Dr John McDonald


Research Interests

My research area is the intersection of particle physics and cosmology, known as particle cosmology or astro-particle physics.

My past and present research interests include:

Dark matter models (thermal relic MSSM neutralino and non-thermal gravitino dark matter, Higgs portal gauge singlet scalar dark matter, RH sneutrino condensate dark matter, alternative thermal production ("freeze-in") mechanisms for MeV and keV dark matter), the cosmology of the minimal supersymmetric standard model (MSSM) and its extensions (flat directions, Affleck-Dine baryogenesis and leptogenesis, Q-ball formation and decay, curvatons and right-handed sneutrinos), inflation models (non-minimally coupled scalar inflation ("Higgs Inflation"), sub-Planckian axion inflation, supersymmetric hybrid inflation models, oscillon formation and reheating) and baryogenesis (Affleck-Dine, electroweak, baryon-to-dark matter ratio ("Baryomorphosis")).

My most recent research focuses on the following themes:

(i) Initial conditions for non-minimally coupled inflation

Inflation models based on conventional particle physics theories are possible if there is a non-minimal coupling between the scalar field responsible for inflation and gravity via the Ricci scalar. The resulting models aare in excellent agreement with measurements of the CMB by the Planck satellite, reproducing the observed spectral index within experimental error and consistent with the upper bound on the amount of gravitational waves generated during inflation (the tensor-to-scalar ratio). However, the models are based on a relatively low energy density during inflation with a flat inflaton potential, which does not explain how the inflationary state was produced in the first place. My research has explored two alternative strategies to explaining the smooth state necessary to start inflation (1) via chaotic initial conditions and (ii) via an early contracting Universe era. It was shown that in both cases the necessary modifications of the non-minimally coupled inflation model can produce observable modifications of the spectral index and, in the case of chaotic initial conditions, of the prediction for the tensor-to-scalar ratio.

[Publications: "Chaotic initial conditions for nonminimally coupled inflation via a conformal factor with a zero", Kim, J. & McDonald, J., Physical Review D. 95 (2017), 103501;"Nonminimally coupled inflation with initial conditions from a preinflation anamorphic contracting era", McDonald, J., Physical Review D. 94 (2016), 043514.]

(ii) Dark matter due to Freeze-in via the Higgs portal

"Freeze-in" is a mechanism for the production of dark matter which can have a very low mass and be very weakly interacting with Standard Model particles. It is based on the slow decay of particles which are in thermal equilibrium in the early Universe, such as the Higgs boson, to particles that interact very weakly and so are out of thermal equilibrium. The original freeze-in model was due to the present author: "Thermally-generated Gauge Singlet Scalars as Self-Interacting Dark Matter", McDonald, J. , Physical Review Letters. 88 (2002), 091304.

(1) Clockwork Freeze-In: Freeze-in generally requires dark matter particles which are every weakly-coupled to Standard Model particles, and very light compared to the electroweak mass scale of the Standard Model. This raises the question of why their mass and coupling are so small. A new theoretical development which may be able to explain this is the "clockwork mechanism", which generates very small masses and couplings from a theory in which the non-zero masses and couplings are of a conventional magnitude. My research applied the clockwork mechanism to generate a scalar dark matter particle with very small mass and very small coupling to the Standard Model via the Higgs portal interaction. It was shown that the model can quite naturally generate of the observed dark matter density via freeze-in, and that the model typically predicts a dark matter particle mass of around 1 MeV.

[Publication:``A Clockwork Higgs Portal Model for Freeze-In Dark Matter'', J.Kim and J.McDonald, arXiv:1709.04105 [hep-ph] (Submitted for publication).]

(2) Ultra-Violet Freeze-In: A variation of freeze-in is "Ultra-Violet Freeze-in", where dark matter is generated at the reheating temperature, which is the temperature at which the Universe enters the conventional Hot Big Bang era following inflation. An important question is the form of the momentum distribution of the dark matter. This plays a critical role in the question of whether the resulting matter can play the role of Warm Dark Matter (WDM), a form of dark matter in which the particles move faster than usual and can modify the process of galaxy formation, which appears to fit better with observations of the dark matter halos of galaxies than conventional Cold Dark Matter (CDM). My research considered singlet fermion dark matter particles produced by annihilation of Higgs bosons via the fermionic Higgs portal interaction at the reheating temeprature, and derived their momentum distribution relative to that expected for conventional thermal WDM. It was shown that the mass of the dark matter particle is larger than that predicted for a conventional thermal fermion (5-7 keV compared to 2-3 keV for thermal), while the momentum distribution of the fermion singlets is quite different from that expected for a thermal fermion, being close to that of a thermal boson.

[Publication: "Warm dark matter via ultra-violet freeze-in: reheating temperature and non-thermal distribution for fermionic Higgs portal dark matter", McDonald, J., Journal of Cosmology and Astroparticle Physics (2016), 035. ]

(iii) Hemispherical asymmetry of the CMB temperature fluctuations

The cosmic microwave background radiation (CMB) is the remnant radiation from the Big Bang. It has a mean temperature T = 2.725 K, on top of which there are temperature fluctuations of magnitude about 1/10000 of the mean temperature. Analysis of the temperature fluctuations observed by NASA's Wilkinson Microwave Anisotropy Probe (WMAP) and ESA's Planck satellite have found that there is an asymmetry in the magnitude of the temperature fluctuations observed on one side of the sky compared to the opposite side, with a 12-14% difference. This is known as the Hemispherical Power Asymmetry. It is not expected to exist in conventional cosmology models, which predict a difference of no more than 6%. I proposed unconventional models based on cosmological phase transitions which occur during inflation, which are able to generate an asymmetric scalar field on the scale of the observed Universe. The hope is that this field may be able modulate the CMB temperature fluctuations across the sky, without introducing unwanted features such as a change in the statistics of the microwave fluctuations or a change in the mean CMB temperature.

[Publications: "Isocurvature and Curvaton Perturbations with Red Power Spectrum and Large Hemispherical Asymmetry", arXiv:1305.0525 [astro-ph.CO], JCAP 1307 (2013) 043; "Hemispherical Power Asymmetry from Scale-Dependent Modulated Reheating", arXiv:1309.1122 [astro- ph.CO], JCAP 1311 (2013) 041; "Hemispherical Power Asymmetry from a Space-Dependent Component of the Adiabatic Power Spectrum", arXiv:1403.2076 [astro-ph.CO], Phys.Rev. D89 (2014) 127303; "Negative Running of the Spectral Index, Hemispherical Asymmetry and the Consistency of Planck with Large r", arXiv:1403.6650 [astro-ph.CO], JCAP 1411 (2014) 012.]

(iv) Sub-Planckian inflation models with a large gravitational wave signature

Inflation might be able to generate primordial gravitational waves, whose detection would serve as a 'smoking gun' for inflation as the explanation of the observed structure of the Universe. However, in order to generate a large gravitational wave signal, the simplest inflation models require scalar fields which are larger than the Planck scale, the scale at which quantum gravity effects become large and the new physics associated with the unification of gravity and other particles and forces is expected to become important. Therefore conventional inflation theories which can generate a large gravitational wave signal are expected to break down. This raises the question of whether it is possible to construct inflation models which can generate a large gravitation wave signal but which do not require scalar field values larger than the Planck scale. I am studying field theory inflation models based on simple axion models which achieve this and which make clear predictions for both the magnitude of the gravitational wave signal ("tensor-to-scalar ratio") and the slope of CMB fluctuation spectrum ("scalar spectral index").

[Publications: "Sub-Planckian Two-Field Inflation Consistent with the Lyth Bound", arXiv:1404.4620 [hep-ph], JCAP 1409 (2014) 09, 027; "A Minimal Sub-Planckian Axion Inflation Model with Large Tensor-to-Scalar Ratio", arXiv:1407.7471 [hep-ph], JCAP 1501 (2015) 018; "Signatures of Planck Corrections in a Spiralling Axion Inflation Model", arXiv:1412.6943 [hep-ph].]

  • Theoretical Particle Cosmology