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{{Short description|Definitions for distance between two objects or events in the universe}}
{{distinguish|Distance measurement}}
{{cosmology}}

'''Distance measures''' are used in [[physical cosmology]] to give a natural notion of the [[distance]] between two objects or events in the [[universe]].  They are often used to tie some ''observable'' quantity (such as the [[luminosity]] of a distant [[quasar]], the [[redshift]] of a distant [[galaxy]], or the angular size of the acoustic peaks in the [[cosmic microwave background]] (CMB) power spectrum) to another quantity that is not ''directly'' observable, but is more convenient for calculations (such as the [[comoving coordinates]] of the quasar, galaxy, etc.). The distance measures discussed here all reduce to the common notion of [[Euclidean distance|Euclidean]] distance at low redshift.

In accord with our present understanding of cosmology, these measures are calculated within the context of [[general relativity]], where the [[Friedmann–Lemaître–Robertson–Walker metric|Friedmann–Lemaître–Robertson–Walker]] solution is used to describe the universe.

==Overview==

There are a few different definitions of "distance" in cosmology which are all [[asymptotic]] one to another for small [[redshift]]s. The expressions for these distances are most practical when written as functions of redshift <math>z</math>, since redshift is always the observable. They can also be written as functions of [[scale factor (cosmology)|scale factor]] <math>a=1/(1+z).</math>

In the remainder of this article, the [[peculiar velocity]] is assumed to be negligible unless specified otherwise.

We first give formulas for several distance measures, and then describe them in more detail further down. Defining the "Hubble distance" as
<math display="block">d_H = \frac{c}{H_0}\approx 3000 h^{-1} \text{Mpc}\approx 9.26 \cdot 10^{25} h^{-1} \text{m}</math>
where <math>c</math> is the [[speed of light]], <math>H_0</math> is the Hubble parameter today, and {{mvar|h}} is the [[dimensionless Hubble constant]], all the distances are asymptotic to <math>z\cdot d_H</math> for small {{mvar|z}}.

According to the [[Friedmann equations]], we also define a dimensionless Hubble ''parameter'':<ref name="peebles1993"/>
<math display="block">E(z) = \frac{H(z)}{H_0}=\sqrt{\Omega_r(1+z)^4+\Omega_m(1+z)^3+\Omega_k(1+z)^2+\Omega_\Lambda}</math>

Here, <math>\Omega_r, \Omega_m,</math> and <math>\Omega_\Lambda</math> are normalized values of the present radiation energy density, matter density, and "[[dark energy]] density", respectively (the latter representing the [[cosmological constant]]), and <math>\Omega_k = 1-\Omega_r-\Omega_m-\Omega_\Lambda</math> determines the curvature. The [[Hubble parameter]] at a given redshift is then <math>H(z) = H_0E(z)</math>.

The formula for comoving distance, which serves as the basis for most of the other formulas, involves an [[integral]]. Although for some limited choices of parameters (see below) the comoving distance integral has a closed analytic form, in general—and specifically for the [[Lambda-CDM model|parameters of our universe]]—we can only find a solution [[Numerical integration|numerically]]. Cosmologists commonly use the following measures for distances from the observer to an object at redshift <math>z</math> along the line of sight (LOS):<ref name=Hogg>{{cite arXiv|title=Distance measures in cosmology |author=David W. Hogg |year=2000 |eprint=astro-ph/9905116v4}}</ref>

*'''Comoving distance:''' <math display="block">d_C(z)  = d_H \int_0^z \frac{dz'}{E(z')}</math>
*'''Transverse comoving distance:''' <math display="block"> d_M(z) = \begin{cases}
 \frac{d_H}{\sqrt{\Omega_k}} \sinh\left(\frac{\sqrt{\Omega_k}d_C(z)}{d_H}\right) & \Omega_k>0\\
 d_C(z) & \Omega_k=0\\
 \frac{d_H}{\sqrt{|\Omega_k|}} \sin\left(\frac{\sqrt{|\Omega_k|}d_C(z)}{d_H}\right) & \Omega_k<0
\end{cases}</math>
*'''Angular diameter distance:''' <math display="block"> d_A(z)  = \frac{d_M(z)}{1+z}</math>
*'''Luminosity distance:''' <math display="block">d_L(z)=(1+z) d_M(z)</math>
*'''Light-travel distance:''' <math display="block">d_T(z) = d_H \int_0^z \frac{d z'}{(1+z')E(z')} </math>

[[Image:CosmoDistanceMeasures z to onehalf.png|thumb|right|400px|A comparison of cosmological distance measures, from redshift zero to redshift of&nbsp;0.5. The background cosmology is Hubble parameter 72 km/s/Mpc, <math>\Omega_\Lambda=0.732</math>, <math>\Omega_{\rm matter}=0.266</math>, <math>\Omega_{\rm radiation}=0.266/3454</math>, and <math>\Omega_k</math> chosen so that the sum of Omega parameters is&nbsp;1. [[Edwin Hubble]] made use of galaxies up to a redshift of a bit over 0.003 ([[Messier 60]]).]]

[[Image:CosmoDistanceMeasures z to 1e4.png|thumb|right|400px|A comparison of cosmological distance measures, from redshift zero to redshift of 10,000, corresponding to the epoch of matter/radiation equality. The background cosmology is Hubble parameter 72 km/s/Mpc, <math>\Omega_\Lambda=0.732</math>, <math>\Omega_{\rm matter}=0.266</math>, <math>\Omega_{\rm radiation}=0.266/3454</math>, and <math>\Omega_k</math> chosen so that the sum of Omega parameters is one.]]

==Alternative terminology==

Peebles calls the transverse comoving distance the "angular size distance", which is not to be mistaken for the angular diameter distance.<ref name="peebles1993">{{cite book |title=Principles of Physical Cosmology |publisher=[[Princeton University Press]] |first=P. J. E. |last=Peebles |pages=[https://archive.org/details/principlesofphys00pjep/page/310 310–320] |year=1993 |bibcode=1993ppc..book.....P |isbn=978-0-691-01933-8 |url-access=registration |url=https://archive.org/details/principlesofphys00pjep/page/310 }}</ref> Occasionally, the symbols <math>\chi</math> or <math>r</math> are used to denote both the comoving and the angular diameter distance. Sometimes, the light-travel distance is also called the "lookback distance" and/or "lookback time".{{cn|date=August 2022}}

==Details==

===Peculiar velocity===

In real observations, the movement of the Earth with respect to the [[Hubble flow]] has an effect on the observed redshift.{{cn|date=February 2023}}

There are actually two notions of redshift. One is the redshift that would be observed if both the Earth and the object were not moving with respect to the "comoving" surroundings (the [[Hubble flow]]), defined by the cosmic microwave background. The other is the actual redshift measured, which depends both on the [[peculiar velocity]] of the object observed and on their peculiar velocity. Since the [[Solar System]] is moving at around 370&nbsp;km/s in a direction between [[Leo (constellation)|Leo]] and [[Crater (constellation)|Crater]], this decreases <math>1+z</math> for distant objects in that direction by a factor of about 1.0012 and increases it by the same factor for distant objects in the opposite direction. (The speed of the motion of the Earth around the Sun is only 30&nbsp;km/s.){{cn|date=February 2023}}

===Comoving distance===
{{Main|Comoving distance}}

The comoving distance <math>d_C</math> between fundamental observers, i.e. observers that are both moving with the [[Hubble's law|Hubble flow]], does not change with time, as comoving distance accounts for the expansion of the universe. Comoving distance is obtained by integrating the proper distances of nearby fundamental observers along the line of sight ('''LOS'''), whereas the proper distance is what a measurement at constant cosmic time would yield.{{cn|date=February 2023}}

In [[Big Bang|standard cosmology]], '''comoving distance''' and '''proper distance''' are two closely related distance measures used by cosmologists to measure distances between objects; the comoving distance is the proper distance at the present time.{{cn|date=February 2023}}

The comoving distance (with a small correction for our own motion) is the distance that would be obtained from parallax, because the parallax in degrees equals the ratio of an [[astronomical unit]] to the circumference of a circle at the present time going through the sun and centred on the distant object, multiplied by 360°. However, objects beyond a [[megaparsec]] have parallax too small to be measured (the [[Gaia space telescope]] measures the parallax of the brightest stars with a precision of 7 microarcseconds), so the parallax of galaxies outside our [[Local Group]] is too small to be measured.

There is a [[closed-form expression]] for the integral in the definition of the comoving distance if <math>\Omega_r=\Omega_m=0</math> or, by substituting the scale factor <math>a</math> for <math>1/(1+z)</math>, if <math>\Omega_\Lambda=0</math>. Our universe now seems to be closely represented by <math>\Omega_r=\Omega_k=0.</math> In this case, we have:
<math display="block">d_C(z)  = d_H \Omega_m^{-1/3}\Omega_\Lambda^{-1/6}[f((1+z)(\Omega_m/\Omega_\Lambda)^{1/3})-f((\Omega_m/\Omega_\Lambda)^{1/3})]</math>
where
<math display="block">f(x)\equiv\int_0^x \frac{dx}{\sqrt{x^3+1}}</math>

The comoving distance should be calculated using the value of {{mvar|z}} that would pertain if neither the object nor we had a peculiar velocity.

Together with the scale factor it gives the proper distance at the time:
<math display="block">d = a d_C</math>

===Proper distance===

Proper distance roughly corresponds to where a distant object would be at a specific moment of [[cosmological time]], which can change over time due to the [[expansion of the universe]]. ''Comoving distance'' factors out the expansion of the universe, which gives a distance that does not change in time due to the expansion of space (though this may change due to other, local factors, such as the motion of a galaxy within a cluster); the comoving distance is the proper distance at the present time.{{cn|date=February 2023}}

===Transverse comoving distance===

Two comoving objects at constant redshift <math>z</math> that are separated by an angle <math>\delta\theta</math> on the sky are said to have the distance <math>\delta\theta d_M(z)</math>, where the transverse comoving distance <math>d_M</math> is defined appropriately.{{cn|date=February 2023}}

===Angular diameter distance===
{{Main|Angular diameter distance}}

An object of size <math>x</math> at redshift <math>z</math> that appears to have angular size <math>\delta\theta</math> has the angular diameter distance of <math>d_A(z)=x/\delta\theta</math>. This is commonly used to observe so called [[standard ruler]]s, for example in the context of [[baryon acoustic oscillations]].

When accounting for the earth's peculiar velocity, the redshift that would pertain in that case should be used but <math>d_A</math> should be corrected for the motion of the solar system by a factor between 0.99867 and 1.00133, depending on the direction. (If one starts to move with velocity {{mvar|v}} towards an object, at any distance, the angular diameter of that object decreases by a factor of <math display="inline">\sqrt{(1+v/c) / (1-v/c)}.</math>)

===Luminosity distance===
{{Main|Luminosity distance}}

If the intrinsic [[luminosity]] <math>L</math> of a distant object is known, we can calculate its luminosity distance by measuring the flux <math>S</math> and determine <math display="inline">d_L(z) = \sqrt{L/4\pi S}</math>, which turns out to be equivalent to the expression above for <math>d_L(z)</math>. This quantity is important for measurements of [[standard candles]] like [[type Ia supernovae]], which were first used to discover the acceleration of the [[Dark Energy|expansion of the universe]].

When accounting for the earth's peculiar velocity, the redshift that would pertain in that case should be used for <math>d_M,</math> but the factor <math>(1+z)</math> should use the measured redshift, and another correction should be made for the peculiar velocity of the object by multiplying by <math display="inline">\sqrt{(1+v/c) / (1-v/c)},</math> where now {{mvar|v}} is the component of the object's peculiar velocity away from us. In this way, the luminosity distance will be equal to the angular diameter distance multiplied by <math>(1+z)^2,</math> where {{mvar|z}} is the measured redshift, in accordance with [[Etherington's reciprocity theorem]] (see below).

===Light-travel distance===
(also known as "[[#Alternative terminology|lookback time]]" or "[[#Alternative terminology|lookback distance]]")<ref name="CALC-2022">{{cite web |author=Staff |title=Cosmology Calculator |url=https://cosmocalc.icrar.org/ |date=2022 |work=[[International Centre for Radio Astronomy Research]] |accessdate=4 August 2022 }}</ref>

This distance <math>d_T</math> is the time that it took light to reach the observer from the object multiplied by the [[speed of light]]. For instance, the radius of the [[observable universe]] in this distance measure becomes the age of the universe multiplied by the speed of light (1 light year/year), which turns out to be approximately 13.8 billion light years.{{cn|date=February 2023}}

There is a closed-form solution of the light-travel distance if <math>\Omega_r = \Omega_m = 0</math> involving the [[inverse hyperbolic function]]s <math>\text{arcosh}</math> or <math>\text{arsinh}</math> (or involving [[inverse trigonometric functions]] if the cosmological constant has the other sign). If <math>\Omega_r = \Omega_\Lambda = 0</math> then there is a closed-form solution for <math>d_T(z)</math> but not for <math>z(d_T).</math>

Note that the comoving distance is recovered from the transverse comoving distance by taking the limit <math>\Omega_k \to 0</math>, such that the two distance measures are equivalent in a [[flat universe]].

There are websites for calculating light-travel distance from redshift.<ref name="UCLA-2015">{{cite web |author=Staff |title=UCLA Cosmological Calculator |url=http://www.astro.ucla.edu/~wright/ACC.html |date=2015 |work=[[UCLA]] |accessdate=6 August 2022 }} Light travel distance was calculated from redshift value using the UCLA Cosmological Calculator, with parameters values as of 2015: H<sub>0</sub>=67.74 and Omega<sub>M</sub>=0.3089 (see Table/Planck2015 at "[[Lambda-CDM model#Parameters]]" )</ref><ref name="UCLA-2018">{{cite web |author=Staff |title=UCLA Cosmological Calculator |url=http://www.astro.ucla.edu/~wright/ACC.html |date=2018 |work=[[UCLA]] |accessdate=6 August 2022 }} Light travel distance was calculated from redshift value using the UCLA Cosmological Calculator, with parameters values as of 2018: H<sub>0</sub>=67.4 and Omega<sub>M</sub>=0.315 (see Table/Planck2018 at "[[Lambda-CDM model#Parameters]]" )</ref><ref name="ICRAR-2022">{{cite web |author=Staff |title=ICRAR Cosmology Calculator |url=https://cosmocalc.icrar.org/ |date=2022 |work=[[International Centre for Radio Astronomy Research]] |accessdate=6 August 2022 }} ICRAR Cosmology Calculator - Set H<sub>0</sub>=67.4 and Omega<sub>M</sub>=0.315 (see Table/Planck2018 at "[[Lambda-CDM model#Parameters]]")</ref><ref name="KEMP-2022">{{cite web |last=Kempner |first=Joshua |title=KEMPNER Cosmology Calculator |url=https://www.kempner.net/cosmic.php |date=2022 |work=Kempner.net |accessdate=6 August 2022 }} KEMP Cosmology Calculator - Set H<sub>0</sub>=67.4, Omega<sub>M</sub>=0.315, and Omega<sub>Λ</sub>=0.6847 (see Table/Planck2018 at "[[Lambda-CDM model#Parameters]]")</ref>

The age of the universe then becomes <math>\lim_{z\to\infty} d_T(z)/c</math>, and the time elapsed since redshift <math>z</math> until now is: <math> t(z) = d_T(z)/c.</math>

===Etherington's distance duality===
{{Main|Etherington's reciprocity theorem}}

The Etherington's distance-duality equation <ref>I.M.H. Etherington, “LX. On the Definition of Distance in
General Relativity”, Philosophical Magazine, Vol. 15, S. 7 (1933), pp. 761-773.</ref> is the relationship between the luminosity distance of standard candles and the angular-diameter distance. It is expressed as follows: <math>d_L = (1+z)^2 d_A </math>

==See also==
{{Portal|Space}}
* [[Big Bang]]
* [[Comoving and proper distances]]
* [[Friedmann equations]]
* [[Parsec]]
* [[Physical cosmology]]
* [[Cosmic distance ladder]]
* [[Friedmann–Lemaître–Robertson–Walker metric]]
* [[Subatomic scale]]

==References==
{{reflist}}

* Scott Dodelson, ''Modern Cosmology.'' Academic Press (2003).

==External links==
*[http://www.atlasoftheuniverse.com/redshift.html 'The Distance Scale of the Universe'] compares different cosmological distance measures.
*[https://arxiv.org/abs/astro-ph/9905116 'Distance measures in cosmology'] explains in detail how to calculate the different distance measures as a function of world model and redshift.
*[http://icosmos.co.uk/ iCosmos: Cosmology Calculator (With Graph Generation )] calculates the different distance measures as a function of cosmological model and redshift, and generates plots for the model from redshift 0 to 20.

[[Category:Physical cosmology]]
[[Category:Physical quantities]]
[[Category:Length, distance, or range measuring devices]]