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Sound waves that have frequencies below 16 Hz are called infrasonic and those above 20 kHz are called ultrasonic. Sound is a mechanical wave and as such consists physically in oscillatory elastic compression and in oscillatory displacement of a fluid .
As the universe expanded, the plasma cooled to below 3000 K—a low enough energy such that the electrons and protons in the plasma could combine to form neutral hydrogen atoms. This recombination happened when the universe was around 379,000 years old, or at a redshift of z = 1089 . [ 4 ]
Gerson Goldhaber (1924–2010) used supernova observations to measure the energy density of the universe; J. Richard Gott (1947–) proposed the use of cosmic strings for time travel; Alan Guth (1947–) explained the isotropy of the universe by theorizing a phase of exponential cosmic inflation soon after the big bang
In the widely accepted ΛCDM cosmological model, dark matter accounts for about 25.8% ± 1.1% of the mass and energy in the universe while about 69.2% ± 1.2% is dark energy, a mysterious form of energy responsible for the acceleration of the expansion of the universe. [17] Ordinary ('baryonic') matter therefore composes only 4.84% ± 0.1% of ...
Estimates put the average energy density of the present day universe at the equivalent of 5.9 protons per cubic meter, including dark energy, dark matter, and baryonic matter (ordinary matter composed of atoms). The atoms account for only 4.6% of the total energy density, or a density of one proton per four cubic meters. [26]
Hugh Everett's universal wavefunction supports the idea that observed and observer are all mixed together: . If we try to limit the applicability so as to exclude the measuring apparatus, or in general systems of macroscopic size, we are faced with the difficulty of sharply defining the region of validity.
The First Three Minutes attempts to explain the early stages of the universe after the Big Bang.Weinberg begins by recounting a creation myth from the Younger Edda and goes on to explain how, in the first half of the twentieth century, cosmologists have come to know something of the real history of the universe.
"One analogy is to think of helium-4 as ash, and the amount of ash that one forms when one completely burns a piece of wood is insensitive to how one burns it." [16] The resort to the BBN theory of the helium-4 abundance is necessary as there is far more helium-4 in the universe than can be explained by stellar nucleosynthesis. In addition, it ...