Missing baryons in the cosmic web revealed by the Sunyaev-Zel'dovich effect
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(2017)cite arxiv:1709.10378Comment: 19 pages, 6 figures, submitted to Nature.

Observations of galaxies and galaxy clusters in the local universe can account for only $10\%$ of the baryon content inferred from measurements of the cosmic microwave background and from nuclear reactions in the early Universe. Locating the remaining $90\%$ of baryons has been one of the major challenges in modern cosmology. Cosmological simulations predict that the 'missing baryons' are spread throughout filamentary structures in the cosmic web, forming a low density gas with temperatures of $10^5-10^7$ K. Previous attempts to observe this warm-hot filamentary gas via X-ray emission or absorption in quasar spectra have proven difficult due to its diffuse and low-temperature nature. Here we report a $5.1 \sigma$ detection of warm-hot baryons in stacked filaments through the thermal Sunyaev-Zel'dovich (SZ) effect, which arises from the distortion in the cosmic microwave background spectrum due to ionised gas. The estimated gas density in these 15 Megaparsec-long filaments is approximately 6 times the mean universal baryon density, and overall this can account for $\sim 30\%$ of the total baryon content of the Universe. This result establishes the presence of ionised gas in large-scale filaments, and suggests that the missing baryons problem may be resolved via observations of the cosmic web.
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