It is well known that a closed universe with a minimally coupled massive scalar field always collapses to a singularity unless the initial conditions are extremely fine-tuned. We show that the corrections to the equations of motion for the massive scalar field, given by loop quantum gravity in the high curvature regime, always lead to a bounce independently of the initial conditions. In contrast with the previous works in loop quantum cosmology, we note that the singularity can be avoided even at the semiclassical level of effective dynamical equations with nonperturbative quantum gravity modifications, without using a discrete quantum evolution.

We construct solutions of plane symmetric wormholes in the presence of a negative cosmological constant by matching an interior spacetime to the exterior anti–de Sitter vacuum solution. The spatial topology of this plane symmetric wormhole can be planar, cylindrical, or toroidal. As usual, the null energy condition is necessarily violated at the throat. At the junction surface, the surface stresses are determined. By expressing the tangential surface pressure as a function of several parameters, namely, the matching radius, the radial derivative of the redshift function, and the surface energy density, the sign of the tangential surface pressure is analyzed. We then study four specific equations of state at the junction: the zero surface energy density, the constant redshift function, the domain wall equation of state, and the traceless surface stress-energy tensor. The equation governing the behavior of the radial pressure, in terms of the surface stresses and the extrinsic curvatures, is also displayed. Finally, we construct a model of a plane symmetric traversable wormhole which minimizes the usage of the exotic matter at the throat, i.e., the null energy condition is made arbitrarily small at the wormhole throat, while the surface stresses on the junction surface satisfy the weak energy condition, and consequently the null energy condition. The construction of these wormholes does not alter the topology of the background spacetime (i.e., spacetime is not multiply connected), so that these solutions can instead be considered domain walls. Thus, in general, these wormhole solutions do not allow time travel.

We investigate the behavior of a dynamical scalar field on a fixed Kerr background in Kerr-Schild coordinates using a (3+1)-dimensional spectral evolution code, and we measure the power-law tail decay that occurs at late times. We compare evolutions of initial data proportional to ${f\left(r\right)Y}_{\mathcal{l}m}(\theta ,\phi ),$ where $Y_{\mathcal{l}m}$ is a spherical harmonic and $(r,\theta ,\phi )$ are Kerr-Schild coordinates, to that of initial data proportional to ${f(r}_{\mathrm{BL}}{)Y}_{\mathcal{l}m}({\theta }_{\mathrm{BL}},\phi ),$ where ${(r}_{\mathrm{BL}},{\theta }_{\mathrm{BL}})$ are Boyer-Lindquist coordinates. We find that although these two cases are initially almost identical, the evolution can be quite different at intermediate times; however, at late times the power-law decay rates are equal.

Generic spherically symmetric self-similar collapse results in strong naked-singularity formation. In this paper we are concerned with particle creation during naked-singularity formation in spherically symmetric self-similar collapse without specifying the collapsing matter. In the generic case, the power of particle emission is found to be proportional to the inverse square of the remaining time to the Cauchy horizon (CH). The constant of proportion can be arbitrarily large in the limit of a marginally naked singularity. Therefore, the unbounded power is especially striking in the case in which an event horizon is very close to the CH, because the emitted energy can be arbitrarily large in spite of the cutoff expected from quantum gravity. The above results suggest the instability of the CH in spherically symmetric self-similar spacetime from quantum field theory and seem to support the existence of a semiclassical cosmic censor. The divergence of redshifts and blueshifts of emitted particles is found to cause the divergence of power to positive or negative infinity, depending on the coupling manner of the scalar fields to gravity. On the other hand, it is found that there is a special class of self-similar spacetimes in which the semiclassical instability of the CH is not efficient. The analyses in this paper are based on the geometric optics approximation, which is justified in two dimensions but needs justification in four dimensions.

One can construct families of static solutions that can be viewed as interpolating between nonsingular spacetimes and those containing black holes. Although everywhere nonsingular, these solutions come arbitrarily close to having a horizon. To an observer in the exterior region, it becomes increasingly difficult to distinguish these from a true black hole as the critical limiting solution is approached. In this paper we use the Majumdar-Papapetrou formalism to construct such quasi-black hole solutions from extremal charged dust. We study the gravitational properties of these solutions, comparing them with the quasi-black hole solutions based on magnetic monopoles. As in the latter case, we find that solutions can be constructed with or without hair.

We analyze the general conditions on the equation of state w required for quantum fluctuations of a scalar field to produce a scale-invariant spectrum of density perturbations, including the possibility of models which (in the four dimensional effective description) bounce from a contracting to an expanding phase. We show that there are only two stable cases: $w\approx -1$ and $w\gg 1.$ The first is utilized in inflationary models and the second in the ekpyrotic/cyclic scenario, assuming the perturbations propagate through the bounce. All other cases, including the $w\approx 0$ case considered by some authors, require extreme fine-tuning of initial conditions and/or the effective potential. For the ekpyrotic/cyclic $\left(w\gg 1\right)$ case, we also analyze the small deviations from scale invariance.

Using only general properties of the tachyon potential we show that inflation may be generic when many branes and antibranes become coincident. Inflation may occur (1) because of the assistance of the many diagonal tachyon fields, (2) when the tachyons condense in a staggered fashion, or (3) when some of them condense very late. We point out that such inflation is in some sense a stringy implementation of chaotic inflation and may have important applications for “regularizing” a lopsided or singular cosmological compact surface.

Big bang nucleosynthesis (BBN) and the cosmic microwave background (CMB) are two major pillars of cosmology. Standard BBN accurately predicts the primordial light element abundances ${(}^4\mathrm{He},$ D, ${}^3\mathrm{He}$ and ${}^7\mathrm{Li}),$ depending on one parameter, the baryon density. Light element observations are used as a baryometer. The CMB anisotropies also contain information about the content of the Universe which allows an important consistency check on the big bang model. In addition CMB observations now have sufficient accuracy to not only determine the total baryon density, but also resolve its principal constituents H and ${}^4\mathrm{He}.$ We present a global analysis of all recent CMB data, with special emphasis on the concordance with BBN theory and light element observations. We find ${\Omega }_B{h}^2{=0.0250\mathrm{}}_{-0.0026}^{+0.0019\mathrm{}}$ and $Y_p{=0.250\mathrm{}}_{-0.014}^{+0.010\mathrm{}}$ (fraction of baryon mass as ${}^4\mathrm{He})$ using CMB data alone, in agreement with ${}^4\mathrm{He}$ abundance observations. The determination of $Y_p$ allows us to constrain the relativistic degrees of freedom during BBN, measured through the effective number of light neutrino species, $N_{\nu ,eff}{=3.02\mathrm{}}_{-0.79}^{+0.85\mathrm{}},$ in accord with the standard model of particle physics. With this concordance established we show that the inclusion of standard, $N_{\nu ,eff}\equiv 3,$ BBN theory priors significantly reduces the volume of parameter space. In this case, we find ${\Omega }_B{h}^2{=0.0245\mathrm{}}_{-0.0028}^{+0.0015\mathrm{}}$ and $Y_p{=0.2493\mathrm{}}_{-0.0010}^{+0.0007\mathrm{}}.$ We also find that the inclusion of deuterium abundance observations reduces the $Y_p$ and ${\Omega }_B{h}^2$ ranges by a factor of $\sim 2.$ Further light element observations and CMB anisotropy experiments will refine this concordance and sharpen BBN and the CMB as tools for precision cosmology.

A new method to study the effects of neutrino masses on a supernova neutrino signal is proposed. The method relies exclusively on the analysis of the full statistics of neutrino events, it is independent of astrophysical assumptions, and does not require the observation of any additional phenomenon to trace possible delays in the neutrino arrival times. The sensitivity of the method to the sub-eV neutrino mass range, defined as the capability of disentangling at 95% C.L. the case $m_{\nu }=1\mathrm{}\mathrm{eV}$ from $m_{\nu }=0,$ is tested by analyzing a set of synthetic neutrino samples modeled according to the signal that could be detected at SuperKamiokande. For a supernova at the Galactic center success is achieved in more than 50% of the cases. It is argued that a future Galactic supernova yielding several thousands of inverse $\beta$ decays might provide enough information to explore a neutrino mass range somewhat below 1 eV.

We study strange quark stars in the framework of the MIT bag model, allowing the bag parameter B to depend on the density of the medium. We also study the effect of Cooper pairing among quarks, on the stellar structure. A comparison of these two effects shows that the former is generally more significant. We study the resulting equation of state of the quark matter, stellar mass-radius relation, mass-central-density relation, radius-central-density relation, and the variation of the density as a function of the distance from the center of the star. We find that the density-dependent B allows stars with larger masses and radii, due to a stiffening of the equation of state. Interestingly, certain stellar configurations are found to be possible only if B depends on the density. We have also studied the effect of the variation of the superconducting gap parameter on our results.

Quartessence is one of the alternatives to $\Lambda \mathrm{CDM}$ that has lately attracted considerable interest. According to this unifying dark matter/energy scenario, the Universe evolved from an early nonrelativistic matter-dominated phase to a more recent accelerated expansion phase, driven by a single fluid component. Recently, it has been shown that some problems of the quartessence model, such as the existence of instabilities and oscillations in the matter power spectrum, can be avoided if a specific type of intrinsic entropy perturbation is considered. In the present article we explore the role of skewness in constraining this nonadiabatic scenario. We show that nonadiabatic quartessence and quintessence have different signatures for the skewness of the density distribution on large scales and suggest that this quantity might prove helpful to break possible degeneracies between them.

In the context of top-color-assisted techicolor (TC2) models, we study the contributions of the neutral top-pion ${\pi }_t^0$ to the lepton flavor violating processes $l_i\to l_j\gamma$ and $l_i\to l_j{l}_k{l}_l.$ We find that the present experimental bound on $\overrightarrow{\mu }e\gamma$ gives severe constraints on the free parameters of TC2 models. Taking into account these constraints, we consider the processes $l_i\to l_j{l}_k{l}_l$ generated by top-pion exchange at the tree level and the one loop level, and obtain the branching ratio $\mathrm{Br}\left(\overrightarrow{\mu }3e\right)\simeq 2.87\times {10}^{-14},$ $1.1\times {10}^{-15}<~\mathrm{Br}\left(\overrightarrow{\tau }3e\right)\simeq \mathrm{Br}\left(\overrightarrow{\tau }2e\mu \right)<~4.4\times {10}^{-15},$ $3.1\times {10}^{-15}<~\mathrm{Br}\left(\overrightarrow{\tau }2\mu e\right)\simeq \mathrm{Br}\left(\overrightarrow{\tau }3\mu \right)<~1.5\times {10}^{-14}$ in most of the parameter space.

New lattice data for the ${\Pi }_u$ and ${\Sigma }_u^{-}$ potentials at short distances are presented. We compare perturbation theory to the lower static hybrid potentials and find good agreement at short distances, once the renormalon ambiguities are accounted for. We use the nonperturbatively determined continuum-limit static hybrid and ground state potentials at short distances to determine the gluelump energies. The result is consistent with an estimate obtained from the gluelump data at finite lattice spacings. For the lightest gluelump, we obtain ${\Lambda }_B^{\mathrm{RS}}\left({\nu }_f{=2.5r}_0^{-1}\right)=[2.25\pm 0.10(\mathrm{latt}.\mathrm{})\pm 0.21(\mathrm{th}.\mathrm{})\pm 0.08({\Lambda }_{\overline{\mathrm{MS}}}\left)\right]r_0^{-1}$ in the quenched approximation with $r_0^{-1}\approx 400\mathrm{MeV}.$ We show that, to quote sensible numbers for the absolute values of the gluelump energies, it is necessary to handle the singularities of the singlet and octet potentials in the Borel plane. We propose to subtract the renormalons of the short-distance matching coefficients, the potentials in this case. For the singlet potential the leading renormalon is already known and related to that of the pole mass; for the octet potential a new renormalon appears, which we approximately evaluate. We also apply our methods to heavy-light mesons in the static limit and from the lattice simulations available in the literature we obtain the quenched result ${\overline{\Lambda }}^{\mathrm{RS}}\left({\nu }_f{=2.5r}_0^{-1}\right)=[1.17\pm 0.08(\mathrm{latt}.\mathrm{})\pm 0.13(\mathrm{th}.\mathrm{})\pm 0.09({\Lambda }_{\overline{\mathrm{MS}}}\left)\right]r_0^{-1}.$ We calculate $m_{b,\overline{\mathrm{MS}}}{(m}_{b,\overline{\mathrm{MS}}})$ and apply our methods to gluinonia whose dynamics are governed by the singlet potential between adjoint sources. We can exclude nonstandard linear short-distance contributions to the static potentials, with good accuracy.

Slow-roll inflation is a beautiful paradigm, yet the inflaton potential can hardly be sufficiently flat when unknown gravitational effects are taken into account. However, the hybrid inflation models constructed in $D=4,$ $\mathcal{N}=1\mathrm{}$ supergravity can be consistent with $\mathcal{N}=2\mathrm{}$ supersymmetry, and can be naturally embedded into string theory. This article discusses the gravitational effects carefully in the string model, using a $D=4\mathrm{}$ supergravity description. We adopt the D3-D7 system of type IIB string theory compactified on a $K3\mathrm{}\times T^2/Z_2$ orientifold for definiteness. It turns out that the slow-roll parameter can be sufficiently small despite the nonminimal Kähler potential of the model. The conditions for this to happen are given in terms of string vacua. We also find that the geometry obtained by blowing up the singularity, which is necessary for the positive vacuum energy, is stabilized by introducing certain three-form fluxes.

We study the spectrum of fluctuations about static solutions in $\mathrm{}(1+1)\mathrm{}$-dimensional noncommutative scalar field models. In the case of soliton solutions noncommutativity leads to the creation of new bound states. In the case of static singular solutions an infinite tower of bound states is produced whose spectrum has a striking similarity to the spectrum of confined quark states.

The computation of the quantum vacuum pressure must take into account the contribution of zero-point oscillations of a rank-three gauge field $A_{\mu \nu \rho }.$ This result was established in a previous paper where we calculated both the Casimir pressure within a region of vacuum simulating a hadronic bag and the Wilson factor for the three-index potential associated with the boundary of the bag. The resulting “volume law” satisfied by the Wilson loop is consistent with the basic confining requirement that the static interquark potential increases with the distance between two test charges. As a sequel to that paper, we consider here the coupling of $A_{\mu \nu \rho }$ to the generic current of a matter field, later identified with the spin density current of a Dirac field. In fact, one of the objectives of this paper is to investigate the impact of the quantum fluctuations of $A_{\mu \nu \rho }$ on the effective dynamics of the spinor field. The consistency of the field equations, even at the classical level, requires the introduction of a mass term for $A_{\mu \nu \rho }.$ In this case, the Casimir vacuum pressure includes a contribution that is explicitly dependent on the mass of $A_{\mu \nu \rho }$ and leads us to conclude that the mass term plays the same role as the infrared cutoff needed to regularize the finite volume partition functional previously calculated in the massless case. Remarkably, even in the presence of a mass term, $A_{\mu \nu \rho }$ contains a mixture of massless and massive spin-0 fields so that the resulting equation is still gauge invariant. This is yet another peculiar, but physically relevant property of $A_{\mu \nu \rho }$ since it is reflected in the effective dynamics of the spinor fields and confirms the confining property of $A_{\mu \nu \rho }$ already expected from the earlier calculation of the Wilson loop.

It is argued here that the quantum computation of the vacuum pressure must take into account the contribution of zero-point oscillations of a rank-three gauge field. The field $A_{\mu \nu \rho }$ possesses no radiative degrees of freedom, its sole function being that of polarizing the vacuum through the formation of finite domains characterized by a non-vanishing, constant, but otherwise arbitrary pressure. This extraordinary feature, rather unique among quantum fields, is exploited to associate the $A_{\mu \nu \rho }$ field with the “bag constant” of the hadronic vacuum, or with the cosmological term in the cosmic case. We find that the quantum fluctuations of $A_{\mu \nu \rho }$ are inversely proportional to the confinement volume and interpret the result as a Casimir effect for the hadronic vacuum. With these results in hands and by analogy with the electromagnetic and string case, we proceed to calculate the Wilson loop of the three-index potential coupled to a “test” relativistic bubble. From this calculation we extract the static potential between two opposite points on the surface of a spherical bag and find it to be proportional to the enclosed volume.

We construct a supermatrix model that has a classical solution representing the noncommutative (fuzzy) two-supersphere. Expanding supermatrices around the classical background, we obtain a gauge theory on a noncommutative superspace on a sphere. This theory has $osp\left(1\mathrm{}|2\right)$ supersymmetry and $u(2L+1\mathrm{}|2L)$ gauge symmetry. We also discuss a commutative limit of the model keeping the radius of the supersphere fixed.