Dark energy

Observations of the speeds of galaxies collected by Edwin Hubble in the 1920s showed that our universe is not static but expanding, providing one of the first solid proofs in favour of the Big Bang theory. However, more recently, in 1998, measurements of supernovae by research groups led by American researchers Saul Perlmutter, Brian Schmidt, and Adam Riess added an unexpected piece of the puzzle. The universe is not just expanding; this expansion seems to be accelerated. To explain this behaviour, the existence of a mysterious “antigravity” that is opposed to the contraction induced by gravitational force, and which is represented by an additional term in Einstein’s field equation called the “cosmological constant”, must be admitted. This form of unknown, invisible, and homogeneous energy was called “dark energy” and constitutes approximately 70% of the density of energy of the universe. Understanding its nature is one of the biggest open challenges in contemporary physics.

Pie chart illustrating the cosmological composition of the universe. (© INFN)

Artist's impression of ESA's Euclid mission in space. (© ESA, ATG)

Over the years, various hypotheses on the nature of dark energy have been advanced. The simplest assumes that the latter coincides with so-called “vacuum energy”. According to the laws of quantum mechanics, empty space is not really empty, but continuously animated by quantum fluctuations that lead to the creation and destruction of “virtual” particles and antiparticles. In short, the vacuum has its own energy, whose effect on the scale of the universe could justify the existence of dark energy. However, the expected value of vacuum energy is dramatically larger compared to that indicated by observations on accelerated expansion, highlighting that the road to a full understanding of dark energy is still a long one.
While other, more “exotic” theoretical hypotheses are not lacking, help might arrive from space experiments. In particular, the Euclid telescope of the European Space Agency, launched in July 2023, has the goal of investigating the nature of dark matter and dark energy, among others. INFN is actively participating in this programme with its researchers.

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Einstein and general relativity

The general theory of relativity, published by Albert Einstein in 1915, is one of the cornerstones of modern physics. It is a theory that describes the gravitational interactions, generalising and overcoming the previous theory of Isaac Newton, developed almost three centuries earlier.

The cosmological standard model

The standard model of cosmology, also called the Lambda-CDM model, is the simplest theoretical framework able to provide a good description of all the observed cosmological phenomena with just 6 free parameters.

The Big Bang and the universe

The Big Bang theory is currently the most reliable scientific theory about the origins of the cosmos. It postulates that our universe started approximately 13.8 billion years ago from an extremely hot and dense state and, since then, has expanded basically continuously.

Astroparticles and cosmic messengers

The universe is continuously traversed by elementary and subatomic particles, which travel through space at very high speeds. Many of these reach Earth, bringing very precious information about astrophysical phenomena that produced them.

Gravitational waves

Gravitational waves are ripples in spacetime produced by large masses in accelerated motion during violent astrophysical phenomena, such as, for example, the merging of pairs of black holes or neutron stars.

Multi-messenger astronomy

On 17 August 2017, a coalescence of neutron stars that occurred in the NGC 4993 galaxy (at approximately 130 million light years from us) was observed at the same time by LIGO and Virgo gravitational wave observers and by numerous electromagnetic telescopes (from radio waves to energetic gamma rays) throughout the world.

Black holes

Black holes are one of the most fascinating and mysterious astronomical objects. They were hypothesised for the first time in 1916, a year after Albert Einstein’s publication of his general theory of relativity, when the German astronomer Karl Schwarzschild presented the first exact solution of the theory’s equations, known as “Einstein equations”.

Dark matter

Dark matter is a kind of matter that is invisible to telescopes, which does not emit electromagnetic radiation and whose (presumed) existence can only be indirectly detected today through its gravitational effects.