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 Noncommutative Geometry, and one in the context of Chern-Simons modified General Relativity.

The kinetic Sunyaev Zel'dovich (kSZ) effect, cosmic microwave background (CMB) anisotropies induced by scattering from free electrons in bulk motion, is a primary target of future CMB experiments. In addition to shedding light on the distribution of baryons and the details of the epoch of reionization, measurements of the kSZ effect have the potential to address fundamental questions about the structure and evolution of our Universe on the largest scales and at the earliest times. This potential is unlocked by combining measurements of small-scale CMB anisotropies with large-scale structure surveys, a technique known as kSZ tomography. Previous work established a quadratic estimator for the remote dipole field, the CMB dipole observed at different locations in the Universe, given a CMB map and a redshift-binned map of large scale structure. This previous work did not include gravitational lensing, redshift space distortions, or non-linear evolution of structure. In this paper, we investigate how well the remote dipole field can be reconstructed in the presence of such effects by using mock data from a suite of simulations of gigaparsec-sized regions of the Universe. To properly model both large and small scales, we develop a novel box-in-box simulation pipeline, where small-scale information is obtained from L-PICOLA N-body simulations, and large-scale information obtained by evolving fields using linear theory and adding the resulting corrections to the N-body particle data. This pipeline allows us to create properly correlated maps of the primary CMB including lensing as well as the kSZ effect and density maps on the past light cone of an observer. Analyzing an ensemble of mocks, we find that the dipole field can be reconstructed with high fidelity over a range of angular scales and redshift bins. However, we present evidence for a bias due to the non-linear evolution of structure. We also analyze correlations with the primary CMB, investigating the ability of kSZ tomography to reconstruct the intrinsic CMB dipole. Our results constitute a proof-of-principle that kSZ tomography is a promising technique for future datasets. * jcayuso@perimeterinstitute.ca † mjohnson@perimeterinstitute.ca ‡ mertens@yorku.ca arXiv:1806.01290v1 [astro-ph.CO] 4 Jun 2018 small angular scales. Although the amplitude of fluctuations is small, of order a microkelvin, the kSZ effect has now been detected at greater than 4σ [2][3][4][5], with future experiments [6] forecasted to achieve signal to noise in excess of 10 2 .The kSZ effect can be expressed as the line of sight integral [7] ∆T T kSZ (n e ) = −σ T χre 0 dχ e a e (χ e )n e (χ e )(1 + δ e (n e , χ e ))v eff (n e , χ e ),where σ T is the Thompson cross-section,n e (χ e ) is the average electron number density at comoving distance χ e , δ e (n e , χ e ) is the electron overdensity field,n e is the angular direction on the sky to the electron, v eff (n e , χ e ) is the projection of the remote CMB dipole field (the CMB dipole observed by each electron along the line of sight), and a...

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