Title: Probing Missing Baryons with X-ray Spectroscopy 

　　Speaker: Wei Cui (Tsinghua & Purdue University) 

　　Time: 3 PM, May 04th (Thursday)    

　　Abstract: 

　　The number of baryons produced in the Big Bang Nucleosynthesis (BBN) is fully accounted for at the redshift of about 1000, with observations and modeling of the cosmic microwave background. Most (if not all) of the BBN baryons are also found at redshifts above 2-4. In the present-day universe, however, only about half of the BBN baryons are detected, thus the global “missing baryon” problem. On smaller scales, observations have revealed that, while the measured baryon fraction of the richest clusters agrees with the cosmic average, it becomes increasingly smaller toward lower mass systems, so there is also a local “missing baryon” problem. Furthermore, about 80-90% of the metals in the universe seem to be “missing”, as far as observations are concerned. A single solution to all three problems may be that the “missing” baryons are hidden in a hot phase of IGM and/or CGM that is significantly enriched (with a metallicity of 0.2-0.3). Such gas has so far evaded observations, but may be detected through the emission or absorption lines of its highly ionized constituents at soft X-ray energies (< 1 keV). 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, and also briefly discuss the design of a satellite experiment that employs a microcalorimeter array to directly image the hot IGM/CGM. The results would likely have significant implications on our understanding of structure formation and evolution.