Astroparticle Physics

284 13 Dark MatterquintessenceBoomerangMaximasurface of last scatteringrecombinationBoomerang, Maxima,WMAPtemperature clustersit is clear that the cosmological constant plays a dominantrôle even today. Even if the repulsive force of the vacuumis small compared to the action of gravity, it will eventuallywin over gravity if there is enough empty space, see also(13.24). The present measurements (status 2005) indicate avalue for Ω Λ of ≈ 0.7. This means that the vacuum is filledwith a weakly interacting hypothetical scalar field, which essentiallymanifests itself only via a repulsive gravitation andnegative pressure, which consequently corresponds to a tension.In this model the field (e.g., ‘quintessence’, see glossary)produces a dynamical energy density of the vacuumin such a way that the repulsive gravity plays an importantrôle also today. This idea is supported by the findings of theAmerican Boomerang 9 and Maxima 10 experiments. Thesetwo balloon experiments measure the blackbody radiationover a limited region of the sky with a much higher precisionthan the COBE experiment. The observed temperatureanisotropies are considered as a seed for galaxy formation.These temperature variations throw light on the structureof the universe about 380 000 years after the Big Bang whenthe universe became transparent. The universe before thattime was too hot for atoms to form and photons were trappedin a sea of electrically charged particles. Pressure waves pulsatedin this sea of particles like sound waves in water. Theresponse of this sea to the gravitational potential fluctuationsallows to measure the properties of this fluid in an expandinguniverse consisting of ordinary and dark matter. Whenthe universe cooled down, protons and electrons recombinedto form electrically neutral hydrogen atoms, thereby freeingthe photons. These hot photons have cooled since then to atemperature of around 2.7 K at present. Temperature variationsin this radiation are fingerprints of the pattern of soundwaves in the early universe, when it became transparent. Thesize of the spots in the thermal map is related to the curvatureor the geometry of the early universe and gives informationsabout the energy density. Both experiments, Boomerang andMaxima, and the satellite experiment WMAP observed tem-perature clusters of a size of about one degree across. Thisinformation is experimentally obtained from the power spectrumof the primordial sound waves in the dense fluid ofparticles (see Chap. 11). The observed power spectrum correspondingto characteristic cluster sizes of the temperature9 Boomerang – Balloon Observations of Millimetric ExtragalacticRadiation and Geophysics10 Maxima – Millimeter Anisotropy Experiment Imaging Array