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Tests of general relativity on the largest scales are not nearly so stringent as Solar System tests. [129] The earliest such test was the prediction and discovery of the expansion of the universe. [130] In 1922, Alexander Friedmann found that the Einstein equations have non-stationary solutions (even in the presence of the cosmological constant).
The waves are sourced by primordial density perturbations, and travel at speed that can be predicted from the baryon density and other cosmological parameters. The total distance that these sound waves can travel before recombination determines a fixed scale, which simply expands with the universe after recombination.
Table of geography, hydrography, and navigation, from a 1728 Cyclopaedia.. Hydrography is the branch of applied sciences which deals with the measurement and description of the physical features of oceans, seas, coastal areas, lakes and rivers, as well as with the prediction of their change over time, for the primary purpose of safety of navigation and in support of all other marine activities ...
A free (=) scalar field has =, and one with vanishing kinetic energy is equivalent to a cosmological constant: =. Any equation of state in between, but not crossing the w = − 1 {\displaystyle w=-1} barrier known as the Phantom Divide Line (PDL), [ 2 ] is achievable, which makes scalar fields useful models for many phenomena in cosmology.
A particle passing through a material at a velocity greater than that at which light can travel through the material emits light. This is similar to the production of a sonic boom when an airplane is traveling through the air faster than sound waves can move through the air.
Physics theories of the 19th century assumed that just as surface water waves must have a supporting substance, i.e., a "medium", to move across (in this case water), and audible sound requires a medium to transmit its wave motions (such as air or water), so light must also require a medium, the "luminiferous aether", to transmit its wave ...
For non-breaking waves, the energy flux associated with the wave motion, which is the product of the wave energy density with the group velocity, between two wave rays is a conserved quantity (i.e. a constant when following the energy of a wave packet from one location to another).
Light waves of all frequencies travel at the same speed of light while matter wave velocity varies strongly with frequency. The relationship between frequency (proportional to energy) and wavenumber or velocity (proportional to momentum) is called a dispersion relation .