Dark matter

What is Dark Matter?

Imagine the universe as a vast ocean, where only 4% of it is visible to us—like fish swimming on the surface. The rest, about 96%, remains hidden beneath the waves, and one of its components is dark matter.

The Invisible Force

Dark matter is an invisible form of matter that does not interact with light or electromagnetic radiation, making it impossible to observe directly. It affects galaxies’ formation, evolution, gravitational lensing, and cosmic microwave background anisotropies. In the standard Lambda-CDM model, dark matter constitutes 85% of total mass.

From Hypothesis to Evidence

Dark matter was first hypothesized by William Thomson (Lord Kelvin) in 1884 and later discussed by Poincaré, Kapteyn, Lundmark, and Oort in the early 20th century. But it wasn’t until Fritz Zwicky studied galaxy clusters that dark matter truly began to take shape.

Observational Evidence

The astrophysics community accepts dark matter’s existence, but some argue for modifications to general relativity to explain specific observations. So far, none of these modified gravity theories can describe all observational evidence at the same time. The term ‘dark matter’ refers to all components that obey ρ ∝ a−3 but are not visible, including non-baryonic dark matter.

Understanding Dark Matter’s Role

Dark matter is crucial for understanding how galaxies form and evolve. It affects the rotation curves of galaxies, causing them to remain flat or increase with distance from the center. This phenomenon cannot be explained by visible matter alone.

The Cosmic Web

After the Big Bang, dark matter clumped into blobs along filaments, forming a cosmic web at scales where galaxies appear as tiny particles. The Bullet Cluster collision demonstrates that dark matter can separate from visible gas, producing a distinct lensing peak. Type Ia supernovae provide standard candles for measuring extragalactic distances, indicating the universe is expanding at an accelerating rate.

Gravitational Lensing and CMB

Gravitational lensing is a consequence of general relativity, causing light to bend around massive objects. Strong lensing distorts background galaxies into arcs, while weak lensing causes minute distortions. Measuring distortion geometry allows for mass determination of intervening clusters.

The Cosmic Microwave Background (CMB)

The CMB contains temperature anisotropies that depend on cosmological parameters. The CMB’s angular power spectrum reveals acoustic peaks with different heights and locations, which match theory to constrain cosmological parameters. Baryon acoustic oscillations (BAO) are fluctuations in the density of visible baryonic matter on large scales and can be observed in the cosmic microwave background angular power spectrum.

Classifying Dark Matter

Dark matter is classified as ‘cold,’ ‘warm,’ or ‘hot’ according to its velocity. Most dark matter is thought to be cold, implying galaxies formed first, followed by clusters and superclusters. The identity of dark matter remains unknown, but many hypotheses exist.

Baryonic vs Non-Baryonic Matter

Dark matter can refer to any substance interacting predominantly via gravity with visible matter. Baryonic matter includes protons, neutrons, atomic nuclei, dust, gas, stars, and MACHOs (Massive Compact Halo Objects). Non-baryonic matter candidates include new particles like WIMPs (Weakly Interacting Massive Particles), axions, sterile neutrinos, primordial black holes, and modified gravity theories.

Experimental Searches

Experiments aim to detect dark matter particles by observing their scattering off atomic nuclei (direct detection) or looking for products of annihilations or decays in outer space (indirect detection). Direct detection experiments such as AMANDA, IceCube, and ANTARES search for high-energy neutrinos produced by dark matter accumulation at the center of celestial bodies.

Alternative Theories

Some alternative theories have emerged to explain the observational evidence for dark matter without introducing a new unknown type of matter. A suitable modification to general relativity can conceivably eliminate the need for dark matter. The best-known theories of this class are MOND and its relativistic generalization tensor–vector–scalar gravity (TeVeS), f(R) gravity, negative mass, dark fluid, and entropic gravity.

Conclusion

The search for dark matter continues to be one of the most intriguing puzzles in modern astrophysics. While we have made significant strides in understanding its role in the universe, the identity of this mysterious substance remains a mystery. As we delve deeper into the cosmos, who knows what secrets it holds? The quest for dark matter is not just about finding an answer but also about expanding our knowledge of the universe’s most fundamental aspects.