There is by now compelling evidence that most of the matter in the Universe is in the form of dark matter, a form of matter quite different from the matter we experience in every day life. The gravitational effects of this dark matter have been observed in many different ways but its true nature is still unknown. In most models, dark matter particles can annihilate with each other into standard model particles; the direct or indirect observation of such annihilation products could give important clues for the dark matter puzzle. For signals from dark matter annihilations to be detectable, typically high dark matter densities are required. Massive objects, such as stars, can increase the local dark matter density both via scattering off nucleons and by pulling in dark matter gravitationally as a star forms. Annihilations within this kind of dark matter population gravitationally bound to a star, like the Sun, give rise to a gamma ray flux. For a star which has a planetary system, dark matter can become gravitationally bound also through gravitational interactions with the planets. The interplay between the different dark matter populations in the solar system is analyzed, shedding new light on dark matter annihilations inside celestial bodies and improving the predicted experimental reach. Dark matter annihilations inside a star would also deposit energy in the star which, if abundant enough, could alter the stellar evolution. This is investigated for the very first stars in the Universe. Finally, there is a possibility for abundant small scale dark matter overdensities to have formed in the early Universe. Prospects of detecting gamma rays from such minihalos, which have survived until the present day, are discussed. / Kosmologiska observationer har visat att större delen av materian i universum består av mörk materia, en form av materia med helt andra egenskaper än den vi upplever i vardagslivet. Effekterna av denna mörka materia har observerats gravitationellt på många olika sätt men vad den egentligen består av är fortfarande okänt. I de flesta modeller kan mörk materia-partiklar annihilera med varandra till standardmodellpartiklar. Att direkt eller indirekt observera sådana annihilationsprodukter kan ge viktiga ledtrådar om vad den mörka materian består av. För att kunna detektera sådana signaler fordras typiskt höga densiteter av mörk materia. Stjärnor kan lokalt öka densiteten av mörk materia, både via spridning mot atomkärnor i stjärnan och genom den ökande gravitationskraften i samband med att en stjärna föds. Annihilationer inom en sådan mörk materia-population gravitationellt bunden till en stjärna, till exempel solen, ger upphov till ett flöde av gammastrålning, som beräknas. För en stjärna som har ett planetsystem kan mörk materia även bli infångad genom gravitationell växelverkan med planeterna. Samspelet mellan de två mörk materia-populationerna i solsystemet analyseras, vilket ger nya insikter om mörk materia-annihilationer inuti himlakroppar och förbättrar de experimentella möjligheterna att detektera dem. Mörk materia-annihilationer inuti en stjärna utgör också en extra energikälla för stjärnan, vilket kan påverka stjärnans utveckling om mörk materia-densiteten blir tillräckligt stor. Denna effekt undersöks för de allra första stjärnorna i universum. Slutligen finns det också en möjlighet att det i det tidiga universum skapades mörk materia-ansamlingar som fortfarande finns kvar idag. Utsikterna att upptäcka dessa genom mätning av gammastrålning diskuteras. / QC 20120130

Observations from a broad range of astrophysical scales have forced us to the realization that the well understood matter comprising the stars and galaxies we see around us accounts for only a small fraction of the total mass of the Universe. An amount roughly five times larger exist in the form of dark matter about which we have virtually no direct evidence apart from its large scale gravitational effects. It is also known that the largest contribution to the energy density of the universe is the dark energy, a negative pressure form of energy which will not be dealt with here. I will present a candidate for the dark matter which is based completely in known physics and which presents several possible observational signatures. In this model the dark matter is composed of dense nuggets of baryonic matter and antimatter in a colour superconducting state. If these object are sufficiently massive their low number density will make them effectively dark in the sense that collisions with visible matter become infrequent. This work presents the basics of dark matter as a colour superconductor and then uses the physical properties of the quark nuggets to extract observational consequences.

I have used an analytic model of tidal interactions to predict the evolution of a substructure in a static dark matter halo. Given the initial conditions of the satellite and background halo, the model predicts with high accuracy the mass loss of the satellite and also its density evolution. The main phenomena taken into account in the model are tidal truncation at the tidal radius of the satellite and heating due to tidal shocks at the pericenter of its orbit. To calibrate and test the model, it has been compared with numerical simulations of a satellite orbiting in a static dark matter halo. The model predicts a set of tidal radii for the satellite in different stages of its evolution. The mass of the satellite is accurately calculated at each stage by truncating an NFW (Navarro, Frenk and White) profile at the tidal radius. The mass lost beyond the tidal limit is scaled by half the instantaneous orbital period of the satellite. The model can also be used to predict analytically the new density profile of the satellite. This new profile is given by a modification of the NFW density profile as a function of radius. The tidal radius is the only parameter going into this modification. The effect of numerical relaxation has been studied and quantified by performing the same simulations in lower resolutions. I find that substructures with less than 1000 particles are artificially relaxed and this process affects their mass loss and results in their premature disruptions. This underlines the utility of an analytic model predicting the evolution of substructures in minor mergers.

We consider the prospects to use polarized dark-matter detectors to discriminate between various dark-matter models. If WIMPs are fermions and participate in parity-violating interactions with ordinary matter, then the recoil-direction and recoil-energy distributions of nuclei in detectors will depend on the orientation of the initial nuclear spin with respect to the velocity of the detector through the Galactic halo. If, however, WIMPS are scalars, the only possible polarization-dependent interactions are extremely velocity-suppressed and, therefore, unobservable. Since the amplitude of this polarization modulation is fixed by the detector speed through the halo, in units of the speed of light, exposures several times larger than those of current experiments will be required to be probe this effect. / text

Despite the great successes of the Cold Dark Matter (CDM) model in explaining a wide range of observations of the global evolution and the formation of galaxies and large-scale structure in the universe, the origin and microscopic nature of this dark matter is still unknown. The most common form of CDM considered to-date is that of Weakly Interacting Massive Particles (WIMPs), but some of the cosmological predictions for this kind of CDM are in apparent conflict with observations (e.g. cuspy-cored halos and an overabundance of satellite dwarf galaxies). For these reasons, it is important to consider the consequences of different forms of CDM. We focus here on the hypothesis that the dark matter is comprised, instead, of ultralight bosons that form a Bose-Einstein Condensate (BEC), described by a complex scalar field. We start from the Klein-Gordon and Einstein field equations to describe the evolution of the Friedmann-Robertson-Walker (FRW) universe in the presence of this kind of dark matter. We find that, in addition to the phases of radiation-domination (RD), matter-domination (MD) and Lambda-domination (LD) familiar from the standard CDM model, there is an earlier phase of scalar-field-domination (SFD) which is special to this model. In addition, while WIMP CDM is non-relativistic at all times after it decouples, the equation of state of BEC-SFDM is found to be relativistic at early times, evolving from incompressible ($\bar{p} = \bar{\rho}$) to radiation-like ($\bar{p} = \bar{\rho}/3$), before it becomes non-relativistic and CDM-like at late times. The timing of the transitions between these phases and regimes is shown to yield fundamental constraints on the particle mass and self-interaction coupling strength. We also discuss progress on the description of structure formation in this model, which includes additional constraints on these parameters. / text

published_or_final_version / Physics / Master / Master of Philosophy

Observations from a broad range of astrophysical scales have forced us to the realization that the well understood matter comprising the stars and galaxies we see around us accounts for only a small fraction of the total mass of the Universe. An amount roughly five times larger exist in the form of dark matter about which we have virtually no direct evidence apart from its large scale gravitational effects. It is also known that the largest contribution to the energy density of the universe is the dark energy, a negative pressure form of energy which will not be dealt with here. I will present a candidate for the dark matter which is based completely in known physics and which presents several possible observational signatures. In this model the dark matter is composed of dense nuggets of baryonic matter and antimatter in a colour superconducting state. If these object are sufficiently massive their low number density will make them effectively dark in the sense that collisions with visible matter become infrequent. This work presents the basics of dark matter as a colour superconductor and then uses the physical properties of the quark nuggets to extract observational consequences.

I have used an analytic model of tidal interactions to predict the evolution of a substructure in a static dark matter halo. Given the initial conditions of the satellite and background halo, the model predicts with high accuracy the mass loss of the satellite and also its density evolution. The main phenomena taken into account in the model are tidal truncation at the tidal radius of the satellite and heating due to tidal shocks at the pericenter of its orbit. To calibrate and test the model, it has been compared with numerical simulations of a satellite orbiting in a static dark matter halo. The model predicts a set of tidal radii for the satellite in different stages of its evolution. The mass of the satellite is accurately calculated at each stage by truncating an NFW (Navarro, Frenk and White) profile at the tidal radius. The mass lost beyond the tidal limit is scaled by half the instantaneous orbital period of the satellite. The model can also be used to predict analytically the new density profile of the satellite. This new profile is given by a modification of the NFW density profile as a function of radius. The tidal radius is the only parameter going into this modification. The effect of numerical relaxation has been studied and quantified by performing the same simulations in lower resolutions. I find that substructures with less than 1000 particles are artificially relaxed and this process affects their mass loss and results in their premature disruptions. This underlines the utility of an analytic model predicting the evolution of substructures in minor mergers.

DEAP (Dark Matter Experiment with Argon and Pulse Shape Discrimination) is an experiment that aims to directly detect dark matter particles via nuclear recoils in liquid argon. The experiment uses the scintillation property of liquid argon as a means to discriminate the γ and β backgrounds from the expected signal. DEAP-1 is a 7 kg single phase liquid argon detector. It was constructed to demonstrate the scalability for a larger (3600 kg) detector. The detector was originally operated at Queen’s University, where the background rejection level achieved was 6.3×10−8 for the recoil detection efficiency of 97.1%. The detector was relocated to SNOLAB, where the background in the energy region of interest was reduced by a factor of 7.7 (from 4.61±0.17 mHz to 0.60±0.05 mHz.). The background rejection level of 9.64×10−9 (10.4 part per billion) was achieved from the combined data set (Queen’s University and SNOLAB) for a recoil detection efficiency of 35.5 ± 1.3 %. With the current background rate, the background rejection level required for the 3600 kg detector (1.8×10−9 ) is projected to be achieved in 382 days at the neutron efficiency of 9.1±0.6 %. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2009-01-26 09:28:25.432

The PICASSO experiment located 2 km underground in SNOLAB is directly searching for dark matter signals by looking for interactions between dark matter particles and an active target made of superheated droplets of freon C4F10. During the interaction, energy is deposited to the freon triggering a phase transition, inducing pressure waves which are detected by piezo-electric sensors. A temperature dependent analysis of the amplitudes of the signals for detector 71 showed that, above 25 ◦C, between 20 and 80 % of the events were saturated implying that the preamplifiers had too high a gain. Decreasing this gain by a fixed factor was not found to be a suitable solution to the problem. Ideally, a temperature dependent gain should be established. In addition, some channels have intrinsic problems and should be repaired. A threshold analysis was used to establish the trigger efficiency which was found to be 90% above 25 ◦C but only 50% at lower temperatures with the current setting of the threshold. A temperature dependent threshold setting has been proposed. A new setup at Queen’s University has been built and a gamma calibration using three different radioactive sources (22Na,137Cs,57Co) was undertaken leading to a new detector response curve for gammas. For a proper analysis, new and more appropriate cuts were implemented. The analysis confirmed the expectation that the PICASSO detectors are mostly blind to gammas below 50 ◦C. However, the detectors appear to be more sensitive to 122 keV gammas than to 622 keV gammas by a factor of about 10. The sensitivity for 22Na also differs by a large factor from what was expected from old calibrations on detectors with much smaller bubbles. The rate plots exhibit a strong exponential increase in rate above 40 ◦C which is not due to any of the gamma sources used, but could be due to neutrons or low energy x-rays. This remains under investigation. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2009-09-24 18:28:21.308