Dark matter makes up roughly 27% of the universe and the dark energy content is almost 68%!
Dark mater has always shown its effects in the interstellar medium. Its gravity drives normal matter (gas and dust) to collect ad build up into stars, galaxies, and massive galaxy clusters. How do we know it really exists?
Gravitational lensing đź”
Although astronomers cannot directly observe dark matter, they can detect its presence through its gravitational effects on light from more distant objects, a phenomenon known as gravitational lensing. This occurs when the gravity of massive objects, such as galaxy clusters that contain both normal and dark matter, bends the light from background galaxies. By examining the area around these massive clusters, astronomers can identify distorted images of background galaxies that have been gravitationally lensed. By analysing the degree and nature of these distortions, researchers can reverse-engineer the distribution of dark matter within the cluster. This method not only helps in mapping dark matter but also enhances our understanding of the large-scale structure of the universe and the role dark matter plays in cosmic evolution.
Galaxy rotation curves🌌
Although its nature is not fully understood, dark matter plays a crucial role in our understanding of the universe. It is primarily inferred through indirect observations, such as galactic rotation curves. Observations confirm that the velocity of the Milky Way galaxy's disk remains relatively constant, even at large distances from the centre of our galaxy. This phenomenon has led to the widely accepted postulation that dark matter exists in a massive, roughly spherical halo surrounding each galaxy.
According to the laws of physics, one would expect the orbital speed of stars to decrease as they move farther from the galactic center. However, studies of rotation curves for various galaxies reveal that the orbital speeds of stars remain constant or even increase at greater distances. This discrepancy suggests that there is more mass—and consequently more gravitational force—than can be accounted for by visible matter alone. This additional mass is attributed to the presence of dark matter within the galaxy, including its clusters.
Cosmic Microwave Background Radiation(CMBR)📡
The Cosmic Microwave Background Radiation (CMBR) provides a crucial snapshot of the universe when it was merely a few hundred thousand years old, a time known as the recombination era. At this stage, the universe was a hot, dense plasma composed of protons, electrons, photons, neutrinos, and dark matter. As the universe expanded and cooled, protons and electrons combined to form neutral hydrogen atoms, allowing photons to travel freely for the first time. This decoupling of matter and radiation marked the release of the CMBR, which fills the universe and can be detected in all directions.
The CMBR is remarkably uniform, with a temperature of approximately 2.7 Kelvin. However, tiny fluctuations—on the order of one part in 100,000—have been observed. These fluctuations are critical for understanding the early universe's structure. They represent regions of slightly varying density, where some areas had more matter than others. These denser regions eventually became the seeds for the formation of galaxies and large-scale structures in the universe.
The study of CMBR fluctuations has profound implications for cosmology. The patterns of these fluctuations provide insights into the universe's composition, including the proportions of normal matter, dark matter, and dark energy. The CMBR data has led to the development of the Lambda Cold Dark Matter (ΛCDM) model, which is the standard model of cosmology. This model describes a universe that is flat, expanding, and composed of approximately 68% dark energy, 27% dark matter, and only about 5% ordinary matter.
The CMBR serves as a critical tool for understanding the universe's evolution. By studying the anisotropies in the CMBR, researchers can infer the rate of cosmic expansion, the geometry of the universe, and the formation of large-scale structures. Additionally, the CMBR provides a window into the physics of the early universe, including conditions that may have existed during the inflationary period—a rapid expansion that occurred just after the Big Bang.
Dark matter remains one of the most intriguing and elusive components of our universe, constituting approximately 27% of its total mass-energy content. Through various indirect observational methods, such as gravitational lensing, galaxy rotation curves, and the analysis of Cosmic Microwave Background Radiation (CMBR), scientists have been able to infer the existence and distribution of dark matter. Gravitational lensing reveals how dark matter influences the path of light from distant galaxies, while the study of galaxy rotation curves highlights the presence of unseen mass surrounding galaxies. Furthermore, the CMBR provides a snapshot of the early universe, offering critical insights into the density fluctuations that led to the formation of cosmic structures.
These findings not only reinforce the existence of dark matter but also enhance our understanding of the universe's large-scale structure and evolution. As we continue to explore the cosmos, the study of dark matter will remain a pivotal area of research, driving advancements in our comprehension of fundamental physics and the nature of the universe itself. The ongoing quest to unravel the mysteries of dark matter and its role in cosmic evolution promises to yield profound insights into the fabric of reality, shaping our understanding of the universe for generations to come.