The cosmological standard model

In the model, the validity of the cosmological principle is assumed (the universe is homogeneous and isotropic on large scales), as well as the general theory of relativity on cosmological distances and the standard model of elementary particles. The universe described by the model has its origin in the Big Bang, followed by a period of cosmic inflation in which the universe exponentially expands. The existence of non-baryonic dark matter that is cold (i.e. whose speed is much less than light’s in the epoch of matter-radiation equality) and with only gravitational interaction is also assumed to explain various gravitational phenomena observed in large-scale structures. These include the rotation curves of galaxies and the “gravitational lensing” effect of galactic masses.

Time Line of the Universe (© NASA/WMAP Science Team)

Map of the anisotropies of the cosmic microwave background seen by the ESA Planck satellite. (© Esa e Planck Collaboration)

Despite the great successes obtained, in particular with the measurements of the cosmic microwave background, some recent observations are not in complete agreement with the predictions of the Lambda-CDM model. These may indicate the need to reconsider the assumptions made or to modify the model. The most well known is the so-called “Hubble tension”. The determination of the Hubble constant (that measures the expansion speed ofthe universe) obtained by analysing the data of the cosmic microwave background in the context of the Lambda-CDM model is in partial disagreement with that obtained by measurements made using the stars and galaxies of the local universe. Various explanations have been put forward for this divergence, including the presence of “new physics” not described by the cosmological standard model.

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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 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.

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.