Pier Stefano Corasaniti

Superacceleration as the signature of a dark sector interaction

We show that an interaction between dark matter and dark energy generically results in an effective dark-energy equation of state of w<-1. This arises because the interaction alters the redshift dependence of the matter density. An observer who fits the data treating the dark matter as noninteracting will infer an effective dark-energy fluid with w<-1. We argue that the model is consistent with all current observations, the tightest constraint coming from estimates of the matter density at different redshifts. Comparing the luminosity and angular-diameter distance relations with {lambda}CDM and phantom models, we find that the three models are degenerate within current uncertainties but likely distinguishable by the next generation of dark-energy experiments.

The Essence of Quintessence and the Cost of Compression

Standard two-parameter compressions of the infinite dimensional dark energy model space show crippling limitations even with current Type Ia supernova (SN Ia) data unless strong priors are imposed. First, they cannot cope with rapid evolutionour best fit to the latest SN Ia data shows late and very rapid evolution to w0 = -2.85. However, all of the standard parameterizations (incorrectly) claim that this best fit is ruled out at more than 2 primarily because they track it well only at very low redshift, z 0.2. Furthermore, they incorrectly rule out the observationally compatible region w L -1 for z > 1. Second, the parameterizations give wildly different estimates for the redshift of acceleration, which vary from zacc = 0.14 to zacc = 0.59. Although these failings are largely cured by including higher order terms (3 parameters), this results in new degeneracies and opens up large regions of previously ruled out parameter space. All of this casts serious doubt on the usefulness of the standard two-parameter compressions in the coming era of high-precision dark energy cosmology and emphasizes the need for decorrelated compressions with at least three parameters.

Model selection as a science driver for dark energy surveys

A key science goal of upcoming dark energy surveys is to seek time evolution of the dark energy. This problem is one of model selection, where the aim is to differentiate between cosmological models with different numbers of parameters. However, the power of these surveys is traditionally assessed by estimating their ability to constrain parameters, which is a different statistical problem. In this paper we use Bayesian model selection techniques, specifically forecasting of the Bayes factors, to compare the abilities of different proposed surveys in discovering dark energy evolution. We consider six experiments — supernova luminosity measurements by the Supernova Legacy Survey, SNAP, JEDI, and ALPACA, and baryon acoustic oscillation measurements by WFMOS and JEDI — and use Bayes factor plots to compare their statistical constraining power. The concept of Bayes factor forecasting has much broader applicability than dark energy surveys.

The impact of cosmic dust on supernova cosmology

Extinction by intergalactic grey dust introduces a magnitude redshift-dependent offset in the standard‐candle relation of supernova Type Ia. This leads to overestimated luminosity distances compared to a dust-free universe. Quantifying the amplitude of this systematic effect is crucial for an accurate determination of the dark energy parameters. In this paper, we model the grey dust extinction in terms of the star formation history of the Universe and the physical properties of the dust grains. We focus on a class of cosmic dust models which satisfy current observational constraints. These can produce an extinction as large as 0.08 mag at z = 1.7 and potentially disrupt the dark energy parameter inference from future SN surveys. In particular depending on the dust model, we find that an unaccounted extinction can bias the estimation of a constant dark energy equation of state w by shifting its best-fitting value up to 20 per cent from its true value. Near-IR broad-band photometry will hardly detect this effect, while the induced decrement of the Balmer lines requires high signal-to-noise spectra. Indeed, IR-spectroscopy will be needed for high-redshift SNe. Cosmic dust extinction may also cause a detectable violation of the distance‐duality relation. A more comprehensive knowledge of the physics of the intergalactic medium is necessary for an accurate modelling of intergalactic dust. Due to the large magnitude dispersion current luminosity distance measurements are insensitive to such possible extinction effects. In contrast, these must be taken into account if we hope to disclose the true nature of dark energy with the upcoming generation of SN Ia surveys.

Gravitational waves, inflation and the cosmic microwave background: towards testing the slow-roll paradigm

One of the fundamental and yet untested predictions of inflationary models is the generation of a very weak cosmic background of gravitational radiation. We investigate the sensitivity required for a space-based gravitational wave laser interferometer with peak sensitivity at ~1 Hz to observe such signal as a function of the model parameters and compare it with indirect limits that can be set with data from present and future cosmic microwave background missions. We concentrate on signals predicted by slow-roll single-field inflationary models and instrumental configurations such as those proposed for the LISA follow-on mission: big bang observer.

Cosmic Dust Induced Flux Fluctuations: Bad and Good Aspects

Cosmic dust extinction alters the flux of Type Ia supernovae (SNe Ia). Inhomogeneities in the dust distribution induce correlated fluctuations of the SN fluxes. We find that such correlation can be up to 60% of the signal caused by gravitational lensing magnification, with an opposite sign. Therefore, if not corrected, cosmic dust extinction is the dominant source of systematic uncertainty for future SNe Ia lensing measurement, limiting the overall S/N to be 10. On the other hand, SN flux correlation measurements can be used in combination with other lensing data to infer the level of dust extinction. This will provide a viable method to eliminate gray dust contamination from the SN Ia Hubble diagram.

Testing dark energy with the advanced liquid-mirror probe of asteroids, cosmology and astrophysics

The Advanced Liquid-Mirror Probe of Asteroids, Cosmology and Astrophysics (ALPACA) is a proposed 8-meter liquid mirror telescope surveying 1000 square degree of the southern-hemisphere sky. It will be a remarkably simple and inexpensive telescope, that nonetheless will deliver a powerful sample of optical data for studying dark energy. The bulk of the cosmological data consists of nightly, high signal-to-noise, multiband light curves of SN Ia. At the end of the three-years run ALPACA is expected to collect 100,000 SNe Ia up to z~1. This will allow us to reduce present systematic uncertainties affecting the standard-candle relation. The survey will also provide several other datasets such as the detection of baryon acoustic oscillations in the matter power spectrum and shear weak lensing measurements. In this preliminary analysis we forecast constraints on dark energy parameters from SN Ia and baryon acoustic oscillations. The combination of these two datasets will provide competitive constraints on the dark energy parameters under minimal prior assumptions. Further studies are needed to address the accuracy of weak lensing measurements.

Indirect limit on the amplitude of primordial gravitational wave background from CMB-galaxy cross correlation

While large scale cosmic microwave background (CMB) anisotropies involve a combination of the scalar and tensor fluctuations, the scalar amplitude can be independently determined through the CMB-galaxy cross correlation. Using recently measured cross correlation amplitudes, arising from the cross correlation between galaxies and the integrated Sachs-Wolfe (ISW) effect in CMB anisotropies, we obtain a constraint r<0.5 at 68% confidence level on the tensor-to-scalar fluctuation amplitude ratio. The data also allow us to exclude gravity waves at a level of a few percent, relative to the density field, in a low-Lambda dominated universe ({omega}{sub {lambda}}{approx}0.5). In future, joining cross correlation ISW measurements, which captures cosmological parameter information, with independent determinations of the matter density and CMB anisotropy power spectrum, may constrain the tensor-to-scalar ratio to a level above 0.05. This value is the ultimate limit on tensor-to-scalar ratio from temperature anisotropy maps when all other cosmological parameters except for the tensor amplitude are known and the combination with CMB-galaxy correlation allows this limit to be reached easily be accounting for degeneracies in certain cosmological parameters.