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:
“Genius is present in every age, but the men carrying it within them remain benumbed unless extraordinary events occur to heat up and melt the mass so that it flows forth.”
—Denis Diderot (1713–1784)
“What will happen once the authentic mass man takes over, we do not know yet, although it may be a fair guess that he will have more in common with the meticulous, calculated correctness of Himmler than with the hysterical fanaticism of Hitler, will more resemble the stubborn dullness of Molotov than the sensual vindictive cruelty of Stalin.”
—Hannah Arendt (1906–1975)
“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)