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Transcribed Image Text:13-4. The Sun is moving with speed 2.5 X 10° m/s in a circular orbit about the center of the
Galaxy. How long (in Earth years) does it take to complete one orbit? How many orbits has it
completed since it was formed?
13-5. The reason that massive neutrinos were considered as a candidate for solving the miss-
ing mass problem is that, at the conclusion of the lepton era, the universe contained about equal
numbers of photons and neutrinos. They are still here, for the most part. The former can be ob-
served and their density is measured to be about 500 photons/cm³; thus, there must be about
that number density of neutrinos in the universe, too. If neutrinos have a nonzero mass and if
the cosmological expansion has reduced their average speed so that their energy is now pri-
marily mass, what would be the individual neutrino mass (in eV/c²) necessary to account for
the missing mass of the universe? Recall that the observed mass of the stars and galaxies (in-
cluding the dust and gas) accounts for only about 4 percent of that needed to close the universe.
13-6. Using data from Table 13-3, construct a graph that demonstrates the validity of Equation
13-17.
13-7. Recalling that the light-year c • y is the distance light travels in one year, compute in me-
ters the distance equivalent to 1 light-second, 1 light-minute, 1 light-hour, and 1 light-day.
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