Editing Cosmic distance ladder (section)

=== Problems ===
Two problems exist for any class of standard candle. The principal one is [[calibration]], that is the determination of exactly what the absolute magnitude of the candle is. This includes defining the class well enough that members can be recognized, and finding enough members of that class with well-known distances to allow their true absolute magnitude to be determined with enough accuracy. The second problem lies in recognizing members of the class, and not mistakenly using a standard candle calibration on an object which does not belong to the class. At extreme distances, which is where one most wishes to use a distance indicator, this recognition problem can be quite serious.

A significant issue with standard candles is the recurring question of how standard they are. For example, all observations seem to indicate that [[Type Ia supernova]]e that are of known distance have the same brightness, corrected by the shape of the light curve. The basis for this closeness in brightness is discussed below; however, the possibility exists that the distant Type Ia supernovae have different properties than nearby Type Ia supernovae. The use of Type Ia supernovae is crucial in determining the correct [[Physical cosmology|cosmological model]]. If indeed the properties of Type Ia supernovae are different at large distances, i.e. if the extrapolation of their calibration to arbitrary distances is not valid, ignoring this variation can dangerously bias the reconstruction of the cosmological parameters, in particular the reconstruction of the matter [[Friedmann equations|density parameter]].<ref name="Lindenetal2009">
{{cite journal |last1=Linden |first1=S. |last2=Virey |first2=J.-M. |last3=Tilquin |first3=A. |date=2009 |title=Cosmological parameter extraction and biases from type Ia supernova magnitude evolution |journal=[[Astronomy and Astrophysics]] |volume=506 |issue=3 |pages=1095–1105 |arxiv=0907.4495 |bibcode=2009A&A...506.1095L |doi=10.1051/0004-6361/200912811 |s2cid=15178494}} (And references therein.)</ref>{{clarify|date=August 2015}}

That this is not merely a philosophical issue can be seen from the history of distance measurements using [[Cepheid variable]]s. In the 1950s, [[Walter Baade]] discovered that the nearby Cepheid variables used to calibrate the standard candle were of a different type than the ones used to measure distances to nearby galaxies. The nearby Cepheid variables were [[Stellar population|population I]] stars with much higher [[metallicity|metal]] content than the distant [[Stellar population|population II]] stars. As a result, the population II stars were actually much brighter than believed, and when corrected, this had the effect of doubling the estimates of distances to the globular clusters, the nearby galaxies, and the diameter of the Milky Way.{{citation needed|date=January 2023}}

Most recently [[kilonova]] have been proposed as another type of standard candle. "Since kilonovae explosions are spherical,<ref name=Spher>{{Cite journal |last1=Sneppen |first1=Albert |last2=Watson |first2=Darach |last3=Bauswein |first3=Andreas |last4=Just |first4=Oliver |last5=Kotak |first5=Rubina |last6=Nakar |first6=Ehud |last7=Poznanski |first7=Dovi |last8=Sim |first8=Stuart |date=February 2023 |title=Spherical symmetry in the kilonova AT2017gfo/GW170817 |url=https://www.nature.com/articles/s41586-022-05616-x |journal=Nature |language=en |volume=614 |issue=7948 |pages=436–439 |doi=10.1038/s41586-022-05616-x |issn=1476-4687 |pmid=36792736 |arxiv=2302.06621 |bibcode=2023Natur.614..436S |s2cid=256846834 }}</ref> astronomers could compare the apparent size of a supernova explosion with its actual size as seen by the gas motion, and thus measure the rate of cosmic expansion at different distances."<ref>{{cite web | url=https://www.universetoday.com/160165/when-neutron-stars-collide-the-explosion-is-perfectly-spherical/#more-160165 | title=When Neutron Stars Collide, the Explosion is Perfectly Spherical | date=17 February 2023 }}</ref>