Isocurvature Perturbations Research Articles

Meso-inflationary Peccei–Quinn Symmetry Breaking with Nonminimal Coupling

We study a realization of the inflationary scenario where the Peccei–Quinn (PQ) symmetry is spontaneously broken during inflation, facilitated by its nonminimal coupling to gravity. This results in, effectively, a two-field inflation: the early stage is driven by an inflaton field with the PQ symmetry intact, and the later stage is driven by the PQ scalar after its effective mass becomes tachyonic, causing destabilization from the origin. The nonminimal coupling serves the dual purpose of restoring the PQ symmetry during early inflation and flattening the PQ potential posttachyonic shift, allowing for continued slow-roll. We analyze the inflationary background solutions and scalar perturbations, which are amplified at small scales via significant isocurvature perturbations generated near the symmetry-breaking epoch. These perturbations lead to second-order gravitational waves, detectable by next-generation space-based experiments.

Constraints on dark matter from dynamical heating of stars in ultrafaint dwarfs. II. Substructure and the primordial power spectrum

There is a large and growing interest in observations of small-scale structure in dark matter. We propose a new way to probe dark matter structures in the ∼10–108M⊙ range. This allows us to constrain the primordial power spectrum over shorter distances scales than possible with direct observations from the CMB. For k in the range ∼10–1000 Mpc−1 our constraints on the power spectrum are orders of magnitude stronger than previous bounds. We also set some of the strongest constraints on dark matter isocurvature perturbations. Our method relies on the heating effect such dark matter substructures would have on the distribution of stars in an ultrafaint dwarf galaxy. Many models of inflation produce enhanced power at these short distance scales and can thus be constrained by our observation. Further, many dark matter models such as axion dark matter, self-interacting dark matter and dissipative dark matter, produce dense structures which could be constrained this way. Published by the American Physical Society 2024

Generalising axion-like particle as the curvaton: sourcing primordial density perturbation and non-Gaussianities

We investigate the non-perturbatively generated axion-like particle (ALP) potential, involving fermions in the dark sector that couple to the ALP, in an early universe cosmological inflationary stage with the ALP being a spectator field. The potential here deviates from the standard cosine nature due to the presence of the two dark sector fermion masses mu\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$m_u$$\\end{document} and md\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$m_d$$\\end{document} which couple to the ALP. The ALP is a spectator field during inflation but it starts to oscillate and dominates the energy density of the universe after inflation ends, thereby sourcing isocurvature perturbations, while standard curvature fluctuations from the inflaton are assumed to be sub-dominant. Subsequently the ALP decays converting the isocurvature perturbations to adiabatic perturbations thereby acting as the origin of the primordial density perturbations. We identify the parameter space involving the axion decay constant fϕ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f_{\\phi }$$\\end{document}, scale of confinement Λ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Lambda $$\\end{document}, ALP mass m and the masses of the fermions, mu\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$m_u$$\\end{document} and md\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$m_d$$\\end{document} where it can satisfactorily behave as the curvaton and source the observed primordial density perturbation. We also predict local non-Gaussianity signals for bi-spectrum and tri-spectrum fNL\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f_{NL}$$\\end{document} and gNL\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$g_{NL}$$\\end{document}, as a function of the ratio mu/md\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$m_u/m_d$$\\end{document}, which are within the allowed range in the latest Planck observations and are detectable with future observations. Particularly we find that the value of fNL\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f_{NL}$$\\end{document} and gNL\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$g_{NL}$$\\end{document} are dependent on the ratio of mu\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$m_u$$\\end{document} and md\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$m_d$$\\end{document}: fNL\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f_{NL}$$\\end{document} is more or less positive for all scenarios except mu=md\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$m_u = m_d$$\\end{document} and gNL\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$g_{NL}$$\\end{document} is always positive irrespective of the ratio between mu\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$m_u$$\\end{document} and md\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$m_d$$\\end{document}. The results of our analysis in the limit mu=md\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$m_u = m_d$$\\end{document} resembles vanilla curvaton scenario while in the limit mu≫md\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$m_u \\gg m_d$$\\end{document} resembles pure axion cosine potential. This is because the model has the unique feature where the extra matter in the dark sector determines the nature and shape of the potential.

Encyclopædia Inflationaris: Opiparous Edition

The current flow of high-accuracy astrophysical data, among which are the Cosmic Microwave Background (CMB) measurements by the Planck satellite, offers an unprecedented opportunity to constrain the inflationary theory. This is however a challenging project given the size of the inflationary landscape which contains hundreds of different scenarios. Given that there is currently no observational evidence for primordial non-Gaussianities, isocurvature perturbations or any other non-minimal extension of the inflationary paradigm, a reasonable approach is to consider the simplest models first, namely the slow-roll single-field models with minimal kinetic terms. This still leaves us with a very populated landscape, the exploration of which requires new and efficient strategies. It has been customary to tackle this problem by means of approximate model-independent methods while a more ambitious alternative is to study the inflationary scenarios one by one. We have developed the publicly available runtime library ASPIC11https://github.com/cosmicinflation/aspic. to implement this last approach. The ASPIC code provides all routines needed to quickly derive reheating-consistent observable predictions within this class of scenarios. ASPIC has been designed as an evolutive code which presently supports 118 different models. In this paper, for each of the ASPIC models, we present and collect new results in a systematic manner, thereby constituting the first Encyclopædia Inflationaris.

Vector dark matter, inflation, and non-minimal couplings with gravity

We propose a cosmological dark matter production mechanism in the form of a longitudinal massive vector boson. We build upon the work [1] including non-minimal couplings of the massive vector with gravity, developing a well motivated set-up from an effective field theory perspective. We carefully track the dynamics of vector field in passing from inflation to radiation dominated universe to show that the late time abundance of longitudinal modes — excited initially by the quantum fluctuations during inflation — can provide the observed dark matter abundance for sufficiently weak non-minimal coupling and wide range of vector masses 5 × 10-7 ≲ m [eV] ≲ 5 × 103. The final abundance of dark matter depends on two parameter, the vector mass and its non-minimal coupling with gravity. We discuss experimental venues to probe this framework, including the production of a stochastic gravitational wave background. The latter is especially interesting, as the same mechanism that generates dark matter can potentially lead to the production of gravitational waves in the LISA frequency band, through the second-order effects of large dark matter iso-curvature perturbations at small scales. We take a first step in this direction, identifying the potential information that gravitational wave experiments can provide on the parameter space of dark matter within this scenario.

Improving constraints on inflation with CMB delensing

The delensing of cosmic microwave background (CMB) maps will be increasingly valuable for extracting as much information as possible from future CMB surveys. Delensing provides many general benefits, including sharpening of the acoustic peaks, more accurate recovery of the damping tail, and reduction of lensing-induced B-mode power. In this paper we present several applications of delensing focused on testing theories of early-universe inflation with observations of the CMB. We find that delensing the CMB results in improved parameter constraints for reconstructing the spectrum of primordial curvature fluctuations, probing oscillatory features in the primordial curvature spectrum, measuring the spatial curvature of the universe, and constraining several different models of isocurvature perturbations. In some cases we find that delensing can recover almost all of the constraining power contained in unlensed spectra, and it will be a particularly valuable analysis technique to achieve further improvements in constraints for model parameters whose measurements are not expected to improve significantly when utilizing only lensed CMB maps from next-generation CMB surveys. We also quantify the prospects of testing the single-field inflation tensor consistency condition using delensed CMB data; we find it to be out of reach of current and proposed experimental technology and advocate for alternative detection methods.

Primordial origin of supermassive black holes from axion bubbles

We study a modification of the primordial black hole (PBH) formation model from axion bubbles.We assume that the Peccei-Quinn scalar rolls down in the radial direction from a large field value to the potential minimum during inflation, which suppresses the axion fluctuations and weakens the clustering of PBHs on large scales.We find that the modified model can produce a sufficient number of PBHs that seed the supermassive black holes while avoiding the observational constraints from isocurvature perturbations and angular correlation of the high-redshift quasars.

Phenomenology of a vector-field-induced and possibly parity breaking compensated isocurvature perturbation

Phenomenology of a vector-field-induced and possibly parity breaking compensated isocurvature perturbation

Anharmonic effects on the squeezing of axion perturbations

It is assumed in standard cosmology that the Universe underwent a period of inflation in its earliest phase, providing the seeds for structure formation through vacuum fluctuations of the inflaton scalar field. These fluctuations get stretched by the quasi-exponential expansion of the Universe and become squeezed. The aim of this paper is to deepen the understanding of the squeezing process, considering the effect of self-interactions. Axion-like particles can provide a useful setup to study this effect. Specifically we focus on the consequences that a non-trivial evolution of the background axion field has on the squeezing of the perturbations. We follow the evolution of the axion's fluctuation modes from the horizon exit during inflation to the radiation-dominated epoch. We compute Bogoliubov coefficients and squeezing parameters, which are linked to the axion particle number and isocurvature perturbation. We find that the quantum mechanical particle production and the squeezing of the perturbations are enhanced, if one accounts for anharmonic effects, i.e., the effect of higher order terms in the potential. This effect becomes particularly strong towards the hilltop of the potential.

Misalignment production of vector boson dark matter from axion-SU(2) inflation

We present a new mechanism to generate a coherently oscillating dark vector field from axion-SU(2) gauge field dynamics during inflation. The SU(2) gauge field acquires a nonzero background sourced by an axion during inflation, and it acquires a mass through spontaneous symmetry breaking after inflation. We find that the coherent oscillation of the dark vector field can account for dark matter in the mass range of 10-13 – 1 eV in a minimal setup. In a more involved scenario, the range can be wider down to the fuzzy dark matter region. One of the dark vector fields can be identified as the dark photon, in which case this mechanism evades the notorious constraints for isocurvature perturbation, statistical anisotropy, and the absence of ghosts that exist in the usual misalignment production scenarios. Phenomenological implications are discussed.

Non-Gaussianity in rapid-turn multi-field inflation

We show that theories of inflation with multiple, rapidly turning fields can generate large amounts of non-Gaussianity. We consider a general theory with two fields, an arbitrary field-space metric, and a potential that supports sustained, rapidly turning field trajectories. Our analysis accounts for non-zero field cross-correlation and does not fix the power spectra of curvature and isocurvature perturbations to be equal at horizon crossing. Using the δN formalism, we derive a novel, analytical formula for bispectrum generated from multi-field mixing on super-horizon scales. Rapid-turn inflation can produce a bispectrum with several potentially large contributions that are not necessarily of the local shape. We exemplify the applicability of our formula with a fully explicit model and show that the new contributions indeed can generate a large amplitude of local non-Gaussianity, f NL loc ∼ 𝒪(1). These results will be important when interpreting the outcomes of future observations.

Power spectrum in the chaotic regime of axionic blue isocurvature perturbations

Power spectrum in the chaotic regime of axionic blue isocurvature perturbations

Diraxiogenesis

The family of Dirac Seesaw models offers an intriguing alternative explanation for the smallness of neutrino masses without necessarily requiring microscopic lepton number violation, when compared to the more familiar class of Majorana Seesaws. A global U(1)D symmetry, that is explicitly broken by a higher dimensional scalar operator, ensures that the right handed neutrino does not couple directly to the Standard Model like Higgs and an exact gauged or residual lepton number symmetry prohibits all Majorana masses. We demonstrate that all three Dirac Seesaws possess a Pseudo-Nambu-Goldstone boson associated with the U(1)D symmetry, that we call the Diraxion, whose cosmological dynamics have so far been left unexplored. Furthermore we illustrate that a Dirac-Leptogenesis version of the recently proposed Lepto-Axiogenesis scenario can be realized in this class of models, leading to a unified origin of the observed baryon asymmetry and dark matter relic abundance. Explaining only the baryon asymmetry can lead to potentially observable amounts of right handed neutrino dark radiation with ∆Neff. ≲ 0.028. On the other hand, if we only fix the dark matter abundance via the kinetic misalignment mechanism, this set-up could lead to detectable signatures in proposed cosmic neutrino background experiments via decays of eV-scale Diraxions to neutrinos. Here there is no domain wall problem, since topological defects decay to a subleading fraction of relic Diraxions. A key ingredient of all Axiogenesis scenarios is the dynamics of relatively light scalar called the Saxion, that in our case has a mass at the GeV-scale and which might reveal itself in heavy meson decays or collider searches. Our setup predicts isocurvature perturbations in baryons, dark matter and dark radiation sourced by fluctuations of the Saxion.

Role of QCD in moduli stabilization during inflation and axion dark matter

Ignorance of the initial condition for the axion dynamics in the early Universe has led us to consider an O(1) valued initial amplitude, and that prefers the decay constant, Fa, of the QCD axion to be an intermediate scale such as 1012 GeV in order to explain the dark matter abundance. We explore a cosmological scenario of Fa being much larger than 1012 GeV by considering the axion and modulus dynamics during inflation to set the initial amplitude. We show that if the volume modulus (radion) of the extra-dimension is stabilized mainly by the QCD contribution to the modulus potential during inflation, the QCD axion with the string-scale decay constant obtains a mass around the inflationary Hubble parameter. This means that the axion rolls down to the θ = 0 minimum during the inflation realizing almost vanishing initial amplitude, and the inflationary quantum fluctuation can be the dominant source of the current number density of axions. We find natural parameter regions where the axion explains the cold dark matter of the Universe, while the constraint on the isocurvature perturbation is avoided. The presence of the axion miniclusters or axion stars are predicted in a wide range of parameters, including the one explains the Subaru-HCS microlensing event.

Impact of freeze-in on dark matter isocurvature

Dark matter freeze-in is a compelling cosmological production mechanism in which all or some of the observed abundance of dark matter is generated through feeble interactions it has with the Standard Model. In this work we present the first analysis of freeze-in dark matter fluctuations and consider two benchmark models: freeze-in through the direct decay of a heavy vector boson and freeze-in through pair annihilation of Standard Model particles in the thermal bath. We provide a theoretical framework for determining the impact of freeze-in on curvature and dark matter isocurvature perturbations. We determine freeze-in dark matter fluid properties from first principles, tracking its evolution from its relativistic production to its final cold state, and calculate the evolution of the dark matter isocurvature perturbation. We find that in the absence of initial isocurvature, the freeze-in production of dark matter does not source isocurvature. However, for an initial isocurvature perturbation seeded by inflation, the nonthermal freeze-in process may allow for a fraction of the isocurvature to persist, in contrast to the exponential suppression it receives in the case of thermal dark matter. In either case, the evolution of the curvature mode is unaffected by the freeze-in process. We show sensitivity projections of future cosmic microwave background experiments to the amplitude of uncorrelated, totally anticorrelated, and totally correlated dark matter isocurvature perturbations. From these projections, we infer the sensitivity to the abundance of freeze-in dark matter that sustains some fraction of the primordial isocurvature.

Adiabatic and isocurvature perturbations in extended theories with kinetic couplings

The scalar field sector in low-energy effective field theories motivated by string theory often contains several scalar fields, some of which possess non-standard kinetic terms. In this paper we study theories with two scalar fields, in which one of the fields has a non-canonical kinetic term. The kinetic coupling is allowed to depend on both fields, going beyond the work in the literature, which usually considers the case of the coupling to depend on the other field only. Our aim is to study adiabatic and isocurvature perturbations in these extended theories. Our results show that the evolution equation for the curvature perturbation does not change when allowing the coupling to depend on both fields, while the effective mass of the entropy perturbation changes. We find expressions for the spectral index and its running at horizon crossing and at the end of inflation. We apply the formalism and study three phenomenological models, with different kinetic couplings.

Clustering of primordial black holes from QCD axion bubbles

We study the clustering of primordial black holes (PBHs) and axion miniclusters produced in the model proposed to explain the LIGO/Virgo events or the seeds of the supermassive black holes (SMBHs) in ref. [1]. It is found that this model predicts large isocurvature perturbations due to the clustering of PBHs and axion miniclusters, from which we obtain stringent constraints on the model parameters. Specifically, for the axion decay constant fa = 1016 GeV, which potentially accounts for the seeds of the SMBHs, the PBH fraction in dark matter should be f PBH ≲ 7 × 10-10. Assuming that the mass of PBHs increases by more than a factor of 𝒪(10) due to accretion, this is consistent with the observed abundance of SMBHs. On the other hand, for fa = 1017 GeV required to produce PBHs of masses detected in the LIGO/Virgo, the PBH fraction should be f PBH ≲ 6 × 10-8, which may be too small to explain the LIGO/Virgo events, although there is a significant uncertainty in calculating the merger rate in the presence of clustering.

Comparable dark matter and baryon abundances with a heavy dark sector

We propose a scenario that explains the comparable abundances of dark matter (DM) and baryons without any coincidence in the corresponding particle masses. Here, DM corresponds to heavy “dark baryons” in a hidden MSSM-like dark sector, where the supersymmetry breaking scale can be several orders of magnitude larger than in the visible sector. In both sectors a baryon asymmetry is generated via the Affleck-Dine mechanism, and the smaller dark baryon-to-entropy ratio partially compensates the larger dark baryon masses to give similar densities in the two sectors. The large mass hierarchy also naturally results in an asymmetric reheating of these sequestered sectors. Moreover, this scenario predicts uncorrelated DM and baryon isocurvature perturbations.

Multifield positivity bounds for inflation

Positivity bounds represent nontrivial limitations on effective field theories (EFTs) if those EFTs are to be completed into a Lorentz-invariant, causal, local, and unitary framework. While such positivity bounds have been applied in a wide array of physical contexts to obtain useful constraints, their application to inflationary EFTs is subtle since Lorentz invariance is spontaneously broken during cosmic inflation. One path forward is to employ a Breit parameterization to ensure a crossing-symmetric and analytic S-matrix in theories with broken boosts. We extend this approach to a theory with multiple fields, and uncover a fundamental obstruction that arises unless all fields obey a dispersion relation that is approximately lightlike. We then apply the formalism to various classes of inflationary EFTs, with and without isocurvature perturbations, and employ this parameterization to derive new positivity bounds on such EFTs. For multifield inflation, we also consider bounds originating from the generalized optical theorem and demonstrate how these can give rise to stronger constraints on EFTs compared to constraints from traditional elastic positivity bounds alone. We compute various shapes of non-Gaussianity (NG), involving both adiabatic and isocurvature perturbations, and show how the observational parameter space controlling the strength of NG can be constrained by our bounds.

Peaky production of light dark photon dark matter

We explore a mechanism to produce a light dark photon dark matter through a coupling between the dark photon field and a spectator scalar field which plays no role in the inflationary expansion of the Universe while rolling down its potential during the inflation. The motion of the spectator field efficiently produces dark photons with large wavelengths which become non-relativistic before the time of matter-radiation equality. The spectrum of the wavelengths is peaky so that the constraint from the isocurvature perturbation can be evaded. The correct relic abundance is then achieved over a wide range of the dark photon mass down to 10-13 eV. Our mechanism favors high-scale inflation models which can be tested in future observations. Furthermore, fluctuations of the dark photon field during inflation produce gravitational waves detectable at future space-based interferometers and/or pulsar timing array experiments.