Before we enter this long page, the writer (NMS) would like to present his own definition of a word - perhaps setting a precedent. Many readers are familiar with the famed astronomer Carl Sagan's TV series on the "Cosmos". But it is not easy to find a good working definition of that term that all agree on. Typing in the word on Google led to many entries not related to astronomy. The Wikipedia entry gave this information (reproduced here as two italicized parts extracted from that Website):
In physical cosmology, the term cosmos is often used in a technical way, referring to a particular space-time continuum within the (postulated) multiverse. Our particular cosmos is generally capitalized as the Cosmos. The philosopher Ken Wilber uses the term "kosmos" to refer to all of manifest existence, including various realms of consciousness.
Using this for support, we will define this use of "Cosmos" in this way: Everything that can be conceived to exist in a real and physical way which includes all that is within our Universe, any other Universe (Multiverse concept) and any of the vacuum containing virtual particles that fills the (non)space between the (possibly infinite number of) multiverses. Thus, when we refer specifically to the "Universe", we will mean this (our) Universe; when we refer to the Cosmos, we are taking into account the speculation by scientists, and by science fiction writers, that there can be more than one Universe, even though at present there is no way to prove the actual existence of anything beyond the limits of our observations through telescopes. So far then, the notion of any physical reality beyond these limits is conceptual rather than factual.
While we are redefining terms, lets consider "space" itself. The Space Program in its broadest connotation refers to monitoring activities connected with Earth, to the exploration of the Solar System, and to the observations of the farthest reaches of the Cosmos (at least to the detectable photons making up the Cosmic Background Radiation and to the first stars [or more properly the first galaxies since individual stars at great distances cannot be resolved]). Space in a "geometric" sense is easily visualized as that which includes all detectable cosmic objects (stars, galaxies, gas clouds, etc.) and everything between any two such objects. The "between" however is not empty. Even if nothing can be detected by present means, quantum theory holds that such space contains vacuum energy and even virtual particles which can "pop" in and out of active existence. In this sense, space is that which resides within the Cosmos regardless of whether there are any markers that establish the "between". It would not be surprising if such 'space' is eventually shown to be infinite.
For now, all we can talk about from direct contact is the Observable Universe. The subject of the Observable Universe will be explored several times in this Section. For those anxious for a preview, check out this Wikipedia website.
Before beginning this Section, we urge you to read through a page called the Preface (once there, hit your BACK button on the browser you use to return to this page). The Preface contains four major topics: 1) the role of remote sensing in astronomy; 2) some suitable references for additional information; and basic principles of 3) Relativity, and 4) Quantum Physics. The Preface contains a list of some very readable books and a number of Internet links to reviews or tutorials on Astronomy/Cosmology. Also, most of the illustrations in this Section were made from images and data acquired by spaceborne Observatories. A brief description of those Observatories is given on this Wikipedia website. Most of the ground-based Observatories are listed in this Caltech site.
As we did in Section 19, we begin with this statement: Astronomy and Cosmology depend almost entirely on remote sensing technology (mainly telescopes with various sensors) to gather the data and mold these into information about every thing in space beyond our Solar System.
Cosmologists - those who study the origin, structure, composition, space-time relations, and evolution of the astronomical Universe (and the possibility of a Cosmos as defined above) - generally agree that the Universe had a finite beginning and that it is expanding at a steady rate so that any two points (e.g., galaxies) move away from each other at speeds proportional to their separation. (The expansion of space has been referred to as the Hubble Flow, to honor Edwin Hubble who first verified the expansion). This beginning is commonly referred to as the Big Bang, which is not an explosion in the sense of, say, the detonation of dynamite but is an "explosion" of space itself as a continuing expansion accompanied at the outset by the creation and release of all energy and matter now occupying the ever growing Universe. (The Big Bang received its descriptive name as a disparaging comment from the astronomer Fred Hoyle, who advocated instead an infinitely large Universe of constant matter density [requiring continuous creation of new particles to maintain the density even as the Universe expanded within its infinite limits] as described in his [now rejected] Steady State model [developed in consort with Hermann Bondi and Thomas Gold]. This model also infers its Universe to have always existed [no creation event] and will exist largely unchanged [except for its expansion] forever; variants of this and other models have been put forth, as described on page 20-9).
As of 1990 the time of the Big Bang had been placed between 12 and 16 Ga ago (Ga = 1 billion years [b.y.]) ; the current best estimate (derived from observations made by the Hubble telescope and WMAP [a cosmic background radiation satellite]) lies close to 14 Ga (13.7 Ga is now recognized as the most accurate value [see page 20-9]). This is derived by measuring the time needed for light to have traveled from the observable outer limit of the Universe to Earth in terms of light years *, which can be converted to distances. In a sense, the term "light year" has a dual meaning. Thus, when the value of 2000 light years is stated for a star or galaxy, one could think in terms of distance: the entity is 2 x 103 x (3600 x 24 x 365.4 [the number of seconds in a year] x 2.998.... x 108 m/sec (see first footnote *), approximately 11.8 quintillion kilometers, away from the Earth as the observing platform. Or, one might think in terms of age: relativistically, we see the entity as it was 2000 years ago when the light first left it; cosmically we always look back in time when observing stars and galaxies. Both distance and age are valid connotations.
At this outset, let us define the term "Universe". The (this; ours) Universe will be specified as everything that lies spatially within the outermost limit of matter and energy that has participated in the expansion of Space since the moment of the Big Bang. In this definition, the Universe (the one we live in; in principle, there may be other Universes [see page 20-10]) is finite in both space and time (note: it had a beginning and seemingly will last in some state for many billions of years to come [possibly infinitely]). This Universe is said to be homogeneous and isotropic. Homogeneity means that the entities involved are the same in all locations. Isotropy means that the entities are the same in all directions. These terms imply uniformity at some scales - generally large (cosmic). Thus, the Universe would appear much the same at any point within it. If we were to observe the Universe around us from a planet in some other galaxy, we would see generally the same set of physical conditions and the same general appearance and distribution of other galaxies elsewhere in the Universe as we now actually do from Earth. This must be modified by the scale of observation. The Universe shows apparent inhomogeneities, such as clumping of energy and clustering of galaxies, in regions that are less than about 200 million light years in size. But at larger scales the Universe approaches a more uniform or smooth status. (A broader meaning often applied to "Universe" holds it to include all that can be conceived to exist either physically and/or metaphysically; but as stated above we prefer using the term "Cosmos" for this idea.)
The Cosmological Principle, which is deducible from the postulates of homogeneity and isotropy, states that the Universe will look the same no matter where the observer is located within it. A corollary of this states that there is no real center for the Universe. But an observer at any location may think he/she is at the center. That notion as applied to Earth dwellers persisted until the 16th Century when Nicolaus Copernicus presented arguments that negated the geocentric view favored by philosophers and theologians and replaced it with the heliocentric view (the Sun is the center for the planets). (Galileo got into deep trouble with the Catholic Church for his support of Copernican centricity). The Sun was dismissed as a candidate for the Universe's center when, first, its place in the Milky Way was determined to be about a third of the way out from our galaxy's center, and then the galaxy itself was shown by Edwin Hubble in 1923 to be just one of many both nearby and far away from the Sun's galaxy.
The physical conditions that guaranteed the present Universe must have burst into existence almost instantaneously. During the first minute of the Universe's history, many of the fundamental principles of both Quantum Physics (or, as applied to this situation, Quantum Cosmology) and Relativity - the two greatest scientific discoveries of the 20th Century (see Preface, accessed by link above) - played key roles in setting up the special conditions of this Universe that have been uncovered and defined in the 20th Century. Quantum processes were a vital governing factor during the buildup and modifications of the particles and subparticles that arose in the earliest stages. Likewise, Relativity influenced the space-time growth of the Cosmos from the very start.
In the most widely accepted current model of the Universe, there is no starting place or time in the conventional sense of human experience. Space**, as now defined and constrained by the outer limits of the observable Universe, did not yet exist (see below); also, sequential events, embedded in a temporal continuum, had not begun. The observable Universe is just the visible or detectable part extending to that part of the Universe where objects or sources of radiation have sent signals traveling at the speed of light over an elapsed time not greater (usually somewhat less) than the time (age) of the start of expansion. Most cosmologists now feel with some confidence that there is something real and physical beyond the observable Universe (be it the unseen parts of our Universe or some other Universe(s) but it is too far away for light to have had enough time to reach Earth's ground or orbiting telescopes). That observed part plus the unobserved part together make up the Cosmos.
Everything that exists physically is included in the Cosmos. (One can debate whether things "spiritual" are only the thought processes that have a physical basis, or do these things really exist independently.) As this Section unfolds, you will come to realize that there is a hierarchy that deals with the physical entities within the Universe, arranged (in part) by a progression of decreasing sizes. That hierarchy, in its simplest form, is:
?? --> THE ABSOLUTE VACUUM (THE COSMOS) --> OUR UNIVERSE (PERHAPS OTHER UNIVERSES)-->THE INTERGALACTIC MEDIUM (VOIDS CONTAINING GAS, PHOTONS, AND OTHER PARTICLES) --> GALACTIC CLUSTERS --> GALAXIES --> INTRAGALACTIC GASES --> STAR CLUSTERS --> STARS --> PLANETARY SYSTEMS --> PLANETS --> SATELLITES (MOONS) --> FIRST ORDER SURFICIAL AND INTERIOR FEATURES ON PLANETS --> LOCAL FEATURES --> INDIVIDUAL ENTITIES (FOR EXAMPLE, HUMANS) --> ATOMS AND MOLECULES --> SUBATOMIC PARTICLES --> SUPERSTRINGS -->?
The mysterious Absolute Vacuum (the writer's term) will be considered later in this Section (suffice to say now that it a rather abstract concept that considers the possibility of a dimensionless emptiness that stretches to infinity; time also is eternal, having no real beginning or end). The initiating event which started our Universe from out of that Vacuum, referred to as the Big Bang (BB), began at a point so small that the notion of spatial three-dimensions [3-D] has no conceptual meaning. The event sprang from some sort of quantum state of still-being-defined nature that marks the inception of space/time (thus, without a preceding "where/when"; philosophically "uncaused"), from which all that was to become the Universe can be mentally envisioned to have been concentrated. This singularity is described as a state that is not quite a point (dimensionless) condition which has extreme curvature and before which there was no "yesterday". The singularity is the first event in Universe history, so that it connotes a beginning-of-time aspect in addition to its beginning-of-space implication. At the very beginning, its physical nature transcends the laws of physics (including relativity); these laws break down, i.e., do not apply, but almost immediately came into existence. This extremely small point condition nevertheless contained all the energy within the eventual Universe. This singularity energy is measured as a temperature that reached to billions of degrees centigrade. The density of the point at the moment of singularity was extremely high - far greater than that characteristic of Black Holes.
At the very beginning of this (our) Universe, multidimensional space and time came into being and began to take on physical characteristics. But at the cosmic scale, these two fundamental properties must, according to Special Relativity, comprise the 4-dimensional spacetime Universe (see Preface for a definition of spacetime) we now observe (according to some theories discussed below and on page 20-10, additional dimensions are possible). The exact nature (concept) of time is still not fully understood and is subject to continuing debate (for an excellent review of time, read About Time: Einstein's Unfinished Revolution by Paul Davies, 1995); also consult his Web site on "What happened before the Big Bang" at this site (the host site contains many interesting and provocative articles; click on Albert Einstein within the page that comes up to get to the parent site). There is, of course, the conventional time of everyday experience on Earth (years, days, seconds, etc.), measured fairly precisely by atomic clocks (e.g., the pulsating beat of a cesium atom, used to define the 'second') and less so by mechanical timepieces or crystal watches. There are the redefining ideas of time consequent upon Special Relativity, in which the perception of time units proceeds faster or slower depending on frames of reference moving at different relative velocities. There is the notion of "eternity" in which time just is - has no specific beginning or ending.
But, all these measures and concepts are difficult to extrapolate to that nebulous temporal state (if real) which was before the singularity at which our Universe came into being. But, time had to separate at that instant and become measurable in terms we have set forth to use its property of steady progression of a temporal nature. If nothing existed prior to the singularity event, then scientists presently have no means to determine and measure the nature of the time that was involved as a prior state. If ours is not the only Universe (see the discussion of multiverses on page 20-10), and other Universes existed before the one we observe, then time in some way can be pushed backward to their inceptions. One possibility is an infinite number of Universes in time and space, with no end points for starts and finishes (read Paul Davies' book for the philosophical as well as physical implications of time, and the still unresolved dilemmas in specifying the meaning of time).
For our purposes in studying the Cosmology of the one known Universe, we will assume time started at the moment the Universe sprang into existence. Arbitrarily, we postulate that time is immutable (a second at the beginning is of the same duration as a second is defined by today); there are models that postulate variable time values but we will ignore these. We accept the subsequent progression of time as being comprehensible in the units we define for Earth living. Thus, the Universe, under this proposition, can be dated as to its age in years - the year is an arbitrary unit, being the present day time involved in the Earth's complete revolution around the Sun.
At the very beginning, the fundamental energy within the singularity point may have been (or been related to) gravitational energy that controlled the nature of what existed at the singularity moment. An alternative driver now being investigated is some form of repulsive energy (similar to that once proposed by Albert Einstein as his 'Cosmological Constant'; but with a different numerical value) such as Quintessence (see page 20-10) which may prove to be related to the "Dark Energy" (page 20-9) that seemingly dominates the present Universe. (It is now customary to think of gravity as a positive effect and this repulsive energy as countering gravity as a negative effect [it may be, or be equivalent to, Dark Energy].) At the instant of singularity, the initial energy (some of which was about to become matter) was compressed into a state of extremely high density (density = mass or amount of matter [or its energy equivalent] per specific [unit] volume), estimated to be about 1090 kg/cc (kilograms per cubic centimeter) and extraordinary temperatures, perhaps in excess of 1032 °K (K = Kelvin = 273 + °C [C = degrees Centigrade]). (Note: the term "vacuum density" has been used in reference to this pre-Big Bang state; the density in this case refers to energy [which is a surrogate for mass according to E = mc2]; this vacuum density is said to be very large.) Both high values are without any counterpart in the presently observed Universe; particle accelerators are not yet close to reproducing these ultrahigh temperatures. As you will see below, certain forms of matter came from the pure energy released during the first fraction of a second of the Universe's history. The famed Einstein equation E = mc2 accounts for the fact that under the right conditions, energy can convert to matter, and vice-versa.
At the instant of the Big Bang's singularity, the particle (whatever its nature; the term "particle" also refers more generally to any of the fundamental entities such as protons, photons, muons, etc. that constitute matter and energy [see below on this page]) proved exceptionally unstable and proceeded to "come apart" by experiencing something that has been likened to an "explosion", which goes under the popular name of the "Big Bang" (BB). In TV shows that have an astronomy theme, such as seen on the History and Discovery channels, the depiction of the BB resembles a detonation or explosion (in the shows you usually hear a banging noise; this is meaningless since the BB moves into a true vacuum, which cannot support sound), and the terms have been applied (incorrectly) to the event. There is this fundamental difference: In a conventional explosion, every thing involved is hurled outward from the point of initiation as an advancing and enlarging front that moves into existing space, leaving the volume between the front and the point devoid of the explosive debris; this volume increases in size as the debris progresses outward. To depict this visually, look at this animation of the explosion around the star Eta Carinae (you must be on the Internet):
In the Big Bang, there is no real hurling away of the material released; instead all the material (from eventual galaxies down to subatomic particles) simply expands around the singularity point creating its own space as it enlarges. The explosion is described as "not into space" but "of space". No center (in space today) can be specified for the BB since all points in the new finite but growing space simply draw apart more or less equally as space stretches under conditions in which pressure and density remain uniform and isotropic everywhere.
Several Internet sites actually have movies that simulate the expansion. This one works on the writer's computer; it should work on yours if you have the right software program installed:
This difference in behavior between conventional and BB is clarified in the two illustrations (read their captions for more information) below taken from (Six) Misconceptions about the Big Bang, by C.H. Lineweaver and T.M. Davis, Scientific American, March 2005, cited again on pages 20-8 - 20-10.