Hamish A. Clark's research while affiliated with The University of Sydney and other places

Publications (9)

Article
An excess of gamma rays has been identified at the centre of the Milky Way, and annihilation of dark matter has been posited as a potential source. This hypothesis faces significant challenges: difficulty characterizing astrophysical backgrounds, the need for a non-trivial adiabatic contraction of the inner part of the Milky Way's dark matter halo,...
Article
Full-text available
An excess of gamma rays has been identified at the center of the Milky Way, and annihilation of dark matter has been posited as a potential source. This hypothesis faces significant challenges: difficulty characterizing astrophysical backgrounds, the need for a non-trivial adiabatic contraction of the inner part of the Milky Way's dark matter halo,...
Article
The existence of substructure in halos of annihilating dark matter would be expected to substantially boost the rate at which annihilation occurs. Ultracompact minihalos of dark matter (UCMHs) are one of the more extreme examples of this. The boosted annihilation can inject significant amounts of energy into the gas of a galaxy over its lifetime. H...
Article
Despite being a major component in the teaching of special relativity, the twin `paradox' is generally not examined in courses on general relativity. Due to the complexity of analytical solutions to the problem, the paradox is often neglected entirely, and students are left with an incomplete understanding of the relativistic behaviour of time. Thi...
Article
Cosmological inflation generates primordial density perturbations on all scales, including those far too small to contribute to the cosmic microwave background. At these scales, isolated ultracompact minihalos of dark matter can form, well before standard structure formation, if the small-scale perturbations have a large enough amplitude. Such mini...
Article
Full-text available
Ultracompact minihaloes (UCMHs) have been proposed as a type of dark matter substructure seeded by large-amplitude primordial perturbations and topological defects. UCMHs are expected to survive to the present era, allowing constraints to be placed on their cosmic abundance using observations within our own Galaxy. Constraints on their number densi...
Article
Small-scale dark matter structure within the Milky Way is expected to affect pulsar timing. The change in gravitational potential induced by a dark matter halo passing near the line of sight to a pulsar would produce a varying delay in the light travel time of photons from the pulsar. Individual transits produce an effect that would either be too r...

Citations

... Known as NPTF, this is currently a leading method for parametric point-source inference in gamma-ray astronomy, and the method has also been applied to the search for astrophysical neutrino point sources in data from the IceCube telescope (Aartsen et al. 2020). The NPTF was primarily developed to analyze the excess of Galactic Center (GCE) gamma-rays observed by the Fermi telescope (Goodenough & Hooper 2009;Hooper & Goodenough 2011;Hooper & Linden 2011;Abazajian & Kaplinghat 2012;Hooper & Slatyer 2013;Gordon & Macias 2013;Abazajian et al. 2014Abazajian et al. , 2015Daylan et al. 2016;Calore et al. 2015;Ajello et al. 2016;Linden et al. 2016;Macias et al. 2018;Clark et al. 2018), and concluded that the observed excess was better described by a population of point sources, in comparison to a dark matter annihilation origin (Lee et al. , 2016; see also Bartels et al. 2016). 9 An approach similar to the NPTF that has also been widely used is the onepoint fluctuation analysis or one-point PDF method; see, for example, Lee et al. (2009), Feyereisen et al. (2015, 2017, Zechlin et al. (2016aZechlin et al. ( , 2016bZechlin et al. ( , 2018, Manconi et al. (2020), and Calore et al. (2021). ...
... [18,19] showed that there are some hints for a statistical preference for point-source emission in the GCE (but this conclusion might be sensitive to structure in the diffuse background that is not modeled by the range of background models considered, see also [21]). The morphology of the excess if pointlike would be in tension with the smooth distribution expected for a CDM annihilation [20,119]. Most physicists would then attribute the observed point-like spectrum to a new population of Millisecond Pulsars (MSPs) [20,22]. ...
... In mixed dark matter scenarios with PBHs and thermal relics, ultradense haloes would also enormously boost the dark matter annihilation rate, potentially leading to detectable gamma rays (Bringmann et al. 2012) or other signatures (e.g. Clark et al. 2017;Silk 2018), although these mixed scenarios are already tightly constrained (Adamek et al. 2019). For broad PBH mass spectra, the presence of ultradense haloes implies that PBHs quickly become highly clustered, even if they are not clustered at formation (Moradinezhad Dizgah et al. 2019). ...
... However, PTAs can also probe perturbations arising from an inhomogeneous matter environment. In particular, pulsar timing holds the potential to detect primordial black holes [2][3][4] and dark matter subhalos [5][6][7][8][9][10][11][12] as small as 10 −11 M ⊙ , which lie far beyond the reach of other astrophysical probes of dark matter. ...
... Numerous previous works have explored the prospects of using the first dark matter halos to probe the smallscale primordial power spectrum [41][42][43][44][45][46][47][48][49]. However, most treatments assumed that these halos, if they form sufficiently early, possess a particularly compact ρ ∝ r −9/4 density profile [50] derived from self-similar theory [51]. ...
... In this paper, we still assume that the inflaton is a real scalar field φ, but we allow a general, renormalizable polynomial potential. This next to simplest scenario has been analyzed many times since 1990 [9][10][11][12][13][14][15][16][17][18]. All these analyses have been performed before the release of the 2018 Planck and BICEP/Keck results [7]. ...
... The majority of this paper will focus on a pair of constraints that weaken only linearly with the DM density for a DM sub-component. These come from heating of neutron stars (NSs) from DM capture, and pulsar timing measurements of transiting DM clumps, where DM passing near pulsars enhance the Doppler effect on the pulsar frequencies [15][16][17][18][19][20][21][22][23][24][25][26]. These two constraints on the DM-baryon fifth force, which we derive in detail below, are summarized in Fig. 1, labeled as "heating" and "PTA." ...
... Note that if the small-scale fluctuations produced by inflation are not large enough to collapse into PBHs, they could create a population of dense low-mass dark matter halos, usually referred to as ultra-compact minihalos (Berezinsky et al. 2003;Bringmann et al. 2012). These objects are found to have a steep inner density profile, q $ r À9=4 or r À1:5 , in simulations (Gosenca et al. 2017;Delos et al. 2018a), and they are expected leave their own distinctive observational signature that could be constrained by microlensing (Ricotti and Gould 2009;Li et al. 2012b), pulsar time delays (Clark et al. 2016), or by (the lack of) a gamma-ray signal if DM selfannihilates (Bringmann et al. 2012;Gosenca et al. 2017;Delos et al. 2018b). ...