Constraints on modified gravity from Planck 2015: when the health of your theory makes the difference

We use the effective field theory of dark energy (EFT of DE) formalism to constrain dark energy models belonging to the Horndeski class with the recent Planck 2015 CMB data. The space of theories is spanned by a certain number of parameters determining the linear cosmological perturbations, while the expansion history is set to that of a standard $\Lambda$CDM model. We always demand that the theories be free of fatal instabilities. Additionally, we consider two optional conditions, namely that scalar and tensor perturbations propagate with subliminal speed. Such criteria severely restrict the allowed parameter space and are thus very effective in shaping the posteriors. As a result, we confirm that no theory performs better than $\Lambda$CDM when CMB data alone are analysed. Indeed, the healthy dark energy models considered here are not able to reproduce those phenomenological behaviours of the effective Newton constant and gravitational slip parameters that, according to previous studies, best fit the data.Comment: 21 pages, 8 figures. Added Mu-Sigma plane in Fig.7 plus some changes in the text with respect to the previous version. This is an author-created un-copyedited version of the article published in JCAP. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscrip

Constraints on Modified Gravity from ACT and SPT

The Atacama Cosmology Telescope (ACT) and the South Pole Telescope (SPT) have recently provided new and precise measurements of the Cosmic Microwave Background anisotropy damping tail. This region of the CMB angular spectra, thanks to the angular distortions produced by gravitational lensing, can probe the growth of matter perturbations and provide a test for general relativity. Here we make use of the ACT and SPT power spectrum measurements (combined with the recent WMAP9 data) to constrain f(R) gravity theories. Adopting a parametrized approach, we obtain an upper limit on the lengthscale of the theory of B_0 < 0.86 at 95% c.l. from ACT, while we get a significantly stronger bound from SPT with B_0 < 0.14 at 95% c.l..Comment: 6 pages, 4 figures, some sentences correcte

Indications of a late-time interaction in the dark sector

We show that a general late-time interaction between cold dark matter and vacuum energy is favoured by current cosmological datasets. We characterize the strength of the coupling by a dimensionless parameter $q_V$ that is free to take different values in four redshift bins from the primordial epoch up to today. This interacting scenario is in agreement with measurements of cosmic microwave background temperature anisotropies from the Planck satellite, supernovae Ia from Union 2.1 and redshift space distortions from a number of surveys, as well as with combinations of these different datasets. We show that a non-zero interaction is very likely at late times. We then focus on the case $q_V\not=0$ in a single low-redshift bin, obtaining a nested one parameter extension of the standard $\Lambda$CDM model. We study the Bayesian evidence, with respect to $\Lambda$CDM, of this late-time interaction model, finding moderate evidence for an interaction starting at $z=0.9$, dependent upon the prior range chosen for the interaction strength parameter $q_V$. For this case the null interaction ($q_V=0$, i.e.$\Lambda$CDM) is excluded at 99% c.l..Comment: version to appear in PRL, 6 pages, 6 figure

Parametrised modified gravity and the CMB Bispectrum

We forecast the constraints on modified theories of gravity from the cosmic microwave background (CMB) anisotropies bispectrum that arises from correlations between lensing and the Integrated Sachs-Wolfe effect. In models of modified gravity the evolution of the metric potentials is generally altered and the contribution to the CMB bispectrum signal can differ significantly from the one expected in the standard cosmological model.We adopt a parametrised approach and focus on three different classes of models: Linder's growth index, Chameleon-type models and f(R) theories. We show that the constraints on the parameters of the models will significantly improve with future CMB bispectrum measurements.Comment: 7 pages, 6 figure

Future constraints on the Hu-Sawicki modified gravity scenario

We present current and future constraints on the Hu and Sawicki modified gravity scenario. This model can reproduce a late time accelerated universe and evade solar system constraints. While current cosmological data still allows for distinctive deviations from the cosmological constant picture, future measurements of the growth of structure combined with Supernova Ia luminosity distance data will greatly improve present constraints.Comment: 8 pages, 12 figure

Multi-Channel Auto-Calibration for the Atmospheric Imaging Assembly using Machine Learning

Solar activity plays a quintessential role in influencing the interplanetary medium and space-weather around the Earth. Remote sensing instruments onboard heliophysics space missions provide a pool of information about the Sun's activity via the measurement of its magnetic field and the emission of light from the multi-layered, multi-thermal, and dynamic solar atmosphere. Extreme UV (EUV) wavelength observations from space help in understanding the subtleties of the outer layers of the Sun, namely the chromosphere and the corona. Unfortunately, such instruments, like the Atmospheric Imaging Assembly (AIA) onboard NASA's Solar Dynamics Observatory (SDO), suffer from time-dependent degradation, reducing their sensitivity. Current state-of-the-art calibration techniques rely on periodic sounding rockets, which can be infrequent and rather unfeasible for deep-space missions. We present an alternative calibration approach based on convolutional neural networks (CNNs). We use SDO-AIA data for our analysis. Our results show that CNN-based models could comprehensively reproduce the sounding rocket experiments' outcomes within a reasonable degree of accuracy, indicating that it performs equally well compared with the current techniques. Furthermore, a comparison with a standard "astronomer's technique" baseline model reveals that the CNN approach significantly outperforms this baseline. Our approach establishes the framework for a novel technique to calibrate EUV instruments and advance our understanding of the cross-channel relation between different EUV channels.Comment: 12 pages, 7 figures, 8 tables. This is a pre-print of an article submitted and accepted by A&A Journa

New constraints on coupled dark energy from the Planck satellite experiment

We present new constraints on coupled dark energy from the recent measurements of the cosmic microwave background anisotropies from the Planck satellite mission. We found that a coupled dark energy model is fully compatible with the Planck measurements, deriving a weak bound on the dark matter-dark energy coupling parameter xi = -0.49(-0.31)(+0.19) at 68% C.L. Moreover if Planck data are fitted to a coupled dark energy scenario, the constraint on the Hubble constant is relaxed to H-0 = 72.1(-2.3)(+3.2) km/s/Mpc, solving the tension with the Hubble Space Telescope (HST) value. We show that a combined PLANCK + HST analysis provides significant evidence for coupled dark energy finding a nonzero value for the coupling parameter xi, with -0.90 < xi < -0.22 at 95% C.L. We also consider the combined constraints from the Planck data plus the baryon acoustic oscillation measurements of the 6dF Galaxy Survey, the Sloan Digital Sky Survey and the Baron Oscillation Spectroscopic Survey