Research Interests

"My goal is simple. It is a complete understanding of the universe, why it is as it is and why it exists at all." - Stephen Hawking

  • Large-Angle Anomalies in the Cosmic Microwave Background

  •       Several anomalies in the cosmic microwave background (CMB) maps, observed by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite, have been discussed in the literature. For example, when derived from the full CMB sky, the two lowest cosmologically interesting multipoles, the quadrupole (l=2) and the octopole (l=3), are unexpectedly aligned with each other (as shown below):

    Quadrupole-Octopole Alignment
                                                                                                                                     Credit: Copi et al. (2006)

    The above figure (presented in Galactic coordinates) shows the l=2+3 multipoles from the TOH cleaned map, computed using the multipole vector formalism (Copi et al. 2006). The quadrupole vectors are plotted as the solid red diamond and their normal is the open red diamond. The octopole vectors are the solid magenta diamonds and their three normals are the open magenta diamonds. One can clearly see that the normal (open red diamond) to the quadrupole plane and the three normals (open magenta diamonds) to the octopole planes are aligned with each other in the above map.

    Another particularly puzzling feature is that the two-point angular correlation function of the temperature of the CMB shows significantly lower large-angle (greater than about 60 degrees) correlations than expected from the standard inflationary cosmological model (as shown below):

    Low Power on Large Angular Scales
                                                                                                                               Source: Sarkar et al. (2010)

    In Sarkar et al. (2010), we conclusively demonstrate that, assuming Gaussian random and statistically isotropic CMB anisotropies, there is no statistically significant correlation between the missing power on large angular scales in the CMB and the alignment of the quadrupole and octopole. The chance to measure the sky with both such a lack of large-angle correlation and such an alignment of the low multipoles is thus less than 0.0001% likely. Given that both anomalies occur at the largest observable scales and are correlated with special directions in the sky (ecliptic and/or dipole), they clearly require a causal explanation.

  • Dark Energy: Uncorrelated Estimates of the Equation of State

  •       In 1998, exactly a decade ago, observations of type Ia supernovae (the best known standardizable candles), led by two independent groups: the High-z Supernova Search Team and the Supernova Cosmology Project, provided the first observational evidences in favour of an accelerating expansion of the universe. One of the most viable explanations for this accelerated expansion entails an additional component in the energy budget of the universe, known as the dark energy. Although the existence of dark energy has been corroborated by several independent studies over the past decade, understanding the fundamental nature of this mysterious component is still one of the major challenges in cosmology. In order to learn more about this mysterious candidate, a number of current and future observational studies are aimed at constraining its equation of state (EOS; the ratio of its pressure to its energy density) with unprecedented precision (please refer to the report of the Dark Energy Task Force for further details).
          In Sarkar, Sullivan, Joudaki, Amblard, Holz, and Cooray (2008), we have tried to motivate the community to use uncorrelated binned estimates of the dark energy EOS by showing that three or more independent decorrelated binned estimates of the EOS can be obtained by combining future surveys.  We encourgae other groups to make use of our wzBinned code, which is capable of extracting decorrelated estimates of the EOS from a combination of data sets.
          The most recent (and much improved) constraint on the Hubble constant and its effect on the measurement of the dark energy equation of state parameters are presented in Riess et al. (2009).
          Applying a similar approach described above to a wide range of cosmological data, we have shown, in Serra et al. (2009), that there is no significant evidence for evolving dark energy; the data remains completely consistent with a cosmological constant.

  • Dark Energy and SNe Ia: Systematics! Systematics! Systematics!

  •       Observations of the type Ia supernovae (SNe Ia) provide the most direct way to probe dark energy. Assuming that they are good standardizable cadles, one can easily determine the expansion history of the universe by mapping out their distance-redshift relation. However, one has to be careful about the possible systematic uncertainties. According to some recent claims, the supernova population consists of two components, with a "prompt" component proportional to the instantaneous host galaxy star formation rate, and a "delayed" component that is delayed by several Gyrs; the prompt component being expected to be more luminous. This systematic difference in the intrinsic luminosity could conceivably be calibrated out using lightcurve calibration technique. However, a residual in the calibrated luminosity could potentially remain, leading to a redshift-dependent shift in the Hubble diagram, and systematic errors in the best-fit cosmological parameters.
          In Sarkar, Amblard, Cooray, and Holz (2008), we have modeled the expected systematic effect, constraining the magnitude of the effect with current data, and exploring the effects of a two population systematic on the determination of cosmological parameters with future data.

  • Weak Lensing of Supernovae

  •      Although type Ia supernovae have been shown to be good standardizable candles, the distance estimate to a given supernova is degraded due to gravitational lensing of its flux. The gravitational magnification and demagnification of type Ia supernovae modify their positions on the Hubble diagram, shifting the distance estimates from the underlying luminosity-distance relation. This can introduce a systematic uncertainty in the dark energy equation of state (EOS) estimated from supernovae, although this systematic is expected to average away for sufficiently large data sets.
          In Sarkar, Amblard, Holz, and Cooray (2008), we have quantified the bias introduced in the estimation of the dark energy EOS due to weak lensing of supernova flux, considering the effects due to the non-Gaussian nature of the lensing magnification distributions.

  • Gravitational Waves and Cosmic Shear

  •       Weak gravitational lensing by foreground density perturbations generates a gradient mode in the shear of background images. In contrast, cosmological tensor perturbations induce a non-zero curl mode associated with image rotations.
          In Sarkar, Serra, Cooray, Ichiki, and Baumann (2008), we have studied the lensing signatures of both primordial gravitational waves from inflation and second-order gravitational waves generated from the observed spectrum of primordial density fluctuations.

  • Weak Lensing and CMB Bispectrum

  •       The cosmic microwave background (CMB) bispectrum is a well-known probe of the non- Gaussianity of primordial perturbations. Just as the intervening large-scale structure modifies the CMB angular power spectrum through weak gravitational lensing, the CMB primary bispectrum generated at the last scattering surface is also modified by lensing. For a high resolution experiment such as Planck, the lensing modification to the bispectrum must be properly included when attempting to estimate the primordial non-Gaussianity.
          In Cooray, Sarkar, and Serra (2008), we have discussed the lensing modification to the CMB bispectrum, showing that lensing leads to an overall decrease in the amplitude of the primary bispectrum at the multipoles of interest.

  • Velocity Fields

  •       The analysis of peculiar velocity fields of galaxies and clusters is one of the most effective ways of probing mass fluctuations on ~100 h-1 Mpc scales. Studies of peculiar velocities can be used to constrain the amplitude of mass power spectrum on scales others than those probed by redshift surveys and those sampled by anisotropies in the CMB. The present generation of redshift-distance surveys consist of larger and higher-quality data sets of both spiral and early-type galaxies. These new samples pave the path toward a possible resolution of many discrepancies found in earlier samples; however, some quantitative disagreements persist.
          Our analysis of the theoretically expected correlation between the estimates of the bulk flows of samples of galaxies in several recent surveys, presented in Sarkar, Feldman, and Watkins (2007), supports the notion that we have reached an era where velocity field data is consistent and robust across morphological types, selection criteria, survey geometry etc.

Yakko's Universe Song; From the TV Show: "Animaniacs"