Dark Matter | Science for ACT PDF Download

Understanding Dark Matter and Its Invisible Nature

• Dark matter, by its very essence, remains invisible to traditional detection methods as it emits neither light nor energy. This invisibility stems from its enigmatic composition, which scientists continue to explore.
• Ordinary matter, also known as baryonic matter, comprises familiar subatomic particles like protons, neutrons, and electrons. However, dark matter's makeup remains speculative, with theories ranging from baryonic to non-baryonic compositions.
• Most scientists lean towards the belief that dark matter consists of non-baryonic matter, with WIMPs (weakly interacting massive particles) emerging as a leading candidate. These particles, hypothesized to possess ten to a hundred times the mass of a proton, interact weakly with conventional matter, rendering them challenging to detect. Neutralinos, theorized as massive particles heavier and slower than neutrinos, are prominent among these candidates, though they have yet to be observed.
• Sterile neutrinos offer another possibility. Unlike conventional neutrinos, which barely interact with ordinary matter, sterile neutrinos interact solely through gravity, presenting a viable dark matter candidate.
• The quest for dark matter also considers smaller entities like neutral axions and uncharged photinos, both theoretical particles with potential implications for the dark matter puzzle.
• It's crucial to distinguish dark matter from antimatter, which consists of particles possessing opposite electrical charges to their ordinary matter counterparts. Antimatter's interaction with matter results in annihilation, highlighting its scarcity in the observable universe. Unlike dark matter, antimatter can be produced in laboratory settings.
• As scientists delve deeper into the mysteries of the cosmos, the search for dark matter persists, with each revelation bringing us closer to unraveling one of the universe's most elusive enigmas.

Does Dark Matter Exist?

The existence of dark matter remains a profound mystery in cosmology. When observing typical galaxies and applying Newton's Laws of Gravity or Einstein's General Relativity to describe their motions, discrepancies emerge. Objects within galaxies, as well as galaxies within clusters, appear to move faster than expected based on visible matter alone, suggesting the presence of unseen mass.

Two potential explanations arise:

• Dark matter: This theory posits the existence of additional matter that eludes detection by conventional telescopes, hence the term "dark." It accounts for the observed gravitational effects on galactic scales and within clusters.
• Modified gravity: Alternatively, some propose that Newton's laws or General Relativity might require modification on cosmic scales. This notion, known as Modified Newtonian Dynamics (MOND) or modified gravity, aims to address the observed discrepancies without invoking unseen matter.

Currently, the prevailing view among cosmologists favors dark matter, as attempts to formulate successful theories of modified gravity have proven challenging. Moreover, observations of the cosmic microwave background (CMB), the relic radiation from the early universe, corroborate the need for dark matter to explain the evolution of cosmic structures. However, the exact nature of dark matter remains elusive. Despite its invisible nature, if dark matter exists, it must possess mass to exert gravitational influence, providing a compelling clue for its detection.

Searching for Dark Matter

• Given the uncertainty surrounding its composition, the search for dark matter encompasses diverse approaches tailored to various hypothetical candidates. Researchers employ massive underground detectors to shield from environmental particles and detect potential dark matter interactions.
• Some researchers, like myself, investigate a specific form of massive dark matter with visible effects, targeting its interactions with ordinary matter. For instance, if such dark matter were to collide with rocks, it would leave discernible scars, which scientists could detect, perhaps even in everyday materials like granite countertops.
• In conclusion, while the quest for dark matter persists, its elusive nature challenges scientists to explore novel detection strategies and probe the mysteries of the universe's hidden realm.

Why Do We Think Dark Matter Exists?

• The existence of dark matter, although elusive to direct observation, is inferred through its gravitational effects on cosmic structures. Since the 1920s, astronomers have postulated the presence of unseen matter to account for gravitational forces surpassing what visible matter alone could explain.
• Observations of celestial motions provide compelling evidence for dark matter. By studying the velocities of stars within galaxies, astronomers discerned that galaxies contain more mass than is apparent from visible matter alone. For instance, in spiral galaxies, stars at the center move at similar speeds to those at the outer edges, suggesting the presence of additional unseen mass.
• Similar conclusions arose from studies of gas within elliptical galaxies and the dynamics of galaxy clusters. Without the gravitational influence of unseen mass, galaxies and galaxy clusters would behave differently, leading to discrepancies between observations and theoretical predictions.
• Remarkably, some galaxies appear to be predominantly composed of dark matter, such as Dragonfly 44 discovered by a team led by Pieter van Dokkum in 2016. Conversely, recent observations have identified galaxies seemingly devoid of dark matter, adding complexity to the understanding of cosmic structures.
• Additionally, gravitational lensing, a phenomenon predicted by Albert Einstein's theory of general relativity, provides further evidence for dark matter. The bending of light by massive objects, such as galaxy clusters, enables astronomers to map the distribution of dark matter in the universe.
• Despite the compelling evidence supporting dark matter's existence, its direct detection remains elusive due to its feeble interactions with ordinary matter. Nevertheless, ongoing efforts to develop increasingly sensitive detectors aim to capture elusive dark matter particles and unravel their properties.
• While dark matter remains a cornerstone of modern cosmology, there persists the possibility that alternative explanations or modifications to the laws of gravity could provide alternative explanations for observed phenomena within the cosmos.

Where Does Dark Matter Come From?

• Dark matter, although invisible, manifests its presence through its gravitational effects on cosmic structures, forming a widespread network across the cosmos. Galaxy clusters emerge at the intersections of these cosmic fibers, reinforcing the evidence for dark matter's existence. Concurrently, scientists validate the consistent operation of gravity both within and beyond our solar system, further corroborating dark matter's role. Additionally, dark energy, an invisible force fueling the universe's expansion, acts in opposition to gravity, adding complexity to cosmic dynamics.
• However, the origins of dark matter remain speculative. A study published in The Astrophysical Journal in December 2021 posits that dark matter could be concentrated within black holes, celestial entities of extreme gravitational influence that engulf surrounding matter. This suggests that dark matter, like other elements of the universe, may have originated during the Big Bang.
• Furthermore, remnants of stellar evolution, such as white dwarfs, neutron stars, and brown dwarfs, are hypothesized to harbor significant quantities of dark matter. Brown dwarfs, failed stars lacking sufficient mass to initiate nuclear fusion, are particularly considered as potential reservoirs of dark matter.

How Do Scientists Study Dark Matter?

• Despite its elusive nature, dark matter is subject to investigation through both astronomical and particle physics approaches. Astronomers analyze the distribution of dark matter in the universe by scrutinizing material clustering and celestial object motions. Conversely, particle physicists endeavor to detect the fundamental particles constituting dark matter.
• In space, the Alpha Magnetic Spectrometer (AMS) aboard the International Space Station tracks antimatter in cosmic rays, offering insights into particle composition traversing the cosmos. Similarly, experiments like XENON1T and the Large Underground Xenon dark-matter experiment (LUX) aim to capture interactions between weakly interacting massive particles (WIMPs) and xenon atoms deep underground.
• Meanwhile, the IceCube Neutrino Observatory in Antarctica searches for sterile neutrinos, hypothesized dark matter candidates that interact solely through gravity. Particle colliders at the European Organization for Nuclear Research (CERN) also contribute to dark matter exploration.
• Space-based telescopes, including Planck and Fermi, probe the cosmos for dark matter's effects. Planck's observations of the cosmic microwave background unveil clues about dark matter distribution, while Fermi's gamma-ray maps reveal excess emissions potentially linked to dark matter annihilations.
• Launching in 2021, the James Webb Space Telescope promises groundbreaking insights into dark matter by tracing galaxy evolution since the universe's infancy. Additionally, ESA's Euclid mission, launched in 2023, aims to map cosmic matter geometry, shedding light on dark matter's elusive nature.