DS10 - Défi des autres savoirs

Cosmological probes of Gravity and Dark Energy – eBOSS

eBOSS : Cosmological probes of gravity and dark energy

eBOSS is measuring the history of cosmic expansion with baryon acoustic oscillations (BAO) in the redshift range from 0.6 to 3.5. It is a 6-year spectroscopic survey of red galaxies, star-forming galaxies and quasars covering about 7000 deg2. It probes the Universe in the turnover epoch when it migrates from dark matter to dark energy. In addition, eBOSS is studying the the growth of structure which provides new tests of General Relativity on cosmological scales

BAO with quasars and their Ly-a forests - Galaxy-Galaxy lensing - Cosmic voids

The strategy of this project is to focus on four topics directly related to the study of dark energy and gravity. The first topic, BAO and Redshift-Space-Distorsion (RSD) with quasars, constitutes the core of the eBOSS science case. The second topic, Lya forest of quasars, represents the main expertise of the French Participation Group (FPG), strongest group on this subject. The third topic, galaxy clustering and lensing cross-correlation, is innovative in eBOSS. Finally, the last topic, cosmic voids that can be studied both with galaxies and Lya forest, allows us to probe Gravity in under-dense regions. In the choice of these four topics for our project, we mixed safe subjects with a high visibility and novel approaches with a large discovery potential.

We structure our effort around 3 geographical poles, driven by 4 scientific projects as explained in previous summary, and corresponding to a post-doc recruitment in each pole:
- Saclay pole (IRFU): Benefitting from the Saclay group experience in quasar (QSO) target selection, the post-doc is focused on the quasar clustering in order to measure both BAO and RSD. At the end of his project, the post-doc will test alternative models of gravity.
- Paris intra-muros pole (APC, IAP and LPNHE): The post-doc works on the absorbers in QSO spectra. The main goal will be to measure BAO with Lya forest autocorrelation or cross-correlation with other tracers (QSO, DLA, C-IV absorber...). A small fraction of the post-doc project will be dedicated to the study of cosmic voids in Lya forests.
- Marseille pole (LAM and CPPM): The post-doc studies the clustering and the weak lensing with the ELG and LRG samples. In parallel, the post-doc will perform a joint-analysis of voids, RSD and Alcock-Paczynski tests.

In addition, we encourage transverse activities. For instance, two distinct poles (Paris/Marseille groups) cover the topics “absorbers in QSO spectra” and “study of cosmic voids”. The purpose of this project is to develop new collaborations between French scientists and to reinforce the synergy between geographically dispersed French groups. The post-doc positions provide the opportunity to foster or enhance collaborations between the French groups.

We are writing this summary, one year and a half after the beginning of the project and we have already substantial results in terms of organization and all the organizational structure proposed in the ANR project is established.
We have already organized three e-BOSS France workshops (two in Paris and one in Marseille). These meetings allow the PhD students and the post-docs to present the status of their works. It is also opportunities to launch common actions, such as the preparation of papers, the validation of quasar catalogs, the preparation of N-body simulations for ELGs and QSOs. Participants beyond those in eBOSS are invited to these workshops, allowing us to share our experience with the members of future projects, such as DESI, CFIS and Euclid.
In December 2018 or in early 2019, we will organize an eBOSS collaboration meeting in France. It will be an opportunity for the whole French community working in cosmology to have face-to-face discussions with other specialists in quasar, galaxy and clustering science. The organization of this eBOSS meeting will also further contribute to the international visibility of the FPG.
Finally, we have almost finished the recruitment of the post-docs and all the all the working groups are in place.

eBOSS is exploring a complete “terra incognita” of cosmology, probing the expansion of the Universe in the transition period between dark matter and dark energy dominating regimes. No other project has achieved before a wide survey (7500 deg2) at this epoch in the Universe history (5-11 billion years ago). There is no competitor to eBOSS and the next generation of BAO projects will not happen before at least 2020 with DESI and Euclid projects.
Complementing the measurement of the BAO scale, other cosmological objectives of eBOSS include measurements of the total mass of neutrinos from the analysis of the IGM clustering, studies of redshift-space distortions (RSD), which tell us how galaxies are moving in response to gravity) and measurements of weak gravitational lensing (which tells us how massive objects bend the path of light). Both of the latter methods can not only measure dark energy but also can independently test deviations from Einstein’s theory of gravity.
In addition, new novel and innovative probes of both cosmology and astrophysics are emerging, such as the study of cosmic voids. eBOSS provides both large samples to test this promising concept, and also rare targets to test for fundamental physics (constraining the fundamental constants of physics, ...).
Finally, beyond the cosmology research fields, eBOSS has a tremendous impact on the physics of quasars with more than 750,000 quasars to be discovered (thus 3 times more than what is known at present) and is opening the completely new field of time-dependent spectroscopy with the sub-program TDSS (Time Domain Spectroscopic Survey), targeting more than 50,000 variable objects.

The SDSS data have been made available to both the wider astronomical community and the general public through regular planned and supported data releases. These publicly released data have been comprehensively documented, enabling users outside the SDSS collaboration to exploit the full scientific potential of the SDSS database, thereby multiplying its scientific productivity. SDSS data are now regarded as an indispensable public resource in astronomy.
For eBOSS, we are continuing this practice with several public data releases. In particular, our group is in charge of the releases of Value-Added Catalog (VAC) for quasars. The first new VAC, DR14Q, was made public during the summer 2017.
With this first catalog, we have already published many papers about the measurement of the BAO scales and the growth of the structure (RSD) with quasars. Our group was leading many of these fundamental papers. Before the end of this project, we expect many other papers with the other tracers of the matter used in eBOSS.

Indications of the accelerating expansion of the Universe have accumulated during the last decade thanks to observations of SNIa, measurements of the anisotropies of the cosmic microwave background and studies of the large-scale structure of the universe. This acceleration could be due to a new energy component called "dark energy", which is neither matter nor radiation.

The cosmic acceleration could also be a sign that Einstein’s theory of gravity breaks down on cosmological scales, which would have strong implications on our understanding of the laws of nature. A way to seek signatures of modified gravity is to test the relationship between the history of expansion and the growth of matter clustering.

The baryon acoustic oscillations (BAO) allow us to study dark energy. Indeed, the acoustic waves that propagated in the early universe were frozen at the epoch of recombination. They remained etched in the distribution of matter observed today, defining a characteristic scale. This was observed for the first time in 2005 by the Sloan Digital Sky Survey (SDSS) in the distribution of 50000 Luminous Red Galaxies (LRG), and by the 2dFGRS program. The results of SDSS-III/BOSS with the same type of galaxies now confirm unambiguously the existence of baryon acoustic oscillations. More recently, BAO have also been observed in the Ly-alpha forest of quasars by the French Participation Group (FPG) of BOSS. In addition, the galaxies that are mapped to enable BAO studies also allow measurements of the growth of structure via redshift-space distortions (RSD), a statistical anisotropy in the clustering of galaxies that arises because the peculiar velocities of galaxies shift their apparent positions along the line of sight.

Since September 2014, eBOSS (Extended Baryon Oscillation Spectroscopic Survey) is using the SDSS facility within the SDSS-IV collaboration to conduct a six-year project. The eBOSS project is building the largest 3D map of the Universe ever made, addressing fundamental questions in unexplored redshift regimes. Critically, the 1<z<2.2 expansion had never been studied using any method at comparable accuracy; yet, it probes the epoch when the expansion transitioned from deceleration to acceleration, i.e. when the Universe began to migrate from dark matter to dark energy. With about 1.5 million galaxies and 0.7 million quasars, eBOSS will constrain with unprecedented accuracy the expansion history of the Universe across all redshifts in the range 0.6<z<3.5 and will provide new tests of General Relativity on cosmological scales. eBOSS is a timely project before the next generation of large spectroscopic surveys such as DESI and Euclid.<br />
The strategy of this ANR project is to focus on four topics directly related to the study of Dark Energy and Gravity. The first topic, BAO and RSD with quasars, constitutes the core of the eBOSS science case. The second topic, Ly-a forest of quasars, represents the main expertise of the FPG, strongest group on this subject. The third topic, galaxy clustering and lensing cross-correlation, is innovative in eBOSS. Finally, the last topic, cosmic voids that can be studied both with galaxies and Ly-a forest, allows us to probe Gravity in under-dense regions. In the choice of these four topics for our ANR project, we mixed safe subjects with a high visibility and novel approaches with a large discovery potential.

We structure our effort around three geographical poles (Saclay, Paris and Marseille), driven by our scientific projects and supported by the recruitment of three post-docs, one for each pole. In addition, we want to encourage transverse activities. The purpose of this ANR project is to develop new collaborations between French scientists and to reinforce the synergy between geographically dispersed French groups. The post-doc positions will be opportunities to foster or enhance collaborations between the French groups.

Project coordinator

Monsieur Christophe YECHE (Commissariat à l'énergie atomique et aux énergies alternatives)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

LPNHE Laboratoire de Physique Nucléaire et de Hautes Energies
CNRS-DR12-LAM Centre National de la recherche Scientifique DR12 - Laboratoire d'Astrophysique de Marseille
APC AstroParticule et Cosmologie
IAP Institut d'Astrophysique de Paris
CNRS DR12_CPPM Centre National de la Recherche Scientifique Délégation Provence et Corse_Centre de Physique des Particules de Marseille
CEA/DRF/Irfu Commissariat à l'énergie atomique et aux énergies alternatives

Help of the ANR 599,880 euros
Beginning and duration of the scientific project: September 2016 - 48 Months

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