Definitions of Dark Matter and Dark Energy
Dark matter is a hypothetical form of matter that does not interact with light or electromagnetic radiation, making it invisible to telescopes, yet it exerts gravitational influence on visible matter. It is estimated to comprise about 27% of the universe's mass-energy content. Dark energy, on the other hand, is an even more mysterious component, thought to be a property of space itself that causes the universe's expansion to accelerate. It accounts for approximately 68% of the universe, with ordinary matter making up the remaining 5%.
Key Principles and Components
Dark matter's primary role is gravitational; it clumps together to form halos around galaxies, providing the extra gravity needed to hold them together. Its composition remains unknown but is likely non-baryonic particles like WIMPs (Weakly Interacting Massive Particles). Dark energy operates through negative pressure, counteracting gravity on cosmic scales, and is often modeled as the cosmological constant in Einstein's general relativity, driving the universe away from collapse.
Practical Example: Galaxy Rotation Curves
A classic example of dark matter's effect is observed in galaxy rotation curves. Stars at the edges of galaxies like the Milky Way orbit at speeds that should fling them out if only visible matter's gravity were at play. Instead, their stable orbits indicate unseen dark matter providing additional gravitational pull. For dark energy, the example is the redshift of distant supernovae, showing the universe's expansion speeding up rather than slowing down as expected.
Importance in Cosmology
Dark matter and dark energy are crucial because they explain the universe's large-scale structure, from galaxy formation to the cosmic web, and determine its ultimate fate—whether it will expand forever or recollapse. Without them, standard models like the Big Bang theory fail to match observations from cosmic microwave background radiation and large-scale surveys, making their study foundational to modern cosmology and the search for a unified theory of the universe.