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.

We meet once a week to discuss some papers of general interest to our research activities in cosmology and gravity, broadly defined. This is typically something recent from the arXiv, but could be anything relevant, even an old “classic.”

The current week's paper(s) will be published here a few days in advance.

For questions contact Frans Pretorius (fpretori@princeton.edu).

##### Mar 9: Light ring stability in ultra-compact objects

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.

##### Mar 2: Limits on the number of spacetime dimensions from GW170817

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.

##### Feb 16: No LIGO MACHO: Primordial Black Holes, Dark Matter and Gravitational Lensing of Type Ia Supernovae

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

##### Feb 9: Cosmological Constraints on Horndeski Gravity in Light of GW170817

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.

##### Nov 17: Probing Primordial-Black-Hole Dark Matter with Gravitational Waves

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.

##### Nov 10: No smooth beginning for spacetime

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.

##### Oct 27: The Gravitational Wave Stress-Energy (pseudo)-Tensor in Modified Gravity

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.

##### Oct 20: GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral

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.

##### Oct 13: Cosmological bounce and Genesis beyond Horndeski

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.

##### Sept 29: First search for nontensorial gravitational waves from known pulsars

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

##### Sept 22: Fixing extensions to General Relativity in the non-linear regime

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.