Experimental Cosmology and Astrophysics


Group lead: Jon E. Gudmundsson, senior research scientist at Stockholm University Physics Department and the Oskar Klein Centre for Cosmoparticle Physics funded by a Career Grant from the Swedish National Space Agency and a Starting Grant from the Swedish Research Council.

We study cosmology, astrophysics, and fundamental physics through observations of the microwave sky. The physical processes that took place in the first epochs in the history of our universe have left an imprint in a 2.7-Kelvin blackbody radiation field that permeates our universe, known as the cosmic microwave background (CMB). During the last 50 years, by carefully observing this radiation with increasingly sensitive instruments, we have succeeded in extracting intricate cosmic detail and established a remarkably successful model of the universe. In particular, theories within the cosmic inflation paradigm — a set of models that aspire to describe the earliest epochs in the history of our universe — predict a faint global background of gravitational waves. These gravitational waves should have left a swirly imprint in the polarization of the CMB that known as primordial B-mode polarization. Many experimental efforts are focused on detecting or putting upper limits on the amplitude of this signal.

The science goals of future CMB experiments, in particular 4th generation satellites planning to supersede Planck, call for an unprecedented control of systematic effects. A significant population of potential systematics effects is related to optics. The CMB community is therefore particularly interested in understanding how best to design mm-wavelenght telescopes that can observe the CMB without generating false B-mode signals that would prevent us from reaching our science goals.

A large component of our work here in Stockholm is devoted to the development of realistic optical models for mm-wavelength telescopes. With accurate models, we can understand how optical non-idealities limit our ability to study minute polarization signals in the cosmic microwave background. Our group has recently written a few papers on this topic (see 1809.05034 and 1911.13153), with more on the way. Our work would be classified somewhere between experiment, instrument design, and data analysis.

Another significant part of our research effort has been devoted to SPIDER (see figures above). The experiment is designed to provide high-fidelity maps of the cosmic microwave background (CMB) polarization over approximately 10% of the sky. We launched the experiment on January 1, 2015 and are now busily analyzing the data. The next flight of the SPIDER payload is scheduled for December 2021. I have also made significant contributions to the calibration of the Planck satellite experiment. More recently, I have become involved in a ground based experiment called the Simons Observatory and a satellite proposal called LiteBIRD.

The faint cosmological signals that we want to probe call for mind-boggling detector sensitivities. These receiver sensitivity goals cannot be achieved without strong research and development efforts that combine models of cryogenics, optics, detectors, and readout. This leads to nice synergies with next-generation ultra sensitive dark matter, neutrino, and axion detection experiments. If you are a Stockholm University or KTH student and want to discuss the science of the cosmic microwave background, please do not hesitate to come find me at A5:1065 in AlbaNova.

A Nordita program named "Advances in Theoretical Cosmology in Light of Data" recently took place in Stockholm. See: http://cosmo-nordita.fysik.su.se/

Experimental Collaborations

SPIDER is a long duration balloon-borne experiment designed to measure the polarization of the cosmic microwave background with unparalleled instantaneous sensitivity (see figures above). Novel detector architecture allows for illumination of approximately 2500 detectors through an effective collecting area of roughly 0.5 m2. SPIDER mapped the polarization of the CMB over a tenth of the sky during a 17-day Antarctic flight that took place in January 2015. A subsequent flight of the SPIDER payload is planned for the 2020–2021 Antarctic season.

SPIDER had a successful flight with an instantaneous sensitivity in line with theoretically informed estimates (Fraisse 2011). During the first flight, SPIDER generated 1.56 TB of raw detector timelines in the form of approximately 400 billion detector timeline samples. This is roughly twice that of the entire Planck data set. You can find some more SPIDER-related videos here. Also, if you are curious, do check out some of the other content on SPIDER available online. For example, the Princeton blog page, our Flickr account, and list of public coverage on our wiki page


The SPIDER flight cryostat

One of the biggest challenges in the search for primordial B-modes is related to complications from Galactic foregrounds; microscopic dust particles in our own Galaxy align themselves with large scale magnetic fields and emit polarised thermal radiation at mm-wavelengths. The LiteBIRD satellite is designed to deploy 15 frequency bands from 30-450 GHz which roughly corresponds to 0.7–10 mm wavelength radiation. This extensive frequency coverage together with a significant improvement in detector sensitivity compared to past missions, will allow the experiment to distinguish Galactic foregrounds from any primordial signal. However, wide frequency coverage and great sensitivity are not the only requirements for a successful mission. The instrument design also needs to steer clear of various systematic effects that can produce false polarized signals in our maps. This aspect becomes more important as experiments push down constraints on primordial B-mode amplitudes. A particularly large class of potential systematic effects is related to optics. I am leading the optical design efforts for the mid- and high frequency telescopes of this proposed satellite mission.


The current LiteBIRD design

The Simons Observatory (SO) will be located in the high Atacama Desert in Northern Chile inside the Chajnantor Science Preserve. At 5,200 meters (17,000 ft) the site hosts some of the highest telescopes in the world. The Atacama Cosmology Telescope (ACT) and the Simons Array are currently making observations of the Cosmic Microwave Background (CMB). Their goals are to study how the universe began, what it is made of, and how it evolved to its current state. The Simons Observatory will add to these several new telescopes and new cameras with state of the art detector arrays. The result will set the stage for the next generation of CMB experiments. Find out more at https://simonsobservatory.org/


The SO Large Aperture Telescope

The Planck satellite launched from French Guiana in May 2009. The primary science goal of the satellite was to measure the temperature and polarization anisotropies of the CMB over the full sky (see e.g. Planck Bluebook). The satellite employed two instruments with different scientific leadership. The High and Low Frequency Instruments (HFI/LFI). I have been a member of the Planck HFI core team since the summer of 2010. The satellite was commissioned by the European Space Agency (ESA) although significant financial and scientific contributions have been made by NASA and other institutions in the US. Both instruments onboard the satellite are coupled to the sky through an off-axis Gregorian telescope (see right). Two mirrors, a 1.5 m primary and a 1.0 m secondary, each cooled to 45 K, focus light into the 52 feedhorn-coupled bolometric receivers that populate the HFI. The spectral coverage of these receivers is spread over six frequencies, with band centers ranging from 100–857 GHz. Parts of the receiver elements were cooled to 100 mK through a series of passive and active coolers (see Triqueneaux2006). These refrigerators made Planck the coldest known object in space while it was operational.

The figure on the right, courtesy of ESA, shows the a cutaway drawing of the Planck satellite, including the cryogenic instruments inside the Service Module (SVM). Two black star cameras, mounted on the side of the SVM, and a set of thrusters that facilitate orbital maneuvers are also visible. Four spherical titanium and Kevlar composite structures, designed for 290 atm maximum pressure, hold the helium-3 and helium-4 needed for the open-cycle dilution refrigerators.


Cutaway through the Planck satellite