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:
“No doubt Jews are most obnoxious creatures. Any competent historian or psychoanalyst can bring a mass of incontrovertible evidence to prove that it would have been better for the world if the Jews had never existed. But I, as an Irishman, can, with patriotic relish, demonstrate the same of the English. Also of the Irish.... We all live in glass houses. Is it wise to throw stones at the Jews? Is it wise to throw stones at all?”
—George Bernard Shaw (1856–1950)
“The mass never comes up to the standard of its best member, but on the contrary degrades itself to a level with the lowest.”
—Henry David Thoreau (1817–1862)
“While this America settles in the mould of its vulgarity, heavily
thickening to empire,
And protest, only a bubble in the molten mass, pops and sighs out,
and the mass hardens,”
—Robinson Jeffers (1887–1962)