But the universe was still not completely clear—only optical light could disperse itself unhindered, but not ultraviolet light, which was absorbed by the hydrogen nuclei that filled the universe to a large extent at that time. The expansion of the universe changed the wavelength of optical light. Initially a bright blaze, the cosmos gradually became a reddish color. The red became darker and darker, then purple, and finally infrared—in other words, black to the naked eye. 13.8 billion years later the cosmic background radiation was in the microwave range. That’s why scientists call it the Cosmic Dark Ages.
The first beacon fires
It was during this era that the first stars flickered into existence. At a very early stage, dark matter increased non-homogeneities in the structure of the universe to such an extent that there were areas with hydrogen in higher concentration. Huge bubbles formed—called halos—imposing a structure on the universe shaped by inflation and dark matter, long before the formation of galaxies.
Although dark matter is invisible, it still has an effect on the matter known to us, through its gravitational force. This meant that the interstellar hydrogen also conglomerated in the halos of dark matter. The closer the particles came to one another, the more the pressure increased in the gas—until pressure and gravitation were in balance. The cloud cooled due to the radiation given out by the collision of particles. Then the pressure was reduced and the cloud could contract further. The more mass that collects in a small space, the stronger its attractive effect on the atoms in its vicinity. Temperature and pressure increased again in the interior of the cloud, which began to radiate infrared.
Thus the stage of the protostar was reached—but there were still no nuclear reactions. The future star consisted of ionized atoms, of plasma. The heavier the cloud became, the faster it contracted—and at some point the temperature of around 3 million Kelvin and the pressure were so high that a process of atomic fusion was triggered—two protons collided, transforming with the release of energy into a deuterium nucleus with a proton and a neutron. The deuterium then reacted with another proton to become a helium nucleus, which in turn released more energy. Once the interior of the star was hot enough—upwards of 10 million Kelvin—the He3 nuclei could then fuse into He4, releasing two protons. The effect of all this was that about 100 million years after the Big Bang, the first stars began to light up.
They died young
Our sun is barely comparable to these very first cosmic beacons, which today are referred to as ‘population III stars.’ First of all, they must have consisted of hydrogen and helium, because there were no other elements in the early universe. That’s also how these stars could be identified—the spectrum of a star reveals its structure. By comparison, about 1.8 percent of the mass of our sun consists of heavy elements. But due to the low mass of its components, a population III star must have been much larger than our sun—it’s estimated that the early stars had at least 100 times the mass of the sun. They burned fast and hot and therefore must have long since turned into supernovas. And yet it’s still possible to observe them, in far distant galaxies whose light takes such a long time to reach us that it gives us a glimpse far back into the past.
The light from these stars had a side effect—the radiation ionized the hydrogen gas in the area. The interstellar gas could therefore no longer absorb any more ultraviolet light, and the universe became brighter again. This re-ionization phase must have begun around 200 million years after the Big Bang. At first this happened in bubbles. It would have taken up to 800 million years for the universe to become completely transparent again. But even today there are molecular clouds whose components are not ionized.
Scientists surmise that there were in fact two re-ionization phases. The first, probably giant and fast-burning stars ended their lives at some point in an enormous explosion, a supernova, which blasted the surrounding gas cloud out into the nothingness. This made the formation of their successors more complicated. The hydrogen and helium nuclei now had an opportunity to capture electrons. It wasn’t until about a billion years later that new stars were able to compensate for this effect.
A fascinating death
It’s also exciting the way the very first stars met their end. If they had 130 to 250 solar masses, this could result in a pair-instability supernova. Because the stars were so deficient in heavy elements, despite their high masses, the pressure in the core was not enough to start the synthesis of other elements after the helium had burned away. Instead, the temperature in the core increased more and more until the existing photons had enough energy for electron-positron pairs to be created. Although these were quickly annihilated, the result of this process was that the radiation couldn’t leave the interior of the star quickly enough. Hence the core heated up again, followed by another electron-positron reaction, until this feedback loop eventually led to the star being completely torn apart. Unlike with ordinary supernovas, there was nothing left to form a black hole. The star’s entire mass was flung out into space. A pair-instability supernova releases 100 times more energy than a typical supernova—which itself is no weak campfire in the sky.
If one of these first stars had more than 250 solar masses, another type of supernova could have resulted—photodisintegration. Particularly high-energy photons can knock protons out of nuclei. This is an endothermic process wherein the star is deprived of energy. The core suddenly becomes colder and can no longer withstand the pressure of its environment—it collapses and finally turns into a black hole. While the released protons can lead to the formation of heavier elements, part of the mass of the dying giant star is discharged into the surrounding space in a jet.
Group dynamics
Both of these processes finally led to a gradual change in the composition of the universe. The next generation of stars benefited from this. We’re now talking about population II. They required less base material to catalyze nuclear fusion, and because of that they burned less brightly, but for much longer. Even these stars eventually turned to ash, from which the population I stars were formed. Our 4.57 billion-year-old sun belongs to this younger generation.
But let’s take a step back. The first stars did not only affect their immediate surroundings. Their gravity influenced others of their kind. About 500 million years after the Big Bang, the first galaxies formed.
Compared to today, the early universe was very lively. Small galaxies combined to form larger ones. There were clashes and hostile takeovers. We now know that this development was not purely random. Rather, star formation was concentrated around subtle dark matter structures in the universe, called filaments. Where these filaments met, galaxies and galaxy clusters emerged. The observable universe today contains several million of these galactic superclusters. One example of them, the Virgo supercluster, with a diameter of 110 million light-years, is home to a few thousand galaxies... including the Milky Way as part of the local group.
Fragile spirals
We don’t know for sure how the different types of galaxies formed. It’s assumed that dark matter played a role in forming the more common spiral galaxies, of which the Milky Way is one. Dark matter only interacts gravitationally, whereas ordinary matter is also affected by the radiation pressure of stars. That’s why dark matter was able to concentrate in the outer reaches of the galaxy, where its halo was. The interstellar gas, simultaneously attracted strongly by the core of the galaxy and weakly by its outer reaches, therefore moved on a spiral course toward the core, as though someone was swinging a cat by its tail.
The problem with this model is that the contraction stopped at some point, and cosmologists still don’t know why. Spiral galaxies are also very fragile structures—galactic collisions should actually have deformed them much more than they apparently have. Scientists now regard the elliptical galaxy as the endpoint. These have arisen mostly as a result of the fusion of smaller galaxies. According to one simulation, this could also be imminent for the Milky Way and our neighbor M31, Andromeda. The simulation
suggests that, in about 3 to 4.5 billion years, there will be a collision between the two roughly equal systems. The result is likely to be a gigantic elliptical galaxy.
It was probably also the particular composition of the earliest galaxies that produced a phenomenon that astronomers initially thought to be a special kind of star—a quasar. Scientists now agree that these are active galactic nuclei, probably substantial black holes. Although our Milky Way has an impressive black hole at its center, compared to a quasar it’s pretty humble.
The 3C 273 quasar, for example, can be seen from Earth through an amateur telescope, even though it’s almost at the edge of the observable universe. From a distance of 33 light-years it would look as bright as the sun looks from the Earth. The quasar is about 100 times as bright as all the stars in the Milky Way combined. The black hole at its center must be correspondingly large. The fact that quasars can be observed only at a considerable distance from us is proof that the conditions for their formation no longer exist.
Planets from the debris
Even today, scattered throughout the universe, new stars continue to ignite. But the birth rate has dropped significantly since the early days. The expansion of space reduces the density of the interstellar gas. The conditions are particularly difficult for infant stars. However, after the birth of a star there is almost always a kind of afterbirth—a hot disk of gas and dust around the star.
And there are non-homogeneities in this, which increase under the influence of gravity. The molecules clump together and form ever-larger bodies, which attract each other. If the process continues uninterrupted for long enough, a hot proto planet is created. Depending on the source material, this may become a gas giant or a rocky planet like Earth. The celestial body eventually cools down and may later be hospitable to life.
Glossary of Acronyms
AGN – Active Galactic Nucleus
AI – Artificial Intelligence
AU – Astronomical Unit (the distance from the Earth to the sun)
DNA – DeoxyriboNucleic Acid
WMAP – Wilkinson Microwave Anisotropy Probe
Metric to English Conversions
It is assumed that by the time the events of this novel take place, the United States will have joined the rest of the world and will be using the International System of Units, the modern form of the metric system.
Length:
centimeter = 0.39 inches
meter = 1.09 yards, or 3.28 feet
kilometer = 1093.61 yards, or 0.62 miles
Area:
square centimeter = 0.16 square inches
square meter = 1.20 square yards
square kilometer = 0.39 square miles
Weight:
gram = 0.04 ounces
kilogram = 35.27 ounces, or 2.20 pounds
Volume:
liter = 1.06 quarts, or 0.26 gallons
cubic meter = 35.31 cubic feet, or 1.31 cubic yards
Temperature:
To convert Celsius to Fahrenheit, multiply by 1.8 and then add 32
To convert Kelvin to Celsius, subtract 273.15
Brandon Q. Morris
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www.hard-sf.com
[email protected]
Translator: Siân Robertson
Editing team: Marcia Kwiecinski, A.A.S., and Stephen Kwiecinski, B.S.
The Death of the Universe: Hard Science Fiction (Big Rip Book 1) Page 27