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 seems to be accounted for at high redshifts (z > 2-3) observationally. Going towards lower redshifts, however, only about two thirds of the BBN baryons are detected; this is the "missing baryon problem". The common wisdom is that those baryons are not actually missing, but are hidden in some warm-hot gas of very low density, which makes it difficult to detect; cosmological hydrodynamic 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. We are developing cryogenic microcalorimeters for a spectroscopic satellite mission that is being conceptualized to significantly advance the study of circumgalactic and intergalactic media (and thus the formation and evolution of galaxies).

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