Cosmic Dark Ages
The Cosmic Dark Ages refer to the period in the early universe from approximately 380,000 to 150 million years after the Big Bang, during which the universe was filled with neutral hydrogen gas and no stars or galaxies had yet formed to illuminate it. During this era, the universe was opaque to most forms of electromagnetic radiation except for the cosmic microwave background (CMB), which had just been released during recombination. This period ended when the first stars and galaxies formed, producing ultraviolet radiation that began reionizing the neutral hydrogen — an event known as the Epoch of Reionization.
Key Milestones Around the Cosmic Dark Ages
| Event | Time After Big Bang | Redshift (z) | Key Feature |
|---|---|---|---|
| Big Bang | 0 seconds | ∞ | Origin of universe |
| Recombination | ~380,000 years | ~1100 | CMB released, neutral H forms |
| Cosmic Dark Ages begin | ~380,000 years | ~1100 | No luminous sources |
| First stars (Pop III) form | ~100–200 million years | ~20–30 | Dark Ages end begins |
| Reionization complete | ~1 billion years | ~6 | Universe becomes transparent |
| Present day | ~13.8 billion years | 0 | Fully ionized intergalactic medium |
Interactive Tools
Khan Academy: History of the Universe
Introductory lessons on Big Bang cosmology and early universe structure
Open Tool
Wikimedia Commons, CC BY-SA
Related Terms
Epoch of Reionization
The Epoch of Reionization (EoR) is a pivotal phase in cosmic history, spanning roughly 150 million to 1 billion years after the Big Bang (redshifts z ≈ 6–20), during which the first stars and quasars emitted enough ultraviolet radiation to ionize the neutral hydrogen that pervaded the universe after recombination. This process transformed the intergalactic medium from a neutral, opaque state back into the fully ionized, transparent plasma we observe today. Understanding the EoR is one of the central goals of modern observational cosmology, as it marks the emergence of the large-scale structure of the universe and the first generation of galaxies.
Cosmic Microwave Background
The Cosmic Microwave Background (CMB) is the thermal electromagnetic radiation permeating the entire observable universe, representing the afterglow of light released approximately 380,000 years after the Big Bang when the universe cooled enough for protons and electrons to combine into neutral hydrogen atoms. It is observed today as a nearly uniform blackbody radiation at a temperature of approximately 2.725 K, with tiny temperature fluctuations of about 1 part in 100,000 that encode the seeds of large-scale cosmic structure. The CMB is considered one of the strongest pieces of evidence for the Big Bang model and provides precise measurements of cosmological parameters.
Big Bang Theory
The Big Bang Theory is the prevailing cosmological model describing the origin and evolution of the universe, proposing that all matter, energy, space, and time originated from an extremely hot, dense singularity approximately 13.8 billion years ago. The rapid expansion from this primordial state led to cooling, the formation of fundamental particles, and eventually atoms, stars, and galaxies. Evidence supporting this model includes the observed expansion of the universe, the cosmic microwave background radiation, and the abundance of light elements like hydrogen and helium.
The term derives from English "cosmic" (from Greek kosmikos, meaning "of the universe") and "Dark Ages" by analogy with the medieval historical period. The phrase was popularized by cosmologists in the 1990s and early 2000s to describe the era before the first light sources appeared in the universe.