* Astronomy

Title: A Detection of the Cold Imprint of Voids on the Microwave Background Radiation
Authors: Yan-Chuan Cai (Durham), Mark C. Neyrinck (JHU), Istvan Szapudi (IfA Hawaii), Shaun Cole (Durham), Carlos S. Frenk (Durham)

We measure the average cold spot on the cosmic microwave background(CMB) produced by voids selected in the SDSS DR7 spectroscopic redshift galaxy catalogue, spanning redshifts from 0 to 0.4. Our detection has a significance of ~3sigma based on the variance of random samples, and has an average amplitude of ~3 muK as viewed through a compensated top-hat filter scaled to the radius of each void. This signal, if interpreted as the late-time Integrated Sachs-Wolfe effect, serves as an evidence for the late-time acceleration of the Universe. The detection is achieved by applying the optimal filter size identified from N-body simulations. Two striking features are found by comparing ISW simulations with our detection, 1.) the void profiles traced by halos in our simulations are very similar to those in the data traced by galaxies. 2.) the same filter radius that gives the largest ISW signal in simulations also yields the largest detected signal in the observations. We model the expected ISW signal using voids from N-body simulations in LCDM, selected in the same way as in the observations. The detected signal, however, is many times larger than that from simulations, discrepant at the ~3sigma level. The large cosmic variance of large-scale modes in the gravitational potential can obscure an ISW measurement such as ours. However, we show how this cosmic variance can be effectively reduced by using a compensated top-hat filter for the detection. We test whether a few possible systematic effects could be producing the signal; we find no evidence that they do.

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Title: A Theory of a Spot
Authors: Niayesh Afshordi (Perimeter/Waterloo), Ane Slosar (Brookhaven), Yi Wang (McGill)
(Version v2)

We present a simple inflationary scenario that can produce arbitrarily large spherical underdense or overdense regions embedded in a standard Lambda cold dark matter paradigm, which we refer to as bubbles. We analyse the effect such bubbles would have on the Cosmic Microwave Background (CMB). For super-horizon sized bubble in the vicinity of the last scattering surface, a signal is imprinted onto CMB via a combination of Sach-Wolfe and an early integrated Sach-Wolfe (ISW) effects. Smaller, sub-horizon sized bubbles at lower redshifts (during matter domination and later) can imprint secondary anisotropies on the CMB via Rees-Sciama, late-time ISW and Ostriker-Vishniac effects. Our scenario, and arguably most similar inflationary models, produce bubbles which are over/underdense in potential: in density such bubbles are characterized by having a distinct wall with the interior staying at the cosmic mean density. We show that such models can potentially, with only moderate fine tuning, explain the cold spot, a non-Gaussian feature identified in the Wilkinson Microwave Anisotropy Probe (WMAP) data by several authors. However, more detailed comparisons with current and future CMB data are necessary to confirm (or rule out) this scenario.

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Title: The Cold Spot as a Large Void: Lensing Effect on CMB Two and Three Point Correlation Functions
Authors: Isabella Masina, Alessio Notari
(Version v3)

The "Cold Spot" in the CMB sky could be due to the presence of an anomalous huge spherical underdense region - a "Void" - of a few hundreds Mpc/h radius. Such a structure would have an impact on the CMB two-point (power spectrum) and three-point (bispectrum) correlation functions not only at low-l, but also at high-l through Lensing, which is a unique signature of a Void. Modelling such an underdensity with an LTB metric, we show that for the power spectrum the effect should be visible already in the WMAP data only if the Void radius is at least L \gtrsim 1 Gpc/h, while it will be visible by the Planck satellite if L \gtrsim 500 Mpc/h. We also speculate that this could be linked to the high-l detection of an hemispherical power asymmetry in the sky. Moreover, there should be non-zero correlations in the non-diagonal two-point function. For the bispectrum, the effect becomes important for squeezed triangles with two very high l's: this signal can be detected by Planck if the Void radius is at least L \gtrsim 300 Mpc/h, while higher resolution experiments should be able to probe the entire parameter space. We have also estimated the contamination of the primordial non-Gaussianity f_NL due to this signal, which turns out to be negligible.

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Title: A Theory of a Spot
Authors: Niayesh Afshordi (Perimeter/Waterloo), Ane Slosar (Brookhaven), Yi Wang (McGill)

present a simple model of inflation that can produce arbitrarily large spherical underdense or overdense regions embedded in a standard Lambda cold dark matter paradigm, which we refer to as bubbles. We analyse the effect such bubbles would have on the Cosmic Microwave Background (CMB). For super-horizon sized bubble in the vicinity of the last scattering surface, a signal is imprinted onto CMB via a combination of Sach-Wolfe and an early integrated Sach-Wolfe (ISW) effects. Smaller, sub-horizon sized bubbles at lower redshifts (during matter domination and later) can imprint secondary anisotropies on the CMB via Rees-Sciama, late-time ISW and Ostriker-Vishniac effects. Our model, and arguably most similar inflationary models, produce bubbles which are over/underdense in potential: in density such bubbles are characterized by having a distinct wall with the interior staying at the cosmic mean density. We show that such models can, with only moderate fine tuning, explain the cold spot, a non-Gaussian feature identified in the Wilkinson Microwave Anisotropy Probe (WMAP) data by several authors.

Read more (459kb, PDF)