1,720,988 research outputs found
New Test of General Relativity: Measurement of de Sitter Geodetic Precession Rate for Lunar Perigee
No full textStochastic gravitational wave background: Upper limits in the 10(-6) to 10(-3) Hz band
No full textWe have used precision Doppler tracking of the Cassini spacecraft during its 2001-2002 solar opposition to derive improved observational limits to an isotropic background of low-frequency gravitational waves. Using the Cassini multilink radio system and an advanced tropospheric calibration system, the effects of heretofore leading noises - plasma and tropospheric scintillation - were, respectively, removed and calibrated to levels lower than other noises. The resulting data were used to construct upper limits to the strength of an isotropic background in the 10-6 to 10-3 Hz band. Our results are summarized as limits on the strain spectrum Sh(f), the characteristic strain (hc = the square root of the product of the frequency and the one-sided spectrum of strain at that frequency), and the energy density (Ω = energy density in bandwidth equal to center frequency assuming a locally white energy density spectrum, divided by the critical density). Our best limits are Sh(f) < 6 × 10-27 Hz-1 at several frequencies in the millihertz band, hc < 2 × 10-15 at about 0.3 mHz, and Ω < 0.025 × h75-2, where h75 is the Hubble constant in units of 75 km s-1 Mpc-1, at 1.2 × 10-6 Hz. These are the best observational limits in the low-frequency band, the bound on Ω, for example, being about 3 orders of magnitude better than previous constraints from Doppler tracking
A test of general relativity using radio links with the Cassini spacecraft
No full textAccording to general relativity, photons are deflected and delayed by the curvature of space-time produced by any mass. The bending and delay are proportional to γ + 1, where the parameter γ is unity in general relativity but zero in the newtonian model of gravity. The quantity γ - 1 measures the degree to which gravity is not a purely geometric effect and is affected by other fields; such fields may have strongly influenced the early Universe, but would have now weakened so as to produce tiny-but still detectable-effects. Several experiments have confirmed to an accuracy of ∼0.1% the predictions for the deflection and delay of photons produced by the Sun. Here we report a measurement of the frequency shift of radio photons to and from the Cassini spacecraft as they passed near the Sun. Our result, γ = 1 + (2.1 ± 2.3) × 10-5, agrees with the predictions of standard general relativity with a sensitivity that approaches the level at which, theoretically, deviations are expected in some cosmological models
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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