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22 (Sun) ~ 25 (Wed), June, 2025 at Congressi Stefano Franscini in Ascona, Switzerland
Observational cosmology has entered a golden age. Windows to probe the universe such as the cosmic microwave background (CMB) and large-scale galaxy surveys (LSS) have become precision science, and new windows such as 21cm redshift surveys with radio telescopes are opening up. In light of these advances in observational cosmology, the challenges on theoretical cosmology have grown rapidly.
The standard first-order linear perturbation theory, which is the current main tool in the literature, has been pushed to its limits, and improved approaches are needed in interpreting precision measurements in observational cosmology. Beyond linear order in perturbations, fluctuations on any particular physical scale obtain contributions from other Fourier modes, which are independent in the linear regime. Of particular interest among these nonlinear effects to cosmological observations are so called "back-reaction’’ effects, by which both short wavelength non-linear fluctuations affect the larger scale modes and hence the background evolution in large-scale structure of the Universe. There are also back-reaction effects of long wavelength linear modes on the modes which are currently measured in observations. The aim of this workshop is to bring together ex- perts working on various aspects of back-reation problems, with particular emphasis on computing the effects on measurable quantities which are based on light cone averages.
There have been initial studies of cosmological back-reaction in the late 1990s. The results of these studies, however, were controversial since these studies did not relate the quantities which were computed to actual observables. The importance of focusing on well-defined physical observables which can be measured from the past light cone has only become clear recently. Hence, our workshop will provide a unique venue in which experts on formal back-reaction computations can exchange their insights with cosmologists working on light cone observables. These interactions promise to lead to important breakthroughs.
Beyond the standard LCDM cosmology, the back-reaction effects can be very large, in particular with the presence of super-horizon scale entropy fluctuations or additional energy components. While the current observational data favors the standard LCDM model, small deviations from the standard model in the upcoming large-scale surveys would provide valuable insight on new physics beyond the standard model. This workshop will play a crucial role in reviewing the status of the field and bringing together experts on different aspects of the problem.
Program: pdf (to be updated)
The individual topics of the entropy perturbations, cosmological observables, gravitational waves, and effective field theory approaches have been developed rather independently from each topic, and there has been little communication between each field. The main objectives of the workshop is to bring the experts in each field together, discuss the recent development and its connection to the back-reaction effects, and generate synergy for future collaboration and opportunity to implement techniques in different fields to the cosmological back-reaction effects on observables.
List of Confirmed Participants: pdf (as of 01.01.2025)
Jaiyul Yoo (Zürich), Robert Brandenberger (ETHZ/McGill), Ruth Durrer (Geneva)
If you have any questions please contact the organizers (jaiyul.yoo @ uzh.ch; rhb @ physics.mcgill.ca)
Congressi Stefano Franscini conference centre at Monte Verità
Hotel rooms at the conference center should be booked directly via link sent to the participants.
Travel Information
Str. Collina 84, 6612 Ascona
»Link to Google Maps
The Mt. Verita conference center can be reached via flights to Zurich airport or Milano-Malpensa airport (Italy).
The option from Zurich is to fly to Zurich International Airport (ZRH) and then take a train across the Alps either through the longest rail tunnel in the world or through the scenic Gotthard route to Locarno. There is an express train roughly every half an hour through the tunnel, and no reservations are required. You will have to change trains in Bellinzona from the express train to the local train. Both the express trains and the scenic route trains leave from the Zurich main train station. You can take any train from the airport train station to the Zurich main station. The express train takes about two hours from Zurich to Locarno. Travelling time via the scenic route is approximately 3 hours. The scenic route trains typically go to Locarno, and no change in Bellinzona is required.
Alternatively, you can fly into Milan Malpensa (MPX). There is a train from Malpensa airport to Locarno with just one change in Lugano. This train runs once an hour (leaving 19 minutes past the hour) and takes just a bit more than two hours to get to Locarno (arrival time 38 minutes past the hour).
For train information in Switzerland see www.sbb.ch.
Once you reach the Locarno railway station, take bus no. 1 to the bus stop "Ascona Centro" (duration: 15 - 20 minutes). From the bus stop "Ascona Centro" you can also reach Monte Verita in 25 - 30 minutes on foot (follow the "Strada Collina") or by taxi located next to the Autosilo Ascona. Parking space is not available at the conference center.
