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    Analysis

    The Hubble tension and evolving dark energy: is cosmology broken?

    Measurements of the universe's current expansion rate from nearby galaxies give about 73.5 km/s/Mpc, while fits to the early universe give about 68, a gap one 2025 community analysis puts at 5 to 7 sigma.[1][2][3] Together with DESI's hint that dark energy evolves, this has made the standard Lambda-CDM model the main open question in cosmology, though one team's measurements still agree with it.[4][5]

    Editor reviewedUpdated Particle physics and cosmologyPhysicsScience
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    The two numbers

    The Hubble constant measures how fast the universe is expanding today. There are two ways to get it. One measures distances to nearby galaxies step by step, a “distance ladder”. The other fits the standard Lambda-CDM model to the early universe, using the cosmic microwave background (CMB) and other data.[6][7]

    The Atacama Cosmology Telescope’s final 2025 analysis found the CMB described by Lambda-CDM. Combined with Planck, CMB lensing and DESI data, it gave a Hubble constant of 68.22 ± 0.36 km/s/Mpc, or 68.43 ± 0.27 with DESI’s 2025 data.[2] A community analysis by the H0 Distance Network collaboration, published in Astronomy & Astrophysics, gave a local value of 73.50 ± 0.81 km/s/Mpc.[1][8] It reports a 7.1-sigma difference from Lambda-CDM fitted to Planck, SPT and ACT data, and 5.0 sigma against a combination of Big Bang nucleosynthesis and DESI data.[3]

    The case that the tension is real

    The H0DN result combines eight independent distance indicators, and no single method or indicator dominates the final value.[8] Its authors say that, if the tension reflects real physics, it may indicate new ingredients beyond the standard cosmological model or require reassessing early-universe inferences.[9]

    Robustness to dropping any one method is the main argument against a simple calibration error. A single faulty rung, such as one type of variable star, would not be expected to shift the whole network. The argument does not rule out a systematic error shared by several methods, for example in how supernovae are standardised.

    The case for systematics

    The Chicago-Carnegie Hubble Program (CCHP) uses Hubble and JWST observations of the tip of the red giant branch. It reports 70.39 ± 1.22 (stat) ± 1.33 (sys) ± 0.70 km/s/Mpc, which it says is consistent with Lambda-CDM without new physics.[5] Its JWST-only J-region asymptotic giant branch method gives 67.80 ± 2.17 (stat) ± 1.64 (sys).[10]

    The disagreement between the CCHP and H0DN values is itself evidence that distance-ladder systematics are not fully understood. But CCHP’s larger error bars mean its values are not strongly inconsistent with either side. The debate is less about any single number than about which error budgets to trust.

    The dark energy connection

    DESI’s March 2025 results, combined with CMB and supernova data, preferred dark energy that changes over time over a cosmological constant at 2.8 to 4.2 sigma, depending on the supernova sample.[4] The preferred models have w0 > -1 and wa < 0.[11] DESI’s data also show a mild 2.3-sigma tension with CMB-inferred Lambda-CDM parameters.[12] In April 2026 DESI completed its five-year survey, with full results expected in 2027.[13][14]

    The two anomalies point in different directions. The Hubble tension concerns the expansion rate today. The DESI hint concerns how dark energy behaved over billions of years. A single fix for both is not obvious, which is one reason many cosmologists treat them as separate problems.

    What may happen next

    DESI’s full five-year analysis, expected in 2027, is the most likely near-term decider for evolving dark energy.[14] If its significance rises past 5 sigma with consistent results across supernova samples, Lambda-CDM’s cosmological constant would be in serious trouble. If it falls, attention returns to the Hubble tension alone. Independent distance measurements that bypass the ladder, such as gravitational-wave “standard sirens” from the ligo-virgo-kagra network, could eventually help, but we expect no decisive verdict from them before 2028.[15] Uncertainty on all of this is high.

    Competing views

    New physics is needed

    A local value of 73.50 ± 0.81 km/s/Mpc, built from eight independent distance methods with no single one dominating, differs from early-universe fits by 5 to 7 sigma. Its authors say that, if real, the gap may point to new ingredients beyond Lambda-CDM.[1][8][3][9]

    Measurement systematics, not new physics

    The Chicago-Carnegie Hubble Program's JWST-based red-giant and AGB-star distances give 67.8 to 70.4 km/s/Mpc, which it says is consistent with Lambda-CDM without new physics.[5][10]

    Lambda-CDM holds for the early universe, but dark energy may evolve

    ACT finds the cosmic microwave background well described by Lambda-CDM, while DESI's galaxy maps prefer evolving dark energy at 2.8 to 4.2 sigma, still short of a discovery.[2][4]

    Questions readers ask

    What is the Hubble tension?

    Nearby measurements of the universe's expansion rate give about 73.5 km/s/Mpc, while the standard model fitted to the early universe gives about 68. One 2025 analysis puts the gap at 5 to 7 sigma.[1][2][3]

    Do all astronomers agree the tension is real?

    No. The Chicago-Carnegie Hubble Program's JWST measurements give values of about 68 to 70, which it says are consistent with the standard model.[5][10]

    Is the Hubble tension linked to DESI's dark energy result?

    Both test the Lambda-CDM model. DESI's 2025 data preferred evolving dark energy at 2.8 to 4.2 sigma and showed a mild 2.3-sigma tension with CMB-based parameters.[4][12]

    Sources

    Each numbered claim is a statement we checked against the sources listed with it. Status shows how well established it is.

    1. [1]

      The H0 Distance Network collaboration's community consensus value for the local Hubble constant is 73.50 ± 0.81 km/s/Mpc, a precision of about 1%. confirmedas of 2026-10-10

    2. [2]

      The Atacama Cosmology Telescope's final (DR6) analysis in March 2025 found the CMB described by Lambda-CDM and, combined with Planck, CMB lensing and DESI, a Hubble constant of 68.22 ± 0.36 km/s/Mpc (68.43 ± 0.27 with DESI DR2). confirmedas of 2025-03-18

    3. [3]

      The H0DN local value differs by 7.1 sigma from flat Lambda-CDM fitted to Planck, SPT and ACT CMB data, and by 5.0 sigma from a combination of Big Bang nucleosynthesis and DESI BAO data. confirmedas of 2026-10-10

    4. [4]

      On 19 March 2025 DESI released baryon acoustic oscillation results from three years of data covering about 14 to 15 million galaxies and quasars; combined with other data they preferred evolving dark energy over a cosmological constant at 2.8 to 4.2 sigma, short of the 5-sigma discovery standard. confirmedas of 2025-03-19

    5. [5]

      The Chicago-Carnegie Hubble Program, using Hubble and JWST tip-of-the-red-giant-branch distances, reported a Hubble constant of 70.39 ± 1.22 (stat) ± 1.33 (sys) ± 0.70 km/s/Mpc, which it says is consistent with Lambda-CDM without new physics. confirmedas of 2026-10-10

    6. [6]

      The standard cosmological model, Lambda-CDM, combines cold dark matter with Lambda, the simplest form of dark energy, which acts as a cosmological constant. confirmedas of 2025-03-19

    7. [7]

      The cosmic microwave background is leftover radiation from the Big Bang, released about 380,000 years after the universe began and now at an effective temperature of 2.7 kelvin. confirmedas of 2026-10-10

    8. [8]

      The H0DN analysis, published in Astronomy & Astrophysics, combined eight independent distance indicators and found that no single method or indicator dominates the result. confirmedas of 2026-10-10

    9. [9]

      The H0DN researchers say that if the tension reflects real physics, it may point to new ingredients beyond the standard cosmological model or require reassessing early-universe inferences. confirmedas of 2026-10-10

    10. [10]

      CCHP's JWST-only J-region asymptotic giant branch method gave a Hubble constant of 67.80 ± 2.17 (stat) ± 1.64 (sys) km/s/Mpc. confirmedas of 2026-10-10

    11. [11]

      The DESI 2025 analysis favours dark energy models with w0 greater than -1 and wa less than 0, meaning dark energy whose strength changes over time. confirmedas of 2025-03-19

    12. [12]

      DESI's 2025 BAO results show a mild 2.3-sigma tension with parameters inferred from the cosmic microwave background under Lambda-CDM. confirmedas of 2025-03-19

    13. [13]

      In April 2026 DESI completed its planned five-year survey ahead of schedule, having observed more than 47 million galaxies and quasars, against an original target of 34 million, plus more than 20 million stars. confirmedas of 2026-04-15

    14. [14]

      DESI will keep observing through 2028, expanding its map from 14,000 to about 17,000 square degrees, and its first dark energy results from the full five-year dataset are expected in 2027. confirmedas of 2026-04-15

    15. [15]

      As of September 2026 the collaborations plan a six-month observing run, IR1, beginning in early to mid-November 2026, while plans and timing for the fifth observing run (O5) are still under discussion. confirmedas of 2026-09-03

    Revision history (1)
    1. Page created.

    Created Oct 10, 2026. Last reviewed by an editor on Oct 10, 2026. Next scheduled review: Jan 10, 2027.

    Cite this page

    "The Hubble tension and evolving dark energy: is cosmology broken?." ContentLora, updated Oct 10, 2026. https://contentlora.com/analysis/hubble-tension-debate

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