The density parameter, Ω

Without going to cosmological distances, it is possible to measurethe density parameter Ω, by means of so-called local tests. Manyof the local tests “weigh” local structures by applying the virial theorem,which states that the kinetic energy of a self-gravitating systemshould be approximately equal to its potential energy. Since themotion of luminous galaxies must be observed to estimate the kineticenergy of the system, only the component of the mass density clusteredwith luminous galaxies can be examined in this fashion. As notedearlier, such measurements tend to give low values of around 0.1to 0.2. However, as mentioned in the discussion of cosmic velocityflows in section III, there may be a component of dark matter clumped in sizes largerthan clusters of galaxies but smaller than superclusters. Cosmicvelocity flows may be detecting structures on this scale, givingvalues of Ω near 1. If there exists a perfectly smooth backgroundof mass density unclustered with the galaxy distribution on any scale,it can be detected only by its effects on the curvature of space,in the global measurements of q0.

Deep Imaging of Galaxies

Galaxies have been used as beacons to map the distribution of matterin the universe ever since they were recognized as independent systemsof stars. As described above, the “local” distribution of galaxies showsa complicated network of structures. When averaged over the largestdistances, many billions of light-years, the distribution of matteris expected to be more homogeneous. Current research programs onvery distant galaxies have two distinct goals. The first is to usethe number of visible galaxies as a measure of the surveyed volume.If galaxies were stationary and the geometry of space were determinedby the rules of euclidean geometry, then the number of galaxies seenwould be roughly proportional to the cube of the distance probed(Nr3). When the effects of redshift and non-euclidean geometry are takeninto account, the number of galaxies is expected to increase moreslowly than r3 at larger distances, as is indeed observed. (If these effects werenot present, the night sky would not be dark! This is known as Olbers's paradox.) Questions about the geometry of the universe—is space positively or negatively curved, infinite or finite?—can be related by general relativity to the dynamics of the expansion(will the universe expand forever, or will it stop expanding andcollapse in a Big Crunch?). Thus, measuring the curvature of theuniverse in the past can be used to predict the expansion of theuniverse in the future.

The second use of distant galaxies is to probe for signs of evolutionof galaxies and of the clustering of galaxies in the universe overthe billions of years during which their light has been travelingto us. The notion is that a galaxy seen at an earlier stage in itslife should have more gas available out of which to form new stars,and consequently it should appear brighter and bluer (when adjustedfor redshift) because of the presence of many massive, hot youngstars. A trend to bluer colors in fainter galaxies has been detected,and its detailed interpretation is a subject of active current research.

Normal nearby galaxies emit most of their light at visible wavelengths,but the light received from the most distant galaxies is redshiftedto infrared wavelengths. In the past decade, detectors with highsensitivity have been developed that are ideal for measuring weakinfrared radiation, and the main technical problem that remains isthe strong emission of infrared radiation from the atmosphere andfrom the telescope itself. This problem is analogous to local interferencein the CMBR measurements, and the solution is similar: go where it's cold, to the South Pole or out in space. Plans are well advancedto deploy a telescope at the South Pole to make deep-sky surveysat near-infrared wavelengths. This approach is less expensive thanbuilding space observatories, but it has the disadvantage of beingrestricted by the atmosphere to a relatively narrow spectral range.This effort in Antarctica is complemented by planned space missionssuch as the Space Infrared Telescope

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