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 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–?)
“The great mass of people judge well of things, for they are in natural ignorance, which is man’s true state.”
—Blaise Pascal (1623–1662)
“Life is a series of diminishments. Each cessation of an activity either from choice or some other variety of infirmity is a death, a putting to final rest. Each loss, of friend or precious enemy, can be equated with the closing off of a room containing blocks of nerves ... and soon after the closing off the nerves atrophy and that part of oneself, in essence, drops away. The self is lightened, is held on earth by a gram less of mass and will.”
—Coleman Dowell (1925–1985)