[Paper Review] Hints of Early Dark Energy in Planck, SPT, and ACT data: new physics or systematics?

Tristan L. Smith, Matteo Lucca+6 more|arXiv (Cornell University)|Feb 18, 2022

Cosmology and Gravitation Theories6 citations

TL;DR

This study investigates early dark energy (EDE) using Planck, ACT DR4, and SPT-3G CMB data, finding a 3.3σ preference for EDE over ΛCDM, with EDE contributing 16.3% of the energy density at z≈3357 and predicting H₀≈74.2 km/s/Mpc. The preference is driven primarily by Planck and ACT polarization data, but systematic uncertainties in Planck's TE polarization efficiency significantly reduce the EDE signal, suggesting possible systematics rather than new physics.

ABSTRACT

We investigate constraints on early dark energy (EDE) using ACT DR4, SPT-3G 2018, Planck polarization, and restricted Planck temperature data (at $\ell<650$), finding a $3.3σ$ preference ($Δχ^2=-16.2$ for 3 additional degrees of freedom) for EDE over $Λ$CDM. The EDE contributes a maximum fractional energy density of $f_{ m EDE}(z_c)=0.163^{+0.047}_{-0.04}$ at a redshift $z_c=3357\pm200$ and leads to a CMB inferred value of the Hubble constant $H_0=74.2^{+1.9}_{-2.1}$ km/s/Mpc. We find that Planck and ACT DR4 data provide the majority of the improvement in $χ^2$, and that the inclusion of SPT-3G pulls the posterior of $f_{ m EDE}(z_c)$ away from $Λ$CDM. This is the first time that a moderate preference for EDE has been reported for these three combined CMB data sets. We find that including measurements of supernovae luminosity distances and the baryon acoustic oscillation standard ruler only minimally affects the preference ($3.0σ$), while measurements that probe the clustering of matter at late times - the lensing potential power spectrum from Planck and $f σ_8$ from BOSS - decrease the significance of the preference to 2.6$σ$. Conversely, adding a prior on the $H_0$ value as reported by the SH0ES collaboration increases the preference to the $4-5σ$ level. In the absence of this prior, the inclusion of Planck TT data at $\ell>1300$ reduces the preference from $3.0σ$ to $2.3σ$ and the constraint on $f_{ m EDE}(z_c)$ becomes compatible with $Λ$CDM at $1σ$. We explore whether systematic errors in the Planck polarization data may affect our conclusions and find that changing the TE polarization efficiencies significantly reduces the Planck preference for EDE. More work will be necessary to establish whether these hints for EDE within CMB data alone are the sole results of systematic errors or an opening to new physics.

Motivation & Objective

• To test whether early dark energy (EDE) provides a better fit to combined CMB data than the standard ΛCDM model.
• To assess whether the observed preference for EDE arises from new physics or systematic errors in CMB data, particularly Planck polarization.
• To evaluate how external datasets—such as supernovae, BAO, lensing, and fσ₈—affect the significance of the EDE preference.
• To determine whether the EDE signal is robust across different data combinations and whether future high-precision CMB data could resolve the ambiguity.

Proposed method

• The analysis uses temperature (TT), polarization (TE, EE), and lensing likelihoods from Planck (up to 𝓁<650), ACT DR4, and SPT-3G 2018 data.
• A scalar field model of early dark energy (EDE) is implemented with a potential V(ϕ)∝[1−cos(ϕ/f)]³, allowing the field to become dynamical at a critical redshift z_c.
• The EDE model introduces three additional parameters: f_EDE(z_c), z_c, and the EDE field’s initial value, which affect the sound horizon and CMB anisotropy power spectrum.
• χ² comparisons are performed between ΛCDM and EDE models, with Δχ² used to quantify statistical preference.
• Systematic effects are tested by varying Planck’s TE polarization efficiency and allowing EE dust amplitudes to vary freely.
• External datasets—SNe, BAO, Planck lensing, and BOSS fσ₈—are sequentially added to assess their impact on the EDE significance.

Experimental results

Research questions

• RQ1 Does the combination of Planck, ACT DR4, and SPT-3G CMB data show a statistically significant preference for early dark energy over ΛCDM?
• RQ2 To what extent is the observed EDE preference driven by systematics in Planck’s polarization data, particularly TE efficiency?
• RQ3 How do external late-time probes (SNe, BAO, lensing, fσ₈) affect the significance of the EDE signal?
• RQ4 Can future high-precision CMB data resolve whether the EDE signal is a statistical fluctuation, systematic error, or evidence of new physics?

Key findings

• A 3.3σ preference for early dark energy is found over ΛCDM, with Δχ² = -16.2 for three additional degrees of freedom, using Planck, ACT DR4, and SPT-3G data.
• The EDE contributes a maximum fractional energy density of f_EDE(z_c) = 0.163⁺⁰.⁰⁴⁷₋₀.⁰⁴ at a critical redshift z_c = 3357 ± 200.
• The CMB-inferred Hubble constant is H₀ = 74.2⁺¹.⁹₋₂.¹ km/s/Mpc, consistent with late-time measurements.
• Planck and ACT DR4 data are primarily responsible for the improved fit, while SPT-3G pulls the EDE posterior away from ΛCDM.
• Including Planck TT data at ℓ > 1300 reduces the preference to 2.3σ and makes f_EDE(z_c) compatible with ΛCDM at 1σ.
• Changing the TE polarization efficiency in Planck data significantly reduces the EDE preference, indicating that systematics may explain the signal.