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Repair Windows errors before they cause bigger problemsFix Now →Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →Astronomers estimate how fast the universe is expanding today by comparing galaxies’ distances with the redshift of their light. The local distance ladder builds those distances from nearby geometric measurements to Cepheid stars and then to Type Ia supernovae. Other methods use the cosmic microwave background or the pattern of baryon acoustic oscillations. Because these approaches rely on different observations—and, in some cases, different assumptions—they do not all yield the same value. That disagreement is known as the Hubble tension.
What the Hubble constant measures
The present expansion rate is called the Hubble constant, written H0 and conventionally expressed in kilometres per second per megaparsec (km/s/Mpc). It describes how the expansion rate relates to distance in the nearby universe: in the simplest low-redshift picture, a galaxy twice as far away has about twice the recession rate attributable to cosmic expansion. The unit is a rate of change of recession speed with distance, not a speed assigned to the universe as a whole.
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A megaparsec is a unit of astronomical distance. So a value in km/s/Mpc tells us how much the inferred expansion-related recession rate changes for each megaparsec of distance. Astronomers do not take two pictures of the entire universe a short time apart and watch it grow. Instead, they infer the present rate from observations of light, calibrated distances, and—depending on the method—cosmological models.
How the local distance ladder works
The local or late-universe distance ladder links objects whose distances can be measured nearby to objects visible much farther away. Each rung calibrates the next: the more distant indicators are useful because earlier measurements establish how bright they really are. NASA outlines the three-rung process in its overview of the Hubble constant distance ladder.
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Anchor the first rung with parallax
Parallax is the apparent shift in a nearby star’s position when observed from different viewpoints as Earth moves around the Sun. The geometry of that shift gives a distance without first needing to know the star’s intrinsic brightness. Those nearby stars act as geometric anchors for calibrating later indicators.
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Use Cepheids to reach other galaxies
Cepheid variable stars brighten and dim in a regular cycle. Their pulsation period is related to their intrinsic brightness. Once that relationship has been calibrated using stars at known distances, astronomers can compare a Cepheid’s intrinsic brightness with how bright it appears to estimate its distance. Cepheids in other galaxies extend the measurement well beyond the nearby stars used for parallax.
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Calibrate Type Ia supernovae
Some galaxies contain both Cepheids and Type Ia supernovae. Cepheid-based distances to those galaxies let astronomers calibrate the supernova brightness scale. Type Ia supernovae are bright enough to be detected much farther away than Cepheids, so they carry the distance ladder outward.
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For a calibrated supernova, astronomers compare its expected intrinsic brightness with its apparent brightness. A dimmer appearance can indicate greater distance, but dust can also dim the light, so its effects must be accounted for. NASA describes using ultraviolet and infrared observations—which dust affects differently—to estimate that correction in its July 12, 2024 explanation of measuring a supernova’s distance.
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Pair distant supernova distances with redshifts
With distances estimated from calibrated supernova brightness, astronomers compare those distances with the redshifts measured from the same galaxies’ spectra. The pattern of distance versus redshift provides an estimate of the present expansion rate. The distance is not read directly from redshift alone: the ladder supplies an independently calibrated distance, while redshift supplies information about how the light has been stretched during its journey.
How redshift is measured—and what it does not tell you alone
Astronomers identify spectral features, such as absorption lines, whose normal wavelengths are known from laboratory measurements. They then measure how far those features have shifted toward longer wavelengths in a galaxy’s observed spectrum. The shift is expressed as redshift. NASA explains that light travelling through expanding space is stretched to longer wavelengths in its account of cosmological redshift.
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Redshift is an observable, but by itself it is not a precise distance measurement. To estimate H0 with the local ladder, researchers need both a redshift and a distance determined through the calibrated indicators. The combination matters: a spectrum provides the shift, while the distance ladder establishes how far away the source is. Confusing the two can make it sound as if measuring a line’s displacement immediately reveals a galaxy’s exact distance; it does not.
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How the methods differ
The local distance ladder is one route to H0, not the only one. Early-universe and standard-ruler approaches use different observables and different chains of inference. Their results should be compared with those distinctions intact rather than treated as interchangeable measurements made under identical assumptions.
| Approach | Primary observable | How it constrains expansion | Main qualification |
|---|---|---|---|
| Local distance ladder | Parallax-calibrated Cepheids, Type Ia supernova brightnesses, and galaxy redshifts | Builds distances outward, then compares distance with redshift to estimate the nearby, present expansion rate. | Calibration uncertainties propagate from rung to rung; dust and crowding can affect brightness measurements. |
| Cosmic microwave background (CMB) | The pattern in the relic radiation from the early universe | Uses early-universe observations to infer today’s expansion rate through a cosmological model. | It is a model-based inference from the early universe, not the same direct local distance-and-redshift procedure. |
| Baryon acoustic oscillations (BAO) | A characteristic standard-ruler pattern in the distribution of matter | Uses the ruler’s apparent scale in observations to constrain cosmic distances and expansion history. | Reported results depend on the assumptions and ruler calibration used in the particular analysis. |
What BAO contributes
Baryon acoustic oscillations provide a standard-ruler approach rather than the parallax-to-Cepheid ladder. A low-redshift analysis by Heavens, Jimenez, and Verde, published in 2014, reported a measured ruler length of 103.9 ± 2.3 h−1 Mpc. The same paper separately gave a Planck-parameterized ΛCDM theoretical prediction of 99.3 ± 2.1 h−1 Mpc. These are values from that specific paper, not universal BAO constants or direct H0 measurements. Its analysis stated assumptions including homogeneity and isotropy, a metric theory of gravity, a smooth expansion history, standard candles, and a standard BAO ruler. The paper is available as “Standard rulers, candles, and clocks from the low-redshift Universe”.
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Why published Hubble-constant values disagree
The Hubble tension is the persistent disagreement between values inferred from local distance-ladder measurements and values inferred from early-universe observations. A local analysis asks how distances and redshifts relate in the nearby universe; a CMB analysis starts with early-universe data and uses a cosmological model to infer the present rate. Both are routes to H0, but they do not use the same observational procedure.
NASA’s explainer summarizes space-telescope results as around 70–76 km/s/Mpc and CMB-derived results as around 67–68 km/s/Mpc; those are broad ranges reported in that page context, not a new combined estimate. NASA’s separate 2022 release gives a more specific comparison: 73 ± 1 km/s/Mpc for SH0ES, versus 67.5 ± 0.5 km/s/Mpc for Planck interpreted with the standard cosmological model. Those values, including their methods and date, belong to that comparison and should not be merged with the broader ranges as though all came from a single analysis. NASA discusses the disagreement in its Hubble constant and tension explainer and the specific comparison in its Hubble milestone release. The SH0ES collaboration’s 2021 local measurement is described in its published paper.
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What can shift a distance-ladder result?
Because each rung depends on earlier calibration, a small bias near the bottom can propagate to the final estimate. The most important checks concern how well the geometric anchors, stellar brightnesses, and supernova observations are understood.
- Calibration through the ladder: Any uncertainty in geometric anchors or in the Cepheid and supernova calibrations carries forward. A change in an earlier rung can alter the distance scale used at later rungs.
- Dust: Intervening dust dims light. If that dimming is mistaken for the effect of greater distance, the inferred supernova distance can be wrong. Ultraviolet and infrared measurements help estimate the correction because dust affects those wavelengths differently.
- Crowding and blending: In a distant host galaxy, light from nearby stars can blend with a Cepheid’s light, making its apparent brightness harder to measure cleanly. That matters because the Cepheid brightness helps set the host galaxy’s distance.
- Model assumptions: The CMB route derives H0 through a cosmological model connecting the early universe to the present. BAO constraints likewise rely on specified assumptions and a standard ruler; the assumptions are part of the result, not incidental details.
What Webb’s Cepheid cross-check established
The James Webb Space Telescope’s sharper infrared view offers a way to test whether crowding and blending could have biased Hubble Space Telescope Cepheid measurements. The European Space Agency reported on March 11, 2024, 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. ESA’s account is “Webb & Hubble confirm Universe’s expansion rate”.
That result is a cross-check of an important part of the local method, not a resolution of the Hubble tension. It does not explain why the local and early-universe inferences differ, nor establish how the expansion evolved between the early-universe and present-day endpoints. ESA said that evolution remained to be established. The remaining scientific question is not merely whether one telescope can repeat another’s Cepheid measurements, but why independently constructed routes to the present expansion rate continue to disagree.
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How to interpret an H0 result
When reading a reported value, look at the measurement route before comparing the number. Ask whether it comes from nearby distance indicators or from early-universe observations interpreted through a model; whether a BAO result depends on a stated standard ruler; and what calibration or observational systematics were tested. A number without its method, uncertainty, source, and date is too incomplete to show what the measurement establishes.
The core idea remains straightforward: measure a distance, measure the redshift of the light from its galaxy, and determine how the two relate. The hard part is building a trustworthy distance scale and deciding how results from different epochs and assumptions fit together. Those calibration tests and independent methods are why H0 remains an active measurement problem rather than a value obtained from one telescope image.
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