ABSTRACT
The LIGO and VIRGO Collaborations have recently announced the detection of gravitational waves from a neutron star-neutron star merger (GW170817) and the simultaneous measurement of an optical counterpart (the γ-ray burst GRB 170817A). The close arrival time of the gravitational and electromagnetic waves limits the difference in speed of photons and gravitons to be less than about 1 part in 10^{15}. This has three important implications for cosmological scalar-tensor gravity theories that are often touted as dark energy candidates and alternatives to the Λ cold dark matter model. First, for the most general scalar-tensor theories-beyond Horndeski models-three of the five parameters appearing in the effective theory of dark energy can now be severely constrained on astrophysical scales; we present the results of combining the new gravity wave results with galaxy cluster observations. Second, the combination with the lack of strong equivalence principle violations exhibited by the supermassive black hole in M87 constrains the quartic galileon model to be cosmologically irrelevant. Finally, we derive a new bound on the disformal coupling to photons that implies that such couplings are irrelevant for the cosmic evolution of the field.
ABSTRACT
If the dark matter consists of primordial black holes (PBHs), we show that gravitational lensing of stars being monitored by NASA's Kepler search for extrasolar planets can cause significant numbers of detectable microlensing events. A search through the roughly 150,000 light curves would result in large numbers of detectable events for PBHs in the mass range 5×10(-10) M(â) to 10(-4) M(â). Nondetection of these events would close almost 2 orders of magnitude of the mass window for PBH dark matter. The microlensing rate is higher than previously noticed due to a combination of the exceptional photometric precision of the Kepler mission and the increase in cross section due to the large angular sizes of the relatively nearby Kepler field stars. We also present a new formalism for calculating optical depth and microlensing rates in the presence of large finite-source effects.
ABSTRACT
Modified gravity theories may provide an alternative to dark energy to explain cosmic acceleration. We argue that the observational programme developed to test dark energy needs to be augmented to capture new tests of gravity on astrophysical scales. Several distinct signatures of gravity theories exist outside the 'linear' regime, especially owing to the screening mechanism that operates inside halos such as the Milky Way to ensure that gravity tests in the solar system are satisfied. This opens up several decades in length scale and classes of galaxies at low redshift that can be exploited by surveys. While theoretical work on models of gravity is in the early stages, we can already identify new regimes that cosmological surveys could target to test gravity. These include: (i) a small-scale component that focuses on the interior and vicinity of galaxy and cluster halos, (ii) spectroscopy of low-redshift galaxies, especially galaxies smaller than the Milky Way, in environments that range from voids to clusters, and (iii) a programme of combining lensing and dynamical information, from imaging and spectroscopic surveys, respectively, on the same (or statistically identical) sample of galaxies.
ABSTRACT
A cross-correlation technique of lensing tomography is developed to probe dark energy in the Universe. The variation of weak shear with redshift around foreground galaxies depends only on the angular distances and is robust to the dominant systematic error in lensing. We estimate the margin-alized accuracies that deep lensing surveys with photometric redshifts can provide on the dark energy density Omega(de), the equation of state parameter w, and its evolution w('): sigma(w) approximately equal 0.01f(-1/2)(sky) and sigma(w(')) approximately equal 0.03f(-1/2)(sky), where a prior of sigma(Omega(de))=0.03 is assumed in the marginalization.
