When the Universe Was Blind: The Long Night Before the First Stars

What are the Dark Ages of the Universe?

A period of cosmic darkness

Following the Big Bang (cosmological model describing the origin of the Universe from an extremely hot and dense state), the Universe went through an extraordinary period known as the Dark Ages. This era extends approximately between 380,000 years and 400 million years after the Big Bang, i.e., between about 13.8 billion and 13.4 billion years before today. During this long cosmic night, no stars, no galaxies, no source of visible light yet existed. The Universe was plunged into near-total darkness, populated only by a vast fog of neutral hydrogen and helium.

A challenge for modern cosmology

The term "dark" refers not only to the absence of visible light: it also reflects the opacity of this era to our observing instruments. Understanding what happened during these 400 million years constitutes one of the major challenges of modern cosmology. What witnesses did this period leave behind? How do astrophysicists manage to probe this primordial darkness?

The context: from recombination to stellar silence

Recombination: a cosmic turning point

About 380,000 years after the Big Bang, a founding event occurred: recombination (process by which protons and electrons combined to form the first neutral hydrogen atoms). Until then, the Universe was so hot and dense that matter existed in the form of plasma: electrons and protons moved freely, making space opaque to photons. When the temperature dropped below 3,000 kelvins, electrons were captured by protons to form neutral hydrogen atoms. The Universe suddenly became transparent, and photons could propagate freely through space.

The cosmic microwave background: a fossil memory

This radiation released during recombination is today detectable as the cosmic microwave background (residual electromagnetic radiation from the Big Bang, observable in microwaves at a temperature of 2.725 K). Its mapping by the COBE (1992), WMAP (2001-2010) and especially Planck (2009-2013) satellites has made it possible to reconstruct with remarkable precision the state of the Universe at this primordial epoch. But after this initial moment of clarity, the Universe sank into silence and darkness.

A starless Universe: the physics of the Dark Ages

The slow growth of structures

After recombination, baryonic matter (neutral hydrogen and helium) began to gather under the effect of gravity in the tiny overdensities revealed by the CMB. These density fluctuations, of the order of \(10^{-5}\) relative to the average density, constituted the seeds of future cosmic structures: filaments, clusters and galaxies.

The wait for nuclear fusion

However, this process of gravitational condensation was extremely slow. It took several hundred million years for these gas clouds to reach sufficient density and temperature to initiate nuclear fusion (nuclear reaction by which light nuclei fuse to form heavier nuclei, releasing energy) and give birth to the first stars. During all this time, the Universe remained plunged in darkness.

The key parameters of the period

The physics of this period is governed by a few key parameters. The temperature of the cosmic gas decreased according to the relation \(T \propto (1+z)\), where \(z\) denotes the cosmological redshift (redshift of a light spectrum due to the expansion of the Universe, used as a measure of distance and cosmic time). At the same time, dark matter (matter that emits nor absorbs light but whose presence is detected through its gravitational effects) played an essential structuring role: its gravitational halos provided the potential wells into which baryonic matter would collapse to form the first structures.

The first stars: the end of darkness

Population III stars

The very first stars, called Population III stars, formed as early as 100 to 200 million years after the Big Bang, marking the beginning of the end of the Dark Ages. Their ultraviolet radiation gradually began to ionize the surrounding neutral hydrogen, initiating the reionization that extended until about 1 billion years after the Big Bang. These primordial stars were probably very massive, of the order of 100 to 1,000 solar masses, because the absence of metals (elements heavier than helium) in the primordial gas prevented the efficient cooling necessary for the formation of low-mass stars.

Theoretical models of the first stellar generation

The theoretical work of Volker Bromm (born 1972) and Richard Larson (1937-2024) greatly contributed to modeling the formation of these first stars in dark matter halos. These stars emitted intense ultraviolet radiation capable of ionizing the surrounding neutral hydrogen, triggering a fundamental process: reionization (cosmological process by which radiation from the first stars and galaxies reionized the neutral hydrogen of the Universe).

Reionization: the official end of the Dark Ages

Reionization officially constitutes the end of the Dark Ages. It occurred progressively between about 150 million years and 1 billion years after the Big Bang. Bubbles of ionized hydrogen around the first light sources grew and merged until the entire Universe was reionized, becoming transparent again to ultraviolet photons.

Witnesses of the invisible Universe

The 21 cm signal from neutral hydrogen

How do we probe an epoch that is by definition dark? Astrophysicists have several indirect tracers that constitute as many windows onto the Dark Ages.

The first and most direct is the 21 cm signal (radio signal at 21 cm emitted by neutral hydrogen during the spin-flip of its electron) from neutral hydrogen. When the electron of a hydrogen atom flips its spin relative to the proton, it emits a photon with a wavelength of 21 centimeters. This signal, redshifted by cosmic expansion, could in principle reveal the distribution of neutral hydrogen during the Dark Ages. Instruments such as the HERA (Hydrogen Epoch of Reionization Array) radio telescope and the future SKA (Square Kilometre Array) are specifically designed to detect this signal.

Gamma-ray bursts: cosmic lighthouses

The second window is provided by distant gamma-ray bursts (extremely intense and brief light bursts from distant galaxies, associated with the collapse of massive stars) (GRBs, Gamma-Ray Bursts). Some of these events, detected at redshifts greater than 6, make it possible to probe the composition and ionization state of the intergalactic gas through which their light passes. They act as cosmic lighthouses briefly illuminating the Dark Ages.

The James Webb Telescope: a direct window

Finally, the James Webb space telescope, operational since 2022, has opened a third path by directly detecting some of the oldest galaxies ever observed, some formed less than 300 million years after the Big Bang. These direct observations make it possible to constrain models of the formation of the first structures and reionization with unprecedented precision.

Chronology of the Dark Ages and reionization

This table summarizes the major stages of cosmic evolution from recombination to the end of reionization, with associated redshifts, key events, observable witnesses, and instruments used to probe this period.

N.B.:
The redshift values indicated are estimates from standard cosmological models (\(\Lambda\)CDM). Uncertainties remain significant for the first few hundred million years, and direct observations at these extreme redshifts remain at the frontier of current instrumental capabilities. The correspondence between age and redshift depends on the adopted cosmological parameters, notably the Hubble constant \(H_0\). Around z ~ 100, the fossil radiation, shifting towards the infrared and then microwaves, leaves the visible domain: the Universe then enters a total darkness that will last until the ignition of the first stars.

Dark matter: invisible architect of the Dark Ages

A period of silent organization

Although the Dark Ages are by definition devoid of visible light, they were not a period of cosmic inactivity. Dark matter, which represents about 27% of the energy density of the Universe, continued to silently organize the cosmic structure.

Hierarchical growth of halos

According to the hierarchical model of structure formation, small dark matter halos formed first, then gradually merged to form increasingly massive structures. This process known as hierarchical growth (formation of large cosmic structures by accretion and successive merging of dark matter halos, from smaller to larger) was entirely driven by gravity, without any light emission.

Cosmological simulations

Large-scale cosmological simulations, such as the Millennium Simulation led by Volker Springel (born 1970) and his collaborators, or the IllustrisTNG project, show that the filaments and nodes of the cosmic web (network of dark matter and gas filaments connecting galaxy clusters on the scale of the observable Universe) were already being organized during the Dark Ages. These structures form the invisible skeleton on which, much later, the vast network of galaxies we observe today would be built.

The 21 cm signal: the next observational frontier

The observational challenge of the 21 cm signal

The direct detection of the 21 cm signal from neutral hydrogen represents today the most ambitious observational challenge in cosmology. In 2018, the EDGES collaboration (Experiment to Detect the Global Epoch of Reionization Signature) announced the detection of a 21 cm signal with an amplitude twice as large as theoretical predictions, centered around 78 MHz, corresponding to a redshift \(z \approx 17\), i.e., about 180 million years after the Big Bang.

The EDGES controversy

This anomaly sparked lively controversy within the scientific community. Some theorists, including Rennan Barkana (born 1972), proposed that this excess could betray an interaction between dark matter and baryons, which would constitute a first direct observational signature of dark matter. Other researchers attributed the anomaly to instrumental effects or poorly subtracted galactic foregrounds. The question remains open and is the subject of intense experimental and theoretical work.

The future Square Kilometre Array

Whatever the outcome of this debate, the three-dimensional mapping of neutral hydrogen during the Dark Ages represents a major scientific objective for the coming decades. The future Square Kilometre Array (SKA), whose first scientific observations are expected around 2030, is designed to meet this challenge.

The Dark Ages in the context of cosmic history

A temporal perspective

The Dark Ages are part of a cosmic chronology that allows us to measure their place in the history of the Universe. If we compress the 13.8 billion years of the Universe into a single calendar year, the Dark Ages extend from January 1st to the first days of February. The Sun and the solar system do not appear until September, and humanity emerges in the last seconds of December 31st. This perspective underscores how fundamental the Dark Ages are as a chapter of cosmic history.

The Gunn-Peterson effect: an observational confirmation

James Gunn (born 1938) and Bruce Peterson (born 1942) predicted as early as 1965 that the spectrum of distant quasars should show complete absorption in the ultraviolet if intergalactic hydrogen was neutral: this is called the Gunn-Peterson effect. Its detection by Robert Becker and his collaborators in the Sloan Digital Sky Survey data in 2001, on quasars at \(z \approx 6.3\), brought the direct observational confirmation that reionization was completed at that time.

Towards a cosmology of the Dark Ages

The fundamental questions

Understanding the Dark Ages is inseparable from understanding the nature of dark matter and dark energy, the two dominant components of the Universe. How did dark matter halos form? What is the minimum mass of the first stars? What was the precise chronology of reionization? These questions remain partially open and constitute active research areas.

The surprises of the James Webb Telescope

Far from closing the debate, the James Webb Space Telescope has reopened the question of the Dark Ages: by detecting galaxies too bright, too early, it suggests that the cosmic dawn was earlier and more tumultuous than anything our models had anticipated.