Douglas Scott


Research Classification

Science and Knowledge

Research Interests


Relevant Degree Programs


Research Methodology

Statistical analysis


Master's students
Doctoral students
Postdoctoral Fellows
Any time / year round

A wide range of topics within observational and theoretical cosmology

I am open to hosting Visiting International Research Students (non-degree, up to 12 months).

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Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Nov 2019)
Searching for hemispheric asymmetry and parity violation with the cosmic microwave background (2018)

The current standard model of cosmology is an extremely successful theory that describes all available data with only half a dozen or so parameters. Nevertheless there are aspects of the model that remain mysterious. In this thesis I test two assumptions with the cosmic microwave background (CMB) as measured by the Planck satellite mission. The first is statistical isotropy, motivated by hints in the temperature anisotropies that power on large scales exhibits a dipolar asymmetry. I confront this claim with data and formulate a mechanism to predict the corresponding asymmetry in different modes given a specific model. I apply this to temperature, CMB lensing, and polarization specifically. I find that while lensing is not constraining enough to help in distinguishing models, cosmic-variance-limited polarization will prove very helpful in doing so. I forecast that if the asymmetry signal is correct, then Planck polarization is quite unlikely to detect it, while a cosmic-variance-limited polarization experiment will increase the probability of a detection greatly. Furthermore via their over production of total power to the CMB, I rule out a class of models that try to explain the asymmetry such as an asymmetry in tensors or isocurvature. The second assumed symmetry is parity. The CMB is sensitive to parity-violation in the electromagnetic sector via correlations of temperature and E modes with B modes. The parity-violation is parameterized by an angle ⍺, defined on the sky. I use polarization data to constrain a uniform ⍺, setting the current best limits on this angle ⍺ = 0°.35 ± 0°.05 (stat.) ± 0°.28 (syst.). I demonstrate that this measurement is now dominated by systematic effects and thus unlikely to be improved upon in the near future. I also set the current best constraints on large-scale anisotropies of ⍺ via a scale-invariant power spectrum l(l+1)CL/2π
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Cosmological Tests of Gravity (2016)

General Relativity (GR) has long been acclaimed for its elegance and simplicity, and has successfully passed many stringent observational tests since it was introduced a century ago. However, there are two regimes in which the theory has yet to be fully challenged. One of them is in the neighbourhood of very strong gravitational fields, and the other is the behaviour of gravity on cosmological scales. While strong field gravity has challenged theorists because of the desire to find consistency between GR and quantum mechanics, cosmology has motivated extensions to GR via the empirical discoveries of dark matter and dark energy. In this thesis, we study a diverse range of modifications to GR and confront them with observational data. We discuss how a generic theory of modified gravity can be parameterized for studies within cosmology, and we introduce a general parameterization that is simpler than those that have been previously considered. This parameterization is then applied to investigate a specific theory, known as ``gravitational aether''. The gravitational aether theory was created to solve one of the theoretical inconsistencies that exists between GR and quantum mechanics, namely the fact that vacuum fluctuations appear not to gravitate. Cosmology is unique in testing this theory, and we find that the gravitational aether solution is excluded when all of the available cosmological data are combined. Nevertheless, a generalization of this theory provides a consistent way to describe the strength of coupling between pressure and gravity, and we present the most accurate measurements of this coupling parameter. In addition, we discuss the constraints that can be placed on modified gravity models using the latest data from cosmic microwave background (CMB) anisotropies, combined with several other probes of large-scale structure. Currently the most accurate CMB anisotropy measurements come from the Planck 2015 CMB power spectra, which we use, along with other cosmological data sets, to perform an extensive study of modified theories of gravity. We find that GR remains the simplest model that can explain all of the data. We end with a discussion of the prospects for future experiments that can improve our understanding of gravity.

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New methods for deblending spectral energy distributions in confused imaging (2016)

The submillimetre band is ideal for studying high-redshift star-forming galaxies, but such studies are hampered by the poor resolution of single-dish telescopes. Interferometric follow-up has shown that many sources are in fact comprised of multiple sources. For many such targets, confusion-limited Herschel observations that target the peak of their far-infrared emission are also available. Many methods for analysing these data have been developed, but most follow the traditional approach of extracting fluxes before model spectral energy distributions are fit, which erases degeneracies among fitting parameters and glosses over the intricacies of confusion noise. We have developed a forward-modelling method in order to tackle this problem in a more statistically rigorous way, which combines source deblending and spectral energy distribution fitting into the same procedure. We adapt our method to three independent projects, all of which benefit from our improved methodology. We investigate a "giant submillimetre arc" behind a massive foreground cluster and uncover seven multiply imaged galaxies, of which six are found to be at a redshift of z~2.9, and possibly constitute an interacting galaxy group. Using our new method, we disentangle the arc into its contributing components and constrain their far-IR properties. Using confusion limited Herschel-SPIRE imaging, the far-IR properties LABOCA detected submillimetre sources can be constrained. Despite such sources often breaking up in high-resolution ALMA imaging, existing studies have implemented traditional fitting methods. We apply our new forward modelling method to re-derive constraints on the far-infrared properties of these sources, exploring selection effects on this sample, while highlighting the benefits of our fitting approach. Finally, we present SCUBA-2 follow-up of 51 candidate proto-cluster fields undergoing enhanced star-formation. With the accompanying Herschel-SPIRE observations and a realistic dust temperature prior, we provide photometric redshift and far-IR luminosity estimates for 172 SCUBA-2 selected sources within the Planck overdensity fields. We find a redshift distribution similar to sources found in cosmological surveys, although our fields are enhanced in both density of sources and star formation rate density over a wide range of redshifts.

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The Faint Extragalactic Radio Sky (2015)

The radio sky covers a large range of sources, from small single galaxies to large clusters of galaxies and the space between them. These sources consist of some of the most powerful objects in the Universe, as well as diffuse weak emitters. Understanding the radio sky tells us about how galaxies have evolved over time, the different kinds of galaxy populations, the star formation history of the Universe, and the role of magnetism, as well as details of large-scale structure and clustering. Advancements in radio telescopes now allow us to push observational limits to new depths, probing fainter galaxies and farther back in cosmic time. We use a multi-pronged approach to examine several aspects of the faint extragalactic radio sky. Using new deep data from the Karl G. Jansky Very Large Array telescope, combined with the confusion analysis technique of P(D), we obtain the deepest estimates of the source count of individual radio galaxies and their contribution to the cosmic radio background temperature. Additionally, these data are used to catalogue the individual galaxies in order to study characteristics such as source size, spectral dependence, galaxy type, and redshift. We then examine the contribution from extended large-scale diffuse emission to the radio sky using data from the Australia Telescope Compact Array. This yields constraints on the total emission from such sources, including galactic halos, galaxy cluster halos and relics, and the inter- and intra-cluster medium. Finally, we investigate the radio angular power spectrum using interferometric data. These measurements show the fluctuations coming from the unresolved radio background as a function of angular scale. Together these studies present the deepest constraints available for the faint radio sky across a range of statistical areas. The measurements obtained here provide constraints on the evolving population of galaxies through their radio emission in order to further our knowledge of galaxy evolution in general.

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Cosmological Recombination (2008)

No abstract available.

Master's Student Supervision (2010 - 2018)
The most luminous galaxies in the universe (2017)

Submillimetre galaxies have become essential tools in studying the high redshift Universe. Reaching luminosities well over 10¹³ L⊙, they constitute the vast majority of star formation during this early epoch. Their combined infrared and submillimetre emission output is comparable in energy density to all of the optical and ultraviolet light emitted by all of the galaxies in the observable Universe.We have used the Submillimeter Array at 860 μm to observe the brightest submillimetre sources in 4 deg² of the Cosmology Legacy Survey. Previous interferometric studies have found a significant amount of multiplicity at the bright end of the single-dish number counts, suggesting a steepening in the drop-off brighter than 10 mJy, but these studies suffered from small-number statistics. We have targeted 75 of the brightest flux density-ordered single-dish SCUBA-2 sources down to approximately 10 mJy, achieving an average synthesized beam size of 2.4 arcsec and an average depth of 1.5 mJy in our primary beam-corrected maps, corresponding to 4σ detections of about 6 mJy. Our data is sufficient to distinguish between intrinsically bright galaxies and systems that break up into two ≳6 mJy galaxies with flux ratios less than 2 and separated by about 2 arcsec or more, corresponding to a physical distance of around 20 kpc at z=2. We include in our study 28 archival observations of similar nature, bringing our sample size to 103. We statistically deboost our flux density measurements and use these to compute the cumulative and differential number counts of our sample, finding them to be consistent with previous single-dish survey number counts within the uncertainties but with a systematic offset between 2 and 20 per cent. We compute the probability that a >10 mJy single-dish submm source resolves into two or more galaxies with a brightest to second-brightest flux density ratio less than 2 to be about 15 per cent. Assuming the remaining 85 per cent of the targets are ultra-luminous galaxies between redshifts 2 and 3, we find the surface density of >500 M⊙ yr-¹ sources to be 8^{+2}_{-1} deg-² and a likely volume density of 660^{+140}_{-120} Gpc-³.

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Contribution to the diffuse radio background from extragalactic radio sources (2011)

We examine the brightness of the Cosmic Radio Background (CRB) by comparing the contribution from individual source counts to absolute measurements. We use a compilation of radio counts to estimate the contribution of detected sources to the CRB in several different frequency bands. Using a Monte Carlo Markov Chain technique, we estimate the brightness values and uncertainties, paying attention to various sources of systematic error. At n = 150MHz, 325MHz, 408MHz, 610MHz, 1:4GHz, 4:8GHz, and 8:4GHz our calculated contributions to the backgroundsky temperature are 18, 2.8, 1.6, 0.71, 0.11, 0.0032, 0.0059 K, respectively.We then compare our results to absolute measurements from the ARCADE 2 experiment. If the ARCADE 2 measurements are correct and come from sources, then there must be an additional population of radio galaxies, fainter than where current data are probing. More specifically, the Euclidean-normalized counts at 1.4 GHz have to have an additional bump below about 10 μJy. We present preliminary results of investigating this new population by use of signal stacking. By stacking onto a very deep 1:4GHz radio map at source positions determined in the infrared and optical we hope to be able to see evidence of this population that would be too faint to be seen individually. Results are currently inconclusive. Future work will consist of modelling radio luminosity functions and new observations with the Extended VLA to continue to search for what may be causing this excess emission.

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Dimensionless cosmology (2011)

The variability of fundamental physical constants has been a topic of interest boththeoretically and experimentally for many years. Although it is interesting to investigate the consequences of such a variation, it is important to realise that onlythe variation of dimensionless combination of constants can be meaningfully measured and discussed. In this thesis, I try to justify this way of thinking and applyit to two basic cosmological observables, Big Bang Nucleosynthesis and CosmicMicrowave Background anisotropies. I will mention some related studies that areeither wrong or not complete because of being dimensionful.Variation of constants could be considered on two different levels. On the firstlevel one assumes that the constants are time invariant but they can assume differentvalues in different Universes or patches of sky. A thought experiment describinga discussion with aliens having a different system of units with different couplingconstants could be helpful, this idea will be returned to throughout the thesis. Onthe next level, the constants can be promoted to being smooth functions of time orspace. It is good to have a firm understanding of what happens on the previous levelbefore trying to consider genuinely variable constants. For variable constants weneed to consider theories beyond the currently accepted ones, which are capable ofconsistently describing such a variation.I briefly review the scalar-tensor theory of gravity as a possible way to describethe variation of the gravitational coupling. I give a brief historical review on thesubject and consider the theory in the two so called ‘frames’, discussing about thebenefits of each frame mathematically and the physical meaning of these frames.Such theories could form the frame work in which further study of variable con-stants could be carried out.

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Extra-galactic and survey science with the submillimeter common user bolometer array-2 (2011)

We have carried out a pilot study for the SCUBA-2 'All-Sky' Survey, SASSy, a wide and shallow mapping project at 850 μm, designed to find rare objects, both Galactic and extragalactic. Two distinct sets of exploratory observations were undertaken, and used to test the SASSy approach and data reduction pipeline.  The first was a 0.5° x 0.5° map around the nearby galaxy NGC 2559.  The galaxy was easily detected at 156 mJy, but no other convincing sources are present in the map.  Comparison with other galaxies with similar wavelength coverage indicates that NGC 2559 has relatively warm dust.  The second observations cover 1 deg² around the W5-E Hɪɪ region. As well as diffuse structure in the map, a filtering approach was able to extract 27 compact sources with signal-to-noise greater than 6.  By matching with data at other wavelengths we can see that the SCUBA-2 data can be used to discriminate the colder cores.  Together these observations show that the SASSy project will be able to meet its original goals of detecting new bright sources which will be ideal for follow-up observations with other facilities.We have also carried out a study of MS 0451-03, a massive galaxy cluster at z=0.5, strongly lensing a group of galaxies at z=3.  Imaging with SCUBA, and more recently with SCUBA-2, shows a prominent arc of submm emission, but lacks the resolution to break up this lensed structure into individual sources.  ALMA, even in Early Science mode, has the ability to finally resolve the giant submm arc.  A lensing reconstruction will allow us to relate the submm sources to lensed objects detected in other wavebands.  The cluster-scale lensing and the extent of the galaxy group make this a relatively simple system to investigate. Lessons learned here could help us understand the effects of differential lensing on the spectral energy distribution in more complicated galaxy-scale lens systems.

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