Dark matter: the invisible component of the universe

Jasmir Aguilera

Physicist-Mathematician

Member of the Nicaraguan Association of Amateur Astronomy “Los Cadejos”

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For nearly a century scientists have been on the hunt for elusive dark matter, a mysterious component that seems to shape the universe

It is estimated that dark matter constitutes 23% of the total matter/energy content of the universe. However, its components are not yet known; its nature is an enigma to modern cosmology and finding it is currently one of the greatest challenges of science.

 The ideas about dark matter date back to the 1930s, when Fritz Zwicky proposed it to explain the movements of galaxies. Zwicky’s calculations indicated that the matter or visible composition of galaxies did not exert enough gravitational force to keep these large structures together, and prevent their stars from scattering across space. Galactic structure and dynamics suggested that they must be made up of more matter that we do not see through our telescopes.

Later, in the 1970s, Vera Rubin measured the spectrum of stars and gas surrounding galactic disks, and observed that the stars were not moving at the expected speed. That is, their speeds did not decrease with distance. Since then, it has been proven that the star velocity curve in the different galaxies remains flat to extreme ends of galaxies.

Observed (red) and theoric (blue) star velocity according to the distance to the galaxy center.

Among the scientific community there is a great acceptance of the existence of dark matter, although the only thing that has been measured is its gravitational interaction with ordinary matter. Dark Matter does not present electromagnetic interaction, and therefore, does not emit photons. Similarly, scientists estimate that it does not interact through strong or weak nuclear force.

Despite the numerous detectors that have been located in different laboratories, some of them underground, no dark matter particles have been detected, nor clues about their interactions. Large particle colliders like the LHC at CERN (Switzerland) have also been searching for dark matter, so far without positive results. Other detectors like DAMA/LIBRA (Italy) have presented data that are open to interpretation, and that fit different dark matter models.

 At the moment, the known data are predictions of fluctuations detected in the cosmic microwave background, which reveal the importance that dark matter has played in the formation of the large structures of the universe. For decades, several dark matter candidates have been proposed. Neutrinos, neutrals, WIMP particles, and axions stand out. However, the standard model of particle physics does not contemplate the existence of these hypothetical constituents of dark matter. On the other hand, some experts believe that there is no need to summon strange particles, but that other objects such as black holes, neutron stars, brown dwarfs, and cold gas may remain invisible to our telescopes.

Dark matter distorts the view of distant galaxies, creating an effect called gravitational lensing. Credit: NASA

Currently, different cosmological models establish that galaxies are formed by giant halos of dark matter that far exceed their radius. Some calculations indicate that, in principle, small dark matter halos filled with gas, dust, and stars, and only after the formation of these small dark matter halos, the larger ones emerge. If this scenario turns out to be true, then dwarf galaxies should be older than the largest galaxies.

 Another problem scientists face is that the distribution of visible matter does not match the predictions either. According to the standard model of cosmology, many smaller galaxies should be gravitationally bound to larger galaxies. Looking at this in perspective, our galaxy, the Milky Way, should have several hundred of small galaxies revolving around it, but only about 24 have been seen.

 An alternative approach suggests that the dark matter particles must have been distributed evenly in the early universe. When our cosmos was young enough, dark matter began to cluster in spherical concentrations forming smaller halos that collided with each other to form larger halos.

The Elucid mission will scan the sky for dark matter starting in 2022. Credit: ESA

In the future, the search for dark matter will be carried out in space with missions like Euclid which will be 1.5 million km away from Earth and is expected to be launched in 2022. This mission aims to map and analyze the positions and movements of a large number of galaxies in the infrared spectrum. It is estimated that the mission will reach 35% of the sky dome and that it will obtain numerous images of deep space in high resolution, pointing its instruments to different places, every 80 minutes, for six consecutive years. Many hope that this mission will allow to restrict theoretical models, and perhaps, find the elusive nature of dark matter.

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