Long-range Scalar Interaction Research Articles

Cold dark matter halos in Multi-coupled Dark Energy cosmologies: Structural and statistical properties

Abstract The recently proposed Multi-coupled Dark Energy (McDE) scenario – characterised by two distinct cold dark matter (CDM) particle species with opposite couplings to a Dark Energy scalar field – introduces a number of novel features in the small-scale dynamics of cosmic structures, most noticeably the simultaneous existence of both attractive and repulsive fifth-forces. Such small-scale features are expected to imprint possibly observable footprints on nonlinear cosmic structures, that might provide a direct way to test the scenario. In order to unveil such footprints, we have performed the first suite of high-resolution N-body simulations of McDE cosmologies, covering the coupling range | β | ≤ 1. We find that for coupling values corresponding to fifth-forces weaker than standard gravity, the impact on structure formation is very mild, thereby showing a new type of screening mechanism for long-range scalar interactions. On the contrary, for fifth-forces comparable to or stronger than standard gravity a number of effects appear in the statistical and structural properties of CDM halos. Collapsed structures start to fragment into pairs of smaller objects that move on different trajectories, providing a direct evidence of the violation of the weak equivalence principle. Consequently, the relative abundance of halos of different masses is significantly modified. For sufficiently large coupling values, the expected number of clusters is strongly suppressed, which might alleviate the present tension between CMB- and cluster-based cosmological constraints. Finally, the internal structure of halos is also modified, with a significant suppression of the inner overdensity, and a progressive segregation of the two CDM species.

DM haloes in the fifth-force cosmology

We investigate how long-range scalar interactions affect the properties of dark matter haloes.For doing so we employ the ReBEL model which implements an additional interaction between dark matter particles.On the phenomenological level this is equivalent to a modification of gravity. We analyse the differences betweenfive ReBEL models and ΛCDM using a series of high resolution cosmological simulations. Emphasis is placed oninvestigating how halo properties change in the presence of a fifth force. We report that the density profileof ReBEL haloes is well described by the NFW profile but with mean concentrations from 5%to a few times higher than the standard ΛCDM value. We also find a slight increase of the halo spin for haloes more massivethan 5 × 1011M☉, reflecting a higher rotational support of those haloes due to scalar forces.In addition, the dark matter haloes in our models are more spherical than their counterparts in ΛCDM.The ReBEL haloes are also more virialised, with a large difference from ΛCDM for strong fifth forces and a much smaller changefor weak scalar interactions.

Structure formation in multiple dark matter cosmologies with long-range scalar interactions

(Abridged) An interaction between Cold Dark Matter (CDM) and a classical scalar field playing the role of the cosmic dark energy (DE) might provide long-range dark interactions without conflicting with solar system bounds. Although presently available observations allow to constrain such interactions to a few percent of the gravitational strength, some recent studies have shown that if CDM is composed by two different particle species having opposite couplings to the DE field, such tight constraints can be considerably relaxed, allowing for long-range scalar forces of order gravity without significantly affecting observations both at the background and at the linear perturbations level. In the present work, we extend the investigation of such Multiple Dark Matter scenarios to the nonlinear regime of structure formation, by presenting the first N-body simulations ever performed for these cosmologies. Our results highlight some characteristic footprints of long-range scalar forces that arise only in the nonlinear regime for specific models that would be otherwise practically indistinguishable from the standard LCDM scenario both in the background and in the growth of linear density perturbations. Among these effects, the formation of "mirror" cosmic structures in the two CDM species, the suppression of the nonlinear matter power spectrum at k > 1 h/Mpc, and the fragmentation of collapsed halos, represent peculiar features that might provide a direct way to constrain this class of cosmological models.

Boosting hierarchical structure formation with scalar-interacting dark matter

Abstract We investigate the effect of long-range scalar interactions in dark matter (DM) models of cosmic structure formation with a particular focus on the formation times of haloes. Utilizing N-body simulations with 5123 DM particles we show that in our models DM haloes form substantially earlier: tracing objects up to redshift z ∼ 6 we find that the formation time, as characterized by the redshift z1/2 at which the halo has assembled half of its final mass, is gradually shifted from z1/2 ≈ 1.83 in the fiducial Λ cold dark matter (ΛCDM) model to z1/2 ≈ 2.54 in the most extreme scalar-interaction model. This is accompanied by a shift of the redshift that marks the transition between merger and steady accretion epochs from z* ≈ 4.32 in the ΛCDM haloes to z* ≈ 6.39 in our strongest interaction model. In other words, the scalar-interacting model employed in this work produces more structures at high redshifts, prolonging at the same time the steady accretion phases. These effects taken together can help the ΛCDM model to account for a high-redshift reionization as indicated by the Wilkinson Microwave Anisotropy Probe (WMAP) data and can alleviate issues related to the survival of the thin-disc-dominated galaxies at low redshifts.

Dark matter gravitational clustering with a long-range scalar interaction

We explore the possibility of improving the $\ensuremath{\Lambda}\mathrm{CDM}$ model at megaparsec scales by introducing a scalar interaction that increases the mutual gravitational attraction of dark matter particles. Using N-body simulations, we study the spatial distribution of dark matter particles and halos. We measure the effect of modifications in the Newton's gravity on properties of the two-point correlation function, the dark matter power spectrum, the cumulative halo mass function, and density probability distribution functions. The results look promising: the scalar interactions produce desirable features at megaparsec scales without spoiling the $\ensuremath{\Lambda}\mathrm{CDM}$ successes at larger scales.

Galaxy satellites and the weak equivalence principle

Numerical simulations of the effect of a long-range scalar interaction (LRSI) acting only on nonbaryonic dark matter, with strength comparable to gravity, show patterns of disruption of satellites that can agree with what is seen in the Milky Way. This includes the symmetric Sagittarius (Sgr) stellar stream. The exception presented here to the Kesden and Kamionkowski demonstration that an LRSI tends to produce distinctly asymmetric streams follows if the LRSI is strong enough to separate the stars from the dark matter before tidal disruption of the stellar component, and if stars dominate the mass in the luminous part of the satellite. It requires that the Sgr galaxy now contains little dark matter, which may be consistent with the Sgr stellar velocity dispersion, for in the simulation the dispersion at pericenter exceeds virial. We present other examples of simulations in which a strong LRSI produces satellites with large mass-to-light ratio, as in Draco, or free streams of stars, which might be compared to ``orphan'' streams.

Structure formation with a long-range scalar dark matter interaction

Numerical simulations show that a long-range scalar interaction in a single species of massive dark matter particles causes voids between the concentrations of large galaxies to be more nearly empty, suppresses accretion of intergalactic matter onto galaxies at low redshift, and produces an early generation of dense dark matter halos. These three effects, in moderation, seem to be improvements over the $\ensuremath{\Lambda}$CDM model predictions for cosmic structure formation. Because the scalar interaction in this model has negligible effect on laboratory physics and the classical cosmological tests, it offers an observationally attractive example of cosmology with complicated physics in the dark sector, notably a large violation of the weak equivalence principle.

Comments on models of scalar-tensorial field equations in general relativity

In this paper we consider some of the proposed models for introducing the long-range scalar interaction in Riemannian space-times. The relationship among these models is discussed. Particular emphasis is placed on the introduction of the scalar interaction via a conformal mapping on the original Riemannian geometry. Following this method we introduce a spinorial model for the coupled system. We also discuss the meaning of the identities satisfied by the left-hand side of the coupled field equations, which are present for any model derivable from an action principle.