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PhD Thesis
PhD thesis director name: Carlo Schimd / Olivier Le Fèvre?
Email: carlo.schimd@lam.fr / olivier.lefevre@lam.fr
Phone number: +33 4 9105 5922 / +33 4 9105 5985
Co-director: Carlo Schimd
PhD title: Mass assembly and peculiar velocities on cosmological scales:
relativistic effects beyond the ΛCDM model
Subject description (1 page max.):
In the era of precision cosmology, aiming at validating or disproving the
cosmological standard model and the theory of general relativity on the largest
scales, it is mandatory to assess the validity of the perturbative scheme and its
Newtonian approximation, on which all the low-redshift observables rely. A
pure general-relativistic mechanism, the so-called kinematical back-reaction,
emerges during the non-linear stages of structure formation; it does mimic
both dark matter (on small/galactic scales) and dark energy (on
large/cosmological scales). No quantitative, observational estimations of this
effect exist so far. Indeed, the unique probe to directly measure this term are
peculiar velocity surveys (difficult to realize in the past, nowadays possible with
much larger accuracy), owing to a well-established relationship between the
back-reaction term and the morphology of the velocity potential, as probed by
the Minkowski functionals (Buchert 2008).
The plan of the thesis is as follows. During first year, using DEUS (Alimi,
Meudon) and CoDECS (Baldi, Bologna) simulations, which realize quintessence
cosmologies with non-trivial back-reaction, the student will calculate the
density field, reconstruct the velocity field applying the Nusser & Davies
technique, and infer the kinematical back-reaction term. The second year will
be devoted to data analysis of redshift surveys (GAMA; VIPERS/final release,
2015-2016) and velocity surveys (Cosmicflow-2, 6dFGSv – tbc), paying attention
to the relationship between the physical (photometric) properties of the
galaxies tracing the reconstructed cosmic-web, their environment, and backreaction. The third year will be dedicated to explore more recent or new
reconstruction schemes, suited when peculiar velocity estimates are not
available, and forecast the potentiality of future velocity surveys like TAIPAN
and redshift surveys such as PFS-SuMIRe and Euclid.
This project is fostered by 1) a funded PNCG project, MorphoLSSisBack (P.I.
C. Schimd; co-I: Alimi, Obs. Meudon/LUTH; Buchert, CRAL; Hawken, INAF
Milan/Brera; Marulli, Univ. Bologna; Branchini, Univ. Roma Tre), aimed at
exploring the morphology of VIPERS galaxies; 2) an ongoing collaboration with
M. Alpaslan & GAMA Team, aimed at investigating the morphology of the local
cosmic-web components; and 3) a new collaboration with H. Courtois (IPNL,
Lyon) on peculiar velocity surveys.
Bibliography:
 Buchert T. 2008, Dark energy from structure: a status report, GRG, 40, 467.
 Rasera Y., Alimi J.-M. et al. 2010, Introducing the Dark Energy Universe Simulation Series
(DEUSS), AIP Conf. Proc., 1241, 1134; Buchert T., Larena J. & Alimi J.-M. 2006,
Correspondence between kinematical back-reaction and scalar-field cosmologies – the
‘morphon field’, Class. Quantum Grav. 23, 6379.
 Baldi, M. 2014, The CoDECS project: a publicly available suite of cosmological N-body
simulations for interacting dark energy models, MNRAS, 422, 1028.
 Nusser A. & Davis M. 1994, On the prediction of velocity fields from redshift space galaxy
samples, Ap.J.Lett. 412, 1; Nusser A., Davis M. & Branchini E. 2014, On the recovery of the
Local Group motion from galaxy redshift surveys, Ap.J. 788, 157.
 Alpaslan M. et al. 2014, Galaxy and Mass Assembly (GAMA): fine filaments of galaxies
detected within voids, MNRAS Lett. 440, 106.
 Marulli F. et al. 2013, The VIMOS Public Extragalactic Redshift Survey (VIPERS): Luminosity and stellar mass dependence of galaxy clustering at 0.5 < z < 1.1, A&A, 557, 17.
 Tully R. B., Courtois H. et al. 2013, Cosmicflow-2: the data, Ap.J. 146, 86.