Title: Acceleration from superhorizon fluctuations

Can the observed acceleration of the universal expansion be explained by superhorizon fluctuations?

We expect superhorizon fluctuations to have an effect on the inferred expansion rate,  with the magnitude of the effect connected tothe time derivative of the fluctuations. In order to explain the accelerated expansion, we probably need time derivatives to be so large that we exit the perturbative regime.

First, investigate the role of superhorizon fluctuations on the apparent background expansion in the linear regime. Second, expand to the non-linear regime using, e.g. the LTB-formalism. Third, investigate the effect on observables such as supernova distances, cosmic microwave background and baryonic acoustic oscillations.

Contact Edvard Mörtsell for more information

 

Title: Development of superconducting magnetic bearing

Future experiments observing the cosmic microwave background (CMB) are considering the deployment of a continuously rotating half-wave plates (HWP) to modulate the polarization of incoming light. In order to minimize the impact of such a mechanism on the noise budget of these experiments, the community is developing techniques that allow the mechanism to spin continuously through superconducting levitation. This project involves the construction and testing of a warm (300 K) prototype mechanism as well as design and preparation for the deployment of a similar mechanism at 77 K.

Left: A very rough design for the proposed mechanism. The inner diameter of the annulus is approximately 220 mm. An annular set of 48 high temperature superconducting disks (YBCO) are mounted to the stator mechanism. This bottom plate also houses 24 wire wound coils that, together with a set of small neodymium magnets, constitute an induction motor. An annular permanent magnet is mounted to the bottom of the rotor directly above the YBCO superconductors. Right: The first parts of a full-scale prototype that is being built at Stockholm University in order to test the proposed control mechanism and relative angle encoding.

 

This research project will help develop skills in electronics, CAD design, thermal modeling, and the development of programmable logic boards​.

Contact Jón Gudmundsson for more information

 

Title: Gravitational waves in ghost-free bimetric gravity

Ghost-free bimetric gravity is an extension of general relativity where, in addition to the normal massless graviton, there are also massive gravitons. The massive gravitons would have implications for the recent gravitational wave detections from LIGO, but possibly also for early-time gravitational waves that can possibly be detected in the cosmological background radiation.The project aims at doing all or part of the following:

- Check the stability properties of tensor perturbations to rule in/out parameter values of the theory.

- Check for possible signatures on early gravitational waves, i.e., the tensor-to-scalar ratio as observed in the cosmic microwave background.

- Check LIGO constraints on propagation of gravitational waves.

- Check effects of gravitational lensing on the massive/mass-less modes (when such a division is possible, i.e., in flat or (A)dS backgrounds).

Contact Edvard Mörtsell for more information

 

Title: Method of moments approach to optical modeling for mm-wavelength telescopes

In this project, we will develop an algorithm that makes use of method of moments approach for electromagnetic scattering—a concept often employed in stealth technology—to study realistic optical systems and their impact on the overall noise budget of our bolometers. Although the method of moments approach to electromagnetic scattering problems has been around for a few decades, it has not seen much use in modeling of large telescope systems operating at radio and mm wavelengths. Some of the basic concepts are introduced in textbooks by Harrington.

We will review the algorithm and implement it on a few simple electromagnetic scattering problems before developing code that applies the algorithm to realistic telescopes used by CMB experiments.

Contact Jón Gudmundsson for more information

 

Title: Modified dispersion relations in cosmology

Can we use cosmological observations to observe, e.g. quantum effects of gravity?

If we can measure the gravitational lensing effect and/or the group velocity through the light travel time for single sources at different energies, we are able to investigate modified dispersion relations.

First, review systems that have observations at different wavelengths. Second, use available data to constrain the energy scale of modifications to the dispersion relation. Last, interpret results in terms of modifications to general relativity.

Contact Edvard Mörtsell for more information