Gravitational waves from a supercooled electroweak phase transition and their detection with pulsar timing arrays

We investigate the properties of a stochastic gravitational wave background produced by a first-order electroweak phase transition in the regime of extreme supercooling. We study a scenario whereby the percolation temperature that signifies the completion of the transition, $T_p$, can be as low as a few MeV (nucleosynthesis temperature), while most of the true vacuum bubbles are formed much earlier at the nucleation temperature, $T_n\sim 50$ GeV. This implies that the gravitational wave spectrum is mainly produced by the collisions of large bubbles and characterised by a large amplitude and a peak frequency as low as $f \sim 10^{-9}-10^{-7}$ Hz. We show that such a scenario can occur in (but not limited to) a model based on a non-linear realisation of the electroweak gauge group, such that the Higgs vacuum configuration is altered by a cubic coupling. In order to carefully quantify the evolution of the phase transition of this model over such a wide temperature range, we go beyond the usual fast transition approximation, taking into account the expansion of the Universe as well as the behaviour of the nucleation probability at low temperatures. Our computation shows that there exists a range of parameters for which the gravitational wave spectrum lies at the edge between the exclusion limits of current pulsar timing array experiments and the detection band of the future Square Kilometre Array observatory.Comment: 20 pages, 3 figures. V2: references added and corrections to match published versio

Low temperature electroweak phase transition in the Standard Model with hidden scale invariance

We discuss a cosmological phase transition within the Standard Model which incorporates spontaneously broken scale invariance as a low-energy theory. In addition to the Standard Model fields, the minimal model involves a light dilaton, which acquires a large vacuum expectation value (VEV) through the mechanism of dimensional transmutation. Under the assumption of the cancellation of the vacuum energy, the dilaton develops a very small mass at 2-loop order. As a result, a flat direction is present in the classical dilaton-Higgs potential at zero temperature while the quantum potential admits two (almost) degenerate local minima with unbroken and broken eletroweak symmetry. We found that the cosmological electroweak phase transition in this model can only be triggered by a QCD chiral symmetry breaking phase transition at low temperatures, $T\lesssim 132$ MeV. Furthermore, unlike the standard case, the universe settles into the chiral symmetry breaking vacuum via a first-order phase transition which gives rise to a stochastic gravitational background with a peak frequency $\sim 10^{-8}$ Hz as well as triggers the production of approximately solar mass primordial black holes. The observation of these signatures of cosmological phase transitions together with the detection of a light dilaton would provide a strong hint of the fundamental role of scale invariance in particle physics

Gravitational Waves and Fundamental Physics

This thesis investigates the implications of gravitational waves (GWs) for particle physics and cosmology. We first give an overview of the current state of general relativity and quantum field theory. We also emphasize where GWs may come into play to shed new light on unsolved problems in physics. First, we make use of GWs to constrain the scale of non-commutative space-time. Assuming such quantum fuzziness, we compute the equations of motion of a binary black hole and the associated generation of GWs. Compared to general relativity, leading non-commutative effects produce a post-Newtonian correction of order (v/c)^4. Using the recent GW150914 signal, we find that the scale of non-commutativity is bounded to be below or at the order of the Planck scale. This represents an improvement of ~15 orders of magnitude compared to previous constraints. Second, we study the production of GWs from cosmological phase transitions. We consider two unrelated extensions of the standard model: a non-linear realization of the electroweak gauge group and a model with hidden scale invariance. In the first case, the Higgs vacuum configuration is altered by a cubic coupling giving the possibility to have a strong and prolonged electroweak first-order transition. In our second model, the electroweak transition cannot proceed until it is triggered by a first-order QCD chiral symmetry breaking around 130 MeV. We compute that the stochastic GW background produced during these two phase transitions is expected to be in the detection range of pulsar timing arrays. Finally, we investigate the backreaction of particle production on false vacuum decay. We present a formalism which makes use of the reduced density matrix of the system to quantify the impact of these particles on the decay rate of a scalar field in flat space-time. We then apply this method to a toy model potential and we exhibit different scenarios with either significant or negligible backreaction

Backreaction of particle production on false vacuum decay

As originally described by Rubakov, particles are produced during the tunneling of a metastable quantum field. We propose to extend his formalism to compute the backreaction of these particles on the semiclassical decay probability of the field. The idea is to integrate out the external bath of particles by computing the reduced density matrix of the system. Following this approach, we derive an explicit correction factor in the specific case of scalar particle production in flat spacetime. In this given framework, we conclude that the backreaction is ultraviolet finite and enhances the decay rate. Moreover, in the weak production limit, the backreaction factor is directly given by one half of the total number of created particles. In order to estimate the importance of this correction, we apply our formalism to a toy model potential which allows us to consider both the decay of a homogeneous bounce and the nucleation of a thin-wall bubble. In the former case, the impact of the created particles is parameter dependent and we exhibit a reasonable choice of variables for which ones the backreaction is significant. In the latter case, we conclude that the backreaction is always negligible

Réflexion et méthodologie: autour d’une prise d’empreinte en papier

Un gisant gravé de la fin du XIIIe siècle a été découvert sous le chœur de la basilique Notre-Dame de Fribourg. Les questions éthiques liées à la prise d’empreinte et à la reproduction de biens culturels sont soulignées. Les auteurs portent un regard critique sur les reproductions et voient la prise d’empreinte comme un support documentaire. L’importance de la découverte de Notre-Dame a conduit à sa dépose. Le moulage de la pièce a été effectué pour assurer sa documentation. Une méthode de moulage empruntée aux épigraphistes a été utilisée: l‘estampage avec du papier buvard. L’adaptation de cette technique de prise d’empreinte a été nécessaire. Le démoulage a été favorisé par l’ajout de cyclododécane sous une stratification de papier, plâtre renforcé, mousse expansée et bois.A stone recumbent figure from the end of the 13th century has been discovered in the chancel of the Notre Dame basilica in Fribourg. In this article, the ethical questions related to the replication or casting of cultural heritage witnesses are underscored. The authors raise their skepticism about reproductions and consider molds as a simple documentary support. The discovered figure has been lifted from its original location. A mold has been applied for documentary purposes. The molding method has been inspired by the epigraphist’s method: a stamping with blotting paper. This technology has been adapted to the current circumstances. The mold removal has been enhanced by adding cyclododecan under an overall compound made of plaster paper, expanded foam and wooden structure

Age-associated differences in the cancer molecular landscape.

Cancer is an age-related disease, as incidence and mortality for most types of cancer increase with age. However, how molecular alterations in tumors differ among patients of different ages remains poorly understood. Recent studies have shed light on the age-associated molecular landscapes in cancer. Here, we summarize the main findings of these current studies, highlighting major differences in the genomic, transcriptomic, epigenetic, and immunological landscapes between cancer in younger and older patients. Importantly, some cancer driver genes are mutated more frequently in younger or older patients. We discuss the potential roles of aging-related processes in shaping these age-related differences in cancer. We further emphasize the remaining unsolved questions that could provide important insights that will have implications in personalized medicine

Intercellular communication analysis of the human retinal pigment epithelial and choroidal cells predicts pathways associated with aging, cellular senescence and age-related macular degeneration.

The retinal pigment epithelium (RPE) and the choroid are ocular tissues with fundamental roles in supporting neuroretinal function. The pathogenesis of age-related macular degeneration (AMD), a leading cause of irreversible blindness for which aging is the highest risk factor is closely linked with progressive impairment of various functions of these tissues. Cellular senescence, marked by cell cycle arrest and secretion of proinflammatory factors, is known to be associated with aging and has been proposed as a potential driver of AMD. Here, we investigated the role played by intercellular communication in the RPE/choroid within the context of aging, senescence and AMD. We inferred cell-cell interactions in the RPE/choroid by applying CellChat and scDiffCom on a publicly available scRNA-seq dataset from three human donors with and without AMD. We identified age-regulated ligand and receptor genes by using limma on a separate publicly available bulk microarray dataset providing RPE/choroid samples at multiple time points. Cellular senescence was investigated by assigning a score to each cell and each sample of these scRNA-seq and microarray datasets, respectively, based on the expression of key signature genes determined by a previous senescence meta-analysis. We identified VEGF-, BMP-and tenascin-mediated pathways supporting some of the strongest cell-cell interactions between RPE cells, fibroblasts and choroidal endothelial cells and as strong intercellular communication pathways related to both aging and senescence. Their signaling strength was enhanced between subpopulations of cells having high senescence scores. Predominant ligands of these pathways were upregulated with age whereas predominant receptors were downregulated. Globally, we also observed that cells from AMD samples presented slightly bigger senescence scores than normal cells and that the senescence score positively correlated with age in bulk samples (R = 0.26, value of p < 0.01). Hence, our analysis provides novel information on RPE/choroid intercellular communication that gives insights into the connection between aging, senescence and AMD