2 00 6 Global monopole , dark matter and scalar tensor theory

In this article, we discuss the space-time of a global monopole field as a candidate for galactic dark matter in the context of scalar tensor theory. It is believed that our Universe in the past was denser and hotter. So the early stage of the Universe was in a systematic phase and there were no topological defects. As the Universe expands, it cools down from its hot initial state [1-3]. So the early Universe had undergone a number of phase transitions. Phase transitions in the early Universe can give rise to various forms of topological defects. They can be monopoles, comic strings or domain walls. Among them monopoles and cosmic string are well studied for their cosmological as well as astrophysical implications. Monopoles are point like topological defects that may arise during phase transitions in the early universe. There has been a fairly large amount of discussions [4-24] on the gravitational field of global monopoles beginning with the work of Barriola and Vilenkin (BV) [25]. According to BV, global monopoles are the configurations with energy density decreasing with the distance as 1 r 2. Recent observations indicate that the galaxies, cluster of galaxies and super clusters are filled out with 90 percent of dark matter [26]. Rubin et al [27] have found that the rotation curve profile of a spiral galaxy is flat outside a central galactic region and they have suggested the energy density varies as 1 r 2 of non luminosity matter (dark matter) which should contribute to the flatness of the rotation curves [28]. Nucamendi and others [29-31] have suggested that the monopole (its energy density proportional to 1 r 2) could be the galactic dark matter in the spiral galaxies. In a recent work, Lee and Lee [32] studied global monopole field (within weak field approximations) as a candidate for galactic dark matter in the scalar tensor theory of gravity inspired by the work of Nucamendi. The measurements, made by Persic et al [33] of rotation curves in spiral galaxies show that the coplanar orbital motion of gas in the outer parts of these galaxies keeps more or less a constant value up to several luminous radii.