Has the time come 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 incomplete understanding of the theory of gravity.
At the origin of the Universe
To date, there is consensus among scientists that the theory that best describes the origin of the Universe is that of the Big Bang. 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 of that has been expanding ever since.
Surprisingly, we only know and understand 5% of the matter that exists in the Universe. Even of this small percentage there are aspects that we have not been able to explain, such as the difference between matter and antimatter.
The physics of elementary particles and the study of the interactions between them try to reveal the state and evolution of the Universe. And among all the unknowns to be resolved, 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 everything made up of leptons (elementary particles) and baryons (made up 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 the moment is attributed to the so-called dark energy.
Vera Rubin’s discovery
Dark matter together with dark energy make up almost 95% of the Universe. We cannot see it, since it does not emit any type of electromagnetic radiation.
Much of the evidence of its existence comes from the study of the movements of galaxies. The analysis of the cosmic microwave background also provides information on the amount of visible and dark matter that exists.
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 rotational speed of stars within spiral galaxies, discovered that these curves remain flat.
Vera Rubin’s finding contradicted the theoretical model that predicted that stars farther from the center of the galaxy would have slower speeds. This fact cannot be explained only with the existence of visible matter and its associated gravitational mass, 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 subsequent decades, more evidence related to dark matter has been collected, to the point that today the vast majority of scientists accept its existence.
First level 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 its composition is unknown.
Scientists have devised different strategies to find these potential dark matter candidate particles. Finding them is one of the biggest challenges in physics today.
There are different search strategies for dark matter, 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 precise instruments.
These experiments are spread all over the world, there is even one on the International Space Station (ISS), and they are part of international collaborations of dozens of scientists.
The ANAIS, 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, for the observation of what happens in nature looking for elementary particles.
In the first case, from direct means, particles that arise from collisions of visible matter particles with dark matter particles are studied, and in the second case, from indirect means, particles from collisions between particles of matter are studied. exclusively dark.
The LHC “makes” dark matter particles
In particle accelerators as energetic as the Large Hadron Collider (LHC) at CERN, the European Organization for Nuclear Research, it is possible to recreate the conditions of seconds after the Big Bang and produce or “manufacture” dark matter particles from collisions of highly energetic protons.
Accelerators are devices that allow the kinetic energy of stable charged particles to be increased.
The LHC is the latest in a chain of accelerators that manages to 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 then be studied.
The ATLAS and CMS experiments are in charge of these searches at CERN’s LHC. These experiments are the same ones that after a long search 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 the Prince of Asturias Award for Scientific and Technical Research.
The difficulty of identifying candidate dark matter particles in these types of experiments is that dark matter interacts very weakly with matter and it is practically impossible in these cases to find its trace.
Digging into unknown processes
Incorporating theories beyond the standard model such as supersymmetry, simplified models with scalar bosons, or models of the hidden sector or dark sector, searches are carried out from their disintegrations 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 exceed the frontier of current knowledge.
To date, nothing has been found about the possible candidates for dark matter, it is a great unknown, a still unsolved mystery that has been unanswered for decades. And among physicists there are beginning to be discordant opinions.
We do not have any conclusive results in any of the search strategies used in accelerated or astroparticles and all possibilities are open.
There are scientists who are beginning to consider that there is no
Last year an article was published in The Astrophysical Journal that proposed to define dark matter as a modification of gravity. This paper proposed that there is really no dark matter, but rather there are parts of the force of gravity that we don’t fully understand. Its publication generated a huge stir 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 coming years, although, most likely, the answer will not come from just one of these studies but from all of them together. The search continues.