Direct reconstruction of dark energy

An important issue in cosmology is reconstructing the effective dark energy equation of state directly from observations. With so few physically motivated models, future dark energy studies cannot only be based on constraining a dark energy parameter space. We present a new non-parametric method which can accurately reconstruct a wide variety of dark energy behaviour with no prior assumptions about it. It is simple, quick and relatively accurate, and involves no expensive explorations of parameter space. The technique uses principal component analysis and a combination of information criteria to identify real features in the data, and tailors the fitting functions to pick up trends and smooth over noise. We find that we can constrain a large variety of w(z) models to within 10-20 % at redshifts z<1 using just SNAP-quality data.Comment: 5 pages, 4 figures. v2 has added refs plus minor changes. To appear in PR

Vector modes generated by primordial density fluctuations

While vector modes are usually ignored in cosmology since they are not produced during inflation they are inevitably produced from the interaction of density fluctuations of differing wavelengths. This effect may be calculated via a second-order perturbative expansion. We investigate this effect during the radiation era. We discuss the generation mechanism by investigating two scalar modes interacting, and we calculate the power of vector modes generated by a power-law spectrum of density perturbations on all scales.Comment: 10 pages, 2 figures, minor changes in main text and new appendix added to match the accepted version for Physical Review D publicatio

The cosmological gravitational wave background from primordial density perturbations

We discuss the gravitational wave background generated by primordial density perturbations evolving during the radiation era. At second-order in a perturbative expansion, density fluctuations produce gravitational waves. We calculate the power spectra of gravitational waves from this mechanism, and show that, in principle, future gravitational wave detectors could be used to constrain the primordial power spectrum on scales vastly different from those currently being probed by large-scale structure. As examples we compute the gravitational wave background generated by both a power-law spectrum on all scales, and a delta-function power spectrum on a single scale.Comment: 8 Page

A litmus test for Lambda

The critical issue in cosmology today lies in determining if the cosmological constant is the underlying ingredient of dark energy. Our profound lack of understanding of the physics of dark energy places severe constrains on our ability to say anything about its possible dynamical nature. Quoted errors on the equation of state, w(z), are so heavily dependent on necessarily over-simplified parameterisations they are at risk of being rendered meaningless. Moreover, the existence of degeneracies between the reconstructed w(z) and the matter and curvature densities weakens any conclusions still further. We propose consistency tests for the cosmological constant which provide a direct observational signal if Lambda is wrong, regardless of the densities of matter and curvature. As an example of its utility, our flat case test can warn of a small transition from w(z)=-1 of 20% from SNAP quality data at 4-sigma, even when direct reconstruction techniques see virtually no evidence for deviation from Lambda. It is shown to successfully rule out a wide range of non-Lambda dark energy models with no reliance on knowledge of Omega_m using SNAP-quality data and a large range for using 10^5 supernovae as forecasted for LSST.Comment: 4 pages, 2 figure

Galaxy correlations and the BAO in a void universe: structure formation as a test of the Copernican Principle

A suggested solution to the dark energy problem is the void model, where accelerated expansion is replaced by Hubble-scale inhomogeneity. In these models, density perturbations grow on a radially inhomogeneous background. This large scale inhomogeneity distorts the spherical Baryon Acoustic Oscillation feature into an ellipsoid which implies that the bump in the galaxy correlation function occurs at different scales in the radial and transverse correlation functions. We compute these for the first time, under the approximation that curvature gradients do not couple the scalar modes to vector and tensor modes. The radial and transverse correlation functions are very different from those of the concordance model, even when the models have the same average BAO scale. This implies that if void models are fine-tuned to satisfy average BAO data, there is enough extra information in the correlation functions to distinguish a void model from the concordance model. We expect these new features to remain when the full perturbation equations are solved, which means that the radial and transverse galaxy correlation functions can be used as a powerful test of the Copernican Principle.Comment: 12 pages, 8 figures, matches published versio

A gravitational wave window on extra dimensions

We report on the possibility of detecting a submillimetre-sized extra dimension by observing gravitational waves (GWs) emitted by pointlike objects orbiting a braneworld black hole. Matter in the `visible' universe can generate a discrete spectrum of high frequency GWs with amplitudes moderately weaker than the predictions of general relativity (GR), while GW signals generated by matter on a `shadow' brane hidden in the bulk are potentially strong enough to be detected using current technology. We know of no other astrophysical phenomena that produces GWs with a similar spectrum, which stresses the need to develop detectors capable of measuring this high-frequency signature of large extra dimensions.Comment: 9 pages, 5 figure

Delocalization of brane gravity by a bulk black hole

We investigate the analogue of the Randall-Sundrum brane-world in the case when the bulk contains a black hole. Instead of the static vacuum Minkowski brane of the RS model, we have an Einstein static vacuum brane. We find that the presence of the bulk black hole has a dramatic effect on the gravity that is felt by brane observers. In the RS model, the 5D graviton has a stable localized zero-mode that reproduces 4D gravity on the brane at low energies. With a bulk black hole, there is no such solution -- gravity is delocalized by the 5D horizon. However, the brane does support a discrete spectrum of metastable massive bound states, or quasinormal modes, as was recently shown to be the case in the RS scenario. These states should dominate the high frequency component of the bulk gravity wave spectrum on a cosmological brane. We expect our results to generalize to any bulk spacetime containing a Killing horizon.Comment: 7 pages, 6 figure

Beyond the plane-parallel and Newtonian approach: Wide-angle redshift distortions and convergence in general relativity

We extend previous analyses of wide-angle correlations in the galaxy power spectrum in redshift space to include all general relativistic effects. These general relativistic corrections to the standard approach become important on large scales and at high redshifts, and they lead to new terms in the wide-angle correlations. We show that in principle the new terms can produce corrections of nearly 10 % on Gpc scales over the usual Newtonian approximation. General relativistic corrections will be important for future large-volume surveys such as SKA and Euclid, although the problem of cosmic variance will present a challenge in observing this.Comment: 14 pages, 5 figures; Typo in equation 5 corrected; results unaffecte

The isotropic blackbody CMB as evidence for a homogeneous universe

The question of whether the Universe is spatially homogeneous and isotropic on the largest scales is of fundamental importance to cosmology, but has not yet been answered decisively. Surprisingly, neither an isotropic primary CMB nor combined observations of luminosity distances and galaxy number counts are sufficient to establish such a result. The inclusion of the Sunyaev-Zel'dovich effect in CMB observations, however, dramatically improves this situation. We show that even a solitary observer who sees an isotropic blackbody CMB can conclude that the universe is homogeneous and isotropic in their causal past when the Sunyaev-Zel'dovich effect is present. Critically, however, the CMB must either be viewed for an extended period of time, or CMB photons that have scattered more than once must be detected. This result provides a theoretical underpinning for testing the Cosmological Principle with observations of the CMB alone.Comment: 5 pages, 1 figur