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There’s no sweet spot – no sweet instrument that we’ve invented yet that can directly observe this gas,” says Richard Ellis at University College London. “It’s been purely speculation until now.” Because it's not quite hot enough for X-ray telescopes to observe.
Both teams took advantage of a phenomenon called the Sunyaev-Zel’dovich effect that occurs when light left over from the big bang passes through hot gas to find another way to definitively show that these threads of gas are really there. As photons of light travel, some of them scatters off the electrons in the gas, leaving a dim patch in the cosmic microwave background from the birth of the cosmos that were to faint to be mapped by the Planck satellite in 2015.
Both teams selected pairs of galaxies from the Sloan Digital Sky Survey that were expected to be connected by a strand of baryons. They stacked the Planck signals for the areas between the galaxies, making the individually faint strands detectable en masse.
Observations of galaxies and galaxy clusters in the local universe can account for only 10% of the baryon content -made of particles called baryons rather than dark matter- 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. The missing links between galaxies have finally been found. This is the first detection of the roughly half of the normal matter in our universe – protons, neutrons and electrons – unaccounted for by previous observations of stars, galaxies and other bright objects in space.
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 ∼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.
Because WIMPs are so "weakly interacting" — that is, they cannot interact with normal matter via the electromagnetic, strong or weak forces — XENON1T can detect them only by looking out for lucky collisions between WIMPs and atoms in a chamber filled with pure liquid xenon cooled to minus 139 degrees Fahrenheit (minus 95 degrees Celsius).
My understanding is quite different from this. I thought the baryons we observed locally accounted for 2.5% of the mass energy content in the corresponding region of space, and that the estimated baryonic content was 5% of the mass energy content the same region. This 10% figure as I understand it applies to a much larger scale and implies that of the 5% of the universe which is baryonic, that only 10% of that or 0.5% is (or was) observable. This is lower than the 2.5% seen locally because most of the universe is much further away. So I think what they claim to have found is some of the gap between the 0.5% baryonic mass energy content previously observed and the 5% of the baryonic mass energy content believed to exist.
originally posted by: TEOTWAWKIAIFF
Baryons are neutrons and protons (or matter that has mass). Just looking into the night sky and seeing all the stars and galaxies, and yes, including black holes, white dwarfs, brown gas giants, etc., still only accounts for 10% of the universe.
What about the other 90%?
Your OP implies maybe this has something to do with dark matter but I'm not clear on that and I'm not clear that it really does, but it seems like it's just about missing baryonic matter though if anybody has a better understanding it's welcome.
Observations of the afterglow of the Big Bang known as the cosmic microwave background (CMB) suggest that protons, neutrons and other (three-quark) baryon particles only account for about 5% of the universe's energy density – the rest is believed to consist of enigmatic dark matter and dark energy. However, the combined mass of all of the stars within a radius of about a billion light-years from Earth only amounts to about 2.5% of the energy density within that region. Computer simulations predict that the missing baryons instead exist within low-density plasma filaments millions of light-years long.
First, let us note that the baryon mass-to-light ratio corresponds to the mass-to-light ratio of stars, and not to the dynamical one which usually includes dark matter (DM). In fact, the presence of DM in the internal parts of spiral galaxies is very evident (Persic & Salucci 1991), so we cannot ignore its contribution to the mass of a galaxy.
a reply to: FocusedWolf
So does non-baryon dark matter even exist? Like if they keep looking and go from 90% to 100% of matter accounted for..