Editing Hubble's law (section)

=== Other kinds of measurements ===
In addition to measurements based on calibrated distance ladder techniques or measurements of the CMB, other methods have been used to determine the Hubble constant.

One alternative method for constraining the Hubble constant involves transient events seen in multiple images of a [[strong gravitational lensing|strongly lensed]] object. A transient event, such as a supernova, is seen at different times in each of the lensed images, and if this ''time delay'' between each image can be measured, it can be used to constrain the Hubble constant. This method is commonly known as "time-delay cosmography", and was first proposed by [[Sjur Refsdal|Refsdal]] in 1964,<ref>{{cite journal |last1=Refsdal |first1=S. |title=On the Possibility of Determining Hubble's Parameter and the Masses of Galaxies from the Gravitational Lens Effect |journal=Monthly Notices of the Royal Astronomical Society |date=1 September 1964 |volume=128 |issue=4 |pages=307–310 |doi=10.1093/mnras/128.4.307|doi-access=free }}</ref> years before the first strongly lensed object was observed. The first strongly lensed supernova to be discovered was named [[SN Refsdal]] in his honor. While Refsdal suggested this could be done with supernovae, he also noted that extremely luminous and distant star-like objects could also be used. These objects were later named [[quasar|quasars]], and to date (April 2025) the majority of time-delay cosmography measurements have been done with strongly lensed quasars. This is because current samples of lensed quasars vastly outnumber known lensed supernovae, of which <10 are known. This is expected to change dramatically in the next few years, with surveys such as [[Vera C. Rubin Observatory|LSST]] expected to discover ~10 lensed SNe in the first three years of observation.<ref>{{cite arXiv|eprint=2504.01068 |last1=Bronikowski |first1=M. |last2=Petrushevska |first2=T. |last3=Pierel |first3=J. D. R. |last4=Acebron |first4=A. |last5=Donevski |first5=D. |last6=Apostolova |first6=B. |last7=Blagorodnova |first7=N. |last8=Jankovič |first8=T. |title=Cluster-lensed supernova yields from the Vera C. Rubin Observatory and Nancy Grace Roman Space Telescope |date=2025 |class=astro-ph.GA }}</ref> For example time-delay constraints on H0, see the results from STRIDES and H0LiCOW in the table below.

In October 2018, scientists used information from [[gravitational wave]] events (especially those involving the [[Neutron star merger|merger of neutron stars]], like [[GW170817]]), of determining the Hubble constant.<ref name="PHYS-20181022">{{cite web |last=Lerner |first=Louise |title=Gravitational waves could soon provide measure of universe's expansion |url=https://phys.org/news/2018-10-gravitational-universe-expansion.html |date=22 October 2018 |work=[[Phys.org]] |access-date=22 October 2018 }}</ref><ref name="NAT-20181017">{{cite journal |last1=Chen |first1=Hsin-Yu |last2=Fishbach |first2=Maya |last3=Holz |first3=Daniel E. |title=A two per cent Hubble constant measurement from standard sirens within five years |date=17 October 2018 |journal=[[Nature (journal)|Nature]] |volume=562 |issue=7728 |pages=545–547 |doi=10.1038/s41586-018-0606-0 |pmid=30333628 |bibcode=2018Natur.562..545C |arxiv=1712.06531 |s2cid=52987203 }}</ref>

In July 2019, astronomers reported that a new method to determine the Hubble constant, and resolve the discrepancy of earlier methods, has been proposed based on the mergers of pairs of [[neutron star]]s, following the detection of the neutron star merger of GW170817, an event known as a [[dark siren]].<ref name="EA-20190708">{{cite news |author=National Radio Astronomy Observatory |author-link=National Radio Astronomy Observatory |date=8 July 2019 |title=New method may resolve difficulty in measuring universe's expansion – Neutron star mergers can provide new 'cosmic ruler' |work=[[EurekAlert!]] |url=https://www.eurekalert.org/pub_releases/2019-07/nrao-nmm070819.php |access-date=8 July 2019}}</ref><ref name="NRAO-20190708">{{cite news |last=Finley |first=Dave |title=New Method May Resolve Difficulty in Measuring Universe's Expansion |url=https://public.nrao.edu/news/new-method-measuring-universe-expansion/ |date=8 July 2019 |work=[[National Radio Astronomy Observatory]] |access-date=8 July 2019 }}</ref> Their measurement of the Hubble constant is {{val|73.3|+5.3|-5.0}} (km/s)/Mpc.<ref name="NAT-20190708">{{cite journal |author=Hotokezaka, K. |display-authors=et al. |title=A Hubble constant measurement from superluminal motion of the jet in GW170817 |date=8 July 2019 |journal=[[Nature Astronomy]] |volume=3 |issue=10 |pages=940–944 |doi=10.1038/s41550-019-0820-1 |bibcode=2019NatAs...3..940H |arxiv=1806.10596 |s2cid=119547153 }}</ref> 

Also in July 2019, astronomers reported another new method, using data from the [[Hubble Space Telescope]] and based on distances to [[Red giant|red giant stars]] calculated using the [[tip of the red-giant branch]] (TRGB) distance indicator. Their measurement of the Hubble constant is {{val|69.8|+1.9|-1.9|u=km/s|up=Mpc}}.<ref name="EA-20190716" /><ref name="SCI-20190719" /><ref name="The Carnegie-Chicago Hubble Program">{{cite journal |last1=Freedman |first1=Wendy L. |author-link1=Wendy Freedman |last2=Madore |first2=Barry F. |last3=Hatt |first3=Dylan |last4=Hoyt |first4=Taylor J. |last5=Jang |first5=In-Sung |last6=Beaton |first6=Rachael L. |last7=Burns |first7=Christopher R. |last8=Lee |first8=Myung Gyoon |last9=Monson |first9=Andrew J. |last10=Neeley |first10=Jillian R. |last11=Phillips |first11=Mark M. |last12=Rich |first12=Jeffrey A. |last13=Seibert |first13=Mark |display-authors=6 |year=2019 |title=The Carnegie-Chicago Hubble Program. VIII. An Independent Determination of the Hubble Constant Based on the Tip of the Red Giant Branch |journal=The Astrophysical Journal |volume=882 |issue=1 |article-number=34 |arxiv=1907.05922 |bibcode=2019ApJ...882...34F |doi=10.3847/1538-4357/ab2f73 |s2cid=196623652 |doi-access=free }}</ref> 

In February 2020, the Megamaser Cosmology Project published independent results based on [[astrophysical masers]] visible at cosmological distances and which do not require multi-step calibration.  That work confirmed the distance ladder results and differed from the early-universe results at a statistical significance level of 95%.<ref name="megamaser" /> 

In July 2020, measurements of the cosmic background radiation by the [[Atacama Cosmology Telescope]] predict that the Universe should be expanding more slowly than is currently observed.<ref>{{Cite journal|last=Castelvecchi|first=Davide|date=2020-07-15|title=Mystery over Universe's expansion deepens with fresh data|journal=Nature|language=en|volume=583|issue=7817|pages=500–501|doi=10.1038/d41586-020-02126-6|pmid=32669728|bibcode=2020Natur.583..500C|s2cid=220583383|doi-access=}}</ref>

In July 2023, an independent estimate of the Hubble constant was derived from a [[kilonova]], the optical afterglow of a [[neutron star merger]], using the [[expanding photosphere method]].<ref name="aanda.org">{{Cite journal |last1=Sneppen |first1=Albert |last2=Watson |first2=Darach |last3=Poznanski |first3=Dovi |last4=Just |first4=Oliver |last5=Bauswein |first5=Andreas |last6=Wojtak |first6=Radosław |date=2023-10-01 |title=Measuring the Hubble constant with kilonovae using the expanding photosphere method |url=https://www.aanda.org/articles/aa/abs/2023/10/aa46306-23/aa46306-23.html |journal=Astronomy & Astrophysics |language=en |volume=678 |article-number=A14 |doi=10.1051/0004-6361/202346306 |issn=0004-6361|arxiv=2306.12468 |bibcode=2023A&A...678A..14S }}</ref> Due to the blackbody nature of early kilonova spectra,<ref>{{Cite journal |last=Sneppen |first=Albert |date=2023-09-01 |title=On the Blackbody Spectrum of Kilonovae |journal=The Astrophysical Journal |volume=955 |issue=1 |article-number=44 |doi=10.3847/1538-4357/acf200 |doi-access=free |issn=0004-637X|arxiv=2306.05452 |bibcode=2023ApJ...955...44S }}</ref> such systems provide strongly constraining estimators of cosmic distance. Using the kilonova [[AT2017gfo]] (the aftermath of, once again, GW170817), these measurements indicate a local-estimate of the Hubble constant of {{val|67.0|+3.6|u=km/s|up=Mpc}}.<ref name="nature.com">{{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 |pmid=36792736 |arxiv=2302.06621 |bibcode=2023Natur.614..436S |s2cid=256846834 |issn=1476-4687}}</ref><ref name="aanda.org"/>
[[File:Hubbleconstants color.png|centre|upright=3.8|thumb|Estimated values of the Hubble constant, 2001–2020. Estimates in black represent calibrated distance ladder measurements which tend to cluster around {{val|73||u=km/s|up=Mpc}}; red represents early universe CMB/BAO measurements with ΛCDM parameters which show good agreement on a figure near {{val|67|u=km/s|up=Mpc}}, while blue are other techniques, whose uncertainties are not yet small enough to decide between the two.]]