arXiv subjects

Nicklas Ramberg

Publications and source records attributed to Nicklas Ramberg.

11 recordsLinked to original sources

arXiv2026

Domain Walls From Confining Bubbles: $SU(N_{c})$ Yang Mills at Finite $\theta$

We study the confinement phase transition in SU($N_{c}$) pure Yang-Mills theory at finite $\theta \neq 0$ using the Improved Holographic QCD (IHQCD) model. We show that the critical temperature, as a function of $\theta$ for large but fixed $N_{c}$, is reduced, thus decreasing the amount of supercooling in the confinement phase transition. Upon completion of the confinement phase transition, a network of domain walls can be produced, owing to the multi-branched vacuum structure of Yang-Mills theory at finite $\theta$. We highlight the potential interplay between the produced domain walls and the confinement phase transition dynamics. We emphasize that DW production from bubble coalescence in strongly coupled non-conformal FOPTs is a dynamical process of vacuum assignment, hydrodynamics, and local reheating effects, all potentially affecting the approach towards the scaling regime. Lastly, we demonstrate the level of tuning necessary for potentially interesting imprints from gravitational waves through domain wall annihilation and its interplay with the confinement PT.

hep-ph↗

arXiv2025

Bubble Nucleation from Boson Star Collapse

We present a new classical mechanism for nucleation of bubbles of true vacuum. The mechanism arises when dense boson stars form in the false vacuum. As the boson stars collapse due to attractive self-interactions, the field inside the star cores is enhanced beyond the potential barrier. Subsequently the stars explode as true vacuum bubbles, and induce a cosmological phase transition. The mechanism raises the possibility that a vacuum that is stable against quantum tunneling can be vulnerable to ``astrophysical'' processes.

hep-ph↗

arXiv2025

Trapped and Unstable: Axion-like particle fragmentation at finite temperature

We investigate the emergence of a resonant behavior in axion-trapped misalignment models featuring finite-temperature potential barriers. As the temperature decreases and the field is released from its trapped configuration, inhomogeneities are exponentially amplified through an instability in their equation of motion, leading to the fragmentation of the axion field. We show that this process constitutes a novel source of gravitational waves (GWs), analogous to those generated in zero-temperature axion fragmentation, but with distinct characteristics. We quantify the resulting GW spectrum, identifying the peak frequency and amplitude associated with the inhomogeneous axion dynamics. Our results indicate that the GW signal can be enhanced by up to two orders of magnitude compared to the standard fragmentation scenario, while exhibiting a markedly different spectral shape. The parameter space featuring both strong GW signals and reproducing the correct dark matter abundance is, however, limited.

hep-ph↗

arXiv2025

Thermodynamical uncertainties for primordial black holes from cosmological phase transitions

Strongly supercooled first-order phase transitions have been proposed as a primordial black hole (PBH) production mechanism. While previous works rely on simplified models with limited thermodynamic precision, we stress that reliable theoretical PBH predictions require precise nucleation dynamics within realistic extensions of the Standard Model. By employing high-temperature dimensional reduction and computing the one-loop fluctuation determinants, we provide a state-of-the-art thermodynamic analysis and obtain an universal lower bound on the transition timescale, $\beta/H_* \simeq 5$. Then, we estimate the corresponding PBH abundance for classically conformal gauge-Higgs theories. Accounting for constraints from successful percolation and QCD chiral symmetry breaking, the parameter space where PBHs are viable dark matter candidates is severely limited.

hep-ph↗

arXiv2023

Primordial gravitational waves in the nano-Hertz regime and PTA data -- towards solving the GW inverse problem

In recent years, several pulsar timing array collaborations have reported first hints for a stochastic gravitational wave background at nano-Hertz frequencies. Here we elaborate on the possibility that this signal comes from new physics that leads to the generation of a primordial stochastic gravitational wave background. We propose a set of simple but concrete models that can serve as benchmarks for gravitational waves sourced by cosmological phase transitions, domain wall networks, cosmic strings, axion dynamics, or large scalar fluctuations. These models are then confronted with pulsar timing data and with cosmological constraints. With only a limited number of free parameters per model, we are able to identify viable regions of parameter space and also make predictions for future astrophysical and laboratory tests that can help with model identification and discrimination.

hep-ph↗

arXiv2023

One $μ$ to rule them all: CMB spectral distortions can probe domain walls, cosmic strings and low scale phase transitions

We present a new probe of purely gravitationally coupled sectors with large anisotropies. These anisotropies are damped via gravitational interactions with the baryon-photon fluid, which is heated up in the process. The injected heat causes measurable distortions of the cosmic microwave background spectrum. We give analytic estimates for the size of the distortions and outline how to calculate them from first principles. These methods are applied to anisotropies in the form of a domain wall/cosmic string network or caused by a first order phase transition or scalar field dynamics. We find that this method can potentially probe large regions of previously unconstrained parameter space and is very much complementary to up-coming searches of gravitational waves caused by such dark sectors.

hep-ph↗

arXiv2023

Echo of the Dark: gravitational waves from dark SU(3) Yang-Mills theory

We analyze the phase transition in improved holographic QCD to obtain an estimate of the gravitational wave signal emitted in the confinement transition of a pure SU(3) Yang-Mills dark sector. We derive the effective action from holography and show that the energy budget and duration of the phase transition can be calculated with minor errors. These are used as input to obtain a prediction of the gravitational wave signal. To our knowledge, this is the first computation of the gravitational wave signal in a holographic model designated to match lattice data on the thermal properties of pure Yang-Mills.

hep-ph↗

arXiv2022

QCD Axion Kinetic Misalignment without Prejudice

The axion field, the angular direction of the complex scalar field associated with the spontaneous symmetry breaking of the Peccei-Quinn (PQ) symmetry, could have originated with initial non-zero velocity. The presence of a non-zero angular velocity resulting from additional terms in the potential that explicitly break the PQ symmetry has important phenomenological consequences such as a modification of the axion mass with respect to the conventional PQ framework or an explanation for the observed matter-antimatter asymmetry. We elaborate further on the consequences of the "kinetic misalignment" mechanism, assuming that axions form the entirety of the dark matter abundance. The kinetic misalignment mechanism possesses a {\it weak} limit in which the axion field starts to oscillate at the same temperature as in the conventional PQ framework, and a {\it strong} limit corresponding to large initial velocities which effectively delay the onset of oscillations. Following a UV-agnostic approach, we show how this scenario impacts the formation of axion miniclusters, and we sketch the details of these substructures along with potential detecting signatures.

hep-ph↗

arXiv2021

The QCD Axion and Gravitational Waves in light of NANOGrav results

The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration has recently reported strong evidence for a stochastic process affecting the 12.5 yr dataset of pulsar timing residuals. We show that the signal can be interpreted in terms of a stochastic gravitational wave background emitted from a network of axionic strings in the early Universe. The spontaneous breaking of the Peccei-Quinn symmetry originate the axionic string network and the QCD axion, the dark matter particle in the model. We explore a non-standard cosmological model driven by an exotic scalar field $ϕ$ which evolves under the influence of a self-interacting potential; the axion field starts to oscillate during the modified cosmology, and provides the dark matter observed. For an equation of state $w_ϕ< 1/3$, the QCD axion mass is smaller than expected in the standard cosmology and the GW spectrum from axionic strings is larger. We assess the parameter space of the model which is consistent with the NANOGrav-$12.5\,$yr detection, which can be explained within 95\% limit by a QCD axion field evolving in a dust-like scenario, as well as within 68\% limit in a cosmology with $w_ϕ< 0$.

astro-ph.CO↗

arXiv2019

Probing the Early Universe with Axion Physics and Gravitational Waves

We show results for the expected reach of the network of experiments that is being set up globally with the aim of detecting the "invisible" axion, in light of a non-standard thermal history of the universe. Assuming that the axion is the dark matter, we discuss the reach of a successful detection by a given experimental setup in a particular axion mass window for different modifications of the cosmological background before primordial nucleosynthesis occurred. Results are presented both in the case where the present energy budget in cold axions is produced through the vacuum realignment mechanism alone, or in the case in which axionic strings also provide with additional contributions to the axion energy density. We also show that in some cosmological models, the spectrum of gravitational waves from the axionic string network would be within reach of the future network of detectors like LISA and DECIGO-BBO. We conclude that some scenarios describing the early universe can be probed jointly by the experimental efforts on axion detection and by gravity wave multi-messenger astronomy.

astro-ph.CO↗

arXiv2019

Environment-assisted holonomic quantum maps

Holonomic quantum computation uses non-Abelian geometric phases to realize error resilient quantum gates. Nonadiabatic holonomic gates are particularly suitable to avoid unwanted decoherence effects, as they can be performed at high speed. By letting the computational system interact with a structured environment, we show that the scope of error resilience of nonadiabatic holonomic gates can be widened to include systematic parameter errors. Our scheme maintains the geometric properties of the evolution and results in an environment-assisted holonomic quantum map that can mimic the effect of a holonomic gate. We demonstrate that the sensitivity to systematic errors can be reduced in a proof-of-concept spin-bath model.

quant-ph↗