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CORNELL LABORATORY FOR ACCELERATOR-BASED SCIENCES AND EDUCATION

CLASSE NEWS | 11 Sep 2014

A promising approach for testing gravity with CMB measurements

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Predicted pairwise velocity as a function of distance between galaxy clusters for different values of the dark energy equation of state and of the modified gravity parameter, γ, where γ=0.55 corresponds to General Relativity. Upcoming measurements of the CMB combined with optical measurements of galaxies should be able to differentiate between these scenarios.

In the standard cosmological model the accelerated expansion of the universe is driven by a dark form of energy, whose nature is not understood. In the last decade several models have been proposed to explain the accelerated expansion without this mysterious form of energy. One of the most appealing possibilities, not ruled out by current data, are models of modified gravity that introduce modification to the law of gravity on cosmological scales. Determining if the accelerated expansion of the universe is due to an exotic form of energy or if it can be explained with modifications to the law of gravity is one of the primary goals of modern cosmology. In a recent paper (available here as a preprint) CLASSE researchers Dr. Francesco De Bernardis and Prof. Michael Niemack worked with graduate student Eva-Maria Mueller and Prof. Rachel Bean (Astronomy) to investigate the possibility of distinguishing modifications to gravity from dark energy using measurements of the cosmic microwave background (CMB). The method uses a combination of CMB data and data from large galaxy surveys. CMB photons passing through hot gas in galaxy clusters undergo Compton scattering that causes a spectral shift of the CMB in the direction of the cluster. This effect, known as the Sunyaev-Zel'dovich (SZ) effect, has a thermal component and a kinematic component (kSZ). The kSZ effect is proportional to the momentum of the galaxy cluster along the line of sight, which is ultimately determined by the way gravity acts on very large scales. In particular the relative motion of pairs of clusters as a function of their separation (pairwise velocity) provides an important test for cosmology and gravity. By identifying galaxy clusters and their redshifts in galaxy surveys and observing the kSZ effect in the direction of these clusters using CMB surveys, it is possible to measure the pairwise velocity as a function of the separation between clusters. This is the same approach used by Niemack and colleagues in the first detection of the kSZ effect in 2012. The new paper describes analysis of the constraints on gravity and dark energy achievable with this approach using current and upcoming CMB and galaxy surveys. Several sources of systematic uncertainties have been considered, such as uncertainties in the electron density inside the cluster, which is necessary in order to extract velocities from the kSZ effect, and uncertainties in the minimum detectable mass of galaxy clusters. The results show that kSZ measurements of cluster peculiar velocities from CMB data like those of the Atacama Cosmology Telescope Polarimeter (ACTPol) and Advanced ACTPol, obtained with this approach, have the potential to provide a useful and independent test of dark energy and theories of gravity on cosmic scales.