The second-order gravitational self-force on a small body is an important problem for gravitational-wave astronomy of extreme mass-ratio inspirals. We give a first-principles derivation of a prescription for computing the first and second perturbed metric and motion of a small body moving through a vacuum background spacetime. The procedure involves solving for a “regular field” with a specified (sufficiently smooth) “effective source”, and may be applied in any gauge that produces a sufficiently smooth regular field.

Generalized teleparallel gravity, also referred to as $f(T)$ gravity, has been recently proposed as an extended theory of gravitation able to give rise to an accelerated expansion in a matter-only universe. The cosmic speedup is driven by an effective torsion fluid whose equation of state depends on the $f(T)$ function entering the modified gravity Lagrangian. We focus on two particular choices for $f(T)$ which share the nice property of emulating a phantom divide crossing as suggested by some recent data. We check their viability, contrasting the predicted background dynamics with the Hubble diagram as traced by both type Ia supernovae and gamma ray bursts, the measurement of the rate expansion $H(z)$, the baryon acoustic oscillations at different redshifts, and the cosmic microwave background radiation distance priors. Both $f(T)$ models turn out to be in very good agreement with this large data set, so we also investigate whether it is possible to discriminate among them, relying on the different growth factors.

I present a solution to the full Einstein-fluid equations representing a self-gravitating Bjorken flow. The motion and the geometry become inhomogeneous in the plane transversal to the flow and the energy density profile acquires, due to gravity, corrections in terms of proper time as compared to the original test hydrodynamics. The transverse distribution of energy density, for example, becomes $ϵ(\tau ,r)/ϵ(\tau ,0)={cosh}^{-4}(3ar)$.

A covariant formulation of a theory with a massive graviton and no negative energy state has been recently proposed as an alternative to the usual general relativity framework. For a spatially flat homogenous and isotropic universe, the theory introduces modified Friedmann equations where the standard matter term is supplemented by four effective fluids mimicking dust, cosmological constant, quintessence and stiff matter, respectively. We test the viability of this massive gravity formulation by contrasting its theoretical prediction to the Hubble diagram as traced by type Ia supernovae and gamma ray bursts, the $H(z)$ measurements from passively evolving galaxies, baryon acoustic oscillations from galaxy surveys and the distance priors from the cosmic microwave background radiation anisotropy spectrum. It turns out that the model is indeed able to very well fit this large data set thus offering a viable alternative to the usual dark energy framework. We finally set stringent constraints on its parameters also narrowing down the allowed range for the graviton mass.

We study some possible observational signatures of $R^n$ gravity at Galactic scales and how these signatures could be used for constraining this type of $f(R)$ gravity. For that purpose, we performed two-body simulations in the $R^n$ gravity potential and analyzed the obtained trajectories of S2-like stars around the Galactic center, as well as the resulting parameter space of the $R^n$ gravity potential. Here, we discuss the constraints on $R^n$ gravity which can be obtained from the observations of orbits of S2-like stars with the present and next generations of large telescopes. We make comparisons between the theoretical results and observations. Our results show that the most probable value for the parameter $r_c$ in the $R^n$ gravity potential in the case of S2-like stars is $\sim 100\mathrm{AU}$, while the universal parameter $\beta$ is close to 0.01. Also, the gravity potential induces the precession of S2-like star orbits in the opposite direction with respect to general relativity; therefore, such a behavior of orbits is qualitatively similar to the behavior of Newtonian orbits with a bulk distribution of matter (including a stellar cluster and dark matter distributions).

We study gravitational waves to first and second order in amplitude in vacuum asymptotically flat spacetimes. The Einstein equations are solved to first order and these solutions are superposed to form a time-symmetric ingoing and then outgoing pulse regular everywhere. The waves are assumed to have odd parity and a nonvanishing angular momentum which keeps them away from the axis at all times. The averaged energy of the waves is evaluated. The relevant Einstein equation is then solved to second order in the amplitude. The influence of the angular momentum of the waves on the rotation of local inertial frames with respect to the frames at great distances is analyzed. The rotation of the frames occurs even in the region around the origin where spacetime is almost flat. The rotation is without time delay as it follows from the constraint equation. The results are illustrated graphically for various values of the “harmonic index $m$” corresponding to azimuthal rotation and the “harmonic index $l$” describing the latitudinal rotation of the waves. The apparent motions of the fixed stars on the celestial sphere as seen through rotating waves from the local inertial frame are calculated and displayed.

We investigate the expansion behavior of Bianchi type I spacetime with a massless scalar field and a cosmological constant using the path integral approach of loop quantum cosmology developed by Ashtekar, Campiglia, and Henderson [Phys. Rev. D 82, 124043 (2010)]. We obtain the effective action for this model and calculate the semiclassical solutions. Although classical general relativity is a good approximation at large scales, it is shown that the loop quantum effect induces a repulsive force at small scales to avoid the big bang singularity as in the isotropic cases. We investigate the evolution of anisotropy in detail and find that the anisotropy cannot increase indefinitely like in the classical case.

Leptogenesis may be induced by the mixing of extra Higgs doublets with experimentally accessible masses. This mechanism relies on diagrammatic cuts that are kinematically forbidden in the vacuum but contribute at finite temperature. A resonant enhancement of the asymmetry occurs generically provided the dimensionless Yukawa and self-interactions are suppressed compared to those of the standard model Higgs field. This is in contrast to typical scenarios of resonant leptogenesis, where the asymmetry is enhanced by imposing a degeneracy of singlet neutrino masses.

The primary observable in dark matter direct detection is the spectrum of scattering events. We simulate multiple positive direct detection signals (on germanium, xenon, and argon targets) to explore the extent to which the underlying particle physics, manifested in the momentum dependence of the operator mediating the scattering, can be extracted. Taking into account realization (Poisson) noise, a single target nucleus with 300 events has limited power to discriminate operators with momentum dependence differing by $q^{\pm 2}$ for a wide range of dark matter masses from 10 GeV to 1 TeV. With the inclusion of multiple targets (or a factor of several more events on a single target), the discrimination of operators with different momentum dependence becomes very strong at the 95% confidence level for dark matter candidates of mass 50 GeV and above. On the other hand, operator discrimination remains poor for 10 GeV candidates until multiple experiments each collect 1000 or more events.

If the standard model is valid up to very high energies it is known that the Higgs potential can develop a local minimum at field values around ${10}^{15}–{10}^{17}\mathrm{GeV}$, for a narrow band of values of the top quark and Higgs masses. We show that in a scalar-tensor theory of gravity such Higgs false vacuum can give rise to viable inflation if the potential barrier is very shallow, allowing for tunneling and relaxation into the electroweak scale true vacuum. The amplitude of cosmological density perturbations from inflation is directly linked to the value of the Higgs potential at the false minimum. Requiring the top quark mass, the amplitude and spectral index of density perturbations to be compatible with observations, selects a narrow range of values for the Higgs mass, $m_H=126.0\pm 3.5\mathrm{GeV}$, where the error is mostly due to the theoretical uncertainty of the 2-loop renormalization group equation. This prediction could be soon tested at the Large Hadron Collider. Our inflationary scenario could also be further checked by better constraining the spectral index and the tensor-to-scalar ratio.

In this paper, the holographic dark energy model, where the future event horizon is taken as an IR cutoff, is confronted by using currently available cosmic observational data sets which include type Ia supernovae, baryon acoustic oscillation, and cosmic microwave background radiation from full information of WMAP 7-yr data. Via the Markov chain Monte Carlo method, we obtain the values of model parameter $c={0.696}_{-0.0737-0.132-0.190}^{+0.0736+0.159+0.264}$ with 1, 2, $3\sigma$ regions. Therefore, one can conclude that at at least $3\sigma$ level the future Universe will be dominated by phantom-like dark energy. It is not consistent with positive energy condition, however this condition must be satisfied to derive the holographic bound. It implies that the current cosmic observational data points disfavor the holographic dark energy model.

We study the linear perturbation of the recently proposed model of inflation where a uniform gauge-kinetic coupling of the inflaton to multiple vector fields breaks the cosmic no-hair conjecture while maintaining the isotropy. We derive the general quadratic action for the perturbation and calculate the power spectra of scalar and tensor modes at the end of inflation by in-in formalism. It is shown that the model predicts slightly red spectra and the tensor-to-scalar ratio tends to be suppressed. The comparison with the data from WMAP 7-year does not impose strong constraints on the parameters and both weak and strong gauge-field regimes are consistent with the current observations.

The cross correlation between the integrated Sachs-Wolfe (ISW) effect and the large-scale structure is a powerful tool to constrain dark energy and alternative theories of gravity. In this paper, we obtain observational constraints on kinetic gravity braiding from the ISW–large-scale-structure cross correlation. We find that the late-time ISW effect in the kinetic gravity braiding model anticorrelates with large-scale structures in a wide range of parameters, which clearly demonstrates how one can distinguish modified gravity theories from the lambda cold dark matter model using the ISW effect.

The cosmic microwave background (CMB) is affected by the total radiation density around the time of decoupling. At that epoch, neutrinos comprised a significant fraction of the radiative energy, but there could also be a contribution from primordial gravitational waves with frequencies greater than $\sim {10}^{-15}\mathrm{Hz}$. If this cosmological gravitational wave background were produced under adiabatic initial conditions, its effects on the CMB and matter power spectrum would mimic massless noninteracting neutrinos. However, with homogenous initial conditions—as one might expect from certain models of inflation, prebig bang models, phase transitions, and other scenarios—the effect on the CMB would be distinct. We present updated observational bounds for both initial conditions using the latest CMB data at small scales from the South Pole Telescope in combination with Wilkinson Microwave Anisotropy Probe, current measurements of the baryon acoustic oscillations, and the Hubble parameter. With the inclusion of the data from the South Pole Telescope, the adiabatic bound on the cosmological gravitational wave background density is improved by a factor of 1.7 to ${10}^6{\Omega }_{\mathrm{gw}}{h}^2\lesssim 8.7$ at the 95% confidence level, with weak evidence in favor of an additional radiation component consistent with previous analyses. The constraint can be converted into an upper-limit on the tension of horizon-sized cosmic strings that could generate this gravitational wave component, with $G\mu \lesssim 2\times {10}^{-7}$ at 95% C. L., for string tension $G\mu$. The homogeneous bound improves by a factor of 3.5 to ${10}^6{\Omega }_{\mathrm{gw}}{h}^2\lesssim 1.0$ at 95% C. L., with no evidence for such a component from current data.

We briefly review an important shortcoming—unearthed in previous works—of the standard version of the inflationary model for the emergence of the seeds of cosmic structure. We consider here some consequences emerging from a proposal inspired on ideas of Penrose and Diósi [R. Penrose, The Emperor’s New Mind. Concerning Computers, Minds and Laws of Physics (1989).][R. Penrose, in Physics meets Philosophy at the Planck Scale: Contemporary Theories in Quantum Gravity, edited by C. Callendar and N. Huggett (2001), pp. 290–+.][L. Diósi, Phys. Lett. A 120, 377 (1987).][L. Diósi, Phys. Rev. A 40, 1165 (1989).] about a quantum-gravity induced reduction of the wave function, which has been put forward to address the shortcomings, arguing that its effect on the inflaton field is what can lead to the emergence of the seeds of cosmic structure [A. Perez, H. Sahlmann, and D. Sudarsky, Classical Quantum Gravity 23, 2317 (2006).]. The proposal leads to a deviation of the primordial spectrum from the scale-invariant Harrison-Zel’dovich one, and consequently, to a different CMB power spectrum. We perform statistical analyses to test two quantum collapse schemes with recent data from the CMB, including the 7-yr release of WMAP and the matter power spectrum measured using LRGs by the Sloan Digital Sky Survey. Results from the statistical analyses indicate that several collapse models are compatible with CMB and LRG data, and establish constraints on the free parameters of the models. The data put no restriction on the timescale for the collapse of the scalar field modes.

A stochastic background of gravitational waves is expected to arise from a superposition of many incoherent sources of gravitational waves, of either cosmological or astrophysical origin. This background is a target for the current generation of ground-based detectors. In this article we present the first joint search for a stochastic background using data from the LIGO and Virgo interferometers. In a frequency band of 600–1000 Hz, we obtained a 95% upper limit on the amplitude of ${\Omega }_{\mathrm{GW}}(f)={\Omega }_3(f/900\mathrm{Hz}{)}^3$, of ${\Omega }_3<0.32$, assuming a value of the Hubble parameter of $h_{100}=0.71$. These new limits are a factor of seven better than the previous best in this frequency band.

Searches for light charged Higgs bosons ($H^{\pm }$) in the decay of top quarks, $t\to H^{\pm }b$, are being carried out at the LHC and at the Tevatron. It is assumed that the dominant decay channels for such an $H^{\pm }$ state are either $H^{\pm }\to \tau \nu$ or $H^{\pm }\to cs$, and separate searches are performed with comparable sensitivity to the parameters $m_{H^{\pm }}$ and $\tan \beta$ of the scalar potential. The branching ratio for the decay $H^{\pm }\to cb$ can be as large as 80% in the Aligned Two Higgs Doublet Model and in models with three or more Higgs doublets with natural flavour conservation, while satisfying the constraint from $b\to s\gamma$ for $m_{H^{\pm }}<m_t$. Although the current search strategy for $H^{\pm }\to cs$ is also sensitive to $H^{\pm }\to cb$, a considerable gain in sensitivity could be obtained by tagging the $b$ quark from the decay $H^{\pm }\to cb$. Such an analysis, which could be readily performed at the Tevatron and in the 7 TeV and 8 TeV runs of the LHC, would probe a parameter space of the fermionic couplings of $H^{\pm }$ in the above models which at present cannot be probed by experimental observables in flavour physics.

O(100 GeV) physics beyond the standard model could be overlooked provided that it is hidden in jets or the untouched Higgs sector. The top-quark forward-backward asymmetry measurements as well as dijet bump, which is observed by CDF in association with charged lepton plus missing energy (supposed arising from $W$ decay), may indicate the existence of a new color-octet axial vector $Z_C$ with a mass of about 145 GeV. Here, $Z_C$ decays into two jets with a certain branching ratio. In this paper, we investigated the possibility to discover $Z_C$ pair via analyzing the four jets as the final states, which are heavily polluted by huge QCD background. Our simulation showed that, however, both Tevatron and LHC have the excellent chance to discover $Z_C$ through analyzing the four-jet events with accessible integrated luminosity and good control of QCD background.

We calculate form-factor ratios between the semileptonic decays ${\overline{B}}^0\to D^{+}{\ell }^{-}\overline{\nu }$ and ${\overline{B}}_s^0\to D_s^{+}{\ell }^{-}\overline{\nu }$ with lattice QCD. These ratios are a key theoretical input in a new strategy to determine the fragmentation fractions of neutral $B$ decays, which are needed for measurements of $\mathrm{BR}(B_s^0\to {\mu }^{+}{\mu }^{-})$. We use the MILC ensembles of gauge configurations with $2+1$ flavors of sea-quarks at two s of approximately 0.12 fm and 0.09 fm. We use the model-independent $z$ parametrization to extrapolate our simulation results at small recoil toward maximum recoil. Our results for the form-factor ratios are $f_0^{(s)}(M_{\pi }^2)/f_0^{(d)}(M_K^2)=1.046(44{)}_{\mathrm{stat}}(15{)}_{\mathrm{syst}}$ and $f_0^{(s)}(M_{\pi }^2)/f_0^{(d)}(M_{\pi }^2)=1.054(47{)}_{\mathrm{stat}}(17{)}_{\mathrm{syst}}$. In contrast to a QCD sum-rule calculation, no significant departure from $U$-spin ($d\leftrightarrow s$) symmetry is observed.