We study Born-Infeld type effective action for unstable D3-brane system including a tachyon and an Abelian gauge field, and find the rolling tachyon with constant electric and magnetic fields as the most general homogeneous solution. Tachyonic vacua are characterized by magnitudes of the electric and magnetic fields and the angle between them. Analysis of small fluctuations in this background shows that the obtained configuration may be interpreted as a fluid consisting of string-like objects carrying electric and magnetic fields. They are stretched along one direction and the rolling tachyon move in a perpendicular direction to the strings. Direction of the propagating waves coincides with that of strings with velocity equal to electric field.
It is suggested that the axino mass in the 1 MeV region and gravitino mass in the eV region can provide an axino lifetime of order of the time of photon decoupling. In this case, some undecayed axinos act like cold dark matters and some axino decay products (gravitinos and hot axions) act like hot dark matters at the time of galaxy formation.
We show that decaying axino dark matter with R-parity violation can explain the observed excess of the 130GeV gamma-ray line from the Galactic center in the Fermi data. The branching fraction of the axino decay into monochromatic photons can be O(1), and constraints from continuum gamma-rays and the anti-proton flux are ameliorated. The Peccei-Quinn scale of O(10 13 − 10 14) GeV and the R-parity violation parameter of O(10 −12 − 10 −11) are cosmologically favored.
Since it became evident that the CDM model for cosmic structure formation predicts smaller power on large scales than observed, many alternatives have been suggested. Among them, the existence of late decaying particle can cure it by delaying the beginning of the matter domination and increasing the horizon length at that time. We discuss the realization of this scenario and present the light neutrino and the light axino as possible examples of working particle physics model. We point out that the increased power at sub-galaxy scale predicted by this scenario could lead to rich sub-galaxy structures.2
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