Editing Hubble's law (section)

== Determining the Hubble constant ==
[[File:Recent Hubble's Constant Values.png|thumb|The value of the Hubble constant in (km/s)/Mpc, including measurement uncertainty, for recent surveys<ref name="planck_overview"/>]]
The value of the Hubble constant, {{math|''H''{{sub|0}}}}, cannot be measured directly, but is derived from a combination of astronomical observations and model-dependent assumptions. Increasingly accurate observations and new models over many decades have led to two sets of highly precise values which do not agree. This difference is known as the "Hubble tension".<ref name="NYT-20170220" /><ref name=VerdeReview2024/>

=== Earlier measurements ===
For the original 1929 estimate of the constant now bearing his name, Hubble used observations of [[Cepheid variable]] stars as "[[Cosmic distance ladder#Standard_candles|standard candles]]" to measure distance.<ref name=Allen/> The result he obtained was {{val|500|u=km/s|up=Mpc}}, much larger than the value astronomers currently calculate. Later observations by astronomer [[Walter Baade]] led him to realize that there were distinct "[[stellar population|populations]]" for stars (Population I and Population II) in a galaxy. The same observations led him to discover that there are two types of Cepheid variable stars with different luminosities. Using this discovery, he recalculated Hubble constant and the size of the known universe, doubling the previous calculation made by Hubble in 1929.<ref>Baade, W. (1944) The resolution of Messier 32, NGC 205, and the central region of the Andromeda nebula. ApJ 100 137–146</ref><ref>Baade, W. (1956) The period-luminosity relation of the Cepheids. PASP 68 5–16</ref><ref name=Allen>{{cite web|last=Allen|first=Nick|title=Section 2: The Great Debate and the Great Mistake: Shapley, Hubble, Baade|url=http://www.institute-of-brilliant-failures.com/section2.htm|website=The Cepheid Distance Scale: A History|access-date=19 November 2011|archive-url=https://web.archive.org/web/20071210105344/http://www.institute-of-brilliant-failures.com/section2.htm|archive-date=10 December 2007|url-status=dead}}</ref> He announced this finding to considerable astonishment at the 1952 meeting of the [[International Astronomical Union]] in Rome.

For most of the second half of the 20th century, the value of {{math|''H''{{sub|0}}}} was estimated to be between {{val|50|and|90|u=km/s|up=Mpc}}.

The value of the Hubble constant was the topic of a long and rather bitter controversy between [[Gérard de Vaucouleurs]], who claimed the value was around 100, and [[Allan Sandage]], who claimed the value was near 50.<ref name="Overbye"/> In one demonstration of vitriol shared between the parties, when Sandage and [[Gustav Andreas Tammann]] (Sandage's research colleague) formally acknowledged the shortcomings of confirming the systematic error of their method in 1975, Vaucouleurs responded "It is unfortunate that this sober warning was so soon forgotten and ignored by most astronomers and textbook writers".<ref name=":0">{{Cite book |last=de Vaucouleurs |first=G. |title=The cosmic distance scale and the Hubble constant |publisher=Mount Stromlo and Siding Spring Observatories, Australian National University |year=1982}}</ref> In 1996, a debate moderated by [[John N. Bahcall|John Bahcall]] between Sidney van den Bergh and Gustav Tammann was held in similar fashion to the earlier Shapley–Curtis debate over these two competing values.

This previously wide variance in estimates was partially resolved with the introduction of the [[Lambda-CDM model|ΛCDM]] model of the universe in the late 1990s. Incorporating the ΛCDM model, observations of high-redshift clusters at X-ray and microwave wavelengths using the [[Sunyaev–Zel'dovich effect]], measurements of anisotropies in the [[cosmic microwave background]] radiation, and optical surveys all gave a value of around 50–70&nbsp;km/s/Mpc for the constant.<ref name=Myers1999>{{cite journal |title=Scaling the universe: Gravitational lenses and the Hubble constant |last=Myers |first=S. T. |date=1999 |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=96 |issue=8 |pages=4236–4239 |doi=10.1073/pnas.96.8.4236 |doi-access=free |pmid=10200245 |pmc=33560|bibcode=1999PNAS...96.4236M }}</ref>

=== Precision cosmology and the Hubble tension {{anchor|Hubble tension}} <!-- Hubble tension redirect here -->===
By the late 1990s, advances in ideas and technology allowed higher precision measurements.<ref name=Turner-2022>{{Cite journal |last=Turner |first=Michael S. |date=2022-09-26 |title=The Road to Precision Cosmology |url=https://www.annualreviews.org/content/journals/10.1146/annurev-nucl-111119-041046 |journal=Annual Review of Nuclear and Particle Science |language=en |volume=72 |pages=1–35 |doi=10.1146/annurev-nucl-111119-041046 |arxiv=2201.04741 |bibcode=2022ARNPS..72....1T |issn=0163-8998}}</ref>
However, two major categories of methods, each with high precision, fail to agree.
"Late universe" measurements using calibrated distance ladder techniques have converged on a value of approximately {{val|73|u=km/s|up=Mpc}}. Since 2000, "early universe" techniques based on measurements of the [[cosmic microwave background]] have become available, and these agree on a value near {{val|67.7|u=km/s|up=Mpc}}.<ref>{{Cite journal |last1=Freedman |first1=Wendy L. |last2=Madore |first2=Barry F. |date=2023-11-01 |title=Progress in direct measurements of the Hubble constant |url=https://doi.org/10.1088/1475-7516/2023/11/050 |journal=Journal of Cosmology and Astroparticle Physics |volume=2023 |issue=11 |article-number=050 |doi=10.1088/1475-7516/2023/11/050 |issn=1475-7516|arxiv=2309.05618 |bibcode=2023JCAP...11..050F }}</ref> (This accounts for the change in the expansion rate since the early universe, so is comparable to the first number.) Initially, this discrepancy was within the estimated [[Measurement uncertainty|measurement uncertainties]] and thus no cause for concern.  However, as techniques have improved, the estimated measurement uncertainties have shrunk, but the discrepancies have ''not'', to the point that the disagreement is now highly [[statistically significant]]. This discrepancy is called the '''Hubble tension'''.<ref name="LS-20190826">{{cite news |last=Mann |first=Adam |date=26 August 2019 |title=One Number Shows Something Is Fundamentally Wrong with Our Conception of the Universe – This fight has universal implications |work=[[Live Science]] |url=https://www.livescience.com/hubble-constant-discrepancy-explained.html |access-date=26 August 2019}}</ref><ref name="di Valentino 2021 153001">{{cite journal |last=di Valentino |first=Eleonora |display-authors=etal |date=2021 |title=In the realm of the Hubble tension—a review of solutions |journal=Classical and Quantum Gravity |volume=38 |issue=15 |article-number=153001 |doi=10.1088/1361-6382/ac086d |doi-access=free |arxiv=2103.01183 |bibcode=2021CQGra..38o3001D |s2cid=232092525}}</ref>

An example of an "early" measurement, the [[Planck (spacecraft)|Planck mission]] published in 2018 gives a value for {{math|1= ''H''{{sub|0}} =}} of {{val|67.4|0.5|u=km/s|up=Mpc}}.<ref name="2018planckcosmos"/> In the "late" camp is the higher value of {{val|74.03|1.42|u=km/s|up=Mpc}} determined by the  [[Hubble Space Telescope]]<ref name="SA2019">{{Cite magazine|url=https://www.scientificamerican.com/article/best-yet-measurements-deepen-cosmological-crisis/|title=Best-Yet Measurements Deepen Cosmological Crisis|last=Ananthaswamy|first=Anil|date=22 March 2019|access-date=23 March 2019|magazine=Scientific American}}</ref> 
and confirmed by the [[James Webb Space Telescope]] in 2023.<ref>{{Citation |last1=Riess |first1=Adam G. |title=Crowded No More: The Accuracy of the Hubble Constant Tested with High Resolution Observations of Cepheids by JWST |date=2023-07-28 |arxiv=2307.15806 |last2=Anand |first2=Gagandeep S. |last3=Yuan |first3=Wenlong |last4=Casertano |first4=Stefano |last5=Dolphin |first5=Andrew |last6=Macri |first6=Lucas M. |last7=Breuval |first7=Louise |last8=Scolnic |first8=Dan |last9=Perrin |first9=Marshall |journal=The Astrophysical Journal |volume=956 |issue=1 |article-number=L18 |doi=10.3847/2041-8213/acf769 |doi-access=free |bibcode=2023ApJ...956L..18R }}</ref><ref>{{Cite web |date=2023-09-12 |title=Webb Confirms Accuracy of Universe's Expansion Rate Measured by Hubble, Deepens Mystery of Hubble Constant Tension – James Webb Space Telescope |url=https://blogs.nasa.gov/webb/2023/09/12/webb-confirms-accuracy-of-universes-expansion-rate-measured-by-hubble-deepens-mystery-of-hubble-constant-tension/ |access-date=2024-02-15 |website=blogs.nasa.gov |language=en-US}}</ref>
The "early" and "late" measurements disagree at the >5 [[Standard deviation|''σ'']] level, beyond a plausible level of chance.<ref name="Riess2019"/><ref>{{Cite journal |last1=Riess |first1=Adam G. |last2=Yuan |first2=Wenlong |last3=Macri |first3=Lucas M. |last4=Scolnic |first4=Dan |last5=Brout |first5=Dillon |last6=Casertano |first6=Stefano |last7=Jones |first7=David O. |last8=Murakami |first8=Yukei |last9=Anand |first9=Gagandeep S. |last10=Breuval |first10=Louise |last11=Brink |first11=Thomas G. |last12=Filippenko |first12=Alexei V. |last13=Hoffmann |first13=Samantha |last14=Jha |first14=Saurabh W. |last15=Kenworthy |first15=W. D’arcy |date=July 2022 |title=A Comprehensive Measurement of the Local Value of the Hubble Constant with 1 km s−1 Mpc−1 Uncertainty from the Hubble Space Telescope and the SH0ES Team |journal=The Astrophysical Journal Letters |language=en |volume=934 |issue=1 |pages=L7 |doi=10.3847/2041-8213/ac5c5b |doi-access=free |bibcode=2022ApJ...934L...7R |issn=2041-8205|arxiv=2112.04510 }}</ref> The resolution to this disagreement is an ongoing area of active research.<ref>{{Cite journal|last1=Millea|first1=Marius|last2=Knox|first2=Lloyd|date=2019-08-10|title=Hubble constant hunter's guide|journal=Physical Review D |volume=101 |issue=4 |page=043533 |doi=10.1103/PhysRevD.101.043533 |language=en|arxiv=1908.03663}}</ref> 

[[File:Measurements of the Hubble constant (H0) by different astronomical missions and groups until 2021.jpg|thumb|upright=2.3|The landscape of H0 measurements around 2021, with the 2018 results from CMB measurements highlighted in pink and 2020 distance ladder values highlighted in cyan.<ref name="di Valentino 2021 153001" />]]

=== Reducing systematic errors ===
Since 2013 much effort has gone in to new measurements to check for possible systematic errors and improved reproducibility.<ref name=VerdeReview2024/>

The "late universe" or distance ladder measurements typically employ three stages or "rungs". In the first rung distances to [[Cepheids]] are determined while trying to reduce luminosity errors from dust and correlations of [[metallicity]] with luminosity. The second rung uses 
[[Type Ia supernova]], explosions of almost constant amount of mass and thus very similar amounts of light; the primary source of systematic error is the limited number of objects that can be observed. The third rung of the distance ladder measures the red-shift of supernova to extract the Hubble flow and from that the constant. At this rung corrections due to [[peculiar velocity|motion other than expansion]] are applied.<ref name=VerdeReview2024/>{{rp|2.1}} 
As an example of the kind of work needed to reduce systematic errors, photometry on observations from the James Webb Space Telescope of extra-galactic Cepheids confirm the findings from the HST. The higher resolution avoided confusion from crowding of stars in the field of view but came to the same value for H<sub>0</sub>.<ref>{{Cite journal |last1=Riess |first1=Adam G. |last2=Anand |first2=Gagandeep S. |last3=Yuan |first3=Wenlong |last4=Casertano |first4=Stefano |last5=Dolphin |first5=Andrew |last6=Macri |first6=Lucas M. |last7=Breuval |first7=Louise |last8=Scolnic |first8=Dan |last9=Perrin |first9=Marshall |last10=Anderson |first10=Richard I. |date=2023-10-01 |title=Crowded No More: The Accuracy of the Hubble Constant Tested with High-resolution Observations of Cepheids by JWST |journal=The Astrophysical Journal Letters |volume=956 |issue=1 |pages=L18 |doi=10.3847/2041-8213/acf769 |doi-access=free |issn=2041-8205|arxiv=2307.15806 |bibcode=2023ApJ...956L..18R }}</ref><ref name=VerdeReview2024>{{Cite journal |last1=Verde |first1=Licia |last2=Schöneberg |first2=Nils |last3=Gil-Marín |first3=Héctor |date=2024-09-13 |title=A Tale of Many H0 |url=https://www.annualreviews.org/content/journals/10.1146/annurev-astro-052622-033813 |journal=Annual Review of Astronomy and Astrophysics |language=en |volume=62 |pages=287–331 |doi=10.1146/annurev-astro-052622-033813 |issn=0066-4146}}</ref>

The "early universe" or inverse distance ladder measures the observable consequences of spherical sound waves on primordial plasma density. These pressure waves – called [[baryon acoustic oscillations]] (BAO) – cease once the universe cooled enough for electrons to stay bound to nuclei, ending the plasma and allowing the photons trapped by interaction with the plasma to escape. The pressure waves then become very small perturbations in density imprinted on the cosmic microwave background and on the large scale density of galaxies across the sky. Detailed structure in high precision measurements of the CMB can be matched to physics models of the oscillations. These models depend upon the Hubble constant such that a match reveals a value for the constant. Similarly, the BAO affects the statistical distribution of matter, observed as distant galaxies across the sky. These two independent kinds of measurements produce similar values for the constant from the current models, giving strong evidence that systematic errors in the measurements themselves do not affect the result.<ref name=VerdeReview2024/>{{rp|Sup. B}}

=== 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.]]

=== Possible resolutions of the Hubble tension ===
The cause of the Hubble tension is unknown,<ref>{{cite web |last1=Gresko |first1=Michael |title=The universe is expanding faster than it should be |url=https://www.nationalgeographic.com/science/article/the-universe-is-expanding-faster-than-it-should-be |archive-url=https://web.archive.org/web/20211217160427/https://www.nationalgeographic.com/science/article/the-universe-is-expanding-faster-than-it-should-be |url-status=dead |archive-date=December 17, 2021 |date=17 December 2021 |website=[[National Geographic]] |access-date=21 December 2021}}</ref> and there are many possible proposed solutions. The most conservative is that there is an unknown systematic error affecting either early-universe or late-universe observations. Although intuitively appealing, this explanation requires multiple unrelated effects regardless of whether early-universe or late-universe observations are incorrect, and there are no obvious candidates. Furthermore, any such systematic error would need to affect multiple different instruments, since both the early-universe and late-universe observations come from several different telescopes.<ref name=VerdeReview2024/>

Alternatively, it could be that the observations are correct, but some unaccounted-for effect is causing the discrepancy. If the [[cosmological principle]] fails (see {{slink|Lambda-CDM model|Violations of the cosmological principle}}), then the existing interpretations of the Hubble constant and the Hubble tension have to be revised, which might resolve the Hubble tension.<ref name="Snowmass21">{{citation |last1=Abdalla |first1=Elcio |title=Cosmology Intertwined: A Review of the Particle Physics, Astrophysics, and Cosmology Associated with the Cosmological Tensions and Anomalies |date=11 Mar 2022 |journal=Journal of High Energy Astrophysics |volume=34 |page=49 |arxiv=2203.06142 |bibcode=2022JHEAp..34...49A |doi=10.1016/j.jheap.2022.04.002 |s2cid=247411131 |last2=Abellán |first2=Guillermo Franco |last3=Aboubrahim |first3=Amin}}</ref> In particular, we would need to be located within a very large void, up to about a redshift of 0.5, for such an explanation to conflate with supernovae and [[baryon acoustic oscillation]] observations.<ref name="di Valentino 2021 153001" /> Yet another possibility is that the uncertainties in the measurements could have been underestimated, but given the internal agreements this is neither likely, nor resolves the overall tension.<ref name=VerdeReview2024/>

Finally, another possibility is new physics beyond the currently accepted cosmological model of the universe, the [[Lambda-CDM model|ΛCDM model]].<ref name="di Valentino 2021 153001"/><ref>{{Cite journal |last=Vagnozzi |first=Sunny |date=2020-07-10 |title=New physics in light of the ''H''<sub>0</sub> tension: An alternative view |url=https://link.aps.org/doi/10.1103/PhysRevD.102.023518 |journal=Physical Review D |volume=102 |issue=2 |article-number=023518 |doi=10.1103/PhysRevD.102.023518|arxiv=1907.07569 |bibcode=2020PhRvD.102b3518V |s2cid=197430820 }}</ref> There are very many theories in this category, for example, replacing general relativity with [[modified Newtonian dynamics|a modified theory of gravity]] could potentially resolve the tension,<ref name="Haslbauer">{{Cite journal |last1=Haslbauer |first1=M. |last2=Banik |first2=I. |last3=Kroupa |first3=P. |date=2020-12-21 |title=The KBC void and Hubble tension contradict LCDM on a Gpc scale – Milgromian dynamics as a possible solution |journal=Monthly Notices of the Royal Astronomical Society |volume=499 |issue=2 |pages=2845–2883 |arxiv=2009.11292 |bibcode=2020MNRAS.499.2845H |doi=10.1093/mnras/staa2348 |issn=0035-8711 |doi-access=free}}</ref><ref name="Mazurenko">{{Cite journal |last1=Mazurenko |first1=S. |last2=Banik |first2=I. |last3=Kroupa |first3=P. |last4=Haslbauer |first4=M. |date=2024-01-21 |title=A simultaneous solution to the Hubble tension and observed bulk flow within 250/h Mpc |journal=Monthly Notices of the Royal Astronomical Society |volume=527 |issue=3 |pages=4388–4396 |arxiv=2311.17988 |bibcode=2024MNRAS.527.4388M |doi=10.1093/mnras/stad3357 |issn=0035-8711 |doi-access=free}}</ref> as can a dark energy component in the early universe,{{efn|In standard ΛCDM, dark energy only comes into play in the late universe – its effect in the early universe is too small to have an effect.}}<ref>{{Cite journal |last1=Poulin|first1=Vivian |last2=Smith|first2=Tristan L. |last3=Karwal|first3=Tanvi |last4=Kamionkowski|first4=Marc |date=2019-06-04 |title=Early Dark Energy can Resolve the Hubble Tension |journal=Physical Review Letters |volume=122 |issue=22 |article-number=221301 |doi=10.1103/PhysRevLett.122.221301 |pmid=31283280 |arxiv=1811.04083 |bibcode=2019PhRvL.122v1301P |s2cid=119233243 }}</ref> dark energy with a time-varying [[Equation of state (cosmology)|equation of state]],{{efn|1=In standard ΛCDM, dark energy has a constant equation of state {{math|1= ''w'' = −1}}.}}<ref>{{cite journal|url=https://www.nature.com/articles/s41550-017-0216-z|title=Dynamical dark energy in light of the latest observations|journal=Nature Astronomy|date=2017|doi=10.1038/s41550-017-0216-z |last1=Zhao |first1=Gong-Bo |last2=Raveri |first2=Marco |last3=Pogosian |first3=Levon |last4=Wang |first4=Yuting |last5=Crittenden |first5=Robert G. |last6=Handley |first6=Will J. |last7=Percival |first7=Will J. |last8=Beutler |first8=Florian |last9=Brinkmann |first9=Jonathan |last10=Chuang |first10=Chia-Hsun |last11=Cuesta |first11=Antonio J. |last12=Eisenstein |first12=Daniel J. |last13=Kitaura |first13=Francisco-Shu |last14=Koyama |first14=Kazuya |last15=l'Huillier |first15=Benjamin |last16=Nichol |first16=Robert C. |last17=Pieri |first17=Matthew M. |last18=Rodriguez-Torres |first18=Sergio |last19=Ross |first19=Ashley J. |last20=Rossi |first20=Graziano |last21=Sánchez |first21=Ariel G. |last22=Shafieloo |first22=Arman |last23=Tinker |first23=Jeremy L. |last24=Tojeiro |first24=Rita |last25=Vazquez |first25=Jose A. |last26=Zhang |first26=Hanyu |volume=1 |issue=9 |pages=627–632 |arxiv=1701.08165 |bibcode=2017NatAs...1..627Z |s2cid=256705070 }}</ref> or [[dark matter]] that decays into dark radiation.<ref>{{cite journal|url=https://journals.aps.org/prd/abstract/10.1103/PhysRevD.92.061303|title=Reconciling Planck results with low redshift astronomical measurements|journal=Physical Review D|date=2015|doi=10.1103/PhysRevD.92.061303 |last1=Berezhiani |first1=Zurab |last2=Dolgov |first2=A. D. |last3=Tkachev |first3=I. I. |volume=92 |issue=6 |article-number=061303 |arxiv=1505.03644 |bibcode=2015PhRvD..92f1303B |s2cid=118169478 }}</ref> A problem faced by all these theories is that both early-universe and late-universe measurements rely on multiple independent lines of physics, and it is difficult to modify any of those lines while preserving their successes elsewhere. The scale of the challenge can be seen from how some authors have argued that new early-universe physics alone is not sufficient;<ref>{{cite web|url=https://astrobites.org/2021/05/17/template-post-5/|title=Solving the Hubble tension might require more than changing the early Universe|author=Laila Linke|publisher=Astrobites|date=17 May 2021}}</ref><ref>{{Cite journal |last1=Vagnozzi|first1=Sunny |date=2023-08-30 |title=Seven Hints That Early-Time New Physics Alone Is Not Sufficient to Solve the Hubble Tension |journal=Universe |volume=9 |issue=9 |article-number=393 |doi=10.3390/universe9090393 |arxiv=2308.16628 |bibcode=2023Univ....9..393V |doi-access=free }} </ref> while other authors argue that new late-universe physics alone is also not sufficient.<ref>{{cite journal|title=Ruling Out New Physics at Low Redshift as a Solution to the H<sub>0</sub> Tension|author=Ryan E. Keeley and Arman Shafieloo|journal=Physical Review Letters |date=August 2023|volume=131 |issue=11 |article-number=111002 |doi=10.1103/PhysRevLett.131.111002 |pmid=37774270 |arxiv=2206.08440 |bibcode=2023PhRvL.131k1002K |s2cid=249848075 }}</ref> Nonetheless, astronomers are trying, with interest in the Hubble tension growing strongly since the mid 2010s.<ref name="di Valentino 2021 153001" />