Cosmology and fundamental physics with the Euclid satellite

TL;DR

Euclid’s Cosmology and Fundamental Physics review outlines how its weak-lensing and redshift surveys will probe dark energy, modified gravity, and the dark sector with unprecedented precision. It articulates a framework based on background evolution and linear perturbations, introducing two key growth parameters (Q and η) to capture deviations from GR, and surveys a broad landscape of DE/MG models (quintessence, k-essence, f(R), DGP, scalar-tensor theories) along with nonlinear tools (N-body simulations, spherical collapse) to predict observable signatures. The paper discusses observational strategies and forecast techniques, including Planck priors and PCA/binned approaches, and emphasizes the role of weak lensing, BAO, RSD, and bulk flows in breaking degeneracies between expansion history and gravity. It also surveys DM and neutrino physics, detailing how Euclid constrains halo functions, neutrino masses and hierarchies, warm DM, ultralight fields, and DM surrogates in MG theories, highlighting the synergy with X-ray surveys and other probes. Overall, Euclid is positioned to decisively test the nature of cosmic acceleration and the physics of the dark sector, with quantitative forecasts for w, γ, μ, Σ, and neutrino properties, ultimately guiding model discrimination and fundamental physics discoveries.

Abstract

Euclid is a European Space Agency medium class mission selected for launch in 2019 within the Cosmic Vision 2015-2025 programme. The main goal of Euclid is to understand the origin of the accelerated expansion of the Universe. Euclid will explore the expansion history of the Universe and the evolution of cosmic structures by measuring shapes and redshifts of galaxies as well as the distribution of clusters of galaxies over a large fraction of the sky. Although the main driver for Euclid is the nature of dark energy, Euclid science covers a vast range of topics, from cosmology to galaxy evolution to planetary research. In this review we focus on cosmology and fundamental physics, with a strong emphasis on science beyond the current standard models. We discuss five broad topics: dark energy and modified gravity, dark matter, initial conditions, basic assumptions and questions of methodology in the data analysis. This review has been planned and carried out within Euclid's Theory Working Group and is meant to provide a guide to the scientific themes that will underlie the activity of the group during the preparation of the Euclid mission.

Figures (57)

• Figure 1: LEFT: the Cosmic Microwave Background angular power spectrum $l(l+1)C_l/(2\pi)$ for TeVeS (solid) and $\Lambda$CDM (dotted) with WMAP 5-year data NoltaEtAl2008. RIGHT: the matter power spectrum $P(k)$ for TeVeS (solid) and $\Lambda$CDM (dotted) plotted with SDSS data.
• Figure 2: The evolution of $w$ as a function of the comoving scale $k$, using only the 5-year WMAP CMB data. Red and yellow are the 95% and 68% confidence regions for the LV formalism. Blue and purple are the same for the flow-equation formalism. From the outside inward, the colored regions are red, yellow, blue, and purple. [Figure from Ilic et al 2010]
• Figure 3: The complete evolution of $w(N)$, from the flow-equation results accepted by the CMB likelihood. Inflation is made to end at $N=0$ where $w(N=0)=-1/3$ corresponding to $\epsilon_H(N=0)=1$. For our choice of priors on the slow-roll parameters at $N=0$, we find that $w$ decreases rapidly towards $-1$ (see inset) and stays close to it during the period when the observable scales leave the horizon ($N\approx 40 - 60$). [Figure from Ilic et al 2010]
• Figure 4: Required accuracy on $w_\text{eff} = -1$ to obtain strong evidence against a model where $-1 - \Delta_{-} \leq w_\text{eff} \leq -1+\Delta_+$ as compared to a cosmological constant model, $w=-1$. For a given $\sigma$, models to the right and above the contour are disfavoured with odds of more than $20:1$.
• Figure 5: Ratio of the total mass functions, which include the quintessence contribution, for $c_s=0$ and $c_s=1$ at $z=0$ (above) and $z=1$ (below). [Figures from Creminelli/etal:2010.]