Browsing by Subject "cosmology"

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  • Mattila, Kalevi; Vaisanen, Petri (2019)
    The Extragalactic Background Light (EBL) stands for the mean surface brightness of the sky as we would see it from a representative vantage point in the intergalactic space outside of our Milky Way Galaxy. Averaged over the whole 4 pi solid angle it represents the collective light from all luminous matter radiated throughout the cosmic history. Part of the EBL is resolved into galaxies that, with the increasing detecting power of giant telescopes and sensitive detectors, are seen to deeper and deeper limiting magnitudes. This resolved part is now known to contribute a substantial or even the major part of the EBL. There still remains, however, the challenge of finding out to what extent galaxies too faint or too diffuse to be discerned individually, individual stars or emission by gas outside the galaxies, or - more speculatively - some hitherto unknown light sources such as decaying elementary particles are accounting for the remaining EBL. We review the recent progress that has been made in the measurement of EBL. The current photometric results suggest that there is, beyond the resolved galaxies, an EBL component that cannot be explained by diffuse galaxy halos or intergalactic stars.
  • Flender, Samuel (Helsingin yliopisto, 2014)
    One of the most important aspects of cosmology is the theory of structure formation, which describes the transition from the early, homogeneous Universe to the inhomogeneous Universe we observe today, i.e. the formation of stars, galaxies and clusters of galaxies. In this thesis, we study structure formation using the Newtonian theory of gravity within an expanding Friedmann-Robertson-Walker spacetime. We use this simple framework in order to learn for instance about the order of structure formation, which is a bottom-up evolution. Further, we introduce relativistic cosmological perturbation theory. We show that the Newtonian and relativistic descriptions of linear perturbations coincide on scales that are well inside the horizon. On larger scales however, we find differences between the two theories, in particular in the obtained linear matter power spectra. Observations indicate that the Universe is today in a phase of accelerated expansion. In the standard model of cosmology, the LCDM model, the accelerated expansion is explained by the existence of dark energy in form of a cosmological constant. Here, we focus on the integrated Sachs-Wolfe effect as a probe of the dynamical effects of dark energy. In particular, this effect causes an imprint of the local large-scale structure into the temperature anisotropies of the cosmic microwave background (CMB). We discuss how this effect arises in theory and how it can be measured in practice. The statistical properties of the temperature fluctuations in the CMB can be remarkably well described within the LCDM model. However, on the largest angular scales some features have been found that are difficult to explain within the standard model, the so-called CMB anomalies. Here, we discuss these anomalies from a statistical point of view. We focus on one particular anomaly, the hemispherical power asymmetry, and explore its connection to the initial conditions of the Universe. In particular, this asymmetry can be related to primordial non-Gaussianity in certain inflation models.
  • Salo, Sirpa (2008)
    This is a study of people’s eclectic understanding of illnesses and inconsistence in illness management in a Nepali village called Bholung. The aim of the discussion is to illustrate and explain how the villagers and local healers make cultural sense of their illnesses which are thought to have a supernatural origin. I aim to explain what kinds of personal and socio-cultural meanings the villagers and the village healers give to experiences of being ill – and why. By analysing how the local Hindu culture and society shape the villagers’ ways of seeing and being in the world I aim to explain how these matters contribute to culturally recognised forms of being ill and getting well in Bholung. I did village based research for seven months in 2003. My fieldwork was focused on the village of thirty-five households and some 180 people. My material consists of structured and unstructured, informal interviews and participant observation. I interviewed fourteen villagers of whom five were women. The interviewees were between forty-five and sixty-six years old. Structured direct observation I practiced during healing sessions, daily puja rituals, purification rituals and for example during annual offering rituals. Hindu rules and restrictions and local household rites and rituals influence how the villagers know and understand their holistic cosmology and their hierarchical social system. Also, they influence how the villagers know and experience their own bodies, how they explain and interpret - depending on their personal motives and needs, social pressure and constant socio cultural changes - the causes and consequences of some of their illnesses, and how they further deal with them at the village level. I suggest that the hierarchical Hindu order is constructed and maintained in Bholung household rites and rituals, and in healing rituals, not only because the maintenance of the holistic social system is believed to require such created differences between people and places, but also because the hierarchical order stands for purity, the ideal order of relations and of being related in Bholung.
  • Annala, Jaakko (Helsingin yliopisto, 2020)
    We study how higher-order gravity affects Higgs inflation in the Palatini formulation. We first review the metric and Palatini formulations in comparative manner and discuss their differences. Next cosmic inflation driven by a scalar field and inflationary observables are discussed. After this we review the Higgs inflation and compute the inflationary observables both in the metric and Palatini formulations. We then consider adding higher-order terms of the curvature to the action. We derive the equations of motion for the most general action quadratic in the curvature that does not violate parity in both the metric and Palatini formulations. Finally we present a new result. We analyse Higgs inflation in the Palatini formulation with higher-order curvature terms. We consider a simplified scenario where only terms constructed from the symmetric part of the Ricci tensor are added to the action. This implies that there are no new gravitational degrees of freedom, which makes the analysis easier. As a new result we found out that the scalar perturbation spectrum is unchanged, but the tensor perturbation spectrum is suppressed by the higher-order curvature couplings.
  • Berlea, Vlad Dumitru (Helsingin yliopisto, 2020)
    The nature of dark matter (DM) is one of the outstanding problems of modern physics. The existence of dark matter implies physics beyond the Standard Model (SM), as the SM doesn’t contain any viable DM candidates. Dark matter manifests itself through various cosmological and astrophysical observations of the rotational speeds of galaxies, structure formation, measurements of the Cosmic Microwave Background (CMB) and gravitational lensing of galaxy clusters. An attractive explanation of the observed dark matter density is provided by the WIMP (Weakly Interacting Massive Particle) paradigm. In the following thesis I explore this idea within the well motivated Higgs portal framework. In particular, I explore three options for dark matter composition: a scalar field and U(1) and SU(2) hidden gauge Fields. I find that the WIMP paradigm is still consistent with the data. Even though it finds itself under pressure from direct detection experiments, it is not yet in crisis. Simple and well motivated WIMP models can fit the observed DM density without violating the collider and direct DM detection constraints.
  • Aghanim, N.; Ashdown, M.; Aumont, J.; Baccigalupi, C.; Ballardini, M.; Banday, A. J.; Barreiro, R. B.; Bartolo, N.; Basak, S.; Benabed, K.; Bernard, J. -P.; Bersanelli, M.; Bielewicz, P.; Bonaldi, A.; Bonavera, L.; Bond, J. R.; Borrill, J.; Bouchet, F. R.; Boulanger, F.; Bracco, A.; Burigana, C.; Calabrese, E.; Cardoso, J. -F.; Chiang, H. C.; Colombo, L. P. L.; Combet, C.; Comis, B.; Crill, B. P.; Curto, A.; Cuttaia, F.; Davis, R. J.; de Bernardis, P.; de Rosa, A.; de Zotti, G.; Delabrouille, J.; Delouis, J. -M.; Di Valentino, E.; Dickinson, C.; Diego, J. M.; Dore, O.; Douspis, M.; Ducout, A.; Dupac, X.; Dusini, S.; Keihänen, E.; Kurki-Suonio, H.; Lähteenmäki, A.; Savelainen, M.; Suur-Uski, A. -S.; Väliviita, J.; Planck Collaboration (2017)
    The characterization of the Galactic foregrounds has been shown to be the main obstacle in the challenging quest to detect primordial B-modes in the polarized microwave sky. We make use of the Planck-HFI 2015 data release at high frequencies to place new constraints on the properties of the polarized thermal dust emission at high Galactic latitudes. Here, we specifically study the spatial variability of the dust polarized spectral energy distribution (SED), and its potential impact on the determination of the tensor-to-scalar ratio, r. We use the correlation ratio of the CBB `angular power spectra between the 217 and 353 GHz channels as a tracer of these potential variations, computed on different high Galactic latitude regions, ranging from 80% to 20% of the sky. The new insight from Planck data is a departure of the correlation ratio from unity that cannot be attributed to a spurious decorrelation due to the cosmic microwave background, instrumental noise, or instrumental systematics. The effect is marginally detected on each region, but the statistical combination of all the regions gives more than 99% confidence for this variation in polarized dust properties. In addition, we show that the decorrelation increases when there is a decrease in the mean column density of the region of the sky being considered, and we propose a simple power-law empirical model for this dependence, which matches what is seen in the Planck data. We explore the effect that this measured decorrelation has on simulations of the BICEP2-Keck Array/Planck analysis and show that the 2015 constraints from these data still allow a decorrelation between the dust at 150 and 353 GHz that is compatible with our measured value. Finally, using simplified models, we show that either spatial variation of the dust SED or of the dust polarization angle are able to produce decorrelations between 217 and 353 GHz data similar to the values we observe in the data.
  • Annila, Arto (2016)
    Rotation of galaxies is examined by the general principle of least action. This law of nature describes a system in its surroundings, here specifically a galaxy in the surrounding Universe. According to this holistic theory the gravitational potential due to all matter in the expanding Universe relates to the universal curvature which, in turn, manifests itself as the universal acceleration. Then the orbital velocities from the central bulge to distant perimeters are understood to balance both the galactic and universal acceleration. Since the galactic acceleration decreases with distance from the galaxy's center to its luminous edge, the orbital velocities of ever more distant stars and gas clouds tend toward a value that tallies the universal acceleration. This tiny term has been acknowledged earlier by including it as a parameter in the modified gravitational law, but here the tiny acceleration is understood to result from the gravitational potential that spans across the expanding Universe. This resolution of the galaxy rotation problem is compared with observations and contrasted with models of dark matter. Also, other astronomical observations that have been interpreted as evidence for dark matter are discussed in light of the least-action principle.
  • Bracho Blok, Fernando Arturo (Helsingin yliopisto, 2020)
    We study single scalar field inflation with the standard model Higgs boson as the inflaton. We first review the homogeneous and isotropic description of the universe given by the FLRW model as well as the inflation scenario. Then we study how this scenario can can be achieved by a single scalar field minimally coupled to gravity in the slow-roll approximation. Next we study linear perturbation theory around the FLRW background. Here the perturbations are decoupled into scalar, vector and tensor perturbations which allows to study them separately. The split of physical quantities into perturbations around a background introduces gauge degrees of freedom which we address by reviewing gauge transformation of the scalar and tensor perturbations (the latter which turns out to be gauge-independent). We then use the comoving gauge and define, for the scalar perturbations, the gauge-invariant quantity known as the comoving curvature perturbation. For scalar perturbations the Einstein Field equation yields the Mukhanov-Sasaki equation, which we solve to first order in the slow-roll approximation in terms of the Mukhanov variable. We then quantize this variable using canonical quantization and calculate the power spectrum from vacuum fluctuations. We also carry the same analysis for tensor perturbations. With the power spectra at hand we introduce the spectral parameters and discuss current observations and constraints on such parameters. In Higgs inflation the Standard Model Higgs boson takes the role of the inflaton. Here the Higgs field is also coupled to the Ricci scalar, giving us a non-minimal coupling to gravity. This coupling can be transformed away using a conformal transformation at the expense of a field re-definition. This enables us to use the results reviewed thus far. At tree level we find the inflationary predictions to be in excellent agreement with current observations. However, quantum corrections complicate this picture. We review the tree level unitarity of the model and examine arguments in favour and against it. We also study how quantum corrections can qualitatively change the shape of the potential and the viability of Higgs inflation in each scenario.
  • Keskitalo, Reijo (Helsingfors universitet, 2005)
    As a full-grown science, cosmology is relatively young. Even though man has pondered the existence and structure of the universe throughout his history, the lack of actual observational data has prevented analytical research. Observational cosmology can be seen to have born in the 1920’s when Edwin Hubble discovered that the galaxies surrounding us are receding in all directions. This led to the conclusion that the universe around us is itself actually expanding. Expansion occurring isotropically in all directions indicates that the universe was once much denser and hotter. So hot that the matter in it has been completely ionized plasma. The decrease in temperature caused by the expansion is calculated to have caused the neutralizing of the plasma, recombination, over thirteen billion years ago. The instant is cosmologically remarkable, since light that until that moment scattered frequently from the charged particles now began to propagate freely. Initially at three thousand Kelvin temperature, the radiation has cooled down due to expansion and is now observed as the three Kelvin cosmic microwave background radiation (CMB). First observations of the existence of the CMB date back to 1965. Since the background radiation has traveled its long journey relatively unchanged, its study can yield direct information on the conditions of the early universe. Theoretically it was expected, well before observational confirmation in 1992, that the CMB should have a structure that reflects those inhomogeneities, that have now undergone their ten billion years of evolution, to become the large scale structure we observe: galaxies, galaxy clusters and the evermore larger entities. In this thesis we examine, how the effects of two cosmological parameters, the matter and baryon densities of the universe, manifest in the pre-recombination dynamics and how these effects are reflected in the structure of the observed CMB anisotropy. Baryons are the “ordinary” matter all around us, protons and neutrons. The concept of “matter” is extended to include the unknown dark matter, the existence of which is only known through its gravitational effects. We will review the equations that are necessary to track the evolution of the primordial perturbations. By a computer program based on those equations we display how the early universe dynamics change with the values of the density parameters. Finally we will show how these effects are reflected in the angular power spectrum that describes the structure of the microwave background.
  • Gibson, Clint (Helsingfors universitet, 2017)
    Albert Einstein’s General Theory of Relativity radically transformed our understanding of gravitation. Along with this transformative view came several powerful predictions. One of these predictions, the deflection of light in a gravitational field, has proven in recent decades to be crucial to the study of cosmology. In this work we present the foundational theory of gravitational lensing, with a particular focus on the weak regime of lensing. Weak gravitational lensing produced by the large scale structure, called cosmic shear, induces percent level distortions in the images of distant galaxies. Gravitational lensing is of particular interest, since the image distortions are due to all of the matter in the large scale structure, including dark matter. We present the definitions of shear and convergence which are used to quantify the source galaxy image distortions, and discuss some techniques shown in literature which are used for measuring these quantities. This includes presenting the necessary derivations which connect these quantities to two particular classes of results: mass map reconstructions and cosmological parameter constraints. We present some results obtained in recent years: mass map reconstructions obtained using the Canada-France-Hawaii-Telescope Lensing Survey (CFHTLenS) and the Cosmological Evolution Survey (COSMOS), and constraints on the parameters Ω_m and σ_8 (the total matter density parameter and the power spectrum normalization) obtained using CFHTLenS, COSMOS, the Kilo Degree Survey (KiDS), and the Dark Energy Survey (DES). This includes some discussion of apparent tensions with results obtained from Planck (using observations of the cosmic microwave background—a completely different cosmological probe) and of some inconsistencies within the more recent survey results.