About

Liam McAllister is a professor in the Department of Physics at Cornell University, with a research focus on string theory, which combines quantum field theory and gravity in a consistent framework. His work aims to understand the early universe and develop compactifications of string theory that lead to realistic four-dimensional physics. He investigates the interface between string theory and inflationary cosmology, exploring how string theory can provide a framework for studying the early universe and addressing long-standing puzzles in inflation. His research includes constructing explicit models of D-brane inflation, applying the AdS/CFT correspondence, developing models of large-field inflation based on axion shift symmetries and axion monodromy, and analyzing signatures of these models in the CMB spectrum and bispectrum. Additionally, he applies techniques from random matrix theory to study problems in string theory, such as the scalar mass spectrum and the stability of de Sitter critical points. McAllister's work aims to connect string theory with cosmological observations, potentially providing a testable interface between fundamental theory and empirical data.

Research topics

• Materials science
• Physics
• Nuclear physics
• Theoretical physics
• Particle physics
• Nuclear engineering
• Mechanics
• Computational physics

Selected publications

• Fuzzy axions and associated relics

  Journal of High Energy Physics · 2025-09-01 · 6 citations

  articleOpen access

  A bstract We study fuzzy axion dark matter in type IIB string theory, for axions descending from the Ramond-Ramond four-form in compactifications on orientifolds of Calabi-Yau hypersurfaces. Such models can be tested by cosmological measurements if a significant relic abundance of fuzzy dark matter arises, which we argue is most common in models with small numbers of axions. We construct a topologically exhaustive ensemble of more than 350,000 Calabi-Yau compactifications yielding up to seven axions, and in this setting we perform a systematic analysis of misalignment production of fuzzy dark matter. In typical regions of moduli space, the fuzzy axion, the QCD axion, and other axions have comparable decay constants of f a ≈ 10 16 GeV. We find that overproduction of heavier axions is problematic, except at special loci in moduli space where decay constant hierarchies can occur: without a contrived reheating epoch, it is necessary to fine-tune initial displacements. The resulting dark matter is typically a mix of fuzzy axions and heavier axions, including the QCD axion. Dark photons are typically present as a consequence of the orientifold projection. We examine the signatures of these models by simulating halos with multiple fuzzy axions, and by computing new cosmological constraints on ultralight axions and dark radiation. We also give evidence that cosmic birefringence is possible in this setting. Our findings determine the phenomenological correlates of fuzzy axion dark matter in a corner of the landscape.

  PublisherOA PDFDOI

• TASI Lectures on de Sitter Vacua

  arXiv (Cornell University) · 2025-12-18

  preprintOpen access1st authorCorresponding

  These lectures provide a self-contained introduction to flux compactifications of type IIB string theory on Calabi-Yau orientifolds. The first lecture begins with geometric foundations, then presents vacuum solutions in Calabi-Yau compactifications, as well as the geometry and physics of the moduli problem. The second lecture develops the classical theory of type IIB flux compactifications, both in ten dimensions and in the four-dimensional effective theory. The third lecture turns to the quantum theory of flux compactifications, including perturbative and non-perturbative corrections. With this foundation, in the fourth lecture we give a detailed treatment of the candidate de Sitter vacua recently constructed in arXiv:2406.13751. These notes are intended to be accessible to graduate students working in adjacent fields, and so extensive background material is included throughout.

  PublisherDOI

• Candidate de Sitter vacua

  Physical review. D/Physical review. D. · 2025-04-11 · 22 citations

  articleOpen access1st authorCorresponding

  We construct compactifications of type IIB string theory that yield, at leading order in the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msup><a:mi>α</a:mi><a:mo>′</a:mo></a:msup></a:math> and <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:msub><c:mi>g</c:mi><c:mi>s</c:mi></c:msub></c:math> expansions, de Sitter vacua of the form envisioned by Kachru We specify explicit Calabi-Yau orientifolds and quantized fluxes for which we derive the four-dimensional effective supergravity theories, incorporating the exact flux superpotential, the nonperturbative superpotential from Euclidean D3-branes, and the Kähler potential at tree level in the string loop expansion but to all orders in <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:msup><e:mi>α</e:mi><e:mo>′</e:mo></e:msup></e:math>. Each example includes a Klebanov-Strassler throat region containing a single anti-D3-brane, whose supersymmetry-breaking energy, computed at leading order in <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:msup><g:mi>α</g:mi><g:mo>′</g:mo></g:msup></g:math>, causes an uplift to a metastable de Sitter vacuum in which all moduli are stabilized. Finding vacua that demonstrably survive subleading corrections, and in which the quantization conditions are completely understood, is an important open problem for which this work has prepared the foundations.

  PublisherOA PDFDOI

• TASI Lectures on de Sitter Vacua

  ArXiv.org · 2025-12-18

  articleOpen access1st authorCorresponding

  These lectures provide a self-contained introduction to flux compactifications of type IIB string theory on Calabi-Yau orientifolds. The first lecture begins with geometric foundations, then presents vacuum solutions in Calabi-Yau compactifications, as well as the geometry and physics of the moduli problem. The second lecture develops the classical theory of type IIB flux compactifications, both in ten dimensions and in the four-dimensional effective theory. The third lecture turns to the quantum theory of flux compactifications, including perturbative and non-perturbative corrections. With this foundation, in the fourth lecture we give a detailed treatment of the candidate de Sitter vacua recently constructed in arXiv:2406.13751. These notes are intended to be accessible to graduate students working in adjacent fields, and so extensive background material is included throughout.

  PublisherOA PDF

• F-theory Axiverse

  ArXiv.org · 2025-11-25

  preprintOpen access

  We compute the couplings of Ramond-Ramond four-form axions in three ensembles of F-theory compactifications, with up to 181,200 axions. We work in the stretched Kähler cone, where $α'$ corrections are plausibly controlled, and we use couplings to certain non-Abelian sectors as a proxy for couplings to photons. The axion masses, decay constants, and couplings to gauge sectors show striking universality across the ensembles. In particular, the axion-photon couplings grow with $h^{1,1}$, and models in our ensemble with $h^{1,1} \gtrsim$ 10,000 axions are in tension with helioscope constraints. Moreover, under mild assumptions about charged matter beyond the Standard Model, theories with $h^{1,1} \gtrsim$ 5,000 are in tension with Chandra measurements of X-ray spectra. This work is a first step toward understanding the phenomenology of quantum gravity theories with thousands of axions.

  PublisherOA PDFDOI

• Candidate de Sitter Vacua

  arXiv (Cornell University) · 2024-06-19 · 1 citations

  preprintOpen access1st authorCorresponding

  We construct compactifications of type IIB string theory that yield, at leading order in the $α^\prime$ and $g_s$ expansions, de Sitter vacua of the form envisioned by Kachru, Kallosh, Linde, and Trivedi. We specify explicit Calabi-Yau orientifolds and quantized fluxes for which we derive the four-dimensional effective supergravity theories, incorporating the exact flux superpotential, the nonperturbative superpotential from Euclidean D3-branes, and the Kähler potential at tree level in the string loop expansion but to all orders in $α'$. Each example includes a Klebanov-Strassler throat region containing a single anti-D3-brane, whose supersymmetry-breaking energy, computed at leading order in $α'$, causes an uplift to a metastable de Sitter vacuum in which all moduli are stabilized. Finding vacua that demonstrably survive subleading corrections, and in which the quantization conditions are completely understood, is an important open problem for which this work has prepared the foundations.

  PublisherOA PDFDOI

• Fuzzy Axions and Associated Relics

  arXiv (Cornell University) · 2024-12-16 · 1 citations

  preprintOpen access

  We study fuzzy axion dark matter in type IIB string theory, for axions descending from the Ramond-Ramond four-form in compactifications on orientifolds of Calabi-Yau hypersurfaces. Such models can be tested by cosmological measurements if a significant relic abundance of fuzzy dark matter arises, which we argue is most common in models with small numbers of axions. We construct a topologically exhaustive ensemble of more than 350,000 Calabi-Yau compactifications yielding up to seven axions, and in this setting we perform a systematic analysis of misalignment production of fuzzy dark matter. In typical regions of moduli space, the fuzzy axion, the QCD axion, and other axions have comparable decay constants of $f_a \approx 10^{16}$ GeV. We find that overproduction of heavier axions is problematic, except at special loci in moduli space where decay constant hierarchies can occur: without a contrived reheating epoch, it is necessary to fine-tune initial displacements. The resulting dark matter is typically a mix of fuzzy axions and heavier axions, including the QCD axion. Dark photons are typically present as a consequence of the orientifold projection. We examine the signatures of these models by simulating halos with multiple fuzzy axions, and by computing new cosmological constraints on ultralight axions and dark radiation. We also give evidence that cosmic birefringence is possible in this setting. Our findings determine the phenomenological correlates of fuzzy axion dark matter in a corner of the landscape.

  PublisherOA PDFDOI

• Compactification of Superstring Theory

  Encyclopedia of Mathematical Physics · 2024-10-03

  book-chapterSenior author
  PublisherDOI

• Glimmers from the axiverse

  Journal of Cosmology and Astroparticle Physics · 2024-09-01 · 39 citations

  articleOpen access

  Abstract We study axion-photon couplings in compactifications of type IIB string theory. We find that these couplings are systematically suppressed compared to the inverse axion periodicity, as a result of two effects. First, couplings to the QED theta angle are suppressed for axion mass eigenstates that are light compared to the mass scale set by stringy instantons on the cycle supporting QED. Second, in compactifications with many axions the intersection matrix is sparse, making kinetic mixing weak. We study the resulting phenomenology in an ensemble of 200,000 toy models constructed from the Kreuzer-Skarke database up to the maximum Hodge number h 1,1 = 491. We examine freeze-in production and decay of thermal axions, birefringence of the cosmic microwave background, X-ray spectrum oscillations, and constraints on the QCD axion from supernovae. We conclude that compactifications in this corner of the landscape involve many invisible axions, as well as a handful that may be detectable via photon couplings.

  PublisherDOI

• Computational Mirror Symmetry

  Journal of High Energy Physics · 2024-01-30 · 8 citations

  articleOpen access

  A bstract We present an efficient algorithm for computing the prepotential in compactifications of type II string theory on mirror pairs of Calabi-Yau threefolds in toric varieties. Applying this method, we exhibit the first systematic computation of genus-zero Gopakumar-Vafa invariants in compact threefolds with many moduli, including examples with up to 491 vector multiplets.

  PublisherOA PDFDOI

Recent grants

• CAREER: String Theory Applied to Precision Cosmology

  NSF · $400k · 2011–2016

Frequent coauthors

• Daniel Baumann

  39 shared

• Mehmet Demirtaş

  The NSF AI Institute for Artificial Intelligence and Fundamental Interactions

  22 shared

• Eva Silverstein

  21 shared

• Cody Long

  Harvard University

  19 shared

• Jakob Moritz

  19 shared

• Shamit Kachru

  19 shared

• Raphael Flauger

  University of California, San Diego

  15 shared

• Manki Kim

  14 shared

Labs

• Liam McAllister GroupPI

Awards & honors

• NSF CAREER Award (2011)
• Simons Fellowship (2014)
• Alfred P. Sloan Research Fellow (2009-2011)