Beyond self-acceleration: force- and fluid-acceleration

The notion of self acceleration has been introduced as a convenient way to theoretically distinguish cosmological models in which acceleration is due to modified gravity from those in which it is due to the properties of matter or fields. In this paper we review the concept of self acceleration as given, for example, by [1], and highlight two problems. First, that it applies only to universal couplings, and second, that it is too narrow, i.e. it excludes models in which the acceleration can be shown to be induced by a genuine modification of gravity, for instance coupled dark energy with a universal coupling, the Hu-Sawicki f(R) model or, in the context of inflation, the Starobinski model. We then propose two new, more general, concepts in its place: force-acceleration and field-acceleration, which are also applicable in presence of non universal cosmologies. We illustrate their concrete application with two examples, among the modified gravity classes which are still in agreement with current data, i.e. f(R) models and coupled dark energy.

As noted already for example in [35, 36], we further remark that at present non-universal couplings are among the (few) classes of models which survive gravitational wave detection and local constraints (see [12] for a review on models surviving with a universal coupling). This is because, by construction, baryonic interactions are standard and satisfy solar system constraints; furthermore the speed of gravitational waves in these models is  cT = 1 and therefore in agreement with gravitational wave detection. It has also been noted (see for example [37–39] and the update in [33]) that models in which a non-universal coupling between dark matter particles is considered would also solve the tension in the measurement of the Hubble parameter [40] due to the degeneracy beta - H0 first noted in Ref. [41].

Reference: L.Amendola, V.Pettorino  "Beyond self-acceleration: force- and fluid-acceleration", Physics Letters B, in press, 2020.

The first Deep Learning reconstruction of dark matter maps from weak lensing observational data

DeepMass: The first Deep Learning reconstruction of dark matter maps from weak lensing observational data (DES SV weak lensing data)


 This is the first reconstruction of dark matter maps from weak lensing observational data using deep learning. We train a convolution neural network (CNN) with a Unet based architecture on over 3.6 x 10^5 simulated data realisations with non-Gaussian shape noise and with cosmological parameters varying over a broad prior distribution.  Our DeepMass method is substantially more accurate than existing mass-mapping methods. With a validation set of 8000 simulated DES SV data realisations, compared to Wiener filtering with a fixed power spectrum, the DeepMass method improved the mean-square-error (MSE) by 11 per cent. With N-body simulated MICE mock data, we show that Wiener filtering with the optimal known power spectrum still gives a worse MSE than our generalised method with no input cosmological parameters; we show that the improvement is driven by the non-linear structures in the convergence. With higher galaxy density in future weak lensing data unveiling more non-linear scales, it is likely that deep learning will be a leading approach for mass mapping with Euclid and LSST.

Reference 1:  N. Jeffrey, F.  Lanusse, O. Lahav, J.-L. Starck,  "Learning dark matter map reconstructions from DES SV weak lensing data", Monthly Notices of the Royal Astronomical Society, in press, 2019.


Diffuse Galactic thermal dust emission: modified black-body parameter maps

Diffuse Galactic thermal dust emission: modified black-body parameter maps

Diffuse emissions are ubiquitous within our Galaxy. They probe star-forming regions, the chemical composition of the Galaxy and the Galactic magnetic field. Conversely, they also obscure cosmological measurements such as the cosmic microwave background and the epoch of reionisation signal. Detailed characterisation of these emissions is of interest to both cosmologists and astrophysicists. In early 2019 CosmoStat released our contribution to the investigation of thermal dust emission in the form of modified black-body temperature, spectral index and optical depth maps (to be found here). These intensity maps are presented at Nside 2048, FWHM 5 arcmin and were made from Planck (data release 2) HFI and IRAS data.

Reference 1: M. Irfan, J. Bobin, M-A. Miville-Deschenes and I. Grenier, "Determining thermal dust emission from Planck HFI data using a sparse parametric technique ", A&A, 623, 03/2019.


Weak Lensing 2D and 3D Density Fluctuation Map Reconstruction

Weak lensing 2D & 3D density fluctuation map reconstruction

The 3D tomographic weak lensing is one of the most important tools for modern cosmology:  Underlying the link between weak lensing and the compressed sensing theory, we have proposed a  new approach to reconstruct the dark matter distribution in two and three dimensions, using photometric redshift information. We have shown  that we can estimate with a very good accuracy the mass and redshift of dark matter haloes, which is crucial for unveiling the nature of the Dark Universe (Leonard et al. 2014). We have shown that it outperforms significantly all existing methods. In particular, we have seen using simulations that we can reconstruct two clusters on the same light of sight, which was impossible with previous methods.  The method has be chosen by the DES consortium to general its weak lensing mass map (Jeffrey et al, 2018).

Reference 1: A. Leonard, F. Lanusse and J.-L. Starck, "GLIMPSE: Accurate 3D weak lensing reconstructions using sparsity", MNRAS, 440, 2, 2014.

Reference 2: F. Lanusse, J.-L. Starck, A. Leonard, S. Pires, "High Resolution Weak Lensing Mass-Mapping Combining Shear and Flexion", Astronomy and Astrophysics, 591, id.A2, 19 pp, 2016.

Reference 3: Niall Jeffrey et al., MNRAS, 479, 2018, arXiv:1801.08945.

Press release: CEA press release


Cosmology and Fundamental Physics with the Euclid Satellite

Cosmology and Fundamental Physics with the Euclid Satellite

Understanding the source of cosmic acceleration in the universe is one of the major challenges that will be addressed by future surveys like the Euclid space mission. Acceleration may be caused by a cosmological constant or by a dynamical fluid (dark energy) or rather be a sign that the laws of gravity themselves are different at very large scales. Euclid data interpretation will aim at discriminating among these scenarios. CosmoStat is active in the Theory Working Group, and V.Pettorino led the update of the Review Cosmology and Fundamental Physics with the Euclid Satellite, published on Living Reviews in Relativity in 2018 The figure below (originally from Hu and Sawicki (2007a), replotted as Fig.19 of the Review, shows constraints expected for Euclid on the growth factor, for different cosmological scenarios.

Reference : Euclid Theory Working Group, Cosmology and Fundamental Physics with the Euclid Satellite,

DOI: published on 12 April 2018.

Radio-Interferometry: Improving the Resolution by a Factor of 4 (2 in each spatial dimension)

Radio-Interferometry: Improving the Resolution by a Factor of 4 (2 in each spatial dimension)

Sparse recovery allows us to reconstruct radio-interferometric images with a resolution increased by a factor two. This has been confirmed by comparing two images of the Cygnus A radio source, the first one from the LOFAR instrument and reconstructed using sparsity, and the second one from the Very Large Array at a higher frequency (and therefore with a better resolution). The contour of the VLA image in Fig 1 right matches perfectly the high resolution features that can be seen in the LOFAR color map image, while in Fig 1 left, we can see what the LOFAR pipeline produces on the same data. All small details which appear in the color image at right, but not in the left image are real, since they are also there in the contours, which corresponds to the VLA image at better resolution.

Reference:  Garsden, Girard, Starck, Corbel et al, A&A, 2015

Press release 1:

Press release 2: French researchers push forward radio image quality in view of SKA,  Thursday 10 November 2016.

Press release 3: Astrophysique et IRM, un mariage qui a du sens, May 17, 2017.

Cosmic Microwave Background: Joint WMAP/Planck CMB Map Recovery

Cosmic Microwave Background: Joint WMAP/Planck CMB Map Recovery

The LGMCA method has been used to reconstruct the Cosmic Microwave Background (CMB) image from WMAP 9 year and Planck-PR2 data. Based on the sparse modeling of signals , the LCS component separation method is well-suited for the extraction of foreground emissions. A joint WMAP9 year and Planck PR2 CMB has been reconstructed produce a very high quality CMB map, especially on the galactic center where it is the most difficult due to the strong foreground emissions of our Galaxy. This LGMCA CMB map estimate exhibits appealing characteristics for astrophysical and cosmological applications: i) it is a full sky map that did not require any inpainting or interpolation post-processing, ii) foreground contamination is showed to be very low even on the galactic center, iii) it does not exhibit any detectable trace of thermal SZ contamination. Furthermore,  following the principle of reproducible research, LCS provides the codes to reproduce the LGMCA map, which makes it the only reproducible CMB map.

Reference 1: J. Bobin, F. Sureau, P. Paykari, A. Rassat, S. Basak and J.-L. Starck, "WMAP 9-year CMB estimation using sparsity", Astronomy and Astrophysics , 553, L4, pp 10, 2013.
Reference 2: J. Bobin, F. Sureau and J.-L. Starck, "CMB reconstruction from the WMAP and Planck PR2 data", Astronomy and Astrophysics, 591, id.A50, 12 pp, 2016.

Press release:


Cosmic Microwave Background: Large Scale Non Gaussianities Studies

Cosmic Microwave Background: Large Scale Non Gaussianities Studies

If the LGMCA map (Bobin et al, A&A, 2014) presented above is used, then LCS has shown that this map is clean enough so no masking  is required anymore for large scale statistical analysis. We have found the most claimed anomalies in the CMB map disappear if i) we do not mask, and ii) we take  properly into account the Integrated Sachs-Wolfe effect (ISW) (Rassat and Starck, 2013). Similar results were obtained with Planck data and CMB large scales are therefore compatible with the standard l-CDM cosmological model (Rassat et al, 2014).  

Reference 1: A. Rassat, J-L. Starck, P. Paykari, F. Sureau and J. Bobin, "Planck CMB Anomalies: Astrophysical and Cosmological Foregrounds and the Curse of Masking",Journal of Cosmology and Astroparticle Physics, 08, id. 006, 2014.

Reference 2: A. Rassat and J-L. Starck, "On Preferred Axes in WMAP Cosmic Microwave Background Data after Subtraction of the Integrated Sachs-Wolfe Effect", Astronomy and Astrophysics, 557, id.L1, pp 7, 2013.

Press release 1: Science et Vie, August 2014.

Press release 2: Défis du CEA, March, 2015.

CFHTLenS: Constraining the Dark Universe with CFHTlenS Weak Lensing Survey

CFHTLenS: Constraining the Dark Universe with weak lensing from the CFHT Lensing Survey

We have taken part in the largest galaxy survey to measure the distribution of dark matter in the Universe using the gravitational lensing effect. More than 4.2 million galaxies have been observed for over 500 nights at the Canada-France Hawaii Telescope (CFHT) with the camera MegaCam, built at the CEA. Measuring the weak-lensing distortions from these galaxies, we determined the fraction of dark matter and dark energy up to 8.8 billion years in the past. Together with other experiments the results showed that the Universe is undergoing a phase of accelerated expansion, due to a yet unknown “dark-energy” component, that makes up around 70% of the cosmos. Those measurements were also use to test the laws of gravity on large scales. Some models of deviations from Einstein’s theory of general relativity could be ruled out by the data, reducing possible alternatives for the cause of the accelerated expansion of the Universe.

Reference 1: Kilbinger et al, CFHTLenS: combined probe cosmological model comparison using 2D weak gravitational lensing, MNRAS, 3, 2200-2220, 2013

Reference 2: Simpson F, Heymans C, Parkinson D, Blake C, Kilbinger M & al. 2013 MNRAS 429, 2249–2263

Press release: