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Dilatonic dark matter and unified cosmology: a new paradigm

We study the possibility that the dilaton - the fundamental scalar field which exists in all existing unified field theories - plays the role of the dark matter of the universe. We find that the condition for the dilaton to be the dark matter strongly restricts its mass to be around 0.5 keV or 270 MeV. For the other mass ranges, the dilaton either undercloses or overcloses the universe. The 0.5 keV dilaton has a free-streaming distance of about 1.4 Mpc and becomes an excellent candidate for warm dark matter, while the 270 MeV one has a free-streaming distance of about 7.4 pc and becomes cold dark matter. We discuss the possible ways to detect the dilaton experimentally.