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:
“After the planet becomes theirs, many millions of years will have to pass before a beetle particularly loved by God, at the end of its calculations will find written on a sheet of paper in letters of fire that energy is equal to the mass multiplied by the square of the velocity of light. The new kings of the world will live tranquilly for a long time, confining themselves to devouring each other and being parasites among each other on a cottage industry scale.”
—Primo Levi (1919–1987)
“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)
“I stand for the heart. To the dogs with the head! I had rather be a fool with a heart, than Jupiter Olympus with his head. The reason the mass of men fear God, and at bottom dislike Him, is because they rather distrust His heart, and fancy Him all brain like a watch.”
—Herman Melville (1819–1891)