We introduce semiclassical methods into the study of the volume spectrum in loop gravity. The classical system behind a 4-valent spin network node is a Euclidean tetrahedron. We investigate the tetrahedral volume dynamics on phase space and apply Bohr-Sommerfeld quantization to find the volume spectrum. The analysis shows a remarkable quantitative agreement with the volume spectrum computed in loop gravity. Moreover, it provides new geometrical insights into the degeneracy of this spectrum and the maximum and minimum eigenvalues of the volume on intertwiner space.

What should be the quasinormal modes associated with a space-time that contains a naked singularity instead of a black hole? In the present work we address this problem by studying the scattering of scalar fields on a curved background described by a Reissner-Nordström space-time with $|q|>m$. We show that there is a qualitative difference between cases with $1<q^2/m^2\lesssim 9/8$ and cases with $q^2/m^2\gtrsim 9/8$. We discuss the necessary conditions for the well-posedness of the problem and present results for the low damped modes in the low $l$ and large $l$ limit. We also consider the asymptotically highly damped quasinormal modes. We present strong evidence that such modes are absent in the case of a naked Reissner-Nordström singularity, corroborating recent conjectures relating them to classical and quantum properties of horizons.

Determining the final result of black-hole–neutron-star mergers, and, in particular, the amount of matter remaining outside the black hole at late times and its properties, has been one of the main motivations behind the numerical simulation of these systems. Black-hole–neutron-star binaries are among the most likely progenitors of short gamma-ray bursts—as long as massive (probably a few percents of a solar mass), hot accretion disks are formed around the black hole. Whether this actually happens strongly depends on the physical characteristics of the system, and, in particular, on the mass ratio, the spin of the black hole, and the radius of the neutron star. We present here a simple two-parameter model, fitted to existing numerical results, for the determination of the mass remaining outside the black hole a few milliseconds after a black-hole–neutron-star merger (i.e., the combined mass of the accretion disk, the tidal tail, and the potential ejecta). This model predicts the remnant mass within a few percents of the mass of the neutron star, at least for remnant masses up to 20% of the neutron star mass. Results across the range of parameters deemed to be the most likely astrophysically are presented here. We find that, for $10M_{\odot }$ black holes, massive disks are only possible for large neutron stars ($R_{\mathrm{NS}}\gtrsim 12\mathrm{km}$), or quasiextremal black hole spins ($a_{\mathrm{BH}}/M_{\mathrm{BH}}\gtrsim 0.9$). We also use our model to discuss how the equation of state of the neutron star affects the final remnant, and the strong influence that this can have on the rate of short gamma-ray bursts produced by black-hole–neutron-star mergers.

We consider the background-free quantum gravity based on conformal gravity with the Riegert-Wess-Zumino action, which is formulated in terms of a conformal field theory. Employing the $R\times S^3$ background in practice, we construct the nilpotent BRST operator imposing diffeomorphism invariance. Physical fields and states are analyzed, which are given only by real primary scalars with a definite conformal weight. With attention to the presence of background charges, various significant properties, such as the state-operator correspondence and the norm structure, are clarified with some examples.

We present a simple dynamical model of symmetric, nondegenerate $n\times n$ matrices of fixed signature defined on an $n$-dimensional hyper-cubic lattice with nearest-neighbour interactions. We show how this model is related to general relativity, and discuss multiple ways in which it can be useful for studying gravity, both classical and quantum. In particular, we show that the dynamics of the model when all matrices are close to the identity corresponds exactly to a finite-difference discretization of weak-field gravity in harmonic gauge. We also show that the action which defines the full dynamics of the model corresponds to the Einstein-Hilbert action to leading order in the lattice spacing, and use this observation to define a lattice analogue of the Ricci scalar and Einstein tensor. Finally, we perform a mean-field analysis of the statistical mechanics of this model.

We study here the phase-transitional evolution between the Eguchi-Hanson soliton, the orbifolded Schwarzschild anti–de Sitter black hole, and orbifolded thermal anti–de Sitter space in Gauss-Bonnet gravity for a small Gauss-Bonnet coefficient $\alpha$. Novel phase structure is uncovered for both negative and positive $\alpha$ with spacetime configurations that are stable in Gauss-Bonnet gravity without being so in Einsteinian gravity. The evolutionary tracks taken towards such stable configurations are guided by quantum tunneling and can be represented with a phase diagram constructed by comparing the Euclidean actions of each of our states as a function of $\alpha$ and the black hole radius $r_b$. According to the AdS/CFT correspondence dictionary, it is expected that some generalized version of closed-string tachyon condensation will exhibit the phase behavior found here.

We investigate in detail the properties of oscillating instanton solutions discussed recently in the literature. We find that the solutions with $N$ nodes contain exactly $N$ homogeneous negative modes in their spectrum of linear perturbations. The existence of extra negative modes for the $N>1$ solutions suggest that they are not final state physical objects resulting from tunneling, but rather unstable intermediate thermal configurations. By contrast, the single negative mode for the $N=1$ instanton confirms its interpretation as mediating the curved-space tunneling between vacua with equal energy densities.

We examine model independent constraints on the high redshift and prerecombination expansion history from upcoming cosmic microwave background observations, using a combination of principal component analysis and other techniques. This can be translated to model independent limits on early dark energy and the number of relativistic species $N_{\mathrm{eff}}$. Models such as scaling (Doran-Robbers), dark radiation ($\Delta N_{\mathrm{eff}}$), and barotropic aether fall into distinct regions of eigenspace and can be easily distinguished from each other. Incoming CMB data will map the expansion history from $z=0–{10}^5$, achieving subpercent precision around recombination, and enable determination of the amount of early dark energy and valuable guidance to its nature.

Recent cosmological observations indicate the existence of extra light species, i.e., dark radiation. In this paper we show that signatures of the dark radiation are imprinted in the spectrum of inflationary gravitational waves. If the dark radiation is produced by the decay of a massive particle, high frequency modes of the gravitational waves are suppressed. In addition, due to the effect of the anisotropic stress caused by the dark radiation, a dip in the gravitational wave spectrum may show up at the frequency which enters the horizon at the time of the dark radiation production. Once the gravitational wave spectrum is experimentally studied in detail, we can infer the information on how and when the dark radiation was produced in the Universe.

We investigate whether the late-time (at $z\le 100$) velocity dispersion expected in warm dark matter scenarios could have some effect on the cosmic web (i.e., outside of virialized halos). We consider effective hydrodynamical equations, with a pressurelike term that agrees at the linear level with the analysis of the Vlasov equation. Then, using analytical methods, based on perturbative expansions and the spherical dynamics, we investigate the impact of this term for a 1 keV dark matter particle. We find that the late-time velocity dispersion has a negligible effect on the power spectrum on perturbative scales and on the halo mass function. However, it has a significant impact on the probability distribution function of the density contrast at $z\sim 3$ on scales smaller than $0.1h^{-1}\mathrm{Mpc}$, which correspond to Lyman-$\alpha$ clouds. Finally, we note that numerical simulations should start at $z_i\ge 100$ rather than $z_i\le 50$ to avoid underestimating gravitational clustering at low redshifts.

In order to satisfy limits on the allowed variation of particle masses from big bang nucleosynthesis until today, the chameleon scalar field is required to reach its attractor solution early on in its cosmological evolution. Brax et al. [Phys. Rev. D 70, 123518 (2004)] have shown this to be possible for certain specific initial conditions on the chameleon field at the end of inflation. However the extreme fine-tuning necessary to achieve this poses a problem if the chameleon is to be viewed a natural candidate for dark energy. In this paper we revisit the behavior of the chameleon in the early universe, including the additional coupling to electromagnetism proposed by Brax et al. [Phys. Lett. B 699, 5 (2011)]. Solving the chameleon evolution equations in the presence of a primordial magnetic field, we find that the strict initial conditions on the chameleon field at the end of inflation can be relaxed, and we determine the associated lower bound on the strength of the primordial magnetic field.

Future redshift-drift measurements (also known as the Sandage-Loeb signal) will be crucial to probe the so called “redshift desert,” thus providing a new tool for cosmological studies. In this paper we quantify the ability of a future measurement of the Sandage-Loeb signal by a Cosmic-Dynamics-Experiment-like spectrograph to constrain a phenomenological parametrization of dynamical dark energy, specifically by obtaining constraints on $w_0$ and $w_a$. We also demonstrate that if used alongside CMB data, the Sandage-Loeb measurements will be able to break degeneracies between expansion parameters, thus greatly improving cosmological constraints.

The Einstein Telescope (ET) is conceived to be a third generation gravitational-wave (GW) observatory. Its amplitude sensitivity would be a factor 10 better than advanced LIGO and Virgo and it could also extend the low-frequency sensitivity down to 1–3 Hz, compared to the 10–20 Hz of advanced detectors. Such an observatory will have the potential to observe a variety of different GW sources, including compact binary systems at cosmological distances. ET’s expected reach for binary neutron star (BNS) coalescences is out to redshift $z\simeq 2$ and the rate of detectable BNS coalescences could be as high as one every few tens or hundreds of seconds, each lasting up to several days. With such a signal-rich environment, a key question in data analysis is whether overlapping signals can be discriminated. In this paper we simulate the GW signals from a cosmological population of BNS and ask the following questions: Does this population create a confusion background that limits ET’s ability to detect foreground sources? How efficient are current algorithms in discriminating overlapping BNS signals? Is it possible to discern the presence of a population of signals in the data by cross correlating data from different detectors in the ET observatory? We find that algorithms currently used to analyze LIGO and Virgo data are already powerful enough to detect the sources expected in ET, but new algorithms are required to fully exploit ET data.

We present a no-go theorem for keV sterile neutrino dark matter: if sterile neutrinos at the keV scale play the role of dark matter, they are typically unstable and their decay produces an astrophysical monoenergetic x-ray line. It turns out that the observational bound on this line is so strong that it contradicts the existence of a quasidegenerate spectrum of active neutrinos in a seesaw type I framework where the Casas-Ibarra matrix $R$ is real. This is the case in particular for models without $CP$ violation. We give a general proof of this theorem. While the theorem (like every no-go theorem) relies on certain assumptions, the situation under which it applies is still sufficiently general to lead to interesting consequences for keV neutrino model building. In fact, depending on the outcome of the next generation experiments, one might be able to rule out whole classes of models for keV sterile neutrinos.

We discuss and clarify the validity of effective single-field theories of inflation obtained by integrating out heavy degrees of freedom in the regime where adiabatic perturbations propagate with a suppressed speed of sound. We show by construction that it is indeed possible to have inflationary backgrounds where the speed of sound remains suppressed and uninterrupted slow-roll persists for long enough. In this class of models, heavy fields influence the evolution of adiabatic modes in a manner that is consistent with decoupling of physical low- and high-energy degrees of freedom. We emphasize the distinction between the effective masses of the isocurvature modes and the eigenfrequencies of the propagating high-energy modes. Crucially, we find that the mass gap that defines the high-frequency modes increases with the strength of the turn, even as the naively heavy (isocurvature) and light (curvature) modes become more strongly coupled. Adiabaticity is preserved throughout, and the derived effective field theory remains in the weakly coupled regime, satisfying all current observational constraints on the resulting primordial power spectrum. In addition, these models allow for an observably large equilateral non-Gaussianity.

After completion of the LHC8 run in 2012, the plan is to upgrade the LHC for operation close to its design energy $\sqrt{s}=14\mathrm{TeV}$, with a goal of collecting hundreds of ${\mathrm{fb}}^{-1}$ of integrated luminosity. The time is propitious to begin thinking of what is gained by even further LHC upgrades. In this report, we compute an LHC14 reach for supersymmetry in the mSUGRA/CMSSM model with an anticipated high luminosity upgrade. We find that LHC14 with 300 $(3000){\mathrm{fb}}^{-1}$ has a reach for supersymmetry via gluino/squark searches of $m_{\tilde{g}}\sim 3.2\mathrm{TeV}$ (3.6 TeV) for $m_{\tilde{q}}\sim m_{\tilde{g}}$, and a reach of $m_{\tilde{g}}\sim 1.8\mathrm{TeV}$ (2.3 TeV) for $m_{\tilde{q}}\gg m_{\tilde{g}}$. In the case where $m_{\tilde{q}}\gg m_{\tilde{g}}$, then the LHC14 reach for chargino-neutralino production with decay into the $Wh+{\mathrm{E̸}}_T$ final state reaches to $m_{\tilde{g}}\sim 2.6\mathrm{TeV}$ for $3000{\mathrm{fb}}^{-1}$.

We suggest that, if current measurements of inclusive jet production for central rapidities at the LHC are extended to lower transverse momenta, one could define a visible cross section sensitive to the unitarity bound set by the recent determination of the inelastic proton-proton cross section.

There has been a lot of recent interest in the experimental hints of $CP$ violation in $B_{d,s}^0$ mixing, which would be a clear signal of beyond the standard model physics (with higher significance). We derive a relation for the mixing parameters, which allows clearer interpretation of the data in models in which new physics enters in $M_{12}$ and/or ${\Gamma }_{12}$. Our results imply that the central value of the D0 measurement of the semileptonic $CP$ asymmetry in $B_{d,s}^0$ decay is not only in conflict with the standard model but in a stronger tension with data on $\Delta {\Gamma }_s$ than previously appreciated. This result can be used to improve the constraint on $\Delta \Gamma$ or $A_{\mathrm{SL}}$, whichever is less precisely measured.

We calculate the masses of bottom mesons using an improved relativistic action for the $b$-quarks and the RBC/UKQCD Iwasaki gauge configurations with $2+1$ flavors of dynamical domain-wall light quarks. We analyze configurations with two lattice spacings: $a^{-1}=1.729\mathrm{GeV}$ ($a\approx 0.11\mathrm{fm}$) and $a^{-1}=2.281\mathrm{GeV}$ ($a\approx 0.086\mathrm{fm}$). We use an anisotropic, clover-improved Wilson action for the $b$-quark, and tune the three parameters of the action nonperturbatively such that they reproduce the experimental values of the $B_s$ and $B_s^{*}$ heavy-light meson states. The masses and mass splittings of the low-lying bottomonium states (such as the ${\eta }_b$ and ${\rm Y}$) can then be computed with no additional inputs, and comparison between these predictions and experiment provides a test of the validity of our method. We obtain bottomonium masses with total uncertainties of $\sim 0.5–0.6\%$ and fine-structure splittings with uncertainties of $\sim 45–55\%$: for all cases we find good agreement with experiment. The parameters of the relativistic heavy-quark action tuned for $b$-quarks presented in this work can be used for precise calculations of weak matrix elements such as $B$-meson decay constants and mixing parameters with lattice discretization errors that are of the same size as in light pseudoscalar meson quantities. This general method can also be used for charmed meson masses and matrix elements if the parameters of the heavy-quark action are appropriately tuned.