arXiv subjects

Graham White

Publications and source records attributed to Graham White.

At least 19 recordsLinked to original sources

arXiv2026

Irreducible Gravitational Wave Background as a Particle Detector

Beyond the standard model particles can imprint a temporary early matter-dominated epoch on a stochastic gravitational-wave background. For a freely propagating background whose production has ceased, we show that the resulting feature contains two characteristic frequencies that determine the particle decay rate $\Gamma$ and the abundance-weighted mass $Y_iM$, independently of the microphysical origin of the gravitational-wave source. Using numerical scans, we derive semi-analytic relations between these characteristic frequencies and the BSM particle parameters, and quantify the parameter ranges over which these relations remain valid. For backgrounds that remain actively sourced, the end of early matter domination continues to define a source-independent cosmological frequency scale controlled by $\Gamma$, although the second feature, and hence the inference of $Y_iM$, is source dependent. Once a particle model and production mechanism are specified, the measured combinations can be translated into masses and microscopic couplings. Remarkably, decay lengths targeted by upcoming long-lived-particle searches map directly onto the nanohertz range in which NANOGrav has reported evidence for a stochastic gravitational-wave background.

hep-ph↗

arXiv2025

Worth the Effort? An Examination on the Effect of Higher Diligence Calculations of the Sound Shell Model

The gravitational wave spectrum arising from using the full velocity profile is well known to differ qualitatively from analytic fits to a broken power law. Former studies have shown that unlike the uncertainties arising from thermal field theory, more diligence in the hydrodynamics can sometimes have limited benefit. However, this was shown in the context of broken power law fits. We test the benefits of some recent calculations in modeling the spectrum, including new developments in adjustments of the low frequency tail to be consistent with causality, but we use the full velocity profile. We find the spectral shape information has a heightened sensitivity to the speed of sound which can be demonstrated analytically, however for our benchmark model this still results in a modest difference. The reason for a heightened sensitivity is because the velocity at the boundary is quite sensitive to the speed of sound, which in turn means a small change to the speed of sound can have a large change to the shape of the velocity profile. Furthermore, even modest changes in the product $\alpha \kappa$ can make non-trivial changes to the shape around the peak. Finally, there are many points where adjusting the infrared behavior to be consistent with causality is affecting the spectrum near its peak. All this implies that the spectrum is sensitive to five thermal parameters rather than four which gives hope that an observation of a gravitational wave spectrum from a first order cosmological phase transition could eventually give even more information about the underlying microphysics responsible.

hep-ph↗

arXiv2025

Cosmic Strings Gravitational Wave Probe of Leptogenesis: Thermal, Non-thermal, Near-resonant and Flavourful

Breaking a global or local $U(1)_{\rm B-L}$ symmetry at high scales simultaneously generates Majorana masses for heavy right-handed neutrinos and produces a network of cosmic strings. The evolution and decay of these strings source a stochastic gravitational-wave background that may be probed by current and future gravitational-wave experiments, while the decays of the resulting massive right-handed neutrinos can generate the baryon asymmetry of the Universe via leptogenesis. We derive analytical bounds for successful leptogenesis with a global and a local $U(1)_{B-L}$ symmetry, separately finding an absolute lower bound on the lightest right-handed neutrino mass $M_1 > 1.74 \times 10^{8}\,\mathrm{GeV}$ for thermal initial conditions and $M_1 > \mathcal{O}(10^{6})\,\mathrm{GeV}$ for non-thermal initial conditions. Allowing for near-resonant leptogenesis relaxes these bounds to TeV scale in both cases making it a viable target at collider searches complementing the GW signals. Full flavour effects are included, and crucially, we determine the region where successful leptogenesis can be probed through gravitational-wave observations in upcoming experiments such as LISA and Einstein Telescope. Importantly, we find that flavour effects rescue regions of the parameter space that are ruled out due to current CMB or gravitational wave measurements.

astro-ph.CO↗

arXiv2025

Gravitational Wave Spectral Shapes as a probe of Long Lived Right-handed Neutrinos, Leptogenesis and Dark Matter: Global versus Local B-L Cosmic Strings

The scale of the seesaw mechanism is typically much larger than the electroweak scale. This hierarchy can be naturally explained by $U(1)_{B-L}$ symmetry, which after spontaneous symmetry breaking, simultaneously generates Majorana masses for neutrinos and produces a network of cosmic strings. Such strings generate a gravitational wave (GW) spectrum which is expected to be almost uniform in frequency unless there is a departure from the usual early radiation domination. We explore this possibility in Type I, II and III seesaw frameworks, finding that only for Type-I, long-lived right-handed neutrinos (RHN) may provide a period of early matter domination for parts of the parameter space, even if they are thermally produced. Such a period leaves distinctive imprints in the GW spectrum in the form of characteristic breaks and a knee feature, arising due to the end and start of the periods of RHN domination. These features, if detected, directly determine the mass $M$, and effective neutrino mass $\tilde m$ of the dominating RHN. We find that GW detectors like LISA and ET could probe RHN masses in the range $M\in[0.1,10^{9}]$ GeV and effective neutrino masses in the $\tilde m\in[10^{-10},10^{-8}]$ eV range. We investigate the phenomenological implications of long-lived right-handed neutrinos for both local and global $U(1)_{B-L}$ strings, focusing on dark matter production and leptogenesis. We map the viable and detectable parameter space for successful baryogenesis and asymmetric dark matter production from right-handed neutrino decays. We derive analytical and semi-analytical relations correlating the characteristic gravitational-wave frequencies to the neutrino parameters $\tilde m$ and $M$, as well as to the relic abundances of dark matter and baryons.

astro-ph.CO↗

arXiv2025

Using Gravitational Wave Signals to Disentangle Early Matter Dominated Epochs

Curvature perturbations induce gravitational waves (GWs) at second order, contributing to the stochastic gravitational wave background. The resulting gravitational wave spectrum is sensitive to the evolutionary history of the universe and can be substantially enhanced by early matter-dominated (eMD) epochs, particularly if they end rapidly. Such epochs can be caused by primordial black holes (PBHs) and non-topological solitons (Q-balls), for example. Prior analysis approximated the end of the eMD epoch as instantaneous or used a Gaussian smoothing. In this work, we present a complete analysis fully incorporating their time-evolving decay rates. We demonstrate that the resulting signal spectra from PBH, thin wall Q-ball, thick wall Q-ball, and delayed Q-ball eMD epochs are distinguishable for monochromatic distributions. We then consider log-normal mass distributions and discuss the distinguishability of the various GW spectra. Importantly we find that the change in the spectrum from a finite mass width is qualitatively different from the change arising from a slower transition to radiation domination.

astro-ph.CO↗

arXiv2025

Cosmological Consequences of Domain Walls Biased by Quantum Gravity

One of the simplest standard model extensions leading to a domain wall network is a real scalar $S$ with a $Z_2$ symmetry spontaneously broken during universe evolution. Motivated by the swampland program, we explore the possibility that quantum gravity effects are responsible for violation of the discrete symmetry, triggering the annihilation of the domain wall network. We explore the resulting cosmological implications in terms of dark radiation, dark matter, gravitational waves, primordial black holes, and wormholes connected to baby universes.

hep-ph↗

arXiv2025

Challenges and Opportunities of Gravitational Wave Searches above 10 kHz

The first direct measurement of gravitational waves by the LIGO and Virgo collaborations has opened up new avenues to explore our Universe. This white paper outlines the challenges and gains expected in gravitational-wave searches at frequencies above the LIGO/Virgo band. The scarcity of possible astrophysical sources in most of this frequency range provides a unique opportunity to discover physics beyond the Standard Model operating both in the early and late Universe, and we highlight some of the most promising of these sources. We review several detector concepts that have been proposed to take up this challenge, and compare their expected sensitivity with the signal strength predicted in various models. This report is the summary of a series of workshops on the topic of high-frequency gravitational wave detection, held in 2019 (ICTP, Trieste, Italy), 2021 (online) and 2023 (CERN, Geneva, Switzerland).

gr-qc↗

arXiv2025

Complementarity between Cosmic String Gravitational Waves and long-lived particle searches in a laboratory

Cosmic strings are powerful witnesses to cosmic events including any period of early matter domination. If such a period of matter domination was catalysed by metastable, long-lived particles, then there will be complementary signals to ascertain the nature of dark sector in experiments detecting primordial features in the gravitational wave (GW) power spectrum and laboratory searches for long-lived particles. We give explicit examples of global and local U(1) gauge extended dark sectors to demonstrate such a complementarity as the union of the two experiments reveals more information about the dark sector than either experiment. Demanding that Higgs-portal long-lived scalar be looked for, in various experiments such as DUNE, FASER, FASER-II, MATHUSLA, SHiP, we identify the parameter space which leads to complementary observables for GW detectors such as LISA and ET.

hep-ph↗

arXiv2024

Investigating the Electroweak Phase Transition with a Real Scalar Singlet at a Muon Collider

A strong first-order electroweak phase transition (SFOEWPT) is essential for explaining baryogenesis and for potentially generating observable gravitational waves. This study investigates the potential of a high-energy muon collider to examine the occurrence of SFOEWPT within the context of a Standard Model extended by a real scalar singlet (xSM). We analyzed all possible decay modes of the singlet to constrain the valid parameter space of SFOEWPT, which was extracted numerically at different renormalization scales to account for theoretical uncertainties, thereby determining the sensitivity of a muon collider to the production and decay channels of novel heavy scalar particles that emerge in the xSM. The findings demonstrate that a 3 TeV muon collider can directly examine the nature of electroweak symmetry breaking by efficiently detecting novel scalar particles associated with a first-order electroweak phase transition through jet-rich final states, thus complementing the indirect constraints from gravitational wave experiments.

hep-ph↗

arXiv2024

Baryogenesis and first-order QCD transition with gravitational waves from a large lepton asymmetry

A large primordial lepton asymmetry can lead to successful baryogenesis by preventing the restoration of electroweak symmetry at high temperatures, thereby suppressing the sphaleron rate. This asymmetry can also lead to a first-order cosmic QCD transition, accompanied by detectable gravitational wave (GW) signals. By employing next-to-leading order dimensional reduction we determine that the necessary lepton asymmetry is approximately one order of magnitude smaller than previously estimated. Incorporating an updated QCD equation of state that harmonizes lattice and functional QCD outcomes, we pinpoint the range of lepton flavor asymmetries capable of inducing a first-order cosmic QCD transition. To maintain consistency with observational constraints from the Cosmic Microwave Background and Big Bang Nucleosynthesis, achieving the correct baryon asymmetry requires entropy dilution by approximately a factor of ten. However, the first-order QCD transition itself can occur independently of entropy dilution. We propose that the sphaleron freeze-in mechanism can be investigated through forthcoming GW experiments such as $\mu$Ares.

hep-ph↗

arXiv2024

A Precise Fitting Formula for Gravitational Wave Spectra from Phase Transitions

Obtaining a precise form for the predicted gravitational wave (GW) spectrum from a phase transition is a topic of great relevance for beyond Standard Model (BSM) physicists. Currently, the most sophisticated semi-analytic framework for estimating the dominant contribution to the spectrum is the sound shell model; however, full calculations within this framework can be computationally expensive, especially for large-scale scans. The community therefore generally manages with fit functions to the GW spectrum, the most widely used of which is a single broken power law. We provide a more precise fit function based on the sound shell model: our fit function features a double broken power law with two frequency breaks corresponding to the two characteristic length scales of the problem -- inter-bubble spacing and thickness of sound shells, the second of which is neglected in the single broken power law fit. Compared to previously proposed fits, we demonstrate that our fit function more faithfully captures the GW spectrum coming from a full calculation of the sound shell model, over most of the space of the thermodynamic parameters governing the phase transition. The physical origins of the fit parameters and their dependence on the thermodynamic parameters are studied in the underlying sound shell model: in particular, we perform a series of detailed scans for these quantities over the plane of the strength of the phase transition ($\alpha$) and the bubble wall velocity ($v_w$). Wherever possible, we comment on the physical interpretations of these scans. The result of our study can be used to generate accurate GW spectra with our fit function, given initial inputs of $\alpha$, $v_w$, $\beta/H$ (nucleation rate parameter) and $T_n$ (nucleation temperature) for the relevant BSM scenario.

hep-ph↗

arXiv2024

A first-order deconfinement phase transition in the early universe and gravitational waves

We clarify the conditions of the cosmic quantum chromodynamics (QCD) first-order phase transition in the early universe by carefully distinguishing the chiral and deconfinement phase transitions. While the chiral one with light quarks at zero chemical potential is unlikely to be first order based on the recent lattice QCD calculations, the latter one can be naturally extended with one extra rolling scalar to be first order. The argument is also valid for the dark QCD theory with arbitrary $N_c$ with a wide range of phase transition temperatures, which can be from hundreds of MeV up to beyond TeV. Notably, here we derive the general formula for the deconfinement phase transition potential of SU($N_c$) gauge theory characterized by the Polyakov loop. With the effective potential in hand, the gravitational wave spectrum is then determined via the sound shell model, which then enables us to give for the first time the quantitative analysis of the gravitational wave signals coming from the QCD deconfinement phase transition and awaits the check from future space interferometers.

hep-ph↗

arXiv2024

Using gravitational waves to see the first second of the Universe

Gravitational waves are a unique probe of the early Universe, as the Universe is transparent to gravitational radiation right back to the end of inflation. In this article, we summarise detection prospects and the wide scope of primordial events that could lead to a detectable stochastic gravitational wave background. Any such background would shed light on what lies beyond the Standard Model, sometimes at remarkably high scales. We overview the range of strategies for detecting a stochastic gravitational wave background before delving deep into three major primordial events that can source such a background. Finally, we summarize the landscape of other sources of primordial backgrounds.

hep-ph↗

arXiv2023

Gravitational Wave Signals From Early Matter Domination: Interpolating Between Fast and Slow Transitions

An epoch of matter domination in the early universe can enhance the primordial stochastic gravitational wave signal, potentially making it detectable to upcoming gravitational wave experiments. However, the resulting gravitational wave signal is quite sensitive to the end of the early matter-dominated epoch. If matter domination ends gradually, a cancellation results in an extremely suppressed signal, while in the limit of an instantaneous transition, there is a resonant-like enhancement. The end of the matter dominated epoch cannot be instantaneous, however, and previous analyses have used a Gaussian smoothing technique to account for this, and consider only a limited regime around the fast transition limit. In this work, we present a study of the enhanced gravitational wave signal from early matter domination without making either approximation and show how the signal smoothly evolves from the strongly suppressed to strongly enhanced regimes.

astro-ph.CO↗

arXiv2023

Quantum Gravity Effects on Fermionic Dark Matter and Gravitational Waves

We explore the phenomenological consequences of breaking discrete global symmetries in quantum gravity (QG). We extend a previous scenario where discrete global symmetries are responsible for scalar dark matter (DM) and domain walls (DWs), to the case of fermionic DM, considered as a feebly interacting massive particle, which achieves the correct DM relic density via the freeze-in mechanism. Due to the mixing between DM and the standard model neutrinos, various indirect DM detection methods can be employed to constrain the QG scale, the scale of freeze-in, and the reheating temperature simultaneously. Since such QG symmetry breaking leads to DW annihilation, this may generate the characteristic gravitational wave background, and hence explain the recent observations of the gravitational wave spectrum by pulsar timing arrays. This work therefore highlights a tantalizing possibility of probing the effective scale of QG from observations.

hep-ph↗

arXiv2023

Sphaleron freeze-in baryogenesis with gravitational waves from the QCD transition

A large primordial lepton asymmetry is capable of explaining the baryon asymmetry of the Universe (BAU) through suppression of the electroweak sphaleron rates (``sphaleron freeze-in") which can lead to a first-order cosmic QCD transition with an observable gravitational wave (GW) signal. With next-to-leading order dimensional reduction and the exact 1-loop fluctuation determinant, we accurately compute the lepton asymmetry needed to realize this paradigm, finding it to be an order of magnitude smaller than previous estimates. Further, we apply an improved QCD equation of state capable of describing the phase transition line together with the critical endpoint leading to better agreement with lattice and functional QCD results. Based on this, we identify the range of lepton flavor asymmetries inducing a first-order cosmic QCD transition. We then extract the parameters relevant to the prediction of GW signal from a first-order cosmic QCD transition. Our result showcases the possibility of probing the sphaleron freeze-in paradigm as an explanation of BAU by future gravitational wave experiments like $\mu$Ares.

hep-ph↗

arXiv2023

Testing high scale supersymmetry via second order gravitational waves

Supersymmetry predicts multiple flat directions, some of which carry a net baryon or lepton number. Condensates in such directions form during inflation and later fragment into Q-balls, which can become the building blocks of primordial black holes. Thus supersymmetry can create conditions for an intermediate matter-dominated era with black holes dominating the energy density of the universe. Unlike particle matter, black holes decay suddenly enough to result in an observable gravitational wave signal via the poltergeist mechanism. We investigate the gravitational waves signatures of supersymmetry realized at energy scales that might not be accessible to present-day colliders.

hep-ph↗

arXiv2023

Quantum Gravity Effects on Dark Matter and Gravitational Waves

We explore how quantum gravity effects, manifested through the breaking of discrete symmetry responsible for both Dark Matter and Domain Walls, can have observational effects through CMB observations and gravitational waves. To illustrate the idea we consider a simple model with two scalar fields and two $\mathcal{Z}_2$ symmetries, one being responsible for Dark Matter stability, and the other spontaneously broken and responsible for Domain Walls, where both symmetries are assumed to be explicitly broken by quantum gravity effects. We show the recent gravitational wave spectrum observed by several pulsar timing array projects can help constrain such effects.

hep-ph↗