Research :: PhD Topics for 2015 :: Cosmology

Topics under the thematic line Unveiling the dynamics of the Universeavailable for the 2015 Call (10 PhD Topics):

  • 2015/408 – Probing fundamental physics with scalar fields and cosmic defects using state of the art cosmological observations
  • 2015/409 – The nature of the Universe’s acceleration
  • 2015/410 – The ESPRESSO Road to Fundamental Cosmology
  • 2015/411 – Astrophysical and cosmological applications of modified theories of gravity
  • 2015/412 – Cosmological weak lensing and galaxy formation effects with Euclid
  • 2015/413 – Unveiling the Dark Side of the Universe
  • 2015/414 – Acceleration of the late universe: Modified gravity theories or Back-reaction?
  • 2015/415 – Polarisation of the Cosmic Microwave Background Radiation
  • 2015/416 – Coding the Cosmos: A New Generation of Superstring Simulations
  • 2015/417 - New Maps of the Dark Side

For details, please see below the abstract and advisors for each topic. Prospective candidates are welcome to contact directly the proposers of the topics for inquiries and further details.

2015/408 – Probing fundamental physics with scalar fields and cosmic defects using state of the art cosmological observations

Advisors: Pedro Avelino (IA-U.Porto & FCUP), Lara Sousa (IA-U.Porto)

The recent discovery of the Higgs Boson at the Large Hadron Collider appears to support the idea that the universe underwent, in its early history, a series of symmetry-breaking phase transitions and that, as a consequence, networks of topological defects could have been generated. These defect networks, although formed in the early universe, are expected to survive throughout the cosmological history, potentially leaving behind a plethora of observational signatures. The study of cosmic defects and their signatures, then offers an insight into the physics of the early universe. Compellingly, the recent suggestion that fundamental strings and 1-dimensional Dirichlet branes – the fundamental objects of Superstring theory – may play the role of cosmic strings, extends this possibility towards very early cosmological times into energy scales far beyond the reach of current particle accelerators.

Surveys of the cosmological 21cm signal – using SKA and LOFAR – will probe the matter distribution of the universe during the “dark ages”, potentially unveiling the role of small-scale density perturbations generated by cosmic strings and other cosmic defects in structure formation. On the other hand, the gravitational wave background will be probed with unprecedented sensitivity by a new generation of gravitational wave experiments (e.g. eLISA, LIGO) and the Cosmic Microwave Background B-mode polarization power spectrum will be determined with unprecedented precision by present and future missions such as Planck and CMBPol. Together these provide new observational windows for the study of cosmic defects and their associated vector and tensor perturbations which will be extensively explored in this project. The opportunities to gain information about the physics of the early universe through the search for topological defects are, thus, manifold.

This PhD project aims at significantly improving current constraints on cosmic defects, by making use of the latest data and realistic numerical and semi-analytical models for defect network evolution. Particular emphasis will be given to the gravitational wave background generated by cosmic strings and domain walls and its potential impact on the B-mode polarization of the Cosmic Microwave Background (CMB), as well as to the characterization of specific string signatures on the 21cm background and their impact on reionization history. The potential role of domain walls as seeds of space-time variations of fundamental couplings shall also be investigated, taking full advantage of the window opened by a new generation of high-resolution ultra-stable spectrographs such as ESPRESSO.

University: Porto

2015/409 – The nature of the Universe’s acceleration

Advisors: Nelson Nunes (IA-U.Lisboa)

Over a decade has passed since supernova observations first indicated a remarkable property of our Universe: it is currently undergoing a period of accelerated expansion. In the intervening time, this acceleration has been confirmed by a range of complementary observational probes, such as those of the cosmic microwave background and of large scale structure and baryonic acoustic oscillations. Yet, despite this plethora of observational evidence, the theoretical model that explains this phase of evolution remains unknown.

In this project we aim to take a step towards determining the theoretical model of present-day acceleration by analysing an extremely general class of models, which can have intriguing theoretical properties, to evaluate if they can also be observationally consistent. This investigation has both a theoretical and a hands on data component. A combination of analytical and numerical methods is, therefore, required to construct and test these models against current and forthcoming data.

This study is extremely timely given the current push to understand theoretical models concerning dark energy and the cosmological constant in time for them to be confronted by the data of the Euclid mission.

University: Lisboa

2015/410c – The ESPRESSO Road to Fundamental Cosmology

Advisors: Carlos Martins (IA-U.Porto)

The full cosmological impact of astrophysical tests of fundamental physics, including those of the spacetime stability of nature’s fundamental couplings, has only emerged recently. In part this was due to the work done within CAUP’s ‘Dark Side’ project, and these tests are now being included in studies for ESA and ESO’s next-generation of facilities.

While a detection of variations of fundamental couplings will be direct evidence of Equivalence Principle violations and a fifth force in Nature, any such measurements (even null results) can provide tight constraints on additional dynamical degrees of freedom (such as fundamental scalar fields) that are now known to be among Nature’s building blocks.

This thesis will explore the role of these astrophysical tests on cosmology and fundamental physics, with some emphasis on dynamical dark energy. Starting with the experience gained from a recent UVES Large Program, the work will concentrate on the ESPRESSO fundamental physics tests, contributing both to the GTO preparation and its subsequent exploitation. The ESPRESSO experience will also lead to more detailed and realistic feasibility studies for ELT-HIRES.

University: Porto

Note: This is a closed topic.

2015/411 – Astrophysical and cosmological applications of modified theories of gravity

Advisors: Francisco Lobo (IA-U.Lisboa), Nelson Nunes (IA-U.Lisboa)

The late-time cosmic accelerated expansion is one of the most important and challenging current problems in cosmology. Although models of dark energy are the most popular candidates responsible for the cosmic expansion, the latter may be due to modifications of General Relativity, which introduce new degrees of freedom to the gravitational sector itself. This research project will explore the viability of a plethora of modified gravity models, consistently analysing the reproduction of all the cosmological epochs. More specifically, we will consider generalizations of the Einstein-Hilbert action by including non-linear curvature invariants and Lagrangians that also include curvature couplings to the matter sector. Another class of theories under our scrutiny will be generalised scalar-tensor or vector-tensor gravity theories, where scalar or vector fields play gravitational roles that can also be perceived as couplings to matter in an appropriate frame. One fundamental goal of this project is to study the theoretical issues of the extra degrees of freedom of the theory and to analyse its astrophysical applications.

University: Lisboa

2015/412 – Cosmological weak lensing and galaxy formation effects with Euclid

Advisors: Ismael Tereno (IA-U.Lisboa), António da Silva (IA-U.Lisboa)

Cosmological weak lensing is a powerful probe of gravity, geometry and mass in the Universe, and is in the core of the forthcoming Euclid space mission. Weak lensing cosmological measurements are however contaminated by astrophysical effects originated by mechanisms that operate during the galaxy formation process. In order to achieve highly precise constraints in the parameters of the dark universe, the contamination must be mitigated. For this, the effects need to be studied and modelled and their impact on the cosmological signal understood.

This project aims to investigate the process of angular momentum acquisition of galaxies using dedicated state-of-the-art N-body simulations of large-scale structure. With this investigation we will be able to design effective models for intrinsic alignments and to estimate their effects in weak lensing data. Finally, we will define estimators to efficiently combine the modelled effects with forecasted Euclid cosmic shear data, in order to mitigate the impact of intrinsic alignments of galaxies in cosmological parameters constraints.

University: Lisboa

2015/413 – Unveiling the Dark Side of the Universe

Advisors: Pedro Avelino (IA-U.Porto & FCUP)

Unveiling the nature of dark matter and dark energy, the main constituents of the Universe, is one of the most ambitious challenges of fundamental physics. The main goal of this project is to provide a contribution towards this major objective through the parameterization, characterization and constraining of coupled dark matter/dark energy models. This project contemplates both theoretical and numerical tasks, including the computation of the cosmological implications of coupled dark matter/dark energy models taking into account nonlinear back reaction effects.. Numerical simulations and semi-analytical methods will be employed in the modeling of the dark matter/dark energy interaction both at microscopic and macroscopic levels. State of the art cosmological observations, using type Ia supernovae, large scale clustering (including baryon acoustic oscillations), cosmic microwave background temperature and polarization anisotropies, and weak lensing, will be used to constrain the parameter space of coupled dark matter/dark energy scenarios. Forecasts of the results to be obtained with future missions, such as EUCLID and ESPRESSO, will also be performed.

University: Porto

2015/414 – Acceleration of the late universe: Modified gravity theories or Back-reaction?

Advisors: José Pedro Mimoso (IA-U.Lisboa & FCUL), Francisco Lobo (IA-U.Lisboa), António da Silva (IA-U.Lisboa & FCUL)

The proposed research plan is aimed at explaining the observed, late acceleration of the Universe, looking beyond the standard theory of gravity, and assessing the implications of in-homogeneities of the space-time. Instead of assuming the existence of some sort of exotic matter, it focus on the approaches that consider extensions of the gravitational theory itself and the assumption of spatial inhomogeneity. The latter envisage modifications of the geometrical sector of the theory. So a possible degeneracy between the predictions arising from modified gravity theory and those of the so-called back-reaction prescription will be investigated. Ultimately this means on the one hand, thoroughly comparing both prescriptions, and, on the other hand, testing extended models both from modified gravity, and back-reaction against astrophysical and laboratory measurements, as well as laboratory and space-based Equivalence Principle experiments to put forward discriminators. The work plan inscribes itself in the goals pursued by the “Galaxies, Cosmology and the Evolution of the Universe” group of the Institute for Astrophysics and Space Sciences (IA), and is part of the contribution of its Thematic Line “Unveiling the Dynamics of the Universe” to the Theory and Cosmological Simulations working groups of the Euclid space mission.

University: Lisboa

2015/415 – Polarisation of the Cosmic Microwave Background Radiation

Advisors: C. Sofia Carvalho (IA-U.Lisboa), António da Silva (IA-U.Lisboa)

Cosmology is presently a very active field because of the large number of observations that are becoming available and that will allow us to characterize with great precision the nature and physical origin of the primordial cosmological perturbations, as well as of dark matter and dark energy. The cosmic microwave background (CMB) radiation provides a wealth of cosmological information that has led to major advances on our knowledge about the origin and evolution of the Universe. One of the ultimate challenges for CMB observations is to fully probe the CMB polarisation spectrum, from large to small angular scales, and to decipher the information encoded in the polarisation signal. This project is centred on cosmological data analysis and modelling in the context of the Planck and Euclid ESA satellite missions. It comprises two main objectives.

The first objective consists in developing and implementing a novel estimator of the primary polarisation signal of the CMB. This signal can be measured from the gravitational lensing field to be extracted from CMB maps. This type of analysis will be particularly important in the sequence of the public release of CMB polarisation data by the Planck Collaboration and in the context of the tasks of the CMB Cross-Correlations Science Working Group of the Euclid Consortium.

The second objective consists in assessing the robustness of the estimator of the primary CMB with respect to contamination by secondary CMB polarisation effects, which is not being considered in the current CMB data analysis. This requires the development of a detailed model of the secondary polarisation signals, such as the CMB quadrupole and double-scattering induced polarisations, which act as contaminants in the detection of the primary signal. This topic is related to the detection of CMB secondary polarisation by galaxy clusters with the ALMA interferometer. Another interesting topic of research that may be addressed within this project is the development of methods for the separation of the different polarisation components at the ALMA frequencies.

University: Lisboa

2015/416 – Coding the Cosmos: A New Generation of Superstring Simulations

Advisors: Carlos Martins (IA-U.Porto)

Cosmic strings arise naturally in many proposed theories of new physics beyond the standard model unifying the electroweak and strong interactions, as well as in many superstring inspired inflation models. In the latter case, fundamental superstrings produced in the very early universe may have stretched to macroscopic scales, in which case they are known as cosmic superstrings. If observed, these objects thus provide a unique window into the early universe and possibly string theory.

The recently released Planck results highlight how some of these scenarios can be constrained by high-resolution data. However, they also show that the current bottleneck is the lack of accurate high-resolution simulations of defect networks that can be used as templates for detailed statistical analysis. This is expected to be an even bigger problem for next-generation facilities such as COrE+ and eLisa. The goal of this thesis will thus be to go significantly beyond the state-of-the-art and develop and implement a new generation of high-scalability HPC defect codes that will be able to match the sensitivity of forthcoming observational searches.

University: Porto

2015/417 – New Maps of the Dark Side

Advisors: Carlos Martins (IA-U.Porto)

The growing amount of observational evidence for the recent acceleration of the universe unambiguously demonstrates that canonical theories of cosmology and particle physics are incomplete—if not incorrect—and that new physics is out there, waiting to be discovered. The most fundamental task for the next generation of astrophysical facilities is therefore to search for, identify and ultimately characterise this new physics. The acceleration is seemingly due to a dark component whose low-redshift gravitational behaviour is very similar to that of a cosmological constant. However, currently available data provides very little information about the high-redshift behaviour of this dark sector or its interactions with the rest of the degrees of freedom in the model.

It is becoming increasing clear that tackling the dark energy enigma will entail significantly extending the redshift range where its behaviour can be accurately mapped. A new generation of ESA and ESO facilities, such as Euclid, the E-ELT, and the SKA have dark energy characterization as a key science driver, and in addition to significantly increasing the range and sensitivity of current observational probes will allow for entirely new tests. The goal of this thesis will be to carry out a systematic exploration of the landscape of physically viable dark energy paradigms and provide optimal discriminating observational tests. The work will initially focus on Euclid (in which the dark side team is more directly involved) and will gradually broaden to explore synergies with other facilities.

University: Porto