What Is Dark Matter?


What Is Dark Matter
Dark matter is an invisible type of matter that exerts gravitational effects on light and regular matter.

Dark matter is a hypothesized form of matter that does not interact with light or other forms of electromagnetic radiation, but exerts gravitational effects on visible matter, light, and the structure of the universe. Scientists calculate this elusive form of matter makes up approximately 27% of the universe, outweighing visible matter nearly six to one. Yet, despite its prevalence, it remains one of the least understood phenomena in modern physics due to its ‘invisible’ nature.

Defining Dark Matter

Dark matter is a hypothetical form of matter that does not absorb, reflect, or emit electromagnetic radiation. This makes it incredibly challenging to detect directly with current technology. It’s “dark” not because it’s black or absence of light but because it does not interact with light or any other form of electromagnetic radiation. In essence, it’s transparent and therefore ‘invisible’ to our current methods of observation.

Properties of Dark Matter

While the specific characteristics of dark matter are still under investigation, scientists generally agree it possesses the following properties:

  1. Non-Baryonic: Dark matter is not made of baryons, which are particles like protons and neutrons that comprise ordinary matter.
  2. Non-Luminous: It does not emit, reflect or absorb light, or any other electromagnetic radiation. It is invisible.
  3. Gravity Interacting: Dark matter interacts gravitationally with ordinary matter and light.
  4. Collisionless: Dark matter particles do not interact with each other or other particles via strong or electromagnetic forces, meaning they pass right through each other and through other particles.

Dark Matter vs Ordinary Matter and Antimatter

Ordinary baryonic matter makes up everything we can see: stars, galaxies, planets, and even us. This matter consists of atoms, which are in turn made up of protons, neutrons, and electrons. Ordinary matter interacts with other matter through electromagnetic forces and absorbs, emits, or reflects light. We detect its presence using various technological instruments.

Antimatter, on the other hand, is like a mirror image of ordinary matter. Its particles have properties opposite to their matter counterparts. For instance, a positron is an antimatter particle with the same mass as an electron but with a positive charge. When matter and antimatter meet, they annihilate each other, releasing energy.

In contrast, dark matter doesn’t interact with electromagnetic forces as ordinary matter and antimatter do. It doesn’t emit, absorb, or reflect light, and we can’t directly observe it. However, it does interact gravitationally with other matter.

The Evidence for Dark Matter

Though we can’t directly observe dark matter, we infer its existence through its gravitational effects. Here are the three primary lines of evidence:

  1. Galactic Rotation Curves: According to the laws of physics, stars at the edges of a spinning galaxy should move slower than stars towards the center. However, observations show that stars at the edges move just as quickly, suggesting the presence of unseen mass (i.e., dark matter) influencing their motion.
  2. Gravitational Lensing: When the light from distant galaxies passes closer massive objects, it bends due to gravity. The name for this phenomenon is gravitational lensing. Observations show that light often bends more than expected, suggesting the presence of additional unseen mass.
  3. Cosmic Microwave Background (CMB): The CMB is the afterglow of the Big Bang. Detailed measurements of CMB indicate the existence of dark matter. The distribution of the tiny temperature fluctuations in the CMB suggests a universe composed of roughly 5% ordinary matter, 27% dark matter, and 68% dark energy.

History

The dark matter hypothesis traces its origins to a debate about the age of the Earth. In 1846, British physicist Lord Kelvin used the laws of thermodynamics to estimate Earth’s age. He determined that the Earth was between 20 to 100 million years old. This was significantly younger than the hundreds of millions to billions of years suggested by geologists and evolutionary biologists. To reconcile this discrepancy, Kelvin suggested the presence of “dark bodies” in the universe that affected Earth’s thermal history through their gravitational influence. According to Kelvin, these bodies could be stars that had cooled and dimmed to the point of invisibility.

French physicist Henri Poincaré also considered the presence of dark matter in the universe. In a speech delivered at the Congress of Arts and Science in St. Louis in 1904, he speculated on “dark stars” that were invisible not because of their distance but due to their inherent lack of brightness. These invisible celestial bodies would have a significant gravitational influence on visible matter.

In 1932, Dutch astronomer Jan Oort analyzed the motions of nearby stars in the Milky Way. He found a discrepancy between the mass of the galaxy inferred from the number of stars and the mass calculated by the motion of these stars. He proposed the existence of “dark matter” that we cannot see or detect through traditional methods to account for this discrepancy.

Fritz Zwicky’s research in 1933 solidified the dark matter hypothesis in the scientific community. Zwicky studied the Coma galaxy cluster and found that the galaxies within the cluster move too fast for the cluster’s observed mass and should have flown apart. He reasoned that there must be some missing mass or dark matter holding the cluster together.

In the 1970s, Vera Rubin and Kent Ford observed the rotation curves of galaxies, reinforced the hypothesis of dark matter. They found that galaxies were spinning so fast that they should have torn themselves apart, absent the gravitational pull of unseen matter. The subsequent research and observations in the following decades further established dark matter as a fundamental component of our current cosmological models.

Hypotheses About Dark Matter

There are several competing theories about what dark matter could be:

  1. Weakly Interacting Massive Particles (WIMPs): WIMPs are the most popular candidate. They are hypothetical particles that interact weakly with ordinary matter and are heavy enough to account for the observed effects of dark matter.
  2. Axions: Axions are hypothetical particles that are light, abundant, and interact weakly with other particles, making them potential candidates for dark matter.
  3. Sterile Neutrinos: These are a hypothetical type of neutrino that interacts even less with ordinary matter than regular neutrinos. They could be a potential source of dark matter.
  4. Modified Newtonian Dynamics (MOND): This hypothesis suggests a modification of the laws of gravity at very large scales for explaining the observations without invoking dark matter.
  5. Quantum Gravity & String Theory: Some theorists speculate that a better understanding of quantum gravity or the implementation of string theory would resolve the mystery of dark matter. The gravitino is a proposed particle that mediates supergravity interactions and is a candidate for dark matter.

Dark Matter Detection Experiments

Many experiments worldwide aim to detect and understand dark matter:

  1. Direct Detection Experiments: These experiments, such as the XENON1T and the Large Underground Xenon experiment (LUX), try to detect the rare collisions between dark matter particles and ordinary matter.
  2. Indirect Detection Experiments: These experiments, like the Fermi Gamma-ray Space Telescope, search for the products of dark matter particle annihilations or decays.
  3. Collider Experiments: These experiments, like those conducted at CERN’s Large Hadron Collider (LHC), aim to produce dark matter particles by smashing together ordinary particles at high energies.

While these experiments have yet to definitively detect dark matter, they continue to place constraints on the properties that dark matter particles can have.

References

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  • Bertone, G.; Hooper, D.; Silk, J. (2005). “Particle dark matter: Evidence, candidates and constraints”. Physics Reports. 405 (5–6): 279–390. doi:10.1016/j.physrep.2004.08.031
  • Cho, Adrian (2017). “Is dark matter made of black holes?”. Science. doi:10.1126/science.aal0721
  • Randall, Lisa (2015). Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe. New York: Ecco / Harper Collins Publishers. ISBN 978-0-06-232847-2.
  • Trimble, V. (1987). “Existence and nature of dark matter in the universe”. Annual Review of Astronomy and Astrophysics. 25: 425–472. doi:10.1146/annurev.aa.25.090187.002233