Chapter 5: The Physical Universe
Section 5.1 Hierarchical Structure
Section 5.2 The geography of matter
Section 5.3 The history and present state of the Universe
Section 5.4 The origin of structure
Section 5.5 Final states
Section 5.6 Cosmic options
References for Chapter 5
This chapter provides an overview of our current understanding of the
nature of the physical Universe. In turn we consider the large-scale
distribution of matter in the Universe, its present expansion and past
evolution (the history of the `Hot Big Bang'), and its possible futures. This
is the larger environment in which we exist.
Section 5.1 Hierarchical Structure
On astronomical scales the matter we can see is hierarchically organised into the structures listed in Table 5.1 where we assume the bottom layers (underlying the atoms that form the chemical elements) as in the last Chapter 3:
|
Hierarchical structure of astronomy |
Processes |
|
The whole Universe |
Formation of the Universe |
|
The observable Universe |
Expansion of the Universe |
|
Large scale structures (walls, voids) |
Structure formation |
|
Cluster of galaxies |
Galaxy cluster formation |
|
Galaxies |
Evolution of Galaxies |
|
Star clusters |
Star cluster evolution |
|
Stars and their planetary systems |
Stellar evolution |
|
Planets, including the Earth |
Planetary formation |
Table 5.1: Hierarchical nature of astronomical structure
This Chapter will examine this hierarchical structuring, looking at the nature of what exists at each level firstly in terms of how it behaves - the kinds of concepts and behavioural patterns occurring at that level - and secondly at the diversity of what exists at that level. It will also consider how causal properties at the different levels relate to each other.
Section 5.2 The geography of matter
The biosphere exists and flourishes on the surface of the Earth [webpage] and in its immediate environment (the rivers, lakes, and seas, and the surrounding atmosphere), depending on them for its existence, so this is the immediate context in which our own existence is based. The Earth was briefly characterised in the last Chapter. We now focus on the broader environment that makes all of this possible: the Solar System, Galaxy, and surrounding Universe, whose nature we explore by using telescopes of various kinds [69,70].
This astronomical environment has been the subject of speculation and study for thousands of years [71,72]; the past three decades have led to unprecedented new understanding of its nature because of images produced by powerful new telescopes such as the Hubble Space Telescope [73,74] [webpage], together with extraordinarily delicate measurements enabled by new instruments such as Charge Coupled Devices (`CCD's'). Astronomical understanding depends on using these instruments to measure spectra (spectroscopy) and intensity (photometry) to great accuracy. In the following, I emphasize the unique identity of our own Earth, Sun, and Galaxy by using capital letters in their names.
In order to get an overall feel for the sizes of things, it is strongly recommended that the reader spend some time contemplating the orders of magnitude of the many different scales of organisation of matter in the universe, see Powers of Ten [17] [webpage].
5.2.1 The Sun and the Solar System.
Earth is surrounded by empty space. It moves in a nearly circular orbit around the Sun, 150 million km away, being held in this orbit by the Sun's gravitational attraction, and taking a year to complete each cycle (Copernicus' great discovery [71]). The center of the Earth is heated by decay of radioactive materials, but the energy source for the biosphere (and so in particular for our lives) is heat radiated to the Earth by the Sun, captured and turned into chemical energy by plants and trees. Earth has one natural satellite, the Moon [webpage] , at a distance of 380,000 km = 1.27 light seconds. The Moon is much smaller than the earth, but has significant gravitational effects on the Earth (it causes the tides) and causes solar eclipses.
The Sun [webpage], [webpage] is very much larger and more massive than the Earth [69]. It is a sphere of very hot gas (mainly hydrogen and helium), with a temperature of 15 million K at the centre and 5,500 K at the surface. The energy source for the Sun is the nuclear fusion of hydrogen nuclei to form helium near its centre (it is a giant free-floating fusion reactor, held together by gravity), the heat generated being radiated away to space from its surface [77,78].
The Earth is one of nine planets circling the Sun, the whole collection (together with asteroids and comets that also orbit round the Sun) forming the Solar System [webpage]. The Sun is the central object in the Solar System, containing most of its mass and providing virtually all the energy pervading the region. Despite appearing so different from them, the Sun is a typical star [77], looking much larger and brighter than all the other stars simply because it is nearer to us than they are. The nearest star - after the Sun - is about 4 light-years away, a light-year being about 90 million billion km. The planets [webpage] are divided into two rather different kinds. The Terrestrial Planets which are relatively small and have a rocky surface and thin atmosphere, with few moons. They are
|
The Terrestrial
Planets |
|||
|
Name |
Distance from
sun |
Moons |
|
|
Mercury |
57.9 x 106km = 0.38 a.u. |
0 |
|
|
Venus |
108.2 x 106km = 0.72 a.u. |
0 |
|
|
Earth |
149.6 x 106km = 1.00 a.u. |
1 |
|
|
Mars |
227.9 x 106km = 1.52 a.u. |
2 |
|
Table 5.2 The Terrestrial Planets
(1 a.u. = 1 astronomical unit = the average distance of the Earth from the Sun = 8.31 light minutes). The Jovian Planets are much larger and are mainly composed of methane and ammonia, and have numerous moons and sometimes ring systems (particularly Saturn). They are as follows:
|
The Jovian
Planets |
|||
|
Name |
Distance from
sun |
Moons |
|
|
Jupiter |
7.78 x 109km = 5.20 a.u |
13 |
|
|
Saturn |
14.27 x 109km = 9.54 a.u |
10 |
|
|
Uranus |
28.70 x 109km = 19.18 a.u |
5 |
|
|
Neptune |
44.97 x 109km = 30.06 a.u |
2 |
|
Table 5.3: The JovianPlanets
The outermost planet -Pluto [webpage], at a distance of 247.7 a.u. - seems to be exceptional and maybe a captured object rather than a true planet. The sun is much larger than the planets [webpage].
5.2.2 Stars
Stars are at large distances relative to the size of the solar system distances to stars [webpage]. They are essentially huge thermonuclear reactors held together by gravity [webpage]. [webpage], [webpage]. In normal stars gas and radiation pressure prevents collapse; the heat needed to maintain the high temperatures required, while radiation escapes and carries energy away from the surface of the star, is provided by nuclear fusion in the centre [webpage]. Hydrogen burns to helium, helium to lithium, and so on up the periodic table of elements, until if the star last long enough the process will come to an end when iron is produced. Thus a star consists of spherical shells around the centre each of different chemical composition. This structure can be probed by helioseismology [webpage]. Many stars are members of binary star systems - they occur in pairs orbiting around each other - and are imbedded in the interstellar medium [webpage], [webpage]
Ordinary stars are classified by their position in the Hertrzprung-Russell Diagram [webpage], [webpage] which plots luminosity (the total radiation emitted) vertically and colour (which represents temperature, and is more formally represented by spectral type) horizontally. The main sequence [webpage] [webpage] is where the majority of stars in a star cluster lie. Above it is where red giants [webpage], [webpage], red supergiants [webpage], and variable stars (Cepheids, RR Lyrae) live, and below is where white dwarfs and neutron stars live. They
|
Source of
pressure |
Types of stars |
|
Gas and Radiation pressure |
Main sequence Red Giants Supergiants |
|
Electron degeneracy pressure |
White dwarfs |
|
Neutron degeneracy pressure |
Neutron Stars |
Table 5.4: Different types of stars, according to their source of support against gravitational collapse.
After stars are born [webpage]) through a process of star formation [webpage], they undergo a relatively well understood process of evolution whereby hydrogen is burnt to helium and then successively heavier elements are burnt up, until they run our of nuclear fuel. The luminosity and colour of the star vary in a well-understood way during this evolutionary process; understanding this relation (observationally seen in the Hertzsprung-Russell diagram [webpage], [webpage] is one of the triumphs of theoretical astrophysics. Eventually their fuel will be burnt out and they will cease shining, perhaps undergoing a dramatic explosion in their death-throes (see Section 4 below and [webpage], [webpage]). The evolutionary sequence from red giants to stellar explosions in nova and on to white dwarfs is shown in [webpage], [webpage], while the evolutionary sequence from red supergiants to vast supernova explosions is shown in [webpage], [webpage], [webpage], often ending as neutron stars [webpage], perhaps a pulsar, or they may end up as black holes.
The basic conclusion is that
· Stars are born, live, and die like everything else ([6], pp134-141).
5.2.3 Star clusters and the Galaxy
Many stars we see around us belong to star clusters [webpage].
They in turn belong to the Galaxy, indeed all the bright stars we see around us
belong to the Galaxy [webpage],
a rotating disk of stars and dust about 60,000 light years across (about 54
thousand billion billion km, compared with the 150 million km from the Earth to
the Sun), with spiral arms surrounding a central region. The disk is visible to
us as the Milky Way [webpage]
(the band of stars clearly visible in the night sky when one is well away from
city lights) [69] [webpage],
[webpage].
Within and around the Galactic disk there are thousands of star clusters [webpage] such as
Messier2 [webpage]
as well as huge clouds of dust and gas. Our Solar System is situated in the
Galactic disk, towards its outer edge, and circles around the centre of the
Galaxy in an orbit that takes about 250 million years to complete.
The Galaxy is held together by gravitational attraction; it does not collapse to the centre because of its rotation There are about 100 billion (100,000,000,000) stars in the Galaxy, each one something like the Sun, often concentrated in star clusters.
Many of these stars may be surrounded by planetary systems like our own Solar System. It is very difficult to detect such planets from Earth, because they do not shine like a star does; their surfaces are only lit by reflection of light from the central star, which will far outshine them. However if there is life elsewhere in the Galaxy, it will have developed on the planets in such planetary systems (the stars themselves being far too hot to support life).
5.2.4 Galaxies
Faint wisps of light we can detect between the stars in the night sky turn out to be other galaxies, that are enormously far away (the nearest is about a million light years away). One of the great triumphs of astronomy is determining the distances of these objects (see [webpage]), thereby showing that they are in fact huge star systems as large as our own Galaxy [69,72], the nearest being Andromeda [webpage]., [webpage]. Indeed our Milky way is a typical spiral galaxy. Like our galaxy, each is held together by gravity and prevented from collapse by rotation (or the orbital motions of its constituent stars around its center).
We can observe the following types of galaxies [webpage], [webpage]:
|
Kinds of Galaxies |
|
elliptical galaxies: 17% |
|
spiral galaxies, barred spirals: 80% |
|
irregular galaxies: 2.5% |
Table 5.5 : Kinds of galaxies.
Ellipticals vary in ellipticity from spherical to very elliptical. Spiral galaxies have a disk, a core (a bulge at the centre of the disk), and a large halo surrounding them both, extending to large distance and containing large amounts of dark matter. The spiral arms maybe tightly would or loosely wound; together with the elllipticals they can be classified in the Hubble tuning fork diagram
[webpage]. Spiral galaxies contain cool gas and young stars, but elliptical contain neither. Many galaxies, including our own, are thought to have massive black holes at their centres. Indeed it has no become clear that many galaxies have high energy processes going on in their cores during some of their life histories, resulting in Active Galactic Nuclei (AGN's) that may be associated black hole formation at the centre.
Galaxies evolve, young blue galaxies being composed of hot young stars which change colour towards the yellow and red as they age. It is thought they may change in type as they evolve. They also undergo collisions with each other often resulting in mergers between two galaxies [webpage]
.
5.2.5 Clusters of galaxies, superclusters, and large scale structures
In the region round us, galaxies are clustered in the local group [webpage], [webpage]. The geography of the local region is beautifully shown at [webpage].
On larger scales, galaxies occur in clusters of galaxies [webpage] , for example Coma [webpage], [webpage] and Virgo [webpage], which in turn appear to form even larger scale structures: superclusters [webpage], which then form huge walls surrounding vast voids [80], giving a foamlike texture on these vast scales [webpage], [webpage].
Associated with these large structures are large scale motions: for example our local group seems to be moving towards the Great Attractor, an overdense region which has exerted a gravitational pull on it sufficient to generate our `peculiar velocity' relative to the universe detected by the Microwave background radiation dipole discussed below.
5.2.6 The Cosmos
We are able to detect about 100 billion galaxies around us , each containing about as many stars as our own but appearing extremely faint and small because of their distance from us. However one should realise that even in the centre of star clusters and galaxy clusters, most of the universe is empty space: there are isolated concentrations of matter (stars and planets) separated by vast distances [webpage]).
We can also detect many other very distant objects such as [70],
· Radio Sources,
· X-ray Sources,
· Quasi-Stellar Objects (QSO's),
· Gamma-Ray Bursters (GRB's).
Many of these may be galaxies at various stages of their evolution. Their astrophysics is under very active discussion.
These objects appear to extend on without end, together forming the Universe, that is, the entirety of that unique interacting distribution of matter and radiation that is the basis of our physical reality [79-81]. Our knowledge of these domains has been vastly extended in the past decades by our extension of observations to all wavelengths [webpage].
As discussed later, the observable region of the Universe is almost certainly a small part of the entire Universe. Cosmology is the subject that studies the distribution and history of matter on these vast scales [webpage].
On the very largest scales (say greater than 300 million light years) galaxies appear to be isotropic about us (that is, the distribution of matter looks the same in all directions we observe). Indeed all the broad features of the Universe appear to be the same everywhere; we cannot for example point to any particular region as being its centre. Consequently we believe matter and radiation are distributed uniformly in space, i.e. the Universe is homogeneous on the largest scales [79,80], so that conditions are the same in every place. The alternative possibility is that we are at the centre of the Universe, which is spherically symmetric about us. This is consistent with the observations, but (for philosophical reasons) is believed to be unlikely [81].
It is difficult not to get overawed and perhaps even dismayed on realising the prodigious number of astronomical objects, their size, and their distance; humanity seems truly insignificant in the face of these vast scales. Certainly the Solar System is physically tiny compared with the size of the Local Cluster of galaxies, which in turn is just a tiny part of the observable region of the Universe; and the Earth is a very small component of the Solar System. However physical size is not the only criterion of importance or of causal significance (this is clearly true on Earth; it is just as true in the Universe as a whole).
Section 5.3 The history and present state of the Universe
The major issue in physical cosmology is, where has all this matter come from, and how has it come to be organised in this particular way? [82,83]. There have been two major viewpoints: that the Universe is unchanging in time, or that it is evolving (i.e. major changes take place in its physical state as time unfolds) [84,85].
5.3.1 The evolving universe
The view put forward by Einstein in 1917 when he formulated the first quantitative universe model was that the Universe is static, that is, is completely unchanging in time (when viewed on the largest scales). This was indeed taken for granted by most of the cosmologists of the period. However it was difficult for them to explain the spectra of light received from distant galaxies, which show systematic redshifts (i.e. a displacement towards the red of spectral lines, the displacement being proportional to wavelength), with the amount of redshift increasing proportionally to the distance of the galaxy, for data and a discussion see [webpage]. Such redshifts do not occur in the Einstein static universe. Furthermore it was shown that the Einstein static universe model is unstable, so one could not expect the Universe to remain in this static state.
Consequently the viewpoint changed [72,85]; the spectral redshifts are now interpreted as being caused by motion of galaxies away from each other (redshifts arising naturally from such a recession). Thus the idea of the Expanding Universe became established [webpage]. The expansion is universal in that every galaxy recedes equally from every other one (there is no centre to the expansion, because the Universe is spatially homogeneous).
At first glance this seems to imply we must abandon the idea that the Universe could be unchanging. However this is not necessarily so, because of one rather exceptional kind of expansion that can occur if the Einstein Field Equations are modified appropriately. This is when the Universe is always expanding, but the rate of expansion and the density of matter are always the same. Clearly this requires a continuous creation of matter to keep the density constant while the Universe continually expands. Such a universe is called a Steady State Universe . For many people this is an attractive possibility, because an unchanging Universe is thought to be "more perfect" than a changing one [79]. However this model cannot explain evidence obtained from optical and radio telescopes that there was a higher density of radio sources and quasars in the past than at present (the densities would have to be unchanging, if the Universe were in a steady state, for then conditions would be the same everywhere); and it does not give a natural explanation of the cosmic background radiation (discussed below). It has therefore been abandoned by almost all cosmologists. The present consensus, then, is that
· The Universe is expanding and evolving . The present expansion phase originated a finite time ago.
It had a higher density of matter in the past (when the galaxies were closer together) than at present [79,86-91]. This is one of the major discoveries of cosmology.
5.3.2 The Hot Big Bang
It has profound consequences: if we apply Einstein's gravitational equations to an evolving universe model, assuming the behaviour of matter is normal, we find that the Universe must have originated in a singular state where the density and temperature were infinite (follow the motion of the galaxies backwards in time; they get closer and closer together until the matter of which they are made reaches an infinite density.
In principle a cosmological constant, that is a uniform force of repulsion proportional to distance and independent of the nature of matter, could avoid this conclusion, but in practice evidence related to the observed background radiation and the largest observed redshifts shows this remedy will not work. That is, the implication is not only that the Universe is evolving, but that it had a beginning a finite time ago . Furthermore, we can determine when this happened: measurement of the rate at which the recession velocity increases with distance ( Hubble's constant ) gives an estimate of about 10 billion years for the age of the Universe.
Because the matter gets hotter and hotter as we go back in time towards this initial state, we can talk of the Hot Big Bang [webpage], and use standard physical laws to examine the physical processes going on in the high density and temperature mixture of matter and radiation in the early Universe [86-89] [webpage]. An important feature is that when the temperature dropped lower than about 3000 K as the Universe expanded, nuclei and electrons combined to form atoms; but at earlier times when the temperature was higher, atoms could not exist, as the radiation then had so much energy it disrupted any atoms that tried to form into their constituent parts (nuclei and electrons). Thus at early times matter was ionised , i.e. it consisted of electrons moving independently of atomic nuclei. Under these conditions, the free electrons interact strongly with radiation. Consequently matter and radiation were tightly coupled together by scattering processes, and the gas in the Universe was opaque to radiation (rather like the interior of the Sun). When the temperature became lower so that atoms formed from the nuclei and electrons, this scattering ceased and the Universe became transparent (today we are able to see galaxies at enormous distances from us). The time when this transition took place is known as the time of decoupling (it was the time when matter and radiation ceased to be tightly coupled to each other).
Radiation was emitted by matter at the time of decoupling, thereafter travelling freely to us through the intervening space. When it was emitted, it had the form of Black Body radiation at 3000K. As it travelled towards us, the Universe expanded by a factor of 1000; consequently by the time it reaches us, the radiation has cooled to 3 K (that is, 3 degrees above absolute zero, with a spectrum peaking in the microwave region), and so is extremely hard to detect. This Cosmic Blackbody Background Radiation (`CBR') was detected in 1965, and its spectrum has since been intensively investigated, its black body nature being confirmed to high accuracy; this is now taken as solid proof that the Universe has indeed expanded from a hot big bang. The radiation is often referred to as Relic Radiation from the Big Bang [webpage], [webpage].
An important feature is its high degree of isotropy: for its temperature is the same in all directions about us, to better than one part in 1,000, with a major hotspot in one part of the sky and cooler part in the opposite direction. We interpret this anisotropy as being due to our motion relative to the cosmos. Subtracting out this motion and the contribution from our galaxy, the background radiation is seen to be isotropic to one part in 10,000. This is the major reason we believe the Universe is uniform and isotropic: any inhomogeneities or anisotropies in the matter distribution lead to anisotropies in this radiation. The very low level of anisotropy is therefore evidence of a very high degree of homogeneity of the universe, on the largest scales (i.e. when we have averaged out the smaller-scale inhomogeneities such as our galaxy and clusters of galaxies).
5.3.3 The history of matter
Before decoupling, the temperature of the Universe exceeded any temperature that can ever be attained on Earth or even in the centre of the Sun; as it dropped towards 3 K, successive physical reactions took place that determined the nature of the matter we see around us today. An important time was the era of nucleosynthesis, the time when the light elements were formed. Above about a billion degrees K, nuclei could not exist because the radiation was so energetic that as fast as they formed, they were disrupted into their constituent parts (protons and neutrons). However below this temperature, once these particles had collided with each other with sufficient energy for nuclear reactions to take place, the resultant nuclei remained intact (the radiation being less energetic and hence unable to disrupt them). Thus the nuclei of the light elements (deuterium and tritium (the heavy forms of hydrogen), helium, lithium) were created by neutron capture.
This process ceased when the temperature dropped below about 100 million degrees K (the nuclear reaction threshold). In this way, the proportions of these light elements at the time of decoupling were determined; they have remained virtually unchanged since. The rate of reaction was extremely high; all this took place within the first three minutes of the expansion of the Universe [86-88]. One of the major triumphs of Big Bang theory is that the predicted abundances of these elements (25% Helium by weight, 75% Hydrogen, the others being less than 1%) agrees very closely with the observed abundances. Thus the standard model explains the origin of the light elements in terms of known nuclear reactions taking place in the early Universe.
In a similar way [89-91], physical processes in the very early Universe (before nucleosynthesis) can be invoked to explain the ratio of matter to anti-matter and the ratio of matter to radiation in the present-day Universe. However other quantities (such as electric charge) are believed to have been conserved even in the extreme conditions of the early Universe, so their present values result from given initial conditions at the origin of the Universe, rather than from physical processes taking place as it evolved. In the case of electric charge, the total conserved quantity appears to be zero: there are equal numbers of positively charged protons and negatively charged electrons, so that at the time of decoupling there were just enough electrons to combine with the nuclei and form uncharged atoms (it seems there is no net electrical charge on astronomical bodies such as our galaxy; were this not true, electromagnetic forces would dominate cosmology, rather than gravity).
After decoupling, matter formed large scale structures (as discussed in the next section) which eventually led to the formation of the first generation of stars. However at that time planets could not form for a very important reason: there were no heavy elements present in the Universe. The material out of which this first generation of stars was made consisted mainly of hydrogen and helium, with a trace of deuterium, tritium and lithium, for this was the result of the process of nucleosynthesis in the early Universe; planets cannot be formed out of these elements alone (nor can complex beings such as humans). The first stars aggregated matter together by gravitational attraction, the matter heating up as it became more and more concentrated, until its temperature exceeded the thermonuclear ignition point and nuclear reactions started burning the hydrogen to form helium (like in the centre of our Sun). Eventually more complex nuclear reactions started in concentric spheres around the centre, leading to a build up of heavy elements (carbon, nitrogen, oxygen for example) [77,78]. These could not form in the early Universe because the whole process there took place so swiftly, but they can form in stars because there is a long time available (millions of years).
Massive stars burn relatively rapidly, and eventually run out of nuclear fuel. At this time a dramatic event happens: the star becomes unstable, and its core rapidly collapses because of gravitational attraction. The consequent rise in temperature blows the star apart in a giant explosion, during which new reactions take place that generate elements heavier than iron; this explosion is seen by us as a Supernova ("New Star") suddenly blazing in the sky, where previously there was just an ordinary star (or perhaps the star was so faint we saw nothing there at all).
This is fundamentally important to us, because such explosions blow into space the heavy elements that had been accumulating in the star's interior, forming vast filaments of dust (such as those we see in the Crab Nebula) around the remnant of the star. It is this material that can later be accumulated, during the formation of second generation stars, to form planetary systems around those stars. Thus the elements of which we are made (the carbon, nitrogen oxygen and iron nuclei) were created in the extreme heat of stellar interiors, and made available for our use by supernova explosions. Without these explosions, we could not exist.
Overall, we obtain a dramatic and compelling picture of the rapid evolution of the primeval fireball that comprised the early Universe, cooling down from extreme temperatures as the matter and radiation expanded [webpage]. The primordially existing gas, initially composed of an equilibrium mixture of particles and radiation, underwent a series of irreversible processes: baryons (essentially, protons and neutrons, providing the materials to make nuclei) were synthesized out of the elementary particles that dominated the earliest epochs; light nuclei were synthesized out of the baryons; decoupling of matter and radiation took place; first stars formed (powered by gravitational attraction). Heavy elements were formed in first generation stars. In this way we can understand the origin of the matter around us, and the chemical elements on Earth and in the Sun.
5.3.4 Dark Matter
It is easy to conceive of matter that is hard to detect (for example, small rocks distributed through space); the data mentioned above implies the existence of huge amounts of dark matter, dominating the dynamics of the Universe [webpage].
Originally this high density was thought to be prohibited because it would ruin the very good agreement of nucleosynthesis calculations with observations. However it has been pointed out this is not necessarily so if the dark matter is non-baryonic, that is it is made of exotic particles (massive neutrinos, magnetic monopoles, axions, and so on) whose existence is predicted by elementary particle theory, rather than the protons and neutrons that are the substance of ordinary matter. A large number of proposals have been made for such dark matter candidates [90,91]. An important distinction is whether they are massive so that they cooled down early, thereafter forming cold dark matter, moving slowly at the time of galaxy formation (and resulting in a bottom up process of large scale structure formation), or have a low mass and cooled slowly, therefore for a long time forming hot dark matter, moving very fast at the time of galaxy formation (and resulting in a top-down galaxy formation scenario) [73,74,90]. Galaxy formation studies support the cold dark matter hypothesis.
5.3.5 The Spatial Curvature of the Universe
It is usually supposed that the inflationary model (discussed above) necessarily implies that the Universe expanded so much at early times as to drive the total density parameter (the value of W due to all sources - visible matter, cold dark matter, and the cosmological constant) arbitrarily close to 1, so that it still remains very close to that critical value today (the theory does not say if it should be ever so slightly over or under this value). That prediction is consistent with the present data which suggests the total value of W is close to 1 (the contribution from matter being about 0.3 and that from the cosmological constant being about 0.7)
Now an important feature of a high-density universe, i.e. one with total W greater than 1, is that it necessarily has closed (finite) spatial sections [31]. If we set off in any direction in space and just keep going, we would end up back where we started, rather like happens on the surface of the Earth when we steer an undeviating course (but the surface of the Earth is a two-dimensional surface; in these cosmologies, the same effect occurs in the curved 3-dimensional space sections of space-time that are surfaces of constant time for all the fundamental observers [31]). There is therefore necessarily a finite amount of matter in such a universe, and a finite number of galaxies exist. By contrast, in universes with total W less than 1, unless the connectivity of the space-sections is unusual (this case will be discussed in the following chapter) the space sections are infinite. When one considers this carefully, it is a rather difficult feature to come to terms with: there are an infinite number of galaxies in such a Universe, an infinite amount of matter is created at the Big Bang, there an infinite number of living beings in it.
Albert Einstein and John Wheeler have pointed out other advantages of closed space sections, specifically that this does away with the problem of setting boundary conditions for physical fields at infinity (for there is no spatial infinity in such Universes). Wheeler claims these advantages are so overwhelming that such Universes are necessarily to be preferred over all the others, indeed that the Universe must be this way.
The present data do not tell us if this is so or not - while W is close to 1, it is unclear if it is greater or less than this value (some CBR data suggested it was about 1.06, but later data puts it closer to 1 and even perhaps less than1). Strong as the boundary condition arguments for a closed space section are, they have left many physicists unconvinced; furthermore there are some low density Universes that also have closed space sections. The issue must be regarded as unresolved: it remains one of the major challenges to cosmologists to determine if the Universe has closed space sections, or not. If we can determine the total effective density of matter and the cosmological constant accurately enough, it will give us the answer.
Section 5.4 The origin of structure
While the basic Big Bang picture is clear and is strongly supported by evidence, the way astronomical structures have arisen is not so clear.
5.4.1 The universe
At the very largest scales, the question is whether the Universe started in a very smooth state and then developed inhomogeneity, or started off very inhomogeneously and then became very smooth. Either way, our problem is to understand how the Universe at present appears so smooth on the very largest scales (particularly as evidenced by the isotropy of the microwave background radiation), and yet has complex structuring on all smaller scales, as outlined in the previous section.
The original viewpoint was that the Universe started off very smoothly, that is the density and temperature were almost uniform everywhere. Then small perturbations grew by gravitational attraction to form the large-scale structures we see. However the origin and nature of the required perturbations was a mystery, as was the reason the Universe appeared so smooth on the very largest scales. Consequently the Chaotic Cosmology idea was advanced, assuming that the Universe started of very inhomogeneously in the large but then physical processes caused it to develop into a very smooth state overall. Initial work on this idea by Charles Misner and others suggested this could happen through the smoothing effects of viscosity; however this did not seem to really work.
5.4.2 The Inflationary Universe
This idea received tremendous impetus when Alan Guth and others pointed out that if a particular form of matter (a scalar field, associated with particles without spin) dominated the expansion of the early Universe, there could be an enormous expansion ("inflation") in a very small period before nucleosynthesis - an expansion much more rapid and of a much greater magnitude than would occur in an ordinary Big Bang model [20,88-91].
The result of this expansion is a dramatic smoothing of the largest scale inhomogeneities - like a balloon being blown up, becoming smoother and flatter as it gets larger and larger. Thus this inflationary Universe idea [webpage] explains the overall smoothness of the Universe as the result of physical processes blowing up a very small region to become very large, thereby smoothing it out [webpage]. This has caused much excitement and a dramatic surge of activity while the consequences of the idea have been worked out. It has in particular led to an interesting proposal: the seeds that form galaxies later on could come from quantum fluctuations in the very early Universe, blown up to enormous size by the expansion that takes place during inflation (superimposing a small roughness on the basic smoothness). This enables us in principle to link the present distribution of galaxies with the properties of elementary particles in the very early Universe: a truly remarkable claim.
While the idea has many supporters, it is not without problems, in particular the failure to identify the field which is supposed to underlie inflation. Furthermore Roger Penrose [34] argues strongly that this model cannot be correct because of the Arrow of Time problem (discussed below), which he argues demands a smooth start to the Universe. This basic issue is at present unresolved. However the predictions of inflation for structure formation and associated Cosmic Background radiation anisotropies have been very successful, so most cosmologists believe some form of inflation took place, even if we do not know what caused it.
5.4.3 The formation of large scale structure
Assuming the Universe has reached a state where it is very smooth in the large, the next question is how galaxies and larger scale structures arose from initial seeds of inhomogeneity. Here again we have a basic dichotomy between two different approaches [20,83,90,91]. On the one hand it is possible these structures were created top-down, with the largest structures (walls and voids) forming first, smaller structures (clusters of galaxies and then galaxies) then coalescing out. On the other hand there could have been a bottom-up process, whereby globular clusters of stars formed first, and then later aggregated together to form the larger structures (galaxies and then clusters of galaxies). The first is more likely if the Universe is presently dominated by Hot Dark Matter, and the latter if it is dominated by Cold Dark Matter (discussed below). The latter seems more likely, and is the currently favoured model for structure formation.
The main question is whether the basic mechanism underlying the start of formation of structures on these scales is gravity alone, or if other processes were significant. For example, vast explosions in the very early Universe, like Supernovae, could possibly sweep up matter around them and so explain the voids; or the creation of vast sheets of galaxies could perhaps be explained by exotic structures known as cosmic strings (energy concentrated on extremely thin string-like regions) - if they exist. However these possibilities are not favoured by observations. Once structure has started to form by whatever method, gravity will then amplify the inhomogeneities, in particular forming galaxies.
On the inflationary universe picture, initial quantum fluctuations at the start of inflation get blown up to very large scales by the inflationary expansion and following expansion of the universe during the hot big bang era and after - they become the seeds for structure formation on galactic scales. During the hot big bang era matter attracts other matter and tries to create ever higher densities, but it is tied in to the radiation by scattering of radiation off electrons. The radiation pressure resists collapse, and on smaller scales causes oscillations (giving `acoustic waves') as it resists the gravity, whereas on large scales gravity wins and density inhomogeneities grow . After decoupling the matter and radiation proceed on their separate ways, the matter now collapsing unimpeded to high densities and the formation of stars. The density inhomogeneities on the surface of last scattering cause gravitational redshifts and so result in the CBR radiation temperature being slightly lower in some directions than in others. A vast amount of work has been put into calculating the resultant pattern of temperature anisotropies on the sky and comparing them with measurement. This project has been a tremendous success- the predicted peaks in the radiation power spectrum, associated with the acoustic waves at the surface of last scattering, have indeed been observed as predicted. Detailed measurement of the shape and angular size of these peaks provides detailed information on the structure formation process in the early universe and also on the major parameters describing the universe - for example the density of matter present in it.
5.4.4 The formation of the Earth
Within galaxies, first generation stars form by gravitational attraction; some of them die as supernovae, and then second generation stars form from the debris, sometimes with planetary systems (as discussed above). Although the broad outlines of the basic process are reasonably clear (see [webpage]), we are still uncertain about the detailed nature of the creation of the Solar System, with two competing theories. One is that the Solar System had a hot origin, the planets being formed from hot gas which coagulated into a disk rotating around the Sun, and ever since has been cooling down, forming the solid planets as molten material cooled down. The other is that the planets had a cold origin, forming from dust accumulating and eventually heating up to give the molten core at the centre of the Earth. Whatever its mode of origin, the Earth was formed about 5 billion years ago, at about the same time as the Sun (the oldest rocks we have found, have been measured to be this old).
5.4.5 The probability of life
One of the most interesting questions we can ask is whether or not there is life on other planets in our Galaxy, or elsewhere in the visible Universe. This has been the subject of ongoing debate, with some protagonists believing it is very probable, so that there are likely to be millions of planets in the Galaxy with life on them; while others maintain we are the only self-conscious beings in the entire visible Universe [93-95].
The problem in making the estimates is the many uncertainties not only in the probability of formation of a planet like Earth, but of the evolution of life (first single celled forms, and then more complex forms) once conditions on the surface of the planet had settled down enough for living systems to have a chance to survive. A view held strongly by some is that there are so many improbable steps in the evolution of complex life forms, that life cannot exist anywhere else in the Universe except on the Earth. In biological terms this places the human race at the centre of the Universe, by stating that the Earth is the one unique planet on which intelligent life has evolved (despite the fact there are something like ten thousand billion billion stars in the observable region of the Universe - and could be an infinite number in the unobserved regions).
In my view we should resist this return to a pre-Copernican view, with ourselves the only conscious beings in the entire Universe and therefore the highest state of organisation that exists, unless it is absolutely necessitated by proper estimates of all the probabilities involved; however these simply are not known at present. Until we are forced to conclude otherwise, I would suggest the presumption must be that we are not unique; the laws of physics and chemistry that created life here would also have done so elsewhere, given this vast array of stars (a reasonable proportion of which must have planetary systems where the same processes that operated to create life here would be at work). Indeed many agree that there is a good probability of extraterrestrial life in our galaxy and in others [webpage], but not necessarily near enough to communicate with.
A final comment on this topic: much has been made of the argument that there cannot be intelligent life elsewhere in the Galaxy that has reached a technological stage, for if there were they would already have visited us [95]. In my view this is an interesting argument but should not be confused with a scientific proof (as some have tried to present it). It has at least two major flaws: firstly, the presumption that we know what the intentions of such other intelligent beings would be. Other beings with the capability to do so might choose to spend time and effort on exploring the Galaxy, but then again they might not. The attempt to claim they inevitably would do so is sociologically and psychologically naive (indeed they might have decided to save themselves the time and expense of doing so precisely because one of their logicians had come up with this very argument, and thereby proved there was no purpose in exploring the Galaxy!). Secondly, it would take a minimum of 200,000 years to explore the Galaxy (and to do so thoroughly could take enormously longer); so even as we are proclaiming they do not exist, they may be heading towards us to investigate the radio signals we have been broadcasting for the past sixty years. Even disregarding the sociological issues, the `proof' cannot be rigorous. If there is any group of beings in the Galaxy that, having reached an advanced technological stage, decides to explore the Galaxy, then there must be a first group to do so; they are the first beings to set out to explore the galaxy. The argument is clearly fallacious when applied to that first group.
5.4.6 The Propensity to Complexity
Despite what is often said in discussions of the Second Law of Thermodynamics, claiming that left to itself matter will always tend to a state of increasing disorder, the opposite is in fact shown in the natural history of the universe. By a spontaneous process, first ordered structures (stars and galaxies) grew through the process of gravitational attraction; then second generation stars with planets arose and provided the habitat for life, which also apparently initiated itself through the long term operation of natural physical and chemical processes, leading to the appearance of biomolecules and then the first living cells, these in turn leading to multi-cellular organisms and eventually emergence of intelligence.
Thus there is in fact an extraordinary propensity of matter to create order, spontaneously generating higher and higher levels of hierarchical structure and concomitant emergent order. This is only possible because of the particular laws of physics and chemistry that operate in the universe, on the one hand, and the specific environmental conditions created by the initial conditions for the universe, on the other.
The theme of how this comes to be the case will be taken up again in Chapter 7.
5.4.7 The structural options
Overall, we see in this discussion much more uncertainty than in the previous two chapters. The reason is straightforward: we are here dealing with historical science, rather than analytic or synthetic science (cf. Chapter 2). Thus there are a series of major dichotomies (Table 5.6); in each case we have many studies of possible modes of evolution, and indications of which is correct, but no final answers.
|
Structural Options |
||
|
Structure |
Major Options |
|
|
The universe as a whole |
From smooth to structure |
From chaotic to smooth |
|
Large scale structure |
Top-down (hot dark matter) |
Bottom-up (cold darkmatter) |
|
The solar system |
Hot origin |
Cold Origin |
|
Intelligent life |
Very rare |
Quite Common |
Table 5.6 Possible options for development of structures .
However despite these uncertainties, we have attained a good degree of certainty about many aspects of what has happened. One feature is well recognised: it is gravity that primarily powers the generation of astronomical structures [34]. The objects about us are essentially the results of non-equilibrium events as the universe expanded from a relatively uniform state (at the time of decoupling) and matter attempted to cluster itself more and more because of gravitational attraction. with various forces successively slowing the process down ([34], Dyson in [35]). Thus while the general tendency implied by the Second Law of Thermodynamics is towards disorder, gravitation creates structures at the astronomical scale, and evolution does so in the biological world.
Section 5.5: Final states
Having attained a partial vision of origins, the issue of endings or final states is also of interest. Predicting the future is of course even more hazardous than charting the past. Nevertheless there are some things we can say about it with a high degree of certainty, and some cases where we can clearly lay out a number of major options with quite different implications.
5.5.1 Stars and Black Holes
One thing we can say with absolute certainty is that the Sun is going to stop shining. The reason is that it has a finite stock of nuclear fuel available, which will eventually run out. The more massive a star is, the more quickly it evolves to its end-state; our own sun is not very massive (as stars go), and will probably survive for another thousand million years or so.
·
It is certain that our Sun will come to the end of
its life, on an astronomical timescale.
·
Then the Earth will become uninhabitable.
At that point, as is the case with any star, there are a number of options available [77,78]. Depending primarily on its mass, a star can
(1) just fizzle out, ending up something like the planet Jupiter;
(2) have a dramatic collapse (perhaps involving an explosion we see as a Supernova), and end up at a very condensed stable state, becoming either
- a white dwarf (a star supported by the degeneracy pressure that results from the Pauli exclusion principle) or
- a neutron star (where the whole stellar interior is like one giant nucleus); or
(3) collapse to a black hole.
A Black Hole [webpage] occurs when space-time becomes so highly curved, due to the high gravitational fields involved, that it traps everything in its vicinity and drags them into itself, arbitrarily large tidal forces then destroying anything that falls in [31,34,79]. As light tries to escape from the interior of a black hole it is pulled back by the massive gravitational field, and falls in; consequently we cannot see to the interior (light cannot convey information to us from the interior, because it cannot reach us). Classically considered, at the centre of the black hole there is a breakdown of the structure of space-time itself (a space-time singularity); however this is hidden from us by the event horizon (the surface separating events from which light can escape the black hole, and those from which it cannot). A black hole eventually radiates its mass away as black-body radiation, because of a remarkable quantum mechanical process discovered by Stephen Hawking, the rate of radiation depending on its mass.
We have seen many white dwarfs and neutron stars, the latter being detected as pulsars like that at the centre of the Crab Nebula, emitting periodic radio signals with incredible precision. Precisely because of their nature, black holes are difficult to detect; however infalling gas will get heated up until it emits radiation which can be detected, and we can also hope to detect them by their gravitational effects. On theoretical grounds we believe there are some hundreds of thousands of black holes in our galaxy, and some reasonably good black hole candidates have been identified, for example the X-ray source Cygnus X-1; there may even be a massive black hole at the centre of the Galaxy. Evidence that these identifications are correct is strong, but not overwhelming.
If a black hole came near the Earth, the effects could be devastating; however there is no reason to believe there is one near enough us to pose any threat. The asteroids in the Solar System are far more problematic (the craters on the Moon are evidence of massive bodies colliding with it in the past), but we have not detected one on a collision course with the Earth. The real problem for the Earth (in the long term) is that when the evolution of the Sun is very advanced, it will probably expand to become a Red Giant, enveloping us in its hot atmosphere; if this gas does not extend far enough to vaporise the Earth's atmosphere, seas, and surface, then the next phase will do so, for the Sun will probably then blow off its outer layers in a massive explosion (forming a Planetary Nebula) while the core collapses to form a white dwarf star. The eventual cooling of the remnant of the Sun will cause any surviving planets to become bitterly cold, their heat source having died out; they will eventually cool to a few degrees centigrade (the temperature of the background radiation). This scenario is fairly certain: the time when it will occur is not well determined, but is certainly a very long way off.
5.5.2 The Universe
The final state of the Universe itself has also been the subject of speculation and investigation [96,97]. There are essentially two options for the present phase of expansion. Either
· the Universe will continue to expand forever, or
·
it will reach a maximum size, and then recollapse to
a hot dense state in the future.
In the first case as the Universe continues its eternal expansion, all the stars and galaxies eventually cool down as all the nuclear fuel is used up, the stars running their life cycles and ending up as white dwarfs, neutrons stars, or black holes. These themselves are probably all unstable too in the long term; not only the black holes radiating away by the Hawking process, but also all the other stellar remnants decaying (because baryons are unstable on a very long timescale). This is the first form of heat death envisaged for the Universe (it should be called a cold death, but this name became established in the 1930's when the possibility was first discovered): the Second Law of Thermodynamics triumphs as everything eventually cools down and radiates away, leaving nothing but the emptiness of interstellar space, with even the last glimmers of light from cooling neutron stars eventually fading out to leave an eternal blackness.
In the second case, by contrast, as the Universe collapse, everything heats up indefinitely and we have in effect a re-run of the Big Bang, but run backwards in time. The matter and radiation in the Universe get hotter and hotter, all the structures that have been built up get destroyed by the increasingly hot radiation (remaining stars explode because they can no longer radiate away their energy to the sky, which is hotter than their surfaces; molecules, atoms, even nuclei are decomposed into their constituent particles by the ceaseless bombardment of radiation). Entropy triumphs as the Universe races to increasing disorder, and accelerates towards a space-time singularity in the future, or at least a region where classical physics breaks down completely because of the extreme conditions prevailing. This is a heat death of the second kind. It could not occur before the Universe is at least ten times as old as the present, and could conceivably take much longer than that to occur; however once started, the final phase (like the Big Bang) will be extremely rapid.
Which of these will actually happen? This has been one of the great unsolved questions in cosmology . The issue hinges firstly on how much matter there is in the Universe [90,91], and secondly on the possible existence of a cosmological constant L. We look at these in turn.
Suppose there is no cosmological constant. If there is a lot of matter present (in terms of the density parameter W measuring the density relative to the square of the Hubble constant, this is when W is greater than 1), then the gravitational attraction of matter will eventually win over the kinetic energy of the expansion: the Universe will halt its expansion and recollapse. If W is less than 1, the kinetic energy will win and the Universe will expand forever. When W equals 1, there is a critical density of matter present; in this borderline case, the matter just succeeds in expanding forever, but only just. Thus the question is, what is the value of W ?
The visible matter in the Universe corresponds to a value for W of about 0.04, and dark matter whose existence can be deduced from its observed gravitational effects amounts perhaps to W of between 0.2 and 0.4, with a `best value' of about 0.3. Taken at face value, this implies we live in a low density universe that will expand forever - there is not enough matter present to slow down the expansion and turn it into a collapse.
The possible existence of a cosmological constant has been a contentious issue since the start of quantitative cosmology. In recent decades it has usually been assumed that though it was a theoretical possibility, it in fact did not occur. However that situation has dramatically changed in the past few years, because of observations of distant galaxies that have use supernovae observations to determine their distances. This has been possible firstly because of much improved observations due to new telescopes and detectors being employed, but also because it was realised that if one measured the decay rate of the light from type Ia supernovae one could determine their intrinsic luminosity. This mean they could be used as standard candles.
Two observational teams have carried out extensive searches for such supernovae and measured their light curves very accurately. As a result, it has become apparent that the expansion of the universe is now speeding up rather than slowing down. That means there must be a cosmological constant present, or else an effective scalar field (called "quintessence") that has a similar dynamical effect; and this has an effective value W of about 0.7, i.e. it is presently twice as important dynamically as matter . This greatly strengthens the conclusion above:
·
It is virtually certain that the universe will
expand forever both because there is not enough matter to cause a recollapse and
because a cosmological constant aids the expansion.
5.5.3 Phoenix Universes and Darwinian Evolution?
The major further question is whether the process of expansion only happens once in the life of the Universe, or occurs repeatedly.
The first option is the standard model, where the entire evolution of the Universe is a once-off affair, with all the objects we see, and indeed the Universe itself, being transient objects that will burn out like dead fireworks after a firework display. In this case everything that ever happens occurs during one expansion phase of the Universe (possibly followed by one collapse phase, as discussed above, but made unlikely by the data).
The alternative is that many such phases have occurred in the past, and many more will occur in the future; that is, the Universe is a Phoenix Universe, new expansion phases repeatedly arising from the ashes of the old. This could conceivably occur in a spatially homogeneous way, the entire Universe undergoing successive phases of expansion and collapse (demanding a high-density with W greater than 1 to cause the collapse), followed by re-expansion; or it could occur inhomogeneously, with small parts of the Universe collapsing and then rebounding to form the seeds of vast new expansion phases. The data make the former option unlikely (that is, the entire universe recollapsing and then bouncing), but the idea of local collapses and re-expansion remains open. Such cyclic universe models combine the advantages of predicting hot big bang evolutionary phases, consistent with our present observations, but also potentially having an eternal character by allowing an overall repetitive pattern of behaviour that might possibly go on for ever. A variant is the chaotic inflation idea of new expanding universe regions arising from vacuum fluctuations in old expanding regions, leading to a universe that has a fractal-like structure at the largest scales, with many expanding regions with different properties emerging out of each other in a universe that lasts forever.
While the idea of one or more bounces is an old one, repeatedly proposed in various forms, actual mechanisms that might allow this bounce behaviour have not yet been elucidated in detail in a fully satisfactory way. If we consider a collapse phase in the future, it is possible that eventually (when conditions are very extreme, for example much hotter than the temperature of nucleosynthesis) quantum fields would eventually dominate and turn the collapse around. Then because of such quantum processes the local collapse of matter to form a black hole will result not in a singularity, but in a re-expansion into a new Universe region to form the seed for a new big bang phase. If so, this process would not be visible from the old universe region (it would be hidden behind the event horizon), so as the old expansion phase decayed away, the new Universes it was creating would be invisible daughters creating new life out of the death of the old.
The proposal remains a hope rather than an established theory. However if it becomes properly established, it opens the way to the concept not merely of evolution of the Universe in the sense that its structure and contents develop in time, but in the sense that the Darwinian selection of Universe models (or rather, of expanding universe regions) could conceivably take place. The idea, proposed by Lee Smolin, is that there could be collapse to black holes followed by re-expansion, but with an alteration of the constants of physics each time, so that each time there is an expansion phase, the action of physics is a bit different. The crucial point then is that some values of the constants will lead to production of more black holes, while some will result in less. This allows for evolutionary selection favouring the expanding universe regions that produce more black holes (because of the favourable values of physical constants operative in those regions), for they will have more daughter expanding universe regions. Thus one can envisage natural selection towards those regions that produce the maximum number of black holes. The idea needs development, but is very intriguing, uniting as it does two great concepts of modern scientific understanding: the expanding universe and the evolution of populations.
These are all fascinating speculations which could conceivably be correct, but they have no experimental basis. The theory that predicts any particular one will have to be extremely convincing, if it is to be generally accepted.
Section 5.6: Cosmic options
The overall possibilities for the Universe can be divided into four cases, as shown in Table 5.7.
|
Options for evolution of the universe |
||
|
Unchanging |
Static
|
Steady
State |
|
Never expands/contracts |
Always expanding |
|
|
Evolving |
Big
Bang |
Phoenix |
|
One expansion phase |
Cycles of expansion/contraction |
|
Table 5.7: Possible evolutions of the Universe
The possibility of an unchanging universe appears to be ruled out by the evidence, although many people find this philosophically attractive. There may or may not be many expansion phases; this depends on operation of physical processes we do not understand well. However we do understand quite well much of the present expansion phase of the Universe [webpage]:
As the Universe expanded from its hot early phase, physical processes led
to the production of the light elements and then of large scale structures,
galaxies, stars, and the Solar system, so eventually providing the habitats in
which our life evolved. The Cosmic Microwave Radiation is fossil radiation left
over from the hot early phase.
The physical scale of the Universe is enormous, and the images of distant objects from which we obtain our information are extremely faint. It is remarkable that we have been able to understand the Universe as well as we do. However there is still much to do, see [webpage].
References for Chapter 5: The Physical Universe
The nature of Astronomical discoveries is presented in
[69] T P Snow: Essentials of the dynamic Universe (West Publishing Company, St. Paul, 1987)
[70] N Henbest and M Marten: The New Astronomy. (Cambridge University Press, 1983).
The historical struggle to understand the solar system is described in
[71] A Koestler: The sleepwalkers (Penguin, 1959)
and the history of the major discoveries in astronomy in
[72] M Harwit: Cosmic Discovery: The search, scope and heritage of astronomy (Harvester Press, 1981).
The recent search for understanding of cosmology is presented in
[73] D Overbye: Lonely hearts of the cosmos: The scientific quest for the secret of the universe (Harper Collins, 1991)
[74] A Lightmann and R Brawer: Origins: The Lives and Worlds of Modern Cosmologists (Harvard University Press, 1991).
Basic astrophysics is discussed in [20] and
[77] I Shlovsky: Stars: Their Birth Life and Death (Freeman, 1978)
[78] F Hoyle and J Narlikar: The physics-Astronomy frontier, (Freeman, 1980)*.
Cosmology is presented in a masterly survey in
[79] Ted Harrison: Cosmology: the science of the universe, (Cambridge University Press, 2000),
with recent observations summarised in
[80] J Cornell (Ed.): Bubbles voids and bumps in time: the new cosmology. (Cambridge University Press, 1989),
and interesting topics discussed in
[81] J Leslie: Physical Cosmology and Philosophy. (MacMillan, 1990).
A general overview of the origins of the solar system and life is given in
[82] J G Eccles: The Human Mystery (Routledge and Kegan Paul, 1984),
[83] A Fabian: Origins (Cambridge University Press, 1988)
(and see also [78,93,95]) while changes in paradigms in cosmology are discussed in
[84] Ted Harrison: Masks of the Universe (Collier Books, New York, 1985)
[85] G F R Ellis: The transition to the expanding universe. In Modern Cosmology in retrospect, Ed. B Bertotti, R Balbinot, S Bergia and A Messina. (Cambridge University Press, 1990), 97-114.
The physics of the hot big bang early universe is presented in
[86] D W Sciama: Modern Cosmology (Cambridge University Press, 1971)
[87] S Weinberg: The first three minutes: A modern view of the origin of the Universe (Basic Books, 1977)
[88] J Silk: The Big Bang (Freeman, 1980).
Recent developments in cosmology are discussed in [20] and
[89] H Pagels: Perfect Symmetry (Penguin, 1985)
[90] L Krauss: The Fifth Essence The Search for Dark Matter in the Universe (Vintage, Basic Books, London, 1989)
[91] J Gribbin and M J Rees: Cosmic coincidences (Black Swan, 1991).
The quantum creation of the universe is discussed in [20] and
[92] S Hawking: The Universe in a Nutshell (Bantam, 2001).
The issue of how many civilisations there are in the universe is considered in
[93] I. S. Shlovsky and C. Sagan: Intelligent life in the universe. (New York: Dell, 1966).
[94] D Goldsmith The Quest for extraterrestrial life: a book of readings (University Science Books, Mill Valley, California, 1980)
[95] J D Barrow and F J Tipler: The Anthropic Cosmological Principle (Oxford, Oxford University Press) 1986*.
The final fate of the universe is discussed in
[96] J N Islam: The ultimate fate of the universe (Cambridge University Press, 1983)
[97] J Gribbin: The Omega Point (Heinemann: 1987).
[Version 2002-07-29].
Chapter 5 of Cosmology Explained by George Ellis