epoch of rapid expansion, called inflation. The recently launched Plancksatellite will produce higher-resolution, all-sky CMB temperature and polarization maps at many frequencies. Complementary observations from the ground will look at patches of the sky with fine angular resolution. These experiments will be able to compare the temperature fluctuations on a range of scales, from those so small that they will grow into only a small group of galaxies today, to the largest-scale fluctuations observable on the whole sky, which will allow scientists to see if the fluctuations are truly random or instead non-Gaussian, as some theories suggest. However, the most ambitious goal of all is to try to detect a particular pattern in the polarization—called B-modes—that is caused by very long wavelength gravitational radiation that would be created at the time of inflation. The B-modes are a window allowing us to peer far back beyond the screen of the CMB into the period of inflation.

The convincing detection of B-mode polarization in the CMB produced in the epoch of reionization would represent a watershed discovery. The strength of the associated fluctuations, now constrained to less than 20 percent, should be measurable by upcoming telescopes at a level as low as 20 times weaker than the current limit. If these fingerprints of inflation are detected, then a decadal survey implementation advisory committee (DSIAC) (as discussed in Chapter 3) could determine whether a technology development program should be initiated with a view to flying a space microwave background mission during the following decade that would be capable of improving the accuracy by a further factor of 10 and elucidating the physical conditions at the end of inflation.

Third, an inescapable consequence of general relativity is the existence of black holes. Once mere conjectures, black holes are now known to be very common. They are found at the centers of normal galaxies like our own Milky Way and as companions to normal stars transferring mass to their neighbors through winds. Gas close to a black hole radiates X-rays prodigiously and offers a quantitative observational test of relativistic theory that would be possible to conduct with the proposed sensitive International X-ray Observatory, IXO. Another general property of black holes is that they create jets of hot plasmas that move at speeds very close to that of light and create intense beams of radiation from the longest radio wavelengths to the highest gamma-ray energies. The proposed Advanced Čerenkov Telescope Array (ACTA) will use high-energy gamma-ray observations to probe the properties of black holes.13


The committee also considered a proposed black hole finder mission called the Energetic X-ray Imaging Survey Telescope (EXIST). This was recommended by AANM and further considered by the NRC’s Beyond Einstein Program Advisory Committee (NRC, NASA’s Beyond Einstein Program: An Architecture for Implementation, The National Academies Press, Washington, D.C., 2007). While it would address important science goals, the high estimated cost of $2.4 billion, well over 10 times the cost indicated in the 2001 decadal survey, AANM, ruled it out for further consideration, and it is no longer recommended.

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