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Home page > PhD fellowships > Proposed PhD subjects 2016-2019 > LSST : Matter and void distribution with the Large Synoptic Survey Telescope

LSST : Matter and void distribution with the Large Synoptic Survey Telescope

by Luc Frappat - 17 December 2015

Topics:  Dark energy

Proponents:  Cécile Renault

Address:  LPSC Grenoble - 53, rue des Martyrs - 38026 GRENOBLE CEDEX

Phone:  + 33 (0)476 28 40 13

Contact Email: rcecile@in2p3.fr


The thesis places itself as part of the LSST Dark Energy Science Collaboration. The Large Synoptic Survey Telescope (LSST) is a fast, wide and large telescope (partly) dedicated to cosmology, in particular, to the understanding of the nature of dark energy which is responsible for the current acceleration of the space expansion in the Universe. The work will be mainly devoted to the comparison of several probes like the Baryonic Acoustic Oscillations (BAOs) or the voids mapping. The study will be done on simulations and on public data in the framework of the LSST software as much as possible. Additionally, part of the work will be devoted to the Camera Calibration Optical Bench (CCOB) device, which is under the LPSC responsibility.

One of the most robust methods for measuring the distance-redshift relation, and so to constrain the dark energy part of the universe equation of state, is to use the Baryon Acoustic Oscillation (BAO) feature(s) in the clustering of galaxies as a “standard ruler”. The acoustic oscillations arise from the tight coupling of baryons and photons in the early Universe: the propagation of sound waves through this medium gives rise to a characteristic scale in the distribution of perturbations corresponding to the distance travelled by the wave before recombination. The BAO feature relies on linear, well understood physics that can be calibrated by relic radiation anisotropy measurements and is quite insensitive to non-linear or astrophysical processing that typically occurs on much smaller scales. An alternative and promising study, also free from astrophysical non-linear behavior, is the geometry of the voids stretched by the dark energy force.

Nevertheless the analyses are delicate, in particular because LSST will only perform photometry and not spectrometry. The team, in collaboration with the D. Boutigny (LAPP), will propose to integrate in the LSST software a method to determine the photometric redshift. Based on the most recent developments at Grenoble (J.S. Ricol) and Marseille (S. Arnoults et O. Ilbert, LAM), this method will allow to test some cosmological capabilities of LSST in realistic conditions using the whole pipeline as much as possible.

On the instrumental side, our laboratory is responsible for the LSST Camera Calibration Optical Bench (CCOB). The CCOB is an important device to test the camera of the LSST before the « in situ » operations in Chile. It is crucial to insure the nominal performances of the LSST and therefore to reach the expected physics goals. It will have basically two operating modes. The first one, referred to as the « wide beam » mode, is expected to provide a 0.2% precise flat field for the full camera. As the camera has 3.2 billion pixels and is by far the most complex ever designed, its calibration is a hard task. The second mode, referred to as the « thin beam » mode, is expected to provide a full commissioning of the integrated camera, including the lenses and filters. During his/her PhD thesis, the student will have the opportunity to actively participate to the final tests of the CCOB « wide beam » during the first year and « thin beam » the second or third year. An optical bench has been developed and many subtle thermal, electrical, and mechanical effects have to be investigated and corrected for. This physicist task requires to deal with quite a lot of intricate deviations from the « ideal » behavior at the same time.