【摘 要】
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The kinematics on spatially flat FLRW spacetimes is presented for the first time in local charts with physical coordinates,i.e.,the cosmic time and proper Cartesian space coordinates of Painlevé-type.It is shown that there exists a conserved momentum that
【机 构】
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West University of Timisoara,V.Parvan Ave.4,RO-300223 Timisoara,Romania
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The kinematics on spatially flat FLRW spacetimes is presented for the first time in local charts with physical coordinates,i.e.,the cosmic time and proper Cartesian space coordinates of Painlevé-type.It is shown that there exists a conserved momentum that determines the form of the covariant four-momentum on geodesics in terms of physical coordinates.Moreover,with the help of this conserved momentum,the peculiar momentum can be defined,thus separating the peculiar and recessional motions without ambiguity.It is shown that the energy and pe-culiar momentum satisfy the mass-shell condition of special relativity while the recessional momentum does not pro-duce energy.In this framework,the measurements of the kinetic quantities along geodesics performed by different observers are analyzed,pointing out an energy loss of the massive particles similar to that producing the photon red-shift.The examples of the kinematics on the de Sitter expanding universe and a new Milne-type spacetime are ex-tensively analyzed.
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