# Shaun Hotchkiss's research while affiliated with University of Auckland and other places

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## Publications (38)

Random, multifield functions can set generic expectations for landscape-style cosmologies. We consider the inflationary implications of a landscape defined by a Gaussian random function, which is perhaps the simplest such scenario. Many key properties of this landscape, including the distribution of saddles as a function of height in the potential,...

Random, multifield functions can set generic expectations for landscape-style cosmologies. We consider the inflationary implications of a landscape defined by a Gaussian random function, which is perhaps the simplest such scenario. Many key properties of this landscape, including the distribution of saddles as a function of height in the potential,...

Landscape cosmology posits the existence of a convoluted, multidimensional, scalar potential -- the "landscape" -- with vast numbers of metastable minima. Random matrices and random functions in many dimensions provide toy models of the landscape, allowing the exploration of conceptual issues associated with these scenarios. We compute the relative...

In simple inflationary cosmological scenarios, the near-exponential growth can be followed by a long period in which the Universe is dominated by the oscillating inflaton condensate. The condensate is initially almost homogeneous, but perturbations grow gravitationally, eventually fragmenting the condensate if it is not disrupted more quickly by re...

In simple inflationary cosmological scenarios the near-exponential growth can be followed by a long period in which the Universe is dominated by the oscillating inflaton condensate. The condensate is initially almost homogeneous, but perturbations grow gravitationally, eventually fragmenting the condensate if it is not disrupted more quickly by res...

We recently showed that postulated ultracompact minihalos with a steep density profile do not form in realistic simulations with enhanced initial perturbations. In this paper we assume that a small fraction of the dark matter consists of primordial black holes (PBHs) and simulate the formation of structures around them. We find that in this scenari...

We recently showed that postulated ultracompact minihalos with a steep density profile do not form in realistic simulations with enhanced initial perturbations. In this paper we assume that a small fraction of the dark matter consists of primordial black holes (PBHs) and simulate the formation of structures around them. We find that in this scenari...

PYULTRALIGHT simulates the dynamics of ultralight dark matter in a non-expanding background. PYULTRALIGHT can describe the evolution of several interacting ultralight dark matter halos or one or more halos orbiting a central, fixed Newtonian potential, the latter scenario corresponding to dwarf galaxies orbiting a massive central galaxy. We verify...

PyUltraLight simulates the dynamics of ultralight dark matter in a non-expanding background. PyUltraLight can describe the evolution of several interacting ultralight dark matter halos or one or more halos orbiting a central, fixed Newtonian potential, the latter scenario corresponding to dwarf galaxies orbiting a massive central galaxy. We verify...

We perform three-dimensional simulations of structure formation in the early Universe, when boosting the primordial power spectrum on approximately kpc scales. We demonstrate that our simulations are capable of producing power-law profiles close to the steep $\rho\propto r^{-9/4}$ halo profiles that are commonly assumed to be a good approximation t...

The use of Eulerian 'standard perturbation theory' to describe mass assembly in the early universe has traditionally been limited to modes with k $\lesssim$ 0.1 h/Mpc at z = 0. At larger k the SPT power spectrum deviates from measurements made using N-body simulations. Recently, there has been progress in extending the reach of perturbation theory...

The properties of large underdensities in the distribution of galaxies in the Universe, known as cosmic voids, are potentially sensitive probes of fundamental physics. We use data from the MultiDark suite of N-body simulations and multiple halo occupation distribution mocks to study the relationship between galaxy voids and the gravitational potent...

Within a cosmological context, we study the behaviour of collisionless
particles in the weak field approximation to General Relativity, allowing for
large gradients of the fields and relativistic velocities for the particles. We
consider a spherically symmetric setup such that high resolution simulations
are possible with minimal computational reso...

Cosmic voids may be very useful in testing fundamental aspects of cosmology.
Yet observationally voids can only be seen as regions with a deficit of bright
galaxies. To study how biased galaxies trace matter underdensities and how the
properties of voids depend on those of the tracer galaxy population, we use a
$\Lambda$CDM N-body simulation popula...

The statistical study of voids in the matter distribution promises to be an important tool for precision cosmology, but there
are known discrepancies between theoretical models of voids and the voids actually found in large simulations or galaxy surveys.
The empirical properties of observed voids are also not well understood. In this paper, we stud...

We discuss the universality and self-similarity of void density profiles, for
voids in realistic mock luminous red galaxy (LRG) catalogues from the Jubilee
simulation, as well as in void catalogues constructed from the SDSS LRG and
Main Galaxy samples. Voids are identified using a modified version of the ZOBOV
watershed transform algorithm, with ad...

The discovery of a void of size $\sim200\;h^{-1}$Mpc and average density
contrast of $\sim-0.1$ aligned with the Cold Spot direction has been recently
reported. It has been argued that, although the first-order integrated
Sachs-Wolfe (ISW) effect of such a void on the CMB is small, the second-order
Rees-Sciama (RS) contribution exceeds this by an o...

The stacked density profile of cosmic voids in the galaxy distribution provides an important tool for the use of voids for
precision cosmology. We study the density profiles of voids identified using the zobov watershed transform algorithm in realistic mock luminous red galaxy (LRG) catalogues from the Jubilee simulation, as well
as in void catalog...

We examine the stacked integrated Sachs-Wolfe (ISW) imprints on the CMB along the lines of sight of voids and superclusters in galaxy surveys, using the Jubilee ISW simulation and mock luminous red galaxy (LRG) catalogues. We show that the expected signal in the concordance \Lam CDM model is much smaller than the primary anisotropies arising at the...

We examine the integrated Sachs–Wolfe (ISW) imprint of voids and superclusters on the cosmic microwave background. We first
study results from the Jubilee N-body simulation. From Jubilee, we obtain the full-sky ISW signal from structures out to redshift z = 1.4 and a mock luminous red galaxy catalogue. We confirm that the expected signal in the con...

We examine the integrated Sachs-Wolfe (ISW) imprint of voids and superclusters on the cosmic microwave background. We first study results from the Jubilee N-body simulation, which models the full-sky ISW signal from structures out to redshift z=1.4 and provides a mock luminous red galaxy (LRG) catalogue, to confirm that the expected signal in the c...

We present initial results from the Jubilee Integrated Sachs-Wolfe (ISW) project, which models the expected Λ cold dark matter
ISW effect in the Jubilee simulation. The simulation volume is (6 h−1 Gpc)3, allowing power on very large scales to be incorporated into the calculation. Haloes are resolved down to a mass of 1.5 × 1012 h−1 M⊙, which allows...

Sutter et al. have responded to the criticisms we made of their cosmic void
catalogue in our recent paper presenting an alternative catalogue (Nadathur &
Hotchkiss 2013). Unfortunately, their response contains several statements
which are incorrect, as we point out in this note.

The study of the interesting cosmological properties of voids in the Universe
depends on the efficient and robust identification of such voids in galaxy
redshift surveys. Recently, Sutter et al. (2012) have published a public
catalogue of voids in the Sloan Digital Sky Survey Data Release 7 main galaxy
and luminous red galaxy samples, using the voi...

We investigate the hemispherical power asymmetry in the cosmic microwave
background on small angular scales. We find an anomalously high asymmetry in
the multipole range l=601-2048, with a naive statistical significance of 6.5
sigma. However, we show that this extreme anomaly is simply a coincidence of
three other effects, relativistic power modula...

A detection of the stacked integrated Sachs-Wolfe (ISW) signal in the CMB of
rare superstructures identified in the SDSS Luminous Red Galaxy catalogue has
been reported at very high statistical significance. The magnitude of the
observed signal has previously been argued to be more than 3 standard
deviations larger than the theoretical \Lambda CDM...

We consider methods with which to answer the question "is any observed galaxy
cluster too unusual for Lambda-CDM?" After emphasising that many previous
attempts to answer this question will overestimate the confidence level at
which Lambda-CDM can be ruled out, we outline a consistent approach to these
rare clusters, which allows the question to be...

A crucial diagnostic of the ΛCDM cosmological model is the integrated Sachs-Wolfe (ISW) effect of large-scale structure on the cosmic microwave background (CMB). The ISW imprint of superstructures of size ∼ 100 h−1Mpc at redshift z ∼ 0.5 has been detected with > 4σ significance, however it has been noted that the signal is much larger than expected...

We critically investigate current statistical tests applied to high redshift clusters of galaxies in order to test the standard cosmological model and describe their range of validity. We carefully compare a sample of high-redshift, massive, galaxy clusters with realistic Poisson sample simulations of the theoretical mass function, which include th...

We address the issue of cosmological backreaction from non-linear
structure formation by constructing an approximation for the time evolved metric of a dust dominated universe based on a gradient expansion. Our metric begins as a perturbation of a flat Friedmann-Robertson-Walker state described by a nearly scale invariant, Gaussian, power-law distr...

The detection of primordial gravitational waves, or tensor perturbations,
would be regarded as compelling evidence for inflation. The canonical measure
of this is the ratio of tensor to scalar perturbations, r. For single-field
slow-roll models of inflation with small field excursions, the Lyth bound
dictates that if the evolution of the slow-roll...

I show that the most common method of quantifying the likelihood that an
extreme galaxy cluster could exist is biased and can result in false claims of
tension with LambdaCDM. This common method uses the probability that at least
one cluster could exist above the mass and redshift of an observed cluster. I
demonstrate the existence of the bias usin...

There are observations of at least 14 high-redshift massive galaxy clusters, which have an extremely small probability with a purely Gaussian initial curvature perturbation. Here we revisit the estimation of the contribution of non-Gaussianities to the cluster mass function and point out serious problems that have resulted from the application of t...

Number counts of massive high-redshift clusters provide a window to study
primordial non-Gaussianity. The current quality of data, however, forces the
statistical analysis to probe a region of parameter space -- the extreme tail
of the mass function -- which is neither accessible in any of the currently
available theoretical prescriptions for calcu...

We consider observational constraints and fine-tuning issues in a
renormalizable model of inflection point inflation, with two independent
parameters. We derive constraints on the parameter space of this model arising
from the WMAP 7-year power spectrum. It has previously been shown that it is
possible to successfully embed this potential in the MS...

Multiple inflation is a model based on N=1 supergravity
wherein there are sudden changes in the mass of the inflaton because
it couples to `flat direction' scalar fields which undergo symmetry
breaking phase transitions as the universe cools. The resulting
brief violations of slow-roll evolution generate a non-gaussian
signal which we find to be os...

The form of the inflationary potential is severely restricted if one requires that it be natural in the technical sense, i.e. terms of unrelated origin are not required to be correlated. We determine the constraints on observables that are implied in such natural inflationary models, in particular on $r$, the ratio of tensor to scalar perturbations...

## Citations

... In an attempt to characterise metastability and slow dynamics in glasses, several tools to count and classify the stationary points (local minima, maxima and saddles) of complicated, very non-convex (free)-energy landscapes have been conceived and developed. Given that corrugated landscapes are ubiquitous, these techniques are expected to play a relevant role in other contexts involving rugged landscapes to optimize, being them fitness landscapes in biology [3,4], utility functions in economics [5], cost functions in constraint satisfaction or inference problems [6][7][8], loss landscapes in supervised learning [9], and obviously energy landscapes in condensed and soft matter physics [10], but also string theory and cosmology [11,12]. Most of these settings are naturally high-dimensional: for instance, the space of genotypes over which fitness landscapes are defined is combinatorially large; in the same vein, training huge artificial neural networks like those used in current deep learning applications requires to find good minima of loss landscapes depending on an extremely large number of parameters. ...

... If the inflaton is decoupled from other fields, it fragments through a process of self-resonance for p > 2, which gives rise to a radiation-dominated universe ∼10 e-folds of expansion after the end of inflation [35,36]. If p = 2, the inflaton fragments instead due to gravitational effects at much longer time scales [37]. ...

... The paper is organized in the following manner. In section 2, we review the accretion process of DM spikes around isolated PBHs by closely following the formalism of ref. [46]. In this section, which can be skipped by the expert reader, we describe the density profile of dark matter spikes expected around PBHs. ...

... We run dynamical simulations of FDM using the Python package PyUltraLight [43]. This package solves the Schrödinger-Poisson equation on a grid, where we choose a spatial resolution of 100 pc. ...

... The formation of dark matter halos has been studied through numerical simulations and analytical approaches, specifically targeting those that developed during the matter-dominated era (see, e.g., Gosenca et al. 2017;Delos et al. 2018;White 2022;). On the other hand, halos forming during the radiation-dominated era have the potential to possess significantly higher densities. ...

... Alkhanishvili et al. 2022 ), but using such a model will incur a significantly lower computational cost. One such analytical model that is gaining in popularity when conducting full-shape analyses is the ef fecti ve field theory of large-scale structure (EFTofLSS, Baumann et al. 2012 ;Carrasco, Hertzberg & Senatore 2012 ;Senatore 2015 ;de la Bella et al. 2017 ;Philcox et al. 2020 ;Ivanov 2022 ;Mergulh ˜ ao, Rubira & Voivodic 2023 ;Moretti et al. 2023 ). This perturbation-theory based model maps predictions for the dark matter clustering to that of galaxies via a series of nuisance parameters φ, that are marginalized o v er when putting constraints on the cosmological parameters θ. ...

... Once the stacked image is computed, it is possible to reconstruct the averaged convergence profile of the voids by averaging the pixel value in concentric shells around the image centre. Furthermore, [105] has shown using simulations that these imprints change according to the void population. In fact, the lensing imprint is directly related to the true underlying gravitational potential, and the inner density of cosmic voids -which depends on size and voids definition [90,106] -will impact on the strength of this correlation signal. ...

... These models are studied with matter distributed not only in a single halo cloud but also in various different ways, for example, in onion-like configurations [23,24,25] or in layers of walls [26,27]. Results of these studies can be compared with N-body relativistic simulations in a weak field approximation [22,24,25,28,29]. Furthermore, exact inhomogeneous models are used for building cosmological models in a Swiss cheese arrangement which allows for light propagation studies in more realistic conditions [30,31,32,33]. ...

... Furthermore, galaxies are often used to identify voids observationally, where galaxies of different types populate halos differently. Since halo masses can have a different impact on clustering properties, the resulting void catalogues, which depend on the minimum halo mass cut, can have different abundances and void profiles [95,338]. As a complementary approach, peaks of the void lensing map can also be used to identify voids, also resulting in a higher signal-to-noise ratio relative to galaxy-identified voids [335,339]. ...

... Thus, it is not unnatural to find some discrepancy between voids in data and the spherical void model. For example, the shell-crossing density threshold derived from the statistics of voids in data is inconsistent with the prediction based on the spherical void approximation (Chan et al. 2014;Achitouv et al. 2015;Nadathur & Hotchkiss 2015). This motivates a search for a new definition and identification method for voids that mitigate the inconsistency between voids in theory and data. ...