Munch

Recently we studied inflation models in which the inflaton potential is characterized by an underlying approximate global symmetry. In the first work we pointed out that in such a model curvature perturbations are generated after the end of the slow-roll phase of inflation. In this work we develop further the observational implications of the model and compute the degree of non-Gaussianity predicted in the scenario. We find that the corresponding nonlinearity parameter, $f_{NL}$, can be as large as 10^2.

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An Observational Test of Holographic Inflation

Authors: James E. Lidsey, David Seery
Comments: 7 pages, 2 figures. Uses RevTeX4 LaTeX class

Observational consequences of inflationary cosmology in the holographic dual of the Randall-Sundrum type II braneworld scenario, as motivated by the AdS/CFT correspondence, are investigated. High energy corrections to the standard four-dimensional Friedmann equation induce a corresponding modification to the form of the single-field inflationary consistency equation based on Einstein gravity. The degree of departure from the standard expression is determined by the ratio, r, of the primordial tensor and scalar perturbation amplitudes and the coefficient, c, of the conformal anomaly in the dual gauge theory. It is found that a necessary condition for detecting such a correction with the next generation of cosmic microwave background (CMB) polarization experiments is that r >= 0.06. The bound is tightened to r > 0.3 for values of the central charge that are compatible with known compactifications of type IIB string theory as parametrized in terms of F-theory compactification on Calabi-Yau four-folds. This is close to the present upper bounds inferred from combined observations of the CMB anisotropy power spectrum and high redshift surveys. We conclude that if such modifications to the inflationary consistency equation are to be observable, the gravitational wave background should be detected in the near future. A further consequence of the non-standard dynamics at high energies is that the initial state of the universe is a quiescent singularity with a finite density and pressure.

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Low Frequency Radio Observations of Galaxy Clusters Can Reveal Global Signatures in the Cosmological 21-cm Background

Authors: Asantha Cooray
Comments: 4 pages, 3 figures

While the low frequency spectrum of the radio sky is rich in features associated with the cosmic 21-cm background from neutral Hydrogen, a direct measurement of this frequency spectrum is challenging. We propose a technique to indirectly establish global signatures in the cosmic 21-cm radiation based on differential observations towards and away from a galaxy cluster such that modifications imposed by inverse-Compton scattering via electrons in galaxy clusters can be detected. While the unscattered spectrum has features with amplitudes up to 40 mK in the brightness temperature relative to the cosmic microwave background, after scattering, the observable spectrum contains decrements and increments with amplitudes around 0.2 to 1 mK in addition to a uniform few mK decrement associated with the black-body CMB. Low frequency radio interferometers that are now planned for 21-cm anisotropy measurements can be easily utilized for the proposed measurement. The signatures towards clusters will also confirm the 21-cm signal is produced at a redshift higher than the cluster, in addition to providing an easy, and a necessary, cross-check on fluctuation measurements.

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Redshift space 21 cm power spectra from reionization

Authors: Xiaomin Wang, Wayne Hu
Comments: 11 pages, 9 figures. Submitted to ApJ

We construct a simple but self-consistent analytic ionization model for rapid exploration of 21cm power spectrum observables in redshift space. It is fully described by the average ionization fraction $x_e(z)$ and HII patch size $R(z)$ and has the flexibility to accommodate various reionization scenarios. The model associates ionization regions with dark matter halos of the number density required to recover $x_e$ and treats redshift space distortions self-consistently with the virial velocity of such halos. Based on this model, we study the line-of-sight structures in the brightness fluctuations since they are the most immune to foreground contamination. We explore the degeneracy between the HII patch size and nonlinear redshift space distortion in the one dimensional power spectrum. We also discuss the limitations experimental frequency and angular resolutions place on their distinguishability. Angular resolution dilutes even the radial signal and will be a serious limitation for resolving small bubbles before the end of reionization. Nonlinear redshift space distortions suggest that a resolution of order 1 -- 10\arcsec and a frequency resolution of 10kHz will ultimately be desirable to extract the full information in the radial field at $z\sim 10$. First generation instruments such as LOFAR and MWA can potentially measure radial HII patches of a few comoving Mpc and larger at the end of reionization and are unlikely to be affected by nonlinear redshift space distortions.

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Testing f(R) Gravity Against the Large Scale Structure of the Universe

Authors: Pengjie Zhang (SHAO & Fermilab)
Comments: 4 pages, 2 figures. Comments welcome

We solve the field equations of the $f(R)$ gravity using the perturbation theory and investigate the effect of $f(R)$ gravity on the large scale structure (LSS) of the Universe. The $f(R)$ gravity we designed has the form $f(R)=-\lambda_1 H_0^2\exp(-R/\lambda_2H_0^2)$, with $\lambda_2\ga 100$ in order to produce ({\it almost}) degenerate expansion history of the Universe to $\Lambda$CDM, and $\lambda_2\ll 10^{10}$ to pass the solar system tests. This exponential $f(R)$ induces scale dependence in the linear density growth factor and thus causes an observable integrated Sachs-Wolfe (ISW) effect at $z\ga 3$ and $l\ga 20$, which can not be produced by most dark energy models and many modified gravities. Future measurements of ISW-LSS cross correlation at high $z$ will then provide unambiguous tests of gravity. Furthermore, we find that, in general, singularities exist in the perturbation equations of $f(R)$ gravity. This may have already ruled out $f(R)$ gravities as alternatives to dark energy or general relativity.

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Impact of Systematic Errors in Sunyaev-Zel'dovich Surveys of Galaxy Clusters

Authors: Matthew R. Francis (Rutgers University), Rachel Bean (Cornell University), Arthur Kosowsky (University of Pittsburgh)
Comments: 12 pages, 6 figures; accepted to JCAP

Future high-resolution microwave background measurements hold the promise of detecting galaxy clusters throughout our Hubble volume through their Sunyaev-Zel'dovich (SZ) signature, down to a given limiting flux. The number density of galaxy clusters is highly sensitive to cluster mass through fluctuations in the matter power spectrum, as well as redshift through the comoving volume and the growth factor. This sensitivity in principle allows tight constraints on such quantities as the equation of state of dark energy and the neutrino mass. We evaluate the ability of future cluster surveys to measure these quantities simultaneously when combined with PLANCK-like CMB data. Using a simple effective model for uncertainties in the cluster mass-SZ flux relation, we evaluate systematic shifts in cosmological constraints from cluster SZ surveys. We find that a systematic bias of 10% in cluster mass measurements can give rise to shifts in cosmological parameter estimates at levels larger than the $1\sigma$ statistical errors. Systematic errors are unlikely to be detected from the mass and redshift dependence of cluster number counts alone; increasing survey size has only a marginal effect. Implications for upcoming experiments are discussed.

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