Astroparticle Physics

14 Astrobiology 289on the chances to create life (see also Sect. 10.5). There aremany other parameters which appear to be fine-tuned to thedevelopment of life in the form that we know.One of the problems of the unification of forces in anall-embracing Theory of Everything is the ‘weakness’ of thegravitational force. If, however, gravitation would be muchstronger, we would live in a short-lived miniature universe.No creatures could grow larger than a few centimeters, andthere would be hardly time for biological evolution.The fact that we live in a flat universe with Ω = 1appearsalso to be important for us. If Ω were much larger thanunity, the universe would have recollapsed long ago, alsowith the impossibility for the development of life. In thiscontext the exact value of the cosmological constant whichdescribes a repulsive gravity, plays an important rôle for thedevelopment of the universe.It is remarkable that the effect of Λ on microscopicalscales, just as that of attractive gravity, is negigible. The discrepancybetween the measured small value of Λ in cosmologyand the extremely large values of the vacuum energy inquantum field theories tells us that important ingredients inthe understanding of the universe are still missing.Another crucial parameter is the number of dimensions.It is true that superstring theories can be formulated ineleven dimensions, out of which seven are compactified, sothat we live in three spatial and one time dimension. But lifewould be impossible, e.g., in two spatial dimensions. Whathas singled out our case?Also the efficiency of energy generation in stars is importantfor the production of different chemical elements.If this efficiency would be much larger than the value thatwe know (0.7%), stars would exhaust their fuel in a muchshorter time compared to that needed for biological evolution.It has become clear that the fine-tuning of parametersin the Standard Model of elementary particle physics andcosmology is very important for the development of stars,galaxies, and life. If some of the parameters which describeour world were not finely tuned, then our universe wouldhave grossly different properties. These different propertieswould have made unlikely that life – in the form we knowit – would have developed. Consequently, physicists wouldnot be around to ask the question why the parameters haveexactly the values which they have. Our universe could havefine-tuned parametersgravitationΩ parametercosmological constantnumber of dimensionsefficiencyof energy generationfine-tuning of parameters