Early Universe with CMB polarization

The Universe is the grandest conceivable scale on which the human mind can strive to understand nature. The amazing aspect of cosmology, the branch of science that attempts to understand the origin and evolution of the Universe, is that it is largely comprehensible by applying the same basic laws of physics that we use for other branches of physics. The observed cosmic microwave background (CMB) is understood by applying the basic laws of radiative processes and transfer, masterfully covered in the classic text by S. Chandrasekhar, in the cosmological context. In addition to the now widely acclaimed temperature anisotropy, there is also linear polarization information imprinted on the observed Cosmic Microwave background. CMB polarization already has addressed, and promises to do a lot more, to unravel the deepest fundamental queries about physics operating close to the origin of the Universe.Comment: 19 pages, 6 figures, Invited contribution : Special Chandra Centennial issue of the Bulletin of the Astronomical Society of India (BASI), [Ed. Virginia Trimble & D J S Saikia

Suppressing CMB low multipoles with ISW effect

Recent results of Planck data reveal that the power in the low multipoles of the CMB angular power spectrum, approximately up to $l=30$, is significantly lower than the theoretically predicted in the best fit $\Lambda$CDM model. In this paper we investigate the possibility of invoking the Integrated Sachs-Wolfe (ISW) effect to explain this power deficit at low multipoles. The ISW effect that originates from the late time expansion history of the universe is rich in possibilities given the limited understanding of the origin of dark energy (DE). It is a common understanding that the ISW effect adds to the power at the low multipoles of the CMB angular power spectrum. In this paper we carry out an analytic study to show that there are some expansion histories in which the ISW effect, instead of adding power, provides negative contribution to the power at low multipoles. Guided by the analytic study, we present examples of the features required in the late time expansion history of the universe that could explain the power deficiency through the ISW effect. We also show that an ISW origin of power deficiency is consistent, at present, with other cosmological observations that probe the expansion history such as distance modulus, matter power spectrum and the evolution of cluster number count. We also show that the ISW effect may be distinguished from power deficit originating from features in the PPS using the measurements of the CMB polarization spectrum at low multipoles expected from Planck. We conclude that the power at low multipoles of the CMB anisotropy could well be closely linked to Dark Energy puzzle in cosmology and this observation could be actually pointing to richer phenomenology of DE beyond the cosmological constant $\Lambda$. (abbreviated)Comment: 20 pages, 7 figure

Litmus Test for Cosmic Hemispherical Asymmetry in the Cosmic Microwave Background B-mode polarization

Recent measurements of the temperature field of Cosmic Microwave Background (CMB) provide tantalising evidence for violation of Statistical Isotropy (SI) that constitutes a fundamental tenet of contemporary cosmology. CMB space based missions, WMAP and Planck have observed a $7\%$ departure in the SI temperature field at large angular scales. However, due to higher cosmic variance at low multipoles, the significance of this measurement is not expected to improve from any future CMB temperature measurements. We demonstrate that weak lensing of the CMB due to scalar perturbations produce a corresponding SI violation in $B$ modes of CMB polarization at smaller angular scales. Measurability of this phenomenon depends upon the scales ($l$ range) over which power asymmetry is present. Power asymmetry which is restricted only to $l<64$ in temperature field cannot lead to any significant observable effect from this new window. However, this effect can put an independent bound on the spatial range of scales of hemispherical asymmetry present in scalar sector.Comment: 7 pages, 2 figures. Matches the published versio

Generation of seed perturbations from Quantum Cosmology

The origin of seed perturbations in the Universe is studied within the framework of a specific minisuperspace model. It is shown that the `creation' of the Universe as a result of a quantum transition from a flat empty spacetime would lead to a flat FLRW (Friedmann Lema\^\i tre Robertson-Walker) Universe with weak inhomogeneous perturbations at large wavelengths. The power spectrum of these perturbations is found to be scale invariant at horizon crossing (i.e., the Harrison-Zeldovich spectrum). It is also recognised that the seed perturbations generated in our model would be generically of the isocurvature kind.Comment: Plain Tex 24 Page

Revised cosmological parameters after BICEP 2 and BOSS

Estimation of parameters of the \lq standard\rq \,model of cosmology have dramatically improved over past few decades due to increasingly exquisite measurements made by Cosmic Microwave Background (CMB) experiments. Recent data from Planck matches well with the minimal $\Lambda$CDM model. A likelihood analysis using Planck, WMAP and a selection of high resolution experiments (highL), tensor to scalar ratio $r_{0.002}$ is found to be $<0.11$ when $dn_{s}/d\ln k = 0$. Planck also imposes an upper bound on neutrino mass $\sum m_\nu<0.23\,$eV using Planck+WMAP+highL+BAO likelihood. However, recently results from BICEP 2 claims the detection of $r= 0.2^{+0.07}_{-0.05}$ from polarization spectra. Further, results from SDSS-III BOSS large scale galaxy survey constrains the total neutrino mass to $\sum m_\nu=0.36 \pm 0.10$ eV. It is important to study the consequences of these new measurements on other cosmological parameters. In this paper we assess the revised constraints on cosmological parameters in light of these two measurements that are in some tension with the constraints from Planck. Using the prior on $\sum m_\nu$ as measured by SDSS-III BOSS and BICEP 2 likelihood, we find that the model with running spectral index ($dn_{s}/d\ln k \neq 0$) leads to a value of $A_L>1$ at $3.1 \sigma$. But, the model with $dn_{s}/d\ln k =0$ makes $A_L$ consistent with $1$, at $2.1\sigma$ and also shows that $N_{\rm eff}$ is consistent with its theoretical value of $3.046$ at around $2\sigma$. Therefore, the analysis in this paper shows that the model with $dn_{s}/d\ln k =0$ gives consistency with other cosmological parameters ($N_{\rm eff}$ and $A_L$ ) when the current limits on $\sum m_\nu$ and $r_{0.05}$ are considered. However, on reducing the value of $r_{0.05}$, the model with non-zero $dn_{s}/d\ln k$ gives consistent result of $A_L =1$ [abridged].Comment: 14 pages, 10 figures. Matches the published versio

Features in the primordial power spectrum? A frequentist analysis

Features in the primordial power spectrum have been suggested as an explanation for glitches in the angular power spectrum of temperature anisotropies measured by the WMAP satellite. However, these glitches might just as well be artifacts of noise or cosmic variance. Using the effective Delta chi^2 between the best-fit power-law spectrum and a deconvolved primordial spectrum as a measure of "featureness" of the data, we perform a full Monte-Carlo analysis to address the question of how significant the recovered features are. We find that in 26% of the simulated data sets the reconstructed spectrum yields a greater improvement in the likelihood than for the actually observed data. While features cannot be categorically ruled out by this analysis, and the possibility remains that simple theoretical models which predict some of the observed features might stand up to rigorous statistical testing, our results suggest that WMAP data are consistent with the assumption of a featureless power-law primordial spectrum.Comment: 17 pages, 3 figures; v2: minor changes, matches published versio