Cosmological fluctuation growth in bimetric MOND
  I look at the growth of weak density inhomogeneities of nonrelativistic
matter, in bimetric-MOND (BIMOND) cosmology. I concentrate on
matter-twin-matter-symmetric versions of BIMOND, and assume that, on average,
the universe is symmetrically populated in the two sectors. MOND effects are
absent in an exactly symmetric universe, apart from the appearance of a
cosmological constant, Lambda~(a0/c)^2. MOND effects-local and cosmological-do
enter when density inhomogeneities that differ in the two sectors appear and
develop. MOND later takes its standard form in systems that are islands
dominated by pure matter. I derive the nonrelativistic equations governing
small-scale fluctuation growth. The equations split into two uncoupled systems,
one for the sum, the other for the difference, of the fluctuations in the two
sectors. The former is governed strictly by Newtonian dynamics. The latter is
governed by MOND dynamics, which entails stronger gravity, and nonlinearity
even for the smallest of perturbations. These cause the difference to grow
faster than the sum, conducing to matter-twin-matter segregation. The
nonlinearity also causes interaction between nested perturbations on different
scales. Because matter and twin matter (TM) repel each other in the MOND
regime, matter inhomogeneities grow not only by their own self gravity, but
also through shepherding by flanking TM overdensitie. The relative importance
of gravity and pressure in the MOND system depends also on the strength of the
perturbation. The development of structure in the universe, in either sector,
thus depends crucially on two initial fluctuation spectra: that of matter alone
and that of the matter-TM difference. I also discuss the back reaction on
cosmology of BIMOND effects that appear as "phantom matter" resulting from
inhomogeneity differences between the two sectors.
