Abstract 

  One of the triumphs of the Big Bang Nucleosynthesis (BBN) theory is that its predicted abundances of primordial isotopes agree with the measured values. Moreover, the predicted baryonic mass is accounted for at high redshifts observationally. Going towards low redshifts, however, only a fraction of the BBN baryons are detected; this is the “missing baryon” problem. The common wisdom is that those baryons are not missing, but are hidden in some warm-hot gas of very low density, which is difficult to detect; cosmological simulations support this view. Such gas may be “seen” through the emission or absorption lines of its highly ionized constituents. For that, an X-ray spectrometer of high throughput and high resolution would likely be required. I will describe the development of microcalorimeters for X-ray spectroscopy. I will also briefly discuss the design of a satellite experiment that employs the microcalorimeters to carry out a survey of warm-hot gas in the universe, addressing a wide range of important issues in astrophysics, including the “missing baryon” problem. (http://www.physics.purdue.edu/~cui/)