Io Odderskov, Sofie Marie Koksbang, Steen Hannestad

The effects of local inhomogeneities on low redshift $H_0$ determinations are studied by estimating the redshift-distance relation of mock sources in N-body simulations. The results are compared to those obtained using the standard approach based on Hubble's law. The comparison shows a clear tendency for the standard approach to yield lower values of $H_0$ than the approach based on the scheme using light rays. The difference is, however, small. More precisely, it is found that the overall effect of inhomogeneities on the determination of $H_0$ is a small increase in the local estimates of about $0.3\%$ compared to the results obtained with Hubble's law, when based on a typical distribution of supernovae in the redshift range $0.01 < z < 0.1$. The overall conclusion of the study is a verification of the results that have earlier been obtained by using Hubble's law: The effects of inhomogeneities on local $H_0$ estimates are not significant enough to make it plausible that differences in high- and low-redshift estimates of $H_0$ are due to small inhomogeneities within the setting of standard cosmology.

Fulvio Melia, Jun-Jie Wei, Xue-Feng Wu

Strongly gravitationally lensed quasar-galaxy systems allow us to compare competing cosmologies as long as one can be reasonably sure of the mass distribution within the intervening lens. In this paper, we assemble a catalog of 69 such systems, and carry out a one-on-one comparison between the standard model, LCDM, and the R_h=ct Universe. We find that both models account for the lens observations quite well, though the precision of these measurements does not appear to be good enough to favor one model over the other. Part of the reason is the so-called bulge-halo conspiracy that, on average, results in a baryonic velocity dispersion within a fraction of the optical effective radius virtually identical to that expected for the whole luminous-dark matter distribution. Given the limitations of doing precision cosmological testing using the current sample, we also carry out Monte Carlo simulations based on the current lens measurements to estimate how large the source catalog would have to be in order to rule out either model at a ~99.7% confidence level. We find that if the real cosmology is LCDM, a sample of ~200 strong gravitational lenses would be sufficient to rule out R_h=ct at this level of accuracy, while ~300 strong gravitational lenses would be required to rule out LCDM if the real Universe were instead R_h=ct. The difference in required sample size reflects the greater number of free parameters available to fit the data with LCDM. We point out that, should the R_h=ct Universe eventually emerge as the correct cosmology, its lack of any free parameters for this kind of work will provide a remarkably powerful probe of the mass structure in lensing galaxies, and a means of better understanding the origin of the bulge-halo conspiracy.

Rajeev Kumar Jain, Chris Kouvaris, Niklas Grønlund Nielsen

Scalar-tensor theories of gravity can lead to modifications of the gravitational force inside astrophysical objects. We exhibit that compact stars such as white dwarfs provide a unique set-up to test such deviations from Newtonian gravitational physics inside the stars. We obtain stringent and independent constraints on the parameter $\Upsilon$ characterizing the deviations from gravity using the mass-radius relation, the Chandrasekhar mass limit and the maximal rotational frequency of white dwarfs. We find that white dwarfs impose stronger constraints on $\Upsilon$ than the red and brown dwarfs.

Austin Joyce, Lucas Lombriser, Fabian Schmidt

Understanding the reason for the observed accelerated expansion of the Universe represents one of the fundamental open questions in physics. In cosmology, a classification has emerged among physical models for the acceleration, distinguishing between Dark Energy and Modified Gravity. In this review, we give a brief overview of models in both categories as well as their phenomenology and characteristic observable signatures in cosmology. We also introduce a rigorous distinction between Dark Energy and Modified Gravity based on the strong and weak equivalence principles.

Nikola Paunković, Marko Vojinović

We argue that Hartle-Hawking states in the Regge quantum gravity model generically contain non-trivial entanglement between gravity and matter fields. Generic impossibility to talk about "matter in a point of space" is in line with the idea of an emergent spacetime, and as such could be taken as a possible candidate for a criterion for a plausible theory of quantum gravity. Finally, this new entanglement could be seen as an additional "effective interaction", which could possibly bring corrections to the weak equivalence principle.

David Wittman, 2), Ramya Bhaskar, Ryan Tobin, 3) ((1) Physics Department, University of California, Davis, (2) Instituto de Astrofísica e Ciências do Espaço, Universidade de Lisboa, (3) Department of Physics and Astronomy, University of Hawaii)

We describe a new test of photometric redshift performance given a spectroscopic redshift sample. This test complements the traditional comparison of redshift {\it differences} by testing whether the probability density functions $p(z)$ have the correct {\it width}. We test two photometric redshift codes, BPZ and EAZY, on each of two data sets and find that BPZ is consistently overconfident (the $p(z)$ are too narrow) while EAZY produces approximately the correct level of confidence. We show that this is because EAZY models the uncertainty in its spectral energy distribution templates, and that post-hoc smoothing of the BPZ $p(z)$ provides a reasonable substitute for detailed modeling of template uncertainties. Either remedy still leaves a small surplus of galaxies with spectroscopic redshift very far from the peaks. Thus, better modeling of low-probability tails will be needed for high-precision work such as dark energy constraints with the Large Synoptic Survey Telescope and other large surveys.

Joseph Polchinski

I expand on some ideas from my recent review "String theory to the rescue," I discuss my use of Bayesian reasoning. I argue that it can be useful but that it is very far from the central point of the discussion. I then review my own personal history with the multiverse. Finally I respond to some criticisms of string theory and the multiverse.

Sean M. Carroll, Grant N. Remmen

We investigate theories in which gravity arises as an entropic force. We distinguish between two approaches to this idea: holographic gravity, in which Einstein's equation arises from keeping entropy stationary in equilibrium under variations of the geometry and quantum state of a small region, and thermodynamic gravity, in which Einstein's equation emerges as a local equation of state from constraints on the area of a dynamical lightsheet in a fixed spacetime background. Examining holographic gravity, we argue that its underlying assumptions can be justified in part using recent results on the form of the modular energy in quantum field theory. For thermodynamic gravity, on the other hand, we find that it is difficult to formulate a self-consistent definition of the entropy, which represents an obstacle for this approach. This investigation points the way forward in understanding the connections between gravity and entanglement.

Tatsuya Narikawa, Hideyuki Tagoshi

We discuss the potential of the advanced ground-based gravitational-wave detectors, such as LIGO, Virgo, and KAGRA, to detect generic deviations of gravitational waveforms from the prediction of General Relativity. We use the parameterized post-Einsteinian formalism to characterize the deviations, and assess how much magnitude of the deviations are detectable by using an approximate decision scheme based on Bayesian statistics. We find that there exist detectable regions of the parameterized post-Einsteinian parameters by using a single gravitational wave event. The regions are not excluded by currently existing binary pulsar observations for the parameterized post-Einsteinian parameters at higher post-Newtonian order.

Yuhei Goto, Kazumi Okuyama

We study the false vacuum decay of a single scalar field $\phi$ coupled to gravity described by the Coleman-de Luccia (CdL) instanton. We show that it is possible to numerically calculate the bounce factor, which is related to the CdL tunneling rate, without using the thin-wall approximation. In this paper, we consider $1/\cosh(\phi)$- and $\cos(\phi)$-type potential as examples, which have cosmological and phenomenological applications. Especially, in the $\cos(\phi)$-type potential we show that the range of values in which axion decay constant can take is restricted by the form of the periodic potential if the CdL tunneling occurs.

Cheng Guo, Eric Chitambar, Runyao Duan

Given a set of multipartite entangled states, can we find a common state to prepare them by local operations and classical communication? Such a state, if exists, will be a common resource for the given set of states. We completely solve this problem for bipartite pure states case by explicitly constructing a unique optimal common resource state for any given set of states. In the multipartite setting, the general problem becomes quite complicated, and we focus on finding nontrivial common resources for the whole multipartite state space of given dimensions. We show that GHZ_3 is a nontrivial common resource for three-qubit systems.

Joseph Polchinski

The search for a theory of quantum gravity faces two great challenges: the incredibly small scales of the Planck length and time, and the possibility that the observed constants of nature are in part the result of random processes. A priori, one might have expected these to be insuperable obstacles. However, clues from observed physics, and the discovery of string theory, raise the hope that the unification of quantum mechanics and general relativity is within reach.

E. Fedorova, V.M. Sliusar, V.I. Zhdanov, A.N. Alexandrov, A. Del Popolo, J. Surdej

Extended dark matter (DM) substructures may play the role of microlenses in the Milky Way and in extragalactic gravitational lens systems (GLSs). We compare microlensing effects caused by point masses (Schwarzschild lenses) and extended clumps of matter using a simple model for the lens mapping. A superposition of the point mass and the extended clump is also considered. For special choices of the parameters, this model may represent a cusped clump of cold DM, a cored clump of self-interacting dark matter (SIDM) or an ultra compact minihalo of DM surrounding a massive point-like object. We built the resulting micro-amplification curves for various parameters of one clump moving with respect to the source in order to estimate differences between the light curves caused by clumps and by point lenses. The results show that it may be difficult to distinguish between these models. However, some region of the clump parameters can be restricted by considering the high amplification events at the present level of photometric accuracy. Then we estimate the statistical properties of the amplification curves in extragalactic GLSs. For this purpose, an ensemble of amplification curves is generated yielding the autocorrelation functions (ACFs) of the curves for different choices of the system parameters. We find that there can be a significant difference between these ACFs if the clump size is comparable with typical Einstein radii; as a rule, the contribution of clumps makes the ACFs less steep.

Bin Hu, Marco Raveri, Matteo Rizzato, Alessandra Silvestri

We show how to fully map a specific model of modified gravity into the Einstein-Boltzmann solver EFTCAMB. This approach consists in few steps and allows to obtain the cosmological phenomenology of a model with minimal effort. We discuss all these steps, from the solution of the dynamical equations for the cosmological background of the model to the use of the mapping relations to cast the model into the effective field theory language and use the latter to solve for perturbations. We choose the Hu-Sawicki f(R) model of gravity as our working example. After solving the background and performing the mapping, we interface the algorithm with EFTCAMB and take advantage of the effective field theory framework to integrate the full dynamics of linear perturbations, returning all quantities needed to accurately compare the model with observations. We discuss some observational signatures of this model, focusing on the linear growth of cosmic structures. In particular we present the behavior of $f\sigma_8$ and $E_G$ that, unlike the $\Lambda$CDM scenario, are generally scale dependent in addition to redshift dependent. Finally, we study the observational implications of the model by comparing its cosmological predictions to the Planck 2015 data, including CMB lensing, the WiggleZ galaxy survey and the CFHTLenS weak lensing survey measurements. We find that while WiggleZ data favor a non-vanishing value of the Hu-Sawicki model parameter, $\log_{10}(-f^0_{R})$, and consequently a large value of $\sigma_8$, CFHTLenS drags the estimate of $\log_{10}(-f^0_{R})$ back to the $\Lambda$CDM limit.

Tomoya Kinugawa, Hiroyuki Nakano, Takashi Nakamura

The existence of the ergoregion of the Kerr space-time has not been confirmed observationally yet. We show that the confirmation would be possible by observing the quasinormal mode in gravitational waves. As an example, using the recent population synthesis results of Pop III binary black holes, we find that the peak of the final merger mass ($M_f$) is about $50~\rm M_{\odot}$, while the fraction of the final spin $q_f = a_f/M_f > 0.7$ needed for the confirmation of a part of ergoregion is $\sim 77\%$. To confirm the frequency of the quasinormal mode, ${\rm SNR} > 35$ is needed. The standard model of Pop III population synthesis tells us that the event rate for the confirmation of more than $50\%$ of the ergoregion by the second generation gravitational wave detectors is $\sim 2.3$~${\rm events~yr^{-1}~(SFR_p/(10^{-2.5}~M_\odot~yr^{-1}~Mpc^{-3}))} \cdot (\rm [f_b/(1+f_b)]/0.33)$ where ${\rm SFR_p}$ and ${\rm f_b}$ are the peak value of the Pop III star formation rate and the fraction of binaries, respectively.

Emmanuel N. Saridakis, Catolica), Minas Tsoukalas

We investigate the cosmological applications of new gravitational scalar-tensor theories, which are novel modifications of gravity possessing 2+2 propagating degrees of freedom, arising from a Lagrangian that includes the Ricci scalar and its first and second derivatives. Extracting the field equations we obtain an effective dark energy sector that consists of both extra scalar degrees of freedom, and we determine various observables. We analyze two specific models and we obtain a cosmological behavior in agreement with observations, i.e. transition from matter to dark energy era, with the onset of cosmic acceleration. Additionally, for a particular range of the model parameters, the equation-of-state parameter of the effective dark energy sector can exhibit the phantom-divide crossing. These features reveal the capabilities of these theories, since they arise solely from the novel, higher-derivative terms.

Do Young Kim, Anthony N. Lasenby, Michael P. Hobson

The $R_H = ct$ cosmological model has received considerable attention in recent years owing to claims that it is favoured over the standard $\Lambda$CDM model by most observational data. A key feature of the $R_H = ct$ model is that the zero active mass condition $\rho + 3p = 0$ holds at all epochs. Most recently, Melia has claimed that this condition is a requirement of the symmetries of the Friedmann-Robertson-Walker spacetime. We demonstrate that this claim is false and results from a flaw in the logic of Melia's argument.

S.I. Kruglov

A new model of nonlinear electrodynamics with a dimensional parameter $\beta$ coupled to gravity is considered. We show that an accelerated expansion of the universe takes place if the nonlinear electromagnetic field is the source of the gravitational field. A pure magnetic universe is investigated and the magnetic field drives the universe to accelerate. In this model, after the big bang, the universe undergoes inflation, and the accelerated expansion and then decelerates approaching Minkowski spacetime asymptotically. We demonstrate the causality of the model and a classical stability at the deceleration phase.