accelerating expansion of the Universe
gostareš-e šetâbande-ye giti
Fr.: expansion accélérée de l'Univers
giti-ye šetâbandé (#)
Fr.: univers en accélération
The deduction based on the observation that the most distant → Type Ia supernovae are fainter than that expected from their → redshifts in a matter-only dominated expanding Universe. The faintness is attributed to larger distances resulting from an accelerating Universe driven by presence of a new component with strongly negative pressure. This component that makes the Universe accelerate is named → dark energy. The deceleration or acceleration of an expanding Universe, given by the general relativistic equation, is: R../R = -(4/3)πGρ(1 + 3w), where R is the linear → cosmic scale factor of the expanding Universe, G the → gravitational constant, ρ the mean density of the Universe, and w the → equation of state parameter representing dark energy. The expansion accelerates whenever w is more negative than -1/3. The Nobel Prize in Physics 2011 was awarded to the initiators of this concept, Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess, for their discovery of the accelerating expansion of the Universe through observations of distant supernovae. See also the original paper: Perlmutter et al. 1999, ApJ 517, 565.
age of the Universe
Fr.: âge de l'Univers
The time elapsed since the → Big Bang.
giti-ye basté (#)
Fr.: Univers fermé
A → cosmological model, first formulated by Friedmann and Lemaître, in which the Universe has a → finite size and lifetime and → space has a → positive → curvature, e.g. a Universe with a density greater than the → critical density. See also → closed space.
Fr.: Univers à densité critique
A Universe whose density is just less than or equal to the critical value and expands forever with no change in the expansion rate.
de Sitter Universe
giti-ye de Sitter
Fr.: Univers de de Sitter
A solution to → Einstein's field equations of → general relativity which contains no ordinary matter (ΩM = 0) or radiation (ΩR = 0), is → Euclidean (k = 0), but has a → cosmological constant (ΩΛ > 0). The Universe expands exponentially forever. This solution was the first model expanding of → expanding Universe. See also → empty Universe, → Milne Universe.
After the Dutch mathematician and physicist Willem de Sitter (1872-1934) who worked out the model in 1917; → universe.
giti-ye âqâzin (#)
Fr.: Univers jeune
giti-ye Eddington-Lemaître (#)
Fr.: Univers d'Eddington-Lemaître
A theoretical model in which the → cosmological constant plays a crucial role by allowing an initial phase that is identical to the Einstein static Universe. After an arbitrarily long time, the Universe begins to expand. The difficulty with this model is that the initiation of galaxy formation may actually cause a collapse rather than initiate an → expansion of the Universe.
Einstein static Universe
giti-ye istâ-ye Einstein
Fr.: Univers stationnaire d'Einstein
A cosmological model in which a static (neither expanding nor collapsing) Universe is maintained by introducing a cosmological repulsion force (in the form of the cosmological constant) to counterbalance the gravitational force.
Einstein-de Sitter Universe
giti-ye Einstein-de Sitter
Fr.: Univers Einstein-de Sitter
The → Friedmann-Lemaitre model of → expanding Universe that only contains matter and in which space is → Euclidean (ΩM > 0, ΩR = 0, ΩΛ = 0, k = 0). The Universe will expand at a decreasing rate for ever.
Fr.: Univers ekpyrotique
A cosmological model in which the → Big Bang is not the beginning of the → Universe, but a transitory phase in a more global scenario. The ekpyrotic Universe model is fundamentally different from the → standard cosmology and offers radically different explanations for the cosmological problems (→ homogeneity, → isotropy, → flatness, → magnetic monopoles, etc.). In this highly speculative model → space-time has five dimensions, four spatial and one temporal. Two three-dimensional → branes, one visible and one hidden, collide following the contraction of the extra dimension. The contraction produces a blue shift effect that converts gravitational energy into brane kinetic energy. Some fraction of this kinetic energy is converted into matter and radiation that can fuel the Big Bang. The movement of the hidden brane prior to the collision is under the influence of a potential created by the exchange of appropriate M-theory fields between the branes. The resulting temperature is finite, so the hot Big Bang phase begins without a → singularity. The Universe is homogeneous because the collision and initiation of the Big Bang phase occur nearly simultaneously everywhere. The energetically preferred geometry for the two branes is flat, so their collision produces a flat Big Bang Universe. According to → Einstein's field equations, this means that the total energy density of the Universe is equal to the → critical density. Massive → magnetic monopoles, which are over-abundantly produced in the standard Big Bang theory, are not produced at all in this scenario because the temperature after collision is far too small to produce any of these massive particles. A new version of the model provides the possibility of a cyclic Universe in which the fifth dimension undergoes a cycle of contraction and expansion a number of times, or indefinitely. The Big Bang is therefore not a special event and can happen again and again. Each cycle begins with a Big Bang and ends in a → Big Crunch. At the transition between the Big Crunch and Big Bang, matter and radiation are created, restoring the Universe to the high density required for a new Big Bang phase. In this scenario, the → dark energy that is causing the cosmic acceleration of the Universe today is inter-brane potential energy. Apart from speculation, this model suffers from several fine tunings (J. Khoury et al. 2001, Phys. Rev. D64, 123522 (hep-th/0103239); P. J. Steinhardt & N. Turok, 2002, Phys. Rev. D65, 126003 (hep-th/0111098), and references therein).
Ekpyrotic is inspired by the ancient Stoic doctrine according to which the world ends in a supreme conflagration, called ekpyrosis, and then reborns from the fire (palingenesis), only to be destroyed again at the end of the new cycle; ekpyrosis, from Gk. ek- "out of," → ex-, + → pyro- combining form of pyr, → fire, + -sis a suffix used to form nouns of action, process, state, condition, such as thesis, analysis, catharsis; → Universe.
Fr.: Univers vide
A → cosmological model based on → Einstein's field equations in which the → Universe is devoid of → matter and → radiation. There are two types of empty Universes: the → de Sitter Universe and the → Milne Universe.
giti-ye hamâré gostarâ
Fr.: Univers en expansion continue
Same as → accelerating Universe.
Fr.: Univers en expansion
The deduction based on the observational fact that the greater the → distance to a → galaxy, the greater the → redshift in its → spectral lines (→ Doppler effect). The observations strongly indicate that galaxies appear to be moving away from us with speeds proportional to their distance. This is in agreement with the overall → expansion of the → Universe.
expansion of the Universe
gostarš-e giti (#)
Fr.: expansion de l'Univers
The receding of galaxies from one another at a speed proportional to their separation, as inferred by Edwin Hubble from the observed Doppler shift of distant galaxies. → Hubble constant describes the local rate of the expansion.
Fr.: univers plat
A Universe where the → geometry is → Euclidean, i.e. parallel lines remain parallel when extended into the distance and the sum of the interior angles of a triangle is 180°. The → space-time in a flat Universe has a null → curvature constant, k = 0. See also → closed Universe, → open Universe.
Fr.: univers Friedmann-Lemaître
heat death of the Universe
marg-e garmâyi-ye giti (#)
Fr.: mort thermique de l'Univers
Assuming that the Universe is a thermodynamically → isolated system, a state of absolute uniformity in the Universe in which all temperature differences would reduce to zero and no energy will be available for use, according to the → second law of thermodynamics. In that condition of maximum → entropy, the Universe would be in a state of unchanging death. First introduced by the German physicist Hermann von Helmholtz (1821-1894) in 1854, on the basis of William Thomson's (1824-1907) idea.
giti-ye hamgen (#)
Fr.: Univers homogène
A model Universe which is homogeneous and → isotropic on large scales. It is modeled by a → Robertson-Walker cosmology. A homogeneous Universe is filled with a constant density and negligible pressure. Any small spatial region is characteristic for the whole Universe.
The hypothesis first put forward by Immanuel Kant (1724-1804) according to which the objects termed "spiral nebulae" were stellar systems comparable to our own → Milky Way galaxy. At the end of the 18th century, William Herschel (1738-1822) using his giant reflectors discovered thousands of such nebulae. However, in spite of advances in observations it was never possible to prove Kant's idea until the second decade of the twentieth century. The observations using the Mount Wilson 2.50m (100 inch) telescope allowed Edwin Hubble in 1924 to firmly establish that the "spiral nebulae" were unquestionably extragalactic.