In the next subsections we explore how dark energy or modified gravity effects can be detected through weak lensing and redshift surveys.
Quite generally, cosmological observations fall into two categories: geometrical probes and structure formation probes. While the former provide a measurement of the Hubble function, the latter are a test of the gravitational theory in an almost Newtonian limit on subhorizon scales. Furthermore, possible effects on the geodesics of test particles need to be derived: naturally, photons follow null-geodesics while massive particles, which constitute the cosmic large-scale structure, move along geodesics for non-relativistic particles.
In some special cases, modified gravity models predict a strong deviation from the standard Friedmann equation as in, e.g., DGP, (1.4.74). While the Friedmann equation is not know explicitly in more general models of massive gravity (cascading gravity or hard mass gravity), similar modifications are expected to arise and provide characteristic features, [see, e.g., 11, 478]) that could distinguish these models from other scenarios of modified gravity or with additional dynamical degrees of freedom.
In general however the most interesting signatures of modified gravity models are to be found in the perturbation sector. For instance, in DGP, growth functions differ from those in dark-energy models by a few percent for identical Hubble functions, and for that reason, an observation of both the Hubble and the growth function gives a handle on constraining the gravitational theory, . The growth function can be estimated both through weak lensing and through galaxy clustering and redshift distortions.
Concerning the interactions of light with the cosmic large-scale structure, one sees a modified coupling in general models and a difference between the metric potentials. These effects are present in the anisotropy pattern of the CMB, as shown in , where smaller fluctuations were found on large angular scales, which can possibly alleviate the tension between the CMB and the CDM model on small multipoles where the CMB spectrum acquires smaller amplitudes due to the ISW-effect on the last-scattering surface, but provides a worse fit to supernova data. An interesting effect inexplicable in GR is the anticorrelation between the CMB temperature and the density of galaxies at high redshift due to a sign change in the integrated Sachs–Wolfe effect. Interestingly, this behavior is very common in modified gravity theories.
A very powerful probe of structure growth is of course weak lensing, but to evaluate the lensing effect it is important to understand the nonlinear structure formation dynamics as a good part of the total signal is generated by small structures. Only recently has it been possible to perform structure formation simulations in modified gravity models, although still without a mechanism in which GR is recovered on very small scales, necessary to be in accordance with local tests of gravity.
In contrast, the number density of collapsed objects relies only little on nonlinear physics and can be used to investigate modified gravity cosmologies. One needs to solve the dynamical equations for a spherically symmetric matter distribution. Modified gravity theories show the feature of lowering the collapse threshold for density fluctuations in the large-scale structure, leading to a higher comoving number density of galaxies and clusters of galaxies. This probe is degenerate with respect to dark-energy cosmologies, which generically give the same trends.
The magnification matrix is a matrix that relates the true shape of a galaxy to its image. It contains two distinct parts: the convergence, defined as the trace of the matrix, modifies the size of the image, whereas the shear, defined as the symmetric traceless part, distorts the shape of the image. At small scales the shear and the convergence are not independent. They satisfy a consistency relation, and they contain therefore the same information on matter density perturbations. More precisely, the shear and the convergence are both related to the sum of the two Bardeen potentials, , integrated along the photon trajectory. At large scales however, this consistency relation does not hold anymore. Various relativistic effects contribute to the convergence, see . Some of these effects are generated along the photon trajectory, whereas others are due to the perturbations of the galaxies redshift. These relativistic effects provide independent information on the two Bardeen potentials, breaking their degeneracy. The convergence is therefore a useful quantity that can increase the discriminatory power of weak lensing.
The convergence can be measured through its effect on the galaxy number density, see e.g. . The standard method extracts the magnification from correlations of distant quasars with foreground clusters, see [804, 657]. Recently, [977, 978] designed a new method that permits to accurately measure auto-correlations of the magnification, as a function of the galaxies redshift. This method potentially allows measurements of the relativistic effects in the convergence.
We are interested in computing the magnification matrix in a perturbed Friedmann universe. The magnification matrix relates the true shape of a galaxy to its image, and describes therefore the deformations encountered by a light bundle along its trajectory. can be computed by solving Sachs equation, see , that governs propagation of light in a generic geometry. The convergence and the shear are then defined respectively as the trace and the symmetric traceless part of
We consider a spatially flat () Friedmann universe with scalar perturbations. We start from the usual longitudinal (or Newtonian) gauge where the metric is given by
Eq. (1.7.3) and the first term in Eq. (1.7.4) are the standard contributions of the shear and the convergence, but expressed here with the full-sky transverse operators
The other terms in Eq. (1.7.4) are relativistic corrections to the convergence, that are negligible at small scales but may become relevant at large scales. The terms in the first line are intrinsic corrections, generated respectively by the curvature perturbation at the source position and the Shapiro time-delay. The terms in the second line are due to the fact that we measure the convergence at a fixed redshift of the source rather that at a fixed conformal time . Since in a perturbed universe, the observable redshift is itself a perturbed quantity, this transformation generates additional contribution to the convergence. Those are respectively the Sachs–Wolfe contribution, the Doppler contribution and the integrated Sachs–Wolfe contribution. Note that we have neglected the contributions at the observer position since they only give rise to a monopole or dipole term. The dominant correction to the convergence is due to the Doppler term. Therefore in the following we are interested in comparing its amplitude with the amplitude of the standard contribution. To that end we define and as
The convergence is not directly observable. However it can be measured through the modifications that it induces on the galaxy number density. Let us introduce the magnification
The two components of the convergence and (and consequently the galaxy number overdensity) are functions of redshift and direction of observation . We can therefore determine the angular power spectrum
So far the derivation has been completely generic. Eqs. (1.7.3) and (1.7.4) are valid in any theory of gravity whose metric can be written as in Eq. (1.7.2). To evaluate the angular power spectrum we now have to be more specific. In the following we assume GR, with no anisotropic stress such that . We use the Fourier transform convention
We evaluate and in a CDM universe with , and . We approximate the transfer function with the BBKS formula, see . In Figure 9, we plot and for various source redshifts. The amplitude of and depends on , which varies with the redshift of the source, the flux threshold adopted, and the sky coverage of the experiment. Since influences and in the same way we do not include it in our plot. Generally, at small redshifts, is smaller than 1 and consequently the amplitude of both and is slightly reduced, whereas at large redshifts tends to be larger than 1 and to amplify and , see e.g., . However, the general features of the curves and more importantly the ratio between and are not affected by .
Figure 9 shows that peaks at rather small , between 30 and 120 depending on the redshift. This corresponds to rather large angle . This behavior differs from the standard term (Figure 9) that peaks at large . Therefore, it is important to have large sky surveys to detect the velocity contribution. The relative importance of and depends strongly on the redshift of the source. At small redshift, , the velocity contribution is about and is hence larger than the standard contribution which reaches . At redshift , is about 20% of , whereas at redshift , it is about of . Then at redshift and above, becomes very small with respect to : . The enhancement of at small redshift together with its fast decrease at large redshift are due to the prefactor in Eq. (1.7.15). Thanks to this enhancement we see that if the magnification can be measured with an accuracy of 10%, then the velocity contribution is observable up to redshifts . If the accuracy reaches 1% then the velocity contribution becomes interesting up to redshifts of order 1.
The shear and the standard contribution in the convergence are not independent. One can easily show that their angular power spectra satisfy the consistency relation, see 
Note that in practice, in weak lensing tomography, the angular power spectrum is computed in redshift bins and therefore the square bracket in Eq. (1.7.17) has to be integrated over the bin
Wide-deep galaxy redshift surveys have the power to yield information on both and through measurements of Baryon Acoustic Oscillations (BAO) and redshift-space distortions. In particular, if gravity is not modified and matter is not interacting other than gravitationally, then a detection of the expansion rate is directly linked to a unique prediction of the growth rate. Otherwise galaxy redshift surveys provide a unique and crucial way to make a combined analysis of and to test gravity. As a wide-deep survey, Euclid allows us to measure directly from BAO, but also indirectly through the angular diameter distance (and possibly distance ratios from weak lensing). Most importantly, Euclid survey enables us to measure the cosmic growth history using two independent methods: from galaxy clustering, and from weak lensing. In the following we discuss the estimation of and from galaxy clustering.
From the measure of BAO in the matter power spectrum or in the 2-point correlation function one can infer information on the expansion rate of the universe. In fact, the sound waves imprinted in the CMB can be also detected in the clustering of galaxies, thereby completing an important test of our theory of gravitational structure formation.
The BAO in the radial and tangential directions offer a way to measure the Hubble parameter and angular diameter distance, respectively. In the simplest FLRW universe the basis to define distances is the dimensionless, radial, comoving distance:
The characteristic scale of the BAO is set by the sound horizon at decoupling. Consequently, one can attain the angular diameter distance and Hubble parameter separately. This scale along the line of sight () measures through , while the tangential mode measures the angular diameter distance .
One can then use the power spectrum to derive predictions on the parameter constraining power of the survey (see e.g., [46, 418, 938, 945, 308]).
In order to explore the cosmological parameter constraints from a given redshift survey, one needs to specify the measurement uncertainties of the galaxy power spectrum. In general, the statistical error on the measurement of the galaxy power spectrum at a given wave-number bin is 
In general, the observed galaxy power spectrum is different from the true spectrum, and it can be reconstructed approximately assuming a reference cosmology (which we consider to be our fiducial cosmology) as (e.g., )linear bias factor between galaxy and matter density distributions, is the linear growth rate,4 and is the linear redshift-space distortion parameter . The linear matter power spectrum in Eq. (1.7.27) takes the form scale independent linear growth-factor in the absence of massive neutrino free-streaming (see Eq. (25) in ), whose fiducial value in each redshift bin is computed through numerical integration of the differential equations governing the growth of linear perturbations in presence of dark energy  or employing the approximation of Eq. (1.3.22). depends on matter and baryon densities5 (neglecting dark energy at early times), and is computed in each redshift bin using a Boltzmann code like camb6  or cmbfast.
In Eq. (1.7.29) a damping factor has been added, due to redshift uncertainties, where , being the comoving distance [940, 815], and assumed that the power spectrum of primordial curvature perturbations, , is
In the limit where the survey volume is much larger than the scale of any features in , it has been shown that the redshift survey Fisher matrix for a given redshift bin can be approximated as 
To minimize nonlinear effects, one should restrict wave-numbers to the quasi-linear regime, e.g., imposing that is given by requiring that the variance of matter fluctuations in a sphere of radius is, for instance, for . Or one could model the nonlinear distortions as in . On scales larger than () where we focus our analysis, nonlinear effects can be represented in fact as a displacement field in Lagrangian space modeled by an elliptical Gaussian function. Therefore, following [338, 816], to model nonlinear effect we multiply by the factor
Finally, we note that when actual data are available, the usual way to measure is by fitting the measured galaxy redshift-space correlation function to a model :7
The bias between galaxy and matter distributions can be estimated from either galaxy clustering, or weak lensing. To determine bias, we can assume that the galaxy density perturbation is related to the matter density perturbation as :
Bias can be derived from galaxy clustering by measuring the galaxy bispectrum:
In general, bias can be measured from weak lensing through the comparison of the shear-shear and shear-galaxy correlations functions. A combined constraint on bias and the growth factor can be derived from weak lensing by comparing the cross-correlations of multiple redshift slices.
Of course, if bias is assumed to be linear () and scale independent, or is parametrized in some simple way, e.g., with a power law scale dependence, then it is possible to estimate it even from linear galaxy clustering alone, as we will see in Section 1.8.3.
As we have seen, the additional redshift induced by the galaxy peculiar velocity field generates the redshift distortion in the power spectrum. In this section we discuss a related effect on the luminosity of the galaxies and on its use to measure the peculiar velocity in large volumes, the so-called bulk flow.
In the gravitational instability framework, inhomogeneities in the matter distribution induce gravitational accelerations , which result in galaxies having peculiar velocities that add to the Hubble flow. In linear theory the peculiar velocity field is proportional to the peculiar acceleration
Constraints on the power spectrum and growth rate can be obtained by comparing the bulk flow estimated from the volume-averaged motion of the sphere of radius :
Over the years the bulk flows has been estimated from the measured peculiar velocities of a large variety of objects ranging from galaxies [397, 398, 301, 256, 271, 788] clusters of galaxies [549, 165, 461] and SN Ia . Conflicting results triggered by the use of error-prone distance indicators have fueled a long lasting controversy on the amplitude and convergence of the bulk flow that is still on. For example, the recent claim of a bulk flow of within , inconsistent with expectation from the CDM model, has been seriously challenged by the re-analysis of the same data by  who found a bulk flow amplitude consistent with CDM expectations and from which they were able to set the strongest constraints on modified gravity models so far. On larger scales,  claimed the detection of a dipole anisotropy attributed to the kinetic SZ decrement in the WMAP temperature map at the position of X-ray galaxy clusters. When interpreted as a coherent motion, this signal would indicate a gigantic bulk flow of within . This highly debated result has been seriously questioned by independent analyses of WMAP data [see, e.g., 699])
The large, homogeneous dataset expected from Euclid has the potential to settle these issues. The idea is to measure bulk flows in large redshift surveys, based on the apparent, dimming or brightening of galaxies due to their peculiar motion. The method, originally proposed by , has been recently extended by  who propose to estimate the bulk flow by minimizing systematic variations in galaxy luminosities with respect to a reference luminosity function measured from the whole survey. It turns out that, if applied to the photo- catalog expected from Euclid, this method would be able to detect at significance a bulk flow like the one of  over independent spherical volumes at , provided that the systematic magnitude offset over the corresponding areas in the sky does not exceed the expected random magnitude errors of 0.02 – 0.04 mag. Additionally, photo- or spectral- could be used to validate or disproof with very large () significance the claimed bulk flow detection of  at .
Closely related to the bulk flow is the Local Group peculiar velocity inferred from the observed CMB dipole 
As for the bulk flow case, despite the many measurements of cosmological dipoles using galaxies [972, 283, 654, 868, 801, 513] there is still no general consensus on the scale of convergence and even on the convergence itself. Even the recent analyses of measuring the acceleration of the Local Group from the 2MASS redshift catalogs provided conflicting results.  found that the galaxy dipole seems to converge beyond , whereas  find no convergence within .
Once again, Euclid will be in the position to solve this controversy by measuring the galaxy and cluster dipoles not only at the LG position and out to very large radii, but also in several independent ad truly all-sky spherical samples carved out from the the observed areas with . In particular, coupling photometry with photo- one expects to be able to estimate the convergence scale of the flux-weighted dipole over about 100 independent spheres of radius out to and, beyond that, to compare number-weighted and flux-weighted dipoles over a larger number of similar volumes using spectroscopic redshifts.
Living Rev. Relativity 16, (2013), 6
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