The period before star formation, when matter began to gravitationally concentrate into denser regions
After the recombination epoch — when the Universe became transparent to radiation and the cosmic microwave background (CMB) appeared — came a long period called In the Dark Ages. At that time, there were no luminous sources (stars or quasars), so the Universe was truly dark. However, even though there was no visible light, important processes were taking place: matter (mainly hydrogen, helium and dark matter) began to to concentrate gravitationally, creating the basis for the first formation of stars, galaxies and large structures.
In this article we will discuss:
- Definition of the Dark Ages
- Cooling of the Universe after Recombination
- Growth of density fluctuations
- The role of dark matter in the formation of structures
- Cosmic dawn: the birth of the first stars
- Monitoring challenges and methods
- Significance for modern cosmology
1. Definition of the Dark Ages
- Time limit: Approximately from 380 thousand years after the Big Bang (the end of recombination) until the formation of the first stars, which began around after 100-200 million years.
- Neutral Universe: After recombination, almost all protons and electrons have combined into neutral atoms (mostly hydrogen).
- No significant light sources: In the absence of stars or quasars, there were no bright sources of radiation, making the Universe almost "invisible" in many ranges of the electromagnetic spectrum.
During the Dark Ages cosmic microwave background photons continued to travel freely and cool as the Universe expanded. However, these photons shifted into the microwave range, providing only a small amount of light at that time.
2. Cooling of the Universe after recombination
2.1 Temperature change
After recombination (when the temperature reached about 3 thousand K) The universe continued to expand, its temperature falling. At the beginning of the Dark Ages, the temperature of background photons reached several tens or hundreds of kelvins. Neutral hydrogen dominated, with helium making up a smaller fraction (~24 % by weight).
2.2 Ionization part
A small fraction of the electrons still remained ionized (about one part in 10 thousand or even less) due to various residual processes and a small amount of hot gas. This small part of the ionization had some influence on energy exchange and chemistry, but overall the Universe was mostly neutral — very different from the previous ionized plasma state.
3. Growth of density fluctuations
3.1 Origins from the early Universe
The small density perturbations seen in the CMF as temperature anisotropies were formed during the early period of quantum fluctuations (for example, during inflation, if that scenario is correct). After recombination, these perturbations represented a small excess or deficiency of matter.
3.2 The Dominance of Matter and Gravitational Collapse
During the Dark Ages, the Universe was already in the realms of matter — dark and baryonic matter, not radiation, played the decisive role here. In areas where the density was slightly higher, gravitational pull gradually accumulated more matter. Over time, these excess centers grew larger, leading to:
- Dark matter halos: Dark matter deposits that formed gravitational wells into which gas could have accumulated.
- Prestellar clouds: Baryonic (ordinary) matter followed the dark matter halos, forming gas clumps.
4.The role of dark matter in forming structures
4.1 Space Network
Simulations of structure formation show that dark matter is crucial in constructing space network — the structure of the filaments. Where the dark matter concentration is highest, baryonic gas also gathers, forming the earliest massive potential “wells.”
4.2 Cold Dark Matter (ΛCDM)
In modern theory ΛCDM Dark matter is thought to have been "cold" (non-relativistic) since early times, and therefore can be efficiently condensed. These dark matter halos grow hierarchically, forming small ones at first, eventually merging into larger ones. By the end of the Dark Ages, many such halos already existed, ready to become the sites where the first stars (Population III stars) would form.
5. Cosmic Dawn: The Birth of the First Stars
5.1 Population III stars
Eventually, in the densest areas, the material collapsed to the first stars — the so-called Population III stars. These stars, composed almost entirely of hydrogen and helium (with no heavier elements), were probably much more massive than modern stars. Their extinction marks the end of the Dark Ages.
5.2 Regionalization
As these stars ignited nuclear reactions, they released abundant ultraviolet rayswho took to rezone surrounding neutral hydrogen. As the stars (and later galaxies) expanded, the reionization zones grew and merged, thus converting the intergalactic medium from a mostly neutral state back to the ionized state of interest. This reionization the era continued at z ~ 6–10 and finally ended the Dark Ages, revealing a new phase of light to the Universe.
6. Monitoring challenges and methods
6.1 Why The Dark Ages is hard to watch
- No bright sources: The fundamental reason why this period is called "dark" is the lack of luminous objects.
- KMF shift: After recombination, the remaining photons cooled and shifted out of the visible region.
6.2 21 cm cosmology
A promising method for studying the Dark Ages is 21 cm hyperfine transition in neutral hydrogen. In the Dark Ages, neutral hydrogen could absorb or emit a 21 cm wave, with the background of the CMB. In essence, by mapping this signal at different cosmic times, one can see the distribution of neutral gas in a "layered" way.
- Challenges: The 21 cm signal is very weak and gets drowned out by strong background sources (e.g. our galaxy).
- Experiments: Projects such as LOFAR, MWA, EDGES and the future Square Kilometre Array (SKA) seeks to detect or refine observations of the 21 cm line from this period.
6.3 Indirect conclusions
Because it is difficult to directly detect electromagnetic radiation from the Dark Ages, scientists make indirect inferences through cosmological simulations and studies the earliest galaxies observed at later times (z ~ 7–10).
7. Significance for modern cosmology
7.1 Testing structure formation models
The transition from the Dark Ages to the cosmic dawn is a great opportunity to examine how matter collapsed to form the first bound objects. Comparing observations (especially the 21 cm signal) with theoretical models can refine our understanding of:
- The nature of dark matter and the properties of its fine-scale aggregates.
- Initial conditions of inflation and their reflections in the KMF data.
7.2 Lessons about cosmic evolution
Cosmologists add to Dark Ages study solid A description of the history of the universe:
- The Hot Big Bang and Inflationary Fluctuations.
- Recombination and separation of KMF.
- The gravitational collapse of the Dark Ages leading to the first stars.
- Reionization and galaxy formation.
- The growth of galaxies and the network of large-scale cosmic structures.
All of these stages are related, and getting to know one better reveals the others more deeply.
Conclusion
Dark Ages – this is a significant period in the evolution of the Universe, when there was no starlight, but there was active gravitational aggregation. It was then that matter began to concentrate into the first bound formations and prepared the ground galaxies and swarm Although this era is difficult to observe directly, it is crucial for understanding how the Universe transitioned from a uniform distribution of matter after recombination to a pronounced structured spacewhich we see now.
Future progress 21 cm in cosmology and highly sensitive radio observation technologies promise to shed light on this little-known "dark" era, showing how primordial hydrogen and helium coalesced to eventually produce the first flashes of light — cosmic dawn, allowing countless stars and galaxies to form.
References and further reading
- Barkana, R., & Loeb, A. (2001). "In the Beginning: The First Sources of Light and the Reionization of the Universe." Physics Reports, 349, 125–238.
- Ciardi, B., & Ferrara, A. (2005). "The First Cosmic Structures and their Effects." Space Science Reviews, 116, 625–705.
- Loeb, A. (2010). How Did the First Stars and Galaxies Form? Princeton University Press.
- Furlanetto, SR, Oh, SP, & Briggs, FH (2006). "Cosmology at Low Frequencies: The 21 cm Transition and the High-Redshift Universe." Physics Reports, 433, 181–301.
- Planck Collaboration. https://www.cosmos.esa.int/web/planck
Based on this research, the Dark Ages are becoming more than just an empty pause, but a very important connection between the thoroughly studied KMF epoch and the bright Universe of stars and galaxies—an epoch whose secrets we are only now beginning to unravel.