Authors: Hee-Jong Seo, Masanori Sato, Scott Dodelson, Bhuvnesh Jain, Masahiro Takada
http://arxiv.org/abs/1008.0349v1
http://arxiv.org/abs/1008.0349v1
Gravitational lensing of distant galaxies can be exploited to infer the convergence field as a function of angular position on the sky. The statistics of this field, much like that of the cosmic microwave background (CMB), can be studied to extract information about fundamental parameters in cosmology, most notably the dark energy in the Universe. Unlike the CMB, the distribution of matter in the Universe which determines the convergence field is highly non-Gaussian, reflecting the nonlinear processes which accompanied structure formation. Much of the cosmic information contained in the initial field is therefore unavailable to the standard power spectrum measurements. Here we propose a method for re-capturing cosmic information by using the power spectrum of a simple function of the observed (nonlinear) convergence field. We adapt the approach of Neyrinck et al. (2009) to lensing by using a modified logarithmic transform of the convergence field. The Fourier transform of the log-transformed field has modes that are nearly uncorrelated, which allows for additional cosmological information to be extracted from small-scale modes.
Pulsar timing array observations of gravitational wave source timing parallax
Pulsar timing arrays act to detect gravitational waves by observing the small, correlated effect the waves have on pulse arrival times at Earth. This effect has conventionally been evaluated assuming the gravitational wave phasefronts are planar across the array, an assumption that is valid only for sources at distances $R\gg2\pi{}L^2/\lambda$, where $L$ is physical extent of the array and $\lambda$ the radiation wavelength. In the case of pulsar timing arrays (PTAs) the array size is of order the pulsar-Earth distance (kpc) and $\lambda$ is of order pc. Correspondingly, for point gravitational wave sources closer than $\sim100$~Mpc the PTA response is sensitive to the source parallax across the pulsar-Earth baseline. Here we evaluate the PTA response to gravitational wave point sources including the important wavefront curvature effects. Taking the wavefront curvature into account the relative amplitude and phase of the timing residuals associated with a collection of pulsars allows us to measure the distance to, and sky position of, the source.
The dynamics of metric-affine gravity
Metric-affine theories of gravity provide an interesting alternative to General Relativity: in such an approach, the metric and the affine (not necessarily symmetric) connection are independent quantities. Furthermore, the action should include covariant derivatives of the matter fields, with the covariant derivative naturally defined using the independent connection. As a result, in metric-affine theories a direct coupling involving matter and connection is also present. The role and the dynamics of the connection in such theories is explored. We employ power counting in order to construct the most general action and search for the minimal requirements it should satisfy for the connection to be dynamical. We find that for the most general action containing lower order invariants the independent connection does not carry any dynamics. It actually reduces to the role of an auxiliary field and can be completely eliminated algebraically in favour of the metric and the matter field, introducing extra interactions with respect to general relativity. However, we also show that including higher order terms in the action radically changes this picture and excites new degrees of freedom in the connection, making it (or parts of it) dynamical. Constructing actions that constitute exceptions to this rule requires significant fine tuned and/or extra {\em a priori} constraints on the connection. We also consider f(R) actions as a particular example in order to show that they constitute a distinct class of metric-affine theories with special properties, and as such they cannot be used as representative toy theories to study the properties of metric-affine gravity.
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