ABSTRACT: In the cyclic model of the universe that arises from combining intervals of slow contraction with a non-singular bounce, the present accelerated expansion driven by dark energy must eventually come to an end, before a phase of slow contraction can begin. At this transition between expansion and contraction, the quintessence-like scalar field responsible for dark energy domination starts to drive slow contraction as it evolves from a constant positive potential (approximating a cosmological constant) to rolling down a negative exponential potential. I will discuss how this transition can be studied numerically and explore different potentials and initial conditions that allow for it, while also commenting on the issues that arise from having very different energy scales and dynamics in the two phases.

ABSTRACT: I will show that it is possible to have a Universe where the phase of slow contraction is followed by a stable bounce. This is achieved using a scalar field + a spinor field that are coupled to the spin-2 and Torsion. Initially, the spinor bilinears have zero VEV and therefore do not contribute to the Hubble evolution, so the scalar field with negative exponential potential is dominant and takes care of the smoothness and flatness of the Universe. By the end of this phase, the metric is spatially flat FRW. The second phase is the simple evolution of FRW in the presence of a scalar field with negative exponential potential. This phase continues until a critical value of the scale factor is reached. At this point, the spinor bilinears acquire a VEV and soon dominate the equation of state, after which a stable bounce is followed. I will justify my claims through both, approximate analytic and numerical solutions.

ABSTRACT: Slow contraction when driven by a canonical scalar field that has a negative exponential potential is a robust and rapid smoothing mechanism. However, it remains to be seen if including an end for the smoothing phase alters the conclusions. In this talk, I will review the case of exponential potentials and generalize it to potentials with a global minimum that terminate the smoothing phase after the Hubble radius has shrank by at least 60 e-foldings. In the second part of my talk, I will explain how certain subtleties in the implementation of the numerical relativity code affect its convergence properties, particularly the unexpected third order convergence during slow contraction.

ABSTRACT: It is possible to achieve slow contraction using a scalar field with a non-canonical kinetic term as an alternative to the usual potential-driven models. A common feature of kinetic contraction theories is the appearance of a superluminal sound speed for perturbations in the field. In this talk, I will review existing arguments about why superluminality may be physically realizable in this context, and I will then outline the traces of a generic connection between superluminality and kinetic contraction. Finally, I will describe how a non-canonical field driving slow contraction may be identified with dark energy in cyclic models of the universe, and I will discuss the implications for choosing initial conditions for the contracting phase.

ABSTRACT: The puzzling coincidence between the maximum dS lifetime predicted by Trans-Planckian Censorship Conjecture (TCC) and the scrambling time suggests a deeper connection between the Swampland program and the thermal aspects of dS. The goal of the talk is to take a step in the direction of bridging this gap. After reviewing TCC and its motivations, I will discuss the thermalization process in dS through different lenses and interpret the results in light of Swampland conditions. I will show the swampland conditions suggest the thermal interpretation of dS is flawed. I will also discuss some of the implications of TCC for inflationary cosmology.

ABSTRACT: Perhaps the greatest challenge for fundamental theories based on compactification from extra dimensions is accommodating a period of accelerated cosmological expansion. Previous studies have identified constraints imposed by the existence of dark energy for two overlapping classes of compactified theories: (1) those in which the higher dimensional picture satisfies certain metric properties selected to reproduce known low energy phenomenology; or (2) those derived from string theory assuming they satisfy the Swampland conjectures. For either class, the analyses showed that dark energy is only possible if it takes the form of quintessence. In this paper, we explore the consequences for theories that belong to both classes and show that the joint constraints are highly restrictive, leaving few options.

Montefalcone et al. Dark energy, extra dimensions, and the Swampland.

ABSTRACT: In [1], the authors argue that it is possible to have a non-singular bounce without the violation of the null energy condition. A bounce is possible for a closed FRW universe if the effective equation of state satisfies ϵ=3/2(1+ω)<1, i.e., the contribution of the spatial curvature component to the total energy density , Ω_k, is dominant as the universe contracts. The matter stress-energy is sourced by a canonical scalar field that is non-minimally coupled to gravity and has a polynomial potential.  In addition to presenting the proposed scenario, we will also compare [1] with the bouncing models (ϵ>3) we've been discussing so far.

[1] Ö. Güngör & G. Starkmann. A classical, non-singular, bouncing universe.

ABSTRACT: After reviewing the basics of mimetic gravity, I will make the case for a version of this theory which introduces a curvature dependence of the gravitational and the cosmological constant but whose equation of motion is free of higher derivatives of the metric. In this model the concept of “asymptotic freedom” gains special importance. This name is awarded to modifications where limiting curvature is achieved by a vanishing of the gravitational constant. I will discuss background solutions of this model in several cosmological and Black Hole settings and finally give a brief outlook on cosmological perturbations and instabilities.

ABSTRACT: In [1], it was shown that by introducing a constrained (mimetic) scalar field to general relativity one can avoid the initial (final) singularities in expanding (contracting) Friedmann and Kasner universes. Instead, one introduces a limiting curvature, which sets an upper bound on the curvature invariants. This in turn allows for a non-singular transition from a contracting to an expanding universe in the above mentioned cases. If the limiting curvature remains well below the Planck scale, the space-time evolution will be dominantly classical. In regions where the curvature is well below the upper bound, the applicability of general relativity remains valid and the introduced corrections are negligibly small. Lastly, the same mechanism which resolves the above mentioned singularities also provides a possible candidate for dark matter without any further modification [2].

[1] Chamseddine, A. H. & Mukhanov, V. Resolving cosmological singularities. J. Cosmol. Astropart. Phys. 2017, (2017).

[2] Chamseddine, A. H. & Mukhanov, V. Mimetic dark matter. J. High Energy Phys. 2013, (2013).

ABSTRACT: After the successful detection of gravitational waves, an unexpected feature of the growing catalog of detected signals is the mass of detected black holes lie in the "intermediate mass" range which consist of black holes either too large to have been produced by known stellar processes and too small to be a supermassive black hole believed to reside in the center of galaxies. This result led to reconsidering a formation scenario in which, during the early universe, regions could collapse to form what are called primordial black holes (PBH) because this scenario has been shown to produce formation rates in the mass ranges detected by LIGO/VIRGO. An important hypothesis relating to PBH is that they could offer an explanation for the existence of dark matter but the current belief is that if primordial black holes made up dark matter, they would predict a formation rate several orders of magnitude higher than the LIGO/VIRGO rate. The paper I will be discussing today revisits the calculation of the formation rate of PBH as dark matter and studies three body interactions in clusters using MCMC simulations. The author finds that, due to these three body interactions, a large fraction of binary systems are destroyed and can reduce the predicted formation rate to below the LIGO/VIRGO rate.

ABSTRACT: I will discuss a paper by Borde, Guth, and Vilenkin [1] that addresses the question of whether a universe inflating eternally into the future may have also been inflating eternally into the past, which would allow for complete geodesics and avoid the cosmic singularity problem. The authors argue, however, that this scenario is impossible as long as the universe has been on average expanding over the course of its lifetime. In particular, null and timelike geodesics are shown to have a finite affine length under this averaged expansion condition. These results imply geodesic incompleteness not only for a broad range of inflationary models, but also for certain cyclic models in which the universe tends to grow between cycles.

[1] Borde, Arvind, Guth, Alan H., and Vilenkin, Alexander. “Inflationary Spacetimes Are Incomplete in Past Directions.” Physical Review Letters 90, no. 15 (2003).

ABSTRACT: We will review Penrose's seminal paper [1] that highlights an often underappreciated aspect of the inflationary homogeneity problem, namely its relationship to the second law of thermodynamics. It is argued that the time-asymmetric character of the second law presents a puzzle: if entropy is non-decreasing in time (and the entropy of the Universe today is significantly lower than it could have been), the Universe must have started with a tiny entropy. Given that gravitational clumping always leads to an increase in entropy, the initial state of the Universe must have been extraordinarily homogeneous to start with. This is problematic for the inflationary model, which attempts to explain the observed homogeneity of the Universe starting from generic initial conditions, and raises concerns of fine-tuning that we will briefly elaborate on.

[1] P. R. Penrose, “Difficulties with Inflationary Cosmology”, Annals N.Y. Acad. Sci. 571 (1989).

ABSTRACT: I will discuss the first-order phase transitions during the early Universe. The talk will be based on the papers [1] and [2]. I will start with a very brief reminder of the first-order phase transitions in statistical mechanics and field theory. Then I will introduce Paul's idea to which he refers as "Regenerative Meta-Cosmology" [1]. The idea is about using a Coleman-Weinberg effective potential (which plays the role of free energy) with a false vacuum to generate an inflation phase. The universe starts at the false vacuum, penetrates through the barrier and gets into de-Sitter phase. I will try to argue that this may be interpreted as the "creation" of the universe from a state that does not admit classical space-time description [2] and will calculate the probability of this "creation".

[1] P. J. Steinhardt, NATURAL INFLATION, Nuffield Workshop on the Very Early Universe (1982), 251-266.

[2] A. Vilenkin, The Birth of Inflationary Universes, Phys.Rev.D 27 (1983) 2848.

ABSTRACT: A mechanism is presented for relaxing an initially large, positive cosmological constant to a value near zero. This is done by introducing a scalar field whose vacuum energy compensates for the initial cosmological constant. The compensating sector involves small mass scales but no unnatural fine-tuning of parameters. It is not clear how to incorporate this mechanism into a realistic cosmology.

See also: Why the cosmological constant is small and positive

ABSTRACT: We present models of realistic globular clusters with post-Newtonian dynamics for black holes. By modeling the relativistic accelerations and gravitational-wave emission in isolated binaries and during three- and four-body encounters, we find that nearly half of all binary black hole mergers occur inside the cluster, with about 10% of those mergers entering the LIGO/Virgo band with eccentricities greater than 0.1. In-cluster mergers lead to the birth of a second generation of black holes with larger masses and high spins, which, depending on the black hole natal spins, can sometimes be retained in the cluster and merge again. As a result, globular clusters can produce merging binaries with detectable spins regardless of the birth spins of black holes formed from massive stars. These second-generation black holes would also populate any upper mass gap created by pair-instability supernovae.

ABSTRACT: We propose that the state of the universe does {\it not} spontaneously violate CPT. Instead, the universe before the Big Bang is the CPT reflection of the universe after the bang. Phrased another way, the universe before the bang and the universe after the bang may be re-interpreted as a universe/anti-universe pair, created from nothing. CPT selects a unique vacuum state for the QFT on such a spacetime, which leads to a new perspective on the cosmological baryon asymmetry, and a new explanation for the observed dark matter abundance. In particular, if we assume that the matter fields in the universe are described by the standard model of particle physics (including right-handed neutrinos), we predict that one of the heavy neutrinos is stable, and that its density automatically matches the observed dark matter density if its mass is 4.8×10^8 GeV. Among other predictions, we have: (i) that the three light neutrinos are majorana; (ii) that the lightest of these is exactly massless; and (iii) that there are no primordial long-wavelength gravitational waves. We mention connections to the strong CP problem and the arrow of time.

See also: The Big Bang, CPT, and neutrino dark matter

ABSTRACT: We present results from the first study of critical behavior in 3-d gravitational collapse. The source of the gravitational field is a massless scalar field. This is a well-studied case for spherically symmetric gravitational collapse, allowing us to understand the reliability and accuracy of the simulations. We study both supercritical and subcritical evolutions to see if one provides more accurate results than the other. We find that even for highly non-spherical initial data, the critical solution is the same as in spherical symmetry.

See also: Hidden-Sector Spectroscopy with Gravitational Waves from Binary Neutron Stars

ABSTRACT: We use the most recent, complete and independent measurements of masses and radii of white dwarfs in binaries to bound the class of non-trivial modified gravity theories, viable after GW170817/GRB170817, using its effect on the mass-radius relation of the stars. We show that the uncertainty in the latest data is sufficiently small that residual evolutionary effects, most notably the effect of core composition, finite temperature and envelope structure, must now accounted for if correct conclusions about the nature of gravity are to be made. We model corrections resulting from finite temperature and envelopes to a base Hamada-Salpeter cold equation of state and derive consistent bounds on the possible modifications of gravity in the stars' interiors, finding that Y < 0.14 at 95 % confidence, an improvement of a factor of three with respect to previous bounds. Finally, our analysis reveals some fundamental degeneracies between the theory of gravity and the precise chemical makeup of white dwarfs.

ABSTRACT: We prove the following theorem: axisymmetric, stationary solutions of the Einstein field equations formed from classical gravitational collapse of matter obeying the null energy condition, that are everywhere smooth and ultracompact (i.e., they have a light ring) must have at least two light rings, and one of them is stable. It has been argued that stable light rings generally lead to nonlinear spacetime instabilities. Our result implies that smooth, physically and dynamically reasonable ultracompact objects are not viable as observational alternatives to black holes whenever these instabilities occur on astrophysically short time scales. The proof of the theorem has two parts: (i) We show that light rings always come in pairs, one being a saddle point and the other a local extremum of an effective potential. This result follows from a topological argument based on the Brouwer degree of a continuous map, with no assumptions on the spacetime dynamics, and hence it is applicable to any metric gravity theory where photons follow null geodesics. (ii) Assuming Einstein's equations, we show that the extremum is a local minimum of the potential (i.e., a stable light ring) if the energy-momentum tensor satisfies the null energy condition.

ABSTRACT: The observation of GW170817 in both gravitational and electromagnetic waves provides a number of unique tests of general relativity. Certain modifications of gravity involve the presence of additional spacetime dimensions. In these models, as the gravitational waves propagate they "leak" into the extra dimensions, leading to a reduction in the amplitude of the observed gravitational waves, and a commensurate systematic error in the inferred distance to the gravitational wave source. Electromagnetic waves would remain unaffected. We compare the inferred distance to GW170817 from the observation of gravitational waves, d_GW,L, with the inferred distance to the electromagnetic counterpart NGC 4993, d_EM,L. The constraints imply that gravitational waves propagate in D=3+1 spacetime dimensions, as expected in general relativity.

ABSTRACT: Black hole merger events detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) have revived dark matter models based on primordial black holes (PBH) or other massive compact halo objects (MACHO). This macroscopic dark matter paradigm can be distinguished from particle physics models through their gravitational lensing predictions: compact objects cause most lines of sight to be demagnified relative to the mean, with a long tail of higher magnifications. We test the PBH model using the lack of lensing signatures on type Ia supernovae (SNe), modeling the effects of large scale structure, allowing for a non-gaussian model for the intrinsic SNe luminosity distribution and addressing potential systematic errors. Using current JLA (Union) SNe data, we derive bounds Ω_PBH/Ω_M < 0.346 (0.405) at 95% confidence, ruling out the hypothesis of MACHO/PBH comprising the totality of the dark matter at 5.01σ (4.28σ) significance. The finite size of SNe limits the validity of the results to M_PBH ≥ 10−2 M_sun, fully covering the black hole mergers detected by LIGO and closing that previously open PBH mass range.

See also the "rebuttal:" LIGO Lo(g)Normal MACHO: Primordial Black Holes survive SN lensing constraints

ABSTRACT: The discovery of the electromagnetic counterpart to GW170817 severely constrains the tensor mode propagation speed, eliminating a large model space of Horndeski theory. We use the cosmic microwave background data from Planck and the joint analysis of the BICEP2/Keck Array and Planck, galaxy clustering data from the SDSS LRG survey, BOSS baryon acoustic oscillation data, and redshift space distortion measurements to place constraints on the remaining Horndeski parameters. We evolve the Horndeski parameters as power laws with both the amplitude and power law index free. We find a 95% CL upper bound on the present-day coefficient of the Hubble friction term in the cosmological propagation of gravitational waves is 2.38, whereas General Relativity gives 2 at all times. While an enhanced friction suppresses the amplitude of the reionization bump of the primordial B-mode power spectrum at ℓ < 10, our result limits the suppression to be less than 0.8%. This constraint is primarily due to the scalar integrated Sachs-Wolfe effect in temperature fluctuations at low multipoles.

ABSTRACT: Primordial black holes (PBHs) have long been suggested as a candidate for making up some or all of the dark matter in the Universe. Most of the theoretically possible mass range for PBH dark matter has been ruled out with various null observations of expected signatures of their interaction with standard astrophysical objects. However, current constraints are significantly less robust in the 20 M_sun < M_PBH < 100 M_sun mass window, which has received much attention recently, following the detection of merging black holes with estimated masses of ∼ 30 M_sun by LIGO and the suggestion that these could be black holes formed in the early Universe. We consider the potential of advanced LIGO (aLIGO) operating at design sensitivity to probe this mass range by looking for peaks in the mass spectrum of detected events. To quantify the background, which is due to black holes that are formed from dying stars, we model the shape of the stellar-black-hole mass function and calibrate its amplitude to match the O1 results. Adopting very conservative assumptions about the PBH and stellar-black-hole merger rates, we show that ∼ 5 years of aLIGO data can be used to detect a contribution of < 20 M_sun PBHs to dark matter down to f_PBH < 0.5 at > 99.9% confidence level. Combined with other probes that already suggest tension with f_PBH = 1, the obtainable independent limits from aLIGO will thus enable a firm test of the scenario that PBHs make up all of dark matter.

ABSTRACT: We identify a fundamental obstruction to any theory of the beginning of the universe, formulated as a semiclassical path integral. Hartle and Hawking's no boundary proposal and Vilenkin's tunneling proposal are examples of such theories. Each may be formulated as the quantum amplitude for obtaining a final 3-geometry by integrating over 4-geometries. We introduce a new mathematical tool - Picard-Lefschetz theory - for defining the semiclassical path integral for gravity. The Lorentzian path integral for quantum cosmology with a positive cosmological constant is meaningful in this approach, but the Euclidean version is not. Framed in this way, the resulting framework and predictions are unique. Unfortunately, the outcome is that primordial tensor (gravitational wave) fluctuations are unsuppressed. We prove a general theorem to this effect, in a wide class of theories.

ABSTRACT: The recent detections of gravitational waves by the advanced LIGO and Virgo detectors open up new tests of modified gravity theories in the strong-field and dynamical, extreme gravity regime. Such tests rely sensitively on the phase evolution of the gravitational waves, which is controlled by the energy-momentum carried by such waves out of the system. We here study four different methods for finding the gravitational wave stress-energy pseudo-tensor in gravity theories with any combination of scalar, vector, or tensor degrees of freedom. These methods rely on the second variation of the action under short-wavelength averaging, the second perturbation of the field equations in the short-wavelength approximation, the construction of an energy complex leading to a Landau-Lifshitz tensor, and the use of Noether's theorem in field theories about a flat background. We find that all methods yield the same rate of energy loss, although the stress-energy pseudo-tensor can be functionally different. We also find that the Noether method yields a stress-energy tensor that is not symmetric or gauge-invariant, and symmetrization via the Belinfante procedure does not fix these problems because this procedure relies on Lorentz invariance, which is spontaneously broken in Einstein-AEther theory. The methods and results found here will be useful for the calculation of predictions in modified gravity theories that can then be contrasted with observations.

ABSTRACT: On August 17, 2017 at 12:41:04 UTC the Advanced LIGO and Advanced Virgo gravitational-wave detectors made their first observation of a binary neutron star inspiral. The signal, GW170817, was detected with a combined signal-to-noise ratio of 32.4 and a false-alarm-rate estimate of less than one per 8.0×104 years. We infer the component masses of the binary to be between 0.86 and 2.26 M⊙, in agreement with masses of known neutron stars. Restricting the component spins to the range inferred in binary neutron stars, we find the component masses to be in the range 1.17 to 1.60 M⊙, with the total mass of the system 2.74+0.04−0.01M⊙. The source was localized within a sky region of 28 deg2 (90% probability) and had a luminosity distance of 40+8−14 Mpc, the closest and most precisely localized gravitational-wave signal yet. The association with the gamma-ray burst GRB 170817A, detected by Fermi-GBM 1.7 s after the coalescence, corroborates the hypothesis of a neutron star merger and provides the first direct evidence of a link between these mergers and short gamma-ray bursts. Subsequent identification of transient counterparts across the electromagnetic spectrum in the same location further supports the interpretation of this event as a neutron star merger. This unprecedented joint gravitational and electromagnetic observation provides insight into astrophysics, dense matter, gravitation and cosmology.

ABSTRACT: We study a “classical” bouncing scenario in beyond Horndeski theory. We give an example of spatially flat bouncing solution that is non-singular and stable throughout the whole evolution. The model is arranged in such a way that the scalar field driving the cosmological evolution initially behaves like full-fledged beyond Horndeski, whereas at late times it becomes a massless scalar field minimally coupled to gravity.

ABSTRACT: We present results from the first directed search for nontensorial gravitational waves. While general relativity allows for tensorial (plus and cross) modes only, a generic metric theory may, in principle, predict waves with up to six different polarizations. This analysis is sensitive to continuous signals of scalar, vector or tensor polarizations, and does not rely on any specific theory of gravity. After searching data from the first observation run of the advanced LIGO detectors for signals at twice the rotational frequency of 200 known pulsars, we find no evidence of gravitational waves of any polarization. We report the first upper limits for scalar and vector strains, finding values comparable in magnitude to previously-published limits for tensor strain. Our results may be translated into constraints on specific alternative theories of gravity.

Potential background reading: arXiv:0909.3328

Detection paper: arXiv:1709.09660

ABSTRACT: The question of what gravitational theory could supersede General Relativity has been central in theoretical physics for decades. Many disparate alternatives have been proposed motivated by cosmology, quantum gravity and phenomenological angles, and have been subjected to tests derived from cosmological, solar system and pulsar observations typically restricted to linearized regimes. Gravitational waves from compact binaries provide new opportunities to probe these theories in the strongly gravitating/highly dynamical regimes. To this end however, a reliable understanding of the dynamics in such a regime is required. Unfortunately, most of these theories fail to define well posed initial value problems, which prevents at face value from meeting such challenge. In this work, we introduce a consistent program able to remedy this situation. This program is inspired in the approach to "fixing" viscous relativistic hydrodynamics introduced by Israel and Stewart in the late 70's. We illustrate how to implement this approach to control undesirable effects of higher order derivatives in gravity theories and argue how the modified system still captures the true dynamics of the putative underlying theories in 3+1 dimensions. We sketch the implementation of this idea in a couple of effective theories of gravity, one in the context of Non-commutative geometry, and one in the context of Chern-Simons modified General Relativity.