New light on the dynamics of dark energy

In physical cosmology, dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the Universe. The cosmological constant is the simplest possible form of dark energy, leading to the current standard model of cosmology.

The EU-funded project 'The dark sector in cosmology and impact on present and future generation experiments' (DEMO) sought to disentangle dark energy from the cosmological constant by identifying dynamical interactions with other forms of matter. The observable Universe contains only a small fraction of ordinary matter, with the rest being dark energy and dark matter.

Scientists explored the possibility that dark energy is dynamical and that gravitational force between dark matter particles is different compared to the general theory of relativity. This was the case of coupled dark energy models, in which dark matter particles feel the presence of a fifth force that is larger than gravity.

Based on this, DEMO implemented a numerical code that allowed test coupling between dark energy and dark matter. Small differences in the Hubble expansion parameter calculations and small fluctuations in the baryonic matter density led scientists to conclude that a non-zero coupling is compatible with data. These cosmological data were produced by the Planck Collaboration in 2013.

Scientists showed for the first time that the amount of dark energy at early stages depends on the epoch at which dark energy started to become relevant. While there are tight constraints regarding the dark energy presence since photon and neutrino decoupling, larger amounts of dark energy are allowed at later times.

DEMO work resulted in formulating a scenario in which dark energy and dark matter are two aspects of the same fluid.

Project work regarding dark energy, dark matter and their comparison to data was published in 12 papers in international journals. Amongst these, there are also works on theoretical models and their comparison to data.