Poster, Monday, January 10, 2005, 9:20am-6:30pm, Exhibit Hall

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*H. L. Helfer (Dept of Physics & Ast; Lab for Laser Energetics, U Rochester Rochester NY 14627)*

We focus on the problem of representing only the dark matter
presently associated with the halos of isolated galaxies.
This manifests itself in gravitational lenses and as the
unexplained source of the extended flat portion of the
rotation curves V_{rot}(r) \approx *const.* of the
Milky Way and other galaxies. For example,any halo fluid
with a simple equation of state must have finite density at
the origin and be isothermal in those regions for which
V_{rot} is flat. Such fluids are represented by
Bonnor-Ebert models; these are gravitationally unstable for
halos extending beyond ~20 kpc. We also note a halo of
exotic non-interacting particles satisfying the
collisionless Boltzman equation cannot be restored to even
approximate spherical symmetry in regions where
galaxy-galaxy collisions have occured. Provisionally, we
suggest that the halos arose as very small amplitude
unstable fluctuation in a classical scalar field of the
generic form c^{-2}\partial^{2}_{tt}\phi -\nabla^{2}\phi=
m^{2}\phi (1-\phi^{2}) where 1/m ~3 kpc; the growth of
spacial fluctuations is limited to finite amplitude by the
non-linear term. From an initial value \delta\phi ~
10^{-4} it takes ~10^{5} yrs to develop to finite
amplitude; they could not have been important in the early
days of the universe. These disturbances today play the role
of `dark' halos around galaxies.The gravitational fields the
produce can well represent the observed rotation curves of
the Milky Way, NGC4605, F583-1, & DDO 154, with 1/m ~
2-5 kpc determining the onset of the flat fortion of
V_{rot}. When galaxies are clustered, the boundaries
between halos is described by discontinuities in the second
derivatives of the \phi-field and a \Lambda-term appears
naturally in representing the field's energy density. The
universe's present energy density associated with these halo
fields is \Omega ~1/2 -2/3. If the universe were
modeled as an enormous cluster of equally spaced galactic
halos, the resulting cosmological fluid's energy density
would scale with the cosmic scale factor as {\cal
R}(t)^{-2}. Using this, a possible scenario in which {\cal
R}(t) would have experienced a recent acceleration,
representing a `quintessence' effect.

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