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: Observable Universe
Famous quotes containing the word mass:
“In the mass of mankind, I fear, there is too great a majority of fools and knaves; who, singly from their number, must to a certain degree be respected, though they are by no means respectable.”
—Philip Dormer Stanhope, 4th Earl Chesterfield (1694–1773)
“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)
“If all feeling for grace and beauty were not extinguished in the mass of mankind at the actual moment, such a method of locomotion as cycling could never have found acceptance; no man or woman with the slightest aesthetic sense could assume the ludicrous position necessary for it.”
—Ouida [Marie Louise De La Ramée] (1839–1908)