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The history of the universe after inflation but before a time of about 1 second is largely unknown. [26] However, the universe is known to have been dominated by ultrarelativistic Standard Model particles, conventionally called radiation, by the time of neutrino decoupling at about 1 second. [27]
The total kinetic energy of an isolated system also depends on the inertial frame of reference: it is the sum of the total kinetic energy in a center-of-momentum frame and the kinetic energy the total mass would have if it were concentrated in the center of mass. Due to the conservation of momentum the latter does not change with time, so ...
The experimental determination of a body's center of mass makes use of gravity forces on the body and is based on the fact that the center of mass is the same as the center of gravity in the parallel gravity field near the earth's surface. The center of mass of a body with an axis of symmetry and constant density must lie on this axis.
But the mass density of the universe can be measured from its gravitational clustering, and is found to have only about 30% of the critical density. [13] Since theory suggests that dark energy does not cluster in the usual way it is the best explanation for the "missing" energy density.
m 1 is the mass of the primary in Earth masses (M E) m 2 is the mass of the secondary in Earth masses (M E) a (km) is the average orbital distance between the centers of the two bodies; r 1 (km) is the distance from the center of the primary to the barycenter; R 1 (km) is the radius of the primary
As such, they have no rest mass, because they can never be measured in a frame where they are at rest. This property of having no rest mass is what causes these particles to be termed "massless". However, even massless particles have a relativistic mass, which varies with their observed energy in various frames of reference.
On the other hand, the steady-state model says while the universe is expanding, it nevertheless does not change its appearance over time (the perfect cosmological principle). E.g., the universe has no beginning and no end. This required that matter be continually created in order to keep the universe's density from decreasing.
Alternatively, some authors have argued that the accelerated expansion of the universe could be due to a repulsive gravitational interaction of antimatter [31] [32] [33] or a deviation of the gravitational laws from general relativity, such as massive gravity, meaning that gravitons themselves have mass. [34]