Etude de bruit de fond induit par les muons dans l'expérience ...

tel-00724955, version 1 - 23 Aug 2012
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12 The Dark Matter problem
Figure 1.7: Hubble diagram of distance vs. velocity by the Hubble Space Telescope
Key Project. A slope of H0 = 72 is shown with its ±10% lines. The bottom box
shows the Hubble constant vs. distance and the horizontal line is the best fit to data.
Figure from [23].
the existence of nonbaryonic dark matter. Big bang nucleosynthesis is a nonequilibrium
process that took place over the course of a few minutes in an expanding,
radiation-dominated plasma with high entropy and many free neutrons [24].
The predictions of big bang nucleosynthesis for the light element abundances
are shown in Figure 1.8, in which the boxes and arrows show the current estimates
for the light element abundances and they are consistent with the corresponding
predictions. The primordial abundances of these elements depend critically upon
the conditions during the period when such fusion was possible, and in particular
on the baryon-to-photon number ratio η ≡ nb nγ × 10 10 .
At times much less than a second after the big bang, there were roughly equal
numbers of electrons, positrons, neutrinos, antineutrinos and photons. The ratio
of photons to nucleons, i.e. protons and neutrons, was more than a billion to one.
The nuclei had not been formed and the ratio of neutrons and protons was unity
due to the weak processes that interconvert them. At about one second, when the
Universe had cooled to around 10 10 K, the weak processes were not able to keep
the same number of neutrons and protons. And at the temperature of ∼ 109 K
the first formation of D, 3H, 3He and 4He took place. As the Universe continued to
expand and cool, the processes maintaining equilibrium slowed down relative to the
temperature evolution and, after five minutes, most neutrons were in 4He nuclei, and
most protons remained free. There was also formation, in much smaller amounts, of
D, 3He, 7Li but the low density and temperature caused the elemental composition
of the Universe to remain unchanged until the formation of the first stars several
billion years later.