Early-Universe Axion Dynamics

Periodic Cosmic String Formation and Dynamics

M.A.F., J. Huang, and N. Siemonsen

JHEP 08 (2025) 080 [arXiv:2503.03116]

We study string formation and dynamics in a scalar field theory with a global U(1) symmetry. If a scalar field Φ is initially displaced from the minimum of a wine-bottle potential, even if uniformly over large spatial patches, small spatial perturbations to Φ grow via parametric resonance as Φ oscillates; this occurs for a wide range of initial U(1) charge densities. The growth of perturbations leads to the formation of spatially coherent, temporally stable “counter-rotating regions” (CRR): spatially connected regions exhibiting Φ evolution with large, opposite-sign rotation speeds in field space which persist over long durations. These CRR are separated by domain boundaries that have a large field gradient and zero rotational speed in field space. String or vortex topological defects form, are confined to, and then annihilate periodically on these boundaries. We demonstrate these periodic dynamics with numerical simulations in both 2+1 and 3+1 dimensions, in both Minkowski spacetime and in a radiation-dominated FLRW universe, and we explain some features of the evolution (semi-)analytically. At late times in an expanding universe, when Φ approaches the potential minimum, the CRR and vortices dissipate into scalar radiation. Phenomenologically, periodic bursts of string formation and annihilation can lead to periodic bursts of gravitational-wave production. For small initial U(1) charge density, these gravitational-wave bursts can be synchronized across the whole Universe. Owing to their periodic nature, they could give rise to a gravitational-wave frequency spectrum consisting of a forest of peaks. These periodic scalar field dynamics also occur with large, untuned initial U(1) charge density; they may thus have implications for models that depend on a coherent field rotation, such as kination and the axion kinetic-misalignment mechanism.

Searching for Primordial Black Hole Dark Matter

Picolensing as a probe of primordial black hole dark matter

M.A.F. and S. Sibiryakov

Phys. Rev. D 111, 063060 (2025) [arXiv:2411.12947]

Gravitational-lensing parallax of gamma-ray bursts (GRBs) is an intriguing probe of primordial black hole (PBH) dark matter in the asteroid-mass window, 2×10^-16M_⊙≲M_PBH≲5×10⁻¹²M_⊙. Recent work in the literature has shown exciting potential reach for this “picolensing” signal if a future space mission were to fly two x-/γ-ray detectors in the Swift/BAT class, with inter-spacecraft separation baselines on the order of the Earth-Moon distance. We revisit these projections with a view to understanding their robustness to various uncertainties related to GRBs. Most importantly, we investigate the impact of uncertainties in observed GRB angular sizes on reach projections for a future mission. Overall, we confirm that picolensing shows great promise to explore the asteroid-mass window; however, we find that previous studies may have been too optimistic with regard to the baselines required. Detector baselines on the order of at least the Earth-L2 distance would make such a mission more robust to GRB size uncertainties; baselines on the order of an astronomical unit (AU) would additionally enable reach that equals or exceeds existing microlensing constraints up to M_PBH∼2×10⁻⁸M_⊙.

Beyond the Standard Model Astrophysics

Fireball anti-nucleosynthesis

M.A.F., D.E. Kaplan, A. Mathur, S. Rajendran, and E. H. Tanin

Phys. Rev. D 109, 123038 (2024) [arXiv:2402.15581]

See also popular science articles in LiveScience and Popular Mechanics.

The tentative identification of approximately ten relativistic anti-helium cosmic-ray events at AMS-02 would, if confirmed, challenge our understanding of the astrophysical synthesis of heavy anti-nuclei. We propose a novel scenario for the enhanced production of such anti-nuclei that is triggered by isolated, catastrophic injections of large quantities of energetic Standard Model (SM) anti-quarks in our galaxy by physics beyond the Standard Model (BSM). We demonstrate that SM anti-nucleosynthetic processes that occur in the resulting rapidly expanding, thermalized fireballs of SM plasma can, for a reasonable range of parameters, produce the reported tentative ∼2:1 ratio of anti-helium-3 to anti-helium-4 events at AMS-02, as well as their relativistic boosts. Moreover, we show that this can be achieved without violating anti-deuterium or anti-proton flux constraints for the appropriate anti-helium fluxes. A plausible BSM paradigm for the catastrophic injections is the collision of macroscopic composite dark-matter objects carrying large net anti-baryon number. Such a scenario would require these objects to be cosmologically stable, but to destabilize upon collision, promptly releasing a fraction of their mass energy into SM anti-particles within a tiny volume. We show that, in principle, the injection rate needed to attain the necessary anti-helium fluxes and the energetic conditions required to seed the fireballs appear possible to obtain in such a paradigm. We leave open the question of constructing a BSM particle physics model to realize this, but we suggest two concrete scenarios as promising targets for further investigation.

Small-scale experiments to search for new physics

Measuring axion gradients with photon interferometry (MAGPI)

M.A.F.*, J.O. Thompson*, R. Cervantes, B. Giaccone, R. Harnik, D.E. Kaplan, S. Posen, and S. Rajendran

* joint first authors

Phys. Rev. D 109, 015025 (2024) [arXiv:2304.11261]; FERMILAB-PUB-23-162-SQMS

We propose a novel search technique for axions with a CP-violating monopole coupling g_Q to bulk SM charges Q ∈ {B, L, B − L}. Gradients in the static axion field configurations sourced by matter induce achromatic circular photon birefringence via the axion–photon coupling g_φγ. Circularly polarized light fed into an optical or (open) radio-frequency (RF) Fabry–Pérot (FP) cavity develops a phase shift that accumulates up to the cavity finesse: the fixed axion spatial gradient prevents a cancellation known to occur for an axion dark-matter search. The relative phase shift between two FP cavities fed with opposite circular polarizations can be detected interferometrically. This time-independent signal can be modulated up to non-zero frequency by altering the cavity orientations with respect to the field gradient. Multi-wavelength co-metrology techniques can be used to address chromatic measurement systematics and noise sources. With Earth as the axion source, we project reach beyond current constraints on the product of couplings g_Q g_φγ for axion masses m_φ < 10⁻⁵ eV. If shot-noise-limited sensitivity can be achieved, an experiment using high-finesse RF FP cavities could reach a factor of ∼ 10⁵ into new parameter space for g_Q g_φγ for masses m_φ < 4 x 10⁻¹¹ eV.

This work also has a related technological spinoff: a new type of high-precision gyroscope based on the spin of light: M. A. Fedderke, R. Harnik, D. E. Kaplan, S. Posen, S. Rajendran, F. Serra, and Y. P. Yakovlev. Phys. Rev. A 111, 043502 (2025) [arXiv:2406.16178].

Low-Frequency Gravitational-Wave Detection

Astrometric gravitational-wave detection via stellar interferometry

M.A.F., P.W. Graham, B. Macintosh, S. Rajendran

Phys. Rev. D 106, 023002 (2022) [arXiv:2204.07677]

We studied a novel approach to astrometric gravitational-wave detection, focusing on extreme-precision measurements of small numbers of stars. This could be enabled by a future space-based stellar interferometry mission, and would enable GW science reach in the band from roughly 10 nHz to roughly 1 µHz.

Asteroids for µHz gravitational-wave detection

M.A.F., P.W. Graham, and S. Rajendran

Phys. Rev. D 105, 103018 (2022) [arXiv:2112.11431]

See also this astrobites blog post.

We proposed a new detection approach for gravitational waves in the band from roughly 0.1 µHz to roughly 10 µHz, based on using inner Solar System asteroids as test masses, and ranging between them using human-deployed base-stations. We showed that such an approach would open up a vast new region of parameter space for GW detection in this challenging band.

Gravity gradient noise from asteroids

M.A.F., P.W. Graham, and S. Rajendran

Phys. Rev. D 103, 103017 (2021) [arXiv:2011.13833]

We pointed out the existence of an irreducible noise background for low-frequency gravitational-wave detection using local test masses in the inner solar system, that severely limits this class of detectors at frequencies below about 1 µHz.

New Physics with Gravitational-Wave Detectors

Asteroids for ultralight dark-photon dark-matter detection

M.A.F. and A. Mathur

Phys. Rev. D 107, 043004 (2023) [arXiv:2210.09324]

Physical Review D Editors’ Suggestion.

Gravitational-wave detectors can also be sensitive to new forces. Ultralight dark-photon dark matter coupled to U(1)_B or U(1)_B−L charges supplies one such force that oscillates with a frequency set by the DM mass. We showed how the asteroid-based detector in the µHz would therefore also enable access to new parameter space for DPDM coupled to B [respectively, B−L] charges in the mass range 5[9]×10⁻²¹eV ≲ m_DM ≲ 2×10⁻¹⁹eV, with peak sensitivities about a factor of 500 [50] beyond current best limits on ε_B [ε_B−L] at m_DM ∼ 2×10⁻¹⁹eV.

Searching for dark clumps with gravitational-wave detectors

S. Baum, M.A.F., and P.W. Graham

Phys. Rev. D 106, 063015 (2022) [arXiv:2206.14832]

Dark compact objects (“clumps”) transiting the Solar System exert accelerations on the test masses in a gravitational-wave detector. We reexamined the detectability of these clump transits in a variety of current and future GW detectors, operating over a broad range of frequencies. We showed that the asteroid-based detector, with μHz sensitivity, is ideal to search for transits of roughly asteroid-mass scale clumps. But we found both analytically and in simulation that purely gravitational clump-matter interactions would yield one detectable transit every ∼ 20 yrs, if clumps with mass m_cl ∼ 10¹⁴ kg saturate the dark-matter (DM) density. Other (proposed) GW detectors operating in higher frequency bands would be sensitive to smaller clump masses and have smaller rates of discoverable signals. We also considered the case of clumps endowed with an additional long-range clump-matter fifth force significantly stronger than gravity (but evading known fifth-force constraints). We used simulations to show that, for example, an attractive clump-matter fifth-force ∼ 10³ times stronger than gravity with a range of ∼ AU would boost the rate of detectable transits to a few per year for clumps in the mass range 10¹¹ kg ≲ m_cl ≲ 10¹⁴ kg, even if they are a ∼1% sub-component of the DM.

Novel Terrestrial Searches for Ultralight Bosonic Dark Matter

A Hunt for Magnetic Signatures of Hidden-Photon and Axion Dark Matter in the Wilderness (SNIPE Hunt)

I. A. Sulai, S. Kalia, A. Arza, I. M. Bloch, E. C. Muñoz, C. Fabian, et al.

Phys. Rev. D 108, 096026 (2023) [arXiv:2306.11575]

Physical Review D Editors’ Suggestion. Article featured in Nature Research Highlights.

Earth as a transducer for axion dark-matter detection

A. Arza, M.A.F., P.W. Graham, D.F. Jackson Kimball, S. Kalia

Phys. Rev. D 105, 095007 (2022) [arXiv:2112.09620]

Search for dark-photon dark matter in the SuperMAG geomagnetic field dataset

M.A.F., P.W. Graham, D.F. Jackson Kimball, S. Kalia

Phys. Rev. D 104, 095032 (2021) [arXiv:2108.08852]

Earth as a transducer for dark-photon dark-matter detection

M.A.F., P.W. Graham, D.F. Jackson Kimball, S. Kalia

Phys. Rev. D 104, 075023 (2021) [arXiv:2106.00022]

In this series of three related papers, we pointed out the existence of a novel signal of dark-photon dark matter and axion-like particle dark-matter: an oscillatory terrestrial magnetic field with a known vectorial spatial pattern that appears globally and in-phase across the entire surface of the Earth, and which is measurable in unshielded magnetometer data recorded at ground level. We also carried out searches for this signal for both DM candidates, making use of a large existing geomagnetic dataset maintained by the SuperMAG Collaboration. Although we report no robust signal candidates, we are able to place the first direct detection bounds on dark-photon dark-matter in a region of low dark-photon mass (these are complementary to existing astrophysical constraints). We are also able to place constraints on axion-like particles that are complementary to, and at some masses comparable to, existing laboratory constraints from the CASE experiment. Future follow-up work is planned.

A Novel Time-Dependent CMB Signal of Axion Dark Matter

Axion dark matter detection with CMB polarization

M.A.F., P.W. Graham, and S. Rajendran

Phys. Rev. D 100, 015040 (2019) [arXiv:1903.02666]

Physical Review D Editors’ Suggestion. See also this popular article.

This work, which was selected as a Physical Review D Editors’ Suggestion, proposes two novel CMB signals of axion-like particle dark-matter. These signals arise from an improved understanding of the well-known effect of axion-induced cosmic birefringence, as applied to dark-matter axion fields. Local axion field oscillations cause a spectacular time-dependent oscillation of the measured CMB polarization pattern at periods accessible to CMB observational campaigns, and in a manner that is in-phase across the entire CMB sky. On the other hand, early-universe axion field oscillations around CMB decoupling cause a slight reduction of the CMB polarisation power as compared to the standard scenario. Both effects allow access to a corner of parameter space in the fuzzy dark-matter regime, at axion-photon coupling parameters that are not yet constrained. The time-dependent effect has been the focus of subsequent follow-up studies by the BICEP/Keck [paper 1, paper 2] and SPT-3G [paper] CMB observational collaborations.