Cosmology Theory



Cosmology is a study of the Universe and its origin. Current progress in theory and cosmological observations marks a new era of high precision and big data with a goal of ultimate understanding of the mysterious nature of the dark energy causing accelerate expansion and the dark matter binding together galaxies. Energetic cosmos laboratory theory and data analysis devision is focused on developing new approaches and statistical methods to test the ingredients of our universe and Einstein’s theory of gravity. We use advanced statistical methods which include nonparametric methods and machine learning techniques to analyze cosmological observations of the cosmic microwave background (CMB), gravitational lensing and large scale structure against theoretical constructions and cosmological models.Please see ECL Publications for many of our papers.

Cosmology Projects

Measuring Cosmic Spatial Curvature of the Universe

Cosmic spatial curvature is a fundamental geometric quantity of the Universe. We develop a model independent geometrical approach to measure spatial curvature directly from cosmological observations. The curvature is determined by employing measurements of strong lensing time delays and supernova distances. We propose two complimentary curvature estimators K and Ω k which differ by error propagation characteristics and redshift dependence, as shown in Figures (a) and (b). Our simulations of redshift distributions and distance measurements of lenses and sources showed that the model independent methods can constrain the curvature to ± 0.006 with next generation measurements. See JCAP 1803, 041 (2018) for more details.

(a) The K curvature estimator gives a specific redshift dependence at non-zero curvature.The black solid curve is the theoretical prediction for K and blue points with error bars show the results of our simulated measurements.

(b) The estimation of the value of the curvature Ω k and its consistency with redshift independence. The solid black curve shows the input value Ω k=−0.02. Blue points with error bars show the results of our simulated measurements.

Probing Modified Gravity with the Large Scale Structure Growth

Cosmic growth of large scale structure encodes the information on the history of cosmic expansion and gravitational coupling. Next generation of galaxy redshift surveys will measure cosmic structure at higher redshift and may enable us to test theories of modified gravity against observational data. In particular, measurements of the redshift space distortions of galaxy clustering can be used to directly probe the growth g and growth rate f of cosmic structure. In our framework we incorporate these cosmic growth observables and propose useful parameterizations to discriminate between different classes and models of modified gravity including f(R) and DGP gravity. We derived from the growth equations a one parameter description of the effects of modified gravity at z>3 on growth observables, and we demonstrated numerically a two parameter description of the effects of modified gravity at z<3 on growth observables. This laid out a clear formalism for testing the properties of gravity from the observational data. We combined analytical and numerical approaches to show the possibility of estimating 5-10% deviations in the amplitude of the modified gravitational strength G based on projected uncertainty measurements of the redshift distortions at the Dark Energy Survey Instrument (DESI), see JCAP 1706, 030 (2017). We applied 2-3 parameter modified gravity description to point to specific characteristics of the underlying theory whether the modification is rising, falling, nonmonotonic or multipeaked, for more information check JCAP 1711, 052 (2017).

(c) Isocontours of the deviation in the growth factor δg/g(z = 0) are plotted in the σ-δ G plane, for the gravitational strength modification peaked at at = 0.1. δG describes the amplitude of the deviation in the gravitational strength and σ measures its duration. Dotted curves for the 0.05, 0.1, 0.15, and 0.2 level contours show the analytic prediction.

(d) The accuracy of fitting the observational RSD factor fσ8 with two late time bins for modified gravity δG(a) is compared to that for the exact theory case. The theory model has a Gaussian δG(a) with parameters δG = 0.05, σ = 0.25, and at = 0.3 (dotted red), 0.5 (solid black), 0.7 (dashed blue). The dot dashed green curve shows the at = 0.3 case fit when allowing for a third, early bin due to the early modification.

Signatures of the Early Universe Physics in the B-mode Polarization of the CMB