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Planck helps solve a long standing mystery

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 :  http://arxiv.org/abs/1511.05156

 

 

 

he Hubble Frontier Fields (HFF) program has been recently extended to include two additional clusters to this spectacular data set. This is great news for science. To date, the HFF has provided the best data set to study the distribution of dark matter in galaxy clusters (the same data set is used for other exciting projects). Currently, the HFF is covering two clusters (from the original set of 4). One of them is MACSJ1149.5+2223. This cluster is interesting for several reasons. One of them is the fact that a supernova at z=1.491  was observed by Hubble in one of the observing campaings. This supernova is seen 4 times, in a configuration known as Einstein cross. The multiple images observed by Hubble are distorted versions of the original supernova that are multiply lensed 4 times (gravitational lensing). One interesting feature of gravitational lensing is that since the paths of photons are distorted by the gravitational potential, and so is their time of arrival to our telescopes on earth. Because of this time difference, multiply lensed images of the same background object are seeing in different epochs. Is like seeing your kids simultaneously when they had different ages, … weird. The supernova observed by Hubble is observed (4 times) in one of the arms of its host galaxy in just one of the counterimages but not in the other two counterimages. This means that that supernova will be observed in the future in the other two counterimages or it has already happened in those two counterimages. Since the lifetime of a supernova is short (days or weeks) and there is no way to predict when a star will go supernova it is nearly imposible to observe a star going supernova before it happens.  The multiply lensed images  of the supernova in this cluster, could in theory, allow us to predcit when a supernova is going to be observed and study the supernova explosion from the very beginning. Using accurate models of the gravitational potential we where able to predict the time difference between the different counterimages and predict when the supernova will be observed next in the different counterimages or when it was happening in those images. The figure shows the predicted time delays for the supernova observed in MACS1149. This supernova is observed now four times. Our model predicts that we are too late to observe one of the counterimages that occurred about 9 years ago but a new chance to see this supernova will take place again around November 1st 2015.  This will be the first time we can point a telescope to a position and wait for the SN to happen (again). Talk about time travel !

Original paper: http://arxiv.org/abs/1504.05953

Published version: http://mnras.oxfordjournals.org/content/456/1/356

SN REFSDAL UPDATE (Dec. 2015)

On December 12th 2015, news broke about the reappearance of SN Refsdal at the exact predicted position posted in this article. The date of the explosion is uncertain by one month but it must have happened between November 15th and December 10th which are the dates when Hubble was observing at this location. On the November 15th observation there was no sign of the explosion but in the December 10th observation the SN had already shown up at the predicted position. The date of the original prediction (November 1st) is based on a value of the Hubble constant that is a bit out of date (h=0.7). Adopting a more recent estimate (h=0.67) and re-scaling the time delay, the best prediction for the reappearance shifts from November 1st 2015 to  November 17th, right in the window of time where we know the explosion had happened.