Critical density is a term used in astrophysics to describe the minimum density of matter that must be present in a region of space to halt the expansion of the universe. The word is spelled as /ˈkrɪtɪkəl dɛnsɪti/ in IPA phonetic transcription. The "c" in "critical" is pronounced as /k/, while the "ti" at the end is pronounced as /ti/ (as in "city"). The "d" in "density" is pronounced as /d/ and the "e" as /ɛ/, while the stress is on the second syllable.
Critical density refers to the specific density of matter and energy required for the universe to expand indefinitely without collapsing on itself or experiencing rapid expansion. In cosmology, it is a crucial concept related to the ultimate fate and evolution of the universe.
The critical density is determined by the balance between the gravitational attraction due to the total mass and the expansion caused by the cosmological constant or dark energy. To achieve this equilibrium, the critical density must reach a specific value.
If the actual density of the universe is greater than the critical density, it implies that the gravitational forces are stronger than the cosmological expansion effects. Consequently, the universe will experience a contraction and eventually collapse in a "Big Crunch." Conversely, if the density is lower than the critical density, the expansion will accelerate indefinitely, leading to a "Big Freeze" scenario where the universe cools down and expands to a point of low energy and activity.
The equation that relates the critical density to the Hubble constant (which determines the expansion rate of the universe) is known as the Friedmann equation. It serves as a fundamental tool in understanding the dynamics of the universe and its potential outcomes.
The concept of critical density plays a significant role in cosmological models and theories, assisting scientists in unraveling the mysteries of the universe's past, present, and future.
The term "critical density" originates from the field of cosmology, specifically in relation to the study of the universe's expansion and its ultimate fate. Here is the etymology of each component of the term:
1. Critical: The word "critical" comes from the Latin word "criticus", which means "able to judge", "discerning", or "decisive". It was derived from the Greek word "kritikos", which is associated with the ability to make judgments or criticisms. In the context of cosmology, "critical" indicates a precise or decisive state that determines the fate of the universe.
2. Density: The word "density" is derived from the Latin word "densitas", which means "thickness" or "closeness". It pertains to the measure of mass per unit volume, indicating the concentration or compactness of a substance or entity.