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Cosmologists summarize the universe’s present expansion with the Hubble constant, H0. They estimate it by comparing galaxy distances with recession rates inferred from redshifts, while correcting for galaxies’ local motions. The leading local method is a calibrated distance ladder—geometric distances, Cepheid variables and Type Ia supernovae. A separate route measures the cosmic microwave background (CMB) and uses a cosmological model to predict today’s value. Baryon acoustic oscillations, gravitational-wave standard sirens, strong-lens time delays and galaxy clusters provide additional checks.
What the Hubble constant measures
H0 is the present-day relationship between distance and expansion-driven recession in a sufficiently nearby, smooth part of the universe. It is reported in kilometres per second per megaparsec (km/s/Mpc). A value of 70 km/s/Mpc means that, after accounting for local motions, the expansion contributes roughly 70 km/s of recession for each additional megaparsec of distance.
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A megaparsec is about 3.26 million light-years. The relationship is statistical: cosmologists fit many galaxies and supernova host systems rather than treating one galaxy as a perfect example. Nearby galaxies can move toward or away from us because of gravity from local groups, clusters and superclusters. Those “peculiar velocities” must be separated from the recession caused by cosmic expansion.
H0 is therefore an inferred parameter, not a speed at which the universe expands into an exterior space. In general relativity, space-time evolves; there is no required outside volume into which the universe is moving.
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Why distances and redshifts are the essential pair
The redshift of a galaxy’s spectrum shows how its light has been stretched. At modest distances, the expansion-related part of redshift increases with distance. To turn that pattern into H0, astronomers need two measurements:
- Distance: how far away the galaxy or its distance indicator is.
- Recession rate: inferred from spectral redshift after correcting, where possible, for local gravitational motions.
Distance is the difficult measurement. A galaxy’s apparent brightness depends both on its intrinsic luminosity and its distance, so cosmologists use objects whose intrinsic properties can be calibrated. Different distance indicators work over different ranges, and the calibration of one becomes the foundation for the next.
The local distance ladder
The distance ladder links methods in successive rungs. Each rung reaches farther than the previous one, but it also carries forward uncertainties in calibration, dust treatment, stellar populations and photometry.
1. Geometric anchors and parallax
Parallax is the geometric starting point. As Earth orbits the Sun, a nearby star appears to shift slightly against much more distant stars. The angle of that shift gives distance through geometry, without assuming the star’s intrinsic brightness. Other geometric techniques, including measurements of certain maser systems, can also anchor nearby galaxies, but the ladder’s basic principle is the same: establish distances independently of a brightness model.
2. Cepheid variables
Cepheids are pulsating stars whose periods are tied to their intrinsic luminosities. Once a sample of Cepheids is calibrated with geometric distances, the measured pulsation period predicts how bright each star truly is. Comparing that predicted brightness with the observed brightness gives the distance to its host galaxy.
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Dust can make a Cepheid look dimmer, and crowding can cause light from neighboring stars to be blended into the measurement. Metallicity, detector calibration and the details of the Cepheid population also enter the error budget. Infrared observations reduce some dust effects and can make crowded stellar fields easier to resolve.
3. Type Ia supernovae
Astronomers then observe galaxies that contain both calibrated Cepheids and Type Ia supernovae. Type Ia events are bright enough to be detected in much more distant galaxies. Their observed peak brightness, after corrections for light-curve shape, color and extinction, provides a standardized luminosity scale. Cepheids in the same host galaxies set that scale: they calibrate the absolute brightness that a Type Ia supernova should have.
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With Type Ia supernovae calibrated, cosmologists measure them across a larger volume. For each host galaxy they combine the supernova distance with a spectroscopic redshift. A fit to many objects yields the local Hubble constant while modeling peculiar velocities and other correlations.
The procedure can be summarized as follows:
- Measure geometric distances to nearby stars or other anchors.
- Use those anchors to calibrate Cepheid luminosities.
- Observe Cepheids in galaxies that later host Type Ia supernovae.
- Use the Cepheids to calibrate the supernova luminosity scale.
- Measure calibrated Type Ia supernovae in more distant galaxies.
- Fit their distances against redshifts, correcting for local motions, to infer H0.
Uncertainty propagates through every step. Important sources include interstellar dust, crowding and blending, photometric zero points, the diversity of supernova populations, host-galaxy properties and peculiar velocities. A change in an early calibration can shift the final expansion-rate estimate even when the distant supernova observations themselves are excellent.
What the CMB route does differently
The cosmic microwave background is relic radiation released when the early universe became transparent. Its temperature and polarization maps contain acoustic peaks produced by interactions among matter, radiation and gravity before that release. The positions and heights of those peaks constrain quantities such as the matter content, baryon density, spatial geometry and the rate at which early perturbations evolved.
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The CMB does not directly measure the expansion rate today. Cosmologists fit a framework such as the standard six-parameter ΛCDM model to the early-universe data, then evolve that model forward to infer what H0 should be at the present epoch. The result is therefore conditional on the model and its assumptions. If the model is incomplete, a very precise CMB fit can still predict the wrong present-day value.
This distinction explains why a CMB-derived number and a distance-ladder number are not simply two instruments measuring the same local quantity. One is a model-based extrapolation from the early universe; the other is a relatively direct nearby distance-and-redshift measurement.
BAO: a large-scale ruler, not a stand-alone H0 meter
Baryon acoustic oscillations (BAO) are a preferred separation scale in the distribution of galaxies. They originated as sound waves in the hot early universe and were frozen into the later matter distribution. Large galaxy surveys measure that feature as a standard ruler.
BAO observations constrain combinations of distance and expansion history at different redshifts. To convert the ruler into an absolute scale, analyses need calibration from information such as the CMB, the baryon abundance or other large-scale-structure measurements. NASA’s LAMBDA methods overview cautions that BAO data alone cannot determine H0. Calling a BAO result “independent” requires stating which calibration and cosmological assumptions were supplied.
Other ways to estimate the expansion rate
| Method | Primary observable | Key dependencies and systematics | How it complements the ladder |
|---|---|---|---|
| Gravitational-wave standard sirens | The waveform of a compact-object merger gives a luminosity distance. | A redshift is needed from an identified electromagnetic counterpart and host, or statistically from possible host galaxies. Host-catalog completeness, peculiar velocities and detection-selection effects affect the inference. | It does not use Cepheid or Type Ia calibration. |
| Strong-lens time delays | Differences in arrival times between multiple images of a variable quasar or other source. | The lens redshift, distance relations and a model of the lens mass distribution are required. Mass modeling and assumed cosmology influence H0. | It probes a different combination of lensing geometry and time-delay information. |
| Galaxy-cluster X-ray and thermal Sunyaev–Zel’dovich observations | Joint gas measurements can yield a cluster’s angular-diameter distance. | Gas-density and temperature profiles, instrument calibration, redshift and cosmological assumptions contribute uncertainty. | It supplies a cluster-based route distinct from stellar candles. |
Gravitational-wave standard sirens in practice
A merger’s gravitational waveform carries an absolute distance scale because the signal’s amplitude and frequency evolution are governed by general relativity. The main challenge is assigning a redshift. A bright electromagnetic counterpart can identify the host galaxy directly. If no counterpart is seen, analysts combine the gravitational-wave localization region with a catalog of candidate galaxies and assign probabilities to their redshifts. This statistical approach can work with many events, but incomplete catalogs and local velocities remain important limitations.
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Strong-lens time delays in practice
When a variable background source is gravitationally lensed, its light follows paths of different lengths and gravitational potentials. The same intrinsic fluctuation therefore arrives at different times in the multiple images. Measuring those delays and modeling the foreground lens constrains a distance combination that is sensitive to H0. The dominant difficulty is reconstructing the lens mass, including effects from nearby structure and the mass-sheet degeneracy.
Clusters as distance indicators
X-ray emission measures hot intracluster gas, while the thermal Sunyaev–Zel’dovich effect measures how CMB photons are altered by that gas. Combining the two can estimate an angular-diameter distance. The method depends on assumptions about gas geometry and clumping, so calibration and astrophysical modeling are central to the result.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to compare apparently different H0 measurements
A useful comparison asks four questions rather than ranking methods by a single quoted error bar:
- Is the route local or early-universe based? Distance ladders and many lens or cluster analyses use relatively late-time observations. CMB inference starts from early-universe conditions and extrapolates through a model.
- What sets the distance scale? It may be geometric parallax, a stellar candle, a gravitational-wave waveform, a lens model or a calibrated acoustic ruler.
- Which assumptions dominate? Dust, crowding, stellar populations, supernova standardization, lens mass structure, gas physics and galaxy catalogs affect different methods.
- How much information is available? Some methods currently rely on smaller samples or rare events, while supernova and CMB programs can contain very large data sets. Statistical precision and systematic control are separate issues.
Methods are complementary, but they are not automatically independent. Two analyses can share a photometric calibration, a redshift catalog, a cosmological prior or a common astrophysical model. Independence must be demonstrated by tracing the data and assumptions, not inferred from the method’s name.
What current measurements show
NASA reported on April 13, 2026, on an international Local Distance Network analysis published April 10 in Astronomy & Astrophysics. Combining several distance indicators, that framework found a nearby-universe expansion rate of about 73.5 km/s/Mpc. NASA’s summary said other estimates discussed in the report typically ranged from 73 to 76 km/s/Mpc. The news summary did not provide the full paper’s uncertainty interval or a method-by-method numerical breakdown, so those details cannot be inferred from the headline value.
For broader context, NASA’s Hubble overview describes space-telescope distance-ladder results as roughly 70–76 km/s/Mpc and CMB-derived results as roughly 67–68 km/s/Mpc. These are explanatory ranges, not a complete meta-analysis or a replacement for the individual papers’ uncertainty budgets.
The persistent difference between local measurements and values inferred from CMB observations under a standard cosmological model is called the Hubble tension. It is a disagreement between methods, not evidence that one instrument has simply “measured the universe wrong.”
Why the tension has not been settled
Several explanations remain possible. Unrecognized systematics could affect a distance-ladder rung, a supernova correction or a CMB analysis. The assumed ΛCDM model might not describe every relevant component of cosmic history. Or new physics could alter the expansion history between the early universe and today. Existing measurements do not establish which explanation is correct.
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NASA and the European Space Agency have used the James Webb Space Telescope to examine Cepheids in infrared light, where dust effects are smaller and crowded stars are easier to distinguish. ESA reports agreement between Webb’s infrared Cepheid observations and Hubble’s optical observations across the range used for the distance ladder. That cross-check reduces concern that a particular Cepheid crowding or measurement problem explains the entire discrepancy. It does not remove every uncertainty, test every rung, or identify the cause of the Hubble tension.
Likewise, additional standard sirens, lens systems, BAO measurements and cluster observations can narrow the space of explanations, but each brings its own calibration and modeling challenges. A future consensus will require agreement among methods with genuinely different data and assumptions, not merely a more precise version of one technique.
How to read an expansion-rate claim
When a paper or news report quotes H0, check the method, the date and the assumptions before comparing numbers. A local value from Cepheids and supernovae should not be read as if it were a direct CMB observation. A CMB value should not be described as a model-free measurement of the current universe. Also check whether a quoted range is a formal confidence interval, a broad explanatory span or a collection of results from different analyses.
The most durable picture is therefore methodological: cosmologists measure distances and redshifts nearby, infer the present rate from calibrated standard candles, infer it indirectly from early-universe maps under a model, and cross-check both routes with rulers, lenses, clusters and gravitational waves. The fact that these routes do not yet converge is itself one of modern cosmology’s central empirical problems.
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