This page lists all my works in the academic literature. This includes published works (such as papers, theses, etc), preprints, and some other documents available in the arXiv, for example. Clicking on a title will take you to a page summarizing the main results.
Níckolas de Aguiar Alves and André G. S. Landulfo
Aguiar Alves, Níckolas de, and André G. S. Landulfo. 2025. “The Sky as a Killing Horizon.” arXiv: 2504.12514 [gr-qc].
Abstract: Symmetries are ubiquitous in modern physics. They not only allow for a more simplified description of physical systems but also, from a more fundamental perspective, can be seen as determining a theory itself. In the present paper, we propose a new definition of asymptotic symmetries that unifies and generalizes the usual notions of symmetry considered in asymptotically flat spacetimes and expanding universes with cosmological horizons. This is done by considering BMS-like symmetries for "asymptotic (conformal) Killing horizons", or A(C)KHs, here defined as null hypersurfaces that are tangent to a vector field satisfying the (conformal) Killing equation in a limiting sense. The construction is theory-agnostic and extremely general, for it makes no use of the Einstein equations and can be applied to a wide range of scenarios with different dimensions or hypersurface cross sections. While we reproduce the results by Dappiaggi, Moretti, and Pinamonti in the case of asymptotic Killing horizons, the conformal generalization does not yield only the BMS group, but a larger group. The enlargement is due to the presence of "superdilations". We speculate on many implications and possible continuations of this work, including the exploration of gravitational memory effects beyond general relativity, understanding antipodal matching conditions at spatial infinity in terms of bifurcate horizons, and the absence of superrotations in de Sitter spacetime and Killing horizons.
Tags: General Relativity, Infrared Structure, and Quantum Field Theory in Curved Spacetime
Níckolas de Aguiar Alves
Aguiar Alves, Níckolas de. 2025. “Lectures on the Bondi–Metzner–Sachs group and related topics in infrared physics.” arXiv: 2504.12521 [gr-qc].
Abstract: These are the extended lecture notes for a minicourse presented at the I São Paulo School on Gravitational Physics discussing the Bondi–Metzner–Sachs (BMS) group, the group of symmetries at null infinity on asymptotically flat spacetimes. The BMS group has found many applications in classical gravity, quantum field theory in flat and curved spacetimes, and quantum gravity. These notes build the BMS group from its most basic prerequisites (such as group theory, symmetries in differential geometry, and asymptotic flatness) up to modern developments. These include its connections to the Weinberg soft graviton theorem, the memory effect, its use to construct Hadamard states in quantum field theory in curved spacetimes, and other ideas. Advanced sections briefly discuss the main concepts behind the infrared triangle in electrodynamics, superrotations, and the Dappiaggi–Moretti–Pinamonti group in expanding universes with cosmological horizons (or "asymptotically de Sitter spacetimes"). New contributions by the author concerning asymptotic (conformal) Killing horizons are discussed at the end.
Tags: General Relativity, Infrared Structure, and Quantum Field Theory in Curved Spacetime
Níckolas de Aguiar Alves and Bruno Arderucio Costa
Aguiar Alves, Níckolas de, and Bruno Arderucio Costa. 2025. “The Measure of a Mass.” Essay written for the Gravity Research Foundation 2025 Awards for Essays on Gravitation. arXiv: 2503.18963 [gr-qc].
Abstract: The concept of mass is central to any theory of gravity. Nevertheless, defining mass in general relativity is a difficult task, and even when it can be accomplished, we still need to investigate whether the typical properties of mass in Newtonian gravity are still true in Einsteinian gravity. In this essay, we discuss "the measure of a mass" in relativity by considering some of the many different definitions (Komar, ADM, and Bondi) and how they are related. Finally, we discuss when and whether the mass is positive, as is usually expected, and which physical properties of matter and gravity can ensure this result.
Tags: General Relativity and Energy Conditions
Níckolas de Aguiar Alves, André G. S. Landulfo, and Bruno Arderucio Costa
Aguiar Alves, Níckolas de, André G. S. Landulfo, and Bruno Arderucio Costa. 2025. “Positive Mass in General Relativity Without Energy Conditions.” Physical Review D 111 (4): 044027. arXiv: 2408.00154 [gr-qc].
Abstract: A long-standing problem in physics is why observed masses are always positive. While energy conditions in quantum field theory can partly answer this problem, in this paper we find evidence that classical general relativity abhors negative masses, without the need for quantum theory or energy conditions. This is done by considering many different models of negative-mass "stars" and showing they are dynamically unstable. A fortiori, we show that any barotropic negative-mass star must be dynamically unstable.
Tags: General Relativity and Energy Conditions
Níckolas de Aguiar Alves
Aguiar Alves, Níckolas de (2023). “Nonperturbative Aspects of Quantum Field Theory in Curved Spacetime.” MSc thesis. Santo André, Brazil: Federal University of ABC. xxiv, 152 pp. arXiv: 2305.17453 [gr-qc].
Abstract: Quantum field theory in curved spacetime is perhaps the most reliable framework in which one can investigate quantum effects in the presence of strong gravitational fields. Nevertheless, it is often studied by means of perturbative treatments. In this thesis, we aim at using the functional renormalization group—a nonperturbative realization of the renormalization group—as a technique to describe nonperturbative quantum phenomena in curved spacetimes. The chosen system is an Unruh–DeWitt particle detector coupled to a scalar quantum field. We discuss how to formulate such a system in terms of an action and how one can compute its renormalization group flow in the case of an inertial detector in flat spacetime, for simplicity. We learn, however, that the results are divergent in the limit in which the detector's energy gap vanishes. Possible workarounds are discussed at the end.
This thesis also presents a review of quantum field theory in curved spacetimes by means of the algebraic approach, although it assumes no previous experience with functional analysis. Hence, it fills a pedagogical gap in the literature. Furthermore, we also review the functional renormalization group and derive the Wetterich equation assuming a general field content that might include both bosonic and fermionic fields. Such a derivation is also hardly found in pedagogical introductions available in the high energy physics literature.
Tags: Quantum Field Theory in Curved Spacetime, Algebraic Quantum Field Theory, and Functional Renormalization Group