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Astronomers estimate the universe’s present expansion rate by comparing how far away galaxies are with how much their light has shifted toward longer wavelengths. The best-known local method builds a calibrated “distance ladder” from nearby stars to distant supernovae; measurements of the early-universe cosmic microwave background and the large-scale baryon acoustic oscillation pattern provide independent, differently derived estimates.
Those methods do not currently yield the same value. Understanding what each one measures—and which assumptions and uncertainties enter along the way—is essential to understanding the Hubble tension.
What astronomers mean by the universe’s expansion rate
The Hubble constant, written H0, describes the present-day expansion rate of the universe. It is conventionally expressed in kilometres per second per megaparsec (km/s/Mpc). A megaparsec (Mpc) is a unit of astronomical distance; the “per megaparsec” in the rate means that, in the simple nearby-universe relation, the expected recession speed increases with distance. H0 is not a speed assigned to every galaxy, and it is not a direct measurement of the universe changing during a brief telescope observation.
Instead, astronomers assemble observations that connect distance and recession. For a nearby galaxy, they need an independently calibrated distance and a measurement of how strongly its light is redshifted. Repeating this for many galaxies lets researchers estimate the relationship between distance and recession, then infer the present expansion rate. The local distance-ladder approach is described in NASA’s three steps to the Hubble constant.
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The distinction matters: redshift is an observation of light, not a precise distance reading by itself. The distance ladder supplies distance information through calibrated astronomical objects; spectral measurements supply the redshift information. The estimate comes from putting the two together.
How the local distance ladder works
The ladder is a sequence of overlapping calibrations. Each rung uses objects whose distances can be established at one scale to calibrate a brighter indicator that can be observed farther away. The key is not that every rung measures distance in the same way, but that the calibration transfers from one rung to the next.
1. Use parallax to anchor nearby distances
Parallax is the apparent change in a star’s position against more distant background objects when the observer views it from different positions. Astronomers can use the known change in viewing position and the observed angular shift to determine the star’s distance geometrically. Because parallax provides a geometric anchor for nearby stars, it helps set the scale used to calibrate more distant indicators.
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Those anchors are not the final answer to the expansion question. They establish distances for nearby calibrators, which can then be used to determine the intrinsic brightness of other kinds of stars. An error at this early stage can propagate to later rungs, which is why calibration is a central source of uncertainty.
2. Use Cepheid variable stars to reach other galaxies
Cepheids are variable stars whose brightness rises and falls in a regular pulsation. Their pulsation period is related to their intrinsic brightness: once the relation is calibrated, the period indicates how bright a Cepheid is in its own right. Astronomers compare that intrinsic brightness with how bright it appears from Earth to estimate its distance.
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When a galaxy contains observable Cepheids, their calibrated distances establish how far away that galaxy is. Cepheids therefore bridge the geometric measurements of nearby stars and the more distant galaxies that host supernovae. NASA’s overview of the distance-ladder steps describes this progression.
3. Calibrate Type Ia supernovae with Cepheid-host galaxies
Some galaxies contain both Cepheids and Type Ia supernovae. The Cepheids provide a distance to the host galaxy; that distance lets astronomers calibrate the supernova’s intrinsic brightness. Type Ia supernovae are bright enough to be detected much farther away than Cepheids, so calibrated supernovae extend the measurement to more distant galaxies.
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For a more distant supernova, astronomers compare its calibrated intrinsic brightness with its observed apparent brightness to estimate its distance. Dust can dim supernova light and make an event appear farther away if the dimming is not accounted for. NASA describes using observations in ultraviolet and infrared wavelengths, where dust affects light differently, to estimate and correct for this effect in its account of Hubble’s supernova-distance measurement.
4. Pair supernova distances with redshifts
Once distances have been established for distant galaxies, astronomers compare those distances with the redshifts measured from their light. A spectrum contains features such as absorption lines whose normal wavelengths are known. If a feature is observed at a longer wavelength than its reference wavelength, its shift provides a measure of redshift. NASA explains that light travelling through expanding space is stretched to longer wavelengths in its cosmological-redshift overview.
Redshift alone is not treated as a precise galaxy-distance measurement in the local ladder. The independently calibrated distance is paired with the spectral shift, and the resulting distance–redshift relationship is used to estimate H0. This is why the calibration steps and the redshift measurement are complementary rather than interchangeable.
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How early-universe and standard-ruler methods differ
The distance ladder measures the nearby, comparatively recent universe using individual stars, supernovae, distances, and redshifts. Other methods use large-scale patterns in the universe or radiation from an earlier epoch. They can constrain H0, but they do not simply repeat the same local measurement with a different telescope.
| Method | Epoch and observable | How it constrains expansion | Model dependence and key qualifications |
|---|---|---|---|
| Local distance ladder | Nearby and distant galaxies; geometric distances, Cepheids, Type Ia supernovae, and galaxy redshifts | Calibrates distances step by step, then compares distant-galaxy distances with their redshifts. | Depends on calibration through the ladder. Important systematic concerns include the geometric anchors, dust corrections, and crowding or blending of Cepheid light. |
| Cosmic microwave background (CMB) | Early-universe relic radiation; its observed pattern | Combines CMB observations with a cosmological model that connects early-universe conditions to the present expansion rate. | H0 is inferred through that model; it is not obtained by the local distance-and-redshift procedure. |
| Baryon acoustic oscillations (BAO) | Large-scale clustering pattern; a characteristic standard-ruler scale | Uses the BAO scale as a ruler to constrain cosmic distances and expansion history. | Published constraints depend on stated assumptions, including homogeneity and isotropy, a metric theory of gravity, a smooth expansion history, standard candles, and a standard BAO ruler. |
Cosmic microwave background: an inference from the early universe
The CMB is relic radiation from the early universe. Researchers analyze its observed properties and infer the present expansion rate using a cosmological model that describes how the universe evolved from that early state to today. This makes the CMB route powerful but distinct from a direct local ladder: its estimate of H0 depends on the model used to connect the early observations to the present.
Consequently, a CMB-derived value and a local distance-ladder value should be described with their methods attached. A comparison that omits the model dependence of the CMB result can make the two estimates sound like identical observations when they are not.
BAO: using a standard ruler
Baryon acoustic oscillations are a pattern in the distribution of matter that can be used as a standard ruler. A low-redshift analysis by Heavens, Jimenez, and Verde combined BAO and Type Ia supernova results. Under its stated assumptions, the 2014 paper reported a measured ruler length of 103.9 ± 2.3 h−1 Mpc. Separately, it gave a Planck-parameterized ΛCDM theoretical prediction of 99.3 ± 2.1 h−1 Mpc. These are results from that particular paper, not universal BAO constants. The analysis and its qualifications are in “Standard rulers, candles, and clocks from the low-redshift Universe”.
The symbol h in h−1 Mpc is part of the reported unit convention; these ruler lengths should not be mistaken for H0 values in km/s/Mpc. BAO is useful here as an independent standard-ruler approach, but the assumptions used in an analysis must travel with the result.
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Why astronomers call it the Hubble tension
Published estimates from local distance-ladder measurements and values inferred from early-universe observations have not agreed. This disagreement is known as the Hubble tension. It is not a dispute over whether the universe expands; it concerns the expansion rate inferred using different methods and their associated calibration or model assumptions.
NASA’s Hubble Constant and Tension explainer summarizes space-telescope results as around 70–76 km/s/Mpc and CMB-derived results as around 67–68 km/s/Mpc (NASA, accessed 2026). Those are ranges summarized on that NASA page, not a single newly combined measurement. See NASA’s explanation of the Hubble constant and tension.
A separate, more specific comparison appeared in a NASA release in 2022: 73 ± 1 km/s/Mpc for SH0ES versus 67.5 ± 0.5 km/s/Mpc for Planck combined with the standard cosmological model. The methods, uncertainty figures, and publication context belong with those numbers; they should not be merged with the broader ranges above into a new “consensus” value. NASA’s Hubble milestone release reports that comparison, and the SH0ES collaboration’s 2021 paper is available at arXiv.
The evidence cited here does not resolve the tension. The gap could reflect unrecognized systematic effects, limitations in how methods are applied, or a need to revise the model connecting early-universe observations to the present. The quoted values establish that methods have yielded different results; by themselves, they do not identify the cause.
Where uncertainty enters the measurements
A reported expansion rate is the end of a chain of observations and interpretations. Its uncertainty is not one generic margin: different methods have different vulnerabilities, and a responsible comparison keeps those vulnerabilities visible.
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Calibration propagates from one rung to the next
In the local ladder, nearby geometric anchors calibrate Cepheids, Cepheids calibrate supernovae in shared host galaxies, and calibrated supernovae extend the distance scale. Uncertainty in an early calibration can carry forward to the final distance–redshift relation. A check on one rung therefore tests a specific link in the chain; it does not automatically validate every step or settle the difference with an early-universe inference.
Dust changes apparent brightness
Dust between a supernova and the observer dims its light. If the dimming is mistaken for greater distance, the inferred distance is biased. Observations at ultraviolet and infrared wavelengths help estimate a correction because dust affects those wavelengths differently. That correction is one part of the supernova measurement, not a replacement for the Cepheid calibration or the redshift observation.
Crowding can alter Cepheid brightness measurements
In a distant galaxy, light from nearby stars can blend with a Cepheid’s light. The blended source may look brighter than the Cepheid alone, affecting the inferred distance. This is a concern when resolving Cepheids in distant supernova-host galaxies and is one reason a sharper view can help test the ladder’s reliability.
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A CMB result for H0 is inferred from early-universe data through a cosmological model. BAO analyses also state assumptions about the universe and the standard ruler. If those assumptions are changed, the inference can change; the numerical result should therefore be read as a constraint within the named analysis and framework, not as an assumption-free direct reading.
What Webb’s Cepheid cross-check established
In a 2024 account, the European Space Agency reported that Webb observations of Cepheids in five Type Ia supernova host galaxies—including a sample of about 1,000 Cepheids—supported the reliability of Hubble’s distance-ladder measurements over the range tested. Webb’s sharper infrared view offered a check on whether crowded fields and blended starlight had biased Hubble’s Cepheid measurements. The account is ESA’s March 11, 2024 report on Webb and Hubble.
This result strengthens a cross-check of the Cepheid-based distance ladder; it does not settle why that ladder’s inferred present rate differs from the CMB-based result. ESA also noted that the evolution of expansion between the early-universe and present-day endpoints remained to be established. Webb’s observations therefore address a possible measurement concern in the local route, not every assumption or question involved in comparing the two methods.
How to read a reported H0 value
When a paper or news report gives an expansion rate, the number is most useful when read alongside its method and provenance. A quick check of the following details helps prevent unlike results from being treated as interchangeable:
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- Units: Is the value reported in km/s/Mpc, or is it a different quantity such as a ruler length in h−1 Mpc?
- Uncertainty: Is an uncertainty stated, and does it belong to this specific result?
- Framework: Does the estimate depend on a cosmological model or stated assumptions?
- Source and date: Is the number a result from a named paper, a dated release, or a broad summary range? Keep that context attached when quoting it.
These checks do not decide which method is right. They make clear what was observed, what was inferred, and what would need to be understood before the disagreement could be explained.
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