Most of the matter in the Universe, we know now, is in the form of mysterious "dark matter", which is probably some as-yet undetected sub-atomic particle. The normal kind of matter which we encounter every day (made of protons, neutrons and other baryons) is only a small fraction of the total Universe mass content. But mysteriously, we're not even sure where most of the normal matter is. Analyses of fluctuations in the Cosmic Microwave Background detected by the Wilkinson Microwave Anisotropy Probe and the Planck spacecraft provide measures of the total baryon content of the Universe. But estimates of the baryon content based on the observed amount of stars and gas in clusters of galaxies find only half the amount of matter that analyses of the Microwave Background require. Where are the rest of the baryons? Astronomers suspect that this "missing normal mass" exists in a warm-hot intergalactic medium (whimsically called the WHIM) located in the strands of the majestic "cosmic web" that connects galaxies together. The image above shows a computer simulation of this "cosmic web", with galaxies appearing in knots and nodes where the web is particularly dense. Now a study by a group in Japan, and an independent study by a group in the UK have claimed the first evidence of the existence of the WHIM. These studies looked at hundreds of thousands, to millions, of pairs of galaxies in the Sloan Digital Sky Survey which were suspected to be connected by strands of the cosmic web. They then studied the properties of the cosmic microwave background between these pairs. Hot gas in the strands will scatter away some of the microwave background, helping to reveal the faint filaments. These studies found that the density of normal matter in these strands is from three to six times higher than the average density in the Universe. These studies thus establish, for the first time, the existence of matter in the WHIM and help reveal the cosmic web.
NS: Half the Universe’s Missing Matter Has Just Been Finally Found
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