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We study structure formation in K-mouflage cosmology whose main feature is the absence of screening effect on quasilinear scales. We show that the growth of structure at the linear level is affected by both a new time dependent Newton constant and a friction term which depend on the background evolution. These combine with the modified background evolution to change the growth rate by up to ten percent since z ~ 2. At the one loop level, we find that the nonlinearities of the K-mouflage models are mostly due to the matter dynamics and that the scalar perturbations can be treated at tree level. We also study the spherical collapse in K-mouflage models and show that the critical density contrast deviates from its A-CDM value and that, as a result, the halo mass function is modified for large masses by an order one factor. Finally we consider the deviation of the matter spectrum from A-CDM on nonlinear scales where a halo model is utilized. We find that the discrepancy peaks around 1 h Mpc-1 with a relative difference which can reach fifty percent. Importantly, these features are still true at larger redshifts, contrary to models of the chameleon-f(R) and Galileon types. [ABSTRACT FROM AUTHOR]

We describe how the kinematic consistency relations satisfied by density correlations of the large-scale structures of the Universe can be derived within the usual Newtonian framework. These relations express a kinematic effect and show how the (ℓ + n)-density correlation factors in terms of the n-point correlation and ℓ linear power spectrum factors, in the limit where the ℓ soft wave numbers become linear and much smaller than the n other wave numbers. We describe how these relations extend to multifluid cases. In the standard cosmology, these consistency relations derive from the equivalence principle. A detection of their violation would indicate non-Gaussian initial conditions, non-negligible decaying modes, or a modification of gravity that does not converge to general relativity on large scales. [ABSTRACT FROM AUTHOR]

We investigate the possible accuracy that can be reached by analytical models for the matter density power spectrum and correlation function. Using a realistic description of the power spectrum that combines perturbation theory with a halo model, we study the convergence rate of several perturbative expansion schemes and the impact of nonperturbative effects, as well as the sensitivity to phenomenological halo parameters. We check that the simple reorganization of the standard perturbative expansion, with a Gaussian damping prefactor, provides a well-ordered convergence and a finite correlation function that yields a percent accuracy at the baryon acoustic oscillation peak (as soon as one goes to second order). Lagrangian-space expansions are somewhat more efficient, when truncated at low orders, but may diverge at high orders. We find that whereas the uncertainty on the halo-profile mass-concentration relation is not a strong limitation, the uncertainty on the halo mass function can severely limit the accuracy of theoretical predictions for P(k) (this also applies to the power spectra measured in numerical simulations). The real-space correlation function provides a better separation between perturbative and nonperturbative effects, which are restricted to x≲10h-1Mpc at all redshifts. [ABSTRACT FROM AUTHOR]

We study the effects of screened modified gravity of the f(R), dilaton and symmetron types on structure formation, from the quasilinear to the nonlinear regime, using semianalytical methods. For such models, where the range of the new scalar field is typically within the Mpc range and below in the cosmological context, nonlinear techniques are required to understand the deviations of the power spectrum of the matter density contrast compared to the ΛCDM template. This is nowadays commonly tackled using extensive N-body simulations. Here we present new results combining exact perturbation theory at the one-loop level (and a partial resummation of the perturbative series) with a halo model. The former allows one to extend the linear perturbative analysis up to k ≲ 0.15h Mpc-1 at the perturbative level while the latter leads to a reasonable, up to a few percent, agreement with numerical simulations for k ≲ 3h Mpc-1 for large-curvature f(R) models, and k ≲ 1h Mpc-1 for dilatons and symmetrons, at z = 0. We also discuss how the behaviors of the perturbative expansions and of the spherical collapse differ for f(R), dilaton, and symmetron models. [ABSTRACT FROM AUTHOR]

We investigate whether the late-time (at z =100) velocity dispersion expected in warm dark matter scenarios could have some effect on the cosmic web (i.e., outside of virialized halos). We consider effective hydrodynamical equations, with a pressurelike term that agrees at the linear level with the analysis of the Vlasov equation. Then, using analytical methods, based on perturbative expansions and the spherical dynamics, we investigate the impact of this term for a 1 keV dark matter particle. We find that the late-time velocity dispersion has a negligible effect on the power spectrum on perturbative scales and on the halo mass function. However, it has a significant impact on the probability distribution function of the density contrast at z ~ 3 on scales smaller than 0.1h-1 Mpc, which correspond to Lyman-ɑ clouds. Finally, we note that numerical simulations should start at zi = 100 rather than zi = 50 to avoid underestimating gravitational clustering at low redshifts. [ABSTRACT FROM AUTHOR]

We propose a new mechanism for the formation of dark matter clumps in the radiation era. We assume that a light scalar field is decoupled from matter and oscillates harmonically around its vacuum expectation value. We include self-interactions and consider the nonrelativistic regime. The scalar dynamics are described by a fluid approach where the fluid pressure depends on both quantum and self-interaction effects. When the squared speed of sound of the scalar fluid becomes negative, an instability arises and the fluctuations of the scalar energy-density field start growing. They eventually become nonlinear and clumps form. Subsequently, the clumps aggregate and reach a universal regime. Afterwards, they play the role of cold dark matter. We apply this mechanism first to a model with a negative quartic term stabilized by a positive self-interaction of order six, and then to axion monodromy, where a subdominant cosine potential corrects a mass term. In the first case, the squared speed of sound becomes negative when the quartic term dominates, leading to a tachyonic instability. For axion monodromy, the instability starts very slowly after the squared speed of sound first becomes negative and then oscillates around zero. Initially the density perturbations perform acoustic oscillations due to the quantum pressure. Eventually, they start growing exponentially due to a parametric resonance. The shape and the scaling laws of the clumps depend on their formation mechanism. When the tachyonic phase takes place, the core density of the clumps is uniquely determined by the energy density at the beginning of the instability. On the other hand, for axion monodromy, the core density scales with the soliton mass and radius. This difference comes from the crucial role that the quantum pressure plays in both the parametric resonance in the linear regime and in the nonlinear formation regime of static scalar solitons. In both scenarios, the scalar-field clumps span a wide range of scales and masses, running from the size of atoms to that of galactic molecular clouds, and from 10-3 gram to thousands of solar masses. Because of finite-size effects, both from the source and the lens, these dark matter clumps are far beyond the reach of microlensing observations. We find that the formation redshift of the scalar clumps can span a large range in the radiation era; the associated background temperature can vary from 10 eV to 105 GeV, and the scalar-field mass from 10-26 GeV to 10 GeV. [ABSTRACT FROM AUTHOR]

We consider scalar dark-matter models where the theory has a shift symmetry only broken by the scalar mass term. We restrict ourselves to k-essence kinetic terms where the shift symmetric part of the Lagrangian is a function of the first derivatives of the scalar field only. When the scalar mass is much larger than the inverse of the astrophysical time and length scales of interest, these models provide a description of dark-matter equivalent to the one given by theories with only polynomial interactions, in the low-amplitude regime where the self-interactions are small contributions to the Lagrangian. In this regime and in the nonrelativistic limit, which apply on large galactic scales, scalar clouds form solitons with a finite core. This provides an adequate model for dark-matter halos with no singular behavior. Close to the center of galaxies, where a supermassive black hole (BH) resides, we analyze the scalar-field distribution and the fate of the dark-matter soliton when subject to the BH gravitational attraction. We show that the scalar-field profile around such a central BH can be described by new oscillatory solutions of a modified Klein-Gordon equation, which generalize the harmonic oscillations of free scalar dark matter in a flat environment and the Jacobi elliptic functions of the ϕ4 model. Moreover, we find that, depending on the form of the k-essence kinetic term, regular solutions can be constructed or not, which connect the relativistic ingoing wavelike profile of the scalar field at the BH horizon to the nearly static nonrelativistic soliton at large distance. These profiles have a constant flux and represent the slow infall of scalar matter into the BH. We show that this regular behavior is only possible for k-essence functions that satisfy the usual conditions for the absence of ghosts and gradient instabilities, together with a new restriction on the growth of the kinetic function K(X) for large argument. It turns out that the same conditions of stability guarantee that quantum corrections are tamed, provided that the mass of the scalar field is less than 10-3 eV and the strong coupling scale of the model Λ is much larger than the scalar mass. [ABSTRACT FROM AUTHOR]

In scalar-field dark matter scenarios, a scalar-field soliton could form at the center of galactic halos, around the supermassive black holes that sit at the center of galaxies. Focusing on the large scalar-mass limit, where the soliton is formed by the balance between self-gravity and a repulsive self-interaction, we study the infall of the scalar field onto the central Schwarzschild black hole. We derive the scalar-field profile, from the Schwarzschild radius to the large radii dominated by the scalar cloud. We show that the steady state solution selects the maximum allowed flux, with a critical profile that is similar to the transonic solution obtained for the hydrodynamic case. This finite flux, which scales as the inverse of the self-interaction coupling, is small enough to allow the dark matter soliton to survive for many Hubble times. [ABSTRACT FROM AUTHOR]

We consider models of scalar dark matter with a generic interaction potential and noncanonical kinetic terms of the K-essence type that are subleading with respect to the canonical term. We analyze the low-energy regime and derive, in the nonrelativistic limit, the effective equations of motions. In the fluid approximation they reduce to the conservation of matter and to the Euler equation for the velocity field. We focus on the case where the scalar field mass 10-21≪m≲10-4 eV is much larger than for fuzzy dark matter, so that the quantum pressure is negligible on cosmological and galactic scales, while the self-interaction potential and noncanonical kinetic terms generate a significant repulsive pressure. At the level of cosmological perturbations, this provides a dark-matter density-dependent speed of sound. At the nonlinear level, the hydrostatic equilibrium obtained by balancing the gravitational and scalar interactions imply that virialized structures have a solitonic core of finite size depending on the speed of sound of the dark matter fluid. For the most relevant potential in λ4ϕ4/4 or K-essence with a (∂ϕ)4 interaction, the size of such stable cores cannot exceed 60 kpc. Structures with a density contrast larger than 106 can be accommodated with a speed of sound cs≲10-6. We also consider the case of a cosine self-interaction, as an example of bounded nonpolynomial self-interaction. This gives similar results in low-mass and low-density halos whereas solitonic cores are shown to be absent in massive halos. [ABSTRACT FROM AUTHOR]

We present a class of modified-gravity theories which we call ultralocal models. We add a scalar field, with negligible kinetic terms, to the Einstein-Hilbert action. We also introduce a conformal coupling to matter. This gives rise to a new screening mechanism which is not entirely due to the nonlinearity of the scalar-field potential or the coupling function but to the absence of the kinetic term. As a result this removes any fifth force between isolated objects in vacuum. It turns out that these models are similar to chameleon-type theories with a large mass when considered outside the Compton wavelength but differ on shorter scales. The predictions of these models only depend on a single free function, as the potential and the coupling function are degenerate, with an amplitude given by a parameter α≲10-6, whose magnitude springs from requiring a small modification of Newton's potential astrophysically and cosmologically. This singles out a redshift zα~α-1/3≳100 where the fifth force is the greatest. The cosmological background follows the Λ cold dark matter (ΛCDM) history within a 10-6 accuracy, while cosmological perturbations are significantly enhanced (or damped) on small scales, k≳2hMpc-1 at z=0. The spherical collapse and the halo mass function are modified in the same manner. We find that the modifications of gravity are greater for galactic or subgalactic structures. We also present a thermodynamic analysis of the nonlinear and inhomogeneous fifth-force regime where we find that the Universe is not made more inhomogeneous before zα when the fifth force dominates, and does not lead to the existence of clumped matter on extra small scales inside halos for large masses while this possibility exists for masses M≲1011M⊙ where the phenomenology of ultralocal models would be most different from ΛCDM. [ABSTRACT FROM AUTHOR]