Cosmological constraints on quintessential halos

A complex scalar field has recently been suggested to bind galaxies and flatten the rotation curves of spirals. Its cosmological behavior is thoroughly investigated here. Such a field is shown to be a potential candidate for the cosmological dark matter that fills up a fraction ΩCDM~0.3 of the Universe (where CDM denotes cold dark matter). However, problems arise when the limits from galactic dynamics and some cosmological constraints are taken simultaneously into account. A free complex field, associated with a very small mass m~10^-23 eV, has a correct cosmological behavior in the early Universe, but behaves today mostly as a real axion, with a problematic value of its conserved quantum number. On the other hand, an interacting field with quartic coupling λ~0.1 has a more realistic mass m~1 eV and carries a quantum number close to the photon number density. Unlike a free field, it would be spinning today in the complex plane---such as the so-called ``spintessence.'' Unfortunately, the cosmological evolution of such a field in the early Universe is hardly compatible with constraints from nucleosynthesis and structure formation.