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:
“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)
“The great mass of people ... will more easily fall victim to a big lie than to a small one.”
—Adolf Hitler (1889–1945)
“No man’s thoughts are new, but the style of their expression is the never-failing novelty which cheers and refreshes men. If we were to answer the question, whether the mass of men, as we know them, talk as the standard authors and reviewers write, or rather as this man writes, we should say that he alone begins to write their language at all.”
—Henry David Thoreau (1817–1862)