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 ${\Omega }_{\mathrm{CDM}}\sim 0.3$ 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 $m\sim {10}^{-23}\hspace{0.5em}\mathrm{eV},$ 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 $\lambda \sim 0.1$ has a more realistic mass $m\sim 1\hspace{0.5em}\mathrm{eV}$ 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.