# Gravitational Waves Provide a New Value for the Hubble Constant
The Hubble constant determines the rate of the Universe's expansion and is estimated at 67 km/s/Mpc from cosmic microwave background data and 73 km/s/Mpc from supernova observations. This discrepancy, known as the Hubble tension, has persisted for about a decade. A new study uses gravitational waves from black hole mergers for an independent estimate: 69.9 km/s/Mpc, with uncertainty decreasing as more data accumulates.
Gravitational waves arise from mergers of compact objects and are detected by the LIGO, Virgo, and KAGRA detectors. The signals contain direct information about the distance to the source—they are called "standard sirens" by analogy with standard candles in electromagnetic astronomy.
Methodology for Analyzing "Dark Sirens"
Most gravitational wave events lack an electromagnetic counterpart, complicating host galaxy identification. Researchers applied a statistical approach:
- Collected 17 events with good localization.
- Integrated data from deep galaxy surveys.
- Used machine learning to calculate the probabilistic distance distribution.
Result: H_0 = 69.9 ± 3.7 km/s/Mpc (68% confidence interval). The value sits in an intermediate position between CMB and SN estimates, boosting confidence in the method.
Detectors capture the merger phase and ringdown modulation, which encode luminosity and distance. For "dark sirens," they compensate for the lack of optical counterparts using galaxy catalogs and Bayesian inference.
Physical Context of the Universe's Expansion
The Hubble constant has evolved: in the early Universe, matter dominated, slowing expansion; about 5 billion years ago, dark energy triggered acceleration. Measurement methods probe different epochs:
- CMB (Planck): early Universe, extrapolation via ΛCDM model.
- Type Ia supernovae: local Universe (z < 0.1).
- Gravitational waves: intermediate redshifts (z ~ 0.01–0.2).
The discrepancy points to possible systematic errors or deviations from ΛCDM: evolving dark energy, new particles in the early Universe, or modified gravity.
Key Points
- Independent channel: Gravitational waves use the fabric of spacetime, minimizing bias from dust or stellar evolution.
- Intermediate value: 69.9 km/s/Mpc agrees with both methods within errors.
- Reducing uncertainty: Future detectors (LIGO A+, Einstein Telescope) will deliver H_0 with <1% precision.
- Implications for ΛCDM: Disagreement could signal physics beyond the Standard Model.
- Statistical power: Machine learning scales analysis to thousands of events.
Prospects for Future Measurements
Since 2015, over 90 mergers have been detected. Thousands are expected in the 2030s. Electromagnetic follow-up (e.g., for neutron stars) calibrates sirens. Multimessenger astronomy combines GW with EM for precise cosmology.
The potential to resolve the tension is growing: if GW confirms the discrepancy, a rethink of cosmological parameters will be needed. The current value underscores the value of diversified methods.
— Editorial Team
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