Editing Hubble's law (section)

=== 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" />