An important open question in cosmology is the degree to which the Friedmann-Lemaître-Robertson-Walker (FLRW) solutions of Einstein’s equations are able to model the large-scale behavior of the locally inhomogeneous observable universe. We investigate this problem by considering a range of exact n-body solutions of Einstein’s constraint equations. These solutions contain discrete masses, and so allow arbitrarily large density contrasts to be modeled. We restrict our study to regularly arranged distributions of masses in topological 3-spheres. This has the benefit of allowing straightforward comparisons to be made with FLRW solutions, as both spacetimes admit a discrete group of symmetries. It also provides a time-symmetric hypersurface at the moment of maximum expansion that allows the constraint equations to be solved exactly. We find that when all the mass in the universe is condensed into a small number of objects ($\lesssim 10$) then the amount of back-reaction in dust models can be large, with $O(1)$ deviations from the predictions of the corresponding FLRW solutions. When the number of masses is large ($\gtrsim 100$), however, then our measures of back-reaction become small ($\lesssim 1\%$). This result does not rely on any averaging procedures, which are notoriously hard to define uniquely in general relativity, and so provides (to the best of our knowledge) the first exact and unambiguous demonstration of back-reaction in general relativistic cosmological modelling. Discrete models such as these can therefore be used as laboratories to test ideas about back-reaction that could be applied in more complicated and realistic settings.

We study cosmological inflation on a warped Dvali-Gabadadze-Porrati braneworld where an inflaton field is nonminimally coupled to induced gravity on the brane. We present a detailed calculation of the perturbations and inflation parameters both in the Jordan and Einstein frame. We analyze the parameter space of the model fully to justify the viability of the model in confrontation with recent observational data. We compare the results obtained in these two frames also in order to judge which frame gives more acceptable results in comparison with observational data.

We consider a relativistic, degenerate electron gas at zero temperature under the influence of a strong, uniform, static magnetic field, neglecting any form of interactions. Since the density of states for the electrons changes due to the presence of the magnetic field (which gives rise to Landau quantization), the corresponding equation of state also gets modified. In order to investigate the effect of very strong magnetic field, we focus only on systems in which a maximum of either one, two, or three Landau level(s) is/are occupied. This is important since, if a very large number of Landau levels are filled, it implies a very low magnetic field strength which yields back Chandrasekhar’s celebrated nonmagnetic results. The maximum number of occupied Landau levels is fixed by the correct choice of two parameters, namely, the magnetic field strength and the maximum Fermi energy of the system. We study the equations of state of these one-level, two-level, and three-level systems and compare them by taking three different maximum Fermi energies. We also find the effect of the strong magnetic field on the mass-radius relation of the underlying star composed of the gas stated above. We obtain an exciting result that it is possible to have an electron-degenerate static star, namely, magnetized white dwarfs, with a mass significantly greater than the Chandrasekhar limit in the range $2.3–2.6M_{\odot }$, provided it has an appropriate magnetic field strength and central density. In fact, recent observations of peculiar type Ia supernovae—SN 2006gz, SN 2007if, SN 2009dc, SN 2003fg—seem to suggest super-Chandrasekhar-mass white dwarfs with masses up to $2.4–2.8M_{\odot }$ as their most likely progenitors. Interestingly, our results seem to lie within these observational limits.

We study the dynamics of a homogeneous and isotropic Friedmann-Robertson-Walker universe in the context of the Eddington-inspired Born-Infeld theory of gravity. We generalize earlier results, obtained in the context of a radiation dominated universe, to account for the evolution of a universe permeated by a perfect fluid with an arbitrary equation of state parameter $w$. We show that a bounce may occur for $\kappa >0$, if $w$ is time dependent, and we demonstrate that it is free from tensor singularities. We argue that Eddington-inspired Born-Infeld cosmologies may be a viable alternative to the inflationary paradigm as a solution to fundamental problems of the standard cosmological model.

Drawing the analogy of replacing the nucleon by heavy chiral quark $Q$, the pion by Goldstone boson $G$, and $\pi NN$ coupling by $GQQ$ coupling, we construct a statistical model for $Q\overline{Q}\to nG$ annihilation, i.e., into $n$ longitudinal weak bosons. This analogy is becoming prescient since the LHC direct bound $m_Q>611\mathrm{GeV}$ implies strong Yukawa coupling. Taking $m_Q\in (1,2)\mathrm{TeV}$, the mean number $⟨n_G⟩$ ranges from 6 to over 10, with negligible two or three boson production. With individual $t^{\prime }$ or $b^{\prime }$ decays suppressed either by phase space or quark mixing, and given the strong Yukawa coupling, $Q\overline{Q}\to n{V}_L$ is the likely outcome for very heavy $Q\overline{Q}$ production at the LHC.

General $\mathrm{SU}(2)\times \mathrm{SU}(2)\times \mathrm{U}(1)$ models represent a well-motivated intermediate step towards the unification of the standard model gauge groups. Based on a recent global analysis of low-energy and LEP constraints of these models, we perform numerical scans of their various signals at the LHC. We show that total cross sections for lepton and third-generation quark pairs, while experimentally easily accessible, provide individually only partial information about the model realized in nature. In contrast, correlations of these cross sections in the neutral and charged current channels may well lead to a unique identification.

We study the Higgs boson mass spectrum of a classical scale invariant realization of the two Higgs doublet model (SI-2HDM). The classical scale symmetry of the theory is explicitly broken by quantum loop effects due to gauge interactions, Higgs self-couplings and top quark Yukawa couplings. We determine the allowed parameter space compatible with perturbative unitarity and electroweak precision data. Taking into account the LEP and the recent LHC exclusion limits on a standard-model-like Higgs boson HSM, we obtain rather strict constraints on the mass spectrum of the heavy Higgs sector of the SI-2HDM. In particular, if MHSM 125 GeV, the SI-2HDM strongly favors scenarios in which at least one of the nonstandard neutral Higgs bosons has a mass close to 400 GeV and is generically degenerate with the charged Higgs boson, whilst the third neutral Higgs scalar is lighter than 500 GeV.

Recent indications of a 125 GeV Higgs boson are challenging for gauge-mediated supersymmetry breaking, since radiative contributions to the Higgs boson mass are not enhanced by significant stop mixing. This challenge should not be considered in isolation, however, as gauge-mediated supersymmetry breaking also generically suffers from two other problems: unsuppressed electric dipole moments (EDMs) and the absence of an attractive dark matter candidate. We show that all of these problems may be simultaneously solved by considering heavy superpartners, without extra fields or modified cosmology. Multi-TeV sfermions suppress the EDMs and raise the Higgs mass, and the dark matter problem is solved by “Goldilocks” cosmology, in which TeV neutralinos decay to GeV gravitinos that are simultaneously light enough to solve the flavor problem and heavy enough to be all of dark matter. The implications for collider searches and direct and indirect dark matter detection are sobering, but EDMs are expected near their current bounds, and the resulting nonthermal gravitino dark matter is necessarily warm, with testable cosmological implications.

The quasiparticle model is extended to investigate the properties of strange quark matter in a strong magnetic field at finite densities. For the density-dependent quark mass, self-consistent thermodynamic treatment is obtained with an additional effective bag parameter, which depends not only on the density but also on the magnetic field strength. The magnetic field makes strange quark matter more stable energetically when the magnetic field strength is less than a critical value of the order ${10}^7\mathrm{Gauss}$ depending on the QCD scale $\Lambda$. Instead of being a monotonic function of the density for the QCD scale parameter $\Lambda >126\mathrm{MeV}$, the effective bag function has a maximum near $0.3–0.4{\mathrm{fm}}^{-3}$. The influence of the magnetic field and the QCD scale parameter on the stiffness of the equation of state of the magnetized strange quark matter and the possible maximum mass of strange stars are discussed.

We deduce the maximum-entropy state of partons created in proton-proton and nucleus-nucleus collisions. This state is characterized by a distribution function $n(x,p_{\perp })$ depending on the light-cone fraction $x$ and the transverse momentum $p_{\perp }$ of forward or backward particles, respectively. The mean transverse momentum $⟨p_{\perp }⟩$ determines the single parameter of the maximum entropy distribution which is constrained by the sum of all light-cone momentum fractions being unity. The total multiplicity is related to the transverse area of the colliding Lorentz-contracted hadrons. Assuming parton-hadron duality, we can compare the model to data from RHIC and LHC.

In this work the nature of the $\sigma$ or $f_0(500)$ resonance is discussed by evaluating its quadratic scalar radius, $⟨r^2{⟩}_s^{\sigma }$. This allows one to have a quantitative estimate for the size of this resonance. We obtain that the $\sigma$ resonance is a compact object with $⟨r^2{⟩}_s^{\sigma }=(0.19\pm 0.02)-i(0.06\pm 0.02){\mathrm{fm}}^2$. Within our approach, employing unitary chiral perturbation theory, the $\sigma$ is a dynamically generated resonance that stems from the pion-pion interactions. Given its small size we conclude that the two pions inside the resonance are merged. A four-quark picture is then more appropriate. However, when the pion mass increases, for pion masses somewhat above 400 MeV, the picture of a two-pion molecule is the appropriate one. The $\sigma$ is then a spread $\pi \pi$ bound state. These results are connected with other recent works that support a nonstandard nature of the $\sigma$ as well, while fulfilling strong QCD constraints, as well as with lattice QCD. We offer a detailed study of the low-energy $S$-wave $\pi \pi$ scattering data from where we extract our values for the threshold parameters of $S$-wave $\pi \pi$ phase shifts, the $\mathcal{O}(p^4)$ chiral perturbation theory low-energy constants as well as the $\sigma$ pole position. From the comparison with other accurate determinations in the literature we obtain the average values for the isospin $0S$-wave $\pi \pi$ threshold parameters, $a_0^0=0.220\pm 0.003$, $b_0^0=0.279\pm 0.003M_{\pi }^{-2}$, and for the real and imaginary parts of the $\sigma$ pole position in $\sqrt{s}$, $458\pm 14-i261\pm 17\mathrm{MeV}$. The quark mass dependence of the size of the $\sigma$, its mass and width are considered too. The latter agree accurately with a previous lattice QCD calculation. The fact that the mass of this resonance tends to follow the threshold of two pions is a clear indication that the $\sigma$ is a dynamically generated meson-meson resonance.

The KamLAND and Borexino experiments have observed, each at $\sim 4\sigma$ level, signals of electron antineutrinos produced in the decay chains of thorium and uranium in the Earth’s crust and mantle (Th and U geoneutrinos). Various pieces of geochemical and geophysical information allow an estimation of the crustal geoneutrino flux components with relatively small uncertainties. The mantle component may then be inferred by subtracting the estimated crustal flux from the measured total flux. To this purpose, we analyze in detail the experimental Th and U geoneutrino event rates in KamLAND and Borexino, including neutrino oscillation effects. We estimate the crustal flux at the two detector sites, using state-of-the-art information about the Th and U distribution on global and local scales. We find that crust-subtracted signals show hints of a residual mantle component, emerging at $\sim 2.4\sigma$ level by combining the KamLAND and Borexino data. The inferred mantle flux slightly favors scenarios with relatively high Th and U abundances, within $\pm 1\sigma$ uncertainties comparable to the spread of predictions from recent mantle models.

We study the implications the recent results from the LHC Higgs searches have on scalar new physics. We study the impact on both the Higgs production and decay from scalars with and without color, and in cases where decoupling does and does not happen. We investigate possible constraints on scalar parameters from the production rate in the diphoton channel, and also the two vector boson channels. Measurements from both channels can help disentangle new physics due to color from that due to charge, and thus reveal the nature of the new scalar states.

In this paper, we first construct a globally well-defined nongeometric background which contains several branes in type II string theory compactified on a 7-torus. One of these branes is called $5_2^2$, which is a codimension-2 object and has a nontrivial monodromy given by a $T$-duality transformation. The geometry near the $5_2^2$-brane is shown to approach the nongeometric background constructed in J. de Boer and M. Shigemori, Phys. Rev. Lett. 104, 251603 (2010). We then construct the solution of a fundamental string rotating along a nontrivial cycle in the $5_2^2$ background. Although the background is not axisymmetric in the usual sense, we show that it is actually axisymmetric as a doubled geometry by explicitly finding a generalized Killing vector. We also show that a vector on the doubled geometry associated with the winding and momentum charges of the rotating string solution is Lie-transported in the generalized sense along the generalized Killing vector.

We reconsider the realization of baryons in the Sakai–Sugimoto model. It has previously been shown that, at least with the standard approach to calculations of baryon properties in the theory, one does not reproduce some model-independent predictions for the behavior of baryon electromagnetic form factors that are connected with long-range pion physics. In the standard approach, considerations involving the consequences of the ${\alpha }^{\prime }$ expansion motivated the use of a linearized approximation. This made it appear that the long-range pion physics of baryons may be hidden in (intractable) ${\alpha }^{\prime }$ corrections in the gravity dual. In this paper, we study the long-range properties of baryons in the Sakai–Sugimoto model without relying on the standard linearization approximations that we suspect appeared to drop pion contributions to baryon properties. The baryon solution we obtain gives the correct result for the model-independent ratio of form factors, implying that the model captures the expected infrared properties of baryons without the need to go beyond the leading order in the ${\alpha }^{\prime }$ expansion.

The baby Skyrme model is a well-known nonlinear field theory supporting topological solitons in two space dimensions. In the limit where the term quadratic in derivatives (the “sigma-model term”) vanishes, some additional structure emerges. The resulting (“extreme” or “restricted” or “BPS”) baby Skyrme model has exact soliton solutions saturating a BPS bound which exists for this restricted model. Further, the restricted model has infinitely many symmetries and infinitely many conservation laws. Here we consider the gauged version of the restricted baby Skyrme model with gauge group U(1) and the usual Maxwell term for the gauge field. We find that, again, there exists a BPS bound and BPS solutions saturating this bound. We further find that the whole problem is essentially determined by a new kind of superpotential equation. The BPS bound and the corresponding BPS solitons only may exist for potentials such that the superpotential equation has a solution which exists globally, i.e., on the whole target space. We also calculate soliton solutions both exactly and numerically, completely confirming our qualitative analytical results.

It is shown that certain aspects of gravitation may be described using a relativistic action-at-a-distance formulation. The equations of motion of the model presented are invariant under Lorentz transformations and agree with the equations of Einstein’s theory of general relativity, at the first post-Newtonian approximation, for any number of interacting point masses.

We consider scalar quantum electrodynamics in the Higgs phase and in the presence of Lorentz violation. Spontaneous breaking of the gauge symmetry gives rise to Lorentz-violating gauge field mass terms. These may cause the longitudinal mode of the gauge field to propagate superluminally. The theory may be quantized by the Faddeev-Popov procedure, although the Lagrangian for the ghost fields also needs to be Lorentz violating.