Age of the universe

The age of the universe represents the duration passed since the Big Bang, i.e. the dense and hot phase of the history of the universe. This term does not prejudge that the Univers is of a finished age, its state former to Big Bang (if there exists) being at present out of reach direct observation.

The age of the universe can be evaluated by several more or less direct methods, which converge all worms a value of about 15 billion years. The most precise estimate today is deduced from the data of the Artificial satellite WMAP and gives 13,7 billion years.

Definition and order of magnitude

The restricted Relativité indicates that the one duration measurement depends on the trajectory followed by the person measuring this duration. To specify what one understands by age of the universe, it is thus necessary to specify the type of measurement used. The universe is a homogeneous medium relatively and isotropic. That means that in a given point, it is always possible to have a trajectory for which the universe appears (with large scales) identical in all the directions. With a relatively good approximation, a galaxy can be regarded as following such a trajectory. The age of the universe is thus the quantity which would have been measured by a clock whose movement follows that of a galaxy or matter which contributed to its formation.

An order of magnitude of the age of the universe can result starting from measurement from sound expansion. It is observed indeed that the remote Galaxie S seem actuated by a movement of recession compared to our galaxy (the Milky Way), and this with a speed all the more large as their distance is important. By Spectroscopie it is possible to measure by Doppler effect the speed of distancing of the galaxies. By various methods, it is also possible to measure their distance. The observation reveals that the speed of distancing of the galaxies is proportional to their distance. The constant of proportionality thus found is called Constante of Hubble, traditionally noted H or H . This constant of Hubble is considered by various method which gives the approximate value of 70 Kilometer S by second and Mégaparsec. If it is considered that the speed of recession of the galaxies is constant during time, then it is possible to estimate when the matter which formed a galaxy given was in our immediate vicinity. This duration T can be calculated and is worth simply

t = \ frac {1} {H_0}

With the numerical value given above, one obtains

t \ sim 14 \ times 10^9 \; {\ rm years}

Actually, the speed of recession of the galaxies is not constant during time. In particular, it was in the past much more important than today and decreased during several billion years. Thereafter occurred the phenomenon of the Accélération of the expansion of the universe, which saw the speed of recession of the galaxies starting to increase. Precise calculations, using the equations of Friedmann and the standard Modèle of cosmology indicate that these two effects are compensated about, and that the real age of the universe very close to the value of 14 billion years is given above.

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