Although the undergoing cosmic acceleration may be explained by a non-vanishing cosmological constant in Einstein gravity, various dynamical effects could very well explain current observations, all dubbed as dark energy. Quintessence, as a light scalar field minimally coupled to gravity, is a dark energy candidate to explain the recent acceleration of the Universe expansion. The Ratra-Peebles potential and its corrected form in supergravity are under study. Using a modified version of CAMB, including perturbations of the scalar field, we use the latest SNIa and CMB observations to select acceptable points in the parameter space. Starting with the associated matter power spectrum, in collaboration with the LUTh (Paris-Meudon Obs., France) we run N-body simulations of growth of large scale structures where the background evolution is modified by quintessence. We are involved in the Dark Energy Universe Simulation Series (DEUSS) collaboration. Another dark energy candiate involves cosmic inflation, currently the best explanation of the origin of large scale structures and CMB anisotropies. Similarly, if dark energy is a light scalar field, the current acceleration can be the consequence of quantum fluctuations during cosmic inflation, provided this one occurs at TeV scale.
External collaborators: Jean-Michel Alimi, Yann Rasera, Pier Stefano Corasaniti (Observatoire de Paris-Meudon, France). Teruaki Suyama (The University of Tokyo, Japan), Tomo Takahashi (Saga University, Japan), Masahide Yamaguchi (Tokyo Institute of Technology, Japan), Shuichiro Yokoyama (Nagoya University, Japan).
The strong equivalence principle (SEP) does not hold anymore in various extensions of General Relativity. Its violations can be revealed by the non-universality of free-falling for compact objects and we have developed a generic and effective way to test the SEP in the Cosmic Microwave Background (CMB). A violation of the SEP indeed alters the amplitude of the acoustic oscillations in the primeval plasma. Using the WMAP data, we have contrained a possible SEP violation for the baryons. Our interests are also focused on the scalar-tensor theories of gravitation and their cosmologies. In a more specific way, we are currently studying a model where the scalar sector is conformally invariant. The effective fluid related to the non-minimally coupled scalar field differs from the other cosmological fluids of radiation by its very particular anisotropic pressure and we are studying its impact on the CMB anisotropies by modifying the CAMB code.
The World LHC Computing GRID (WLCG) is the worldwide distributed computing infrastructure controlled by software middleware that allows a seamless usage of shared storage and computing resources. One PByte of data is expected to be produced every year by the CMS detector at the LHC collider. This data must be processed (iterative and refined calibration and analysis) by a large scientific community that is widely distributed geographically. Instead of concentrating all necessary computing resources in a single location, the LHC experiments have decided to set-up a network of computing centres distributed all over the world. The overall WLCG computing resources needed by CMS alone in 2010 amount to about 25,000 CPUs, 25,000 TB of disk storage and 35,000 TB of tape storage. Working in the context of the WLCG translates into seamless access to shared computing and storage resources. End users do not need to know where their applications run. The choice is made by the underlying WLCG software on the basis of availability of resources, demands of the user application (CPU, input and output data,..) and privileges owned by the user. Back in 2005 UCL proposed the WLCG Belgian Tier2 project that was endorsed by the 6 Belgian Universities involved in CMS. The Tier2 project consists of contributing to the set-up of the WLCG by building two computing centres, one at UCL and one at the IIHE (ULB/VUB). The UCL site of the WLCG Belgian Tier2 is deployed in a dedicated room close to the cyclotron control room of the IRMP Institute and is currently a fully functional component of the WLCG. The UCL Belgian Tier2 project also aims at integrating, bringing on the GRID and sharing resources with other scientific projects. The scientific projects related to or directly integrated on the UCL computing cluster are the following: MadGraph/MadEvent, NA62 and Cosmology.
External collaborators: CISM (UCL), Pascal Vanlaer (Belgium, ULB), Lyon computing centre, CERN computing centre.
