The dark ages of the universe

Astronomers are trying to fill the blank pages in the universe's childhood photo album 

by Avraham Leib, appeared in the Israeli edition of Scientific American magazine February-March 2007

Introduction to Israeli readers: For the first time in human history, scientists are studying the story of Genesis through telescopes. The article I wrote describes the research front of the young universe which is centered on the verse "And let there be light". As always, science does more than it answers - it opens a window to more questions, which maybe one of my young readers will solve in the future. As a young child in Moshav Beit Hanan, I loved to ride a tractor to the fields and debate philosophical questions about the universe there. I still haven't been able to solve most of these questions.

When I look up at the night sky, I often wonder if we humans are not too self-absorbed. Isn't there so much in the universe beyond what we can see on Earth. I've been lucky, and as an astrophysicist I get paid to think about this subject, an occupation that gives me a different perspective on things. If not, there would surely be other issues that would bother me - my death, for example. Every person will die one day, but when I look at the entire universe, I am filled with a sense of longevity. Thanks to the big picture, I find myself less troubled by my private existence than I might be under other circumstances.

Cosmologists deal with some fundamental questions that humans have tried to solve for centuries through philosophical thinking, but we do so by relying on systematic observations and quantitative methodology. Perhaps the greatest achievement of the last century was the construction of a model of the universe, based on an extensive body of knowledge. When I look through the newspaper, as part of my morning routine, I often come across long descriptions of conflicts between people over borders, property or freedoms. The news of the day is often forgotten a few days later, but what do you usually find in the first chapter of ancient writings, which focus the interest of wide audiences over long periods of time, such as the Bible? A discussion on the way the elements of the universe were formed - light, stars, life. Although humans are often preoccupied with day-to-day problems, they are curious about the big picture. As citizens of the universe, it is only natural for us to wonder how the first sources of light were created, how life was formed and whether there are no intelligent beings apart from us in this great space. Astronomers in the 21st century are in a unique position that allows them to find answers to these big questions.

What makes modern cosmology an experimental science is our ability to peer into the past, literally. When you look at your image reflected in a mirror placed a meter away from you, you see yourself as you looked 6 nanoseconds ago - the time when the light traveled to the mirror and back. Similarly, cosmologists do not need to guess how the universe evolved; We are able to watch its history through telescopes. Since statistically the universe looks the same in all directions, what we see billions of light years away is probably a pretty good representation of the state of our segment of the universe billions of years ago.

The ultimate goal of observational cosmology is to describe the entire history of the universe, providing a complete picture of our evolution since it was all a formless gas of subatomic particles. We have a momentary picture of the universe as it was 400,000 years after the big bang - from the cosmic background radiation - and also pictures of individual galaxies, about a billion years later. By the middle of the next decade, NASA intends to launch a new space telescope called the "James Webb Space Telescope" (JWST), which will be able to observe the first galaxies, which were formed, according to theorists, at a cosmic age of several hundred million years.

But all this still leaves us with a huge gap. In the period between the release of the cosmic background radiation, and the first rays of light from the stars, darkness reigned in the universe, and the cosmic radiation did not follow the distribution of matter. This period of time may sound bleak, a kind of boring interlude between the immediate results of the Big Bang, and the busy cosmos of today. But in fact, in those days, significant events took place: the primordial mixture developed into the rich variety of celestial bodies that we see today. In the thick darkness, the forces of gravity engaged in the assembly of bodies in the cosmos. 

The astronomers are in a situation similar to flipping through a photo album of a person that contains his first ultrasound image as an unborn fetus, and several photos of him as a teenager and as an adult. If you tried to guess from these pictures what happened between the two periods, you might have been very wrong. A child is not a fetus that has undergone enlargement, or an adult who has undergone reduction, and the same is true for galaxies. They did not follow a self-evident path of development from the same primary material that was recorded in the cosmic background radiation. Observations suggest that the universe underwent a certain deformation process during that dark age.

Astronomers today are looking for those missing pages in the cosmic photo album, which will show how the universe developed during its infancy, and how the building blocks of galaxies like our Milky Way were formed. Ten years ago, when I joined this effort, only a handful of researchers showed interest in the subject. Today, a considerable part of the future observational projects deal with this, and the field seems to be one of the most exciting fields of action in cosmology in the coming decade.
Ions to Ions

According to the big bang theory, the early universe was full of hot plasma - a cauldron of protons, electrons, photons, and a handful of other particles. The freely moving electrons reacted with the photons in a process called Thomson scattering, thus creating a strong coupling between matter and radiation. As it expanded and expanded, the universe cooled, and when the temperatures dropped below 3,000 degrees Kelvin, the protons bonded to the electrons and formed electrically neutral hydrogen atoms. The Thomson scattering process ended, and the reaction between photons and matter weakened greatly and became the cosmic background radiation. The expansion of the universe continued to cool the gas, so we could expect the cosmic gas to be cold and neutral even today.

But surprisingly the situation is different. Although the world around us is made of atoms, most of the ordinary matter in the universe today is in the form of plasma, found deep in intergalactic space. Observations of the light spectrum of quasars, galaxies and the most distant gamma radiation flashes known to us (and therefore also the oldest) indicate that the same diffuse cosmic hydrogen was all ionized when the universe was a billion years old. A hint of what happened came three years ago, when the Wilkinson Cosmic Background Radiation Inhomogeneities (WMAP) spacecraft found a slight polarization in the cosmic background radiation. Neutral hydrogen cannot polarize this radiation, only ionized hydrogen can. The degree of polarization indicates that the gas was already ionized several hundred million years after the Big Bang. That is, the atoms must have broken down back into their components, the protons and electrons, when the Dark Age came to an end.

Most researchers associate this process of reionization with the first creation of stars. Ionization of a hydrogen atom requires an energy of 13.6 electron volts, the amount carried by a photon in the ultraviolet range. This is not a lot of energy - it is equivalent to about 109 joules per kg of hydrogen, much less than the amount of energy of 1015 joules released in nuclear fusion of the same amount of hydrogen. If only one millionth of the amount of gas in the universe were to undergo fusion within stars, enough energy would be released to ionize all the rest of the gas. Other researchers speculate that matter sucked into black holes released the energy required for ionization. Falling into a black hole releases up to 1016 joules per kg, so only one ten-millionth of the cosmic hydrogen would have to fall into black holes to ionize everything else.

Stars and black holes form inside galaxies, so before reionization could occur, galaxies had to form. Although galaxies are commonly thought of as constellations, cosmologists define them as large clusters of matter. Usually, stars in galaxies formed only at a relatively late stage. And in fact, most of the matter in galaxies is dark matter - a type of matter that we have not yet identified, and by definition it is invisible. Some believe that galaxies were formed when a region of the universe, which was denser than average, became even more crowded due to its own gravity. Although the region initially expanded, like the rest of the universe, the excess gravity slowed its rate of expansion, reversed its direction, and caused the region to collapse and form a bound celestial body - a galaxy.

According to the models available today, dwarf galaxies began to form when the universe was 100 million years old. Over time, they merged together and formed larger and larger galaxies. A modern galaxy, like the Milky Way, was formed after about a million such mergers. Inside the embryonic galaxies, the gas cooled and clustered until stars were formed. The stars' ultraviolet radiation leaked into intergalactic space, stripping electrons from their atoms and creating an expanding bubble of ionized gas. More and more such bubbles appeared with the formation of new galaxies, and the intergalactic space began to look like Swiss cheese. The bubbles began to overlap each other and finally filled the entire space.

This sequence of events does sound possible, but until now it only existed in the minds of theorists. Practical cosmologists would be interested in finding direct proof of the reionization period before adding the missing chapter to the textbooks. Moreover, only observations can determine whether stars or black holes dominated the reionization era and what the properties of dark matter were. But how are such observations even possible if, at least at the beginning, the Dark Age was dark?
see in the dark

Fortunately, even cold hydrogen is able to emit some kind of light. Subatomic particles have an internal property, called spin, which can point in one of two directions and which scientists agree to call "up" and "down". The electron and proton in a hydrogen atom can point in the same direction (parallel state) or in opposite directions (inverted state). The energy of an atom in the inverted state is lower. If, for example, initially both the electron and the proton point up, then the electron turns around and points down, the entire atom drops to a lower energy level, releasing the energy difference in a photon whose wavelength is 21 centimeters. And in the reverse process, if the atom absorbs a photon of such a wavelength, the downward-facing electron will reverse its direction.

The energy of a photon with a wavelength of 21 centimeters is much lower than that of photons characteristic of the hopping of an electron between the orbitals in a hydrogen atom. Therefore, the spin reversal process could have occurred even when the stars had not yet radiated. The energy of the cosmic background radiation and the collisions between the atoms was enough to reverse the spin of electrons and cause the hydrogen to emit weak radiation. The ratio between the number of atoms in a parallel state, to the number of atoms in an inverted state defines the "spin temperature" of the gas. A high spin temperature, for example, indicates a high proportion of atoms in a parallel state.

Therefore, according to the theory, the dark age is defined by three types of temperatures: the spin temperature (a measure of the proportion of atoms in different spin states), the normal kinetic temperature (a measure of the movement of the atoms) and the radiation temperature (a measure of the energy of the background photons). These three temperatures could have been different from each other due to the different physical processes that occurred.

The three temperatures created a strange triangle of relationships: first the spin temperature was similar to the kinetic temperature, then to the radiation temperature, and finally to the kinetic temperature again (see the extension in the text box). As space expanded, the gas and radiation cooled. If the gas had been insulated, it would have cooled faster, but due to the small number of free electrons left after the hydrogen was formed, the cooling would have been slower. The free electrons were used as mediator. They transferred energy from the cosmic background radiation to the atoms, thus maintaining equality between the three temperatures. Only ten million years after the big bang, the free electrons stopped acting as mediators, due to the dilution of the background radiation. The equilibrium between the gas and radiation was broken, and the gas began to cool rapidly. Atomic collisions compare the spin temperature with the kinetic temperature. At this point the hydrogen has only swallowed photons with a wavelength of 21 centimeters, absorbing the energy of the cosmic background radiation (though never enough to return the system to equilibrium).

One hundred million years after the big bang another change took place. Cosmic expansion thinned the gas until the collisions occurred too infrequently for the spin temperature and kinetic temperature to compare. The spins began to gain energy from the cosmic radiation, and as the spin temperature returned to equilibrium with the radiation temperature, the hydrogen both absorbed and emitted photons with a wavelength of 21 centimeters. During this period, the gas could not be seen against the background of cosmic radiation.

When the first stars and black holes appeared, the third change took place. The x-rays they emitted raised the kinetic temperature. Their ultraviolet light was absorbed and re-emitted by the hydrogen, and the repeated transitions of electrons between the atomic orbitals brought the spin temperature and kinetic temperature into equilibrium. The spin temperature rose above the temperature of the cosmic background radiation, and the hydrogen radiated more intensely than the background. Since flipping the spin of electrons requires much less energy than ionizing atoms, the galaxies caused the hydrogen to spin long before they caused it to re-ionize. Finally, as the hydrogen became ionized, it emitted radiation in other ways, and the intergalactic emission at a wavelength of 21 cm faded.
Ancestral tomography

Due to the same triangle of relations, the sky will be illuminated by a radiation of 21 centimeters or darker than the background radiation, depending on the time and place. Another phenomenon that observers should take into account is the diversion of photons to longer wavelengths, brought about by the cosmic expansion. Since the beginning of the dark age, the universe has increased 1,000 times in diameter, so a photon of 21 centimeters emitted at the same time, will reach the earth with a wavelength of 210 meters. A photon emitted near the end of the dark age is deflected along a wavelength of one to two meters.

This is a range of wavelengths found in the radio wave field of the electromagnetic spectrum. The emission can be picked up by an array of low-frequency antennas, such as those used for radio and television broadcasts. Several research groups are currently engaged in building such arrays. The "Milora" field array (MWA) in Western Australia will include 8,000 antennas spread over an area one and a half kilometers in diameter and sensitive to wavelengths of one meter to 3.7 meters. The array has an angular separation capability of a few minutes of arc, corresponding to a physical scale of about three million light years during the Dark Age. Other initiatives in planning are the Low Frequency Array (LOFAR), the Early Structure Telescope (PaST) and, in the more distant future, the Square Kilometer Array (SKA).

These arrays will scan frequencies to map the 21 cm radiation at different times in cosmic history. The astronomers will be able to compile a three-dimensional map of the distribution of neutral hydrogen. They will be able to observe how slight density differences in the universe at a rate of one part in 100,000 (as in the cosmic background radiation) were magnified by several orders of magnitude. In regions of higher density, we hope to see galaxies begin to take shape and create bubbles of ionized hydrogen. The bubbles will multiply and merge until they finally clear intergalactic space of neutral hydrogen (see text box on the right). The sharpness of the contours of the bubbles will answer the question of what caused the reionization, heavy stars or black holes? Massive stars release most of their energy in the ultraviolet light range, radiation that is easily blocked by the intergalactic hydrogen, while black holes mainly create x-rays, which penetrate deep into the gas. That's why black holes create blurrier outlines.

The radiation map at a wavelength of 21 centimeters is expected to give more information than any previous map in cosmology, including the cosmic background radiation map. There are several reasons for this. First, while the cosmic background radiation provides a two-dimensional image, since it was created at a single moment in time (when the universe cooled below 3,000 Kelvin), the 21 centimeter map will, as mentioned, be three-dimensional. Second, the background radiation is a little fuzzy, because it was not released simultaneously everywhere. The universe went through an intermediate age in which it was neither impervious to the passage of radiation nor transparent, like a kind of fog that gradually dissipated. During this period, the radiation spread over short distances and left a thin imprint on the cosmic background radiation. On the other hand, when the 21 cm radiation was emitted from the hydrogen atoms, nothing hindered its progress in space, and thus it reflects the distribution of the gas without blurring. A third reason is the information carried by the background radiation. This radiation carries information about the density of the material that formed the seeds for the galaxies, while the 21 centimeter radiation maps both the seeds of the galaxies and the effect of the galaxies on their environment, right from the moment they were formed.

To detect the radiation signals at a wavelength of 21 centimeters, several challenges will have to be overcome. First, the broadband and low-frequency radio transmissions on Earth must be filtered. Second, it will be necessary to deal with the emission of radio waves from our galaxy, an emission 10,000 times stronger than the signals received from the reionization era. Fortunately, the shape of the galactic noise is more or less uniform at wavelengths close to each other, whereas the 21 cm radiation signals vary with wavelength according to the spatial structure of the ionized bubbles. This difference will make it possible to extract the radiation signals from the galactic noise. Astronomers will be able to compare the 21 cm maps with images obtained by other means, such as JWST. The galaxies visible in infrared light should correspond to the ion bubbles scattered in the neutral hydrogen spaces. 

Besides the observational challenges we mentioned, there are several other tasks facing the theorists. First and foremost, they will have to run larger computer simulations - so that they can track events in a volume large enough to be a statistically representative sample of the universe (a billion light-years wide), and with a high enough resolution - to also track dwarf galaxies. The simulations will also have to follow the progress of the ionizing radiation from the galaxies into the surrounding gas, a process that the simulations have so far only given a very rough description of. It is very possible that the observers will notice the reionization, even before the theorists can predict what exactly they are supposed to see.

The combined effort - both experimental and theoretical - should dispel some of the fog that prevails in some troubling issues in the theory of galaxy formation. One such issue concerns the massive black holes at the centers of galaxies. In the last ten years, astronomers realized that almost every galaxy in the universe today, including the Milky Way, has a massive black hole. These black holes are thought to swallow gas during events that occur when galaxies merge. When this happens, the collapsing gas shines much more brightly than the rest of the galaxy, forming a quasar. The Sloan Digital Sky Survey revealed that quasars with black holes larger than a billion solar masses existed as early as the cosmic age of a billion years. How did such massive black holes form so early? And why did they stop growing?

Another issue concerns the size distribution of galaxies. Theorists believe that the ultraviolet radiation emitted by dwarf galaxies during reionization heated the cosmic gas and suppressed the formation of new low-mass galaxies. How has this situation evolved over time? Which of the dwarf galaxies we see today were there in the beginning? These are just some of the many questions to which the answers are found in the Dark Age.
 Overview - The Reionization Era
In recent years, much attention has been paid in cosmology to the cosmic background radiation, which provides a momentary picture of the universe at the age of 400,000 years. But between this time and the time of the appearance of the first galaxies there was a period of almost total darkness, interrupted only by faint starlight. In this period lies the secret of the formation of galaxies.
It is obviously difficult to study a period, which by its very nature is almost invisible. The key lies in looking for the faint signals of radio waves emitted by neutral hydrogen gas as it reacted with the background radiation. Today, scientists are beginning to conduct such a search.
The result will be an even more interesting map than the cosmic background radiation. It will be a three-dimensional map that will describe step by step how patterns and order were created out of formless matter.

About the author
Avraham Leib (Loeb) is a leading scientist in the world in theoretical research of the first stars and the age of reionization. According to him, what drives him is an interest in age-old philosophical questions. These questions inspired him to enter the field of physics in his youth. He is currently a professor of astronomy at Harvard University and a visiting professor at the Weizmann Institute of Science in Rehovot. Leib was also a pioneer in the discovery of extrasolar planets using gravitational microlensing and gamma ray generation in intergalactic space. He participated in the first scientific working group on the James Webb Space Telescope and received the 2002 Guggenheim Fellowship.

Although there were no stars yet, the Dark Age was not completely dark. A rare process caused the hydrogen gas to glow dimly.
For the hydrogen to shine, an energy source is required. The only available sources were the kinetic energy of the atoms themselves (released in collisions between the atoms) and the photons of the cosmic background radiation. A handful of free electrons helped transfer the energy between the atoms and photons.
However, no energy source was strong enough to make hydrogen glow by the known means, as happens when an electron is thrown into a higher atomic orbit (called an "excited state") and then returns to its original orbit emitting a photon.
The collisions and photons provided enough energy to reverse the spin of the electron and move it into a state parallel to the spin of the proton. When the electron's spin flipped back, it released a photon with a wavelength of 21 centimeters.
The kinetic energy, the photon energy and the spin energy were used as three energy baths that exchanged energy with each other in different processes.
The amount of energy in each bath can be represented by temperature: the higher the temperature, the more energy. At the beginning of the dark ages all three temperatures were the same (a). Then the kinetic temperature and the spin temperature began to drop faster than the photon energy (b). After some time, the spin temperature returns to equilibrium with the photon temperature (c). Finally, stars and quasars heated the gas and raised the kinetic temperature and the spin temperature (d). The relative temperatures determine how (and if) hydrogen can be observed.

And more on the subject

Measuring the Small-Scale Power Spectrum of Cosmic Density Fluctuations through 21cm Tomography Prior to the Epoch of Structure Formation. Abraham Loeb and Matias Zaldarriage in Physical Review Letters, Vol. 92, no. 21, Paper No. 211301; May 25, 2004, Preprint available at arxiv.org/abs/astro-ph/0312134

The State of the Universe, Peter Coles in Nature, Vol. 433, pages 248-256; January 25, 2005.

First Light, Abraham Loeb, Lecture Notes for the SAAS-Fee Winter School, April 2006, arxiv.org/abs/astro-ph/0603360

Chasing Hubble's Shadows: The Search for Galaxies at the Edge of Time, Jeff Kanipe, Hill and Wang, 2006.

Cosmology at Low Frequencies: The 21 cm Transition and the High-Redshift Universe, Steven Furlanetto, S. Peng Oh and Frank Briggs in Physics Reports (forthcoming).
arxiv.org/abs/astro-ph/0608032