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Topic: Comoving coordinates

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	Comoving distance - Definition, explanation
		The comoving distance or conformal distance of two objects in the universe is the distance divided by a time-varying scale factor representing the expansion of the universe.
		The most common notion of such coordinates is that of comoving coordinates, where the spatial reference frame is attached to the average positions of galaxies (or any large lumps of matter which are at most moving slowly).
		However, while cosmological time is identical to locally measured time for an observer at a fixed comoving spatial position, the comoving distance is not, in the general case, identical to a distance as physically experienced by a particle moving slower than or at the speed of light.
	www.calsky.com /lexikon/en/txt/c/co/comoving_distance.php (1052 words)

	How does the cosmological constant fit into general relativity?
		A comoving coordinate system is one which expands with the universe.
		It is the cosmic scale factor a which relates the comoving coordinates to physical distances, through the relation: d = a x.
		With the metric defined with comoving coordinates, the time evolution of the universe is described by the time evolution of the cosmic scale factor.
	super.colorado.edu /~michaele/Lambda/gr.html (608 words)

	What Causes the Hubble Redshift?
		The comoving coordinates of the speckles don't change as the balloon expands, but the distance between the speckles steadily increases.
		In comoving coordinates, light is "stretched" during its journey.
		For example, a comoving observer could set her clock by the average density of surrounding speckles, or by the temperature of the Cosmic Background Radiation.
	www.desy.de /pub/www/projects/Physics/Relativity/GR/hubble.html (740 words)

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	6.4 Newtonian limit
		are the baryonic comoving coordinates and peculiar velocities,
		The standard Zel’dovich solution [165, 68] is commonly used to generate initial conditions satisfying observed or theoretical power spectra of matter density fluctuations.
		Comoving physical displacements and velocities of the collisionless dark matter particles are set according to the power spectrum realization
	relativity.livingreviews.org /Articles/lrr-2001-2/articlesu18.html (968 words)

	The Golden Age of Cosmology
		Because the recessional velocities are a significant fraction of the speed of light, the relativistic Doppler equation is used to estimate the redshift of a quasar.
		Comoving coordinates of these objects are estimated using the luminosity distance since they are located at large cosmological distances.
		is the number density of clouds per comoving volume and sigma is the cross section (assumed constant) of a cloud.
	www.markelowitz.com /cosmology.htm (1086 words)

	SNAPCOMOVE(1NEMO) manual page
		snapcomove scale the coordinates and velocities to comoving coordinates.
		In a future version one should also be able to scale back to real-world coordinates.
		It's value is always taken to be 1.
	bima.astro.umd.edu /nemo/man_html/snapcomove.1.html (107 words)

	Cosmology Tutorial - Part 2
		Since the age of the Universe is one of the measurable quantities, the homogeneity of the Universe must be defined on a surface of constant proper time since the Big Bang.
		Worldlines of comoving observers are plotted and decorated with small, schematic lightcones.
		But in special relativistic coordinates the velocities are less than c.
	www.astro.ucla.edu /~wright/cosmo_02.htm (1576 words)

	Cosmology Tutorial - Part 3
		Note that this is not a Lorentz transformation, and that these coordinates are not the special relativistic coordinates for which a Lorentz transformation applies.
		The Galilean transformation which could be done by skewing cards in this way required that the edge of the deck remain straight, and in any case the Lorentz transformation can not be done on cards in this way because there is no absolute time.
		If every coordinate system is a distorted representation of the Universe, we may as well use a convenient coordinate system and just keep track of the distortion by following the lightcones.
	www.astro.ucla.edu /~wright/cosmo_03.htm (1521 words)

	Energy Citations Database (ECD) - - Document #40204993
		For hydrodynamics, comoving coordinates are chosen and the momentum phase space for the radiation particles is described in comoving frame four-momenta.
		We also investigate a description of the momentum phase space in terms of the particle impact parameter and energy at infinity and derive a simple approximation to the general relativistic Boltzmann equation.
		Further developed are, however, the exact equations in comoving coordinates since the description of the interaction between matter and radiation particles is best described in the closely related orthonormal basis comoving with the fluid elements.
	www.osti.gov /energycitations/product.biblio.jsp?osti_id=40204993 (303 words)

	Peculiar Velocity Fields
		By introducing inhomogeneities, as in the standard model, the Hubble flow will be perturbed, resulting in peculiar velocities of the particles.
		The linearized mass conservation equation in comoving coordinates is given by
		The linearized Poisson equation in comoving coordinates is
	trond.hjorteland.com /thesis/node13.html (371 words)

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