THE FIVE AGES OF THE UNIVERSE
I show this picture to begin with because of the caption that goes with it: Alfonso the Xth of Spain, king of Castile and Leon from 1252-1282, is said to have remarked, after the Ptolemaic system had been explained to him, “If the Almighty had consulted me before embarking upon Creation, I should have recommended something simpler.” But the Earth-centred world view of the Greek had endured for a thousand years and continued to persist until 1543 when it was finally replaced by the heliocentric Copernican solar system. Even then, when it was superseded, it was not because the Copernican system was better in explaining physical observations, in fact it was less accurate, but purely for aesthetic reasons. It just made more sense.
Within the province of cosmology, a great deal of scientific progress has been made in the past few decades. It is now generally accepted that the universe has been expanding since its conception during a violent explosion – the big bang itself. (Although to me, the big bang theory is a simplistic solution to a problem requiring still fuller understanding of the laws of nature.) What has made the theory establish itself so solidly is our ability to fit selected facts into it that seem to explain the nature of the universe expanding and cooling over the last ten to fifteen billion years. We can put together a story of our universe as currently interpreted according to the Big-Bang Theory, from beginning to end. This book is an attempt by the Authors to put together such a story, using the facts generally accepted today to create a time-line over the life of the Universe. They have looked into the physics of eternity and found five distinct phases of evolution and grouped the book accordingly.
We are all familiar with the Eras into which the geological history of the Earth is divided. The Paleozoic era, including the subdivisions from Cambrian to Permian, the Mesozoic that contains the now famous Jurassic dinosaur period, and the Cenozoic which covers the last 50 or so million years to the present. Each era defined by a specific geological process that places it in its correct chronological position in the geological column.
O/H-1 Geology History
In a similar fashion we can look at the universe as a whole and classify periods, or eras in its evolution, by the physical processes that took place at the time. Not only can we thus organise the past evolution leading up to the present, but by applying current scientific knowledge we can extrapolate into the far distant future and use the known physical laws to predict future changes. According to the second law of thermodynamics even the inevitable fate of our universe. The recurring theme throughout the life of the universe is the continual struggle between the force of gravity and the tendency for physical systems to evolve toward more disorganized conditions. The amount of disorder in a physical system is measured by its entropy content. In the broadest sense, gravity tends to pull things together and thereby organizes physical structures. Entropy production works in the opposite direction and acts to make physical systems more disorganized and spread out. The interplay between these two competing tendencies provides much of the drama in astrophysics.
This biography of the universe stretches over a long time and the we need an easily understandable measure that extends to over 10100 years. The number 10100 is big. Very big. The ten billion years already gone by represent an utterly insignificant fragment of this time. Written down without the benefit of scientific notation, this number consists of a 1 followed by one hundred zeros and it looks like this:
O/H-2, Cosmological Decade
Not only is this number rather cumbersome to write out, but it is difficult to obtain an accurate feeling for just how tremendously gigantic it is. Attempts to visualize it by imagining collections of familiar objects are soon thwarted. For example, the number of grains of sand on all of the beaches in the world is often trotted out as an example of an incomprehensibly large number. However, a rough estimate shows that the total number of sand grains is about 1023, a 1 followed by 23 zeros, a big number but still hopelessly inadequate to the task. How about the number of stars in the sky? The number of stars in our galaxy is close to one hundred billion, again a relatively small number. The number of stars in all the galaxies in our observable universe is about 1022, still far too small. In fact, in the entire visible universe, the total number of protons, the fundamental building blocks of ordinary matter, is only 1078, still a factor of ten billion trillion times too small! The number of years between here and eternity is truly immense.
In order to describe the time scales involved in the future evolution of the universe, without becoming completely bewildered, let us use a new unit of time called a cosmological decade. For example, the universe is currently about ten billion years old, which corresponds to 1010 years, or 10 cosmological decades. In the future, when the universe is 100 billion years old, the time will be 11 cosmological decades. The power of this scheme is that each successive cosmological decade represents a tenfold increase in the total age of the universe. The concept of a cosmological decade thus provides us with a way to think about immensely long time spans. Our aggressively large example, the number10100, thus corresponds to the rather more tractable hundredth cosmological decade.
O/H-3 the expanding Universe
We can also use cosmological decades to refer to the very short periods of time which came immediately after the big bang. We simply allow the cosmological decade to become a negative number. With this extension, one year after the big bang corresponds to 100 years, or the zeroth cosmological decade. One-tenth years is thus cosmological decade -1, and one-hundredth years is cosmological decade -2, and so on. In terms of cosmological decades, the big bang is understood to have occurred at the cosmological decade corresponding to negative infinity.
As the universe passes from one era to the next, the inventory and character of the universe change rather dramatically, and in some ways almost completely. The FIVE distinctively different Eras of Activity are analogous to geological eras, where a series of natural disasters shape the environment and drive its subsequent evolution. Here then is a broad outline of the life of the universe.
O/H-4, The five Ages of the Universe
you will note that this time-line starts and finishes with a question mark, confessing to the fact our current knowledge of physics can only take a guess at the events.
The Primordial Era is the time from -50 to +5. This era encompasses the early phase of the universe. While the universe is less than 10,000 years old, most of the energy density of the universe is in the form of radiation and this early time is often called the radiation dominated era. No astrophysical objects, like stars or galaxies, have been able to form.
During this early epoch, many important events took place which served to set the future course of the universe. The synthesis of light elements, such as helium and lithium, occurred during the first few minutes. Even closer to the beginning, complex physical processes set up a small excess of ordinary baryonic matter over antimatter. The antimatter annihilated almost completely with most of the matter and left behind the small residue of matter which makes up the universe of today.
In turning back the clock even further, our understanding becomes shakier. At extraordinarily early times, when the universe was unbelievably hot, it seems that very high energy quantum fields created an Inflationary Period of fantastically rapid expansion and produced very small density fluctuations in the otherwise featureless universe. These tiny fluctuations have survived and grown into the large-scale structures that populate the universe today.
Near the end of the Primordial Era, due to continued expansion, the energy density of the radiation became less than the energy density associated with the matter. (I would like to come back to this point if we have time, to point out what seems to me an interesting anomaly). This crossover took place when the universe was about ten thousand years old. Shortly thereafter, another watershed event took place as the temperature of the universe became cool enough for atoms (or more specifically, hydrogen atoms) to exist. The first appearance of neutral hydrogen atoms is known as recombination time. After recombination took place, fluctuations in the density of matter in the universe allowed it to grow into clumps without being affected by the pervasive sea of radiation. Familiar astrophysical objects, like galaxies and stars, began forming for the first time.
O/H-5 Eta Carinia
We now enter The Stelliferous Era, which lasts from the 6th to the 14th cosmological decade. Stelliferous means "filled with stars." During this era, most of the energy generated in the universe arises from nuclear fusion in conventional stars. We live in the middle of the Stelliferous Era, a time period when stars are actively forming, living, and dying. In the earliest part of the Stelliferous Era, when the universe was only a few million years old, the first generation of stars was born. During the first billion years, the first galaxies appeared and began organizing themselves into clusters and super-clusters.
Many freshly formed galaxies experience violent phases in connection with their rapacious central black holes. As the black holes rip apart stars and surround themselves with whirlpool-like disks of hot gas, vast quantities of energy are released. Overtime, these quasars and active galactic nuclei slowly die down.
In the future, near the end of the Stelliferous Era, a key role will be played by the universe's most ordinary stars – the low-mass stars known as red dwarfs. Red dwarf stars have less than half the mass of the Sun, but they are so numerous that their combined mass easily exceeds the mass of all the larger stars in the universe. These red dwarfs are true misers when it comes to fusing hydrogen into helium. They hoard their energy and will still be around ten trillion years from now, after the larger stars have long since exhausted their nuclear fuel and evolved into white dwarfs or exploded as supernovae. This Era comes to a close when the galaxies run out of hydrogen gas, star formation ceases, and the longest-lived red dwarfs slowly fade away. When the stars finally stop shining, the universe will be about one hundred trillion years old.
After star formation and conventional stellar evolution have ended, the Universe slides again into a completely different age where radically different physical processes dominate. In line with these processes it is called The Degenerate Era. It lasts from the 15th to the 39th cosmological decade. Most of the ordinary mass in the universe is now locked up in the degenerate remnants which remain after stellar evolution has run its course. In this context, degeneracy connotes a peculiar quantum mechanical state of matter, rather than a state of moral decadence. The inventory of degenerates includes brown dwarfs, white dwarfs, neutron stars, and black holes. During the Degenerate Era, the universe looks very different from the way it appears now. No visible radiation from ordinary stars can light up the night skies, warm the planets, or endow galaxies with the faint glow they have today. The universe is colder, darker, and more diffuse.
Nevertheless, events of astronomical interest continually sparkle against the darkness. Chance close encounters scatter the orbits of dead stars, and the galaxies gradually readjust their structure. Some stellar remnants are ejected far beyond the edge of the galaxy, while others fall in towards the centre. A rare beacon of light can emerge when two brown dwarfs collide to create a new low-mass star, which will subsequently live for trillions of years. On average, at any given time, a few such stars will be shining in a galaxy the size of our Milky Way.
O/H-5 – Collision between two brown dwarfs
Our society is steeped in the uneasy awareness that human extinction is not a particularly farfetched possibility. Nuclear Armageddon, ecological catastrophes, and rampant viruses are among the doomsday prospects thrust forward by the prudent, the paranoid, and the profit minded. – But if we adopt a somewhat outdated, yet decidedly more romantic, outlook of rocket ships, space colonies, and galactic conquest, humankind could easily outlast Earth’s and our sun’s demise by moving onward to other solar systems. As this slide shows such star-hopping migration could enable Life as we know it to survive for billions of years well into the Degenerate Era. I’ll come back to the subject of life at the end.
Then, by the 30th cosmological decade the supply of the dark matter particles becomes depleted and this avenue of energy generation comes to an end. The matter inventory of the universe is then limited to brown dwarfs, neutron stars, and dead, widely scattered planets. Proton and Neutron decay now remain as the main source of radiation. A brown dwarf fuelled by proton decay generates approximately 400 watts, enough power to run a few light bulbs. An entire galaxy of these erstwhile stars has a total luminosity smaller than the current Sun. As the proton decay process grinds to completion, the Degenerate Era draws to a close. The universe-ever darker, ever more rarefied, changes its character yet again.
And we enter the Era of The Black Holes, from the 40th to the 100th cosmological decade. After the epoch of proton decay, the only stellar-like objects remaining are the black holes. These fantastic objects have such strong gravitational fields that even light cannot escape from their surfaces. The black holes are unaffected by proton decay and survive unscathed through the end of the Degenerate Era. Yet even black holes cannot last forever. They eventually evaporate away through gravitational radiation and a very slow quantum mechanical process known as Hawking radiation. In spite of their name, black holes are not completely black. In reality, they shine ever so faintly by emitting a thermal spectrum of light and other decay products. After the protons are gone, the evaporation of black holes, almost by default, provides the universe with its primary source of energy. A black hole with the mass of the Sun lasts for about 65 cosmological decades. All black holes are thus slated for destruction. The Black Hole Era is over when the largest black holes have evaporated.
After a hundred cosmological decades, the protons have long since decayed and black holes have evaporated we come into The Dark Era. The only leftover waste products from these processes remaining are photons of colossal wavelength, neutrinos, electrons, and positrons. An odd parallel exists between the Dark Era and the Primordial Era, when the universe was less than a million years old. In each of these eras, distantly separated in time, no stellar-like objects of any kind are present to generate energy.
In this cold and distant future, activity in the universe has tailed off dramatically. Energy levels are low and the expanses of time are mind-boggling. Electrons and positrons drifting through space encounter one another and occasionally form positronium atoms. These late-forming structures are unstable, however, and their constituent particles must eventually annihilate.
In comparison with its profligate past, the universe now lives a relatively conservative and low-profile existence. Or does it? The seeming poverty of this distant epoch could be due to our uncertain extrapolation, rather than an actual slide into senescence. The very fact that space, devoid of limits and content is meaningless could allow unrestricted quantum effects beyond our imagination come into play.
So, let us look at life once more. In the first and most obvious case, we consider life based on a biochemistry that is roughly similar to that on Earth. Life of this sort will presumably arise on terrestrial planets or perhaps on large moons in other solar systems. In keeping with a time-honoured tradition among biologists, we assume that as long as liquid water is present on a planet, then carbon-based life can evolve. The requirement that water must be in liquid form places a rather severe temperature constraint on any potential environment. For example, the temperature range must not exceed 100 degrees from 273 degrees kelvin. This range of temperatures excludes most astrophysical environments.
The second class of life-forms is based on a much more abstract model. In this latter case Freeman Dyson, an influential physicist, has put forth an hypothesis for abstract life-forms. The underlying idea is that at any temperature, one can imagine some kind of abstract life-form which thrives at that temperature – at least in principle. Furthermore, the rate at which this abstract creature uses energy is in direct proportion to its temperature. If we imagine a Dyson organism living at a set temperature, then another similar life form that happens to thrive at half the temperature would have all of its vital functions slowed down by this same factor of two. In particular, if the Dyson organisms in question are intelligent, and have some form of consciousness, then the effective rate at which they experience events is not given by the real physical time, but rather by a scaled time which is proportional to temperature. In other words, the rate of consciousness for a Dyson organism operating at a low temperature is slower than that of an otherwise comparable life-form operating at a higher temperature.
This abstract approach moves the discussion far beyond the familiar carbon-based life of our planet, but nevertheless it makes certain assumptions about the nature of life in general. Most importantly, we must assume that the ultimate basis for consciousness lies in the structure of the life-form and not in the matter that makes it up, For instance, in human beings, consciousness somehow arises through a series of complex biochemical processes. Since the present-day universe is rather convenient for life as we know it we have a natural tendency to think of the present epoch as privileged in some manner. Resisting this tendency, we must adopt the idea of a Copernican time principle, which states quite simply that the current cosmological epoch has no special place in time. In other words, interesting things will continue to happen as the universe evolves and changes. As the available levels of both energy and entropy production become increasingly lower, this effect is compensated by the increasingly long time scales available in the future. Stating this idea yet another way, we claim that the laws of physics do not predict that the universe ever reaches a final quiescent state, but rather that interesting physical processes continue to operate as far into the future as we dare to imagine.
The close of the 20th century is an appropriate time to reflect upon our place in the universe. Using the breadth of understanding gained during this century, we can take an unprecedented look at our position in both time and space. In accordance with the Copernican time principle and the rich variety of astrophysical events yet to take place in the vast expanses of future time, we argue, as a new millennium starts, that the end of the universe is not very near. Armed with an awareness of the forces that balance nature, a new view the universe, and a new calendar that measures time in cosmological decades, we carry on our journey across the grand eras of time.
Points for Discussion
1. The Mass of the annihilated Baryonic matter. Energy Density. Where did it go?
2. The isolation of Galaxies from the overall expansion of the Universe
3. Black Holes and the expansion of Space
4. Inward expansion of Space, the truly created space that makes either distance or time irrelevant.