We know that the rate of expansion of the observable universe over the last few billion years was discovered in the late 1990s. This discovery is often simplified by saying that it is the discovery of dark energy, but in fact, all it has been uncovered by two teams of astrophysicists, one of which was led by Saul Perlmutter, is that acceleration. . We can explain this without introducing the concept of dark energy and, in fact, the Nobel committee that rewarded the two leaders of the teams for this discovery also talks about the revelation of the existence of expansion acceleration.
Technically, this phenomenon is described to a first approximation by adding a constant called the cosmological constant to Einstein’s equations of general relativity. Einstein did this in 1917 when he proposed the first cosmological model based on the relativistic theory of gravity. This constant was already manifested by a repulsive effect counteracting the attraction of masses of matter to give a stable and balanced universe.
The Discovery of the Acceleration of the Cosmic Expansion and Its Explanations, Interviewed by Jonathan Sarre for the Futura-Sciences website, September 2014. © Jean-Pierre Luminet
The Sun and the Extended Solar System?
In fact, this equilibrium was unstable, and from the 1930s until the 1990s, when Hubble discovered the expansion of the universe, Einstein’s cosmological constant was abandoned by almost everyone. Virtually the only one to maintain this constant was Georges Lemaître, who was the first to correctly interpret Hubble’s discovery of the relationship between the spectral redshift effect of light from galaxies and their distance to the galaxy – in terms of the expansion of space.
Hubble did not understand Einstein’s theory and to him, the change was produced by the displacement movements of the galaxies resulting in the Doppler effect. Lemaître had in fact demonstrated that it was the stretching effect of the wavelength of light during its travel, the effect being even more significant the longer the travel lasted.
With or without the cosmological constant, the discovery of expansion raised the problem of knowing how far back it appeared. Should it be concluded that the radii of the orbits of the planets of the Solar System, the size of the Sun, increased over time?
The first element of the answer was given by Einstein and his assistant Ernst G. Strauss as early as 1945. The gravity of a body can be strong enough to completely counteract the effects of expansion. Calculations on this topic were refined over the following decades and showed that the potential effects of expansion on the level of the planets of the Solar System or the stars in the Milky Way were almost negligible.
But these calculations were done before the expansion of the observable universe was discovered. That’s why a group of researchers from the University of Cambridge, England, Anne Davis and Wynn Evans of the legendary DAMTP (where Hawking worked)Institute of Astronomy, Also revisited this question with my colleague David Benisty Department of Applied Mathematics and Theoretical Physics, They did this by performing analytical calculations taking into account the cosmological constant, the motion of the Andromeda galaxy relative to the Milky Way, as they explain in an article to be published in Science. The Astrophysical Journal Letters,
Direct measurement of dark energy and new physics?
It turns out that the Andromeda Galaxy has been known for some time to be on a trajectory that would lead it to collide with the Milky Way, and that this event probably occurred many years ago. billions of years ago. This was already known before the discovery of dark energy and was already explained by explaining that the effect of expansion was only really noticeable when we started going to scales a little beyond the cluster of galaxies and therefore had no Wasn’t a problem. The light from the Andromeda galaxy is actually blue-shifted, this time due to a real Doppler shift caused by Andromeda’s motion on its trajectory toward our galaxy.
Cambridge astrophysicists have discovered that the motions of Andromeda and its predicted collision within 4.5 to 5 billion years are in fact sensitive to the value and nature of the cosmological constant, that is, to say that these motions are negligible affected by the nature of dark matter. There are energy, each calculation gives different results depending on the modeling of the cosmological constant, either its value or its origin from new physics, for example, tensor–scalar theory.
, Dark energy affects every pair of galaxies: gravity tends to bring galaxies together, while dark energy tends to push them apart. In our model, if we change the value of the cosmological constant, we can see how it changes the orbit of the two galaxies. Based on their mass, we can put an upper limit on the cosmological constant, which is about five times higher than what we can measure from the rest of the universe. explains David Benisty in a press release from the University of Cambridge.
And the researchers say: Dark energy is one of the biggest puzzles in cosmology. Its effects may vary with distance and time, but hopefully this technique can help solve the mystery. Since data from the James–Webb Telescope (JWST) will provide more precise measurements of Andromeda’s mass and momentum, this may help to narrow down the upper bound on the value of the cosmological constant, just to the point of everything else. both in detecting and measuring it, and in obtaining other information by studying pairs of distant galaxies affected by dark energy.
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