We find a new family of ${\mathrm{AdS}}_4$ vacua in IIA string theory. The internal space is topologically either the complex projective space ${\mathbb{C}\mathbb{P}}^3$ or the “flag manifold” $\mathrm{SU}(3)/(\mathrm{U}(1)\times \mathrm{U}(1))$, but the metric is in general neither Einstein nor Kähler. All known moduli are stabilized by fluxes, without using quantum effects or orientifold planes. The analysis is completely ten-dimensional and does not rely on assumptions about Kaluza-Klein reduction.

The stress-energy tensor of a quark moving through a strongly-coupled $\mathcal{N}=4$ supersymmetric Yang-Mills plasma, at large $N_{\mathrm{c}}$, is evaluated using gauge/string duality. The accuracy with which the resulting wake, in position space, is reproduced by hydrodynamics is examined. Remarkable agreement is found between hydrodynamics and the complete result down to distances less than $2/T$ away from the quark. In performing the gravitational analysis, we use a relatively simple formulation of the bulk to boundary problem in which the linearized Einstein field equations are fully decoupled. Our analysis easily generalizes to other sources in the bulk.

A class of large $N$ $U(N)$ gauge theories on a compact manifold $S^3\times \mathbb{R}$ (with possible inclusion of adjoint matter) is known to show first-order deconfinement transition at the deconfinement temperature. This includes the familiar example of pure $YM$ theory and $\mathcal{N}=4$ Supersymmetric Yang-Mills theory. Here we study the effect of introduction of $N_f$ fundamental matter fields in the phase diagram of the above mentioned gauge theories at small coupling and in the limit of large $N$ and finite $N_f/N$. We find some interesting features like the termination of the line of first-order deconfinement phase transition at a critical point as the ratio $N_f/N$ is increased and absence of deconfinement transition thereafter (there is only a smooth crossover). This result may have some implication for QCD, which unlike a pure gauge theory does not show a first-order deconfinement transition and only displays a smooth crossover at the transition temperature.

We study a simple class of unitary renormalization group transformations governed by a parameter $f$ in the range [0, 1]. For $f=0$, the transformation is one introduced by Wegner in condensed matter physics, and for $f=1$ it is a simpler transformation that is being used in nuclear theory. The transformation with $f=0$ diagonalizes the Hamiltonian but in the transformations with $f$ near 1 divergent couplings arise as bound-state thresholds emerge. To illustrate and diagnose this behavior, we numerically study Hamiltonian flows in two simple models with bound states: one with asymptotic freedom and a related one with a limit cycle. The $f=0$ transformation places bound-state eigenvalues on the diagonal at their natural scale, after which the bound states decouple from the dynamics at much smaller momentum scales. At the other extreme, the $f=1$ transformation tries to move bound-state eigenvalues to the part of the diagonal corresponding to the lowest momentum scales available and inevitably diverges when this scale is taken to zero. Intermediate values of $f$ cause intermediate shifts of bound-state eigenvalues down the diagonal and produce increasingly large coupling constants to do this. In discrete models, there is a critical value $f_c$ below which bound-state eigenvalues appear at their natural scale, and the entire flow to the diagonal is well behaved. We analyze the shift mechanism analytically in a $3\times 3$ matrix model, which displays the essence of this renormalization group behavior, and we compute $f_c$ for this model.

If a primordial magnetic field (PMF) is present during photon decoupling and afterward, a finite neutrino mass can affect all modes of the CMB. In this work, we expand on earlier studies of the scalar mode effects by constructing the vector and tensor-mode equations in the presence of massive neutrinos and a PMF. We compute the power spectrum of the various modes in an illustrative example and find that the neutrino mass can significantly affect the vector and tensor modes when a PMF exists, while the effects are negligible for no PMF. The most prominent result of the present analysis is the behavior of the EE (grad-like polarization power spectrum) component of the tensor mode at low multipoles. For massive neutrinos the EE mode can become comparable to the observed primary anisotropy. Therefore, if and when the EE mode power spectrum is measured at low multipoles the possibility exists to place a strong constraint on the sum of the neutrino masses.

TeVeS uses a dynamical vector field with timelike unit-norm constraint to specify a preferred local frame. When matter moves slowly in this frame—the so-called quasistatic regime—modified Newtonian dynamics results. Theories with such vectors (such as Einstein-Aether) are prone to the vector dynamics forming singularities that render their classical evolution problematic. Here, we analyze the dynamics of the vector in TeVeS in various situations. We begin by analytically showing that the vacuum solution of TeVeS forms caustic singularities under a large class of physically reasonably initial perturbations. This shows the classical evolution of TeVeS appears problematic in the absence of matter. We then consider matter by investigating black hole solutions. We find large classes of new black hole solutions with static geometries, where the curves generated by the vector field are attracted to the black hole and may form caustics. We go on to consider the full dynamics with matter by numerically simulating, assuming spherical symmetry, the gravitational collapse of a scalar, and the evolution of an initially nearly static boson star. We find that in both cases our initial data evolves so that the vector field develops caustic singularities on a time scale of order the gravitational in-fall time. Having shown singularity formation is generic with or without matter, Bekenstein’s original formulation of TeVeS appears dynamically problematic. We argue that by modifying the vector field kinetic terms to the more general form used by Einstein-Aether, this problem may be avoided.

We revise the imposition of various constraints in spin foam models of four-dimensional general relativity. We argue that the usual simplicity constraint must be supplemented by a constraint on holonomies and together they must be inserted explicitly into the discretized path integral. At the same time, the closure constraint must be relaxed so that the new constraint expresses covariance of intertwiners assigned to tetrahedra by spin foam quantization. As a result, the spin foam boundary states are shown to be realized in terms of projected spin networks of the covariant loop approach to quantum gravity.

Graphity models are characterized by configuration spaces in which states correspond to graphs and Hamiltonians that depend on local properties of graphs such as the degrees of vertices and numbers of short cycles. As statistical systems, graphity models can be studied analytically by estimating their partition functions or numerically by Monte Carlo simulations. Results presented here are based on both of these approaches and give new information about the high- and low-temperature behavior of the models and the transitions between them. In particular, it is shown that matter degrees of freedom must play an important role in order for the low-temperature regime to be described by graphs resembling interesting extended geometries.

For a uniform population of neutron stars whose spin-down is dominated by the emission of gravitational radiation, an old argument of Blandford states that the expected gravitational-wave amplitude of the nearest source is independent of the deformation and rotation frequency of the objects. Recent work has improved and extended this argument to set upper limits on the expected amplitude from neutron stars that also emit electromagnetic radiation. We restate these arguments in a more general framework, and simulate the evolution of such a population of stars in the gravitational potential of our galaxy. The simulations allow us to test the assumptions of Blandford’s argument on a realistic model of our galaxy. We show that the two key assumptions of the argument (two dimensionality of the spatial distribution and a steady-state frequency distribution) are in general not fulfilled. The effective scaling dimension $D$ of the spatial distribution of neutron stars is significantly larger than two, and for frequencies detectable by terrestrial instruments the frequency distribution is not in a steady state unless the ellipticity is unrealistically large. Thus, in the cases of most interest, the maximum expected gravitational-wave amplitude does have a strong dependence on the deformation and rotation frequency of the population. The results strengthen the previous upper limits on the expected gravitational-wave amplitude from neutron stars by a factor of 6 for realistic values of ellipticity.

The fundamental theory and the semiclassical description of loop quantum cosmology (LQC) have been studied in the Friedmann-Robertson-Walker and Bianchi I models. As an extension to include both anisotropy and intrinsic curvature, this paper investigates the cosmological model of Kantowski-Sachs spacetime with a free massless scalar field at the level of phenomenological dynamics with the LQC discreteness corrections. The LQC corrections are implemented in two different improved quantization schemes. In both schemes, the big bang and big crunch singularities of the classical solution are resolved and replaced by the big bounces when the area or volume scale factor approaches the critical values in the Planck regime measured by the reference of the scalar field momentum. Symmetries of scaling are also noted and suggest that the fundamental spatial scale (area gap) may give rise to a temporal scale. The bouncing scenarios are in an analogous fashion of the Bianchi I model, naturally extending the observations obtained previously.

We discuss a cosmology in which cold dark matter begins to decay into relativistic particles at a recent epoch ($z<1$). We show that the large entropy production and associated bulk viscosity from such decays leads to an accelerating cosmology as required by observations. We investigate the effects of decaying cold dark matter in a $\Lambda =0$, flat, initially matter dominated cosmology. We show that this model satisfies the cosmological constraint from the redshift-distance relation for type Ia supernovae. The age in such models is also consistent with the constraints from the oldest stars and globular clusters. Possible candidates for this late decaying dark matter are suggested along with additional observational tests of this cosmological paradigm.

We estimate the strength of large-scale magnetic fields produced during inflation in the framework of Dirac-Born-Infeld (DBI) theories. This analysis is sufficiently general in the sense that it covers most of conformal symmetry breaking theories in which the electromagnetic field is coupled to a scalar field. In DBI theories there is an additional factor associated with the speed of sound, which allows a possibility to lead to an extra amplification of the magnetic field in a ultrarelativistic region. We clarify the conditions under which seed magnetic fields to feed the galactic dynamo mechanism at a decoupling epoch as well as present magnetic fields on galactic scales are sufficiently generated to satisfy observational bounds.

We calculate the expectation value of the coincident product of two field strength tensors at two loop order in scalar electrodynamics on de Sitter background. The result agrees with the stochastic formulation which we have developed in a companion paper [T. Prokopec, N. C. Tsamis, and R. P. Woodard, Ann. Phys. (N.Y.) 323, 1324 (2008)] for the nonperturbative resummation of leading logarithms of the scale factor. When combined with a previous computation of scalar bilinears [T. Prokopec, N. C. Tsamis, and R. P. Woodard, Classical Quantum Gravity 24, 201 (2007)], our current result also gives the two loop stress-energy tensor for inflationary scalar electrodynamics. This shows a secular decrease in the vacuum energy which derives from the vacuum polarization induced by the inflationary production of charged scalars.

In this paper, the following two propositions are proven under the dominant energy condition for the matter field in the higher-dimensional spherically symmetric spacetime in Einstein-Gauss-Bonnet gravity in the presence of a cosmological constant $\Lambda$. First, for $\Lambda \le 0$ and $\alpha \ge 0$ without a fine-tuning to give a unique anti–de Sitter (AdS) vacuum, where $\alpha$ is the Gauss-Bonnet coupling constant, vanishing generalized Misner-Sharp mass is equivalent to the maximally symmetric spacetime. Under the fine-tuning, it is equivalent to the vacuum class I spacetime. Second, under the fine-tuning with $\alpha >0$, the asymptotically AdS spacetime in the higher-dimensional Henneaux-Teitelboim sense is only a special class of the vacuum class I spacetime. This means the universal slow fall-off to the unique AdS infinity in the presence of physically reasonable matter.

We derived simple analytical parametrizations for energy distributions of photons, electrons, and neutrinos produced in interactions of relativistic protons with an isotropic monochromatic radiation field. The results on photomeson processes are obtained using numerical simulations of proton-photon interactions based on the public available Monte Carlo code SOPHIA. For calculations of energy spectra of electrons and positrons from the pair-production (Bethe-Heitler) process we suggest a simple formalism based on the well-known differential cross section of the process in the rest frame of the proton. The analytical presentations of energy distributions of photons and leptons provide a simple but accurate approach for calculations of broadband energy spectra of gamma rays and neutrinos in cosmic proton accelerators located in radiation dominated environments.

We study the mass of the state $Y(2175)$ of $J^{PC}=1^{--}$ in the QCD sum rule. We construct both the diquark-antidiquark currents $(ss)(\overline{s}\overline{s})$ and the meson-meson currents $(\overline{s}s)(\overline{s}s)$. We find that there are two independent currents for both cases and derive the relations between them. The operator product expansion convergence of these two currents is sufficiently fast, which enables us to perform good sum rule analysis. Both the SVZ (Shifman-Vainshtein-Zakharov) sum rule and the finite energy sum rule lead to a mass around $2.3\pm 0.4\mathrm{GeV}$, which is consistent with the observed mass within the uncertainties of the present QCD sum rule. The coupling of the four-quark currents to lower lying states such as $\varphi (1020)$ turns out to be rather small. We also discuss possible decay properties of $Y(2175)$ if it is a tetraquark state.

An exactly solvable quantum field theory (QFT) model of Lee type is constructed to study how neutrino flavor eigenstates are created through interactions and how the localization properties of neutrinos follows from the parent particle that decays. The two-particle states formed by the neutrino and the accompanying charged lepton can be calculated exactly as well as their creation probabilities. We can show that the coherent creation of neutrino flavor eigenstates follows from the common negligible contribution of neutrino masses to their creation probabilities. On the other hand, it is shown that it is not possible to associate a well-defined “flavor” to coherent superpositions of charged leptons.

We investigate a possible new technique for microwave detection of cosmic-ray extensive air showers which relies on detection of expected continuum radiation in the microwave range, caused by free-electron collisions with neutrals in the tenuous plasma left after the passage of the shower. We performed an initial experiment at the Argonne Wakefield Accelerator laboratory in 2003 and measured broadband microwave emission from air ionized via high-energy electrons and photons. A follow-up experiment at the Stanford Linear Accelerator Center in the summer of 2004 confirmed the major features of the previous Argonne Wakefield Accelerator observations with better precision. Prompted by these results we built a prototype detector using satellite television technology and have made measurements suggestive of the detection of cosmic-ray extensive air showers. The method, if confirmed by experiments now in progress, could provide a high-duty cycle complement to current nitrogen fluorescence observations.

We study the future evolution of quintessence/phantom-dominated epoch in modified $F(R)$ gravity which unifies the early-time inflation with late-time acceleration and which is consistent with observational tests. Using the reconstruction technique it is demonstrated that there are models where any known (big rip, II, III, or IV type) singularity may classically occur. From another side, in Einstein frame (scalar-tensor description) only IV type singularity occurs. Near the singularity the classical description breaks up, and it is demonstrated that quantum effects act against the singularity and may prevent its appearance. The realistic $F(R)$ gravity which is future singularity free is proposed. We point out that additional modification of any $F(R)$ gravity by the terms relevant at the early universe is possible, in such a way that future singularity does not occur even classically.

We study a modification of the five-dimensional description of dynamical electroweak symmetry breaking inspired by the AdS/CFT correspondence. Conformal symmetry is broken in the low-energy region near the IR brane by a power-law departure from the pure AdS background. Such a modification—while not spoiling the identification of the IR brane with the scale of confinement—has a dramatic effect on both the coupling of the first composite states to the standard model currents and their self-couplings. Chiral symmetry breaking can take place at a scale larger than the IR cutoff. This study shows that observables, such as the precision parameter $\widehat{S}$, which depend on the couplings of the lightest composite states to the currents, are very sensitive to the details of the dynamics in the low-energy region where conformal symmetry is lost and electroweak symmetry is broken just above the scale of confinement. Therefore results of calculations of these observables in AdS/CFT inspired scenarios should be interpreted conservatively. The most important phenomenological consequence for physics at the Large Hadron Collider is that the bound on the mass scale of the heavy excitations (technirho mesons) in a realistic model is in general lower than in the pure AdS background with a simple hard-wall cutoff in the IR.