Thématique : Cosmologie observationnelle
<bold>Scientific context of the project</bold>
Voids are very low-density environments in the large-scale distribution of matter in the Universe. As they are nearly devoid of matter, voids constitute a promising new probe for testing cosmological models and theories of gravity (Baker et al. 2018; Cai 2018; Hamaus et al. 2016). With the tremendous progress of large galaxy surveys in recent years, the study of cosmic voids has entered the observational domain of precision cosmology (Lavaux and Wandelt 2012).
Following on clustering studies applied to cosmic voids, as Alcock-Paczynski effect (Hamaus et al. 2016; Sutter et al. 2012) and redshift-space distortions (RSD) around voids (Hamaus et al. 2017; Hawken et al. 2017), a powerful approach consists in cross-correlating cosmic voids with weak-lensing (WL) or cosmic microwave background (CMB). Recently the Dark Energy Survey collaboration has published results on the weak-lensing and ISW imprints of voids obtained from photometric galaxy catalogs (Kovács et al. 2019; Sanchez et al. 2017). Indeed, the measurement of weak gravitational lensing probes matter inhomogeneities in the Universe through the deflections of the light of background sources.
Although the weak lensing measurements are widely performed to map matter overdensities, it has been shown that they should be affected by underdensities in the matter distribution as well (Amendola, Frieman, and Waga 1999). The key point is that underdense regions like cosmic voids are more subject to differences between dark energy and modified gravity models due to the screening mechanism of the hypothetical fifth force (Baker et al. 2018).
However, as the tangential shear signal is expected to be very small for individual voids, theoretical calculations (Krause et al. 2013) and numerical simulations (Higuchi, Oguri, and Hamana 2013) suggest that stacking many voids will enhance the significance of the lensing measurement. In addition, the stacked weak lensing signal from voids is a direct measurement of the void density profile which can be used to constrain cosmological models without requiring any assumption about the galaxy bias. The recent lensing measurement with 87 cosmic voids from DES-SV data is very encouraging with a 4? significance (Sanchez et al. 2017), which confirms that measurements that will be obtained with spectroscopic surveys should provide stronger constraints on modified gravity models.
With the imminent massive influx of cosmological data like the DESI ground-based dark energy experiment, the number of observed objects is expected to be at least one order of magnitude. Indeed, the DESI instrument will conduct a five-year survey designed to cover 14,000 deg2 and to map the large-scale structure of the Universe from redshift 0 to 3.5 by measuring 34 million redshifts (Aghamousa et al. 2016). The expected number of voids is several tens of thousands. The DESI project will begin the survey in June 2020 after 4 months of Survey Validation. At CPPM we were involved in the construction and testing of the 10 DESI spectrometers. We are full member of DESI with guaranteed access to the DESI private data.
<bold>Description of the thesis work </bold>
The aim is to apply this kind of analysis on the DESI spectroscopic data, using DESI photometric surveys used for DESI targeting (DECALs).
In addition, we propose to constrain the velocity distribution around the same voids via redshift-space distortion (RSD) measurements. The combination of these two methods will allow implications on competing models of gravity and dark matter from a yet unexplored domain of the cosmic web. This approach is particularly timely, as lensing and RSDs around voids have only recently been detected individually, but have never been analyzed jointly.