Dark Energy and Cosmic Acceleration

Dark energy and accelerated cosmic expansion

Dark energy is a poorly understood component of the universe proposed to explain the observed acceleration of cosmic expansion. In the standard Lambda-CDM model, it accounts for approximately 68% of the present-day universe's total mass-energy, while dark matter contributes about 27% and ordinary matter about 5%. Although its density is extremely low, dark energy is thought to be nearly uniform throughout space, so its total influence becomes dominant on cosmic scales. Its strong negative pressure can cause the expansion of an already expanding universe to accelerate and also slows the growth of cosmic structures. The strongest initial evidence for dark energy came from observations of distant Type Ia supernovae in 1998, which showed that the universe has expanded more rapidly in its later history than expected from matter's gravitational attraction alone. Independent evidence comes from cosmic microwave background measurements, baryon acoustic oscillations, large-scale galaxy surveys, gravitational lensing, and other probes of expansion history. The leading explanation is a cosmological constant, representing constant vacuum energy, although alternatives include dynamic scalar fields such as quintessence, modified gravity, interacting dark energy, and other proposals. The nature of dark energy remains unresolved, and recent observations have investigated whether its density changes over time.

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Dark energy’s role in cosmic structure formation

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