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:
“I am not a very sentimental man; and the best sentiment I can think of is, that if you collect the signatures of all persons who are no less distinguished than I, you will have a very undistinguishing mass of names.”
—Abraham Lincoln (1809–1865)
“... 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)
“At first, he savored only the material quality of the sounds secreted by the instruments. And it had already been a great pleasure when, beneath the tiny line of the violin, slender, resistant, dense and driving, he noticed the mass of the piano’s part seeking to arise in a liquid splashing, polymorphous, undivided, level and clashing like the purple commotion of wave charmed and flattened by the moonlight.”
—Marcel Proust (1871–1922)