Astronomers call it the dark ages. It began around 380,000 years after the Big Bang, when atoms had just formed and X-ray radiation permeated space. There were no galaxies, no stars, no planets at that time—nothing that could shine light into the cosmos. Very gradually, gravitational attraction drew together clouds of matter and eventually the first celestial objects were born, sometime between 200 million and one billion years after the Big Bang.
Whatever the first celestial objects were, they pumped so much light energy into space that they ionized almost every atom in the Universe by blasting away the electrons from their atomic nuclei. This created giant clouds of glowing gas that further lit up space. Thanks to the time it takes light to cross the vast tracts of space, those first luminous sources, more than 13 billion light years away, should still be visible with a sufficiently large telescope as tiny pinpricks of light.
The first attempt to see into the furthest reaches of the Universe was in 1995, when the orbiting Hubble Space Telescope pioneered the technique of “deep field” astronomy. A ten-day observation was conducted that consisted of looking at a single patch of sky no larger than a tennis ball placed 100 meters away. The area chosen was one near the constellation of Ursa Major (also known as The Plough or Big Dipper) that appeared to be completely empty—nothing had ever been found there. After ten days of collecting light, however, the Hubble Telescope produced an image revealing 3000 celestial objects. The vast majority of them were small galaxies, at distances greater than 10 billion light years.
GALAXY FORMATION
The image became known as the “Hubble Deep Field” and gave astronomers their first real look at such extremely distant realms. Previously, observations with ground-based telescopes had only detected galaxies within 7 billion light years, about halfway across the Universe. These galaxies seemed indistinguishable from present-day galaxies, suggesting to astronomers that, however galaxies formed, they did it relatively quickly, building themselves into their mature shapes within the first six billion years of the Universe’s history. Today’s galaxies are classified according to their shape (see What is the Universe?). Elliptical galaxies are elongated balls of stars; spiral galaxies are flat with a central hub of stars and arms that spiral around the center; barred-spiral galaxies each have an elongated central hub connecting to the spiral arms. There are also irregularly shaped galaxies.
The large numbers of small, distant galaxies in the Hubble Deep Field confirmed that today’s large galaxies began as much smaller collections of a few million or fewer stars. They were either irregularly shaped or elliptical, and built themselves into larger galaxies by colliding and merging with their neighbors. As they grew in size, they eventually accumulated enough mass to develop appreciable gravitational fields with which to pull in gas from intergalactic space. As the gas plummeted toward the galaxy, it fell into orbit in a disk around the central hub of stars. When the disk accumulated enough gas, star formation spontaneously began within it and surrounded the galaxy with sweeping arms of new stars.
There are two subtly different patterns of spiral arms that can form. Each betrays the behavior of the gaseous disk in that particular galaxy. First there are the “grand design” spirals, consisting of two dominant spiral arms that can be traced outward from the center of the galaxy. Grand design spirals are caused by a rippling wave of matter that rotates around the center of the galaxy. These ripples, or “density waves,” compress dust and gas as they pass, triggering star formation. By contrast, the second type of spiral galaxy, the “flocculent” spiral, has a messy whorl of stars, which form without the intervention of density waves. As a cluster of bright stars forms, those stars closest to the center of the galaxy complete their small orbits in a short time, while those further out take longer. This stretches the star-forming regions into truncated spirals; thousands of these contribute to the “woolly” appearance of a flocculent spiral galaxy.
If a spiral galaxy is left on its own, it will continually accumulate gas from its surroundings. Perhaps it will occasionally cannibalize a much smaller galaxy, but neither process will affect its overall spiral shape. However, should it veer too close to a similarly sized galaxy, the resultant collision will destroy the delicate spiral shape. As they merge, the two galaxies will lose all structure and the result will be a large fuzzy cloud of stars: an elliptical galaxy. This mighty collision will also force all remaining gas in the merged galaxies to transform into new stars, triggering a sudden explosion of star formation known as a “starburst.” In a few hundred million years, a multitude of brilliant star clusters will be created until all of the gas is exhausted.
TYPES OF SPIRAL GALAXIES: GRAND DESIGN SPIRAL GALAXIES (LEFT) HAVE WELL DEFINED SPIRAL ARMS WHEREAS FLOCCULENT SPIRALS (RIGHT) DO NOT.
At the center of the merging galaxies other dramatic events will be afoot. Both galaxies will contain a central supermassive black hole (see What is a Black Hole?) and these will both sink toward the center of the merged galaxy, where they will draw each other into spiraling orbits. Their huge gravitational fields will interact, swallowing stars and throwing others into eccentric orbits. Eventually the black holes will meet and plunge together, releasing a torrent of energy that sweeps through the galaxy as a burst of radiation. The newly enlarged black hole will continue to consume clouds of gas, stars or planets that haplessly stray into its gravitational reach. For the first few million years following a merger, this can be a massive amount of material, and the merging galaxies will most likely become a quasar: a highly active, tremendously luminous galaxy. These once populated the Universe in great numbers but have dwindled to extinction, no doubt because the black holes that power them have devoured everything within their gravitational grasp.
Once the quasar eventually dies down, it becomes an ordinary galaxy with a dormant central black hole. Cosmologists believe that the Universe built its current quota of galaxies in this way. But the nature of the first step of the sequence—the origin of the collections of a few million stars—remains elusive.
When the Hubble Space Telescope was upgraded with a new camera, astronomers tried another deep field observation. The “Hubble Ultra Deep Field,” as the new shot was called, covers an area of about one-tenth that of the full Moon and revealed 10,000 small galaxies. Most of them appear as they looked around 800 million years after the Big Bang, which is staggering, but still there was no sign of the very first, individual celestial objects. They must be more distant still, and too faint to be seen by the Hubble Telescope.
So astronomers have had to turn to the theorists, who use computers to model what were likely to have been the first celestial objects drawn together by gravity. There seem to be two possibilities: either they were stars, gigantic by our modern standards; or they were black holes, already busily sucking in gas that would radiate furiously as it fell into oblivion. Whichever they were, they were the objects that clustered together to become the galaxy building blocks, and prepared the Universe for the formation of other celestial objects. As stars and black holes would do these jobs in different ways, it is crucial for cosmologists to determine which it was, in order to understand the subsequent development of the Universe.
Of all the celestial objects, stars are the ones that exert the biggest influence over the Universe’s chemical composition (see What Are Stars Made From?), and none have affected it more than the earliest stars. During the “dark ages” before the first luminous objects, all that existed was a diffuse sea of atoms: roughly three-quarters of it hydrogen, one quarter helium, with a seasoning of lithium. No other chemicals yet existed, and computer models suggest that this lack of variety had a tremendous effect on the first generation of stars.
As gravity pulls gas together to form a star, so the gas naturally heats up as its atoms are confined. This heat resists further compression and must be radiated into space in order for the star to continue pulling itself together. Calculations show that the heavy chemical elements are highly efficient radiators, whereas the light gaseous elements find it difficult to dissipate their energy. So in star formation today, the presence of elements heavier than lithium speeds up the collapse, allowing stars to form from relatively compact pockets of gas. This results in most stars containing less mass than the Sun. Back in the dark ages, however, the forming stars did not have the help of the heavy elements in losing heat, and this meant much more gas had to build up in order for gravity to become the overwhelming force. The first stars were therefore much larger than those found in the present Universe, with masses from several hundred to a thousand times that of the Sun. One of these megastars would be big enough to engulf all the planets in the Solar System, were it placed at the Sun’s location.
It was first thought that these early megastars were the chemical factories of the Universe, with their vast nuclear furnaces transforming a fraction of their hydrogen and helium into the other chemical elements and then scattering it into space, where it could be incorporated into the next generation of smaller stars. But there is a flaw in this theory. Hydrogen is transformed by nuclear fusion in one of two ways: either in a series of collisions called the “proton-proton chain” or through a reaction sequence known as the “carbon-nitrogen-oxygen cycle” (CNO cycle for short). The proton-proton chain is the less efficient of the two but, because in any early megastar there was no carbon to begin the CNO cycle, it would have been forced to rely on the proton-proton chain. The difficulty is that the proton-proton chain would not have been able to generate enough energy to counteract the gravity of a star of that size, and so the star should have simply collapsed—and become a black hole. A black hole made in this way would have possessed from several hundred to a thousand solar masses and could certainly have been the seed around which a galaxy began to form. In effect, these early black holes would have been quasars in the making. Yet this picture cannot be completely right either, because light collected from a handful of extremely distant quasars that were shining just 900 million years after the Big Bang shows the telltale pattern of absorption lines (see What Are Stars Made From?) that betrays the presence of iron—an element that can only be formed in the heart of a massive star. So, at least some stars must have formed before these quasars.
“All truths are easy to understand once they are discovered; the point is to discover them.”
GALILEO GALILEI 17TH CENTURY ASTRONOMER
It seems likely then that a mixture of stars and black holes constituted the first celestial objects. The only way to understand exactly what was going on back in those distant times is to find a way of seeing all the way back to the dark ages.
In an attempt to detect the heat from the first stars, astronomers launched a high-altitude balloon-borne experiment in 2006. It was intended to measure the infrared radiation from the first stars, which had been redshifted into radio waves by the expansion of the Universe (see How Big is the Universe?); instead, the experimenters found that a mysterious wall of radio noise deafened their detectors.
This cosmic static was six times louder than anything the astronomers were expecting and completely prevented them from observing the heat from the first stars. Cosmologists speculate that this mysterious radiation may be coming from the death throes of the earliest stars. They have good reason to suspect this because when massive stars explode, they become billions of times brighter than normal. So it seems likely that the first glimpse of something from just after the dark ages will not be an ordinary large star, but the brilliant explosion that marked its death.
Indeed, the most distant object currently known is a type of celestial explosion called a “gamma ray burst” (GRB). These stellar explosions appear to be more energetic than any supernova in the modern (nearby) Universe and add weight to the idea that the earlier generation of stars were more massive than those of today and so experienced more violent deaths. Each gargantuan star is calculated to have died with a titanic outburst that released as much energy as ten trillion Sun-like stars. Much of this energy was packed into a sudden burst of gamma rays that shot off across the Universe. Watching from Earth, we have no idea where the next gamma ray burst will come from. One arrives every day or two, but from a completely unpredictable direction. Not only that, but astronomers have to be really quick to spot them. Having taken billions of years to travel to us, they arrive and pass by in just a few seconds. Highly sophisticated spacecraft are lying in wait; the instant they detect a burst, they can pinpoint the explosion and, within a second, send out signals to guide other orbiting and ground-based telescopes to look at the correct location.
The gamma ray burst GRB090423 is the record holder for distance, having been calculated to be 13.1 billion light years away when it exploded. Its outburst was detected on Earth on April 23, 2009 and allowed astronomers to calculate that it must have blown up just 600 million years after the Big Bang, making it an excellent candidate for being one of the Universe’s earliest stars.
Another strategy for investigating the dark ages is to look for the signal from the hydrogen gas that existed throughout space at that time. Hydrogen atoms can spontaneously emit radio waves with a wavelength of 21 centimeters. As that signal travels through the expanding Universe it is stretched to about two meters and can be picked up by an ordinary radio receiver. But in order to process the received data, tens of thousands of radios are needed, all working together, and a supercomputer. Astronomers are now building such “hydrogen telescopes” and expect to use them to map the distribution of the first celestial objects in the surrounding hydrogen gas. A hydrogen atom can only emit radio waves when it has an electron in orbit around its nucleus; since these atomic electrons would have been stripped away by the ionizing radiation from the first luminous objects, the hydrogen signal would have disappeared at the end of the dark ages. A radio-quiet “bubble” would have formed around each new celestial object, and the hope is that these will appear as dark holes on the hydrogen maps from the new radio telescopes.
The theory goes that at the center of each hole on the map will be either a megastar or a black hole. Computer models predict that megastars will “blow bubbles” in subtly different ways from black holes, and so astronomers believe that they will be able to determine the nature of each central object when they analyze their first results from these telescopes, expected sometime around 2015.
Astronomers dream that eventually they will be able to actually see and take pictures of these first objects, and to analyze each object’s chemical composition and physical conditions. Only this information will tell them definitively what the first celestial objects were, and how the first wave of heavy chemical elements was generated. Then they can move on to investigating how these individual objects gathered together to become the small early galaxies seen in the Hubble Deep and Ultra Deep Fields. Our current understanding of galaxy formation is still at an early stage: difficulties encountered indicate that either there must be “dark matter” that we cannot see in the galaxies or our understanding of gravity is wrong.