⚛️ phenomenology

Constraints on Ultra-Light Axions from the DESI DR1 Full Shape, Planck and ACT

This paper presents the most stringent constraints to date on ultra-light axions as a dark matter subcomponent in the mass range $10^{-32}$ to $10^{-24}$ eV, derived from a full-shape analysis of DESI DR1 galaxy power spectra combined with Planck and ACT CMB data, which significantly improves upon CMB-only limits and reveals that a mild preference for axions seen in galaxy clustering data disappears when CMB data is included.

Original authors: Francesco Verdiani, Emanuele Castorina, Ennio Salvioni, Emiliano Sefusatti, Matteo Viel

Published 2026-07-14

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CC BY 4.0

Original authors: Francesco Verdiani, Emanuele Castorina, Ennio Salvioni, Emiliano Sefusatti, Matteo Viel

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe is a giant, invisible ocean made mostly of "Dark Matter." For decades, scientists have assumed this ocean is made of heavy, slow-moving particles, like cold, dense fog that just sits there. But what if some of that ocean is actually made of something much stranger: Ultra-Light Axions (ULAs)? Think of these not as heavy fog, but as ghostly, invisible waves rippling through the cosmos.

This paper is like a massive, high-tech detective story where scientists use the newest, sharpest telescopes to see if these ghost waves are hiding in the dark matter ocean. They didn't just look at the sky; they used a super-advanced mathematical toolkit called the "Effective Field Theory of Large Scale Structure" (EFTofLSS) to predict exactly how these waves should mess up the patterns of galaxies.

The Big Hunt: Two Telescopes, One Mystery
The team combined data from two massive projects:

DESI (Dark Energy Spectroscopic Instrument): This is like a giant 3D mapmaker. It took a snapshot of nearly 6 million galaxies, measuring their positions and how they clump together.
CMB (Cosmic Microwave Background): This is the "baby picture" of the universe, captured by the Planck and ACT telescopes, showing the heat left over from the Big Bang.

They were looking for a specific "fingerprint" that ULAs would leave behind. Because these axions are so light, they have a huge "wavelength" (imagine a wave so long it stretches across millions of light-years). This creates a weird effect: on small scales, the axions act like a fluid that resists clumping, creating a "Jeans scale" (a minimum size for a clump). If ULAs exist, the galaxy map should look like a staircase that suddenly flattens out at a certain size, rather than a smooth ramp.

The Verdict: The Ghosts Are Mostly Gone
After crunching the numbers with their fancy math, the results are a bit of a bummer for the ghost hunters, but a huge win for our understanding of the universe:

The Main Finding: The data shows no evidence that the universe is filled with these ghost waves. In fact, the scientists set the strictest limits ever on how much of these axions could exist.
The Numbers: For the lightest axions they tested (around $10^{-29}$ eV), the paper proves that if they exist at all, they can make up no more than 0.3% of the total dark matter. That's like saying if the dark matter ocean were a giant swimming pool, the axions would be less than a single drop of water. For slightly heavier axions ( $10^{-27}$ eV), the limit is even tighter, under 1%.
The "Almost" Moment: There was a tiny, fleeting moment of excitement. When they looked at just one specific type of galaxy (Luminous Red Galaxies) from the DESI data, the pattern mildly suggested a ghost wave with a mass of about $10^{-26}$ eV. It was like hearing a faint creak in an empty house. However, as soon as they added the other galaxy types and the cosmic background data, the creak vanished. The authors conclude this was likely just a statistical fluke (a random noise glitch), not a sign of new physics.

Why This Matters
Before this study, some older data (from a project called BOSS) had hinted that these axions might be hiding in the $10^{-26}$ eV range. This new paper, using the much larger and sharper DESI DR1 data, effectively rules that out. It's like upgrading from a blurry security camera to a 4K HD camera and realizing the "intruder" you thought you saw was just a shadow.

What's Next?
The scientists are honest about the limits of their game. They can't yet rule out axions that are slightly heavier (above $10^{-25}$ eV) because the math gets too messy and the "ghost waves" would be too small to see with their current tools. To catch those, they'd need to look at even smaller scales and improve their theoretical models.

But for the mass window they checked ( $10^{-32}$ eV to $10^{-24}$ eV), the message is clear: The universe is mostly cold, clumpy dark matter, and the "ghost wave" version is either non-existent or hiding so quietly it makes up less than a drop in the cosmic bucket.

Technical Summary: Constraints on Ultra-Light Axions from DESI DR1, Planck, and ACT

Problem Statement
Dark matter (DM) remains one of the most significant open questions in cosmology. While the standard $\Lambda$ CDM model assumes cold, collisionless DM, alternative candidates such as Ultra-Light Axions (ULAs) are motivated by the strong CP problem and string theory. ULAs possess a macroscopic de Broglie wavelength that suppresses the growth of cosmological perturbations below a characteristic Jeans scale ( $k_{drop}$ ). This suppression creates a distinct signature in the matter power spectrum, differentiating ULAs from standard Cold Dark Matter (CDM).

The challenge lies in constraining the abundance of ULAs ( $f_{ax}$ ) as a subcomponent of DM across a wide mass window ( $10^{-32} \text{ eV} \leq m_a \leq 10^{-24} \text{ eV}$ ). Previous constraints relied heavily on Cosmic Microwave Background (CMB) data or older Large Scale Structure (LSS) datasets (e.g., BOSS). The advent of the Dark Energy Spectroscopic Instrument (DESI) Data Release 1 (DR1) offers a significantly larger volume and higher precision, necessitating a re-evaluation of ULA constraints using state-of-the-art theoretical frameworks that can handle mixed DM scenarios without relying on phenomenological assumptions calibrated solely on $\Lambda$ CDM.

Methodology
The authors perform an independent re-analysis of the DESI DR1 full-shape galaxy power spectra, combined with CMB data from the Atacama Cosmology Telescope (ACT) DR6 and Planck.

Theoretical Framework (EFTofLSS):
- The analysis employs the Effective Field Theory of Large Scale Structure (EFTofLSS) extended to a "two-fluid" scenario, where the total matter field consists of CDM+baryons and ULAs.
- Unlike standard pipelines hard-coded for pure CDM, this framework explicitly models the distinct linear transfer functions and nonlinear evolution of the ULA component.
- The nonlinear ULA density contrast is evaluated by rescaling the CDM+baryon perturbation by the ratio of their linear transfer functions (Eq. 7), allowing for computationally efficient 1-loop calculations suitable for Markov-Chain Monte Carlo (MCMC) inference.
- The galaxy bias expansion includes a separate linear bias parameter ( $b_a$ ) and counterterms for the ULA component to avoid biased inference.
Data Processing:
- DESI DR1: Power spectra were measured for Luminous Red Galaxies (LRG), Emission Line Galaxies (ELG), Quasars (QSO), and Bright Galaxy Survey (BGS) tracers using pypower. The analysis utilizes monopole ( $\ell=0$ ) and quadrupole ( $\ell=2$ ) multipoles with a scale cut of $k_{max} = 0.2 \, h \text{ Mpc}^{-1}$ .
- CMB: The study combines Planck legacy data with ACT DR6 temperature, polarization, and lensing data.
- Nonlinear Modeling: For CMB lensing, the authors adopt a perturbative 1-loop EFTofLSS approach for multipoles $L < 763$ . This choice is conservative; it marginalizes over theoretical systematics associated with different halo-model prescriptions (e.g., axionHMcode vs. HMcode), which show divergent predictions for heavier axions ( $m_a \gtrsim 10^{-25} \text{ eV}$ ).
Parameter Space:
- The analysis scans a logarithmic grid of axion masses from $10^{-32}$ to $10^{-24}$ eV.
- Priors are set on the axion fraction $f_{ax} \equiv \Omega_a / (\Omega_c + \Omega_a)$ and the axion density $\Omega_a h^2$ .
- In specific tests, the authors apply theoretically motivated priors of order $\mathcal{O}(f_{ax})$ on the axion bias and counterterm parameters to reflect the expectation that galaxy properties should have suppressed sensitivity to a vanishingly small subcomponent.

Key Contributions

First Joint Analysis: This is the first application of DESI DR1 full-shape data to constrain non-cold DM components using a self-consistent two-fluid EFTofLSS framework.
Improved Modeling: The work moves beyond standard $\Lambda$ CDM pipelines by explicitly modeling the nonlinear dynamics, redshift-space distortions, and galaxy bias for mixed DM scenarios from first principles.
Conservative Systematics Handling: By using perturbative modeling for CMB lensing rather than relying on specific halo models for the nonlinear regime, the authors ensure robust constraints that are less sensitive to theoretical uncertainties in the high-mass regime.

Results

Constraints on Abundance: The joint analysis of DESI, Planck, and ACT establishes the most stringent limits to date on the ULA fraction.
- For $m_a \sim 10^{-29} \text{ eV}$ , ULAs are constrained to be less than 0.3% of the total matter energy density ( $f_{ax} < 0.003$ ).
- For masses $m_a \lesssim 10^{-27} \text{ eV}$ , the ULA fraction is constrained to be below 1%.
- Compared to CMB-only constraints, the addition of DESI DR1 improves the bounds by more than a factor of 2 for the smallest masses.
Mass Dependence:
- Low Mass ( $m_a \lesssim 10^{-27} \text{ eV}$ ): Constraints are driven primarily by the modification of the distance to the last scattering surface and the Integrated Sachs-Wolfe effect in the CMB. DESI breaks geometric degeneracies (specifically between $\Omega_a h^2$ and $h$ ), tightening the bounds.
- High Mass ( $m_a \gtrsim 10^{-25} \text{ eV}$ ): Constraints are driven by the shape suppression of the galaxy power spectrum. For $m_a = 10^{-25} \text{ eV}$ , the DESI full-shape analysis alone is more constraining than the combination of primary CMB anisotropies and BAO.
Hints of Preference: The DESI LRG sample (specifically the LRG2 bin) shows a mild preference for a non-zero ULA fraction at $m_a \approx 10^{-26} \text{ eV}$ , mirroring previous hints from BOSS. However, this preference vanishes when combining all DESI tracers and disappears entirely upon combination with CMB data. The authors interpret this as a statistical fluctuation rather than evidence for new physics.
Theoretical Limitations: For masses $m_a \gtrsim 10^{-25} \text{ eV}$ , the allowed fraction $f_{ax}$ is large enough that genuine ULA nonlinearities become significant. The current constraints in this regime are subject to theoretical systematics due to the lack of a unified nonlinear model, and the authors present these limits as dashed lines in their exclusion plots.

Significance
The paper claims to provide the most stringent limits to date on the abundance of Ultra-Light Axions as a subcomponent of dark matter within the perturbative regime. By leveraging the high-precision, large-volume data from DESI DR1 and combining it with state-of-the-art CMB data, the study demonstrates that LSS surveys have reached a competitive position with the CMB in probing the nature of dark matter. The work validates the utility of the two-fluid EFTofLSS framework for isolating beyond-Standard-Model dynamics from astrophysical systematics. While the results currently favor the standard $\Lambda$ CDM model, the methodology establishes a robust pipeline for future investigations, particularly as data analysis extends to smaller scales and theoretical modeling of nonlinear axion dynamics improves.

Technical Summary: Constraints on Ultra-Light Axions from DESI DR1, Planck, and ACT

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