We consider one of the simplest time-dependent backgrounds in M theory, describing the shrinking away of the M-theory dimension with the other spatial dimensions static. As the M-theory dimension becomes small, the situation becomes well described by string theory in a singular cosmological background where the string coupling tends to zero but the ${\alpha }^{\prime }$ corrections become large, near the cosmic singularity. We compute these ${\alpha }^{\prime }$ corrections, both for the background and for linearized perturbations, in heterotic string theory, and show that they may be reproduced by a map from 11-dimensional M theory.

We exploit the Seiberg-Witten map technique to formulate the theory of gravity defined on a Lie algebraic noncommutative space-time. Detailed expressions of the Seiberg-Witten maps for the gauge parameters, gauge potentials, and the field strengths have been worked out. Our results demonstrate that notwithstanding the introduction of more general noncommutative structure there is no first order correction, exactly as happens for a canonical (i.e. constant) noncommutativity.

A method is given which renders indirect detection of strong gravitational waves possible. This is based on the reflection (collision) of a linearly polarized electromagnetic shock wave from (with) a cross polarized impulsive and shock gravitational waves in accordance with the general theory of relativity. This highly nonlinear process induces a detectable Faraday rotation in the polarization vector of the electromagnetic field.

In Bousso-Polchinski models with hundreds of fluxes, we compute the effects of cosmological dynamics on the probability distribution of landscape vacua. Starting from generic initial conditions, we find that most fluxes are dynamically driven into a different and much narrower range of values than expected from landscape statistics alone. Hence, cosmological evolution will access only a tiny fraction of the vacua with small cosmological constant. This leads to a host of sharp predictions. Unlike other approaches to eternal inflation, the holographic measure employed here does not lead to staggering, an excessive spread of probabilities that would doom the string landscape as a solution to the cosmological constant problem.

We derive some new constraints on single-field inflation from the Wilkinson Microwave Anisotropy Probe 3-year data combined with the Sloan Luminous Red Galaxy survey. Our work differs from previous analyses by focusing only on the observable part of the inflaton potential, or in other words, by making absolutely no assumption about extrapolation of the potential from its observable region to its minimum (i.e., about the branch of the potential responsible for the last $\sim 50$ inflationary e-folds). We only assume that inflation starts at least a few e-folds before the observable Universe leaves the Hubble radius, and that the inflaton rolls down a monotonic and regular potential, with no sharp features or phase transitions. We Taylor-expand the inflaton potential at order $v=2$, 3 or 4 in the vicinity of the pivot scale, compute the primordial spectra of scalar and tensor perturbations numerically and fit the data. For $v>2$, a large fraction of the allowed models is found to produce a large negative running of the scalar tilt, and to fall in a region of parameter space where the second-order slow-roll formalism is strongly inaccurate. We release a code for the computation of inflationary perturbations which is compatible with cosmomc.

We discuss the gravitational wave background generated by primordial density perturbations evolving during the radiation era. At second order in a perturbative expansion, density fluctuations produce gravitational waves. We calculate the power spectra of gravitational waves from this mechanism, and show that, in principle, future gravitational wave detectors could be used to constrain the primordial power spectrum on scales vastly different from those currently being probed by a large-scale structure. As examples we compute the gravitational wave background generated by both a power-law spectrum on all scales, and a delta-function power spectrum on a single scale.

We show that the difference between the theoretically expected and measured by WMAP amplitude of the quadrupole fluctuations of the cosmic microwave background (CMB) can be related to the impact of the anisotropic curvature of the homogeneous universe dominated by dark energy. In such a universe the matter expansion becomes practically isotropic just after the period of inflation, and only at small redshifts is the anisotropic expansion generated again by the small curvature ${\Omega }_K=1-{\Omega }_m-{\Omega }_{\Lambda }\le {10}^{-4}$. For such models the possible deviations from the parameters derived for the standard cosmological model are evidently negligible but the correlations of large scale perturbations and distortions of their Gaussianity are possible. Such models are also compatible with the existence of a homogeneous magnetic field and matter rotation which contribute to the low $\ell$ anisotropy and can be considered as “hidden parameters” of the model. Their influence can be observed as, for example, special correlations of small scale fluctuations and the Faraday rotation of the CMB and radiation of the farthest quasars. However, both the magnetic field and matter rotation also require modifications of the simple models of isotropic inflation, and they change the evolutionary history of the early Universe.

The abnormally weighting energy hypothesis consists of assuming that the dark sector of cosmology violates the weak equivalence principle (WEP) on cosmological scales, which implies a violation of the strong equivalence principle for ordinary matter. In this paper, dark energy is shown to result from the violation of WEP by pressureless (dark) matter. This allows us to build a new cosmological framework in which general relativity is satisfied at low scales, as WEP violation depends on the ratio of the ordinary matter over dark matter densities, but at large scales, we obtain a general relativity-like theory with a different value of the gravitational coupling. This explanation is formulated in terms of a tensor-scalar theory of gravitation without WEP for which there exists a revisited convergence mechanism toward general relativity. The consequent dark energy mechanism build upon the anomalous gravity of dark matter (i) does not require any violation of the strong energy condition $p<-\rho c^2/3$, (ii) offers a natural way out of the coincidence problem thanks to the nonminimal couplings to gravitation, (iii) accounts fairly for supernovae data from various simple couplings and with density parameters very close to the ones of the concordance model $\Lambda \mathrm{CDM}$, and therefore suggests an explanation to its remarkable adequacy. Finally, (iv) this mechanism ends up in the future with an Einstein–de Sitter expansion regime once the attractor is reached.

We study a scale-invariant $SU(2)\times U(1{)}_Y\times U(1{)}_s$ model which has only dimensionless couplings. The shadow $U(1{)}_s$ is hidden, and it interacts with the standard model (SM) solely through mixing in the scalar sector and kinetic mixing of the $U(1)$ gauge bosons. The gauge symmetries are broken radiatively by the Coleman-Weinberg mechanism. Lifting of the flat direction in the scalar potential gives rise to a light scalar, the scalon, or the shadow Higgs, and a heavier scalar which we identify as the SM Higgs boson. The phenomenology of this model is discussed. In particular, the constraints on the shadow Higgs in different mass ranges, and the possibility of discovering a shadow Higgs with a mass a few tens of GeV in precision $t$-quark studies at the LHC, are investigated.

The $a_0^0(980)$-$f_0(980)$ mixing intensity has been predicted to be in the range of 0.01 to 0.2 by various theoretical models, but it lacks firm experimental observation. We examine the possibility of extracting the $a_0^0(980)$-$f_0(980)$ mixing from the $J/\psi \to \varphi f_0(980)\to \varphi a_0^0(980)$ reaction at the upgraded Beijing electron positron collider with the BESIII detector. While the branching ratio of this process through the $a_0^0(980)$-$f_0(980)$ mixing is expected to be about $O({10}^{-6})$, similar to the estimated total amount from two background reactions $J/\psi \to {\gamma }^{*}\to \varphi a_0^0(980)$ and $J/\psi \to K^{*}\overline{K}+\mathrm{c}.\mathrm{c}.\to \varphi a_0^0(980)$, the peak width from the $a_0^0(980)$-$f_0(980)$ mixing is about 8 MeV, much smaller than that from other mechanisms. With ${10}^9$ $J/\psi$ events at BESIII, the $a_0^0(980)$-$f_0(980)$ mixing intensity is expected to be unambiguously and precisely measured.

We present a search for $B^0$ decays to charmless final states involving an $\eta$ meson, a charged pion, and a second charged pion or kaon. The data sample corresponds to $383\times {10}^6$ $B\overline{B}$ pairs collected with the BABAR detector operating at the PEP-II asymmetric-energy $B$ factory at SLAC. We find no significant signals and determine the following 90% C.L. upper limits: $\mathcal{B}(B^0\to a_0^{-}{\pi }^{+})\times \mathcal{B}(a_0^{-}\to \eta {\pi }^{-})<3.1\times {10}^{-6}$, $\mathcal{B}(B^0\to a_0^{-}{K}^{+})\times \mathcal{B}(a_0^{-}\to \eta {\pi }^{-})<1.9\times {10}^{-6}$, $\mathcal{B}(B^0\to a_0(1450{)}^{-}{\pi }^{+})\times \mathcal{B}(a_0(1450{)}^{-}\to \eta {\pi }^{-})<2.3\times {10}^{-6}$, $\mathcal{B}(B^0\to a_0(1450{)}^{-}{K}^{+})\times \mathcal{B}(a_0(1450{)}^{-}\to \eta {\pi }^{-})<3.1\times {10}^{-6}$, $\mathcal{B}(B^0\to \eta {\rho }^0)<1.5\times {10}^{-6}$, and $\mathcal{B}(B^0\to \eta f_0(980))\times \mathcal{B}(f_0(980)\to {\pi }^{+}{\pi }^{-})<0.4\times {10}^{-6}$.

The conserved charges for $p$-form gauge fields coupled to gravity are defined using Lagrangian methods. Our expression for the surface charges is compared with an earlier expression derived using covariant phase space methods. Additional properties of the surfaces charges are discussed. The proof of the first law for gauge fields that are regular when pulled back on the future horizon is detailed and is shown to be valid on the bifurcation surface as well. The formalism is applied to black rings with dipole charges and is also used to provide a definition of energy in plane wave backgrounds.

We investigate the effect of $K^{-}$ condensed matter on bulk viscosity and $r$-mode instability in neutron stars. The bulk viscosity coefficient due to the nonleptonic process $n\rightleftharpoons p+K^{-}$ is studied here. In this connection, equations of state are constructed within the framework of relativistic field theoretical models where nucleon-nucleon and kaon-nucleon interactions are mediated by the exchange of scalar and vector mesons. We find that the bulk viscosity coefficient due to the nonleptonic weak process in the condensate is suppressed by several orders of magnitude. Consequently, kaon bulk viscosity may not damp the $r$-mode instability in neutron stars.

We study the implications on experimental data and theoretical models of the positivity constraints on spin observables for the reaction $A(\mathrm{\text{spin}}1/2)+B(\mathrm{\text{unpolarized}})\to C(\mathrm{\text{spin}}1/2)+\mathrm{\text{anything}}$.

The construction of a resonance theory involving hadrons requires implementing the information from higher scales into the couplings of the effective Lagrangian. We consider the large-$N_C$ chiral resonance theory incorporating scalars and pseudoscalars, and we find that, by imposing LO short-distance constraints on form factors of QCD currents constructed within this theory, the chiral low-energy constants satisfy resonance saturation at NLO in the $1/N_C$ expansion.

We explore a possibility of measuring the absolute magnitude and the nature (Majorana vs Dirac) of neutrino masses, by using a novel process of neutrino pair emission from metastable excited atoms. Except lepton number nonconserving processes, the neutrino pair ($\nu \overline{\nu }$) emission is the unique process to directly distinguish the Majorana neutrino from the Dirac neutrino, using the interference effect of identical fermions. The small energy difference between atomic levels makes it easier to measure small neutrino masses as indicated by neutrino oscillation experiments. The crucial point is how to enhance the rate of pair emission without enhancing the radiative decay. We discuss two particular cases; (1) laser irradiated pair emission from metastable atoms, and (2) microwave irradiated emission from circular Rydberg states. A new mechanism of the parametric amplification to enhance the neutrino pair emission is pointed out when Rydberg atoms are irradiated by microwave, while the radiative process may be inhibited by the cavity QED effect. A great variety of measurable neutrino parameters and a variety of experimental methods make this investigation attractive.

A recently proposed approach to the construction of the Vaidya metric in double-null coordinates for generic mass functions is extended to the $n$-dimensional ($n>2$) case and to allow the inclusion of a cosmological constant. The approach is based on a qualitative study of the null geodesics, allowing the description of light cones and revealing many features of the underlying causal structure. Possible applications are illustrated by explicit examples. Some new exact solutions are also presented and discussed. The results presented here can simplify considerably the study of spherically symmetric gravitational collapse and mass accretion in arbitrary dimensions.

We have investigated the thermodynamical properties of dark energy. Assuming that the dark energy temperature $T\sim a^{-n}$ and considering that the volume of the Universe enveloped by the apparent horizon relates to the temperature, we have derived the dark energy entropy. For dark energy with constant equation of state $w>-1$ and the generalized Chaplygin gas, the derived entropy can be positive and satisfy the entropy bound. The total entropy, including those of dark energy, the thermal radiation, and the apparent horizon, satisfies the generalized second law of thermodynamics. However, for the phantom with constant equation of state, the positivity of entropy, the entropy bound, and the generalized second law cannot be satisfied simultaneously.

We show that, in quadratic Lagrangian theories of gravity, isotropic cosmological singularities are stable to the presence of small scalar, vector, and tensor inhomogeneities. We study the effects of the quadratic Ricci term on the dynamics of the Universe at early times. Unlike in general relativity, a particular exact isotropic solution is shown to be the stable attractor on approach to the initial cosmological singularity. This solution is also known to act as an attractor in Bianchi universes of types I, II, and IX, and the results of this paper reinforce the hypothesis that small inhomogeneous and anisotropic perturbations of this attractor form part of the general cosmological solution to the field equations of quadratic gravity. Implications for the existence of a “gravitational entropy” are also discussed.