Benjamin Elder, Justin Khoury, Philipp Haslinger, Matt Jaffe, Holger Müller, Paul Hamilton

Atom interferometry experiments are searching for evidence of chameleon scalar fields with ever-increasing precision. As experiments become more precise, so too must theoretical predictions. Previous work has made numerous approximations to simplify the calculation, which in general requires solving a 3-dimensional nonlinear partial differential equation (PDE). In this paper, we introduce a new technique for calculating the chameleonic force, using a numerical relaxation scheme on a uniform grid. This technique is more general than previous work, which assumed spherical symmetry to reduce the PDE to a 1-dimensional ordinary differential equation (ODE). We examine the effects of approximations made in previous efforts on this subject, and calculate the chameleonic force in a set-up that closely mimics the recent experiment of Hamilton et al. Specifically, we simulate the vacuum chamber as a cylinder with dimensions matching those of the experiment, taking into account the backreaction of the source mass, its offset from the center, and the effects of the chamber walls. Remarkably, the acceleration on a test atomic particle is found to differ by only 20% from the approximate analytical treatment. These results allow us to place rigorous constraints on the parameter space of chameleon field theories, although ultimately the constraint we find is the same as the one we reported in Hamilton et al. because we had slightly underestimated the size of the vacuum chamber. This new computational technique will continue to be useful as experiments become even more precise, and will also be a valuable tool in optimizing future searches for chameleon fields and related theories.

Steven Weinberg

It is assumed that in a measurement the system under study interacts with a macroscopic measuring apparatus, in such a way that the density matrix of the measured system evolves according to the Lindblad equation. Under an assumption of non-decreasing von Neumann entropy, conditions on the operators appearing in this equation are given that are necessary and sufficient for the late-time limit of the density matrix to take the form appropriate for a measurement. Where these conditions are satisfied, the Lindblad equation can be solved explicitly. The probabilities appearing in the late-time limit of this general solution are found to agree with the Born rule, and are independent of the details of the operators in the Lindblad equation.

T. N. Ukwatta, D. R. Stump, J. T. Linnemann, J. H. MacGibbon, S. S. Marinelli, T. Yapici, K. Tollefson

Many early universe theories predict the creation of Primordial Black Holes (PBHs). PBHs could have masses ranging from the Planck mass to 10^5 solar masses or higher depending on the size of the universe at formation. A Black Hole (BH) has a Hawking temperature which is inversely proportional to its mass. Hence a sufficiently small BH will quasi-thermally radiate particles at an ever-increasing rate as emission lowers its mass and raises its temperature. The final moments of this evaporation phase should be explosive and its description is dependent on the particle physics model. In this work we investigate the final few seconds of BH evaporation, using the Standard Model and incorporating the most recent Large Hadron Collider (LHC) results, and provide a new parameterization for the instantaneous emission spectrum. We calculate for the first time energy-dependent PBH burst light curves in the GeV/TeV energy range. Moreover, we explore PBH burst search methods and potential observational PBH burst signatures. We have found a unique signature in the PBH burst light curves that may be detectable by GeV/TeV gamma-ray observatories such as the High Altitude Water Cerenkov (HAWC) observatory. The implications of beyond the Standard Model theories on the PBH burst observational characteristics are also discussed, including potential sensitivity of the instantaneous photon detection rate to a squark threshold in the 5 -10 TeV range.

David J. Jackson

Four-dimensional spacetime, together with a natural generalisation to extra dimensions, is obtained through an analysis of the structures and symmetries deriving from possible arithmetic expressions for one-dimensional time. On taking the infinitesimal limit this simple one-dimensional structure can be consistently equated with a homogeneous form of arbitrary dimension possessing both spacetime and more general symmetries. An extended 4-dimensional manifold, with the associated spacetime symmetry, provides a natural breaking mechanism for a higher-dimensional form and symmetry of time. It will be described how this symmetry breaking leads to a series of distinct properties of the Standard Model of particle physics, deriving directly from the natural mathematical development of the theory.

Ali Masoumi, Sonia Paban, Erick J. Weinberg

Using numerical and analytic methods, we study quantum tunneling from a Minkowski false vacuum to an anti-de Sitter true vacuum. Scanning the parameter space of theories with quartic and non-polynomial potentials, we find that for any given potential tunneling is completely quenched if gravitational effects are made sufficiently strong. For potentials where $\epsilon$, the energy density difference between the vacua, is small compared to the barrier height, this occurs in the thin-wall regime studied by Coleman and De Luccia. However, we find that other potentials, possibly with $\epsilon$ much greater than the barrier height, produce a new type of thin-wall bounce when gravitational effects become strong. We show that the critical curve that bounds the region in parameter space where the false vacuum is stable can be found by a computationally simple overshoot/undershoot argument. We discuss the treatment of boundary terms in the bounce calculation and show that, with proper regularization, one obtains an identical finite result for the tunneling exponent regardless of whether or not these are included. Finally, we briefly discuss the extension of our results to transitions between anti-de Sitter vacua.

P. Gnaciński, T. Młynik

It is common to attribute a flat rotation curve to our Galaxy. However Galazutdinov et al. (2015) in a recent paper have obtained a Keplerian rotation curve for interstellar clouds in outer parts of the Galaxy. They have calculated the distances from equivalent widths of interstellar CaII lines. The radial velocity was also measured on the interstellar CaII absorption line. We verify the result by Galazutdinov et al. (2015) basing on observations of old open clusters. We explain the determinations of flat rotation curves assuming elliptical orbits of stars in our Galaxy. The application of formulas derived with the assumption of circular orbits to elliptical ones mimics the flat rotation curve. The interstellar clouds with cross-sections larger than stars may have almost circular orbits. The rotation curve derived from the interstellar clouds, without the assumption of dark matter, will be Keplerian.

Aaron M. Levy, Gustavo Turiaci

We propose a mechanism to generate a nearly scale-invariant spectrum of adiabatic scalar perturbations about a stable, ekpyrotic background. The key ingredient is a coupling between a single ekpyrotic field and a perfect fluid of ultra-relativistic matter. This coupling introduces a friction term into the equation of motion for the field, opposing the Hubble anti-friction, which can be chosen such that an exactly scale-invariant (or nearly scale-invariant) spectrum of adiabatic density perturbations is continuously produced throughout the ekpyrotic phase. This mechanism eliminates the need for a second (entropic) scalar field and hence any need for introducing a second phase for converting entropic into curvature fluctuations. It also reduces the constraints on the equation of state during the ekpyrotic phase and, thereby, the need for parametric fine-tuning.

C.P. Burgess, M. Cicoli, S. de Alwis, F. Quevedo

Successful inflationary models should (i) describe the data well; (ii) arise generically from sensible UV completions; (iii) be insensitive to detailed fine-tunings of parameters and (iv) make interesting new predictions. We argue that a class of models with these properties is characterized by relatively simple potentials with a constant term and negative exponentials. We here continue earlier work exploring UV completions for these models, including the key (though often ignored) issue of modulus stabilisation, to assess the robustness of their predictions. We show that string models where the inflaton is a fibration modulus seem to be robust due to an effective rescaling symmetry, and fairly generic since most known Calabi-Yau manifolds are fibrations. This class of models is characterized by a generic relation between the tensor-to-scalar ratio $r$ and the spectral index $n_s$ of the form $r \propto (n_s -1)^2$ where the proportionality constant depends on the nature of the effects used to develop the inflationary potential and the topology of the internal space. In particular we find that the largest values of the tensor-to-scalar ratio that can be obtained by generalizing the original set-up are of order $r \lesssim 0.01$. We contrast this general picture with specific popular models, such as the Starobinsky scenario and $\alpha$-attractors. Finally, we argue the self consistency of large-field inflationary models can strongly constrain non-supersymmetric inflationary mechanisms.

Eugen Radu, D. H. Tchrakian, Yisong Yang

A known feature of electrically charged Reissner-Nordstr\"om-anti-de Sitter planar black holes is that they can become unstable when considered as solutions of Einstein--Yang-Mills theory. The mechanism for this is that the linearized Yang-Mills equations in the background of the Reissner-Nordstr\"om (RN) black holes possess a normalizable zero mode, resulting in non-Abelian (nA) magnetic clouds near the horizon. In this work we show that the same pattern may occur also for asymptotically flat RN black holes. Different from the anti-de Sitter case, in the Minkowskian background the prerequisite for the existence of the nA clouds is $i)$ a large enough gauge group and $ii)$ the presence of some extra interaction terms in the matter Lagrangian. To illustrate this mechanism we present two specific examples, one in four and the other in five dimensional asymptotically flat spacetime. In the first case, we augment the usual $SU(3)$ Yang-Mills Lagrangian with higher order (quartic) curvature term, while for the second one we add the Chern-Simons density to the $SO(6)$ Yang-Mills system. In both cases, an Abelian gauge symmetry is spontaneously broken near a RN black hole horizon with the appearance of a condensate of nA gauge fields. In addition to these two examples, we review the corresponding picture for anti-de Sitter black holes. All these solutions are studied both analytically and numerically, existence proofs being provided for nA clouds in the background of RN black holes. The proofs use shooting techniques which are suggested by and in turn offer insights for our numerical methods. They indicate that, for a black hole of given mass, appropriate electric charge values are required to ensure the existence of solutions interpolating desired boundary behavior at the horizons and spatial infinity.

Giorgio Sironi

Spectral distortions of the Cosmic Microwave Background (CMB) offer the possibility of probing processes which occurred during the evolution of our Universe going back up to Z$\simeq 10^7$. Unfortunately all the attempts so far carried out for detecting distortions failed. All of them were based on comparisons among absolute measurements of the CMB temperature at different frequencies. We suggest a different approach: measurements of the frequency derivative of the CMB temperature over large frequency intervals instead of observations of the absolute temperature at few, well separated, frequencies as frequently done in the past. The best observing conditions can be found in space. We discuss therefore the perspectives of new observations in the next years from the ground, at very special sites, and in space as independent missions or as part of other CMB projects

Daniel Kapec, Ana-Maria Raclariu, Andrew Strominger

The area of a cross-sectional cut $\Sigma$ of future null infinity ($\mathcal{I}^+$) is infinite. We define a finite, renormalized area by subtracting the area of the same cut in any one of the infinite number of BMS-degenerate classical vacua. The renormalized area acquires an anomalous dependence on the choice of vacuum. We relate it to the modular energy, including a soft graviton contribution, of the region of $\mathcal{I}^+$ to the future of $\Sigma$. Under supertranslations, the renormalized area shifts by the supertranslation charge of $\Sigma$. In quantum gravity, we conjecture a bound relating the renormalized area to the entanglement entropy across $\Sigma$ of the outgoing quantum state on $\mathcal{I}^+$.