We show how one can test the cosmological Poisson equation by requiring only the validity of three main assumptions: the energy-momentum conservation equations of matter, the equivalence principle, and the cosmological principle. We first point out that one can only measure the combination <math><mi>M</mi><mo>≡</mo><msubsup><mrow><mi>Ω</mi></mrow><mrow><mi>m</mi></mrow><mrow><mo>(</mo><mn>0</mn><mo>)</mo></mrow></msubsup><mi>μ</mi></math>, where μ quantifies the deviation of the Poisson equation from the standard one and <math><msubsup><mrow><mi>Ω</mi></mrow><mrow><mi>m</mi></mrow><mrow><mo>(</mo><mn>0</mn><mo>)</mo></mrow></msubsup></math> is the fraction of matter density at present. Then we employ a recent model-independent forecast for the growth rate <math><mi>f</mi><mo>(</mo><mi>z</mi><mo>)</mo></math> and the expansion rate <math><mi>E</mi><mo>(</mo><mi>z</mi><mo>)</mo></math> to obtain constraints on <math><mi>M</mi></math> for a survey that approximates a combination of the Dark Energy Spectroscopic Instrument (DESI) and Euclid. We conclude that a constant <math><mi>M</mi></math> can be measured with a relative error <math><msub><mrow><mi>σ</mi></mrow><mrow><mi>M</mi></mrow></msub><mo>=</mo><mn>4.5</mn><mtext>%</mtext></math>, while if <math><mi>M</mi></math> is arbitrarily varying in redshift, it can be measured only to within 13.4% (1 σ c.l.) at redshift <math><mi>z</mi><mo>=</mo><mn>0.9</mn></math>, and 15-22% up to <math><mi>z</mi><mo>=</mo><mn>1.5</mn></math>. We also project our constraints on some parametrizations of <math><mi>M</mi></math> proposed in literature, while still maintaining model-independence for the background expansion, the power spectrum shape, and the non-linear corrections. Generally speaking, as expected, we find much weaker model-independent constraints than found so far for such models. This means that the cosmological Poisson equation remains quite open to various alternative gravity and dark energy models.

Germany

Ziyang Zheng

Ziad Sakr

Luca Amendola

Testing the cosmological Poisson equation in a model-independent way

We introduce a novel theory of gravity based on the inverse of the Ricci tensor, that we call the anticurvature tensor. We derive the general equations of motion for any Lagrangian function of the curvature and anticurvature scalars. We then demonstrate a no-go theorem: no Lagrangian that contains terms linear in any positive or negative power of the anticurvature scalar can drive an evolution from deceleration to acceleration, as required by observations. This effectively rules out many realizations of this theory, as we illustrate in detail in a particular case. Finally, we speculate on how to circumvent the no-go theorem.

Brazil

Leonardo Giani

Giorgio Laverda

Ricci-inverse gravity: A novel alternative gravity, its flaws, and how to cure them

We investigate the cosmological perturbations in the context of a Scalar-Tensor-Vector theory of Gravity known as MOG in the literature. Recent investigations show that MOG reproduces a viable background cosmological evolution comparable to ΛCDM. However, the matter dominated era is slightly different. In this paper, we study the linear matter perturbations and estimate the relevant modified gravity parameters. We show that MOG reduces the growth rate of the perturbations and comparing with the RSD data reveals that MOG suggests a higher value for <math><msub><mrow><mi>σ</mi></mrow><mrow><mn>8</mn></mrow></msub></math>, compare to ΛCDM. This point, constitute a powerful challenge to the cosmological viability of MOG.

Iran

Sara Jamali

Mahmood Roshan

Linear cosmological perturbations in scalar-tensor-vector gravity

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 <math><mi>f</mi><mo>(</mo><mi>R</mi><mo>)</mo></math> 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. <math><mi>f</mi><mo>(</mo><mi>R</mi><mo>)</mo></math> models and coupled dark energy.

France

Valeria Pettorino

Beyond self-acceleration: Force- and fluid-acceleration

Many theories of modified gravity with higher order derivatives are usually ignored because of serious problems that appear due to an additional ghost degree of freedom. Most dangerously, it causes an immediate decay of the vacuum. However, breaking Lorentz invariance can cure such abominable behavior. By analyzing a model that describes a massive graviton together with a remaining Boulware-Deser ghost mode we show that even ghostly theories of modified gravity can yield models that are viable at both classical and quantum levels and, therefore, they should not generally be ruled out. Furthermore, we identify the most dangerous quantum scattering process that has the main impact on the decay time and find differences to simple theories that only describe an ordinary scalar field and a ghost. Additionally, constraints on the parameters of the theory including some upper bounds on the Lorentz-breaking cutoff scale are presented. In particular, for a simple theory of massive gravity we find that a breaking of Lorentz invariance is allowed to happen even at scales above the Planck mass. Finally, we discuss the relevance to other theories of modified gravity.

Frank Könnig

Henrik Nersisyan

Yashar Akrami

Sweden

Miguel Zumalacárregui

A spectre is haunting the cosmos: quantum stability of massive gravity with ghosts

Modified gravity theories predict in general a non-standard equation for the propagation of gravitational waves. Here we discuss the impact of modified friction and speed of tensor modes on cosmic microwave polarization B modes. We show that the non-standard friction term, parametrized by αM , is degenerate with the tensor-to-scalar ratio r , so that small values of r can be compensated by negative constant values of αM . We quantify this degeneracy and its dependence on the epoch at which αM is different from the standard, zero, value and on the speed of gravitational waves cT . In the particular case of scalar-tensor theories, αM is constant and strongly constrained by background and scalar perturbations, 0≤αM&lt;0.01 and the degeneracy with r is removed. In more general cases however such tight bounds are weakened and the B modes can provide useful constraints on early-time modified gravity.