Browsing by Author "Shiomi, Sachie"

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Geophysical constraint on a relic background of the dilatons
(2010)
Shiomi, Sachie
;
土木工程學系
;
Department of Civil Engineering
According to a scenario in string cosmology, a relic background of light dilatons can be a significant component of the dark matter in the Universe. A new approach of searching for such a dilatonic background by observing Earth's surface gravity was proposed in my previous work. In this paper, the concept of the geophysical search is briefly reviewed, and the geophysical constraint on the dilaton background is presented as a function of the strength of the dilaton coupling, q(b)(2). For simplicity, I focus on massless dilatons and assume a simple Earth model. With the current upper limit on q(b)(2), we obtain the upper limit on the dimensionless energy density of the massless background, Omega(DW)h(100)(2) <= 6 x 10(-7), which is about one-order of magnitude more stringent than the one from astrophysical observations, at the frequency of similar to 7 x 10(-5) Hz. If the magnitude of q(b)(2) is experimentally found to be smaller than the current upper limit by one order of magnitude, the geophysical upper limit on Omega(DW)h(100)(2) becomes less stringent and comparable to the one obtained from the astrophysical observations.
Proposal for a geophysical search for dilatonic waves
(2008-08-01)
Shiomi, Sachie
;
土木工程學系
;
Department of Civil Engineering
We propose a new method of searching for the composition-dependent dilatonic waves, predicted by unified theories of strings. In this method, Earth's surface-gravity changes due to translational motions of its inner core, excited by dilatonic waves, are searched for by using superconducting gravimeters. This method has its best sensitivity at the frequency of similar to 7 X 10(-5) Hz, which is lower than the sensitive frequencies of previous proposals using gravitational wave detectors: similar to 10 to 1000 Hz. Using available result,,; of surface-gravity measurements with superconducting gravimeters and assuming a simple Earth model, we present preliminary upper limits on the energy density of a stochastic background of massless dilatons at the low frequency. Though the results are currently limited by the uncertainty in the Earth model, this method has a potential of detecting dilatonic waves in a new window.
Testing gravitational physics with superconducting gravimeters
(2008)
Shiomi, Sachie
;
土木工程學系
;
Department of Civil Engineering
Superconducting gravimeters are the most sensitive instruments to measure surface gravity changes at low frequencies. Currently, about twenty five superconducting gravimeters are operating in the world and their global network has been developed. We investigate possible applications of the superconducting gravimeters to tests of gravitational physics. Previous experimental searches for spacial anisotropies in the gravitational constant G and for gravitational waves, performed with gravimeters in 1960's to 1970's, can be improved by applications of the current superconducting gravimeters. Also, we describe other proposed applications of testing the universality of free-fall and searching for composition-dependent dilatonic waves, and discuss future works necessary for these geophysical tests.