We explore a minimal scenario where the sole Standard-Model Higgs is responsible for reheating the Universe after inflation, produces a significant background of gravitational waves and maintains the full classical stability of the electroweak vacuum. As the Higgs self-coupling runs toward negative values at high energy scales, a non-minimal interaction with curvature during a stiff background expansion era drives the Higgs fluctuations closer to the instability scale. This curvature-induced tachyonic instability leads to an intense production of Higgs particles, accompanied by a stochastic gravitational-wave background. The characteristic features of such signal can be directly correlated to the inflationary scale, the non-minimal coupling parameter and the top-quark Yukawa coupling. We distinguish between three possible scenarios: absolute stability with low top-quark masses, potential vacuum instability, and absolute stability with new physics above the instability scale. Our findings suggest that the detection of a peaked background of gravitational waves together with its inflationary tail has the potential to unveil the features of the Higgs effective potential at very high energy scales while providing a minimal explanation for the reheating phase and the emergence of the Standard-Model plasma in the early Universe. Unlike other studies in the literature, the generation of gravitational waves in our scenario does not depend on the quantum instability of the Standard Model vacuum.

Portugal

Giorgio Laverda

Spain

Javier Rubio

Higgs-induced gravitational waves: the interplay of non-minimal couplings, kination and top quark mass

The transition from the end of inflation to a hot, thermal Universe, commonly referred to as (re)heating, is a critical yet often misunderstood phase in early Universe cosmology. This short review aims to provide a comprehensive, conceptually clear, and accessible introduction to the physics of (re)heating, tailored to the particle physics community. We critically examine the standard Boltzmann approach, emphasizing its limitations in capturing the intrinsically non-perturbative and non-linear dynamics that dominate the early stages of energy transfer. These include explosive particle production, inflaton fragmentation, turbulence, and thermalization; phenomena often overlooked in perturbative treatments. We survey a wide range of theoretical tools, from Boltzmann equations to lattice simulations, clarifying when each is applicable and highlighting scenarios where analytic control is still feasible. Special attention is given to model-dependent features such as (pre)heating, the role of fermions, gravitational couplings, and the impact of multifield dynamics. We also discuss exceptional cases, including Starobinsky-like models and instant (pre)heating, where (re)heating proceeds through analytically tractable channels without requiring full non-linear simulations. Ultimately, this review serves both as a practical guide and a cautionary tale, advocating for a more nuanced and physically accurate understanding of this pivotal epoch within the particle physics community.

India

Basabendu Barman

United Arab Emirates

Nicolás Bernal

Two or three things particle physicists (mis)understand about (pre)heating

We consider a mechanism for the production of scalar dark matter during a first-order phase transition in the very early Universe. Seeking for a dark matter energy density in agreement with observations, we study different renormalizable and non-renormalizable interactions between the dark matter species and the field undergoing the transition, considering all possible regimes for the Lorentz boost factor associated with the motion of the bubble wall. By employing a combination of analytical and numerical techniques, we demonstrate that sufficient dark matter production is achievable even in the previously unexplored low-velocity bubble expansion regime, enlarging the parameter space and possibilities of the scenario. Notably, for the non-renormalizable interactions it is found that the produced dark matter abundances exhibit a similar qualitative behavior to the renormalizable case, even for low Lorentz boost factors. Furthermore, for a transition around the electroweak scale, the associated gravitational wave spectrum is within the reach of future detectors.

Jose Cembranos

Jesús Luque

Scalar dark matter production through the bubble expansion mechanism: the role of the Lorentz factor and non-renormalizable interactions

Sterile neutrinos are a compelling candidate for generating neutrino masses and for elucidating the nature of dark matter. Astrophysical X-ray constraints on sterile neutrino dark matter decays, however, largely exclude the active-sterile mixing required to produce $\textit{simultaneously}$ the correct left-handed neutrino spectrum and keV-scale right-handed neutrino dark matter within a type-I seesaw framework. In this study, we demonstrate how these X-ray constraints can be circumvented through a time-dependent approach, thereby reviving a broad range of active-sterile mixing scenarios. Our minimal model incorporates two right-handed neutrinos, which form a two-component dark matter candidate, and an auxiliary scalar field that experiences a very late and still ongoing phase transition, leading to the spontaneous breaking of a global U(1) symmetry. Prior to this phase transition, only the right-handed neutrinos are massive, while the left-handed neutrinos remain massless because of the scalar field’s vanishing expectation value. As the phase transition develops, the growing expectation value of the scalar field increases the active-sterile mixing, thereby opening dark matter decay channels and inducing neutrino masses. The time dependence allows the scenario to be consistent with X-ray constraints as well as current measurements of left-handed neutrino masses. The anticipated level of active-sterile mixing today is within the detection capabilities of the forthcoming TRISTAN (KATRIN) tritium-beta decay project. Additionally, cosmological surveys such as DESI or EUCLID and supernova neutrino observations can test the prediction of massless left-handed neutrinos prior to the phase transition.

Germany

Florian Goertz

Maya Hager

Phasing out of darkness: from sterile neutrino dark matter to neutrino masses via time-dependent mixing

In this paper we investigate the vacuum stability of the non-minimally coupled Standard-Model Higgs during a phase of kinetic domination following the end of inflation. The non-minimal coupling to curvature stabilises the Higgs fluctuations during inflation while driving them towards the instability scale during kination, when they can classically overcome the potential barrier separating the false electroweak vacuum from the true one at super-Planckian field values. Avoiding the instability of the Standard-Model vacuum sets an upper bound on the inflationary scale that depends both on the strength of the non-minimal interaction and on the top quark Yukawa coupling. Classical vacuum stability is guaranteed if the gravitationally-produced energy density is smaller than the height of barrier in the effective potential. Interestingly enough, thanks to the explosive particle production in the tachyonic phase, the Higgs itself can be also appointed to the role of reheaton field responsible for the onset of the hot Big Bang era, setting an additional lower bound on the inflationary scale   <math> <msub> <mi>H</mi> <mi>inf</mi> </msub> </math>  $$ {\mathcal{H}}_{\textrm{inf}} $$  ≳ 105.5 GeV. Overall, these constraints favour lower masses for the top quark, in agreement with the current measurements of the top quark pole mass. We perform our analysis semi-analytically in terms of the one-loop and three-loop running of the Standard-Model Higgs self-coupling and make use of lattice-based parametric formulas for studying the (re)heating phase derived in arXiv:2307.03774. For a specific choice of m  t  = 171.3 GeV we perform also an extensive numerical scanning of the parameter space via classical lattice simulations, identifying stable/unstable regions and supporting the previous analytical arguments. For this fiducial value, the heating of the Universe is achieved at temperatures in the range 10−2–109 GeV.

The rise and fall of the Standard-Model Higgs: electroweak vacuum stability during kination

The Eisenhart lift allows formulating the dynamics of a scalar field in a potential as pure geodesic motion in a curved field-space manifold involving an additional fictitious vector field. Making use of the formalism in the context of inflation, we show that the main inflationary observables can be expressed in terms of the geometrical properties of a two-dimensional uplifted field-space manifold spanned by the time derivatives of the scalar and the temporal component of the vector. This allows to abstract from specific potentials and models and describe inflation solely in terms of the flow of geometric quantities. Our findings are illustrated through several inflationary examples previously considered in the literature.

Georgios K. Karananas

Israel

Marco Michel

The geometry of inflationary observables: Lifts, flows, equivalence classes

The detection of a stochastic Gravitational Wave (GW) background sourced by a cosmological phase transition would allow us to see the early Universe from a completely new perspective, illuminating aspects of Beyond the Standard Model (BSM) physics and inflationary cosmology. In this study, we investigate whether the evolution of the scalar potential of a minimal SM extension after inflation can lead to a strong first-order phase transition. In particular, we focus on a BSM spectator scalar field that is non-minimally coupled to gravity and has a dynamical double-well potential. As inflation ends, the potential barrier diminishes due to the evolution of the curvature scalar. Therefore, a phase transition can proceed through the nucleation of true-vacuum bubbles that collide as they fill the Universe and produce GWs. We consider high and low scales of inflation, while also taking into account a kination period between inflation and the onset of radiation domination. With this prescription, we showcase a proof-of-concept study of a new triggering mechanism for BSM phase transitions in the early Universe, whose GW signatures could potentially be probed with future detectors.

Poland

Maciej Kierkla

Marek Lewicki

Andreas Mantziris

Matteo Piani

Mateusz Zych

From Hubble to Bubble

We point out that the successful generation of the electroweak scale via gravitational instanton configurations in certain scalar-tensor theories can be viewed as the aftermath of a simple requirement: the existence of a quadratic pole with a sufficiently small residue in the Einstein-frame kinetic term for the Higgs field. In some cases, the inflationary dynamics may also be controlled by this residue and therefore related to the Fermi-to-Planck mass ratio, up to possible uncertainties associated with the instanton regularization. We present here a unified framework for this hierarchy generation mechanism, showing that the aforementioned residue can be associated with the curvature of the Einstein-frame target manifold in models displaying spontaneous breaking of dilatations. Our findings are illustrated through examples previously considered in the literature.

Finland

One residue to rule them all: Electroweak symmetry breaking, inflation and field-space geometry

It has been argued that oscillatory features from spectator fields in the primordial power spectrum could be a probe of alternatives to inflation. In this work, we soften this claim by showing that the frequency and amplitude dependence of the patterns appearing in these scenarios could be mimicked by field interactions during inflation. The degeneracy of the frequency holds for the n-point correlation functions, while the degeneracy of the amplitude is broken at the level of non-gaussianities.

Guillem Domènech

Julius Wons

Mimicking features in alternatives to inflation with interacting spectator fields

We present a novel Affleck–Dine scenario for the generation of the observed baryon asymmetry of the Universe based on the non-trivial interplay between quintessential inflationary models containing a kinetic dominated post-inflationary era and a non-minimally coupled <math><mi>U</mi><mo>(</mo><mn>1</mn><mo>)</mo></math> field with a weakly broken <math><mi>B</mi><mo>−</mo><mi>L</mi></math> symmetry. The non-minimal coupling to gravity renders heavy the Affleck–Dine field during inflation and avoids the generation of isocurvature fluctuations. During the subsequent kinetic era the Ricci scalar changes sign and the effective mass term of the Affleck–Dine field becomes tachyonic. This allows the field to dynamically acquire a large expectation value. The symmetry of the Affleck–Dine potential is automatically restored at the onset of radiation domination, when the Ricci scalar approximately equals zero. This inverse phase transition results in the coherent oscillation of the scalar field around the origin of its effective potential. The rotation of the displaced Affleck–Dine field in the complex plane generates a non-zero <math><mi>B</mi><mo>−</mo><mi>L</mi></math> asymmetry which can be eventually converted into a baryon asymmetry via the usual transfer mechanisms.