INTERNATIONAL SPACE SCIENCE INSTITUTE SPATIUM Published by the Association Pro ISSI No. 47, May 2021 Modern Cosmology – nearly perfect but incomplete 2101267_[211415]_Spatium_47_2021_(001_016).indd 1 27.04.21 13:58 Editorial ”Cosmologists are often in error, of the computer age allowing the but never in doubt”. This aphorism simulation of more and more com- Impressum by Lev Davidovich Landau (1908- plex cosmological as well as astro- 1968) may have been applied to physical models. This is why dis- cosmology before the discovery of criminating the “traditional” from ISSN 2297–5888 (Print) the cosmic microwave background the “modern” cosmology is justi- ISSN 2297–590X (Online) (CMB), predicted in 1933 by Er- fied. Moreover, modern cosmol- ich Regener, vaguely indicated by ogy is built on physical, astrophys- Spatium Andrew McKellar in 1940/41, pos- ical, and cosmological concepts Published by the tulated in the 1940s by George and parameters which are con- Association Pro ISSI Gamow, Ralph Alpher and Rob- firmed by experiments and meas- ert Herman, and definitely de- urements to a high degree of tected in 1964 by Arno Penzias and precision. Robert Woodrow Wilson. Cos- The content of this issue resulted mology was a rather theoretical and from the talk “Cosmology Today” Association Pro ISSI perhaps highly speculative branch by Prof. Dr. Bruno Leibundgut Hallerstrasse 6, CH-3012 Bern of astrophysics at the time. Several held on October 14, 2020, in the Phone +41 (0)31 631 48 96 cosmological models were dis- Pro ISSI seminar series. In his pre- see cussed based on and resulting from sentation Prof. Leibundgut briefly www.issibern.ch/pro-issi.html Albert Einstein’s General Theory reviewed the major steps made in for the whole Spatium series of Relativity. However, observa- cosmology from earlier times to tions and, particularly, accurate today. He addressed the methods President distance measurements on large and results of modern cosmology Prof. Dr. Christoph Mordasini scales were missing or too uncer- and pointed out still unresolved University of Bern tain to confirm either the one or problems. Although becoming an the other model. These models established exact science has made Editor concerned the steady-state theory, modern cosmology “nearly per- PD Dr. Andreas Verdun, the pulsating Universe, and the Big fect”, these open issues are the rea- 3086 Zimmerwald Bang theory implying an ever ex- son why it is still “incomplete”. It panding Universe. It was even im- may well be that new space-borne Printing possible to make any reliable state- and ground-based instruments Stämpf li AG ments on the size, age, evolution, providing refined observational CH-3001 Bern structure, and topology of the data will allow us to answer many Universe. of the open questions in near fu- Since then, however, cosmology ture. However, a central issue will has undergone two dramatic shifts remain perhaps forever, as the cos- in paradigm, namely from a math- mologist Dennis William Sciama ematical to a physical science and (1926-1999) stated in 1978 without from a physical to an observational any doubt: “None of us can under- science, thus turning it into an ex- stand why there is a Universe at all, Title Caption act science that follows the method why anything should exist; that’s Hubble Space Telescope-Image of the Type Ia Supernova 1994D (SN1994D) of prediction (modelling) and fal- the ultimate question.” Was he in in galaxy NGC 4526 (SN1994D is the sification (comparing with obser- error, too? bright spot on the lower left). The vations). The main reasons for this supernova was discovered with a small development were the beginning Andreas Verdun (70cm) telescope and observed by chance by HST. of the space age enabling the launch Zimmerwald, May 2021 (Credits: Peter Challis (CfA) and the of dedicated satellite missions, and High-z Supernova Search Team) SPATIUM 47 2 2101267_[211415]_Spatium_47_2021_(001_016).indd 2 27.04.21 13:58 Modern Cosmology – nearly perfect but incomplete Bruno Leibundgut, European Southern Observatory (ESO) and Technical University Munich (TUM) Johannes Geiss Fellow 2019 Cosmologists map the past evolu- of the cosmic microwave back- measured with high precision. A tion of the Universe through di- ground in the 1960s implied a hot prime example is the value of the verse observations probing the past of the Universe and strength- current expansion rate, the Hub- state of the Universe at a given ened the idea of nucleosynthesis in ble constant. Thirty years ago, it epoch. A theoretical model is then the very early Universe. Simula- was still uncertain by almost a fac- used to tie these vantage points tions of the growth of large-scale tor of two, but it has now been de- into a coherent evolution. There structure and the formation of termined to a few percent. New are four important epochs which clusters of galaxies tried to repro- parameters have been added to astrophysics can probe: (1) the cur- duce the distribution of galaxies describe various effects that have rent Universe, presumably at an in the Universe. Some important become observable. Among them age of about 13.8 billion years, (2) additions and extensions to the is the strength of the growth of the evolution of structure over the emerging picture of the Universe structure or the equation of state past 12 billion years, (3) a snapshot were made at the start of the new parameter of an additional ingre- of the plasma state of the Universe millennium. The status of cosmo- dient to the universal contents. at an age of about 350,000 years, logy some 20 years ago can be The current model, for the first and (4) nuclear reactions taking found in the excellent exposés by time, yields a dy namical age that is place within the first three min- Johannes Geiss (Spatium 1, April larger than the oldest stars and pro- utes of the Big Bang. The theory 1998) and Gustav Andreas Tam- vides a nearly complete description connecting these observations is mann (Spatium 3, May 1999). Since of many observables. This “con- based on the assumption that the then, new observations have im- cordance” model goes under the known laws of physics hold proved our knowledge concerning name of CDM. stands for the throughout the observable Uni- verse, and that we can describe “The enormous progress in the construction of the Universe essentially as homo- modern telescopes in space and on the ground geneous and isotropic. This as- is therefore driven not only by the aim to use sumption is called the “cosmolog- them as ‘space ships’, bringing distant objects ical principle”. The cosmological closer to the observer, but also as ‘time ships’, principle leads to a simplification bringing past events into the present.” which allows us to describe the evolution of the Universe with its Gustav Andreas Tammann, Spatium-No. 3, May 1999 (energy) contents. Our view of the cosmos has the geometry of space and added cosmological constant, a late addi- changed dramatically in the past the discovery of a current acceler- tion by Albert Einstein into his three decades. Many basic ideas ation of the expansion of the Uni- field equations, and the remaining had already been in place for most verse. Theorists have made signif- letters for Cold Dark Matter. The of the 20th century, starting with icant progress in simulating the ingredients of this model are the General Relativity as a theory that formation and growth of structures validity of General Relativity as describes a dynamic universe at on almost all scales. The particle the description of gravity, three large scales, the realisation of large physics standard model has been (energy) constituents, and the sim- distances between galaxies, and completed with the detection of plifying assumptions of isotropy the observation of expansion be- the Higgs particle and helps in and homogeneity. The latter as- tween them. During the first half firming up our views of the sumptions are to ensure that we are of the 20th century the evolution Universe. “neutral” observers and can ob- of elemental abundances was pro- serve a “typical” part of the Uni- posed and the existence of dark Over recent years, many cosmo- verse. The Universe contains mat- matter postulated. The discovery logical parameters have been ter in two forms: baryonic matter, SPATIUM 47 3 2101267_[211415]_Spatium_47_2021_(001_016).indd 3 27.04.21 13:58 which encompasses the observable The Universe galaxies to their distances. This Universe (all known elementary describes the cosmic expansion as particles and the matter built up today a function of distance and yields from it: dust, flora and fauna, plan- the Hubble constant, commonly ets, stars, galaxies), and dark mat- denominated H0. Due to the cos- ter, which only acts gravitationally. mic expansion, the observed ve- So far, dark matter has been in- At an age of 13.8 billion years, we locity vectors of the galaxies point ferred only indirectly (Klaus Pretzl, observe a universe that is fairly away from us and the photons are 2001, Spatium 7) and has been de- quiet and settled. Many galaxies shifted to larger wavelengths. The duced by high velocity dispersions are in groups or clusters. Most of expansion rate (velocity per dis- in galaxy clusters (first by Fritz them have had one or two inten- tance) is a critical parameter for all Zwicky almost 90 years ago), the sive phases of star formation and cosmological models as it sets the mass distribution in galaxy clusters galaxy encounters, and mergers are absolute scale and age of the Uni- as observed through their gravita- fairly rare today. Galaxies vigor- verse. The cosmological redshift is tional effects by lensing the light ously forming stars or disturbed by a relativistic effect coming from of background galaxies (see massive mergers are observed at General Relativity related to the Georges Meylan 2013, Spatium 32), large distances and large lookback expansion of space and is not a and extended rotation curves ob- times, when the galaxies were con- Doppler shift as it is sometimes served in nearby galaxies.