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Topic: Comoving distance

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In the News (Wed 22 May 19)

  Comoving distance: Definition and Links by Encyclopedian.com
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.
If you consider a distance to the "galaxy" which follows the light path from that "galaxy", this cannot be done for a "galaxy" beyond the horizon, and for a "galaxy" inside the horizon, use of this definition of distance instead of comoving distance yields a velocity less than the speed of light.
The ordinary distance as experienced by particles travelling slower than or at the speed of light is simply the comoving distance multiplied by the value of the scale factor at the cosmological time studied.
www.encyclopedian.com /co/Comoving-distance.html   (967 words)

 Comoving distance Information
In cosmology, the comoving distance or conformal distance is a convenient way to define distances between objects in a way that is independent of time: it is the separation the objects would have today if both objects did not move (i.e.
The comoving distance is defined as the proper distance divided by a time-varying scale factor representing the expansion of the universe.
However, while conformal 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.bookrags.com /Comoving_coordinates   (1051 words)

 Universe: Comoving distance
An umambiguous rewording of the phrase can now be made: for a "galaxy" towards or beyond the horizon, its "velocity", defined as comoving distance from the observer divided by the present cosmological time, can be greater than the speed of light This is a correct statement.
* If you consider a distance to the "galaxy" which follows the light path from that "galaxy", this cannot be done for a "galaxy" beyond the horizon, and for a "galaxy" inside the horizon, use of this definition of distance instead of comoving distance yields a velocity less than the speed of light.
distances useful on small - galaxy or cluster of galaxies - scales The ordinary distance as experienced by particles travelling slower than or at the speed of light is simply the comoving distance multiplied by the value of the scale factor at the cosmological time studied.
www.historyfocal.com /Universe/Comoving_distance.shtml   (855 words)

 Psychology of... Metric expansion of space   (Site not responding. Last check: 2007-10-09)
Because it is the metric defining distance that is changing rather than objects moving in space, this expansion (and the resultant movement apart of objects) is not restricted by the speed of light upper bound that results from special relativity.
Theory and observations suggest that very early in the history of the universe, there was an "inflationary" phase where this metric changed very rapidly, and that the remaining time-dependence of this metric is what we observe as the so-called Hubble expansion, the moving apart of all gravitationally unbound objects in the universe.
Comoving distance defines this connecting curve to be a curve of constant cosmological time.
www.psychologyof.com /wiki/Metric_expansion_of_space   (2074 words)

 Reference.com/Encyclopedia/Comoving distance
The velocity of an observer relative to the local comoving frame is called the peculiar velocity of the observer.
Each observer measures his distance to the nearest observer in the chain, and the length of the chain, the sum of distances between nearby observers, is the total comoving distance.
It is important to the definition of comoving distance that all observers have the same cosmological age.
www.reference.com /browse/wiki/Comoving_distance   (1030 words)

 Distance - Wikipedia, the free encyclopedia
One might attempt to define the distance between two non-empty subsets of a given set as the infimum of the distances between any two of their respective points, which would agree with the every-day use of the word.
Distance is a scalar quantity, containing only a magnitude, whereas displacement is an equivalent vector quantity containing both magnitude and direction.
The distance covered by a vehicle (often recorded by an odometer), person, animal, object, etc. should be distinguished from the distance from starting point to end point, even if latter is taken to mean e.g.
en.wikipedia.org /wiki/Distance   (811 words)

 The Distance Scale of the Universe   (Site not responding. Last check: 2007-10-09)
This is the problem of defining a distance in an expanding universe: Two galaxies are near to each other when the universe is only 1 billion years old.
The Comoving Distance is the distance scale that expands with the universe.
Comoving Distance is the opposite of the Angular Diameter Distance - it tells us where galaxies are now rather than where they were when they emitted the light that we now see.
www.anzwers.org /free/universe/redshift.html   (775 words)

 Dark energy is superfluous - SciForums.com
Comoving coordinates are taken from a frame of reference in which the cosmic microwave background is at rest (no Doppler shift in any direction) and clocks are unaffected by gravitational redshifts.
Distance is contracted and time is dilated in both frames, but only for the object that is moving.
The distance to the beacon that the crew subsequently measures while they move relative to the comoving frame is irrelevant.
www.sciforums.com /showthread.php?t=61429   (8265 words)

Figure 1: The distance between two events, A (at the origin) and B. The vertical axis indicates the direction of time, and the horizontal axis represents space (squashed into a single dimension rather than the usual three).
Distances outside the light cone are called space like: the best you can do is travel until the time of the event, but finding yourself still some distance away (a bit like the start of a lecture).
Note that these distances are taken from the origin: if you want to know the distance from another event C to B, you should draw a light cone with C at its origin.
iapetus.phy.umist.ac.uk /Teaching/Cosmology/Metric.html   (2264 words)

 A look at tethered and untethered galaxies
However, this is due to the definition of distance (comoving distance), and here we demonstrate that 'radar distance' is a more reasonable definition for which the effect disappears in an empty universe.
Clavering [3] has argued why radar distance is the correct definition (and this paper duplicates his work to some degree), but comoving distance (aχ) also seems reasonable.
In the matter dominated universe, each comoving observer may think of themselves at the bottom of a gravitational potential well, and so would expect a signal from an object at constant distance to be blueshifted.
www.chronon.org /papers/tethered_galaxy.html   (1450 words)

 Quasars and looking "back in time" to early universe - Bad Astronomy and Universe Today Forum   (Site not responding. Last check: 2007-10-09)
Emission distance -- the distance the source was when the light we SEE NOW left the source (there and then).
The co-moving radial distance (or reception distance) -- this is the tape measure distance if we could somehow at this cosmic instant in time measure the distance to the object when the light we see arrived here (roughly: how far is it away now?).
Then there is the "distance" you see quoted in the popular print (because the above two are too complicated to explain in a quick article or interview).
www.bautforum.com /showthread.php?p=52165#post52165   (1517 words)

 Cosmos & Basic Cosmology - Numericana
Distance: In a cosmological context, there are several flavors to the concept.
For cosmological distances and/or curved signal propagation, this is defined in terms of the velocities of the observer and the source with respect to local comoving points at rest in the CMB: The "radial" speed v is actually the difference between the projections of these velocities on the local tangents to the signal's "path".
This observed cosmological distance is thus not a simple concept and it's fairly useless in theoretical speculations, where the distance of an object to the [arbitrary] origin is best defined as the value of a space-coordinate when the time-coordinate is the same [we're talking curvilinear coordinates in curved space, here].
home.att.net /~numericana/answer/cosmos.htm   (3473 words)

 Distance Measures in Cosmology
The unifying aspect is that all distance measures somehow measure the separation between events on radial null trajectories, ie, trajectories of photons which terminate at the observer.
For small v / c, or small distance d, in the expanding Universe, the velocity is linearly proportional to the distance (and all the distance measures, eg, angular diameter distance, luminosity distance, etc, converge)
The comoving volume element and its integral are both used frequently in predicting number counts or luminosity densities.
www.astro.ufl.edu /~guzman/ast7939/projects/project01.html   (1088 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.
The actual distance between the Lyman alpha clouds were they to survive to the present epoch fixed in cosmological coordinates is the splitting in the S
www.markelowitz.com /cosmology.htm   (1086 words)

 Amazon.com: "comoving distance": Key Phrase page   (Site not responding. Last check: 2007-10-09)
The comoving distance between points on a hypothet- ical grid remains constant as the universe expands.
The constant x is called the comoving distance and is equal to the physical distance at the present epoch.
The radial comoving distance at the time of observation to to a source that emitted radiation at time t is _ to dt _...
www.amazon.com /phrase/comoving-distance   (557 words)

 Distance Summary
The abstract problem of distance reversal is made concrete in the technological device itself, which increasingly hides its own near inner workings in favor of unhindered delivery of some commodity.
Formulas for distance define what the shortest route between two points is. For example, if you traveled a distance of 10 on one axis, and then travelled a distance of 5 on a perpendicular axis, you would have travelled a total distance of 15.
In neutral geometry, the distance between two points is the length of the line segment between them.
www.bookrags.com /Distance   (3726 words)

It is presently unknown whether or not this is correct, since according to studies of the shape of the Universe, it is possible that the observable universe is of nearly the same size as the whole of space, but the question remains under debate.
The speed with which a galaxy recedes from the earth increases as the distance from the Earth to that galaxy.
The comoving distance to the edge of the visible universe is about 46.5 billion light years in all directions from the earth; thus the visible universe may be thought of as a perfect sphere with the earth at its center and a diameter of about 93 billion light years.
libraryoflibrary.com /E_n_c_p_d_Universe.html   (2620 words)

 Talk:Comoving distance
I think the suggestion that there should be some kind of notions of distance in cosmology page is a good one.
There is now a page on Distance measures (cosmology) and a redirection from notions of distance in cosmology.
Specifically, I think a bunch of the material in the "other distances used in cosmology" might be better suited to the new page, since it discusses things beyond the scope of the title of this article.
www.danceage.com /biography/sdmc_Talk:Comoving_distance   (491 words)

 [No title]
This metric is different than the one for euclidian geometry -- the distance between two events depends on the curvature of spacetime.
Comoving coordinates are coordinates that move along with the overall expansion of the universe
But the change in the true distance is completely described by the change in the expansion factor R(t).
burro.astr.cwru.edu /Academics/Astr328/Notes/Metrics/metrics.html   (303 words)

The distance they are at ``now'' is called the ``comoving'' distance.
It is the distance you would have to travel to make the trip between galaxies instantaneously.
Suppose that a galaxy is at a conformal distance
www.physics.louisville.edu /astro/students/590/sdss/node10.html   (336 words)

 Astro::Cosmology - calculate cosmological distances, volumes, and times
If greater than zero, distances are measured in Mpc, volumes in Mpc^3/steradian, and time in years.
Distance measures in cosmology, Hogg, D.W., astro-ph/9905116 Perlmutter et al.
The comoving distance is always calculated by numerical integration of the above formula.
hea-www.harvard.edu /~dburke/PDL/Cosmology-0.90.html   (1468 words)

 Evolution of the Observable Universe
Speaking about comoving or proper distances is a matter of definitions and conventions.
The point is that, due to the definition of comoving distance, comoving objects and crud located at a comoving distance Dc will be always located at the comoving distance Dc during the expansion of space.
They will be located at a larger proper distance in new spatial hypersurfaces, but their comoving distance will be the same.
www.physicsforums.com /showthread.php?t=121090&page=2   (1073 words)

 SciForums.com - Most distant galaxy cluster revealed
I suppose that they mean comoving distance, but I'm not sure.
While the `comoving radial distance` is shown as 13.728 Gly.
03-02-05 07:02 PM yeah comoving radial distance is the same as comoving distance.
www.sciforums.com /printthread.php?t=45400   (252 words)

 Cosmology Tutorial - Part 2   (Site not responding. Last check: 2007-10-09)
The time and distance used in the Hubble law are not the same as the x and t used in special relativity, and this often leads to confusion.
The redshift is such a useful distance indicator that it is a shame that science journalists conspire to leave it out of stories: they must be taught the "5 w's but no z" rule in journalism school.
Current density estimates suggest that the matter density is between 0.2 to 1 times the critical density, and this does require that most of the matter in the Universe is dark.
www.astro.ucla.edu /~wright/cosmo_02.htm   (1576 words)

 Assignment 2
In a galaxy in the Virgo cluster (18 Mpc distant)?
If you know about the technique of distance determination by surface brightness fluctuations, consider the implications of this calculations.
the distance needed to convert bolometric luminosity to bolometric flux.
cmb.as.arizona.edu /~eisenste/ast541/assignment2/assignment2.html   (540 words)

 Distance Scales
Additional details about the distance scale and lookback time are provided below in a more detailed addendum to these instructions, but for now you need to know that the distances shown in Partiview are called ``comoving'' distances.
comoving coordinates provide a way to interpret distances without having to worry that we are looking back in time as we observe out to high
The comoving distance in the Partiview database has been calculated with cosmological parameters derived from the Wilkinson Microwave Anisotropy Probe (WMAP) data on the cosmic background radiation.
www.physics.uofl.edu /astro/students/590/sdss/node3.html   (227 words)

 Astron. Astrophys. 325, 954-960 (1997)
4 shows the histogram of the distribution in comoving distance of the 3342 galaxies with measured redshift (panel a) and the corresponding wedge diagram (panel b).
The weighting with the selection function allows to detect structures even at large distances, which would be otherwise "washed out"; however, as any magnitude-limited sample, such structures are inferred from the few high luminosity galaxies that can still be detected, and are therefore affected by a large uncertainty.
Note also that the most prominent structure at large comoving distance in the small strip B appears to be elongated along the line of sight.
aa.springer.de /papers/7325003/2300954/sc4.htm   (1095 words)

 A Cosmology and Cosmography
For the proper distance between observer and object today, one takes the proper distance at the time the light was emitted times the ratio of scale factors now to then.
Next thing to be defined is the transverse comoving distance, which is a quantity used to get the comoving distance perpendicular to the line of sight.
The comoving volume is the volume measure in which the number density of objects remains constant with epoch, assuming no evolution and that the objects are locked in the Hubble flow.
www.mpifr-bonn.mpg.de /staff/hvoss/DiplWeb/DiplWebap1.html   (1490 words)

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