Today, modern physics and cosmology has been able to construct what we believe is a relatively accurate model of the history of the universe to date, as well as what we believe are more or less viable guesses at what is likely to occur over the next several trillion years. Obviously, such a model will doubtless be revised substantially in coming decades, just as it has been substantially modified in the recent past. However, the most commonly accepted model of the history and future of the universe today suggests that it was formed through a period of massive initial inflation, known as the Big Bang; is currently enjoying a period of rapid expansion but also rapid and common star formation due to the extremely large number of free-floating molecular clouds; but will eventually run out of material to form new stars, will then come to be dominated by black holes and the wandering husks of dead stars, and will finally reach a point of either maximum entropy where no further work or heat transfer is possible (the “heat-death” of the universe), or a point at which continuing expansion has dispersed matter to such an extent that the universe grows cold, dark, and ultimately dead.
– Big Bang and Primordial Universe –
In the beginning, about 13.7 billion years ago, all of the matter currently present in the universe (and then some) was probably confined at a single point of extraordinarily great mass and density. This suddenly expanded outwards, causing an initial period of massive and rapid inflation. This initial spark is known as the “Big Bang,” although current models suggest that the effect was more akin to a balloon rapidly filling with air than to an actual explosion or “bang.” In the first few hundred thousand years, matter and antimatter emerged, annihilated one another (leaving a small surplus of matter left over which now makes up the physical universe as we know it), and then formed into the first atoms and molecules.
The result was a universe smaller than it is today, but continuing to expand outwards. Within that universe, matter was almost – but not quite – evenly distributed. Gradually, over time, areas of greater density began to attract the matter from around them, and then to collapse inwards into more and more dense structures. This process, in theory, led to the creation of the first galaxies and to the first stars.
– Stelliferous Era –
The first stars themselves probably would have emerged when the universe was about 150 million years old. These first stellar ignitions marked the beginnings of what some term the stelliferous era: the period in the universe’s history when star formation is frequent, and commonplace, so that the universe is a bright place filled with stars and galaxies. Although as a whole the universe continues to expand during this period, in particular regions bodies may be drawn together by gravity. For example, the Milky Way and Andromeda galaxies are on a collision course and will merge in several billion years. Tens of billions of years after that, the galaxies which comprise the entire Local Group will gradually be drawn together and merge in the same way.
When we look through our telescopes, the universe seems to be a bright and active place, filled with light if not perhaps filled with life. However, thanks to the inescapable logic of the second law of thermodynamics, this period of light and life is sowing the seeds of its own destruction. Stars produce their energy by fusing hydrogen into helium in their cores, but eventually their supplies of hydrogen are exhausted. Smaller stars, like our suns, shed their outer layers to form planetary nebulae and then contract into glowing embers called white dwarfs. Much larger stars undergo gravitational collapse to form neutron stars or – if they are large enough – black holes.
– The Big Crunch –
Moreover, even as this occurs, the process of expansion means that most galaxies are still flying farther apart from one another. Within about one trillion years, it will probably be impossible even with powerful telescopes to see anything beyond our own Local Group. The universe will begin to seem like an exceptionally lonely place. Until recently, scientists believed that the inexorable force of gravity would eventually win out over the process of expansion, and the galaxies would begin to draw together again, ultimately leading to a so-called “big crunch” in which all matter is compressed together again. This could lead to the re-birth of the universe in the form of another Big Bang. Indeed, this early model suggested that our universe might be one of only an infinite number of iterations in a cycle of “big bangs” and “big crunches.”
New data, however, suggests the big crunch – and future inflations – will never happen. Instead, the evidence now suggests that our rate of expansion is accelerating instead of slowing down. If this is true, it means that some other force is counteracting that of gravity; scientists have called this mysterious force dark energy. A universe powered by dark energy will continue to expand forever – and this holds gloomy consequences for those of us who live within it.
– Degenerate Era –
As the universe continues to expand, stars continue to use up their fuel, explode, and fuel the next generation of stars. Eventually, however, there is simply not enough free-floating material left over to give birth to new stars. The rate of star formation will slow and then, in about 100 trillion years, stop entirely. The last stars alive will be the red dwarfs, small, relatively cool stars who burn through their hydrogen reserves so slowly that a single star can live for as much as ten trillion years (compared with about ten billion for our own Sun).
Once stars stop forming, the universe will be dominated by the remains of previous stars, and so this period is known as the Degenerate Era. Red and yellow stars, like our own, will cool into embers known as white dwarfs. Stars about ten times as massive as ours undergo gravitational collapse when they run out of fuel, creating either neutron stars or, if they are large enough, black holes. Black holes are regions of space so massive that they can attract anything nearby, including light itself.
The Degenerate Era is not the end of light. Occasionally, white dwarfs will collide, causing a supernova and lighting up their corner of the universe for a few years. Brown dwarfs – masses similar to stars, but too small to undergo stellar ignition and fuse hydrogen into helium in their cores – will occasionally collide and form red dwarfs, which survive for trillions more years. Once in a while the white dwarf collisions will result in new masses large enough to form stars as well, known theoretically as either helium stars or carbon stars after their major components.
– Black Hole Era –
The Degenerate Era will be trillions or more as long as the Stelliferous Era before it, but eventually, only one type of remnant will be left: the black hole. Black holes are created when very large stars collapse, creating regions of space so dense that they continually draw in everything nearby, including light itself. Even as the other degenerate remnants go dark, black holes will still be feeding on the debris of the earlier eras.
In this period, atomic-level structures will eventually cease to exist. Much will be devoured by black holes. However, protons themselves are believed to decay over time. If this happens relatively rapidly, then the black hole era will come and pass more quickly. If it occurs more slowly, then the remnants of the degenerate stars may form one final generation of stars, this one based on iron (and called iron stars). However, the iron stars will collapse to form black holes. In the end, the black holes to win.
– Dark Era –
Even black holes, however, are not immortal. Their period of dominance will last as much longer than the Degenerate Era as that era was longer than the Stelliferous Era. However, black holes are known to “leak” small amounts of energy in the form of Hawking radiation. Today, this is not a problem for the black holes, since they consume much more matter than they lose through radiation. Eventually, however, the black holes will evaporate away.
In the resulting period, known as the Dark Era, all light and life has more or less ceased. The last occasional dim flickers of light are the result of subatomic particles meeting, colliding, and releasing energy. These too, however, grow more rare and eventually cease. The universe itself, moreover, continues to expand. Eventually all remaining matter will be so dispersed that no further heat transfers are possible – either because everything has finally averaged out to the same temperature, or because everything has grown too far apart to interact, whichever comes first. Respectively, these end-state scenarios are known as the heat death of the universe, and the Big Freeze. Despite the different names, the result is essentially the same: a dark, cold universe forever from that point onwards.
