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Accueil > Thèses, Stages, Formation et Enseignement > Propositions de thèses 2023 > Joint supernovae and galaxy clustering analysis with ZTF and the DESI BGS

Joint supernovae and galaxy clustering analysis with ZTF and the DESI BGS

par Tristan Beau - 7 novembre

Titre : Joint supernovae and galaxy clustering analysis with ZTF and the DESI BGS

Directrice/directeur de thèse : Nicolas Regnault

Co-encadrant.e : Pauline Zarrouk

Groupe d’accueil :Cosmologie et Énergie Noire

Collaboration : ZTF and DESI

Description :

The measurement of the growth rate of structures (fs8) as a function of cosmic time is a direct test of the predictions of General Relativity (GR) at cosmological scales. Since deviations to GR at large scales are a plausible explanation to the existence of Dark Energy, such measurements count among the cornerstone tests of this decade. Precision measurements of fs8 have become possible only recently, with the advent of massive spectroscopic surveys, gathering redshifts of tens of millions of galaxies. With enough statistics it is indeed possible to infer fs8 from the distortions of the galaxy correlation function in redshift space (RSD, for Redshift Space Distortions).

Such fs8 measurements are among the key goals of the new generation Dark Energy Spectroscopic Instrument (DESI) which uses the 4m Mayall telescope at Kitt Peak in Arizona. DESI is the first new generation survey that is actually taking data (the main survey started on May 2021) and it will obtain about 40 million spectra of galaxies over 5 years. The sample of 13 million galaxies at low redshift (Bright Galaxy Survey, BGS) will probe the local universe when it is currently dominated by the cosmic acceleration. Together with its high density sampling, it makes it the ideal dataset to study modifications of general relativity. The precision on the cosmological parameters obtained from RSD will not be limited by statistics but by cosmic variance (limited amount of information available in a small volume of the universe), especially at low redshift z < 0.1 where fs8 will be poorly constrained by RSD alone from the BGS. Combining RSD with other probes becomes essential. A promising probe is the study of the spatial correlations of supernova peculiar velocities (PV) : residuals to the SN Hubble diagram encode peculiar motions at the SN location. The large-scale spatial correlations of these motions are directly proportional to fs8. Again, such measurements necessitate large samples of low-redshift supernovae O(1000), which are just becoming available. For example, the Zwicky Transient Facility (ZTF) is currently accumulating a sample of 6000 SNe at z<0.1.

The LPNHE cosmology team is involved in the DESI and ZTF projects. We propose to conduct a joint ZTFxDESI analysis to measure the growth rate of structure from both density and velocity fields. During the pre-thesis internship, the candidate will first work on improving the standardisation of supernova distances, in order to decrease the dispersion in the Hubble diagram due to SN modelling noise. Recent studies have shown that it is possible to decrease the dispersion in the Hubble diagram by a factor 2 with an improved modelling of supernova diversity. The candidate will develop an improved SN distance estimator and train it on the ZTF dataset. Then, the candidate will use the DESI BGS simulations as input to generate SN at galaxies’ positions. It will result in developing realistic cosmological simulations that contain both probes in order to perform joint analysis and infer both the density and velocity fields. The overall pipeline will be applied to the sample of ZTF SNe and DESI BGS galaxies to obtain the most precise constraints on the nature of dark energy and gravity at z<0.1.

Lieu(x) de travail : LPNHE

Déplacements éventuels : Lyon, Stockholm/Berlin, Berkeley

Stage proposé avant la thèse : Oui

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