![]() The second important characteristic of the CMBR is the variationin intensity (or temperature) from place to place on the sky. Why are “bumps” in the CMBR so important? Thisis exactly the value expected for the CMBR at that redshift. The temperature of the cloud, which shouldhave been the same as that of the CMBR, was found to be 7.6 K. The shift of the spectral lines (redshift) of thecloud was also measured. Using the light from a distant,bright, quasi-stellar source, astronomers were able to measure thetemperature of carbon atoms in an intergalactic cloud between Earthand the source. Recent observations from the new Keck telescope have providedthe first direct evidence that the CMBR temperature has indeed decreasedover (relatively recent) cosmic time. For example, energy-releasingprocesses that would have reheated the universe at critical epochsin its history can now be ruled out.Ĭosmological theories based on the Big Bang unambiguously predictthat the temperature of the universe will fall with time as the universeexpands. The accuratefit of the measured CMBR spectrum to a thermal shape sets extremelytight bounds that limit the variety of hypothetical physical processesthat could have taken place in the early universe. Theoretical modeling of the thermal history of the universe has developedconcurrently with progress in the spectral measurements. The design, operation, and calibrationof the COBE instrument all had their technological heritage in ground-basedand balloon-based instruments, and the experience gained in theseearlier experiments laid the foundation for the highly successfulsatellite measurement. It required the use of a satelliteto get the instrument above the atmosphere of Earth, and the entireinstrument was cooled to a temperature of 1.5 K to reduce radiationfrom the instrument itself. That a single experiment can have such profound implications is unusual.But the measurement was not easy. The recognition that such an epochexisted in the early history of the universe is a cornerstone ofmodern cosmological models. ![]() The simplest and most compelling explanationfor the thermal shape of the CMBR is that the universe was all atthe same temperature for some very early part of its early history.Space was uniformly filled with hot (but rapidly expanding and cooling)ionized gas and thermal radiation. Intensity profile of the universal CMBR-a curious connection between the smallest and the largest realmsof physical theory.
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