Cosmological Data Prefer An Interacting Vacuum Energy
Authors: Najla Said1, Valentina Salvatelli1,2, Marco Bruni3, David Wands3
1Physics Department and INFN, Universita di Roma "La Sapienza", Rome, Italy
2Aix-Marseille Universite, Centre de Physique Theorique UMR 7332, France
3Institute of Cosmology and Gravitation, University of Portsmouth, UK.
Over recent years, a growing number of high precision observations of the primordial Universe and our local cosmic neighbourhood have established a standard cosmological model, called LCDM. In LCDM, the Universe is filled with a constant dark energy, or cosmological constant (Lambda) that is responsible for the present accelerated expansion. Besides dark energy, a similar density of cold dark matter (CDM) supports the growth of large-scale structures (galaxies and clusters of galaxies) in broad agreement with the observations.
A crucial dataset for cosmological research is the Cosmic Microwave Background (CMB). This faint thermal radiation, which fills the whole sky, is a relic of the primordial Universe; it has been fundamental in supporting the homogeneity and isotropy of the Universe and in determining its composition. At the beginning of this year the Planck satellite of the European Space Agency (ESA) released a new set of CMB observations at striking precision, tightly constraining the parameters of the standard model .
These primordial Universe observations can be combined with a series of local Universe observations, coming from a number of galaxy surveys (SDSS , WiggleZ  and others). With these measurements we can map the distribution of structures at different epochs and use these maps to study their evolution up until the present. In particular, spectroscopic galaxy surveys enable us to quantify the clustering of the large-scale structure by measuring the so-called Redshift-Space Distortions (RSD) .
Since the late 1990s, the LCDM model has provided a simple framework to consistently describe local and primordial observations, but the latest precise measurements suggest that we may need to review and possibly revise this standard model. The clustering of structures, appears to be weaker than the one inferred from CMB observations assuming the standard LCDM evolution . A common extension to the LCDM model is to allow the dark energy to have a non-constant density that evolves with time, but this, on its own, does not appear to resolve the conflict between different datasets .
In our recent work we investigated how the growth of structure is affected by an interaction between dark energy and dark matter [5,6,7]. These two components are usually assumed to evolve independently, and indeed an interaction between the two is not favored if one looks only at CMB measurements, for which the simple LCDM model fits perfectly the observations. We used the formalism of the Interacting Vacuum models, as developed in .
Our idea was to study a time-dependent interaction, but without imposing a-priori a specific time dependence. We parametrized the interaction in a simple, linear way, and studied its value at different epochs using a binned function, as sketched in Figure 1. On the x-axis we show the redshift z (a measure of time, where z=0 is today and z>0 is the past), while on the y-axis we show the dimensionless coupling parameter qv. The purple line represents the binned model. The datasets we used for the analysis were the current CMB measurements from Planck and the RSD measurements from a number of surveys (refer to our Letter  for details).
The results we found suggested that an interaction at primordial epochs is not favored, but a zero coupling at recent times is excluded at 99% confidence level.
Starting from here we determined a best-fit model in which an interaction in the dark sector began around redshift z=1 (6 billion years ago) finding that, from a Bayesian point of view, this model is favored with respect to LCDM. This best-fit model is represented with the blue line in Figure 1.
We also compared our model to a cosmology with massive neutrinos , which is another extension of LCDM that has been proposed to solve the tension between primordial and local observations. In this case too we found that the observational evidence favors an interaction in the dark sector.
In Figure 2 we show the RSD dataset used in our work. On the x-axis we show the redshift z and on the y-axis the fσ8 value, a typical measure of the growth of structure. The grey line, representing the LCDM model, predicts a higher growth than that observed, while our late-time interaction model, the blue line, fits the data much better.
The latest observations thus not only confirm that dark matter decay into dark energy is allowed, but also suggests that this interaction is actually favored with respect to the standard LCDM model.
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