple 3-D sinusoidal structure to $\mathcal{R}_c$, we then have a lattice of quasi-spherical over-densities representing idealised dark matter halos connected through filaments and surrounded by voids. This structure is implemented in the synchronous-comoving gauge, using a pressureless perfect fluid (dust) description of CDM, and then it is fully evolved with the Einstein Toolkit code. With this, we look into whether the Top-Hat spherical and homogeneous collapse model provides a good description of the collapse of over-densities. We find that the Top-Hat is an excellent approximation for the evolution of peaks, where we observe that the shear is negligible and collapse takes place when the linear density contrast reaches the predicted critical value $\delta^{(1)}_C =1.69$. Additionally, we characterise the outward expansion of the turn-around boundary and show how it depends on the initial distribution of matter, finding that it is faster in denser directions, incorporating more and more matter in the infalling region. Using the EBWeyl code [1] we look at the distribution of the electric and magnetic parts of the Weyl tensor, finding that they are stronger along and around the filaments, respectively. We introduce a method to dynamically classify different regions in Petrov types. With this, we find that the spacetime is of Petrov type I everywhere, as expected, but we can identify the leading order type, finding a transition between different types as non-linearity grows, with production of gravitational waves.

Cosmic voids and the kinetic analysis. II. Link to Hubble tension

Abstract: We consider a principal problem, that of the possible dominating role of self-consistent gravitational interaction in the formation of cosmic structures: voids and their walls in the local Universe. It is in the context of the Hubble tension as a possible indication of the difference in the descriptions of the late (local) and early (global) Universe. The kinetic Vlasov treatment enables us to consider the evolution of gravitating structures where the fundamental role has the modified gravitational potential with a cosmological constant, leading to the prediction of a local flow with a Hubble parameter that is nonidentical to that of the global Hubble flow. The Poisson equation for a potential with an additional repulsive term, including an integral equation formulation, is analyzed, and we predict the appearance of multiply connected two-dimensional gravitating structures and voids in the local Universe. The obvious consequence of the developed mechanism is that the cosmological constant poses a natural scaling for the voids, along with the physical parameters of their local environment, which can be traced in observational surveys.

Halo Formation from Yukawa Forces in the Very Early Universe

Abstract: If long-range attractive forces exist and are stronger than gravity then cosmic halo formation can begin in the radiation-dominated era. We study a simple realization of this effect in a system where dark matter fermions have Yukawa interactions mediated by scalar particles, analogous to the Higgs boson in the standard model. We develop a self-consistent description of the system including exact background dynamics of the scalar field, and precise modelling of the fermion density fluctuations. For the latter, we provide accurate approximations for the linear growth as well as quantitative modelling of the nonlinear evolution using N-body simulations. We find that halo formation occurs exponentially fast and on scales substantially larger than simple estimates predict. The final fate of these halos remains uncertain, but could be annihilation, dark stars, primordial black holes, or even the existence of galaxy-sized halos at matter-radiation equality. More generally, our results demonstrate the importance of mapping scalar-mediated interactions onto structure formation outcomes and constraints for beyond the standard model theories.

Cosmic Web Dissection in Fuzzy Dark Matter Cosmologies

Abstract: On large cosmological scales, anisotropic gravitational collapse is manifest in the dark cosmic web. Its statistical properties are well known for the standard $\Lambda$CDM cosmology, yet become modified for alternative dark matter models such as fuzzy dark matter (FDM). In this work, we assess for the first time the relative importance of cosmic nodes, filaments, walls and voids in a cosmology with small-scale suppression of power such as FDM. By applying the NEXUS+ Multiscale Morphology Filter technique to cosmological $N$-body simulations of FDM-like cosmologies, we quantify the mass and volume filling fractions of cosmic environments at redshifts $z\sim 3.4-5.6$ and find that 2D cosmic sheets host a larger share of the matter content of the Universe ($\sim 5$% increase for the $m=7 \times 10^{-22}$ eV model compared to CDM) as the particle mass $m$ is reduced. We find that the suppression of node-, filament-, wall- and void-conditioned halo mass functions at the low-mass end can occur well above the half-mode mass $M_{1/2}$. We show that log overdensity PDFs are more peaked in FDM-like cosmologies with medians shifted to higher values, a result of the suppression of the low overdensity tail as $m$ is reduced. Skewness estimates $S_3$ of the unconditioned overdensity PDF $P(\delta)$ in FDM-like cosmologies are systematically higher than in CDM, more so at high redshift $z\sim 5.5$ where the $m=10^{-22}$ eV model differs from CDM by $\sim 2 \sigma$. Accordingly, we advocate for the usage of $P(\delta)$ as a testbed for constraining FDM and other alternative dark matter models.

Structure formation and quasispherical collapse from initial curvature perturbations with numerical relativity simulations

Abstract: We use numerical relativity simulations to describe the spacetime evolution during nonlinear structure formation in $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ cosmology. Fully nonlinear initial conditions are set at an initial redshift $z\ensuremath{\approx}300$, based directly on the gauge invariant comoving curvature perturbation ${\mathcal{R}}_{c}$ commonly used to model early-universe fluctuations. Assigning a simple 3-D sinusoidal structure to ${\mathcal{R}}_{c}$, we then have a lattice of quasispherical overdensities representing idealized dark matter halos connected through filaments and surrounded by voids. This structure is implemented in the synchronous-comoving gauge, using a pressureless perfect fluid (dust) description of CDM, and then it is fully evolved with the einstein toolkit code. With this, we look into whether the top-hat spherical and homogeneous collapse model provides a good description of the collapse of overdensities. We find that the top-hat is an excellent approximation for the evolution of peaks, where we observe that the shear is negligible and collapse takes place when the linear density contrast reaches the predicted critical value ${\ensuremath{\delta}}_{C}^{(1)}=1.69$. Additionally, we characterize the outward expansion of the turn-around boundary and show how it depends on the initial distribution of matter, finding that it is faster in denser directions, incorporating more and more matter in the infalling region. Using the EBWeyl code we look at the distribution of the electric and magnetic parts of the Weyl tensor, finding that they are stronger along and around the filaments, respectively. We introduce a method to dynamically classify the different regions of the simulation box in Petrov types. With this, we find that the spacetime is of Petrov type I everywhere, as expected, but we can identify the leading order type in each region and at different times. Along the filaments, the leading Petrov type is D, while the center of the overdensities remains conformally flat, type O, in line with the top-hat model. The surrounding region demonstrates a sort of peeling-off in action, with the spacetime transitioning between different Petrov types as nonlinearity grows, with production of gravitational waves.

Early Structure Formation from Primordial Density Fluctuations with a Blue, Tilted Power Spectrum -- II. High-Redshift Galaxies

Abstract: The first series of observations by the James Webb Space Telescope (JWST) discovered unexpectedly abundant luminous galaxies at high redshift, posing possibly a serious challenge to popular galaxy formation models. We study early structure formation in a cosmological model with a blue, tilted power spectrum (BTPS) given by $P(k) \propto k^{m_{\rm s}}$ with $m_{\rm s} > 1$ at small length scales. We run a set of cosmological $N$-body simulations and derive the abundance of dark matter halos and of galaxies under simplified assumptions on star formation efficiency. The enhanced small-scale power allows rapid formation of nonlinear structure at $z>7$, and galaxies with stellar mass exceeding $10^{10}\,M_\odot$ can be formed by $z=9$. Because of frequent mergers, the structure of galaxies and galaxy groups appears overall clumpy. The BTPS model reproduces the observed stellar mass density at $z=7-9$, and thus eases the claimed tension between galaxy formation theory and recent JWST observations. Large-scale structure of the present-day Universe is largely unaffected by the modification of the small-scale power spectrum. Finally, we discuss the formation of the first stars and early super-massive black holes in the BTPS model.

Fully nonlinear gravitational instabilities for expanding Newtonian universes with inhomogeneous pressure and entropy: Beyond the Tolman’s solution

Abstract: Nonlinear gravitational instability is a crucial way to comprehend the clustering of matter and the formation of nonlinear structures in both the Universe and stellar systems. However, with the exception of a few exact particular solutions for pressureless matter, there are only some approximations and numerical and phenomenological approaches to study the nonlinear gravitational instability instead of mathematically rigorous analysis. We construct a family of particular solutions of the Euler-Poisson system that exhibits the nonlinear gravitational instability of matter with inhomogeneous pressure and entropy (i.e., the cold center and hot rim) in the expanding Newtonian universe. Despite the density perturbations being homogeneous, the pressure is not, resulting in significant nonlinear effects. By making use of our prior work on nonlinear analysis of a class of differential equations, we estimate that the growth rate of the density contrast is approximately $\ensuremath{\sim}\mathrm{exp}({t}^{\frac{2}{3}})$, much faster than the growth rate anticipated by classical linear Jeans instability ($\ensuremath{\sim}{t}^{\frac{2}{3}}$). Our main motivation for constructing this family of solutions is to provide a family of reference solutions for conducting a fully nonlinear analysis of inhomogeneous perturbations of density contrast. We will present the general results in a mathematical article separately. Additionally, we emphasize that our model does not feature any shell-crossing singularities before mass accretion singularities since we are specifically interested in analyzing the mathematical mechanics of a pure mass accretion model, which poses limitations on the applicability of our model for understanding the realistic nonlinear structure formation.