After decades of searching, with the most advanced technological instruments in the world, the particle that shapes the dark matter of the Universe is still missing. Scientists consider for the first time that they may be after a ghost
Barbara Alvarez Gonzalez, University of Oviedo
Is it time to consider that dark matter does not exist? Is it time to look for other alternatives to explain what 80% of the unknown Universe is made of? At the moment, some scientists are considering the possibility that dark matter is not matter, but an artifact caused by an incomplete understanding of the theory of gravity.
At the origin of the Universe
Today, there is consensus among scientists that the theory that best describes the origin of the Universe is that of big explosion. Thus, the Universe was born about 13.8 billion years ago from an infinitely dense singularity that exploded. The explosion generated a large amount of energy and matter. And all this has been expanding ever since.
Surprisingly, we only know and understand 5% of the matter that exists in the Universe. Even from this small percentage there are aspects that we cannot explain, such as the difference between matter and antimatter.
The physics of elementary particles and the study of interactions between them try to reveal the state and evolution of the Universe. And among all the unknowns to be solved, determining the nature of dark matter is one of the most important in modern cosmology and particle physics.
We know that the Universe is made of visible matter and dark matter. In particular, visible matter, also known as ordinary matter or baryonic matter, is all composed of leptons (elementary particles) and baryons (compounds of quarks, which are also elementary particles). Of this type is only 20% of the matter in the Universe, the remaining 80% is dark matter.
In addition to its composition, we know that there must be an agent that explains the accelerated expansion of the Universe, which for now is attributed to the so-called dark energy.
The Discovery of Vera Rubin
Dark matter along with dark energy make up almost 95% of the Universe. We cannot see it, as it does not emit any kind of electromagnetic radiation.
Much of the evidence for their existence comes from studying the movements of galaxies. Analyzing the cosmic microwave background also provides information about how much visible and dark matter there is.
in 1933 Fritz Zwicky proposed the existence of an invisible mass that could influence the speed of rotation of galaxies. The pioneer Vera Rubin, with her measurements of the curvature of the rotation speed of stars within spiral galaxies, found that these curves remain flat.
Vera Rubin’s discovery contradicts the theoretical model that predicted that stars further away from the center of the galaxy would have slower speeds. This fact cannot be explained by the existence of visible matter and its associated gravitational mass alone, but there must be another form of matter that also provides gravitational energy. This is the most direct and robust evidence for the existence of dark matter.
From that moment, and during the following decades, more evidence related to dark matter was collected, to the point that today the vast majority of scientists accept its existence.
First-tier experiments in search of dark matter
Dark matter is composed of particles that do not absorb, reflect or emit light, cannot be seen directly, and their composition is unknown.
Scientists have developed different strategies to find these potential dark matter candidate particles. Finding them is one of the biggest challenges in physics today.
There are different dark matter search strategies, direct, indirect or with particle accelerators.
Technological progress in recent decades has been enormous. There are dozens of active experiments dedicated to understanding the nature of dark matter with highly sensitive and accurate instruments.
These experiments are spread all over the world, even there is one on the International Space Station (ISS)and are part of international collaborations of dozens of scientists.
The ANAL, DAMA, XENON100 and LUX experiments use direct detection techniques; MAGIC, HESS, VERITAS, Fermi and AMS (on the ISS), among others, are based on indirect techniques to observe what happens in nature in search of elementary particles.
In the first case, from direct means, particles that arise from collisions of particles of visible matter with particles of visible matter are studied.
dark matter, and in the second case, by indirect means, only particles of collisions between dark matter particles are studied.
The LHC “creates” dark matter particles
In particle accelerators as energetic as the Large Hadron Collider (LHC) at CERNEuropean Organization for Nuclear Research, it is possible to recreate the conditions of seconds after the Big Bang and produce or “make” dark matter particles from collisions of very energetic protons.
Accelerators are devices that increase the kinetic energy of stable charged particles.
The LHC is the latest in a chain of accelerators that can reach energies of up to almost 7 TeV (tera electron volts) for each proton beam. Around the collision points, detectors are placed that can measure and identify the particles that are produced in each collision, so that they can be studied.
The ATLAS and CMS experiments are responsible for these searches at CERN’s LHC. These experiments are the same as a long research discovered the Higgs boson in 2012 thus completing the Standard Model of Particle Physics and opening a new era in the field.
This achievement was recognized in 2013 with the Nobel Prize in Physics and with the Prince of Asturias Award for Scientific and Technical Research.
The difficulty in identifying candidate dark matter particles in these types of experiments is that dark matter interacts very weakly with matter and it is virtually impossible in such cases to find its trace.
CERN
Diving into unknown processes
Incorporating theories beyond the standard model, such as the supersymmetry, simplified models with scalar bosons, or models of the hidden sector or dark sector, the searches are carried out from their disintegration into ordinary particles that can be observed. These searches lead us to study unknown processes that may be the ones that will finally allow us to understand the composition of dark matter, something that would go beyond the frontier of current knowledge.
So far, nothing has been found about possible dark matter candidates, it’s a big unknown, an as-yet-unsolved mystery that hasn’t been answered for decades. And among physicists there are beginning to be dissenting opinions.
We do not have conclusive results in any of the search strategies used in accelerated or astroparticle and all possibilities are open.
There are scientists who begin to consider that there is no
Last year an article was published in The Astrophysical Journal who proposed defining dark matter as a modification of gravity. This article argued that there really is no such thing as dark matter, but that there are parts of the force of gravity that we don’t quite understand. Its publication generated great commotion and enthusiasm, but responses were immediately published pointing out inconsistencies that the authors of the article did not take into account. So for now, we still think there is dark matter.
There are great expectations for the detection of dark matter in the next few years, although, most likely, the answer does not come from just one of these studies, but from all of them together. The search continues.
Barbara Alvarez GonzalezRamón y Cajal Researcher at the High Energy Experimental Group at the University of Oviedo, ICTEA, Department of Physics, University of Oviedo
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