accelerating expansion of the Universe
gostareš-e šetâbande-ye giti
Fr.: expansion accélérée de l'Univers
Fr.: expansion binomiale
Fr.: expansion cosmique
Same as the → expansion of the Universe.
1) General: The act or process of expanding; the state or quality of being
Noun from → expand.
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.: paramètre d'expansion
A → scale factor that relates the size of the Universe R = R(t) at time t to the size of the Universe R0 = R(t0) at time t0 by R = aR0. The expansion parameter represents the history of expansion of the Universe.
free expansion phase
fâz-e gostareš-e âzâd
Fr.: phase d'expansion libre
The first phase of → supernova remnant (SNR) evolution in which the surrounding → interstellar medium (ISM) has no influence on the expansion of the → shock wave, and the pressure of the interstellar gas is negligible. The shock wave created by the → supernova explosion moves outward into the interstellar gas at highly → supersonic speed. Assuming that most of the → supernova energy ESN is transformed into → kinetic energy of the ejected gas, the ejection velocity ve can be estimated from ESN by using ESN = (1/2) Meve2, which leads to ve = (2ESN / Me)(1/2), where Me is the ejected mass. The schematic structure of the SNR at this phase can be described as follows: behind the strong → shock front which moves outward into the ISM, compressed interstellar gas accumulates forming a → shell of interstellar gas. This shell of swept-up material in front of shock does not represent a significant increase in the mass of the system. After some time the accumulated mass equals the ejected mass of stellar material, and it will start to affect the expansion of the SNR. By definition, this is the end of the free expansion phase, and the corresponding radius of the SNR, called → sweep-up radius, RSW, is defined by Me = (4π/3) RSW3ρ0, that is RSW = (3Me / 4πρ0)(1/3), where ρ0 is the initial density of the ISM. This radius is reached at the sweep-up time tSW = RSW/ve. The free expansion phase lasts some 100-200 years until the mass of the material swept up by the shock wave exceeds the mass of the ejected material. Then the following → snowplow phase starts.
Fr.: développement post-newtinien
Fr.: expansion thermique
The change in dimensions of a material resulting from a change in temperature.