In physical cosmology and celestial mechanics, dark energy is a hypothetical form of energy that permeated all of space and appeared to increase the universe’s expansion rate. Dark energy is now a normal way to explain what, according to observations and experiments, is interpreted as an accelerating expansion of the universe, i.e., the space-time seems to expand faster and faster.
The most recently presented research indicates that dark energy constitutes 73 percent, dark matter is 22 percent, neutrinos is less than 1 percent and baryons matter, that is ordinary atoms, is only 4 percent of the total energy available in the universe.
Dark energy was introduced in 1998 after studies of supernovae of type Ia distant galaxies of two international research groups by the High-z Supernova Search Team and the Supernova Cosmology Project. This work was rewarded in 2011 with the Nobel Prize in physics.
The groups used both the Hubble Space Telescope and several telescopes on Jorden to observe this type of supernovae in distant galaxies. Since all supernovae of Ia type have virtually the same luminosity, apart from a weak dependence of metallicity one can use the observed brightness to calculate the distance to the supernova. By measuring the redshift of the supernova’s host galaxy a relationship may also be found between the distance to the galaxy and its redshift. Since the redshift measures how much the universe has expanded since the light emitted, these observations allow to calculate how the universe’s expansion rate has changed over time. The results of the measurements were interpreted in the way that expansion of the Universe has not slowed down under the influence of gravity, which had been buy zithromax online expected, but rather accelerating. Later and more accurate measurement data from both ground space-based telescopes confirmed this. The results were surprising, but may be explained by postulating the existence of a dark energy, the nature of which has not yet been explained on a fundamental level. The results are also supported by observations of galaxies large scale distribution and statistics on the occurrence of gravitational lenses.
It seems that the universe started to accelerate at a relatively late time, for nearly 5 billion years ago, when the universe supposed age was 9 billion years.
One of the way to explain this is the Big Bang model, which tell us that the universe is dominated energetically ten thousand years of radiation that has both a positive energy density and pressure, and then of matter that has a positive density and virtually no pressure. Both of these provide an attracting gravitational gradually decelerating expansion of the universe, but at the same time lead to the expansion of their density, which gradually decreases until the dark energy instead begin to dominate. This happened about five billion years ago, and led to the expansion began to accelerate.
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