Search my publications on INSPIRE, ADS, or arxiv

  • Known unknowns of dark matter annihilation over cosmic time (KJM)

    arXiv:1309.7783; submitted to MNRAS

    Abstract: Dark matter self-annihilation holds promise as one of the most robust mechanisms for the identification of the particle responsible for the Universe's missing mass. In this work, I examine the evolution of the dark matter annihilation power produced by smooth and collapsed structures over cosmic time, taking into account uncertainties in the structure of dark matter halos. As we search for observational signatures of annihilation, an understanding of this time evolution will help us to best direct our observational efforts, either with local measurements or investigation of the effects of annihilation on the intergalactic medium at high redshift. As I show in this work, there are several key sources of uncertainty in our ability to estimate the dark matter annihilation from collapsed structures, including: the density profile of dark matter halos; the small-scale cut-off in the dark matter halo mass function; the redshift-dependent mass-concentration relation for small halos; and the particle-velocity dependence of the dark matter annihilation process. Varying assumptions about these quantities can result in annihilation power predictions that differ by several orders of magnitude. These uncertainties must be resolved, through a combination of observation and modeling, before robust estimations of the cosmological annihilation signal can be made.

  • Detecting the redshifted 21cm forest during reionization (KJM & J.S.B. Wyithe)

    Monthly Notices of the Royal Astronomical Society 425, 2988 (2012) [arXiv:1101.5431]

    Summary: The epoch of reionization was the transition between a neutral intergalactic medium (IGM) during the cosmic Dark Ages and the mostly ionized intergalactic medium of the present day. Understanding this crucial period of cosmic time would give us great insight into the nature of the first sources of light in the Universe and how cosmic structure evolved. A key observational tool in this effort is the 21cm line of neutral hydrogen, which can be studied at high redshift using a number of large-scale radio interferometry observatories currently under construction around the world. In this paper, we discuss how the 21cm forest -- the absorption signature of the pre-reionization IGM against high-redshift radio sources -- can allow us to study the thermal history of the IGM in an unprecedented way. We also discuss the observational challenges of this measurement, including the difficulty of finding radio-loud background sources at sufficiently high redshift.

  • Axions, Inflation and the Anthropic Principle (KJM)

    Journal of Cosmology and Astroparticle Physics 1107, 021 (2011) [arXiv:0911.0421]

    Summary: The QCD axion is a hypothetical particle that plays an important role in particle physics (as a solution to a major problem in the theory of the strong force, quantum chromodynamics), and that has been proposed as a candidate for the dark matter that makes up more than 80% of the matter in the Universe. Axions also occur naturally in string theory, but finding theories in which they are produced at the proper scale to explain dark matter has proved challenging. In this paper, I show that if axions are produced before inflation (as would be expected in string theory), either the axion properties or the inflationary model must be highly tuned in order to evade observational constraints, and the attempts by some authors to use the anthropic principle to explain this disparity are ultimately insufficient.

  • Cosmological problems with multiple axion-like fields (KJM & P.J. Steinhardt)

    Journal of Cosmology and Astroparticle Physics 1105, 001 (2011) [arXiv:0911.0418]

    Summary: In string theory, axion-like fields are commonplace, occuring as the result of compactification from higher dimensions. It has often been hypothesized that one of these axion-like fields could be the QCD axion, responsible for solving the strong-CP problem in quantum chromodynamics and possibly accounting for dark matter. However, as we discuss in this work, the tuning required for a generic string theory axion to exist in the context of inflation without violating observational bounds is extreme: in general, the level of tuning is greater than the tuning in QCD that motivated the axion hypothesis in the first place. We show that this tuning is exponentially more extreme if multiple axion-like fields are considered, and that the introduction of anthropic arguments do not alleviate the problem.

  • Primordial black holes in the Dark Ages: Observational prospects for future 21cm surveys (KJM & D.H. Wesley)

    arXiv:0805.1531; submitted to Physical Review D; publication delayed due primarily to co-author's withdrawal from academia

    Summary: If primordial black holes were evaporating during the cosmological Dark Ages when the universe was dominated by neutral hydrogen gas, the effects of the Hawking radiation on the neutral hydrogen may be detectable in future high-redshift 21cm radio interferometry surveys. We present the signal that would be seen in the 21cm fluctuation power spectrum and discuss the instrumental requirements and challenges for such an observation. We show that the high-redshift 21cm fluctuation power spectrum has the potential to place strong limits on evaporating primordial black holes and other exotic energy sources in the Dark Ages.

  • Effect of primordial black holes on the cosmic microwave background and cosmological parameter estimation (M. Ricotti, J.P. Ostriker & KJM)

    The Astrophysical Journal 680, 829 (2008) [arXiv:0709.0524]

    Summary: Massive, non-evaporating primordial black holes can ionize and heat the intergalactic medium in the epoch before the formation of stars and galaxies by accreting baryonic matter and producing X-rays. We show that failing to account for a non-standard recombination history can lead to a misestimation of cosmological parameters. We also discuss the effect of primordial black hole baryon accretion on the primordial molecular hydrogen abundance, which may affect the rate of formation of the first stars and galaxies.

  • Observing cosmic string loops with gravitational lensing surveys (KJM, D.H. Wesley & L.J. King)

    Physical Review D 76, 123515 (2007) [arXiv:astro-ph/0702648]

    Summary: Current constraints on the existence of cosmic strings mainly rely on an understanding of their emission of gravitational radiation. Taking a different approach, we start with an estimate of the gravitational lensing effect of a typical cosmic string loop against a background radio source and use this to predict the constraints that might be reached with extant and future radio surveys.

  • Growth of structure seeded by primordial black holes (KJM, J.P. Ostriker & M. Ricotti)

    The Astrophysical Journal 665, 1277 (2007) [arXiv:astro-ph/0608642]

    Summary: We discuss the constraints on massive (non-evaporating) primordial black holes (PBHs) as a component of dark matter. We also explore the possibilities for PBHs to contribute to the early growth of supermassive black holes or to become the intermediate-mass black holes whose existence may account for ultraluminous x-ray sources seen today.

  • The Lyth bound revisited (G.P. Efstathiou & KJM)

    Journal of Cosmology and Astroparticle Physics 0505, 008 (2005) [arXiv:astro-ph/0503360]

    Summary: The Lyth bound relates the tensor-scalar ratio to the variation in the inflaton field over the course of inflation. We study a population of inflationary models produced stochastically using the inflationary flow equations and with a minimal set of assumptions to test this relationship and comment on its implications for the future detection of a primordial tensor signal.

  • Foregrounds for 21-cm observations of neutral gas at high redshift (S.P. Oh & KJM)

    Monthly Notices of the Royal Astronomical Society 346, 871 (2003) [arXiv:astro-ph/0302099]

    Summary: One of the biggest challenges for observing the cosmological Dark Ages with redshifted 21cm radio observations will be overcoming a wide range of foregrounds. We investigate the effects of foregrounds on spatial tomography of neutral gas at high redshift and on observations of spectral features along a pencil-beam slice.