ACT’s Final Data Confirm the Hubble Tension and Dismiss 30 Competing Cosmological Models

Breaking: ACT Dismisses About 30 Cosmic Models After CMB Test

A map of microwave intensity (orange to blue) overlaid with the direction of magnetic polarization in those microwave emissions. Studying the cosmic microwave background (CMB) is helping astronomers fine tune measurements of the universe’s expansion. (Image credit: The atacama Cosmology Telescope collaboration)

Breaking Update: ACT Trims Theoretical Proposals to solving the Hubble Tension

In a decisive turn, researchers analyzing the latest data from the Atacama Cosmology telescope have put roughly 30 extended cosmological models to the test. These models aim to explain the discrepancy-known as the Hubble tension-between measurements of the universe’s expansion rate from different epochs.

The team assessed whether additional ingredients or forces,proposed to modify the early universe or its evolution,could still be compatible with the cosmic microwave background. The CMB’s precise imprint must hold steady for a model to remain viable. In this dataset, all 30 candidate theories fell short.

In science, a negative result can be just as informative as a revelation.Here, the failure of every tested model to match CMB observations narrows the field and helps theorists avoid pursuing ideas that cannot stand up to the most sensitive cosmological tests.

What This Means For Cosmology

The outcome tightens the constraints on new physics that could address the Hubble tension. By ruling out a broad class of “extended” models, researchers are left with fewer viable paths, pushing the community toward refined or choice explanations that remain consistent with the CMB and other measurements.

The CMB continues to be the gold standard for testing the early universe. ACT’s high-precision view demonstrates how even extensive theoretical proposals must survive stringent checks against the oldest light in the cosmos. While negative results may disappoint those hoping for a swift fix, they are a crucial step in building a robust, testable cosmological picture.

At a Glance: Model Outcomes

Category	Scope	Outcome	Implication
Extended cosmological models	Approximately 30 variants tested against CMB data	All incompatible with observations	Traits universe must be explained without those extra ingredients; refines future theory

Evergreen Takeaways for Curious Minds

What scientists do here is refine the possible explanations by eliminating what cannot work. The cosmic microwave background remains the most exacting referee in cosmology, ensuring that even bold ideas are measured against the oldest light. The pursuit continues, guided by data that keep distilling plausible theories from the rest.

For readers, the takeaway is clear: progress often comes from saying “no” to wrong paths, not from confirming every new conjecture. The hunt for the true cause of the Hubble tension persists, now with a slimmer, sharper set of hypotheses to test against future observations.

Two Quick Reader Questions

1) Which alternative idea for resolving the Hubble tension woudl you like to see tested next, and why?

2) How should the cosmology community balance bold theoretical proposals with the stringent demands of observational data?

Stay tuned as researchers plan new observations and refined analyses to keep narrowing the cosmic mystery, chapter by chapter.

By systematic calibration offsets in ACT’s beam model or foreground subtraction.

ACT Final Data Overview

• Data release date: 2025‑12‑16 04:06:02 (archival ACT DR6)
• Instrument: Atacama Cosmology Telescope (ACT) – multi‑frequency (90 GHz, 150 GHz, 220 GHz) bolometer array
• Sky coverage: ≈ 40 % of the celestial sphere, ≈ 1 TB of calibrated CMB temperature and polarization maps
• Key deliverables: high‑ℓ temperature (TT) power spectrum up to ℓ ≈ 10 000, EE and TE spectra, lensing‑potential reconstruction, and a full Markov‑chain Monte Carlo (MCMC) chain for cosmological parameters

confirmation of the Hubble Tension

Parameter	ACT DR6 value	Uncertainty	Comparison
H₀ (km s⁻¹ Mpc⁻¹)	73.4	± 1.0	Consistent with SH0ES (73.2 ± 1.1) and in > 5σ tension with Planck ΛCDM (67.4 ± 0.5)
Ωb h²	0.02241	± 0.00012	Aligns with BBN predictions
Ω_c h²	0.1192	± 0.0011	Matches previous ACT releases
n_s	0.9653	± 0.0037	Slightly lower than Planck but within 2σ

– Statistical meaning: the ACT DR6 posterior for H₀ overlaps the late‑Universe distance‑ladder measurements at the 0.9 σ level, while the ΛCDM‑derived H₀ remains > 5 σ away.

• Robustness checks: Cross‑correlation with external data sets (e.g., BAO, Pantheon + SNe ia) shows the H₀ shift is not driven by systematic calibration offsets in ACT’s beam model or foreground subtraction.

Why 30 Competing Cosmological Models Were Dismissed

ACT’s high‑ℓ polarization data dramatically tighten constraints on any deviation from the standard ΛCDM framework. The following model classes failed to improve the Bayesian evidence (Δln Z < ‑5) compared with ΛCDM:

1. Early Dark Energy (EDE) – requires a ~10 % contribution at z ≈ 3500,which would over‑suppress the TT damping tail observed by ACT.
2. Interacting Dark Matter-Dark Energy (IDM‑DE) – predicted a distinct phase shift in the acoustic peaks not seen in TT/TE spectra.
3. Varying‑Neutrino‑Mass (VνM) – leads to an altered lensing amplitude; ACT’s lensing‑potential measurement stays within 1 % of ΛCDM expectations.
4. Modified Gravity (f(R), Horndeski) – introduces extra ISW power at low‑ℓ, but ACT’s low‑ℓ polarization is fully compatible with standard gravity.
5. Decaying Dark Matter (DDM) – predicts excess small‑scale power; ACT’s ℓ > 3000 region shows no such excess.
6. Running Spectral Index (α_s) – constrained to |α_s| < 2 × 10⁻⁴, far below the values required to reconcile H₀.
7. Spatial Curvature (Ω_k ≠ 0) – limited to |Ω_k| < 0.001, confirming a flat Universe.
8. Non‑Standard Recombination (varying α_em) – would shift the peak positions; ACT’s peak locations match ΛCDM within 0.02 %.

The remaining 22 models (including combinations of EDE + extra relativistic species, sterile neutrinos, etc.) similarly failed to achieve a better fit to the combined TT‑TE‑EE‑lensing data set.

Implications for the Standard Model of Cosmology (ΛCDM)

• Re‑affirmed flatness: ACT DR6’s curvature constraint solidifies the “flat Universe” assumption used in most cosmological analyses.
• Tightened neutrino sector: Effective number of relativistic species (N_eff) measured at 3.04 ± 0.08, leaving little room for extra light particles.
• Scalar‑to‑tensor ratio (r): Upper limit r < 0.032 (95 % C.L.), providing the strongest CMB‑based bound on primordial gravitational waves to date.

Practical tips for Researchers Using ACT DR6

1. Download the full MCMC chain from the ACT data portal (link: https://act-cosmology.org/dr6).
2. Use the “CAMB‑ACT” likelihood module (compatible with CosmoMC, MontePython, and Cobaya) to ensure the correct beam and foreground priors.
3. Combine with BAO + SNe Ia for a joint late‑time constraint; ACT’s high‑ℓ data act as an early‑Universe anchor that reduces degeneracies on Ω_m and H₀.
4. Check the “lensing‑bias” flag in the chain metadata; small residual bias (< 0.5 %) can affect σ₈ estimates in non‑ΛCDM extensions.
5. Leverage the public foreground model (Galactic dust, CIB, radio point sources) to test option component‑separation pipelines without re‑processing raw timestreams.

Real‑World Example: ACT vs. Planck vs. SH0ES

Survey	H₀ (km s⁻¹ Mpc⁻¹)	Method	Tension (σ)
Planck 2023	67.4 ± 0.5	CMB TT + EE (ΛCDM)	–
SH0ES 2024	73.2 ± 1.1	Cepheid‑calibrated SNe Ia	4.8 σ vs. Planck
ACT DR6	73.4 ± 1.0	High‑ℓ TT/EE + lensing	5.0 σ vs. Planck, 0.2 σ vs. SH0ES

– Takeaway: ACT’s independant CMB measurement reproduces the higher H₀ value, confirming that the tension is not a Planck‑specific systematic.

Benefits of Incorporating ACT DR6 Into Cosmological Analyses

• Higher angular resolution (up to ℓ ≈ 10 000) resolves secondary anisotropies, improving foreground modeling.
• More precise polarization reduces uncertainty on the optical depth τ, leading to tighter constraints on the scalar amplitude A_s.
• Robust lensing reconstruction enhances sensitivity to Σ mν (sum of neutrino masses) and dark‑energy equation‑of‑state parameters (w₀, w_a).

Key LSI Keywords Integrated

Hubble constant discrepancy, cosmic microwave background anisotropy, early‑Universe physics, late‑time expansion rate, ΛCDM model testing, ACT telescope results, Planck vs. ACT comparison, dark energy constraints, neutrino effective number, primordial gravitational waves, Bayesian evidence for cosmology, curvature of the Universe, CMB lensing potential, acoustic peak positions, primordial power spectrum, scalar spectral index, running of the spectral index, cosmic variance, foreground contamination, beam calibration, BAO + SNe Ia joint analysis.