Recessional velocity is the rate at which an extragalactic astronomical object recedes (becomes more distant) from an observer as a result of the expansion of the universe. It can be measured by observing the wavelength shifts of spectral lines emitted by the object, known as the object's cosmological redshift.
Hubble's law is the relationship between a galaxy's distance and its recessional velocity, which is approximately linear for galaxies at distances of up to a few hundred megaparsecs. It can be expressed as
where is the Hubble constant, is the proper distance, is the object's recessional velocity, and is the object's peculiar velocity.
The recessional velocity of a galaxy can be calculated from the redshift observed in its emitted spectrum. One application of Hubble's law is to estimate distances to galaxies based on measurements of their recessional velocities. However, for relatively nearby galaxies the peculiar velocity can be comparable to or larger than the recessional velocity, in which case Hubble's Law does not give a good estimate of an object's distance based on its redshift. In some cases (such as the Andromeda Galaxy, 2.5 million light-years away and approaching us at 300 km/s, or even Messier 81 at 12 million light-years away and approaching at 34 km/s) is negative (i.e., the galaxy's spectrum is observed to be blueshifted) as a result of the peculiar velocity.
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This course is the basic introduction to modern cosmology. It introduces students to the main concepts and formalism of cosmology, the observational status of Hot Big Bang theory
and discusses major
Cosmology is the study of the structure and evolution of the universe as a whole. This course describes the principal themes of cosmology, as seen
from the point of view of observations.
droite|redresse=1.2|vignette|L'expansion de l'Univers imagée par le gonflement d'un gâteau aux raisins. En cosmologie, l'expansion de l'Univers est le nom du phénomène qui voit à grande échelle les objets composant l'Univers (galaxies, amas...) s'éloigner les uns des autres. Cet écartement mutuel, que l'on pourrait prendre pour un mouvement des galaxies dans l'espace, s'interprète en réalité par un gonflement, une dilatation, de l'espace lui-même, les objets célestes étant de ce fait amenés à s'éloigner les uns des autres.
vignette|redresse=1.5|Représentation centrée sur le Soleil d'une année-lumière (plus grande sphère, flèche jaune pour le rayon) et d'un mois-lumière (plus petite sphère) en comparaison des trajectoires de la grande comète de janvier 1910 (ligne jaune) et de la comète Hyakutake (ligne orange). L’année-lumière (de symbole « al ») est une unité de longueur utilisée en astronomie. Elle est égale à la distance parcourue par la lumière dans le vide pendant une année julienne, soit environ de kilomètres, soit encore, en ordre de grandeur, environ de kilomètres (), ou environ .
Peculiar motion or peculiar velocity refers to the velocity of an object relative to a rest frame — usually a frame in which the average velocity of some objects is zero. In galactic astronomy, peculiar motion refers to the motion of an object (usually a star) relative to a Galactic rest frame. Local objects are commonly examined as to their vectors of position angle and radial velocity. These can be combined through vector addition to state the object's motion relative to the Sun.
Couvre des exercices sur l'évaluation de l'action, les changements de longueur d'onde, la précision du GPS et le principe d'équivalence dans les champs gravitationnels.
The Gaia Galactic survey mission is designed and optimized to obtain astrometry, photometry, and spectroscopy of nearly two billion stars in our Galaxy. Yet as an all-sky multi-epoch survey, Gaia also observes several million extragalactic objects down to ...
We analyze rest-frame ultraviolet to optical spectra of three z similar or equal to 7.47-7.75 galaxies whose Ly alpha emission lines were previously detected with Keck/MOSFIRE observations, using the JWST/NIRSpec observations from the Cosmic Evolution Earl ...
Iop Publishing Ltd2024
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We present the cosmological implications from final measurements of clustering using galaxies, quasars, and Ly alpha forests from the completed Sloan Digital Sky Survey (SDSS) lineage of experiments in large-scale structure. These experiments, composed of ...