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:
“The great mass of people ... will more easily fall victim to a big lie than to a small one.”
—Adolf Hitler (1889–1945)
“A serious problem in America is the gap between academe and the mass media, which is our culture. Professors of humanities, with all their leftist fantasies, have little direct knowledge of American life and no impact whatever on public policy.”
—Camille Paglia (b. 1947)
“In really hard times the rules of the game are altered. The inchoate mass begins to stir. It becomes potent, and when it strikes,... it strikes with incredible emphasis. Those are the rare occasions when a national will emerges from the scattered, specialized, or indifferent blocs of voters who ordinarily elect the politicians. Those are for good or evil the great occasions in a nation’s history.”
—Walter Lippmann (1889–1974)