Niayesh Afshordi

Cross - correlation of the Cosmic Microwave Background with the 2MASS galaxy survey: Signatures of dark energy, hot gas, and point sources

We cross-correlate the Cosmic Microwave Background (CMB) temperature anisotropies observed by the Wilkinson Microwave Anisotropy Probe (WMAP) with the projected distribution of extended sources in the Two Micron All Sky Survey (2MASS). By modelling the theoretical expectation for this signal, we extract the signatures of dark energy (Integrated Sachs-Wolfe effect;ISW), hot gas (thermal Sunyaev-Zeldovich effect;thermal SZ), and microwave point sources in the cross-correlation. Our strongest signal is the thermal SZ, at the 3.1-3.7 \sigma level, which is consistent with the theoretical prediction based on observations of X-ray clusters. We also see the ISW signal at the 2.5 \sigma level, which is consistent with the expected value for the concordance LCDM cosmology, and is an independent signature of the presence of dark energy in the universe. Finally, we see the signature of microwave point sources at the 2.7 \sigma level.

Primordial non-Gaussianity, statistics of collapsed objects, and the integrated Sachs-Wolfe effect

Any hint of non-Gaussianity in the cosmological initial conditions will provide us with a unique window into the physics of the early Universe. We show that the impact of a small local primordial non-Gaussianity (generated on superhorizon scales) on the statistics of collapsed objects (such as galaxies or clusters) can be approximated by using slightly modified, but Gaussian, initial conditions, which we describe through simple analytic expressions. Given that numerical simulations with Gaussian initial conditions are relatively well studied, this equivalence provides us with a simple tool to predict signatures of primordial non-Gaussianity in the statistics of collapsed objects. In particular, we describe the predictions for non-Gaussian mass function, and also confirm the recent discovery of a nonlocal bias on large scales [N. Dalal, O. Dore, D. Huterer, and A. Shirokov, Phys. Rev. D 77, 123514 (2008).][S. Matarrese and L. Verde, Astrophys. J. 677, L77 (2008).], as a signature of primordial non-Gaussianity. We then study the potential of galaxy surveys to constrain non-Gaussianity using their autocorrelation and cross correlation with the cosmic microwave background (CMB) (due to the integrated Sachs-Wolfe effect), as a function of survey characteristics, and predict that they will eventually yield an accuracy of

Extended Limber Approximation

\ensuremath{\Delta}{f}_{NL}\ensuremath{\sim}0.1

On the stability of Dark Energy with Mass-Varying Neutrinos

and 3, respectively, which will be better than or competitive with (but independent of) the best predicted constraints from the CMB. Interestingly, the cross correlation of the CMB and the NRAO VLA Sky Survey galaxy survey already shows a hint of a large local primordial non-Gaussianity:

Missing Thermal Energy of the Intracluster Medium

{f}_{NL}=236\ifmmode\pm\else\textpm\fi{}127

Cuscuton and low-energy limit of Horava-Lifshitz gravity

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Integrated Sachs-Wolfe effect in cross-correlation: The observer's manual

We develop a systematic derivation for the Limber approximation to the angular cross-power spectrum of two random fields, as a series expansion in (l+1/2){sup -1}. This extended Limber approximation can be used to test the accuracy of the Limber approximation and to improve the rate of convergence at large ls. We show that the error in ordinary Limber approximation is O(l{sup -2}). We also provide a simple expression for the 2nd order correction to the Limber formula, which improves the accuracy to O(l{sup -4}). This correction can be especially useful for narrow redshift bins, or samples with small redshift overlap, for which the 0th order Limber formula has a large error. We also point out that using l instead of l+1/2, as is often done in the literature, spoils the accuracy of the approximation to O(l{sup -1})

Primordial black holes as dark matter: The Power spectrum and evaporation of early structures

An interesting dynamical model for dark energy which does not require extremely light scalar fields such as quintessence, and at the same time explains the (near) coincidence between the neutrino and dark energy densities is the model of dark energy coupled to mass-varying neutrinos (MaVaNs). Despite the attractions of this model, we show that, generically, this model contains a catastrophic instability which occurs when neutrinos become nonrelativistic. As a result of this instability, as neutrinos become nonrelativistic, they condense into neutrino nuggets which redshift away similar to cold dark matter, and thus cease to act as dark energy. Any stable MaVaNs dark energy model is extremely contrived and is virtually indistinguishable from a cosmological constant.

Cuscuton: A Causal Field Theory with an Infinite Speed of Sound

The Sunyaev‐Zel’dovich (SZ) effect is a direct probe of thermal energy content of the Universe, induced in the cosmic microwave background (CMB) sky through scattering of CMB photons off hot electrons in the intracluster medium (ICM). We report a 9σ detection of the SZ signal in the CMB maps of Wilkinson Microwave Anisotropy Probe (WMAP) three-year data, through study of a sample of 193 massive galaxy clusters with observed X-ray temperatures greater than 3 keV. For the first time, we make a model-independent measurement of the pressure profile in the outskirts of the ICM, and show that it closely follows the profiles obtained by X-ray observations and numerical simulations. We find that our measurements of the SZ effect would account for only half of the thermal energy of the cluster, if all the cluster baryons were in the hot ICM phase. Our measurements indicate that a significant fraction, 35 ± 8 per cent, of baryonic mass is missing from the hot ICM, and thus must have cooled to form galaxies, intracluster stars, or an unknown cold phase of the ICM. There does not seem to be enough mass in the form of stars or cold gas in the cluster galaxies or intracluster space, signalling the need for a yet-unknown baryonic component (at 3σ level), or otherwise new astrophysical processes in the ICM.

Bypass to Turbulence in Hydrodynamic Accretion: Lagrangian Analysis of Energy Growth

Horava-Lifshitz theory has been recently put forth as a proposal for a renormalizable theory of quantum gravity 1. It explicitly breaks Lorentz invariance, introducing an apparent extra scalar degree of freedom. I show that the low energy limit of (non-projectible) Horava-Lifshitz gravity is uniquely given by the quadratic cuscuton model: a covariant scalar field theory with an infinite speed of sound and a quadratic potential, minimally coupled to Einstein gravity. This implies that the extra scalar is nondynamical to all orders in perturbation theory. Cosmological constraints on the quadratic cuscuton model constrain the low energy Lorentz breaking parameter of Horava-Lifshitz theory to |{lambda}-1|<0.014. We also point out that the spatial hypersurfaces are constant mean curvature or uniform expansion surfaces and introduce geometrical symmetries that can protect the nondynamical nature of these theories.