Loop quantum cosmology (LQC) seems to be predicting modified effective Friedmann equations without extra degrees of freedom. A puzzle arises if one decides to seek for a covariant effective action which would lead to the given Friedmann equation: The Einstein-Hilbert action is the only action that leads to second order field equations and, hence, there exists no covariant action which, under metric variation, leads to a modified Friedmann equation without extra degrees of freedom. It is shown that, at least for isotropic models in LQC, this issue is naturally resolved and a covariant effective action can be found if one considers higher order theories of gravity but faithfully follows effective field theory techniques. However, our analysis also raises doubts on whether a covariant description without background structures can be found for anisotropic models.

Motivated by the cosmological wormhole solutions obtained from our recent numerical investigations, we provide a definition of a wormhole which applies to dynamical situations. Our numerical solutions do not have timelike trapping horizons but they are wormholes in the sense that they connect two or more asymptotic regions. Although the null energy condition must be violated for static wormholes, we find that it can still be satisfied in the dynamical context. Two analytic solutions for a cosmological wormhole connecting two Friedmann universes without trapping horizons are presented.

Major observational efforts in the coming decade are designed to probe the equation of state of dark energy. Measuring a deviation of the equation-of-state parameter $w$ from $-1$ would indicate a dark energy that cannot be represented solely by a cosmological constant. While it is commonly assumed that any implied modification to the $\Lambda \mathrm{CDM}$ model amounts to the addition of new dynamical fields, we propose here a framework for investigating whether or not such new fields are required when cosmological observations are combined with a set of minimal assumptions about the nature of gravitational physics. In our approach, we treat the additional degrees of freedom as perturbatively constrained and calculate a number of observable quantities, such as the Hubble expansion rate and the cosmic acceleration, for a homogeneous universe. We show that current observations place our universe within the perturbative validity of our framework and allow for the presence of nondynamical gravitational degrees of freedom at cosmological scales.

We derive an expression for the luminosity distance as a function of redshift for a flat Robertson-Walker spacetime perturbed by arbitrary scalar perturbations possibly produced by a modified gravity theory with two different scalar perturbation potentials. Measurements of the luminosity distance as function of redshift provide a constraint on a combination of the scalar potentials and so they can complement weak lensing and other measurements in trying to distinguish among the various alternative theories of gravity.

We discuss the synchrotron radiation flux from the Galactic center in the unstable dark matter scenario. Motivated by the anomalous excess of the positron fraction recently reported by the PAMELA Collaboration, we consider the case that the dark matter particle is unstable (and long-lived) and that energetic electrons and positrons are produced by the decay of dark matter. Then the emitted $e^{\pm }$ becomes the source of the synchrotron radiation. We calculate the synchrotron radiation flux for models of decaying dark matter, which can explain the PAMELA positron excess. Taking the lifetime of the dark matter of $O({10}^{26}\sec )$, which is the suggested value to explain the PAMELA anomaly, the synchrotron radiation flux is found to be $O(1\mathrm{kJy}/\mathrm{str})$ or smaller, depending on the particle-physics and cosmological parameters.

Recently, Valiviita et al. [J. Cosmol. Astropart. Phys. 07 (2008) 020] have reported a large-scale early-time instability in coupled dark energy and dark matter models. We take the same form of energy-momentum exchange and specialize to the case when the interaction rate is proportional to Hubble’s parameter and the dark energy density only. Provided the coupling is made small enough for a given equation of state parameter, we show that the instability can be avoided. Expressions are derived for nonadiabatic modes on superhorizon scales in both the radiation and matter dominated regimes. We also examine the growth of dark matter perturbations in the subhorizon limit. There we find that the coupling has almost no effect upon the growth of structure before dark energy begins to dominate. Once the Universe begins to accelerate, the relative dark matter density fluctuations not only cease to grow as in uncoupled models, but actually decay as the Universe continues to expand.

We exhibit a varying speed of light (VSL) theory that implements the recently proposed decaying speed of sound mechanism for generating density fluctuations. We avail ourselves of bi-metric VSL theories, where the speed of gravity differs from that of light. We first show that a DBI-like type of $K$-essence model has the necessary speed of sound profile to produce (near) scale-invariant fluctuations. We then examine the map between bimetric and $K$-essence models: typically the bi-scalar connecting the two metrics is a $K$-essence field in one of them. Remarkably, the DBI (Dirac-Born-Infeld) model is found to perturbatively represent the minimal bimetric model, where the bi-scalar is Klein-Gordon in the matter frame. But the full nonperturbative bimetric structure is even simpler: the bi-scalar dynamics should be simply driven by a cosmological constant in the matter frame, balanced by an opposite cosmological constant in the gravity frame. Thus the problem of structure formation receives an elegant and universal solution within bimetric VSL theories, which are known to also solve the flatness and entropy problems and evade a plethora of causality concerns.

We present a novel cosmological model in which scalar field matter in a biaxial Bianchi IX geometry leads to a nonsingular “pancaking” solution: the hypersurface volume goes to zero instantaneously at the “big bang”, but all physical quantities, such as curvature invariants and the matter energy density remain finite, and continue smoothly through the big bang. We demonstrate that there exist geodesics extending through the big bang, but that there are also incomplete geodesics that spiral infinitely around a topologically closed spatial dimension at the big bang, rendering it, at worst, a quasiregular singularity. The model is thus reminiscent of the Taub-NUT vacuum solution in that it has biaxial Bianchi IX geometry and its evolution exhibits a dimensionality reduction at a quasiregular singularity; the two models are, however, rather different, as we will show in a future work. Here we concentrate on the cosmological implications of our model and show how the scalar field drives both isotropization and inflation, thus raising the question of whether structure on the largest scales was laid down at a time when the universe was still oblate (as also suggested by [T. S. Pereira, C. Pitrou, and J.-P. Uzan, J. Cosmol. Astropart. Phys. 9 (2007) 6.][C. Pitrou, T. S. Pereira, and J.-P. Uzan, J. Cosmol. Astropart. Phys. 4 (2008) 4.][A. Gümrükçüoğlu, C. Contaldi, and M. Peloso, J. Cosmol. Astropart. Phys. 11 (2007) 005.]). We also discuss the stability of our model to small perturbations around biaxiality and draw an analogy with cosmological perturbations. We conclude by presenting a separate, bouncing solution, which generalizes the known bouncing solution in closed FRW universes.

We study the evolution of both scalar and tensor cosmological perturbations in a Randall-Sundrum braneworld having an arbitrary expansion history. We adopt a four dimensional point of view where the degrees of freedom on the brane constitute an open quantum system coupled to an environment composed of the bulk gravitons. Because of the expansion of the universe, the brane degrees of freedom and the bulk degrees of freedom interact as they propagate forward in time. Brane excitations may decay through the emission of bulk gravitons which may escape to future infinity, leading to a sort of dissipation from the four dimensional point of view of an observer on the brane. Bulk gravitons may also be reflected off of the curved bulk and reabsorbed by the brane, thereby transformed into quanta on the brane, leading to a sort of nonlocality from the four dimensional point of view. The dissipation and the nonlocality are encoded into the retarded bulk propagator. We estimate the dissipation rates of the bound state as well as of the matter degrees of freedom at different cosmological epochs and for different sources of matter on the brane. We use a near-brane limit of the bulk geometry for the study when purely nonlocal bulk effects are encountered.

Cosmic strings are predicted to form kinks that travel along the string, and cusps, where a small region of the string moves at close to the speed of light. These disturbances are radiated away as highly beamed gravitational waves that produce a burst like pulse as the cone of emission sweeps past an observer. Gravitational wave detectors such as the Laser Interferometer Space Antenna (LISA) and the Laser Interferometer Gravitational wave Observatory (LIGO) will be capable of detecting these bursts for a wide class of string models. Such a detection would illuminate the fields of string theory, cosmology, and relativity. Here, we develop template based Markov chain Monte Carlo (MCMC) techniques that can efficiently detect and characterize the signals from cosmic string cusps. We estimate how well the signal parameters can be recovered by the advanced LIGO-Virgo network and the LISA detector using a combination of MCMC and Fisher matrix techniques. We also consider joint detections by the ground- and space-based instruments. We show that a parallel tempered MCMC approach can detect and characterize the signals from cosmic string cusps, and we demonstrate the utility of this approach on simulated data from the third round of mock LISA data challenges.

In this work, we report the first lattice calculation of hyperon axial couplings, using the $2+1$-flavor MILC configurations and domain-wall fermion valence quarks. Both the $\Sigma$ and $\Xi$ axial couplings are computed for the first time in lattice QCD. In particular, we find that $g_{\Sigma \Sigma }=0.450(21{)}_{\mathrm{stat}}(27{)}_{\mathrm{syst}}$ and $g_{\Xi \Xi }=-0.277(15{)}_{\mathrm{stat}}(19{)}_{\mathrm{syst}}$.

We consider two well-known classes of unitarization of Born amplitudes of hadron elastic scattering. The standard class, which saturates at the black-disk limit includes the standard eikonal representation, while the other class, which goes beyond the black-disk limit to reach the full unitarity circle, includes the $U$ matrix. It is shown that the basic properties of these schemes are independent of the functional form used for the unitarization, and that $U$ matrix and eikonal schemes can be extended to have similar properties. A common form of unitarization is proposed interpolating between both classes. The correspondence with different nonlinear equations are also briefly examined.

We study aspects of the pion condensation in two-flavor neutral quark matter using the Nambu–Jona-Lasinio model of QCD at finite density. We investigate the role of electric charge neutrality, and explicit symmetry breaking via quark mass, both of which control the onset of the charged pion (${\pi }^c$) condensation. We show that the equality between the electric chemical potential and the in-medium pion mass, ${\mu }_e=M_{{\pi }^{-}}$, as a threshold, persists even for a composite pion system in the medium, provided the transition to the pion condensed phase is of the second order. Moreover, we find that the pion condensate in neutral quark matter is extremely fragile with respect to the symmetry breaking effect via a current quark mass $m$, and is ruled out for $m$ larger than the order of 10 keV.

We use a sample of $384\times {10}^6$ $B\overline{B}$ events collected with the BABAR detector at the PEP-II $e^{+}{e}^{-}$ collider to study angular distributions in the rare decays $B\to K^{*}{\ell }^{+}{\ell }^{-}$, where ${\ell }^{+}{\ell }^{-}$ is either $e^{+}{e}^{-}$ or ${\mu }^{+}{\mu }^{-}$. For low dilepton invariant masses, $m_{\ell \ell }<2.5\mathrm{GeV}/c^2$, we measure a lepton forward-backward asymmetry ${\mathcal{A}}_{\mathrm{FB}}={0.24}_{-0.23}^{+0.18}\pm 0.05$ and $K^{*}$ longitudinal polarization $F_L=0.35\pm 0.16\pm 0.04$. For $m_{\ell \ell }>3.2\mathrm{GeV}/c^2$, we measure ${\mathcal{A}}_{\mathrm{FB}}={0.76}_{-0.32}^{+0.52}\pm 0.07$ and $F_L={0.71}_{-0.22}^{+0.20}\pm 0.04$.

Dark matter is examined within the “golden region” of the minimal supersymmetric standard model. This region satisfies experimental constraints, including a lower bound on the Higgs mass of 114 GeV, and minimizes fine-tuning of the $Z$ boson mass. Here we impose additional constraints (particularly due to experimental bounds on $b\to s\gamma$). Then we find the properties of the dark matter in this region. Neutralinos with a relic density that provides the amount of dark matter required by cosmological data are shown to consist of a predominant gaugino (rather than Higgsino) fraction. In addition, the $\mathrm{U}(1{)}_Y$ gaugino mass parameter must satisfy $M_1\lesssim 300\mathrm{GeV}$.

We consider the interaction of two perfectly conducting plates of arbitrary shape that are inside a nonsimply connected cylinder with transverse section of the same shape. We show that the existence of transverse electromagnetic (TEM) modes produces a Casimir force that decays only as $1/a^2$, where $a$ is the distance between plates. The TEM force does not depend on the area of the plates and dominates at large distances over the force produced by the transverse electric and transverse magnetic modes, providing in this way a physical realization of the $1+1$-dimensional Casimir effect. For the particular case of a coaxial circular cylindrical cavity, we compute the transverse electric, transverse magnetic and TEM contributions to the force, and find the critical distance for which the TEM modes dominate.

Motivated by experimental probes of general relativity, we adopt methods from perturbative (quantum) field theory to compute, up to certain integrals, the effective Lagrangian for its $n$-body problem. Perturbation theory is performed about a background Minkowski space-time to $\mathcal{O}[(v/c{)}^4]$ beyond Newtonian gravity, where $v$ is the typical speed of these $n$ particles in their center of energy frame. For the specific case of the 2-body problem, the major efforts underway to measure gravitational waves produced by inspiraling compact astrophysical binaries require their gravitational interactions to be computed beyond the currently known $\mathcal{O}[(v/c{)}^7]$. We argue that such higher order post-Newtonian calculations must be automated for these field theoretic methods to be applied successfully to achieve this goal. In view of this, we outline an algorithm that would in principle generate the relevant Feynman diagrams to an arbitrary order in $v/c$ and take steps to develop the necessary software. The Feynman diagrams contributing to the $n$-body effective action at $\mathcal{O}[(v/c{)}^6]$ beyond Newton are derived.

Models in which dark energy interacts with dark matter have been proposed in the literature to help explain why dark energy should only come to dominate in recent times. In this paper, we present a dynamical framework to calculate cosmological perturbations for a general quintessence potential and interaction term. Our formalism is built upon the powerful phase-space approach often used to analyze the dynamical attractors in the background. We obtain a set of coupled differential equations purely in terms of dimensionless, bounded variables and apply these equations to calculate perturbations in a number of scenarios. Interestingly, in the presence of dark-sector interactions, we find that dark energy perturbations do not redshift away at late times, but can cluster even on small scales. We also clarify the initial conditions for the perturbations in the dark sector, showing that adiabaticity is no longer conserved in the presence of dark-sector interactions, even on large scales. Some issues of instability in the perturbations are also discussed.

Dark matter (DM) annihilation could in principle contribute to the diffuse cosmic gamma-ray background. While with standard assumptions for cosmological and particle physics parameters this contribution is expected to be rather small, a number of processes could boost it, including a larger-than-expected DM annihilation cross section, or the occurrence of DM substructures such as DM minispikes around intermediate-mass black holes. We show that angular correlations of the cosmic gamma-ray background provide a tool to disentangle the signal induced by DM annihilation in minispikes from a conventional astrophysical component. Treating blazars as a known background, we study the prospects for detecting DM annihilations with the Fermi Gamma-Ray Space Telescope for different choices of DM mass and annihilation channels.