arXiv subjects

João F. Melo

Publications and source records attributed to João F. Melo.

5 recordsLinked to original sources

arXiv2025

EFT & Species Scale: Friends or foes?

Recently the notion that quantum gravity effects could manifest at scales much lower than the Planck scale has seen an intense Swamplandish revival. Dozens of works have explored how the so-called species scale -- at which an effective description of gravity must break down -- relates to String Theory and the Swampland conjectures. In particular, the interplay between this scale and the abundant towers of states becoming lighter in asymptotic regions of moduli spaces has proved to be key in understanding the real scale of quantum gravity. Nevertheless concerns have been raised regarding the validity of using infinite towers of states when estimating this scale within Effective Field Theory and, more precisely, the consistency of cutting the tower part way through in a framework that relies on a clear separation of scales. In this work we take an EFT point-of-view and provide a detailed perturbative derivation of the species scale -- by computing the 1-loop graviton propagator in the presence of many fields -- thereby clarifying common sources of confusion in the literature. Not only do we clarify the setup, assumptions and regimes of validity of the result, but more importantly apply the same methods to an infinite tower of states. We show how each state in the tower contributes to the species scale and how the procedure of counting only ''light fields'' can be compatible with not cutting the tower, thereby maintaining the harmony between infinite towers and EFTs even in the context of the species scale.

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arXiv2023

The Propagator Matrix Reloaded

The standard way to perform calculations for quantum field theories involves the S-matrix and the assumption that the theory is free at past and future infinity. However, this assumption may not hold for field theories in non-trivial backgrounds such as curved spacetimes or finite temperature. In fact, even in the simple case of finite temperature Minkowski spacetime, there are a lot of misconceptions and confusion in the literature surrounding how to correctly take interactions into account when setting up the initial conditions. The objective of this work is to clear up these misconceptions and provide a clean and simple derivation of a formalism which includes interactions in the initial conditions and assesses whether or not it is legitimate to ignore them. The ultimate conclusion is that we cannot ignore them: quantum field theories at finite temperature are not free in the infinite past.

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arXiv2022

Stringy corrections to the entropy of electrically charged supersymmetric black holes with $\mathrm{AdS}_5\times S^5$ asymptotics

We study the leading $α^\prime$ corrections to the entropy of certain black holes with AdS$_5\times S^5$ asymptotics. We find that, in the supersymmetric limit, the entropy does not receive $α^\prime$ corrections. This result strengthens recent calculations that match the index of $\mathcal{N}=4$ Super-Yang-Mills with the corresponding partition function in the supersymmetric limit. In the small temperature regime, we find that the entropy corrections are concordant with the weak gravity conjecture.

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arXiv2020

Developing local RG: quantum RG and BFSS

In this paper we study various forms of RG, applying them to the BFSS model of $N$ coincident D0-branes. Firstly, as a warm-up, we perform standard Wilsonian RG, investigating the conditions under which supersymmetry is preserved along the flow. Next, we develop a local RG scheme such that the cutoff is spacetime dependent, which could have further applications to studying QFT in curved spacetime. Finally, we test the conjecture put forward in arXiv:1305.3908 that the method of quantum RG could be the mechanism responsible for the gauge/gravity duality by applying it to the BFSS model, which has a known gravitational dual. Although not entirely conclusive some questions are raised about the applicability of quantum RG as a description of the AdS/CFT correspondence.

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arXiv2019

Introduction to Renormalisation

These are introductory lecture notes aimed at beginning graduate students covering fundamental concepts and ideas behind the renormalisation group. Our main goal is to motivate it and then explore its consequences, in the context of quantum field theory. We shall reproduce the standard results without needing any mention of cancellation of infinities. To keep the story simple and compact, we'll ignore the role of symmetry in this game. We'll also restrict to scalar fields, and not mention neither fermions nor gauge fields. Moreover, we'll only briefly mention the connection with statistical field theory. Knowledge of quantum mechanics and special relativity is essential. Prior knowledge of quantum field theory is not required, however, this is not designed to be an introduction to QFT, so it may not be the most pedagogical way to introduce the topic. Previous familiarity with path integrals is also not assumed.

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