About

Professor Peter Graham is the Wells Family Director of the Stanford Institute for Theoretical Physics and the Dr. William S. & Carol A. Davies Professor of Physics at Stanford University. He earned his Ph.D. in Physics from Stanford University in 2007, and his undergraduate and master's degrees from Harvard University in 2002 and 2002, respectively. His research broadly focuses on theoretical physics beyond the Standard Model, involving cosmology, astrophysics, general relativity, and atomic physics. He investigates questions related to dark matter, the origins of the weak scale, the cosmological constant, and fundamental fermion masses, aiming to develop new theories beyond the Standard Model. Professor Graham has proposed solutions to the hierarchy problem using dynamical relaxation in the early universe and is interested in inventing novel experiments to discover new physics, often utilizing techniques from astrophysics, condensed matter, and atomic physics. He is a proposer and co-PI of the Cosmic Axion Spin Precession Experiment (CASPEr) and the DM Radio experiment, which search for axion dark matter and hidden photon dark matter, respectively. His current research areas include theories beyond the Standard Model, dark matter models and detection, experimental proposals for discovering axions and gravitational waves, and understanding results from experiments ranging from the LHC to early universe cosmology. Professor Graham has received several honors, including the Frontiers of Science Award in Theoretical Physics from the International Congress of Basic Science in 2024, the Simons Investigator award in 2021, the New Horizons Prize in Physics from the Breakthrough Foundation in 2017, the DOE Early Career Award in 2014, and the Terman Fellowship at Stanford.

Research topics

• Physics
• Quantum mechanics
• Engineering
• Computer Science
• Nuclear physics
• Particle physics
• Theoretical physics
• Optics
• Astronomy
• Atomic physics
• Materials science
• Mathematics
• Astrophysics
• Philosophy
• Nuclear engineering
• Classical mechanics
• Thermodynamics
• Mechanics

Selected publications

• Revisiting isocurvature bounds on the minimal QCD axion

  Journal of High Energy Physics · 2025-12-03 · 4 citations

  articleOpen access1st authorCorresponding

  A bstract The QCD axion has important connections to early universe cosmology. For example, it is often said that isocurvature limits rule out a combination of high axion decay constant, f a , and high inflationary Hubble scale, H I . High scales are theoretically motivated, so it is important to ask how robust this constraint is. We demonstrate that this constraint is naturally evaded when the quartic coupling of the complex U(1) PQ -breaking field is small (e.g. ≲ 10 −6 ). In this case, f a changes from a larger value during inflation to a smaller value in the later universe, suppressing isocurvature perturbations. Importantly, we show that in large parts of parameter space this solution is not jeopardised by overproduction of the axion through parametric resonance. The isocurvature bounds are thus dependent on UV physics. We have found that, even for the minimal QCD axion, large parts of UV parameter space at both high f a and high H I are in fact allowed, not ruled out by isocurvature constraints.

  PublisherOA PDFDOI

• Robust bounds on MACHOs from the faintest galaxies

  ArXiv.org · 2025-10-01

  preprintOpen access1st authorCorresponding

  We use the dynamical heating of stars in ultrafaint dwarf (UFD) galaxies to set limits on Massive Compact Halo Objects (MACHOs). In our analysis we study the robustness of the bounds under uncertainties in key UFD parameters, such as the half-light radius, stellar velocity dispersion, total halo mass and dark matter and stellar density profiles. We apply this framework to both well-established UFD candidates, as well as the recently discovered UFD candidate Ursa Major III/UNIONS 1. We find that multiple UFDs yield consistently strong limits in the mass range $10\, M_\odot \lesssim M_{\rm MACHO} \lesssim 10^9\, M_\odot$, underscoring the robustness of a previous analysis solely based on Segue I. We also demonstrate that Ursa Major III, if confirmed as an UFD, would improve the constraints significantly, providing the strongest constraints on MACHO dark matter in the mass range $1\, M_{\odot}\lesssim M_{\rm MACHO} \lesssim 10^5\, M_\odot$.

  PublisherOA PDFDOI

• Status of the Proton EDM Experiment (pEDM)

  ArXiv.org · 2025-04-17

  preprintOpen access

  The Proton EDM Experiment (pEDM) is the first direct search for the proton electric dipole moment (EDM) with the aim of being the first experiment to probe the Standard Model (SM) prediction of any particle EDM. Phase-I of pEDM will achieve $10^{-29} e\cdot$cm, improving current indirect limits by four orders of magnitude. This will establish a new standard of precision in nucleon EDM searches and offer a unique sensitivity to better understand the Strong CP problem. The experiment is ideally positioned to explore physics beyond the Standard Model (BSM), with sensitivity to axionic dark matter via the signal of an oscillating proton EDM and across a wide mass range of BSM models from $\mathcal{O}(1\text{GeV})$ to $\mathcal{O}(10^3\text{TeV})$. Utilizing the frozen-spin technique in a highly symmetric storage ring that leverages existing infrastructure at Brookhaven National Laboratory (BNL), pEDM builds upon the technological foundation and experimental expertise of the highly successful Muon $g$$-$$2$ Experiments. With significant R\&D and prototyping already underway, pEDM is preparing a conceptual design report (CDR) to offer a cost-effective, high-impact path to discovering new sources of CP violation and advancing our understanding of fundamental physics. It will play a vital role in complementing the physics goals of the next-generation collider while simultaneously contributing to sustaining particle physics research and training early-career researchers during gaps between major collider operations.

  PublisherOA PDFDOI

• Revisiting Isocurvature Bounds on the Minimal QCD Axion

  ArXiv.org · 2025-06-03

  preprintOpen access1st authorCorresponding

  The QCD axion has important connections to early universe cosmology. For example, it is often said that isocurvature limits rule out a combination of high axion decay constant, $f_a$, and high inflationary Hubble scale, $H_I$. High scales are theoretically motivated, so it is important to ask how robust this constraint is. We demonstrate that this constraint is naturally evaded when the quartic coupling of the complex $U(1)_\mathrm{PQ}$-breaking field is small. In this case, $f_a$ changes from a larger value during inflation to a smaller value in the later universe, suppressing isocurvature perturbations. Importantly, we show that in large parts of parameter space this solution is not jeopardised by overproduction of the axion through parametric resonance. The isocurvature bounds are thus dependent on UV physics. We have found that, even for the minimal QCD axion, large parts of UV parameter space at both high $f_a$ and high $H_I$ are in fact allowed, not ruled out by isocurvature constraints.

  PublisherOA PDFDOI

• Cosmological Limits on Strong Dark Forces

  ArXiv.org · 2025-11-12

  preprintOpen access1st authorCorresponding

  We showcase cosmology's ability to constrain long-range forces between dark matter particles. Specifically, we consider a fermionic dark matter interacting via a Yukawa-coupled light scalar, focusing on regimes where the dark forces are stronger than gravitational and yet unconstrained. We show that the dark sector dynamics, both at the background and perturbation levels, is far richer than what can be captured with just the static interparticle Yukawa potential. The background dynamics includes an attractor that funnels a wide range of initial conditions onto an evolution unique to each parameter space. In a large swath of parameter space beyond existing limits, the dark sector deviates drastically from cold dark matter in observable epochs. We rule out this parameter space using existing constraints on dark-sector equation of state and small-scale cosmic perturbations, thus setting the strongest constraints yet on dark matter self-interactions at length scales shorter than 100 kpc. In addition, we briefly discuss repulsive dark forces and place cosmological limits that are stricter than in the attractive case.

  PublisherOA PDFDOI

• Constraints on long-ranged interactions between dark matter and the Standard Model

  Journal of Cosmology and Astroparticle Physics · 2025-04-01 · 3 citations

  articleOpen access

  Abstract Dark matter's existence is known thanks to its gravitational interaction with Standard Model particles, but it remains unknown whether this is the only force present between them. While many searches for such new interactions with dark matter focus on short-range, contact-like interactions, it is also possible that there exist weak, long-ranged forces between dark matter and the Standard Model. In this work, we present two types of constraints on such new interactions. First, we consider constraints arising from the fact that such a force would also induce long range interactions between Standard Model particles themselves, as well as between dark matter particles themselves. Combining the constraints on these individual forces generally sets the strongest constraints available on new Standard Model-dark matter interactions. Second, we consider the possibility of constraining new long-ranged interactions between dark matter and the Standard Model using the effects of dynamical friction in ultrafaint dwarf galaxies, especially Segue I. Such new interactions would accelerate the transfer of kinetic energy from stars to their surrounding dark matter, slowly reducing their orbits; the present-day stellar half-light radius of Segue I therefore allows us to exclude new forces which would have reduced stars' orbital radii below this scale by now.

  PublisherOA PDFDOI

• Highly excited electron cyclotron for QCD axion and dark-photon detection

  Physical review. D/Physical review. D. · 2025-04-16 · 7 citations

  articleOpen access

  We propose using highly excited cyclotron states of a trapped electron to detect meV axion and dark-photon dark matter, marking a significant improvement over our previous proposal and demonstration [One-electron quantum cyclotron as a milli-ev dark-photon detector, .]. When the axion mass matches the cyclotron frequency <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msub><a:mi>ω</a:mi><a:mi>c</a:mi></a:msub></a:math>, the cyclotron state is resonantly excited, with a transition probability proportional to its initial quantum number, <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:msub><c:mi>n</c:mi><c:mi>c</c:mi></c:msub></c:math>. The sensitivity is enhanced by taking <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:msub><e:mrow><e:mi>n</e:mi></e:mrow><e:mrow><e:mi>c</e:mi></e:mrow></e:msub><e:mo>∼</e:mo><e:msup><e:mrow><e:mn>10</e:mn></e:mrow><e:mrow><e:mn>6</e:mn></e:mrow></e:msup><e:msup><e:mrow><e:mo stretchy="false">(</e:mo><e:mfrac><e:mrow><e:mn>0.1</e:mn><e:mtext> </e:mtext><e:mtext> </e:mtext><e:mi>meV</e:mi></e:mrow><e:mrow><e:msub><e:mrow><e:mi>ω</e:mi></e:mrow><e:mrow><e:mi>c</e:mi></e:mrow></e:msub></e:mrow></e:mfrac><e:mo stretchy="false">)</e:mo></e:mrow><e:mrow><e:mn>2</e:mn></e:mrow></e:msup></e:mrow></e:math>. By optimizing key experimental parameters, we minimize the required averaging time for cyclotron detection to <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:msub><i:mi>t</i:mi><i:mi>ave</i:mi></i:msub><i:mo>∼</i:mo><i:msup><i:mn>10</i:mn><i:mrow><i:mo>−</i:mo><i:mn>6</i:mn></i:mrow></i:msup></i:math> s, permitting detection of such a highly excited state before its decay. An open–end-cap trap design enables the external photon signal to be directed into the trap, rendering our background-free detector compatible with large focusing cavities, such as the BREAD proposal, while capitalizing on their strong magnetic fields. Furthermore, the axion conversion rate can be coherently enhanced by incorporating layers of dielectrics with alternating refractive indices within the cavity. Collectively, these optimizations enable us to probe the QCD axion parameter space from 0.1 to 2.3 meV (25–560 GHz), covering a substantial portion of the predicted postinflationary QCD axion mass range. This sensitivity corresponds to probing the kinetic mixing parameter of the dark photon down to <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mi>ε</k:mi><k:mo>≈</k:mo><k:mn>2</k:mn><k:mo>×</k:mo><k:msup><k:mn>10</k:mn><k:mrow><k:mo>−</k:mo><k:mn>16</k:mn></k:mrow></k:msup></k:math>.

  PublisherOA PDFDOI

• Detecting White Dwarf Binary Mergers with Gravitational Waves

  ArXiv.org · 2025-10-22

  preprintOpen accessSenior author

  Mergers of white dwarf binaries are a possible progenitor channel for Type Ia supernovae. While white dwarfs are abundant in the universe and relatively well understood, their gravitational wave signals have not yet been directly observed. In order to detect gravitational waves from merging white dwarf binaries, a detector in the mid-band between LVK and LISA appears necessary. In this paper, we compute and discuss the gravitational waves emitted by inspiraling and merging white dwarf binaries, and assess their detectability with proposed space-based atom-interferometer detectors such as MAGIS Space and AEDGE. Gravitational waves from massive white dwarf binaries can be observed for many years before merger, offering a unique early warning of their final explosion. Our projections suggest that MAGIS Space could detect signals from Type Ia supernova progenitors at least once every four years, while AEDGE could observe at least a few hundred such events annually. The prolonged gravitational wave emission captured by atom-interferometers provides precise sky localisation and can allow observation of the final explosion with electromagnetic telescopes. The combined observation with electromagnetic radiation from the white dwarf binary coalescence could open a new pathway for multi-messenger astronomy involving some of the brightest transient events in the universe.

  PublisherOA PDFDOI

• Coherent self-interactions of dark matter in the Bullet Cluster

  Journal of Cosmology and Astroparticle Physics · 2025-03-01 · 5 citations

  articleOpen access

  Abstract Many models of dark matter include self-interactions beyond gravity. A variety of astrophysical observations have previously been used to place limits on the strength of such self-interactions. However, previous works have generally focused either on short-range interactions resulting in individual dark matter particles scattering from one another, or on effectively infinite-range interactions which sum over entire dark matter halos. In this work, we focus on the intermediate regime: forces with range much larger than dark matter particles' inter-particle spacing, but still shorter than the length scales of known halos. We show that gradients in the dark matter density of such halos would still lead to observable effects. We focus primarily on effects in the Bullet Cluster, where finite-range forces would lead either to a modification of the collision velocity of the cluster or to a separation of the dark matter and the galaxies of each cluster after the collision. We also consider constraints from the binding of ultrafaint dwarf galaxy halos, and from gravitational lensing of the Abell 370 cluster. Taken together, these observations allow us to set the strongest constraints on dark matter self-interactions over many orders of magnitude in range below ∼10 kpc, surpassing existing limits by orders of magnitude throughout.

  PublisherOA PDFDOI

• Enhancing direct detection of Higgsino dark matter

  Physical review. D/Physical review. D. · 2025-03-27 · 6 citations

  articleOpen access1st authorCorresponding

  While much supersymmetric weakly interacting massive particle (WIMP) parameter space has been ruled out, one remaining important candidate is Higgsino dark matter. The Higgsino can naturally realize the “inelastic dark matter” scenario, where the scattering off a nucleus occurs between two nearly-degenerate states, making it invisible to WIMP direct detection experiments if the splitting is too large to be excited. It was realized that a “luminous dark matter” detection process, where the Higgsino upscatters in the Earth and subsequently decays into a photon in a large neutrino detector, offers the best sensitivity to such a scenario. We consider the possibility of adding a large volume of a heavy element, such as Pb or U, around the detector. We also consider the presence of U and Th in the Earth itself, and the effect of an enhanced high-velocity tail of the dark matter distribution due to the presence of the Large Magellanic Cloud. These effects can significantly improve the sensitivity of detectors such as JUNO, <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mi>SNO</a:mi><a:mo>+</a:mo></a:mrow></a:math>, KamLAND, and Borexino, potentially making it possible in the future to cover much of the remaining parameter space for this classic supersymmetric WIMP dark matter.

  PublisherDOI

Recent grants

• Searching for Physics Beyond the Standard Model

  NSF · $120k · 2013–2016

• Research in Particle Theory, Cosmology, and Quantum Gravity

  NSF · $2.3M · 2023–2026

Frequent coauthors

• Surjeet Rajendran

  105 shared

• Dmitry Budker

  University of Applied Sciences Mainz

  43 shared

• K. D. Irwin

  27 shared

• M. Kœnig

  Institut Universitaire de Recherche Clinique

  26 shared

• Jason M. Hogan

  Stanford University

  24 shared

• Michael A. Fedderke

  24 shared

• Mark A. Kasevich

  Stanford University

  22 shared

• Derek F. Jackson Kimball

  California State University, East Bay

  21 shared

Awards & honors

• Frontiers of Science Award in Theoretical Physics from the I…
• Simons Investigator 2021
• New Horizons Prize in Physics from Breakthrough Foundation 2…
• DOE Early Career Award 2014
• Terman Fellowship, Stanford