In this first paper we discuss the linear theory and the background evolution of a new class of models we dub SCDEW: Strongly Coupled DE, plus WDM. In these models, WDM dominates todays matter density; like baryons, WDM is uncoupled. Dark Energy is a
scalar field $Phi$; its coupling to ancillary CDM, whose todays density is $ll 1, %$, is an essential model feature. Such coupling, in fact, allows the formation of cosmic structures, in spite of very low WDM particle masses ($sim 100$ eV). SCDEW models yields Cosmic Microwave Background and linear Large Scale features substantially undistinguishable from $Lambda$CDM, but thanks to the very low WDM masses they strongly alleviate $Lambda$CDM issues on small scales, as confirmed via numerical simulations in the II associated paper. Moreover SCDEW cosmologies significantly ease the coincidence and fine tuning problems of $Lambda$CDM and, by using a field theory approach, we also outline possible links with inflationary models. We also discuss a possible fading of the coupling at low redshifts which prevents non linearities on the CDM component to cause computational problems. The (possible) low-$z$ coupling suppression, its mechanism, and its consequences are however still open questions -not necessarily problems- for SCDEW models. The coupling intensity and the WDM particle mass, although being extra parameters in respect to $Lambda$CDM, are found to be substantially constrained a priori so that, if SCDEW is the underlying cosmology, we expect most data to fit also $Lambda$CDM predictions.
Models including an energy transfer from CDM to DE are widely considered in the literature, namely to allow DE a significant high-z density. Strongly Coupled cosmologies assume a much larger coupling between DE and CDM, together with the presence of
an uncoupled warm DM component, as the role of CDM is mostly restricted to radiative eras. This allows us to preserve small scale fluctuations even if the warm particle, possibly a sterile neutrino, is quite light, O(100 eV). Linear theory and numerical simulations show that these cosmologies agree with LCDM on supergalactic scales; e.g., CMB spectra are substantially identical. Simultaneously, simulations show that they significantly ease problems related to the properties of MW satellites and cores in dwarfs. SC cosmologies also open new perspectives on early black hole formation, and possibly lead towards unificating DE and inflationary scalar fields.
Circular polarization of the Cosmic Microwave Background (CMB) offers the possibility of detecting rotations of the universe and magnetic fields in the primeval universe or in distant clusters of galaxies. We used the Milano Polarimeter (MIPOL) insta
lled at the Testa Grigia Observatory, on the italian Alps, to improve the existing upper limits to the CMB circular polarization at large angular scales. We obtain 95% confidence level upper limits to the degree of the CMB circular polarization ranging between 5.0x10^{-4} and 0.7x10^{-4} at angular scales between 8 and 24 deg, improving by one order of magnitude preexisting upper limits at large angular scales. Our results are still far from the nK region where today expectations place the amplitude of the V Stokes parameter used to characterize circular polarization of the CMB but improve the preexisting limit at similar angular scales. Our observations offered also the opportunity of characterizing the atmospheric emission at 33 GHz at the Testa Grigia Observatory.
The properties of dynamical Dark Energy (DE) and, in particular, the possibility that it can form or contribute to stable inhomogeneities, have been widely debated in recent literature, also in association to a possible coupling between DE and Dark M
atter (DM). In order to clarify this issue, in this paper we present a general framework for the study of the nonlinear phases of structure formation, showing the equivalence between two possible descriptions of DE: a scalar field phi self-interacting through a potential V(phi) and a perfect fluid with an assigned negative equation of state w(a). This enables us to show that, in the presence of coupling, the mass of DE quanta may increase where large DM condensations are present, so that also DE may partake to the clustering process.
The dual axion model (DAM), yielding bot DM and DE form a PQ-like scalar field solving the strong CP problem, is known to allow a fair fit of CMB data. Recently, however, it was shown that its transfer function exhibits significant anomalies, causing
difficulties to fit deep galaxy sample data. Here we show how DAM can be modified to agree with the latter data set. The modification follows the pattern suggested to reconcile any PQ-like approach with gravity. Modified DAM allows precise predictions which can be testable against future CMB and/or deep sample data.
