Editing Cosmic distance ladder (section)

== Standard ruler ==

Another class of physical distance indicator is the [[standard ruler]]. In 2008, galaxy diameters have been proposed as a possible standard ruler for cosmological parameter determination.<ref name="Marinonietal2009">{{cite journal |last1=Marinoni |first1=C. |display-authors=etal |year=2008 |title=Geometrical tests of cosmological models. I. Probing dark energy using the kinematics of high redshift galaxies |url=https://www.research.manchester.ac.uk/portal/en/publications/a-gravitationalwave-standard-siren-measurement-of-the-hubble-constant(f8b9915b-5717-45bc-acea-54e277df11a1).html |journal=[[Astronomy and Astrophysics]] |volume=478 |issue=1 |pages=43–55 |arxiv=0710.0759 |bibcode=2008A&A...478...43M |doi=10.1051/0004-6361:20077116 |doi-access=free |access-date=2019-10-23 |archive-date=2022-01-26 |archive-url=https://web.archive.org/web/20220126213113/https://www.research.manchester.ac.uk/portal/en/publications/a-gravitationalwave-standard-siren-measurement-of-the-hubble-constant%28f8b9915b-5717-45bc-acea-54e277df11a1%29.html |url-status=live}}</ref> More recently the physical scale imprinted by [[baryon acoustic oscillations]] (BAO) in the early universe has been used. 
In the early universe (before [[Recombination (cosmology)|recombination]]) the baryons and photons scatter off each other, and form a tightly coupled fluid that can support sound waves. The waves are sourced by primordial density perturbations, and travel at speed that can be predicted from the baryon density and other cosmological parameters.

The total distance that these sound waves can travel before recombination determines a fixed scale, which simply expands with the universe after recombination. BAO therefore provide a standard ruler that can be measured in galaxy surveys from the effect of baryons on the clustering of galaxies. The method requires an extensive galaxy survey in order to make this scale visible, but has been measured with percent-level precision (see [[baryon acoustic oscillations]]). The scale does depend on cosmological parameters like the baryon and matter densities, and the number of neutrinos, so distances based on BAO are more dependent on cosmological model than those based on local measurements.

[[Light echo]]s can be also used as standard rulers,<ref>{{cite press release |date=11 February 2008 |title=Light echoes whisper the distance to a star |publisher=[[European Southern Observatory]] |url=http://www.eso.org/public/news/eso0805/ |access-date=2015-10-18 |archive-date=2015-09-24 |archive-url=https://web.archive.org/web/20150924065025/http://www.eso.org/public/news/eso0805/ |url-status=live}}</ref><ref>{{cite journal |title=The long-period Galactic Cepheid RS Puppis I. A geometric distance from its light echoes |first1=Pierre |last1=Kervella |first2=Antoine |last2=Mérand |first3=László |last3=Szabados |first4=Pascal |last4=Fouqué |first5=David |last5=Bersier |first6=Emanuela |last6=Pompei |first7=Guy |last7=Perrin |journal=[[Astronomy and Astrophysics]] |volume=480 |issue=1 |date=2 March 2008 |pages=167–178 |doi=10.1051/0004-6361:20078961 |doi-access=free |bibcode=2008A&A...480..167K |arxiv=0802.1501 |quote=we derive a geometric distance of {{val|1992|28 |u=pc}} to RS Pup}}</ref> although it is challenging to correctly measure the source geometry.<ref>{{cite journal |title=On geometric distance determination to the Cepheid RS Puppis from its light echoes |first1=Howard E. |last1=Bond |first2=William B. |last2=Sparks |journal=[[Astronomy and Astrophysics]] |volume=495 |issue=2 |date=4 March 2009 |pages=371–377 |doi=10.1051/0004-6361:200810280 |doi-access=free |bibcode=2009A&A...495..371B |arxiv=0811.2943 |quote=We conclude that most of the knots are in fact likely to lie in front of the plane of the sky, thus invalidating the Kervella et al. result. [...]  Although the Kervella et al. distance result is invalidated, we show that high-resolution polarimetric imaging has the potential to yield a valid geometric distance to this important Cepheid.}}</ref><ref>{{cite journal |title=The long-period Galactic Cepheid RS Puppis. III. A geometric distance from HST polarimetric imaging of its light echoes |first1=Pierre |last1=Kervella |first2=Howard E. |last2=Bond |first3=Misty |last3=Cracraft |first4=László |last4=Szabados |first5=Joanne |last5=Breitfelder |first6=Antoine |last6=Mérand |first7=William B. |last7=Sparks |first8=Alexandre |last8=Gallenne |first9=David |last9=Bersier |first10=Pascal |last10=Fouqué |first11=Richard I. |last11=Anderson |journal=[[Astronomy and Astrophysics]] |volume=572 |date=December 2014 |page=A7 (13 pp.) |doi=10.1051/0004-6361/201424395 |doi-access=free |bibcode=2014A&A...572A...7K |arxiv=1408.1697 |quote=We obtain a distance of {{val|1910|80 |u=pc}} (4.2%)}}</ref>