11/19/2023 0 Comments What is the dark energy![]() When astronomers measure the amount of matter and energy in the universe today they only come up with about ~30% of what is needed to make the universe flat. For a matter dominated Universe the critical density (equivalent to about 6 protons per m 3) sits exactly between the density required for a heavy universe that will eventually collapse, and a density required for a light universe that will expand forever. If the density of the universe exactly equals the critical density, then the geometry of the universe is flat like a sheet of paper. Inflation, occurring about 10 −36 seconds after the Big Bang, acted to smooth out the universe and make it geometrically flat. It is thought that the very early universe also went through a period of rapid expansion, called inflation. Other types of dark energy have been proposed, including a cosmic field associated with inflation and a different, low-energy field dubbed “quintessence”. ![]() However if dark energy is described by something similar to Einstein’s cosmological constant it doesn’t just balance gravity to keep a static universe but has negative pressure to cause the expansion to accelerate. Once the universe was observed to be expanding Einstein hastily removed his cosmological constant. ![]() It provided a way of balancing the gravitational contraction caused by matter. It was a vacuum energy of empty space, which kept the universe (predicted by his field equations of the General Theory of Relativity) static, rather than contracting or expanding. In the early part of the 20th century Albert Einstein had invoked a ‘cosmological constant’, (usually symbolized by the Greek letter lambda, Λ). Dark Energy was invoked to drive this acceleration. Both teams agreed that the universe is going through a phase of accelerated expansion. To be fainter, the supernovae must be farther away and this requires that the expansion of the Universe was slower in the past. ![]() In 1998 two teams of astronomers announced that distant, z~1 type Ia supernovae were slightly too faint than model predictions of an expanding (yet slowing) universe. ![]() The other dominant contributor is Dark Matter, and a small amount is due to atoms or baryonic matter. Credit: Swinburneĭark Energy makes up 72% of the total mass-energy density of the universe. Schematic representation of the total mass-energy density in the universe. ![]()
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