Cosmic Reionization and Intergalactic Medium Dynamics
Summary
The epoch of cosmic reionization marks the transformation of the intergalactic medium (IGM) from a neutral, opaque state into an ionised, transparent one, driven by the emergence of the first stars, galaxies and quasars. Beginning at redshifts above z ∼ 15 and concluding by z ∼ 5, this process imprinted distinctive signatures in the thermal and ionisation structure of the IGM and influenced subsequent galaxy formation through radiative feedback. Reionization proceeded inhomogeneously, with ionised bubbles expanding around early sources and eventually percolating to fill the cosmos. The evolving ultraviolet background governed the balance between ionisation and recombination, while the IGM’s density fluctuations gave rise to the Lyman-α forest observed in quasar spectra. Understanding the timing, topology and drivers of reionization is essential to reconstructing the formation of cosmic structures, interpreting cosmic microwave background measurements and modelling the thermal history of baryons in the Universe.
Research from Nature Portfolio
No recent Nature Portfolio content available.
Cosmic Reionization and Intergalactic Medium Dynamics publication trend
The graph below shows the total number of articles in cosmic reionization and intergalactic medium dynamics across all publications each year (not limited to Nature Index journals).
Technical terms
Cosmic reionization: The epoch during which the hydrogen in the intergalactic medium transitioned from neutral to ionised due to ultraviolet radiation from the first luminous sources.
Intergalactic medium (IGM): The diffuse gas that occupies the space between galaxies, composed primarily of hydrogen and helium.
Lyman-α forest: The series of absorption lines in quasar spectra caused by intervening neutral hydrogen along the line of sight.
Gunn–Peterson trough: A region of near-zero flux in high-redshift spectra indicating a large fraction of neutral hydrogen.
Neutral hydrogen fraction (x_HI): The proportion of hydrogen atoms in the IGM that remain un-ionised.
Mean free path: The average distance an ionising photon travels in the IGM before being absorbed.
Photoionisation rate: The frequency at which hydrogen atoms in the IGM are ionised by incident ultraviolet photons.
References
- Damping wings in the Lyman-α forest: A model-independent measurement of the neutral fraction at 5.4. Astronomy & Astrophysics (2024).
- EIGER. I. A Large Sample of [O iii]-emitting Galaxies at 5.3. The Astrophysical Journal (2023).
- Measuring the photoionization rate, neutral fraction, and mean free path of H i ionizing photons at 4.9 ≤ z ≤ 6.0 from a large sample of XShooter and ESI spectra. Monthly Notices of the Royal Astronomical Society (2023).
About these summaries
This Nature Research Intelligence Topic summary is created with the cited references and a large language model. We take care to ground generated text with facts, and have systems in place to gain human feedback on the overall quality of the process in line with our AI principles. We strive to create accurate and useful summaries for people unfamiliar with the research topic and that supports this goal. These pages are a beta release and will be updated as we learn how best to help people gain value from a research topic summary.
Turn complex research questions into confident strategic decisions
When you're under pressure to set direction, justify investment, or understand your competitive position, you need more than raw data — you need trusted insights you can act on.
Benchmark your performance against global peers using robust, methodologically sound analysis.
Combine quantitative metrics with qualitative expert insight to uncover strengths, gaps and emerging opportunities.
Gain tailored, decision-ready recommendations aligned to your strategic priorities.
Talk to us to learn more about our data dashboards and bespoke strategy reports.
Grow research skills, confidence and careers with training built for every stage of the research lifecycle.
Developed with Nature Portfolio journal Editors and internationally renowned experts. Discover three ways to learn:
Self-paced, online courses in convenient bite-sized units, covering key skills across scientific writing, publishing, grant writing, data analysis, and more.
Expert trainer-led workshops with hands-on exercises and real-time feedback across core research skills, delivered via interactive group sessions.
Editor-led workshops combining core principles in writing and publishing, personalised 1:1 feedback from Nature Portfolio Editors and hands-on exercises.
Explore course catalogues and workshop agendas, enquire about the options or request institutional pricing.