Solving a long standing mystery

Planck helps solve a long standing mystery

Virgo cluster (marked with a big circle) is near the centre of the image. The signal detected by Planck extends well beyond the limits of the cluster probing part of the missing baryons.

One of the puzzles of modern astronomy is what is known as the missing baryon problem. Baryons are the ordinary matter we are familiar with. You are made of baryons as it is everything you touch, eat and see. The best known form of baryons are electrons and protons. Together with neutrons (another form of baryons) they form atoms and atoms form molecules and molecules form … well, everything else. Detailed observations of the distant Universe tell us how many baryons are out there and the amount we can see agrees very well with what is expected from the standard model that describes the Universe so there is nothing surprising there. The story changes when we look at the Universe but at distances much closer to us. In theory, we should see the same amount or proportion of baryons that we see in the distant Universe right here, in our neighborhood but they are no where to be found, so where are they?

Baryons follow a similar law than energy, they don’t get created nor destroyed (for the most part), they transform  (with the transformation between a neutron and an electron plus a proton or viceversa being a classical example). If there were baryons in the early universe, pretty much the same number of baryons should exist today. Instead, observations of the local Universe reveal a significant deficit of baryons when compared with the expectations and the observed number of varions in the most distant Universe.   It is commonly believed that most of these missing baryons are in the form of a plasma which emits very small amounts of light (mostly at high energies like UV or X-rays) which has not been detected so far. Howevere, the same plasma produces also a distortion in the light that originated soon after the Big Bang (more rpecisely, 300000 years after the Big Bang). This light, known as the Cosmic Microwave Background, or CMB,  has been travelling through the Universe since the time it was first produced and permeates the entire Universe. When the CMB light crosses a region filled with plasma, it gains a small amount of energy. This small gain of energy can be measured with current telescopes like the Planck satellite through an effcet known as the Sunyaev-Zel’dovich, or SZ ,effect. The SZ effect has been studied with Planck in dense and hot plasma regions, usually found at the centre of galaxy clusters. In a recent work, we have focused our attention to one particular cluster, the Virgo galaxy cluster. This cluster is special because it is the closest cluster to us. In fact, it is so close that  our galaxy is falling towards the centre of this cluster due to its ginat gravitational attraction. The distance from our galaxy to the centre of Virgo is only about six times  the distance from our galaxy to our closest sister galaxy, the Andromeda galaxy. The apparent size of Virgo in the sky is about 15 times larger than the apparent size of the full moon. This large size, allowed us to do a detailed statistical analysis that takes advantage of the large size of Virgo and maximizes the small distortion that the missing baryons around Virgo produce over the CMB light.  Our findings (summarized in the figure accompanying this post) reveal vast amounts of plasma beyond the previously established limits of the Virgo cluster. The signal around Virgo observed by Planck coincides with what was the expected signal emerging from the missing baryons around galaxy clusters confirming that the missing baryons are probably forming diffuse clouds of plasma around the biggest structures in the Universe, like galaxy clusters. Although the missing baryons found by Planck don’t account for all the missing baryons, it does reduce the amount of baryons that are still evading a firm detection. Future analyses based on Planck and ground-based experiments will continue  the hunt for the few remaining missing baryons …


The paper with all the details and results can be found in the following link :




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