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:
“It is almost impossible to be a doctor and an honest man, but it is obscenely impossible to be a psychiatrist without at the same time bearing the stamp of the most incontestable madness: that of being unable to resist that old atavistic reflex of the mass of humanity, which makes any man of science who is absorbed by this mass a kind of natural and inborn enemy of all genius.”
—Antonin Artaud (1896–1948)
“The mass of men lead lives of quiet desperation.”
—Henry David Thoreau (1817–1862)
“It is strange to contemplate how little sympathy or encouragement the great mass of people have with one who differs from them in tastes, to the extent of desiring an education, while they are content with little or none.”
—Caroline Nichols Churchill (1833–?)