Lars Bergström

 

In recent years, the connections between the science of the smallest constituents of the Universe (particles or perhaps strings) and the science of the largest structures in the Universe - including the Universe itself and its evolution, cosmology - have become ever stronger. There are many reasons for this. One is the fact that the current cosmological ``standard model'' - the Big Bang model - implies that the earliest stages of the Universe (the first fractions of a second) were dominated by the particles (quarks, leptons and force carriers) whose properties can only be intensively studied at accelerators. 

An intriguing fact, which has become ever more evident during the last years, is that the matter mass density in the Universe is by a factor of around 5 dominated by so-called dark matter. The nature of the dark matter particles is still largely unknown, except that that some properties are required by the fact that their effect is only so far shown by gravitational influence. For instance, the coupling to electromagnetism (i.e., to photons) and to the strong interaction (i.e., to gluons) must be very much suppressed. In the Standard Model of particle physics there then still remain the weak interactions, and in fact a weakly interacting massive particle (a "WIMP") seems to very well fit the bill. To find the feebly interacting WIMPs, various methods from astrophysics, nuclear and atomic physics, and particle physics such as at CERN are presently being combined for an exciting hunt.

Another interesting dark matter particle, the axion, was proposed by my present colleague here at Stockholm University (SU), Nobel Laureate in Physics 2004, Frank Wilczek. We recently (December 2016) hosted a Nordita Workshop on this theme, and this is a topic that will certainly flourish at SU during the coming years.

With the help of particles such as gamma rays, neutrinos, protons and antiprotons, there is a possibility to learn, besides tha dark matter hunt,  more about many astrophysical processes which have traditionally only been studied by low-energy electromagnetic radiation (light and radiowaves).

Using the energetic cosmic rays that come to the Earth from outer space new information about particles and their interactions can also be obtained. One recent example is the discovery of a non-zero mass of the neutrino by studying solar or cosmic ray induced muon and electron neutrinos (in the Super-Kamiokande experiment in Japan, Nobel Prize to T. Kajita and A. MacDonald, 2015).

The astroparticle theory part of the CoPS group consists presently of Lars Bergström (professor), Joakim Edsjö (professor), Katie Freese (professor), Frank Wilczek (professor) and Doug Spolyar (assistant professor, presently absent due to illness). Thomas Schwetz-Mangold just left for KIT Karlsruhe in 2016. Present graduate students are Sebastian Baum, Jessica Elevant, Carl Niblaeus, Sunny Vagnozzi, and Axel Widmar. Recently graduated include Michael Gustafsson (postdoc at University of Göttingen), Torsten Bringmann (PhD 2005  now lecturer at Oslo University), Anders Pinzke (postdoc in Copenhagen, has now moved to industry), Pat Scott (now Ernest Rutherford Fellow at Imperial College, London), Yashar Akrami (postdoc in Leiden), Sara Rydbeck (Hamburg) and Natallia Karpenka (Southampton). Postdocs: Stefan Hofmann (now at LMU Munich). Anne Green (now permanent position at Nottingham University). Former graduate student Edvard Mörtsell is now lecturer in observational cosmology here in the CoPS group. Some other postdocs: Lidia Pieri (now at Udine/Trento/Padova/Paris) and Malcolm Fairbairn (now at Kings College London), Gabrijela Zaharijas (now at SISSA, Trieste). In the Oskar Klein Centre, we have had Rachel Rosen, Chris Savage, Antje Putze, Fabio Iocco, Alessandro Cuoco, Timur Delahaye, Miguel Pato, Abram Krislock and Florian Kühnel as postdocs.

The group is involved in several activities. One of the main lines of research is of course to investigate the nature of dark matter which dominates the mass density of the Universe. In particular, we have been focusing on a class of candidate WIMPs called supersymmetric particles, which are predicted to exist in superstring models, but also extended Higgs models, for example. Lately, as no supersymmetry has yet been found, we also have started to investigate models of axions.

A large computer package, DarkSUSY, has been developed with our participation and is currently maintained by Joakim Edsjö and Torsten Bringmann. We have also investigated so-called Kaluza-Klein models for dark matter. (By the way, did you know that Oskar Klein was a professor at Stockholm University? He has, of course, given the name to our Centre)

We work in close contact with other theory groups in the world and with the experimental astroparticle physics groups at Fysikum (Fermi-LAT Gamma-ray Telescope: see below, IceCUBE neutrino experiment at the South Pole: Klas Hultqvist and others) and the KTH (antimatter searches, PAMELA: M. Pearce) which aim at detecting or putting limits on these dark matter candidates. We have performed theoretical calculations of the fluxes of neutrinos, gamma-rays, positrons antiprotons which are the result of annihilations of supersymmetric particles, if they make up the dark matter halo of the Milky Way.

Other projects that we currently work on include Big Bang nucleosynthesis, the Cosmic Microwave Background, transplanckian physics, physics of branes in extra dimensions and gravitational lensing of quasars and supernovas.

We are especially involved in theoretical analyses in connection with the Fermi-LAT Gamma-ray satellite (launched in June, 2008 and still taking data), which has a substantial Swedish participation. Th experimental leader of the dark matter detection effort was for a long time Jan Conrad here in CoPS, with whom we collaborate a lot (see, e.g. this paper). He has now moved to the interesting field of direct detection of dark matter in the XENON experiment, and has a group funded by the Wallenberg Foundation. With former graduate student, Anders Pinzke, (Copenhagen) we have used a large N-body simulation of gamma-rays formed in formation of structures like galaxy clusters, to get an idea of realistic backgrounds for the dark matter search. There are also now concrete plans for a new satellite Euclid for which we are making pre-studies in collaboration with Ariel Goobar.

 
A text-book on astroparticle physics, Cosmology and Particle Astrophysics, by Lars Bergström and Ariel Goobar was published in an enlarged second edition, by Springer/Praxis (Germany) in December 2003. A student-priced (paperback) edition appeared in 2006.

I have been a member of the Royal Swedish Academy of Sciences since 2001, and was Scientifc Secretary of its Nobel Committee for Physics during 2004-2015.

             This text was updated on February 14, 2017. I apologize if it is incomplete and already obsolete in this quickly moving field!