Early Dark Energy
A pre-recombination scalar field that mimics a brief burst of dark energy near matter-radiation equality, shrinks the sound horizon, and raises the CMB-inferred H0 toward the local value.
Placeholder for a 3D visualisation of Hubble Tension Solutions. The interactive scene will land in Phase 3. The Hubble tension is the disagreement between the local-distance-ladder measurement of the present-day expansion rate (SH0ES collaboration, around 73 km/s/Mpc) and the value inferred from the cosmic microwave background under ΛCDM (Planck, around 67 km/s/Mpc). The discrepancy has grown to roughly 5 sigma by 2024-2025 and is one of the most-studied current problems in cosmology. Solutions divide into pre-recombination physics (modify the sound horizon at decoupling) and late-time physics (modify the local expansion rate). The Early Dark Energy variant is the most-developed pre-recombination proposal and serves as the family's primary variant. Late-time proposals (interacting dark energy, decaying dark matter) await their own variants in future expansions.
§1 · The claim, in one sentence
Early Dark Energy (EDE) is a pre-recombination scalar-field component that mimics a brief burst of dark energy near matter-radiation equality, then dilutes away faster than radiation. The effect on cosmology is to shrink the sound horizon at recombination, which raises the CMB-inferred Hubble constant H0 toward the local-distance-ladder value. Karwal and Kamionkowski 2016 proposed the framework in connection with the string axiverse; Poulin, Smith, Karwal, and Kamionkowski 2019 showed the construction can resolve the Hubble tension while remaining consistent with the cosmic microwave background spectrum.
§2 · Why it might be true
The Hubble tension is the disagreement between H0 from the local-distance-ladder measurement (SH0ES collaboration, around 73 km/s/Mpc) and the H0 inferred from the cosmic microwave background under ΛCDM (Planck, around 67 km/s/Mpc). The discrepancy reached 5 sigma by 2024-2025 and has resisted easy explanation by systematics on either side. Some new physics modifying the expansion history is needed if the tension is real.
Early Dark Energy posits that a scalar field with a specific potential becomes dynamically important just before matter-radiation equality, behaves like dark energy briefly (with an equation of state w near -1), then transitions to a faster-than-radiation dilution. The energetic contribution at the moment near recombination shrinks the comoving sound horizon r_s by a few percent. Because H0 in the CMB-based inference is anti-correlated with r_s, a smaller sound horizon yields a larger inferred H0, moving the Planck value upward toward the local measurement.
Poulin-Smith-Karwal-Kamionkowski 2019 (arXiv:1811.04083, 1,091 INSPIRE citations) made the framework quantitative. They showed that EDE with an energy fraction f_EDE of around 10% near matter-radiation equality, decaying with appropriate redshift dependence, brings H0 inference into agreement with SH0ES while remaining consistent with the CMB spectrum. The proposal generated substantial follow-up activity. The DESI 2024 BAO data and JWST 2024 distance measurements have constrained the parameter space but the framework remains viable in restricted regions.
The family stance
The Hubble tension is real and physical, not a calibration error. The standard ΛCDM cosmology is incomplete; some new physics, either pre-recombination or late-time, is needed to reconcile the local and CMB measurements. Different variants disagree on which class of new physics is required.
§2.5 · Evidence
- The 2019 Poulin-Smith-Karwal-Kamionkowski paper demonstrated quantitatively that EDE can resolve the Hubble tension within the parameter space allowed by the CMB power spectrum
- Several independent groups (Kamionkowski's, Hill's, Smith's) have confirmed the basic finding using different parameterizations and analysis pipelines
- The framework connects to motivated UV physics: the string axiverse (Karwal-Kamionkowski 2016) provides a natural source for the EDE scalar field
- Current parameter constraints from DESI 2024 BAO data still allow EDE with restricted parameter ranges, indicating the framework has not been ruled out by precision data
§3 · What you'd need to test it
- A scalar-field energy density of order 10% of the total at redshifts around recombination (z ~ 3000) shrinks the sound horizon by a few percent
- CMB-inferred H0 rises from the Planck value (~67) toward the SH0ES local value (~73) under appropriate EDE parameters
- The σ8 (sigma-8) value increases under EDE, worsening the secondary σ8 tension with weak-lensing observations; this is a known cost of the framework
- Specific signatures in the CMB acoustic peaks (small shifts in the locations and amplitudes of secondary peaks) and in the matter power spectrum are predictions that DESI, Euclid, and CMB-S4 can constrain
§4 · Where it breaks
- EDE worsens the σ8 tension with weak-lensing data; resolving one ΛCDM tension at the cost of another is editorially uncomfortable
- The framework requires tuning the scalar field's potential to produce a brief burst at precisely the right cosmological epoch (around recombination); this is a fine-tuning concern
- DESI 2024 BAO data and JWST 2024 distance measurements have pushed the viable EDE parameter space into corners; some authors argue the framework is effectively constrained out
- Alternative explanations of the Hubble tension (systematics in the local-distance ladder, late-time interacting dark energy, decaying dark matter) remain live; EDE is one option among several
Go deeper
The sound horizon at last scattering r_s is the comoving distance a sound wave can travel before recombination. It is one of the most precisely measured quantities in cosmology and serves as a standard ruler for BAO. A few-percent change in r_s translates to a similar change in inferred H0; this is why EDE works in principle. The detailed implementation requires the EDE energy to inject at the right epoch with the right magnitude and decay rate.
Karwal and Kamionkowski 2016 connected EDE to the string axiverse, the spectrum of light scalar fields that arise generically in string-theory compactifications. The axiverse provides many candidate scalars with the right potential structure; EDE could be one such field becoming dynamically important at the relevant epoch. This is the motivated-UV-physics angle the framework leans on.
Cross-references: the existing Standard Cosmological Model family in Ch.5 contains the Plain ΛCDM, wCDM, and w0waCDM variants that EDE modifies. The Modified Gravity (MOND) family contains alternative explanations of dark sector phenomena. The Dark Energy Skeptics family contains the Sarkar anisotropic universe variant that questions the dark-energy interpretation itself. The Hubble tension can in principle be resolved by changes in any of these directions; EDE represents the pre-recombination-physics path. Future variants in this family will cover late-time-physics alternatives.
Variants in this family
▸§5 · Who built it, and when(2 sources, 2 established)
- EstablishedKarwal, T. & Kamionkowski, M. (2016). 'Dark energy at early times, the Hubble parameter, and the string axiverse.' Phys. Rev. D 94, 103523
- EstablishedPoulin, V., Smith, T. L., Karwal, T. & Kamionkowski, M. (2019). 'Early Dark Energy Can Resolve The Hubble Tension.' Phys. Rev. Lett. 122, 221301
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