About

Professor Peter Adshead received his PhD in Physics in 2010 from Yale University after earning his BSc. with first class honors at the University of Canterbury in Christchurch, New Zealand in 2004. He was a KICP fellow at the Kavli Institute for Cosmological Physics at the University of Chicago and a postdoctoral associate at the University of Cambridge before joining the faculty at the University of Illinois in 2014. His research focuses on a broad range of topics in the very early universe, primarily on the physics of inflation and their imprint on the cosmic microwave background radiation, the reheating phase transition that must end inflation and begin the hot Big Bang, and the connections between high-energy particle physics and the early universe.

Research topics

• Physics
• Astrophysics
• Astronomy
• Optics
• Theoretical physics
• Quantum mechanics

Selected publications

• Twisting inflation to sub-Planckian axion decay constants

  Journal of Cosmology and Astroparticle Physics · 2026-03-01

  articleOpen access1st authorCorresponding

  Abstract We study pseudoscalar inflation in the Einstein-Cartan-Palatini (first-order) formulation of gravity while allowing for torsion. We introduce two non-minimal interactions in the gravitational sector — pseudoscalar couplings to the Pontryagin density (Chern-Simons term) and the Nieh-Yan topological invariant. In the presence of these terms the rolling pseudoscalar sources non-trivial torsional fields during inflation. We show that pathological gradient and ghost instabilities limit the strength of the coupling to the Pontryagin density during inflation. Furthermore, we show that the interaction with the Nieh-Yan term induces a new contribution to the pseudoscalar kinetic term which parametrically increases its decay constant and allows for inflation on steep potentials. The torsion field generated by the background is parity violating, which is manifest in the resulting chiral gravitational wave spectrum. We find that the scalar sector is largely unaffected beyond the remapping of the axion decay constant to a larger value. Consequently, we demonstrate that Generalized Natural Inflation, D-brane models, and Hilltop Squared Inflation can satisfy current observational constraints with sub-Planckian decay constants.

  PublisherDOI

• Neutrino Mass Constraints from kSZ Tomography

  ArXiv.org · 2025-02-07

  preprintOpen access

  We forecast neutrino mass constraints using Stage IV CMB and large-scale structure surveys, focusing on kSZ tomography as an independent probe of the growth of cosmic structure. We take into account several realistic factors, including the kSZ optical depth degeneracy. Our baseline setup consists of CMB S4 temperature and polarization (but not lensing) information, DESI BAO, the LSST galaxy power spectrum, and a Planck like $τ$ prior, yielding $σ(\sum m_ν) = 32\, \rm{meV}$. Adding kSZ tomography improves this by a few percent, while a kSZ optical depth prior can push this improvement to over $15\%$, giving $σ(\sum m_ν) = 27\, \rm{meV}$. When CMB lensing is included in the baseline setup, kSZ does not further improve neutrino mass constraints. We find promising prospects for a scenario combining futuristic CMB and galaxy surveys.

  PublisherOA PDFDOI

• Mechanical cosmology: Simulating scalar fluctuations in expanding universes using synthetic mechanical lattices

  Physical Review Research · 2025-04-03 · 2 citations

  articleOpen access

  Inspired by recent advances in observational astrophysics and continued explorations in the field of analog gravity, we discuss the prospect of simulating models of cosmology within the context of synthetic mechanical lattice experiments. We focus on the physics of expanding universe scenarios described by the Friedmann-Lemaître-Robertson-Walker (FLRW) metric. Specifically, quantizing scalar fluctuations in a background FLRW spacetime leads to a quadratic bosonic Hamiltonian with temporally varying pair production terms. Here we present a mapping that provides a one-to-one correspondence between these classes of cosmology models and feedback-coupled mechanical oscillators. As proof of principle, we then perform experiments on a synthetic mechanical lattice composed of such oscillators. We simulate two different FLRW expansion scenarios with universes dominated by vacuum energy and matter and discuss our experimental results.

  PublisherDOI

• Probing beyond local-type non-Gaussianity with kinematic Sunyaev-Zeldovich tomography

  Physical review. D/Physical review. D. · 2024-11-27 · 3 citations

  article1st authorCorresponding

  Non-Gaussianity of the primordial curvature perturbations may arise from a variety of well motivated early Universe scenarios. In particular, inflationary theories with additional light degrees of freedom can generate a bispectrum that is peaked in the squeezed limit. While the presence of an additional massless scalar can produce local-type primordial non-Gaussianity, in general the squeezed limit of the bispectrum depends on the mass of the new degree of freedom, and can deviate from the local shape. The resulting bispectrum leaves a distinct imprint on the amplitude and scale dependence of the galaxy bias, which for massive fields can differ from the ${k}^{\ensuremath{-}2}$ scaling from local type non-Gaussianity, providing an observational window into the physics of the early Universe. In this work we demonstrate that kinematic Sunyaev-Zeldovich (kSZ) tomography with next generation cosmological surveys will offer significant additional constraining power for both the shape and amplitude of scale-dependent bias arising from primordial non-Gaussianity beyond the local type. We show that this improved constraining power is robust to various obstacles such as the optical depth degeneracy, photometric redshift errors, and uncertainty in the galaxy bias model. With cosmic microwave background S4 and the Large Synoptic Survey Telescope, we forecast that compared to the galaxy survey alone the addition of kSZ tomography will offer a roughly factor of 2 reduction in the measurement uncertainty of the amplitude ${f}_{NL}$ of primordial non-Gaussianity well beyond the local (massless) limit. We find that kSZ tomography extends the range of masses for which order unity constraints on ${f}_{NL}$ are achievable, as well as extending the range of masses for which the late time probes of the matter power spectrum outperform the sensitivity of the cosmic microwave background itself.

  PublisherDOI

• Probing beyond local-type non-Gaussianity with kSZ tomography

  arXiv (Cornell University) · 2024-07-30

  preprintOpen access1st authorCorresponding

  Non-Gaussianity of the primordial curvature perturbations may arise from a variety of well motivated early Universe scenarios. In particular, inflationary theories with additional light degrees of freedom can generate a bispectrum that is peaked in the squeezed limit. While the presence of an additional massless scalar can produce local-type primordial non-Gaussianity, in general the squeezed limit of the bispectrum depends on the mass of the new degree of freedom, and can deviate from the local shape. The resulting bispectrum leaves a distinct imprint on the amplitude and scale-dependence of the galaxy bias, which for massive fields can differ from the $k^{-2}$ scaling from local type non-Gaussianity, providing an observational window into the physics of the early Universe. In this work we demonstrate that kinematic Sunyaev-Zeldovich tomography with next generation cosmological surveys will offer significant additional constraining power for both the shape and amplitude of scale-dependent bias arising from primordial non-Gaussianity beyond the local type. We show that this improved constraining power is robust to various obstacles such as the optical depth degeneracy, photometric redshift errors, and uncertainty in the galaxy bias model. With CMB S4 and the Large Synoptic Survey Telescope, we forecast that compared to the galaxy survey alone the addition of kSZ tomography will offer a roughly factor of two reduction in the measurement uncertainty of the amplitude $f_{NL}$ of primordial non-Gaussianity well beyond the local (massless) limit. We find that kSZ tomography extends the range of masses for which order unity constraints on $f_{NL}$ are achievable, as well as extending the range of masses for which the late time probes of the matter power spectrum outperform the sensitivity of the CMB itself.

  PublisherOA PDFDOI

• Gauge preheating with full general relativity

  Journal of Cosmology and Astroparticle Physics · 2024-03-01 · 16 citations

  articleOpen access1st authorCorresponding

  Abstract We study gauge preheating following pseudoscalar-driven inflation in full general relativity. We implement the Baumgarte-Shapiro-Shibata-Nakamura (BSSN) scheme to solve the full nonlinear evolution of the metric alongside the dynamics of the pseudoscalar and gauge fields. The dynamics of the background and emission of gravitational waves are broadly consistent with simulations in a Friedmann-Lemaître-Robertson-Walker (FLRW) spacetime. We find large, localized overdensities in the BSSN simulations of order δ = δρ / ρ ∼ 30, and the dimensionless power spectrum of δ peaks above unity. These overdense regions are seeded on length scales only slightly smaller than the horizon, and have a compactness C ∼ 0.1. The scale of peak compactness is shorter than the Jeans length, which implies that pressure of the matter fields plays an important role in the evolution of these objects.

  PublisherOA PDFDOI

• Gravitational Waves from Kinetic Preheating

  arXiv (Cornell University) · 2024-02-25

  preprintOpen access1st authorCorresponding

  We study gravitational wave production during kinetic preheating after inflation with a focus on scenarios that arise in $α$-attractor models where a scalar dilaton-like inflaton is kinetically coupled to a second scalar field. We present high-resolution lattice simulations of three $α$-attractor models for a range of parameters to probe regions where preheating is efficient. We find that preheating in these models can be extremely violent, resulting in gravitational wave energy densities that can be constrained by cosmic microwave background measurements of the effective number of relativistic species, $N_{\rm eff}$

  PublisherOA PDFDOI

• Gravitational waves from kinetic preheating

  Physical review. D/Physical review. D. · 2024-08-30 · 4 citations

  articleOpen access1st authorCorresponding

  We study gravitational wave production during kinetic preheating after inflation with a focus on scenarios that arise in $\ensuremath{\alpha}$-attractor models where a scalar dilatonlike inflaton is kinetically coupled to a second scalar field. We present high-resolution lattice simulations of three $\ensuremath{\alpha}$-attractor models for a range of parameters to probe regions where preheating is efficient. We find that preheating in these models can be extremely violent, resulting in gravitational wave energy densities that can be constrained by cosmic microwave background measurements of the effective number of relativistic species, ${N}_{\mathrm{eff}}$.

  PublisherOA PDFDOI

• Kinetic preheating after α-attractor inflation

  Physics Letters B · 2024-08-07 · 5 citations

  articleOpen access1st authorCorresponding

  We study preheating via kinetic couplings after dilaton-axion α-attractor inflation. We focus on E-model α-attractor driven inflation where the inflaton is kinetically coupled to an ultralight axion. In this class of models, the kinetic coupling is related to the form of the potential, and once the amplitude of the scalar curvature spectrum as well as the tensor-to-scalar ratio are specified, the model has no free parameters. We find that the kinetic couplings can lead to extremely efficient tachyonic preheating, with stronger preheating occurring at parameter values corresponding to smaller values of the tensor-to-scalar ratio. Preheating becomes extremely efficient below r≲1.6×10−5.

  PublisherDOI

• Dark photon dark matter from an oscillating dilaton

  Physical review. D/Physical review. D. · 2023-04-19 · 24 citations

  articleOpen access1st authorCorresponding

  We present a mechanism for generating ultralight dark photon dark matter in the early Universe via a dilatonlike scalar field coupled to the dark photon's kinetic term. Energy is initially stored in the condensate of the dilaton, which resonantly produces dark photons when it begins oscillating in the early Universe. While similar scenarios with axion--dark-photon couplings require large coupling coefficients to fully populate the dark photon, the dilatonic coupling features a unique regime: When the dark photon's mass is half that of the dilaton, dark photons are copiously produced even when the dilaton undergoes small-amplitude oscillations. Scenarios consistent with the cosmic microwave background allow for ultralight vector dark matter with mass as light as ${10}^{\ensuremath{-}20}\text{ }\text{ }\mathrm{eV}$.

  PublisherOA PDFDOI

Frequent coauthors

• Wayne Hu

  28 shared

• John T. Giblin

  27 shared

• Mark Wyman

  22 shared

• Raphael Flauger

  University of California, San Diego

  18 shared

• Zachary J. Weiner

  17 shared

• Cora Dvorkin

  17 shared

• C. Baccigalupi

  16 shared

• Scott Watson

  16 shared

Education

• PhD, Physics

  Yale University

  2010

• BSc. (hons), Department of Physics and Astronomy

  University of Canterbury

  2004

Awards & honors

• KICP fellow at the Kavli Institute for Cosmological Physics…