Mass
Some care is required in defining what is meant by the total mass of the observable universe. In relativity, mass and energy are equivalent, and energy can take on a variety of forms, including energy that is associated with the curvature of spacetime itself, not with its contents such as atoms and photons. Defining the total energy of a large region of curved spacetime is problematic because there is no single agreed-upon way to define the energy due to gravity (the energy associated with spacetime curvature); for example, when photons are redshifted due to the expansion of the universe, they lose energy, and some physicists would say the energy has been converted to gravitational energy while others would say the energy has simply been lost. One can, however, derive an order-of-magnitude estimate of the mass due to sources other than gravity, namely visible matter, dark matter and dark energy, based on the volume of the observable universe and the mean density.
Read more about this topic: Cosmic Web
Famous quotes containing the word mass:
“Self-determination, the autonomy of the individual, asserts itself in the right to race his automobile, to handle his power tools, to buy a gun, to communicate to mass audiences his opinion, no matter how ignorant, how aggressive, it may be.”
—Herbert Marcuse (1898–1979)
“... in the happy laughter of a theatre audience one can get the most immediate and numerically impressive guarantee that there is nothing in one’s mind which is not familiar to the mass of persons living at the time.”
—Rebecca West (1892–1983)
“It is a mass language only in the same sense that its baseball slang is born of baseball players. That is, it is a language which is being molded by writers to do delicate things and yet be within the grasp of superficially educated people. It is not a natural growth, much as its proletarian writers would like to think so. But compared with it at its best, English has reached the Alexandrian stage of formalism and decay.”
—Raymond Chandler (1888–1959)