Cosmology

In the wake of modern Cosmology, the work of the COSMOS team of the IP2I contributes to a better understanding of the general properties of the Universe, its energy content, its structure on large scales as well as the physical laws governing its evolution over time.

Our work is being carried out within the scope of the cosmological concordance model, the most complete model available to cosmologists. This model describes an isotropic and homogeneous Universe in accelerated expansion from an extremely dense and hot primordial state. It is the result of a progressive construction from the theory of general relativity (1915) to the discovery of the accelerated expansion of the Universe (Nobel Prize in Physics 2011) which brought cosmology into a new era, with many fundamental questions still open:

• What is the history of the expansion of the Universe?
• What is the nature of the “dark matter” and “dark energy” that represent about 95% of the energy content of the Universe according to the current cosmological model?
• What is the origin of large structures in the Universe and the history of their formation?

To try to answer these questions, we use a variety of cosmological probes.

The distribution of galaxy velocities in the near Universe allows us to understand the distribution of matter in our galactic environment and to test different theories of gravity – see Cosmic Flow & Simulations (IDEX).

Thermonuclear supernovae make it possible to rigorously constrain cosmological parameters, in particular the nature of the mysterious “dark energy” that would be responsible for the acceleration of its expansion – see ERC-USNAC.

Our group is also widely involved in major international collaborations in observational cosmology, including the Euclid and LSST projects – See LSST and Euclid.

Finally, we are participating in the development of new instruments on the ground and in space with particular emphasis on near infrared detectors and slit-free spectroscopy – See Euclid.

Near-field Cosmology/Cosmological Probes

Cosmic Flows/field of galaxy velocities

Galaxies acquire motions that deviate from the universal expansion through gravitational interactions on a wide range of scales. The radial component of these deviant motions can be mapped with accurate measurements of distances. One of a variety of ways to measure distances makes use of the correlation between the luminosities of galaxies and their rotation rates. With appropriate photometric and spectroscopic information, the method can be applied to a majority of spiral galaxies. Samples of many thousands of galaxies can be acquired, giving the dense spatial coverage required to study the streams and eddies in the Cosmic Flow.

Mapping the large-scale structure of the nearby region of the Universe is important for several reasons. First, it reveals details of the large-scale cosmic structures that surround the Milky Way. These details are nearly impossible to observe for systems far away from Earth. Second, the morphology of the nearby Universe is essential for a precise determination of cosmological parameters such as the density of dark energy, which is thought to drive the acceleration of the expanding Universe. Third, examination of cosmic structures around the Milky Way will help us to understand how the Galaxy formed and evolved, and galaxy formation processes in general.

• For a quick update visit the website of the Project
• Release of the observational surveys through the Extragalactic Distance Database
• Discovery of Laniakea Supercluster

Simulations (IDEX)

The purpose of the project is to run constrained cosmological simulations of the local universe and to use it to test our cosmological model. Our simulations differ from random simulations in such a way that the local cosmography is forced to be reproduced. This is done in a process known as constrained simulations which starts with the Cosmic-Flows catalogue, an observed list of galaxy distances and peculiar velocities. Since the peculiar velocity field is engendered by the total gravitating mass in the universe, the peculiar velocity field can be used to reconstruct the full mass distribution in the local universe. Such a reconstruction can be used for many cosmographic purposes including discovering superclusters and identifying basins of gravitational attraction in the local universe. They may also be used to generate initial conditions for numerical simulation. In such simulations, the local large-scale structure is forced to be reproduced. On large scales we accurately simulate the main sources of gravity – the super clusters that dominate the peculiar velocity field. On smaller scales, around the Milky Way we accurately simulate the cosmographic landscape: the local void, the virgo cluster, and the local filament as well as a tidal field consistent with observations, emerge naturally through this process. Part of the project is then focused on hydrodynamic simulations of the Local Group producing a pair of galaxies that are roughly identical to the observed ones and in the correct environment. We can thus study the effect of the local cosmography on galaxy formation in the cosmic near field, specifically examining its effect on dwarf galaxies and on the galaxy formation in general.

type Ia Supernovae (ERC USNAC)

The team uses Type Ia Supernovae to measure the properties of the constituents of the Universe, including the physics of dark energy. These supernovae allow us to measure distances of billions of light-years thanks to their unique properties: they are extremely bright and all almost identical. However, despite 30 years of research, we still do not know the exact physics that explains the “Supernovae type Ia” event. This lack of knowledge of all the effects that could be hidden in the “quasi”, limits today our progress in measuring the properties of the Universe. Our team, funded by the European ERC program (USNAC project; PI Rigault), is tackling this issue by comparing the properties of Supernovae according to their galactic environment. We are using data from the new Zwicky Transient Facility survey and also from SNfactory’s 10 years of observation. Soon we will have access to the Large Survey of Space and Time (LSST).

LSST Project

Characterization of atmospheric transmission by slit-free spectrophotometry

The Large Synoptic Survey Telescope (LSST) is one of the large new-generation telescopes currently under construction in Chile. It will observe every night the entire visible sky, and will produce a catalogue of stars and galaxies of unprecedented extent. In order to use this survey for cosmology measurements (SNe Ia, weak lensing, etc.), it is necessary to constrain the systematic calibration errors to 0.1%. In particular those resulting from fluctuations in atmospheric transmission which are becoming preponderant. The 1.2 m Auxiliary Telescope (AuxTel), built on the same site as LSST and put on the sky in the summer of 2019, will observe reference stars jointly with the latter using a slitless spectrograph in order to follow these variations in real time.

Slitless spectrography is currently undergoing a revival, due to its practical advantages (instrumental simplicity, multiplexing, ease of pointing), and will be used in all future cosmological surveys (JWST, Euclid, WFIRST). However, due to the spectro-spatial degeneration it implies, it is severely affected by cross-contamination effects (neighbouring sources may overlap), and self-contamination (the intrinsic size of the source degrades spatial resolution), which compromises the final performance. Forward modelling of the whole observation chain should significantly improve instrumental performance and provide optimal results to meet scientific objectives.

Brocker

One of the challenges of modern cosmology will be to make use of the huge amount of data generated by the new instruments, especially the LSST. In particular, type Ia Supernovae are transient objects: a white dwarf explodes, the luminosity of the event increases for 15 days and then decreases for several weeks.

For cosmology, it is necessary to observe this supernovae before maximum brightness and, if possible, to obtain a spectrum at this time. But LSST discovers millions of transient objects per day and among them, only a few are supernovae. How to find them in time? How do we know which ones are most likely to be interesting for cosmology? When to trigger spectroscopic tracking? For all this, we are creating “Brokers” who will be in charge of monitoring the production of alerts provided not only by LSST, but also by neutrino and gravitational wave telescopes. Our group works in AMPEL, one of the most developed today.

ESA Euclid project

Euclid is a multi-probe observational cosmology satellite (acoustic oscillations of baryons, gravitational shear, redshift distortions, etc.) selected by the European Space Agency (ESA) in 2012 for a launch now scheduled for 2022. To carry out its sounding mission, the 1.2 m telescope is equipped with two panoramic instruments, VIS – an optical imager – and NISP – both an imager and a slitless spectrograph working in the infrared range. In the end, the survey is planned to last 6 years and cover a field of 15,000 deg².

The Euclid consortium, the body with scientific responsibility for the mission, is structured around a complex organisation. In relation with its various partners, international (e.g. ESA and NASA), national (e.g. CNES), and aerospace industry (e.g. Thales), it ensures the definition and implementation of the scientific objectives and requirements, as well as the design, construction and validation of the VIS and NISP instruments before delivery to ESA. In particular, the consortium’s Science Ground Segment (SGS) is responsible for the development of the algorithms for processing and reduction of observations, their implementation in dedicated computing centres, and finally the production, analysis and interpretation of the scientific survey data. To date, the consortium has about 1500 members from 220 European and American laboratories.

Detector activities

Activity on the NISP instrument is focused on testing and validating the performance of the focal plane detection chain. This consists of 16 HgCdTe infrared sensors (H2RG Teledyne, working up to the wavelength of 2.3 um) connected to the 16 reading ROICs and 16 ASICs (SIDECAR Teledyne). A flexible cryogenic cable connects the ROIC to the ASIC allowing operating temperatures of the detector between 80K and 90K and of the ASIC between 130K and 140K. CNES has provided the means for the ground characterization campaign of the flight detectors, after selection by Jet Propulsion Laboratory (JPL) and Detector Characterisation Lab (DCL) of NASA (GSFC NASA), to be handled by IN2P3 at the CPPM.

We participate in the testing of flight detectors and bring our expertise in the interpretation of the results. The algorithms used for the characterization of ground-based detectors and the methods for correcting detector effects are adapted to the reduction of scientific data in the NIR/SIR pipeline and to the monitoring of the behaviour of the detectors in flight.

In particular we have in charge:

• The position of Detector Scientist for monitoring the performance of detection chains from their production by Teledyne to their integration and validation on the focal plane,
• Design, production and maintenance of acquisition software,
• Design, production and maintenance of the online data quality verification software,
• The development of methods for analysis and correction of the pixel response :
  • Noise, common modes and reference pixel subtraction algorithms,
  • Measurement of signal slope and quality from photometric and spectrometric readout modes,
  • Studies of hidden pixels,
  • Study of the effects of trapping and de-trapping of load carriers on the measurements of signal slopes under integrated load,
• Ongoing support for the characterization procedure implemented at the CPPM

Slotless spectrography

The participation of the team is focused on spectroscopic (OU-spectro = OU-SIR & OU-SPE) and software activities in the broad sense (SGS-Fr correspondent), with in particular :

• Co-responsibility for the OU-SIR activity,
• Participation in the development of the photometric (OU-NIR) and spectroscopic (OU-SIR) NISP data reduction pipeline,
• Participation in the Spectroscopic Analysis Pipeline (OU-SPE),
• The establishment of effective modelling of the >NISP instrument,
• Direct modeling of self-contamination effects,
• Galaxy kinematics in slotless spectrography.