Cosmology
6 min read
Why people disagree on the value of the Hubble constant
What is H_0 and why does it matter?
H_0 (“H nought”) is the present-day expansion rate of the Universe: how quickly galaxies recede from us per unit distance. If H_0 is larger, the Universe is expanding faster today, and the time taken to reach its current size tends to be shorter (though the full story depends on how expansion changed over time). H_0 is a cornerstone number in cosmology because it links observations of galaxies and the CMB to the overall history of the Universe.
The tension
When we measure H_0 using the nearby Universe, we get a higher value (roughly low 70s km/s/Mpc). When we infer H_0 from the early Universe using the cosmic microwave background (CMB) and the standard cosmological model, we get a lower value (roughly high 60s km/s/Mpc). The disagreement is several times larger than the quoted uncertainties, which is why it is called the “Hubble tension”.
Two ways to measure (or infer) H_0
1) The distance ladder (late-Universe, “local” H_0)
This route is conceptually straightforward: measure distances to galaxies, measure their redshifts (which tell you recession speed), and fit the slope of speed versus distance.
The problem is that distance is hard. So astronomers build a ladder with calibrated “rungs”:
Geometric anchors: direct distances, for example from parallax (nearby stars) or geometric maser measurements in galaxies. These set the absolute scale.
Standard candles in nearby galaxies: objects with known intrinsic brightness once calibrated. The classic example is Cepheid variable stars. Another popular method is the tip of the red giant branch (TRGB).
Type Ia supernovae: very bright events that can be seen far into the smooth “Hubble flow”. If you calibrate their true brightness using galaxies that also contain Cepheids or TRGB stars, supernovae become long-range distance markers.
Once supernova distances reach far enough that local gravitational motions average out, the expansion signal dominates and you extract H_0.
2) The CMB inference (early-Universe H_0)
The CMB is the afterglow from when the Universe became cool enough for electrons and protons to form neutral hydrogen and light could travel freely. The key point is that the CMB contains a built-in “standard ruler”: the typical distance that sound waves could travel in the hot early plasma before that moment.
The CMB measures this ruler as an angle on the sky. To turn that angular scale into distances and then into H_0, you must assume a model for what the Universe contains and how it expands. Using the standard model (often called Lambda-CDM), the best-fit H_0 comes out lower than the local measurements.
So the CMB does not measure H_0 directly; it measures early-Universe patterns extremely precisely, and H_0 is what you infer once you commit to a specific cosmic recipe.
What could be going on?
There are two broad options: (A) hidden measurement systematics, or (B) new physics.
A) Systematics: could one method be subtly biased?
Possible issues include:
Cepheid or TRGB calibration (dust, crowding, metallicity effects).
Supernova standardisation (selection effects, population differences).
Local motions and “cosmic variance” (whether our region of the Universe is slightly atypical).
These effects are actively tested by using multiple independent distance indicators and by cross-checking with alternative methods like strong gravitational lens time delays and gravitational-wave “standard sirens”.
B) New physics: could the early-Universe model be incomplete?
To bring the CMB-inferred H_0 up towards the local value, you typically need the early-Universe “standard ruler” to be smaller than in the standard model, or you need the CMB-to-distance mapping to change. Examples include:
Early dark energy (EDE): a temporary extra energy component before the CMB is released. It would make the Universe expand slightly faster at that time, reducing the sound-horizon ruler.
Extra relativistic species (“dark radiation”): additional light particles increase the early expansion rate, again tending to shrink the ruler.
Modified recombination: if the details of how and when the primordial plasma became neutral are different (for example due to exotic energy injection or new interactions), the inferred distances can shift.
Late-time changes (like evolving dark energy or modified gravity) can also move inferred distances, but they often run into tight consistency checks from galaxy clustering (BAO) and supernova distance–redshift data.
What to conclude
The Hubble tension is not just a squabble over one number. It is a stress test of two enormous pieces of physics: precision astrophysical calibration in the nearby Universe, and the early-Universe model that translates CMB patterns into today’s expansion rate. If the tension survives every cross-check, it is a strong hint that something in our cosmological picture is missing.