Editing Hubble's law (section)

=== 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" />]]