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Dark energy

Dark energy and accelerating expansion

 Automatic translation  Automatic translation Updated June 01, 2013
We know that the universe is expanding and that expansion is not slowing down but rather accelerates. What is this force that drives all galaxies moving away from each other at a rate more and more rapid ?
Scientists call dark energy. The existence of this dark energy is needed to explain the accelerating expansion of the Universe. This hypothetical energy uniformly filling the entire universe and with negative pressure which acts as a repulsive gravitational force. They'll even discover the very special value of this vacuum energy. This energy would be an order of magnitude which is written with a zero, followed by 122 zero before the number one. However, this extremely small value, much lower than expected by the equations, explain why the matter of our universe exists. If only we remove 3 or 4 zeros, which is very little, the acceleration would be so fast that the matter could not organize itself to form stars and galaxies. In this concept each value of the dark energy represents a different universe.

Video: simulation of the structure of the Universe, represented by 550 billion dark matter particles, each corresponding to a galaxy. The film shows how the universe goes from a uniform soup its current state, where immense voids are crossed by filaments which include galaxies. Credit: CNRS-picture film 9 minutes on DEUS (Dark Energy Universe Simulation) experiment conducted by the team of Jean-Michel Alimi.


Standard candles


In 1929, Edwin Hubble discovered that galaxies are receding more quickly they are distant. Our universe is expanding, what is the Hubble constant (H = 70 km / s / Mpc), which gives this speed depending on the distance at which it is observed (speed = H x distance). This is not an extraordinary expansion because it gives only an increase of the distance of 7% per billion years. Since several methods of observation have corroborated the hypothesis of the accelerating expansion of the universe (standard candles, standard rules,...).
A supernova is a gravitational collapse of stars with a mass greater than 8 solar masses. These events are rare in a galaxy (1 per century), but when the star explodes it is as bright as the galaxy itself. In addition they are true standard candles because they shine up to 400 days after explosion with a perceived brightness which allows us to determine the distance and speed of the galaxies. Thus identifying extremely distant galaxies and in particular those that host cosmic standard candles we can study the evolution of the universe in the past.


The method of standard candles confirms the accelerating expansion of the universe and thus the existence of a new component called dark energy. The Universe is incredibly "empty" of matter, there is one atom per cubic meter. It is composed of 4% baryonic matter, 23% cold dark matter and non-baryonic and 73% black or dark energy.

NB: to search supernovae, we photograph a region of galaxies and a month later we take again a picture, then by treating the two images, the bright objects are subtracted to identify the supernova present in the second picture. This search is performed with larger telescopes Planets (Gemini, CFHT, VLT).

Image: BX442 spiral galaxy located at 10.7 billion years. Image of the Hubble Space Telescope (HST). HST / Keck false color composite image of the galaxy BX442 credit: David Law / Dunlap Institute for Astronomy & Astrophysics.

 Standard candles

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