Dark energy

Dark energy is a form of energy that exerts a negative, repulsive pressure, behaving like the opposite of gravity. It has been hypothesised to account for the observational properties of distant type Ia supernovae, which show the universe going through an accelerated period of expansion. Like dark matter, dark energy is not directly observed, but rather inferred from observations of gravitational interactions between astronomical objects.

Dark energy makes up 68% of the mass-energy of the universe, whose share that would escape the current methods of detection would rise to about 95% including also the dark matter.

Evolutionary models of the current structure of the Universe refer to the fact that observations of distant supernovae have shown that they are at a greater distance than expected, as a result of a still accelerated expansion of space. This expansion was interpreted as being due to the action of a negative pressure form of energy. The analysis of the cosmic background radiation has also shown that the Universe has a Euclidean geometry, that is flat.

To account for such a geometry, it is necessary to integrate the amount of matter present in the Universe (composed mainly of dark matter) with a contribution of about 68% of dark energy. The presence of such energy, and the knowledge of its nature, has profound repercussions on the fate of the Universe, i.e. the link between the geometry of space and the way it expands.

Since the discovery of the expansion of the Universe we tried to measure the slowdown, which should be caused by the gravitational attraction of matter and radiation, the two constituents of the Universe. The expansion rate is obtained by measuring at the same time the receding velocity and the distance of galaxies; the first measure is easily obtained from the spectral analysis of the emitted radiation, the second is very difficult to obtain, because there is no reference point for comparison.

To investigate the deepest spaces of the Universe we have used the development of advanced technologies, in particular CCD (Charge coupled devices), able with their sensitivity to monitor large sectors of the sky and explore thousands of galaxies: this has allowed us to measure in significant numbers a parameter that gives a very precise indication of large distances, the brightness of supernovae Ia. From these measurements, at the end of the last century it was unexpectedly realized that supernovae were less bright than expected, and therefore farther away than would have been expected in the hypothesis of a slow expansion of space.

In subsequent years, further confirmation came from observations of more distant supernovae, including those from the Hubble Space Telescope in 2003 and the ESSENCE (Equation of state: supernovae trace cosmic expansion) Supernova Survey in 2007. The U.S. Department of Energy and NASA are developing the Joint dark energy mission (JDEM) science project to observe even more distant supernovae; it is scheduled to launch in 2016.

The cause of dark energy has been sought in both the theory of general relativity and quantum mechanics. Relativity allows for energy forms that produce repulsive gravity, which was already introduced by Einstein in an attempt to keep his early static model of the Universe in balance. We also speak in quantum terms of vacuum energy, which should not be thought as really empty, but made up of virtual particles capable of exerting repulsive forces.

Contrary to matter, that tends to aggregate, dark energy is uniformly distributed in space and has deep repercussions on the evolution of the Universe. After a first phase of slowdown, when gravitational energy of matter was dominant, the Universe has started to accelerate, some billion years ago, when with the expansion matter has become more rarefied and its gravitational attraction has weakened, leaving dark energy to prevail.

In the future, driven by dark energy, if it will remain constant or even increase, space will expand and empty more and more: with the passage of time our galaxy will be isolated from all others, which will gradually disappear from the cosmic horizon. It is also hypothesized that the role of dark energy will become so relevant, because of its repulsive action, to tear the matter even in its most intimate structures.

Research

  • Dark energy survey internal consistency tests of the joint cosmological probes analysis with posterior predictive distributions. Monthly Notices of the Royal Astronomical Society, Volume 503, Issue 2, May 2021, Pages 2688–2705, https://doi.org/10.1093/mnras/stab526