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Home , Age of the universe, Cosmic age problem, John Huchra, Robert Woodrow Wilson

INTERNATIONAL SPACE SCIENCE INSTITUTE SPATIUM Published by the Association Pro ISSI No. 47, May 2021

Modern – 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 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. . 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 ’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 , 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 -Image of the 1994D (SN1994D) of prediction (modelling) and fal- the ultimate question.” Was he in in 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)

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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 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 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 , (2) additions and extensions to the is the strength of the growth of the evolution of structure over the emerging picture of the Un­ iverse 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 and pro- utes of the . 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 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 , 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 . The of the , starting with icant progress in simulating the ingredients of this model are the 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,

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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 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 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 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. The in- siderably younger than the ones we portrayed. While the redshift clusion of a cold dark matter par- observe nearby. measurement is rather simple – one ticle in the simulations of the observes the shifted wavelength of evolution of the growth of struc- The Hubble-Lemaître law relates a known atomic transition in an ture can reproduce the observed the observed recession velocities of – deter- distribution of galaxies on large scales. Further ingredients of the Figure 1: Evolution of the measured value of the Hubble constant (H0) over cosmological model are radiation, the past century. Based on data collected by John Huchra observed today mostly in the form (2010: https://www.cfa.harvard.edu/~dfabricant/huchra/hubble/) of microwave photons distributed throughout the Universe, and , an enigmatic component 1000 that accelerates cosmic expansion.

800 This article presents the main cos- ) mological observables which led to 1

the current cosmological model. 600

Main measurements are (a) the ex- Mpc

pansion rate today (Hubble’s con- 1 stant), (b) the growth of structure 400 from tiny fluctuations in the early km s ­Universe to today’s distribution ( 0

of ­galaxies and galaxy clusters, (c) H 200 a remnant glow from a hot phase in the early Universe, and (d) the abundances of various elements 0 and their evolution over the age of 1920 1940 1960 1980 2000 the Universe. Publication

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2101267_[211415]_Spatium_47_2021_(001_016).indd 4 27.04.21 13:58 mining an accurate cosmological Wirtz, Georges Lemaître and Ed- Hubble constant can be observed distance is very difficult. The win Hubble. It is Hubble’s famous (figure 1) just before 1960. Some problem lies in the absolute cali- diagram showing the increased re- 60 years ago, the measurements bration of distance indicators and cession velocity for objects at larger settled to values somewhere be- has occupied for distances that demonstrated this tween 50 and 100 km s–1 Mpc–1. decades. expansion. The value of the expan- Alan Sandage and Gustav Andreas sion rate derived by Lemaître and ­Tammann set out to measure the Hubble was of a size that the ex- ­Hubble constant through a series View 25 years ago pansion would of increasing distance determina- (Spatium 1 and 3) have been about a quarter of the tions. This was done by calibrating known age of the – clearly a new distance indicators based on The cosmic expansion was estab- contradiction that was not tenable. previously determined distances lished observationally just over It took several decades for new and establishing a “distance lad- 90 years ago by , measurements during which a sig- der” to step further out into the Knud Lundmark, Carl Wilhelm nificant decrease in the value of the Universe. Thus, they took “Steps

Figure 2: of a Type Ia supernova at high redshift. The fading of the supernova can be appreciated in the images after 16 March.

SupernovaSupernova 11.11. M Marcharch

13.13. M Marcharch

16.16. March March

29.29. M Marcharch

SNSN 1997as1997as 5.5. April April z=0.51z=0.51

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2101267_[211415]_Spatium_47_2021_(001_016).indd 5 27.04.21 13:58 towards the Hubble Constant”, mained fairly stable although with It was a key project for HST to de- which was the title of a series of a significant scatter. The dynami- termine the cosmic expansion rate publications over nearly two dec- cal age of the Universe still re- with an accuracy of about 10%, ades. Of course, it is advantageous mained lower than its oldest stars which was achieved by two sepa- to keep the number of steps as small for the favourite cosmological rate teams. One (identified as as possible in order to minimise model. This model had a flat space “Members of HST Key Project” in the accumulating uncertainties. geometry and was dominated by figure 3) originally favoured several Today, two important classes of matter. The theoretical bias was so different secondary distance indi- variable stars play a crucial role: strong that the discrepancy to the cators. The second team led by Cepheids and Type Ia supernovae observed mean matter density was Sandage and Tammann used (SN Ia). Cepheid stars display a pe- mostly ignored at the time. mainly Type Ia supernovae as the riodic variation in their luminos- secondary distance indicator and ity and colour. Henrietta Swan For many years at the beginning derived somewhat smaller values Leavitt found in 1917 that Cep- of this century the value of the for the Hubble constant. While su- heids in the Small Magellanic ­Hubble constant hovered be­ pernovae had been used earlier for Cloud display a tight correlation tween 50 and 80 km s–1 Mpc–1 (see distance measurements, their accu- between their luminosities and figure 3). There was a major im- racy was mostly established in their periods. Cepheids with longer provement in the measurement small samples, and only dedicated periods are more luminous. Once through the observation of Cep- searches yielded sizeable samples of this period-luminosity relation is heid stars in nearby galaxies with well-observed supernovae. calibrated the distance can be in- the Hubble Space Telescope (HST) ferred by simply measuring their reducing the number of rungs in period. Hubble observed a Cep- the distance ladder dramatically. heid star in the Andromeda Neb- ula in 1924 and could show that this galaxy lies outside the Milky Figure 3: Recent evolution of the Hubble constant (H0) according to research Way. Type Ia supernovae – the su- group (based on data from John Huchra) pernova types were originally de- fined by Zwicky, who dedicated most of his observational activities to the study of galaxies and super- 100 novae – show a rather small scatter in observed luminosities, and their ) 1 peak brightness can be used to de- 80 rive cosmic distances for individ-

ual objects. Most critical is the cov- Mpc

erage of the maximum in the light 1 60 curves (figure 2), which occurs two to three weeks after explosion, but km s often was not observed. Tammann ( 0 40 had recognised the potential of H Sandage and Tammann ­supernovae early on and was in- de Vaucouleur or van den Bergh strumental in calibrating them as Members of HST Key Project reliable distance indicators. 20 Theory (cosmology) 1996 1998 2000 2002 2004 2006 2008 2010 2012 For several decades the measured Publication Year value of the Hubble constant re-

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2101267_[211415]_Spatium_47_2021_(001_016).indd 6 27.04.21 13:58 Changes in the past tances. Alternatively, the superno- ratios of these distances provides a 25 years vae can be calibrated by the tip of measurement of the Hubble con- the red giant branch, where stars stant. This method has led to very The Calán-Tololo supernova reach a unique luminosity, when competitive determinations of the search was the first survey to over- helium burning starts in their cen- Hubble constant. A further geo- come the problem of incomplete tre, or by some other methods. metric method is the measurement light curve coverage and provided Large samples of distant superno- of water (mega-)masers orbiting a a suitable sample of Type Ia super- vae can be used to measure dis- galactic centre. The radial velocity novae. Cepheids were observed tances where the cosmic expansion and the angular separation define with HST in galaxies where Type dominates the movement of the the disk rotation and yield a Ia supernovae had exploded before galaxies, the so-called “Hubble distance. and hence the supernova peak lu- flow”, and where the Hubble con- minosity could be calibrated. stant is derived. Today, many tran- All methods converge to values sient searches are active (Zwicky for the Hubble constant of about The measurement uncertainties Transient Factory, Pan-STARRS, 73 km s–1 Mpc–1 with uncertainties for the local Hubble constant have ATLAS) and several thousand su- of about 2 to 3 percent. now been reduced to only a few pernovae are discovered every percent. This has mostly come year. Supernovae are now the pre- The observation of cosmic accel- about through the reduction to ferred secondary distance indica- eration in the expansion of the late three rungs in the distance ladder. tors to reach the Hubble flow. Universe by Type Ia supernovae introduced an important paradigm shift. The addition of an accelera- “Then, as precision increases, the array of possible tion in the form of a cosmological interpretations permitted by uncertainties in the constant increases the dynamical ­observation will be correspondingly reduced. age of the Universe dramatically, ­Ultimately, when a definite formulation has been and in one fell swoop the age ­prob- achieved, free from systematic errors and with lem with the oldest stars disap- ­reasonably small probable errors, the number of peared. This can be observed in competing interpretations will be ­reduced to a ­figure 3 where the few theoretical predictions were pushing the val- minimum.” ues of the Hubble constant to very , The Law of Red-Shifts, 1953, MNRAS 113, 658 low values to accommodate a mat- ter-dominated, flat universe (the so-called Einstein-de Sitter model).

The luminosity of Cepheid stars is In recent years, new methods in- calibrated in the by ge- dependent of the distance ladder ometric methods, mostly trigono- have been proposed and employed. metric , or in the Magel- They include the time delay ob- lanic Clouds. With calibrated served in gravitationally Cepheid luminosities such stars are lensed by a foreground galaxy clus- observed in local galaxies in which ter or a galaxy. Since the path a Type Ia supernova has been ob- lengths for the individual lensed served. The maximum luminosity images are different, the time lag of the supernovae can then be cal- determines the distances between ibrated through the Cepheid dis- us, the source and the lens, and the

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2101267_[211415]_Spatium_47_2021_(001_016).indd 7 27.04.21 13:58 The Universe The changes indicate an evolution extragalactic nature of the “nebu- towards more structure. Stars in lae” as galaxies was recognised, and over the past our Milky Way which map its the second one in 1963 with the ­history and tell us about the for- discovery of the high in 12 billion years mation and evolution of galaxies spectra. It can be seen how and stars in general provide a fur- the known universe increased by a ther tracer. factor of 1,000 in the 1930s and again by a factor of 10 with the The evolving Universe is revealed large distances of the quasars. By in many observations. The appear- Views 25 years ago 1995 quasars were observed out to ance of galaxies changes dramati- redshifts of 5, which corresponds cally when we look back to earlier Quasi-stellar objects (quasars) are to a lookback time of about 12.3 times, i.e. when we look into the extremely luminous cores of mas- billion years or 91% of the observ- deep Universe. A marked change sive galaxies. They were discov- able Universe, while galaxies were in the rate of star formation within ered in the 1960s and were the not observed that far away and galaxies is observed, and the abun- most distant objects known at the were mostly restricted to smaller dance of various elements changes time. There are two major steps redshifts around 3. over time. Clusters of galaxies visible in the diagrams shown in formed only late in the Universe. figure 4: one around 1930, when the

Figure 4: Evolution of the most distant known objects. (Left panel) Graph with redshifts and the distances provided on the right axis. (Right panel) Linear distances to highlight the jump with the discovery of quasars.

101 9440 Distance (Mpc) 100 3304

10 1 419 Redshift

Galaxies 10 2 42.7 Quasars Gamma-Ray Bursts

1920 1940 1960 1980 2000 2020 Year

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2101267_[211415]_Spatium_47_2021_(001_016).indd 8 27.04.21 13:58 Extensive spectroscopic observa- of structure from the early Uni- still remained unclear. The meas- tions of distant galaxies showed verse (see below). Simulations of urements from galaxy clusters in- that the rate at which stars form the emergence of this large-scale dicated about one third of the crit- within a galaxy is not constant and structure were based on a model ical density, whereas an inflationary in fact has been decreasing for the involving dark matter particles in- period at the Big Bang predicted a past 10 billion years. HST provided creasing the gravitational potential flat universe with a critical den- the first detailed images of galax- wells, which allowed the structures sity – a factor-of-3 difference. The ies at ultraviolet wavelengths and to grow fast enough within the matter-filled, flat Einstein-de Sit- images of very distant galaxies. available time. Dark matter had ter model was favoured by theory, been implied by the observation of but still had an age problem with The distribution of galaxies is not rotation curves of galaxies, which some stars older than the dynami- uniform. Most galaxies are found remain constant beyond the (opti- cal expansion age. As a conse- either in clusters or aligned along cally) observable edges, and hence quence, the Hubble constant had filaments delineating huge empty require additional, unseen mass to be very low (see cyan points in volumes (“voids”), where there ap- to explain the high rotation figure 3). pear to be no galaxies. The recog- velocities. nition of the inhomogeneous ­distribution of galaxies was only At the end of the last millennium slowly realised as due to the growth the matter content of the universe

Both panels display the same data and the relation between linear (proper motion) distance and redshift changes the ­appearance. The relation between redshifts and distances is calculated for a flat universe with a Hubble constant of 70 km s–1 Mpc–1, 30% of matter and 70% of dark energy (CDM model). (Data from Wikipedia; 4 February 2021)

10000 13.2

8000 5.45

6000 2.65 Redshift

4000 1.31

Distance (Mpc) Distance 2000 0.53 Galaxies Quasars 0 Gamma-Ray Bursts 0

1920 1940 1960 1980 2000 2020 Year

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2101267_[211415]_Spatium_47_2021_(001_016).indd 9 27.04.21 13:58 Changes in the past lengths, with redshifts larger than cal) distance indicator out to very 25 years about 12. Only radio observation large distances and have indepen- will be able to detect galaxies at dently confirmed the cosmic ac- Over the past couple of decades the higher redshifts as hydrogen gas celeration found by supernovae. A record holder for the most distant clouds when the required sensitiv- new discrepancy has surfaced from objects observed changed from ity will be achieved. the comparison of the strength of quasars to galaxies to gamma-ray the clustering at scales of about bursts and back to galaxies. The The simulations of large-scale 8 Mpc. This parameter can be re- ­effective increase in “explored vol- have made liably measured through distance ume” at this point is relatively ­tremendous progress. With the in- samples, which are now assembled modest (about 70% over the past clusion of dark energy, the simula- with large imaging surveys, like 25 years, see figure 4), simply be- tions can reproduce the statistical the Kilo Degree Survey (KiDS) at cause objects over most of the ob- matter distribution on large scales the VLT Survey Telescope or the servable Universe can now be seen. By observing “deep fields”, with “I don’t know if we have dark matter or need extremely long exposure times, of a change in gravity or need something else; a single “empty” region in the sky, we know so little about our universe. extremely faint and distant galax- It is a strange and mysterious universe. ies could be detected. New sensi- tivity limits could be reached and But that’s fun.” revealed objects significantly older Vera Cooper Rubin, quoted by Neta Bahcall in Physics Today, than the or even our Milky March 2017, p. 74 Way. HST and the Chandra X-ray telescope were detecting many ­distant galaxies or active galactic nuclei (AGN). Since the activity of rather well. The simulations are (DES) at the a galaxy nucleus is typically lim- working on much finer grids (mass, Victor Blanco Telescope at the ited in time, quasars are only a sub- time and space) and the latest in- Cerro Tololo Observatory. These set of galaxies and are observed carnations involve baryonic phys- will be superseded in the coming much less frequently. Due to the ics, which makes for some truly years by the ESA satellite, Euclid, high redshifts and additional ­realistic looking modelling of and the Legacy Survey of Space blocking of light in the ultraviolet galaxies. and Time (LSST) at the Vera C. by absorbing hydrogen atoms, Rubin Observatory. ­galaxy spectra appear redder and Huge redshift surveys have found redder with distance and display no statistical homogeneity on large or only negligible flux at observed scales again, i.e. the local inhomo- blue wavelengths – they “drop geneities are smoothed out over out” in the blue. This signature can large enough volumes. They also be used to discover ever more dis- confirmed a prediction of the cold tant and fainter galaxies. The same dark matter model, namely that method was employed for a there should be an increased clus- gamma-ray burst found at a red- tering at specific scales, which shift of 8.2. From measurements were originally set by the sound of the cosmic microwave back- horizon at the time of reionisation. ground, it is now clear that it will These Baryonic Acoustic Oscilla- be impossible to “see” galaxies, i.e. tions (BAO) have indeed been to observe them at optical wave- found and provide a new (statisti-

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2101267_[211415]_Spatium_47_2021_(001_016).indd 10 27.04.21 13:58 The Universe at Views 25 years ago at a contrast of 0.00001. What was missing was a measurement of the 350,000 years The cosmic background was pre- typical angular separation between dicted by in the the fluctuations to determine the 1940s to solve the problem of the geometry of the Universe. creation of the elements. The pre- One of the main sources of our un- dictions were mostly forgotten derstanding of the Universe comes ­until re-derived them. Changes in the past from a faint glow from the early The background radiation was 25 years universe. Today, this is observed as found accidentally by Arno Pen- the Cosmic Microwave Back- zias and Robert Wilson while test- Since the microwave radiation is ground (CMB) with a black body ing a sensitive . The mostly blocked by Earth’s atmo­ temperature of about 2.73 Kelvin. discovery of the CMB was taken sphere, its observations require bal- It reaches us from all directions and as a clear sign of a hot past of the loons and satellites. Balloons have provides an imprint of the status of Universe, and is generally used as the advantage that they are much radiation and matter some 350,000 the strongest indication of a Big cheaper and can be deployed faster years after the Big Bang. This ra- Bang. The CMB is highly iso- than satellites, but due to the lim- diation is currently the oldest im- tropic, i.e. it is the same in all di- ited flight times they can observe age we can make of the Universe. rections to better than 1 per mill. only small patches of sky. Two bal- It is the signature of the hot Big The Cosmic Background Explorer loon experiments in the 1990s Bang and has cooled down contin- (COBE) satellite measured a per- (MAXIMA: Millimeter wave An- uously throughout the history of fect black body spectrum and isotropy eXperiment IMaging Ar- the Universe. found fluctuations in temperature ray and BOOMERanG: Balloon

Figure 5: Temperature map of the Cosmic Microwave Background (CMB) as measured by the ESA satellite. (© ESA and the Planck Consortium)

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2101267_[211415]_Spatium_47_2021_(001_016).indd 11 27.04.21 13:58 Observations Of Millimetric Ex- from the fluctuations are the bar- The first tragalactic Radiation ANd Geo- yonic density and the density of physics) found that the angular dark matter. These are imprinted minutes – spacings of the fluctuations in the in the fluctuations during the first CMB indicated a flat space geom- 350,000 years, and with the rich- Big Bang etry. This was important informa- ness of the CMB and in combina- tion as it showed that the Universe tion with the observed structure at nucleosynthesis was likely to be at the critical en- later times provide accurate values. ergy density and is also seen as All measurements are consistent proof of an inflationary period in with the CDM model and suc- A universe dominated by matter the very early Universe. However, cessfully connect the state of the consists of stars, gas and dust. These it was in contradiction to the meas- early Universe to what is observed are typically combined in the term ured matter density. Dark energy today. In particular, the growth of “baryons” defined in particle phys- driving an accelerated expansion structure is confirmed successfully ics as all particles containing makes up for exactly this missing when applied to the simulations quarks. The most common bary- energy. within CDM. ons are protons and neutrons, which form the atomic nuclei. Stars are mostly made of hydrogen “The effort to understand the universe is one and helium, and contain very small of the few things that lifts a little contributions of elements beyond above the level of farce, and gives it some of helium. This was realised less than the grace of tragedy.” 100 years ago by Cecilia Payne- in : Gaposchkin in her PhD thesis. The A Modern View of the Origin of the Universe (1977), p. 155 stellar composition contrasts the composition of Earth, where the inner core is mostly composed of iron and nickel, and the crust is dominated by oxygen and silicon. The CMB has been measured with The microwave background still The human body on the other increasing angular resolution and holds some secrets. Polarisation hand consists mostly of oxygen (by exquisite accuracy by the Wilkin- provides information on the for- weight). Where did the elements son Microwave Anisotropy Probe mation of the fluctuations them- come from and how were they (WMAP) and then by the ESA selves and would give an indication formed? The simplest assumption ­satellite Planck (figure 5). The tem- of gravitational waves interacting is that they always have been there perature fluctuations shown in during the Big Bang. Ground- and do not change. That would ­figure 5 display a typical scale be- based measurements are already immediately beg the question, why tween cold and warm regions. This trying to observe the small-scale the abundances would be as dispa- physical scale is set by primordial polarisation, and future all-sky rate as they are? The evolution of fluctuations, and their observed ­satellite missions are proposed. abundances and the structure of size gives us an indication of the the atoms indicate the build-up of path traveled by photons from the larger and heavier atoms (Rudolf recombination until now. The ob- von Steiger, 2004, Spatium 13). In served scale tells us the geometry a first attempt by Gamow and his of space. The flatness of space has collaborators, all elements would been determined to a very high have been built in the early, hot precision. Other cosmological pa- Universe. But atomic structure rameters that could be derived prevented the formation of stable

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2101267_[211415]_Spatium_47_2021_(001_016).indd 12 27.04.21 13:58 elements with atomic numbers 5 wind sail developed by Geiss and ever, the predicted and measured or 8, and no elements heavier than put up by the Apollo missions on abundance of lithium, as measured lithium and boron could be formed. the moon, confirming this picture in stars today, is significantly dif- The early Universe produced, es- beautifully. ferent from the CMB predictions. sentially, helium from hydrogen This “Lithium Problem” has been with very small additions of deu- Only refinements have been made addressed in theoretical studies. A terium and lithium. Free neutrons to this general picture in the past potential solution could be if there decayed after the equilibrium be- decades. An accurate determina- were more than three families of tween protons and neutrons was tion of the baryonic matter density elementary particles, or a particle lifted, and the remaining neutrons in the early Universe through the that changes the radiation field in were bound in helium nuclei. The CMB has significantly decreased the early Universe but does not ratio of helium to hydrogen is set the range of the allowable helium ­interact with the rest of matter at by the balance of the neutron de- abundance. At the same time meas- later times. Potentially, we are cay time, and when the helium urements of the deuterium abun- missing a piece of physics here. ­nuclei can form. This all happened dance in distant gas clouds with the during the first three minutes of 8m and 10m telescopes on the the Big Bang, and the helium to ground have set severe constraints hydrogen ratio set then can still on the baryonic density. Overall, be observed today. The helium-3 a consistent picture of Big Bang nuclei from the Sun were first nucleosynthesis has emerged and measured directly by the solar strengthened considerably. How-

Figure 6: Combination of different observations pointing towards the “concordance” model (from Goobar and Leibundgut 2011). The contribu- tion of the cosmological constant (, ordinate) is plotted against the matter density (, abscissa) for different measurement methods. The original likelihood regions from distant super- novae showing the accelerated expan- sion of the Universe are the grey con- tours (Supernova Cosmology Project) and dark ellipses (High-z Supernova Search Team). The modern supernova measurements are the blue contours. The possible solutions provided by the fluctuations in the cosmic microwave background are displayed as the or- ange region closely following solutions of flat models. A method to measure density fluctuations of the galaxy dis- tribution (baryonic acoustic oscilla- tions – BAO) is shown in green. The three measurements single out a unique region near ( ≈ 0.7 and  ≈ 0.3), which is consistent with CDM.

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2101267_[211415]_Spatium_47_2021_(001_016).indd 13 27.04.21 13:58 A nearly perfect sion and 2019 for “theoretical dis- Dark matter and dark energy make coveries of ”). up 95% of the energy content to- cosmological day (figure 7), yet we do not have The concordance model has sev- an explanation of what they are. model eral appealing characteristics. The Universe has a flat geometry and a A new problem appeared in recent critical density, which seems to years with an apparent disagree- We have arrived at a rather com- ­follow from an inflationary period ment between the Hubble constant plete view of the Universe today. in the Big Bang that also explains as derived from the CMB in the The CDM model connects the the fluctuations in the CMB. The early Universe and the direct meas- physics at our observing windows various measurements indepen- urements today. Many different (first minutes, 350,000 years, and dently single out a unique set of distance indicators have been em- after one billion years until today) ­parameters, which now makes the ployed to measure and check the with only a few free parameters. It Universe old enough to form the local Hubble constant, and all point has been called a “concordance” oldest known stars. The predic- towards a value that is significantly model, because many disparate tions for the cosmic abundance larger than predicted by the CDM measurements converge to a small of deuterium and helium are a model when calibrated by the region of parameter space that ­natural consequence of this model, CMB. It is at the moment unclear fits (almost) all observations. The and the growth of large-scale whether systematic uncertainties combination of the measurements structure is easily explained with in the measurements or an in­ of acceleration by supernovae the composition of the density complete (or wrong) model is the (SNe Ia), curvature by the CMB ingredients. culprit. The “Hubble tension” has and matter density through the ob- become the focus of most cosmo- servation of baryonic acoustic os- Yet it is clear that we have not logical discussions today and a cillations (BAO) leads to a unique reached the end of cosmological ­resolution may point towards set of allowed parameters (figure 6). studies. There are important limi- “new” physics, if the differences These measurements are com- tations of this model, which re- between the early and the late Uni- pletely independent of each other quire future observations and re- verse cannot be reconciled. and use widely different tracers finements. A glaring problem is the and methods (supernovae, micro- lack of physical characterisation of There are other discrepancies that wave background, clusters of gal- most of the contents in this model. may indicate deficiencies of the axies). This “concordance” model model. The measured normalisa- has become the currently most fa- tion of the matter density in the voured description of the Universe Figure 7: Composition of the Uni- ­local Universe deviates from the and is encapsulated in the CDM verse. Not shown is the contribution of predictions made by the CMB. model. radiation, which is less than 0.00001. The abundance of lithium from the (© ESA and the Planck Consortium) does not The successes in cosmology have correspond to the model predic- been celebrated in different forms. tions and the growth of structure The Nobel Prizes in Physics have may still hold surprises when the been awarded for cosmological new imaging and spectroscopic topics over the years (1978 for the surveys have been analysed in the discovery of the CMB; 2006 for coming years. the COBE measurement of the black body and the fluctuations of Are these discrepancies due to lim- the CMB; 2011 for the discovery ited accuracy of the measurements of the accelerated cosmic expan- or do they point to deficiencies of

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2101267_[211415]_Spatium_47_2021_(001_016).indd 14 27.04.21 13:58 the model? Are basic assumptions were used to predict a Hubble con- Space Telescope) and will certainly in the model inadequate or even stant that was too small compared change our view of the Universe. wrong? It is currently not possible to all measurements (see figure 3). It Another avenue is extensions to to give a definite answer to these was resolved by an observation the physical laws, in particular our questions. that originally wanted to confirm understanding of gravity. Other the existing model and ­ultimately questions include whether we are How will cosmology look like changed the paradigm that allowed actually observing a “typical” re- 25 years from now? If the progress only a decelerated expansion. gion of the Universe or whether of the past 25 years is any indica- we are confined to a special place. tion, much improved observations Cosmology is the attempt to con- This questions the validity of the of a large fraction of the accessible struct a worldview based on the , which has Universe will have challenged some known laws of physics and the some support through the smooth- of the current views. After all, no- available observations. Clearly, ness of the CMB but requires fur- body had predicted an accelerated progress is driven by the available ther testing. expansion of the universe. In fact, data, and we are expecting an ex- it was against the accepted theory, plosion of new measurements of The answers will lie in the resolu- although indications of a paradigm the critical cosmological parame- tion of the current “tension” and change were present as all measure- ters in the coming decade. Several new and improved observations. ments indicated a matter density large survey facilities dedicated to The field is vibrant and has at- too low for the favoured Einstein- cosmo­logy have been in develop- tracted wide-spread interest. Many de Sitter model. The tension be- ment for the past decade (Euclid new missions and projects are un- tween the dynamical age, the mat- satellite, Vera Cooper Rubin Ob- der way to answer some of the out- ter density and the expansion rate servatory, Nancy Grace Roman standing questions.

Further reading/literature Spatium No 1, Geiss, J.: Entstehung De Grijs, R. (ed.): Advancing the Nussbaumer, H., Bieri, L.: des Universums, April 1998 Physics of Cosmic Distances. ­Discovering the Expanding Universe, Spatium No 3, Tammann, G. A.: Proceedings of the 289th symposium Cambridge University Press, Birth, Age and the Future of the of the International Astronomical ISBN 9780521514842, 2009 ­Universe, May 1999 ­Union held in Beijing, China, Panek, E.: The 4% Universe, August 27-31, 2012. Cambridge, UK: Spatium No 7, Pretzl, K.: In Search of Houghton Mifflin Harcourt, Cambridge University Press, 2013. ISBN 978-0-618-98244-8, 2011 the Dark Matter in the Universe, ISBN: 9781107033788. May 2001 Peebles, P. J. E.: Cosmology’s Century, Goldsmith, D.: The Runaway Spatium No 13, von Steiger, R.: Princeton University Press, ­Universe, Perseus Books, ISBN 978-0-691-19602-2, 2020 ­Woher kommen Kohlenstoff, Eisen ISBN 0-7382-0068-9, 2000 und Uran, November 2004 Schneider, P.: Extragalactic Astron- Goobar, A., Leibundgut, B: Supernova omy and Cosmology, 2nd Edition. Spatium No 32, Meylan, G.: Mirages Cosmology: Legacy and Future, in the Universe, November 2013 Springer 2015. ­Annual Reviews of Nuclear and ISBN: 978-3-642-54082-0. Bartelmann, M.: Das kosmologische ­Particle Physics, 61, 251, 2011 Standardmodell. Springer Spektrum, Singh, S.: Big Bang, Fourth Estate, Kirshner, R. : The extravagant ISBN 0-00-719382-3, 2004 2019. ISBN 978-3-662-59626-5. ­Universe, Princeton University Press, https://doi.org/10.1007/ ISBN 0-691-05862-8, 2002 Webb, S.: Measuring the universe: the 978-3-662-59627-2. cosmological distance ladder Springer, Kragh, H. S.: Conceptions of cosmos: 1999. ISBN: 1852331062. David Christian, TED talk 2011, from myths to the accelerating uni- http://www.ted.com/talks/david_ verse: a history of cosmology Oxford christian_big_history?language=en University Press, 2006. Also: The Big History Project ISBN: 9780199209163. (http://www.bighistoryproject.com)

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The Author

became a post-doctoral research been allocated observing time at associate, first at the Harvard- the world’s leading optical and in- Smithsonian Center for Astro­ frared telescopes (VLT, HST, physics, Cambridge Massachusetts, Keck, Gemini and 4m telescopes from 1989 to 1992, and later at the in the USA and Chile). He has University of California, Berkeley, published, as first author and co- from 1992 to 1993. Since Decem- author, about 140 refereed papers ber 1993, he has been staff astro­ and is the co-editor of three books. nomer of the European Southern As a member of the High-z Super- Observatory (ESO). He has held nova Search Team he has received several positions within the ESO, the Science Magazine Break- starting with supporting the VLT through of the Year (1998), the Programme Scientist from 1995 to Gruber Cosmology Prize (2007), 1997 and as the head of the VLT and the Breakthrough Prize in Data Flow Groups from 1998 to Fundamental Physics (2015). Since

© Marc McCaughrean 2001. During this period he con- 2019, he has held an honorary pro- tributed significantly to the devel- fessorship at the Physics Depart- opment of the VLT operations. ment of the Technical University From 2001 to 2009, he led the Munich. Prof. Dr. Bruno Leibundgut stud- ­Office for Science in Garching and ied physics, , and math- Santiago de Chile and was the ESO ematics from 1979 to 1983 at the Director for Science from 2008 to University of Basel, where he 2014. Since August 2014, he has ­obtained his PhD (supervised by been the VLT Programme Scien- G. A. Tammann) at the Astro­ tist. Prof. Leibundgut has re- nomical Institute in 1989. After a searched extensively on supernova brief post-doctoral stint there, he physics and cosmology and has

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