We propose a phenomenological model for the scattering of particles on space-time defects in a treatment that maintains Lorentz invariance on the average. The local defects considered here cause a stochastic violation of momentum conservation. The scattering probability is parametrized in the density of defects and the distribution of the momentum that a particle can obtain when scattering on the defect. We identify the most promising observable consequences and derive constraints from existing data.

We study the general relativistic periastron advance in spinning black hole binaries on quasicircular orbits, with spins aligned or antialigned with the orbital angular momentum, using numerical-relativity simulations, the post-Newtonian approximation, and black hole perturbation theory. By imposing a symmetry by exchange of the bodies’ labels, we devise an improved version of the perturbative result and use it as the leading term of a new type of expansion in powers of the symmetric mass ratio. This allows us to measure, for the first time, the gravitational self-force effect on the periastron advance of a nonspinning particle orbiting a Kerr black hole of mass $M$ and spin $S=-0.5M^2$, down to separations of order $9M$. Comparing the predictions of our improved perturbative expansion with the exact results from numerical simulations of equal-mass and equal-spin binaries, we find a remarkable agreement over a wide range of spins and orbital separations.

It is a common misconception that spacetime discreteness necessarily implies a violation of local Lorentz invariance. In fact, in the causal set approach to quantum gravity, Lorentz invariance follows from the specific implementation of the discreteness hypothesis. However, this comes at the cost of locality. In particular, it is difficult to define a “local” region in a manifoldlike causal set, i.e., one that corresponds to an approximately flat spacetime region. Following up on suggestions from previous work, we bridge this lacuna by proposing a definition of locality based on the abundance of $m$-element order-intervals as a function of $m$ in a causal set. We obtain analytic expressions for the expectation value of this function for an ensemble of causal set that faithfully embeds into an Alexandrov interval in $d$-dimensional Minkowski spacetime and use it to define local regions in a manifoldlike causal set. We use this to argue that evidence of local regions is a necessary condition for manifoldlikeness in a causal set. This in addition provides a new continuum dimension estimator. We perform extensive simulations which support our claims.

Starting from a classical-mechanics stochastic model encoded in a Langevin equation, we derive the natural diffusion equation associated with three classes of multiscale spacetimes (with weighted, ordinary, and “$q$-Poincaré” symmetries). As a consistency check, the same result is obtained by inspecting the propagation of a quantum-mechanical particle in a disordered environment. The solution of the diffusion equation displays a time-dependent diffusion coefficient and represents a probabilistic process, classified according to the statistics of the noise in the Langevin equation. We thus illustrate, also with pictorial aids, how spacetime geometries can be more completely catalogued not only through their Hausdorff and spectral dimension, but also by a stochastic process. The spectral dimension of multifractional spacetimes is then computed and compared with what was found in previous studies, where a diffusion equation with some open issues was assumed rather than derived. These issues are here discussed and solved, and they point towards the model with $q$-Poincaré symmetries.

In this work we take moves from the debate triggered by Melia et al. in [J. Cosmol. Astropart. Phys. 09 (2012) 029; Mon. Not. R. Astron. Soc. 421, 3356 (2012)] and followed by opposite comments by Lewis and Oirschot in [Mon. Not. R. Astron. Soc. Lett. 423, 26 (2012); 431, 25 (2013)]. The point in question regards the role of the Hubble horizon as a limit for observability in a cosmological setting. We propose to tackle the issue in a broader way by relating it to the causal character of the Hubble surface and to the tracing of null trajectories, focusing on both three-fluids and generalized Chaplygin gas models. The results should make clear that for quite reasonable and physically motivated models, light rays reaching a comoving observer at $R(t_0)=0$ have never traveled a distance greater than the proper radius of the horizon until $t_0$.

We study the stability of thin-shell wormholes in Einstein–Maxwell–Gauss-Bonnet gravity. The equation of state of the thin-shell wormhole is considered first to obey a generalized Chaplygin gas, and then we generalize it to an arbitrary state function that covers all known cases studied so far. In particular, we study the modified Chaplygin gas and give an assessment for a general parotropic fluid. Our study is in $d$ dimensions, and with numerical analysis in $d=5$, we show the effect of the Gauss-Bonnet parameter in the stability of thin-shell wormholes against the radial perturbations.

In this paper we argue the possibility that fermion masses, in particular quarks, originate through the condensation of a fourth family that interacts with all of the quarks via a contact four-fermion term coming from the existence of torsion on the spacetime. Extra dimensions are considered to avoid the hierarchy problem.

The divergence structure of spin foam models and its relation to diffeomorphism symmetry has attracted renewed interest. We will discuss in detail the (nonoccurrence of) divergencies in the Barrett-Crane spin foam model, which with our choice of weights can be understood as an integral of delta functions only. We will present furthermore a simple method to estimate the occurrence of so-called bubble divergencies for general spin foam models. We expect divergencies in spin foams related to the existence of (diffeomorphism) gauge symmetries. Thus we have to conclude that such gauge symmetries are not (fully) present in the model we consider. But we will identify a class of gauge symmetries that occur at special solutions of equations imposed by the delta function weights. This situation is surprisingly similar to the case of broken diffeomorphism symmetries in discrete gravity, which are present around flat solutions. We introduce a method to derive (Ward identity-like) equations for the vertex amplitudes of the model in the case of broken gauge symmetries.

We study Galileon theories that emerge in ghost-free massive gravity. In particular, we focus on a subclass of these theories where the Galileons can be completely decoupled from the tensor Lagrangian. These Galileons differ from generic ones—they have interrelated coefficients of the cubic and quartic terms, and most importantly, a nonstandard coupling to external stress-tensors, governed by the same coefficient. We show that this theory has no static stable spherically symmetric solutions that would interpolate from the Vainshtein region to flat space; these two regions cannot be smoothly matched for the sign of the coefficient for which fluctuations are stable. Instead, for this sign choice, a solution in the Vainshtein domain is matched onto a cosmological background. Small fluctuations above this solution are stable, and sub-luminal. We discuss observational constraints on this theory, within the quantum effective Lagrangian approach, and argue that having a graviton mass of the order of the present-day Hubble parameter, is consistent with the data. Last but not least, we also present a general class of cosmological solutions in this theory, some of which exhibit the demixing phenomenon, previously found for the self-accelerated solution.

We explore the shadow of a certain class of rotating traversable wormholes within classical general relativity. The images depend on the angular momentum of the wormhole and the inclination angle of the observer. We compare the results with the case of the Kerr black hole. For small angular momenta, the shadows for the two solutions are nearly identical; however, with the increasing of the angular momentum, they start to deviate considerably.

We obtain an explicit two-point function for the Maxwell field in flat Robertson-Walker spacetimes, thanks to a new gauge condition which takes the scale factor into account and assumes a simple form. The two-point function is found to have the short distance Hadamard behavior.

We analyze the behavior of axion dark matter before it falls into a galactic gravitational potential well. The axions thermalize sufficiently fast by gravitational self-interactions that almost all go to their lowest-energy state consistent with the total angular momentum acquired from tidal torquing. That state is a state of rigid rotation on the turnaround sphere. It predicts the occurrence and detailed properties of the caustic rings of dark matter for which observational evidence had been found earlier. We show that the vortices in the axion Bose-Einstein condensate (BEC) are attractive, unlike those in superfluid ${}^4\mathrm{He}$ and dilute gases. We expect that a large fraction of the vortices in the axion BEC join into a single big vortex along the rotation axis of the galaxy. The resulting enhancement of caustic rings explains the typical size of the rises in the Milky Way rotation curve attributed to caustic rings. We show that baryons and ordinary cold dark matter particles are entrained by the axion BEC and acquire the same velocity distribution. The resulting baryonic angular momentum distribution gives a good qualitative fit to the distributions observed in dwarf galaxies. We give estimates of the minimum fraction of dark matter that is composed of axions.

We show that anthropic selection emerges inevitably in the general framework for prediction in quantum cosmology. There the predictions of anthropic reasoning depend on the prior implied by the universe’s quantum state. To illustrate this we compute the probabilities specified by the no-boundary wave function for our observations at the present time of the values of $\Lambda$ and $Q$ in an inflationary landscape model in which both quantities vary. Within the anthropic range of values, the no-boundary state yields an approximately flat distribution on $\Lambda$ and strongly favors small values of $Q$. This restores Weinberg’s successful prediction of $\Lambda$.

We present an analysis of high-resolution $N$-body simulations of decaying dark matter cosmologies focusing on the statistical properties of the transmitted Lyman-$\alpha$ ($\mathrm{Ly}\alpha$) forest flux in the high-redshift intergalactic medium (IGM). In this type of model a dark matter particle decays into a slightly less massive stable dark matter daughter particle and a comparably light particle. The small mass splitting provides a nonrelativistic kick velocity $V_k=c\Delta M/M$ to the daughter particle resulting in free-streaming and subsequent damping of small-scale density fluctuations. Current $\mathrm{Ly}\alpha$ forest power spectrum measurements probe comoving scales up to $\sim 2–3h^{-1}\mathrm{Mpc}$ at redshifts $z\sim 2–4$, providing one of the most robust ways to probe cosmological density fluctuations on relatively small scales. The suppression of structure growth due to the free-streaming of dark matter daughter particles also has a significant impact on the neutral hydrogen cloud distribution, which traces the underlying dark matter distribution well at high redshift. We exploit $\mathrm{Ly}\alpha$ forest power spectrum measurements to constrain the amount of free-streaming of dark matter in such models and thereby place limits on decaying dark matter based only on the dynamics of cosmological perturbations without any assumptions about the interactions of the decay products. We use a suite of dark-matter-only simulations together with the fluctuating Gunn-Peterson approximation to derive the $\mathrm{Ly}\alpha$ flux distribution. We argue that this approach should be sufficient for our main purpose, which is to demonstrate the power of the $\mathrm{Ly}\alpha$ forest to constrain decaying dark matter models. We find that Sloan Digital Sky Survey 1D $\mathrm{Ly}\alpha$ forest power spectrum data place a lifetime-dependent upper limit $V_k\lesssim 30–70\mathrm{km}/\mathrm{s}$ for decay lifetimes $\lesssim 10\mathrm{Gyr}$. This is the most stringent model-independent bound on invisible dark matter decays with small mass splittings. For larger mass splittings (large $V_k$), $\mathrm{Ly}\alpha$ forest data restrict the dark matter lifetime to ${\Gamma }^{-1}\gtrsim 40\mathrm{Gyr}$. We leave the calibration of IGM properties using high-resolution hydrodynamic simulations for future work, which might become necessary if we consider data with higher precision such as the Baryon Oscillation and Spectroscopic Survey (BOSS) $\mathrm{Ly}\alpha$ data. Forthcoming BOSS data should be able to provide more stringent constraints on exotic dark matter, mainly because the larger BOSS quasar spectrum sample will significantly reduce statistical errors.

We constrain $f(R)$ and chameleon-type modified gravity in the framework of the Berstchinger-Zukin parametrization using the recently released Planck data, including both the cosmic mircowave background radiation (CMB) temperature power spectrum and the lensing potential power spectrum. Some other external data sets are included, such as baryon acoustic oscillation (BAO) measurements from the 6dFGS, SDSS DR7 and BOSS DR9 surveys; Hubble Space Telescope (HST) $H_0$ measurements, and supernovae from the Union2.1 compilation. We also use WMAP9 data for a consistency check and comparison. For $f(R)$ gravity, WMAP9 results can only give a quite loose constraint on the modified gravity parameter $B_0$, which is related to the present value of the Compton wavelength of the extra scalar degree of freedom, $B_0<3.37$ at 95% C.L. We demonstrate that this constraint mainly comes from the late integrated Sachs-Wolfe effect. With only Planck CMB temperature power-spectrum data, we can improve the WMAP9 result by a factor 3.7 ($B_0<0.91$ at 95% C.L.). If the Planck lensing potential power-spectrum data are also taken into account, the constraint can be further strengthened by a factor 5.1 ($B_0<0.18$ at 95% C.L.). This major improvement mainly comes from the small-scale lensing signal. Furthermore, BAO, HST and supernovae data could slightly improve the $B_0$ bound ($B_0<0.12$ at 95% C.L.). For the chameleon-type model, we find that the data set that we used cannot constrain the Compton wavelength $B_0$ or the potential index $s$ of the chameleon field, but it can give a tight constraint on the parameter ${\beta }_1={1.043}_{-0.104}^{+0.163}$ at 95% C.L. (${\beta }_1=1$ in general relativity), which accounts for the nonminimal coupling between the chameleon field and the matter component. In addition, we find that both modified gravity models we consider favor a relatively higher Hubble parameter than the concordance $\Lambda \mathrm{CDM}$ model in general relativity.

We generalize the time-variable dark energy scalar field $\Phi$ model ($\Phi \mathrm{CDM}$) to nonflat space. We show that even in the space-curvature-dominated epoch the scalar field solution is a time-dependent fixed point or attractor, with scalar field energy density that grows relative to the energy density in spatial curvature. This is the first example of a physically consistent and complete model of dynamical dark energy in a nonflat geometry.

All-sky, broadband, coherent searches for gravitational-wave pulsars are computationally limited. It is therefore important to make efficient use of available computational resources, notably by minimizing the number of templates used to cover the signal parameter space of sky position and frequency evolution. For searches over the sky, however, the required template density (determined by the parameter-space metric) is different at each sky position, which makes it difficult in practice to achieve an efficient covering. Previous work on this problem has found various choices of sky and frequency coordinates that render the parameter-space metric approximately constant but that are limited to coherent integration times of either less than a few days or greater than several months. These limitations restrict the sensitivity achievable by hierarchical all-sky searches and hinder the development of follow-up pipelines for interesting gravitational-wave pulsar candidates. We present a new flat parameter-space metric approximation and associated sky and frequency coordinates, which do not suffer from these limitations. Furthermore, the new metric is numerically well conditioned, which facilitates its practical use.

We revise the conditions for the physical viability of a cosmological model in which dark matter has bulk viscosity and also interacts with dark energy. We have also included radiation and baryonic matter components; all matter components are represented by perfect fluids, except for the dark matter one that is modeled as an imperfect fluid. We impose upon the model the condition of a complete cosmological dynamics that results in an either null or negative bulk viscosity, but the latter also disagrees with the local second law of thermodynamics (LSLT). The model is also compared with cosmological observations at different redshifts: type Ia supernova, the acoustic peak of baryon acoustic oscillation, the Hubble parameter $H(z)$, and the angular scale of the cosmic microwave background encoded in the first peak. Taken together, observations consistently point to a negative value of the bulk viscous coefficient, that is in disagreement with the LSLT. From the different cases that we study, the best model that we find corresponds to the case of a dark matter with a null viscosity, interacting with a phantom dark energy. Also, overall the fitting procedure shows no preference for the model over the standard $\Lambda \mathrm{CDM}$ model.

We investigate scenarios in which dark matter interacts with the Standard Model primarily through electroweak gauge bosons. We employ an effective field theory framework wherein the Standard Model and the dark matter particle are the only light states in order to derive model-independent bounds. Bounds on such interactions are derived from dark matter production by weak boson fusion at the LHC, indirect detection searches for the products of dark matter annihilation and from the measured invisible width of the $Z^0$. We find that limits on the UV scale, $\Lambda$, reach weak scale values for most operators and values of the dark matter mass, thus probing the most natural scenarios in the weakly interacting massive particle dark matter paradigm. Our bounds suggest that light dark matter ($m_{\chi }\lesssim m_Z/2$ or $m_{\chi }\lesssim 100–200\mathrm{GeV}$, depending on the operator) cannot interact only with the electroweak gauge bosons of the Standard Model, but rather requires additional operator contributions or dark sector structure to avoid overclosing the Universe.

Adding right-handed neutrino singlets and/or fermion triplets to the particle content of the standard model allows for the implementation of the seesaw mechanism to give mass to neutrinos and, simultaneously, for the construction of anomaly-free gauge group extensions of the theory. We consider Abelian extensions based on an extra $U(1{)}_X$ gauge symmetry, where $X$ is an arbitrary linear combination of the baryon number $B$ and the individual lepton numbers $L_{e,\mu ,\tau }$. By requiring cancellation of gauge anomalies, we perform a detailed analysis in order to identify the charge assignments under the new gauge symmetry that lead to neutrino phenomenology compatible with current experiments. In particular, we study how the new symmetry can constrain the flavor structure of the Majorana neutrino mass matrix, leading to two-zero textures with a minimal extra fermion and scalar content. The possibility of distinguishing different gauge symmetries and seesaw realizations at colliders is also briefly discussed.