The Net Advance of Physics: The Nature of Dark Matter, by Kim Griest -- Section 7I.
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The microlensing experiments have given robust and strong limits
on the baryonic content of the halo. Much more data from the
LMC and SMC will be available soon, so we expect the statistics to
improve in the near future. The LMC events, if interpreted as due to
halo microlensing, allow a measurement of the baryonic
contribution to the halo, which is around 20% for a standard halo.
In this case, the most likely Macho contribution to the Milky Way
halo mass is about ,
which is roughly the same as the disk
contribution to the Milky Way mass. However, the whole story has
been made more complicated (and exciting) by the much larger
than expected number of bulge microlensing events. These events
imply a new component of the Galaxy, and until the nature of this
new component is known, unambiguous conclusions concerning the
LMC events will not be possible. For example, if the Milky Way disk
is much larger than usually considered, a much smaller total halo
mass will be required, and so even an all-Macho halo might be
allowed. Alternatively, the new Galactic component which is giving
rise to the bulge events, may also be giving rise to the LMC events,
and the Macho content of the halo could be zero. Fortunately,
much more data is forthcoming, and many new ideas have been
proposed. Microlensing is fast becoming a new probe of Galactic
structure, and, beside the original potential to discover or limit dark
matter, may well produce discoveries such as extra-solar planetary
systems.
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Macho Conclusion
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