Abstract. Interactions between dark matter and dark energy, allowing both conformal and and disformal couplings, are studied in detail. We discuss the background evolution, anisotropies in the cosmic microwave background and large scale structures. One of our main findings is that a large conformal coupling is not necessarily disallowed in the presence of a general disformal term. On the other hand, we find that negative disformal couplings very often lead to instabilities in the scalar field. Studying the background evolution and linear perturbations only, our results show that it is observationally challenging to disentangle disformal from purely conformal couplings.

Certain modified gravity theories predict the existence of an additional, non-conformally coupled scalar field. A disformal coupling of the field to the Cosmic Microwave Background (CMB) is shown to affect the evolution of the energy density in the radiation fluid and produces a modification of the distribution function of the CMB, which vanishes if photons and baryons couple in the same way to the scalar. We find the constraints on the couplings to matter and photons coming from the measurement of the CMB temperature evolution and from current upper limits on the µ-distortion of the CMB spectrum. We also point out that the measured equation of state of photons differs from wγ = 1/3 in the presence of disformal couplings. PACS numbers:Among the extensions of General Relativity (GR) which have attracted a lot of attention in the literature are scalar-tensor theories, which are also well motivated by some extensions of the standard model such as theories with extra spatial dimensions. In cosmology, those theories have been studied in the context of inflation and models for dark energy as they provide a natural mechanism to drive a period of accelerated expansion in the very early or late universe, see e.g. [1][2][3]. There are some open questions regarding scalar-tensor theories, such as whether the Einstein frame (EF) or Jordan frame (JF) formulation of a given theory is the most "physical", see e.g. [4] and references therein for a discussion.The EF is the frame in which the gravity and scalar sector look like GR plus a scalar field, but matter fields feel a metric which is usually conformally related to the EF metric g µν . In the JF formulation of the theory, matter fields are uncoupled to the scalar, but the spin-2 and spin-0 fields are coupled in a non-trivial way. One could also consider more general transformations of the metric, in which the EF and the JF metrics are not related by a conformal transformation, but a so-called disformal transformation, which includes derivatives of the scalar field (see the definition in Eq. (2) In this paper, we consider scalar-tensor theories with disformally coupled scalar fields and study in particular the consequences on the properties of the CMB. We express observables in terms of the metric a (baryonic) observer feels, to side-step the question about a frameindependent formulation for disformally coupled fields. * Email address: C.vandeBruck@sheffield.ac.uk † Email address: app12jam@sheffield.ac.uk ‡ Email address: Susan.Vu@sheffield.ac.ukFrame-independence in conformally coupled theories has been discussed in [14,15]. The theory we consider is given in the EF aswhere the different matter fields χ i propagate on geodesics for metrics given bỹFrom now on, reduced Planck units are assumed: c = = k B = M Pl = 1. We consider a cosmological setting for eq. (1), where g µν is the standard flat FRW metric, with dsVariation of the action with respect to the fields yields the Klein-Gordon equation for the scalar field φ and the energy conservation equations:where the couplin...

We consider a cosmology in which dark matter and a quintessence scalar field responsible for the acceleration of the Universe are allowed to interact. Allowing for both conformal and disformal couplings, we perform a global analysis of the constraints on our model using Hubble parameter measurements, baryon acoustic oscillation distance measurements, and a Supernovae Type Ia data set. We find that the additional disformal coupling relaxes the conformal coupling constraints. Moreover, we show that, at the background level, a disformal interaction within the dark sector is preferred to both ΛCDM and uncoupled quintessence, hence favoring interacting dark energy.

The simplest way to modify gravity is to extend the gravitational sector to include an additional scalar degree of freedom. The most general metric that can be built in such a theory includes disformal terms, so that standard model fields move on a metric which is the sum of the space time metric and a tensor constructed from first derivatives of the scalar. In such a theory gravitational waves and photons can propagate at different speeds, and these can in turn be different from the maximum speed limit for matter particles. In this work we show that disformal couplings can cause charged particles to emit Cherenkov radiation and bremsstrahlung apparently in vacuum, depending on the background evolution of the scalar field. We discuss the implications of this for observations of cosmic rays, and the constraints that arise for models of dark energy with disformal couplings.

The prevalence of congenital hearing impairment (HI) is highest in Africa. Estimates evaluated genetic causes to account for 31% of HI cases in Africa, but the identification of associated causative genes mutations have been challenging. In this study, we reviewed the potential roles, in humans, of 38 novel genes identified in a murine study. We gathered information from various genomic annotation databases and performed functional enrichment analysis using online resources i.e. genemania and g.proflier. Results revealed that 27/38 genes are express mostly in the brain, suggesting additional cognitive roles. Indeed, HERC1- R3250X had been associated with intellectual disability in a Moroccan family. A homozygous 216-bp deletion in KLC2 was found in two siblings of Egyptian descent with spastic paraplegia. Up to 27/38 murine genes have link to at least a disease, and the commonest mode of inheritance is autosomal recessive (n=8). Network analysis indicates that 20 other genes have intermediate and biological links to the novel genes, suggesting their possible roles in HI. This study will contribute to advance our knowledge in unravelling the biological roles of novel murine HI genes in humans and could enhance the understanding of the genetic causes of HI in Africans.