Théorie

Soft theorems for boosts and other time symmetries

I will derive new classes of soft theorems for theories in which time symmetries (i.e., symmetries that involve the transformation of time, an example of which are Lorentz boosts) are spontaneously broken. The soft theorems involve unequal-time correlation functions with the insertion of a soft Goldstone in the far past. I will discuss explicit examples, which include the effective theory of a relativistic superfluid and inflationary cosmology.

Selecting Horndeski theories without apparent symmetries and their black hole solutions

Since the no-scalar-hair theorems of the 1970s, it has long been thought that four-dimensional, asymptotically flat black holes cannot support any kind of non-minimally coupled real scalar hair, if not for the controversial Bocharova-Bronnikov-Melnikov-Bekenstein (BBMB) black hole. However, the 2010s have seen renewed interest in the healthy, higher-order scalar-tensor theories which were described by Horndeski in 1974, and easily escape the assumptions of the no-hair arguments.

From the tabletop to the Big Bang: Quantum simulators of false vacuum decay

False vacuum decay (FVD) plays a vital role in many models of the early Universe, with important implications for inflation, the multiverse, and gravitational waves. However, we still lack a satisfying theoretical understanding of this process, with existing approaches working only in imaginary (Euclidean) time, and relying on numerous assumptions that have yet to be empirically tested. An exciting route forward is to use laboratory experiments which undergo transitions analogous to FVD, allowing nature to simulate all of the non-perturbative quantum effects for us.

Primordial Magnetic Fields in Cosmic Web and Galaxy Clusters

Magnetic fields are ubiquitous on astrophysical and cosmological scales: from planets and stars to galaxies and galaxy clusters. Different observational methods infer a field strength of the order of microGauss and coherence scales reaching a few tens of kiloparsecs in galaxy clusters. Despite their ubiquity, the origin of these fields still remains unknown. It is commonly assumed that the observed fields are originated from either astrophysical or cosmological (primordial) weak seed magnetic fields that undergo efficient growth during structure formation.

Relativistic superfluids, and the connection between finite density and spontaneous symmetry breaking for interacting scalar fields

We study the low-energy effective action for relativistic superfluids obtained by integrating out the heavy fields of a UV theory. A careful renormalization procedure is required if one is interested in deriving the effective theory to all orders in the light fields (but still to fixed order in the derivative expansion). The result suggests a general relation between finite density and spontaneous symmetry breaking for QFTs of interacting scalars with an internal global symmetry.

Towards the detection of nonclassical primordial waves

One of the cornerstones of inflationary cosmology is that the large
scale structure of the Universe has a quantum mechanical origin. This
invites the question of whether compelling observational evidence for
the quantum nature of the origin can be found. Primordial
gravitational waves are generated directly from quantum fluctuations
during inflation. Since they interact with matter very weakly, travel
through the Universe virtually unimpeded, it is expected that they
keep their nonclassicality until today. In this talk, I will present

Positivity Constraints on Lorentz-breaking EFTs

The coefficients of the operators of an effective field theory (EFT) are constrained to satisfy certain inequalities, under the (mild) assumption that the UV completion satisfies general requirements of causality and unitarity. We begin the extension of these ideas to theories where the Lorentz symmetry is spontaneously broken, as it happens in cosmology and condensed matter physics. Constraints are derived using dispersive arguments for the 2-point function of conserved currents and of the stress-energy tensor.

Quantum effects on the inner (Cauchy) horizon of rotating black holes

All black holes in the Universe are believed to be rotating. This poses interesting questions, since rotating black hole solutions of Einstein’s equations of General Relativity possess a so-called Cauchy horizon in their interior, which threatens the predictability of Einstein’s theory. However, these exact solutions may not model sufficiently accurately black holes in Nature, which have classical matter in their neighbourhood and, furthermore, are inevitably surrounded by a quantum vacuum (which is responsible for Hawking radiation).

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