Chapter 1.The Transient Certainty of Cosmological Models: What History Tells Us
Executive Summary
In this opening chapter we discuss the historical evolution of cosmological models to demonstrate this evolving process. Deatils of this process will be covered in subsequent chapters. Here we wish to confront the notion that, at any given time in history people always think their view of the world is correct, with the actual real uncertainties in that world view. Science is never divorced from culture and significant challanges arise when scientific discovery is not commenusate with the believed "cultural" truth. While this may be come quite evident from a historical view, this conflicts have certainy not subsided in our modern world. |
From the Beginning
Cosmology stems from the Greek word Kosmos. In its philosophical context, Kosmos means the world, or the "orderly universe". Through application of this term, the Greek's have presupposed that nature indeed is orderly and furthermore, if it is orderly, then humans have the capacity to acquire "Truth". This bias that nature is logically constructed largely continues today, even with our recent developments in quantum mechanics and in chaos. An important culture lesson that can therefore be gleamed form a historical overview of cosmology is that our faith in the certainty of cosmological models has always been challanged and we should not be so quick to assume that indeed, nature is easy to understand. Indeed, about 100 years ago, Physicis Max Planck made a very astute observation when he said:
We have no right to assume that any physical laws exist, of if they have existed up to now, that they will continue to exist in a similar manner in the future |
In turn, human fascination with its origins and its place in the Cosmos seems innate to our species. Equally innate is our pre-disposition for acquiring the Truth as fast as possible and then tenaciously hanging on to that Truth in the face of conflicting data. As discussed below, this dynamic has been occurring for millennia and from the purely philosophical point of view, there should be no expectation that the cosmological truth of 2012 will be the same truth a decade, century or millennia later. This sense is readily confirmed through a quick review of history and the application of corrections (i.e. epicycles) to adjust the core model to remain consistent with observations.
Prior to the Greeks there were surely many different kinds of cosmology models that were likely driven by myth and legend. However, the first "scientific" cosmological model occurs around 350 BC as documented by Aristotle and Heraclides. These philsophers developed the first geocentric Solar System model. Geocentric means Earth-centered such that all other objects in the Universe (including the Sun) were positited to revolve around a stationary Earth. That the Earth was stationary was intuitively obvious to the ancient Greeks as how could such a heavy and dense thing as the Earth actually move. In the words of Ptolemy:
it is manifest to any observer that the earth occupies the middle place in the cosmos, and that all weights move toward it...that the earth is spherical and situated in the middle of the cosmos. |
It is via this mechanism that Greek philsophers brought order to their world view such that events could be explained via the intearction of fundamenal forces rather than the whims of the Gods. For instance, lightning might arise as some interaction between air, water and fire instead of arising because Zeus got pissed off. By the time of Aristotle, all of these elements were incorporated into the fully geocentric cosmological model:
The elements of this model are the following:
- All celestial objects are fixed to crystalline spheres (so we can see through each sphere)
- Each sphere containing each object revolves around the Earth
- The stars are through to be all on one sphere at a fixed distance from the Earth but outside the planetary realm.
- To produce motion, Aristotle invokes a character known as the Prime Mover, who apparently just shows up one day and kicks the spheres so that that they all revolve around the earth in the same direction and as perfect circles
The allure of this model is so powerful, due to its natural resonance with the cultural truth of the time (Humans are special) that even though anamolies will appear (an anomaly is a consistent observation that can not readily be explained by the model) they will not be of sufficient concern to suggest that this geocentric model is wrong. For instance, the basic prediction of Prime Mover model is that all objects should revolve around the earth with the same sky motion. That is, all objects should appear to move as rising in the East and setting in the West. However, it was well observed at this time that Mars appears to move in the opposite direction at certain times (i.e. from West to East). This anamolous motion is known as retrograde motion . This observed retrograde motion violates the prediction from the Prime Mover model and will require a mechanical invention in order to preserve the model (see chapter 2 for more detail on the Ptolemaic epicyle). The alternative view, that the Sun was the center of the Solar System was advanced by Aristarchus (270 B.C.) but was not accepted at the time, largely on the grounds that in such a model the Earth actually moves (because it is in orbit about the Sun) yet no one could actually feel the Earth move and therefore the notion that it was moving was logically absurd. As explained further in Chapter 2, this expectation of "feeling the earth move" is a direct reflection of the failure to understand that motion is relative and not absolute
The Copernicun Revolution?
We can now advance our story to the 15th century where thinkers like Da Vinci and Nicholas of Cusa led the way for the adoption of the heliocentric model as published by Copernicus. While many textbooks speak of the Copernicun model as some kind of paradigm shift, now that the Earth is no longer the center of the Universe, this model merely relocates the Sun into the center of the Universe and everything else revolves in perfect circles around the Sun. While this model does eliminate the Ptolemaic epicycle, it provides only a qualatiavely superior solution to the observed motions of the celestial bodies when compared to the Ptolemaic system. Indeed, the details of the Copernicun model do involve motions as compounded circular motions (these are Copernicun epicyles - see Chapter 2). so as to keep all the motion perfectly circular so perhaps not much of a real revolution actually occurred.
As Copernicus writes in this executive summary
Some will say that the ideas I put here are mad, so I dedicate this work to you, Pope Paul III, so that, as a lover of learning, you may protect it from enemies ... Until now it as been accepted that the earth is the centre of the universe. But why, then, do the planets seem to follow such odd paths through the sky, when it is obvious that their motion must the perfection of a circle? ... The ancients thought the earth must be the centre because all heavy things fall towards centres, and earth is most heavy, and that the earth cannot be in motion for otherwise clouds would drift away as it spun. But if the air of the earth moves too, then clouds will hold their places. I follow ancient Martianus Capella, and think that the beautiful sun is at the centre, ... I declare also that the earth varies in the angle of its axis as it goes round the sun, which accounts for the differences of the seasons. There is no clearer way of understanding the apparent motions of the heavenly orbs |
The principle objections, at the time of the publication of this model (1543) arise not from putting the Sun at the center of the solar system, but rather from endowing motion to the Earth. One of more biting critisms comes from a 1549 textbook by Melanchthon:
Out of love for novelty or in order to make a show of their cleverness, some people have argued that the earth moves. They maintain that neither the eighth sphere nor the sun moves, whereas they attribute motion to the other celestial spheres, and also place the earth among the heavenly bodies. Nor were these jokes invented recently. There is still extant Archimedes' book in which he reports that Aristarchus of Samos propounded the paradox that the sun stands still and the earth revolves around the sun.Even though subtle experts institute many investigations for the sake of exercising their ingenuity, nevertheless public proclamation of absurd opinions is indecent and sets a harmful example. Encouraged by this divine evidence, let us cherish the truth and let us not permit ourselves to be alienated from it by the tricks of those who deem it an intellectual honor to introduce confusion into the arts. |
And from French Philosopher Jean Bodin in 1596 we have the following tweet:
No one who is in his right mind or who has had the slightest training in the physical sciences will ever believe that the dense and solid earth with its heaviness and weight simultaneously moves up and down, about its own center, and around the sun, while performing a libration (oscillation around an axis) |
Clearly, the Copernicun model which requires the Earth to move meet with a lot of philosophical objection. However, over the course of the next 100 years the model would be fully supported (details in Chapter 2) through a) Galileo's discovery of a mini-solar system centered on Jupiter, tb) Kepler's elliptical revision of Copernicus's perfect circles and yet Kepler has no idea of why the planet moves faster when closer to the sun (e.g. Kelper's second Law) as is required for the orbits to be elliptical, and c) Newton's dynamical description of the system -- no epicycles are needed anymore - every aspect of the observed motion of the planets can now be accounted for by Newton. Now we have a good working scientific model that doesn't require any ad hoc devices to make it work. Are we done now? It would seem so.
From Copernicus to Einstein to Inflation, Dark Matter and Dark Energy
Still, the nature of the Stars eludes us at this time and the Newtonian universe requires an infinite and homogenous distribution of mass so as not to have collapsed. In 1839 measurement by Bessel of stellar parallax now moves the scale of the Universe well beyond the solar system and raises the issue of the energy source for stars. The 1859 discovery of the non-Newtonian orbit of Mercury also reveals the incompleteness of the Newtonian model (see Chapter 2). Hence by the middle of the 19th century, humans a profoundly ignorant of some basics: we don't know how stars shine; we can't explain the orbit of Mercury. At this time, we therefore don't seem to be possessing very much of the Truth.
The development of general relativity (GR) as well as the concept of E=mc2 by Einstein serves to explain both the orbital aberration of Mercury as well the energy source of stars. Moreover, Einstein initially had the same dilemma as Newton and had to resort to introducing the concept of a negative pressure field (e.g. an epicycle) in the Universe in order to exactly balance it against gravitational collapse. This places the Universe at a point of unstable equilibrium; any small perturbation in this static universe would cause it to either collapse or start expanding as the gravitational energy density would no longer exactly balance the negative pressure (e.g. the cosmological constant) energy density. With the confirmation of the expanding Universe (1929) came the realization that the Universe was not static and that the Cosmological constant could now be set to zero (it will return to non-zero status in 1998). This then marks the transition between historical cosmological models and our modern cosmology. While GR has passed virtually all tests to date their remains the possibility that GR is again a subset of more fundamental description of space time.
The remaining big questions associated with the expanding Universe are a) its expansion age and b) whether or not the Universe was open or closed. This meant designing cosmological observations to test for large scale geometry and/or to directly measure the mass density. These large scale observations begin in the early 1960's and for approximately 20 years there was a strong disagreement of opinion if the data supported a young (10 billion year) or and old (20 billion year Universe). However, most all data agreed that the cosmological mass density was insufficient for closure which raised the philosophical difficulty of an unending universe that originated at a specific point in time
The last major development in our modern cosmological framework occurred in 1980 by Alan Guth (and others) that introduced the idea of inflationary cosmology (details in Chapter 2. The exact mechanism of inflation is still largely unknown (another epicycle) but simple inflation makes a strong prediction: all initial curvature of space-time should have been inflated out and the large scale geometry of the Universe should be flat. A flat universe carries with it a specific constraint, namely that the sum of all possible energy densities must = 1. In cosmologically normalized units, this requirement is expressed as:
- Ωb = the energy density of normal matter
- ΩDM = the energy density of dark matter which is some mysterious form of matter that gravitates but does not emit any light.
- ΩΛ = the energy density of some completely bizzarre component of the Universe known as Dark Energy. Dark Energy was initially hinted at in 1998 and has been confirmed by previous measures. Where as dark matter acts to collapse the Universe, dark energy acts to pull the Universe apart at an increasing rate, resulting in what is now called the "accelearating" Universe.
At the time of the introduction of inflationary cosmology, available observations indicated that ΩΛ = 0 and Ωb =0.1. By simple arithmetic this requires ΩDM = 0.99 in order to satisfy the prediction of inflation. This simple mathematical situation produced a very large paradigm shift in our cosmological view. Rather than stars and galaxies being the dominant matter in the Universe, it had to be dark matter that was dominant. Our ignorance is now extremely high -- we have no clue what the Universe is made of!
By 1985, we had arrived at a new Aristotelian cosmological truth: space was flat and all of the energy density was in some form of dark matter. But this particular truth only lasts for about 15 years compared to the previous 1800 year Aristotelian truth. Around 2000, the concordant cosmology emerged from various observations such as WMAP and distant supernovae. The most prominent change in the new cosmology is the resurrection of the cosmological constant oringally invoked by Einstein. We now call this cosmological constant dark energy. Morever, the dark energy term is dominant in energy density But, now this term is dominate and so we have completely jettisoned the dark matter dominated collpasing Universe of the mid 1980s to our new cosmology that requires the Universe to be accelerating in direct response to the presence of dark energy.
Like dark matter, physicists have even less of a clue on the physical nature of dark energy. In addition, both dark matter and dark energy arise if we assume that we have a complete understanding of the way that gravity works. Newton once thought he had a complete understanding of gravity as a force. Einstein later showed that gravity is more fundamentally related to the curvature of space time (see Chapter 2) and that Newton's theory is a subset of a larger theory. If indeed we later find out that GR is not the complete theory, then a future historian might then refer to both dark matter and dark energy as epicycles designed to save the current cosmology when in fact, we were just all ignorant of the way gravity operates. Just like Aristotle's crystalline universe, the current cosmology can be easily summarized in the form of a new Aristotelian Pie:
Given the history of our evolving cosmologies, it indeed is unwise to assume that now, finally, we know the Truth. Indeed, Newton speaks specifically to this when he says:
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In experimental philosophy, propositions gathered from phenomena by induction should be considered either exactly or very nearly true notwithstanding any contrary hypotheses, until yet other phenomena make such propositions either more exact or liable to exceptions.
We should therefore expect everything in our modern cosmology to be overturned by future data an future theories and we should be prepared for that. Culturally, it is much better to remain humbled by nature and to remain in awe of its complexity, rather than to aarogantly assume that, after just a couple of thousand years of inquiry and measurement, we have now figured out all of this complexity.
End Thoughts
The strong desire to understand our place in the Cosmos is likely endemic to all cultures as they reach out and form some grand set of ideas about the nature of the World. While the details of various cosmologies may be quite different, their existence is a testimony of the tenacity in which intelligent life strives for true understanding.
In the study of Cosmology, all we have available to us is observation since we can not (yet) physically explore areas beyond our Earth. As our observational tools have become increasingly more sophisticated, new discoveries are made which alter our Cosmology. Often times these new discoveries have occurred as the result of new instrumentation which has opened up windows into the electromagnetic spectrum that had been previously closed.
Still, even though we now live in a highly technical world and have wonderful cosmological instruments such as the Keck 10-m telescope or the Hubble Space Telescope , it would be extremely presumptuous to think that we have now arrived at a thorough understanding of the Universe. We are just in the beginning data acquisition stages in our knowledge of the Universe and these data have provided far more questions than answers:
- How did galaxies form?
- Why is the galaxy distribution so clustered?
- How much mass is there in the Universe?
- What is the evidence for dark matter?
- What is the nature of dark matter?
- Does gravity behave the same on all size scales?
- Does vacuum energy drive the current kinematics of the Universe?
- What caused inflation?
- Why is there any mass at all in the Universe?
- What is the ultimate
fate of the Universe?