The Standard Model of particle physics does a good job of explaining the interactions between matter\u2019s basic building blocks. But it\u2019s not perfect. It struggles to explain dark matter. Dark matter makes up most of the matter in the Universe, yet we don\u2019t know what it is.<\/p>\n
The Standard Model says that whatever dark matter is, it can\u2019t interact with itself. New research may have turned that on its head. <\/p>\n
<\/p>\n Physicists propose many different candidates for dark matter, including dark photons, weakly interacting massive particles (WIMPs), primordial black holes, and more. Each one is intriguing in its own way, but there\u2019s no confirmation regarding any of them. And each one is a proposed part of the Standard Model.<\/p>\n New research in the journal Astronomy and Astrophysics suggests we may be barking up the wrong tree. It suggests that another model, called the Self-Interacting Dark Matter model, can explain dark matter while the Standard Model and its Lambda Cold Dark Matter (Lambda CDM) simply can\u2019t. <\/p>\n The paper is \u201cAn N-body\/hydrodynamical simulation study of the merging cluster El Gordo: A compelling case for self-interacting dark matter?\u201d The lead author is Riccardo Valdarnini of SISSA\u2019s (Scuola Internazionale Superiore di Studi Avanzati) Astrophysics and Cosmology group.<\/p>\n El Gordo is an extremely massive, extremely distant galaxy cluster more than seven billion light-years away from Earth. It\u2019s comprised of two galaxy sub-clusters that are colliding with one another at several million kilometres per hour. It\u2019s at the center of a back-and-forth over dark matter and the Lambda CDM. <\/p>\n A 2021 paper claimed that El Gordo presents a challenge for the Lambda-CDM model because it appeared so early in cosmic history, is extremely massive, and has such a high collisional velocity. \u201cSuch a fast collision between individually rare massive clusters is unexpected in Lambda cold dark matter cosmology at such high z,\u201d the authors of that paper wrote. <\/p>\n A later paper from 2021 arrived at a lower mass estimate for El Gordo, one that was consistent with Lambda CDM. \u201cSuch an extreme mass of El Gordo has stimulated a number of discussions on whether or not the presence of the cluster is in tension with the Lambda CDM paradigm,\u201d those authors wrote. \u201cThe new mass is compatible with the current Lambda CDM cosmology.\u201d<\/p>\n A key part of Lambda CDM is that dark matter is both cold and collisionless. In that model, it\u2019s impossible for dark matter particles to collide with one another; they can only interact through gravity and possibly the weak force. This study challenges that notion. <\/p>\n Proving that dark matter can interact with itself via collisions is difficult and complicated. El Gordo is a good place to study the Self-Interacting Dark Matter (SIDM) idea. \u201cThere are, however, unique Galaxy clusters like El Gordo can be divided into three components: the galaxies, the dark matter, and the gas mass. The Standard Model says that the colliding gas loses some of its initial energy during the collision. \u201cThis is why, after the collision, the peak of gas mass density will lag behind those of dark matter and galaxies,\u201d Valdarnini explained. <\/p>\n But the SIDM says something different. It says that the points where the dark matter reaches its maximum density, called centroids, should be physically separated from the other mass components. The peculiarities of that separation are a signature of SIDM. <\/p>\n Observations of El Gordo show that it consists of two large sub-clusters, the northwest (NW) and the southeast (SE), which are merging into one. <\/p>\n X-ray images show different peak locations for the different mass components. The X-ray image below shows a single X-ray emission peak in the SE subcluster and two faint tails elongated beyond the X-ray peak. The X-ray peak precedes the dark matter peak. The Brightest Cluster Galaxy (BCG) is also offset from the SE mass centroid. BCGs are the brightest galaxies in a given cluster, are extremely massive, and are centers of mass in clusters. <\/p>\n \u201cAnother notable aspect can be seen in the NW cluster, where the galaxy number density peak is spatially offset from the corresponding mass peak,\u201d Valdarnini explained. <\/p>\n But those observations alone aren\u2019t enough. In the new paper in Astronomy and Astrophysics, Valdarnini employed a large number of N-body\/hydrodynamical simulations to study El Gordo\u2019s physical properties. The systematic simulations aim to match the observations. Each simulation has slightly different parameters, and when a simulation matches observations, those parameters are likely to offer some explanation of the observations. <\/p>\n Valdarnini explains it clearly in the paper. \u201c\u2026 the aim of this paper is to determine whether it is possible to construct merger models for the El Gordo cluster that can consistently reproduce the observed X-ray morphology, as well as many of its physical properties.\u201d<\/p>\n The critical part of this work and its simulations concerns the separations between the centers of mass in El Gordo. If simulations can produce that, it\u2019s evidence in favour of SIDM.<\/p>\n \u201cThe most significant result of this simulation study is that the relative separations observed between the different mass centroids of the \u201cEl Gordo\u201d cluster are naturally explained if the dark matter is self-interacting,\u201d states Valdarnini. <\/p>\n \u201cFor this reason, these findings provide an unambiguous signature of a dark matter behaviour that exhibits collisional properties in a very energetic high-redshift cluster collision,\u201d he continued.<\/p>\n It\u2019s a classic \u201ctip of the iceberg scenario.\u201d While these results are in favour of the Self Interacting Dark Matter model, they\u2019re nowhere near conclusive, as Valdarnini makes clear when he talks about inconsistencies in the results.<\/p>\n Valdarnini\u2019s work shows that while the results are an approximation of how dark matter may behave during cluster mergers, there\u2019s a lot more to it. The \u201cunderlying physical processes\u201d are extremely complex. <\/p>\n \u201cThe study makes a compelling case for the possibility of self-interacting dark matter between colliding clusters as an alternative to the standard collisionless dark matter paradigm,\u201d he concludes. <\/p>\n For most of the eight billion human beings alive today, dark matter is of little consequence in daily life. But if we want to entertain hopes and enjoy daydreams of human civilization lasting for centuries, millennia, or even longer, expanding into space and travelling to other stars, it\u2019s critical that we understand everything we can about nature. The history of human progress parallels our growing understanding of nature. <\/p>\n Understanding dark matter is critical to understanding nature. If we want civilization to persist, a better understanding of everything about nature is the best way forward. <\/p>\n Now, back to our daily lives under the Standard Model. <\/p>\n
laboratories that can prove very useful for this purpose, many light years away from us,\u201d said lead author Valdarnini. \u201cThese are the massive galaxy clusters, gigantic cosmic structures that, upon collision, determine the most energetic events since the Big Bang.\u201d El Gordo is one of them. <\/p>\n![\"This](\"https:\/\/www.universetoday.com\/wp-content\/uploads\/2024\/06\/elgordo_525.jpg\")
![\"This](\"https:\/\/www.universetoday.com\/wp-content\/uploads\/2024\/06\/El-Gordo-SIDM-sims.jpg\")
Like this:<\/h3>\n