Big
Bang Cosmology Primer
Dr. Michael Sudduth
(Revised 5/5/01)
I. The Universe
We live on the earth. The earth is one of the several planets that orbit the sun along with thousands of other smaller chunks of matter, stellar dust, and ice particles. Our sun, a medium sized star, is one of the 100-200 billion other stars that constitute our galaxy, the Milky Way. Our galaxy belongs to a cluster of nearer galaxies, and there are millions of such galaxies and galaxy clusters strewn across hundreds of millions of light years of space. The distances are great, often unfathomable, and reveal the vastness of the physical system of which we occupy a miniscule part, much like a grain of sand on an infinite shore. This is the Universe. It is a complex physical system of material objects of varying sizes and characteristics that are connected in space and time, and which behave and interact according to a relatively small number of physical laws.
The dominant theory in contemporary cosmology regarding the origin of our Universe is the Hot Big Bang theory. It maintains that the Universe came into existence rather abruptly between 15 to 20 billion years ago in a kind of cosmic “explosion” from a very simple state in which time and space were infinitely shrunk, or from a very dense concentration of matter-energy. This compact, ultra-microscopic particle of pure energy “exploded,” starting an expansion that continues to this day. (Click here for diagram). As the fireball cooled over time matter formed from energy and took the form of heavier particles, clouds of gas, and eventually stars and galaxies and all that they contain.
There are two alternate cosmological theories, neither one of which is considered very plausible anymore (for reasons that will be apparent below).
Steady State Theory: This theory, which dominated prior to Big Bang cosmology, asserts that the Universe has no beginning or is temporally infinite. Although the Universe is expanding, its large-scale features have remained relatively the same. New matter is continuously created out of nothing to fill the space in between galaxies as that space stretches with expansion. Fred Hoyle was the well-known advocate of this theory at the time that Laimatre, Hubble, and others were proposing Big Bang cosmology. Hoyle actually coined the term “Big Bang,” a term he used in derision of the theory.
Oscillating Universe Theory: Similar to Big Bang Cosmology, except that it postulates a number of Big Bangs followed by Big Crunches, then followed by Big Bangs.
For more on Big Bang cosmology and alternate models, see David R. Gilson’s The Ever Expanding Universe in Modern Cosmology.
III. Evidence for Big Bang Cosmology
What are the grounds for believing in Big Bang Cosmology?
(1) The Universe is expanding.
In the 1920s astronomers discovered that the observable galaxies were each moving away from each other. It is not as though they are rushing through empty space, but the space between galaxies is actually stretching. Astronomers arrived at this conclusion by an examination of the light spectra of galaxies. As with sound waves, light waves change depending on whether an object is moving away or moving toward an observer. Sound waves shift to a lower frequency as objects move away from an observer, and they shift to a higher frequency as objects move toward an observer. This is known as the “Doppler” effect. (The sound of a siren rushing by an observer provides a good example of this). Something similar is observed in the case of light waves. In the 1920s Edwin Hubble discovered that light from distant galaxies is shifted to longer wavelengths, to the red end of the light spectrum, whereas light from nearby stars is shifted to shorter wavelengths, to the blue end of the light spectrum. Distant galaxies exhibit what is called a cosmological red shift, and Hubble claimed that the red shift of a galaxy was proportional to its distance. Although Hubble’s observations originally applied only to fairly nearby galaxies that he observed, subsequent observations have extended Hubble’s observation to galaxies and galactic clusters to billions of light years away from us.
The crucial point to note here is that the data implies that galaxies or galaxy clusters are moving away from us. More precisely, ever cluster is moving away from every other cluster – much like the movements of dots on a balloon that is being filled with air. Big Bang cosmology of course predicts that the Universe is and has been expanding from its inception. So expansion is important evidence that confirms the Hot Big Bang theory.
Technical note: Light arriving at earth from distant galaxies provides
information about objects at the time the light left them. Owing to the
distances between galactic objects, we see them literally as they were in the
past. Modern telescopes can see galaxies as far as 10 billion light years away.
Thus, these telescopes give us a glimpse of the Universe 10 billion years ago.
This provides one reason among many for being quite sure that expansion is not
merely a local phenomenon. It can observationally be traced back 10 billion
years. And if the Universe is not expanding, we must suppose that 10 billion
years ago these galaxies all mysteriously turned on in their present positions.
Moreover, general relativity implies that the universe must either be expanding
or contracting, so expansion must be viewed as a feature of the universe from
the inception of its existence.
(2) Uniform Background Microwave Radiation
The entire visible cosmos is bathed in a sea of background microwave radiation. This discovery emerged as the result of observations made by Arnold Penzias and Robert Wilson in the Bell Telephone Laboratories in 1964-65, and the insights of Robert Dicke. It was confirmed to a high degree of precision in 1990 by COBE, the Cosmic Microwave Background Explorer, a satellite designed to measure the microwave background radiation. The data from COBE and more recent experiments shows that microwave radiation is uniformly distributed throughout the visible cosmos, present in all directions at the same temperature, about 2.7 degrees Kelvin. This observation further implies that the Universe is, in its large-scale features, uniform and smooth. Big Bang cosmology leads us to expect all this. The primordial fireball would leave a permanent, uninform microwave signature as it cooled.
(3) Uniform Abundance of Hydrogen and Helium
In our cosmos there is a universal abundance of hydrogen and helium. Stars run on hydrogen, and although stars produce helium, they cannot be the source of all the helium in the Universe. There is too much helium, and uniformly distributed throughout the cosmos, to be accounted for through mere stellar processes. Although stars produce a lot of helium, most of this helium is processed into heavier elements. But the abundance of hydrogen and helium, as well as its uniform distribution, is precisely what we would expect given Big Bang cosmology, which postulates the emergence of the lighter elements out of the fires of the Big Bang, through a process called nucleosynthesis, a process that took place in the first few minutes of the Universe’s existence.
(4) Conforms to some of Einstein’s Equations in General Relativity
According to equations in Einsteinian General Relativity, the Universe must either be expanding or collapsing. When Einstein discovered this consequence of his theory, he was profoundly disturbed, as there was at that time no experimental evidence suggesting that the Universe is expanding. Einstein was so sure that this was incorrect that he added the idea of a cosmological constant in order to have a force that would keep the Universe from either expanding or contracting. Edwin Hubble’s observations of the cosmos later confirmed that the Universe is indeed expanding, and Einstein quickly retracted his idea of a cosmological constant, saying that it was the biggest mistake of his career.
IV. Implications of Big Bang Cosmology
These bits of evidence provide strong support for the idea that in the distant past the entire Universe was compressed into a single point of infinite density, what mathematicians call a singularity, or at any rate originated from an ultra-microscopic, quantum state. (Hawking and Penrose have argued that there would be no singularity). Space, time, and matter-energy all have their origin in that great cosmic event known as the Big Bang. This implies that the Universe is not infinitely old, that it has a beginning.
Of course, several other independent facts and scientific laws also preclude an infinitely old Universe. First, the second law of thermodynamics entails that all physical systems move increasingly toward greater states of disorder or entropy. If the Universe is infinitely old, then it would have reached thermodynamic equilibrium or maximal entropy a finite time ago, which clearly it has not. Among other things, we would not be here if it did. Secondly, the night sky is largely dark. This would not be the case if the Universe had existed for infinite time. For in that case, the night sky would be filled with starlight, as infinite time would have allowed light from all points of the sky to reach us. Third, no star is older than 15-20 billion years old, a rather odd fact if the Universe is infinitely old. If the Universe is infinitely old and there are stars, there should be stars older than 20 billion years of age. These reasons against an infinite universe also lend support to Big Bang cosmology, since the latter postulates a temporally finite universe.
V. Inflationary Cosmology
Inflationary Cosmology developed in the last 20 years by scientists such as Guth and Linde, inflationary theories of the Universe are supplements to big bang cosmology (not alternatives).
Explanations of the classical hot big bang model often include extrapolations back to a zero-point in the form of a singularity. This is understandable given that such an extrapolation seems implied by cosmic expansion. In fact, though, the original hot big bang model is really an account of what happened after a hypothesized bang, not necessarily an extrapolation to times earlier than this. It gives us a description of the way the Universe was immediately after its creation. This is because the predictive elements of the theory only take us back to right after the first second of the Universe's existence or perhaps at 10-35 seconds. The microwave background radiation (part of the original theory) takes us back to 300,000 thousand years after the beginning. Our knowledge of nucleosynthesis allows big bang extrapolations back to one second after the beginning when helium began to form.
Big bang cosmology does not tell us about the conditions that prevailed prior to the first second or millisecond. Hence, the theory must assume and leave unexplained several important features to the Universe at that time. Around 10-35 sec many of the crucial conditions that would determine the later structure of the Universe were determined. For instance, the expansion rate of the cosmos was imprinted at this time. The conditions that dictated the large-scale isotropy and small-scale irregularities of the cosmos were also imprinted at this time. In other words, big bang cosmology begins with an incredibly fine-tuned Universe and does not attempt to explain the mechanism involved in the very big bang itself. Inflationary cosmology attempts to provide an explanation of the big bang itself.
Inflationary theory postulates the emergence of the observable universe through an exponential rapid expansion phase from an unstable vacuum state. (A "vacuum" is the state of lowest possible energy density). More specifically, the universe emerges from a "false vacuum," a temporary vacuum with enormous energy density. Matter at high densities experiences negative pressure. If this negative pressure sufficiently outweighs the positive density, it can create a repulsive force. This repulsive gravitational field can drive an exponential expansion, like a balloon suddenly being filled with air at an incredible rate.
Classical big bang cosmology and inflationary cosmology describe the same sort of Universe at all times after 10-35 seconds. However, unlike big bang cosmology that starts the Universe off at about 10-5 meters in radius, inflationary theory begins at even early epoch of time when the Universe was around 10-52 meters in radius. The radius of the nucleus of an atom is by comparison 10-12 meters. During inflation the size of the Universe is increased by a factor of 1050. (These numbers indicating the size of the universe should not be taken too literally, as the actual values are not certain. The numbers are designed to illustrate how inflation can operate, and to note the relative size difference of the Universe between inflationary and standard big bang models). Inflationary theory also predicts some of the values involved in fine-tuning. For instance, inflation pushes the Universe toward flatness, and so explains why omega should be close to 1. It also explains why the Universe is isotropic but with sufficient irregularities to generate cosmic structures. Minor fluctuations in the vacuum are exponentially increased during expansion, leading to variations in density levels in certain regions of the Universe that function as the seeds of galactic structure.
For more on Inflationary Cosmology, see Sten
Odenwald’s FAQ: What
are the basic ideas and issues in contemporary Inflationary cosmology?
VI. Other Important Terms in Cosmology
Cosmic Expansion: Although predicted by Einstein's theory of general relativity, in 1929 Edwin Hubble confirmed that the Universe is expanding. More specifically, he confirmed that observable galaxies were moving away from us. He extrapolated that observable galaxies were each moving away from each other. They are not rushing through empty space, but the space between galaxies is actually stretching. (An analogy to this is the movement of dots on the surface of a balloon that is inflated with air.) This picture when backwards implies that the Universe was at one time in the distant past much smaller and extremely dense, as all of its matter would have been compressed into an ultra-microscopic region. Ordinarily this picture is interpreted as implying a point of origin in a singularity, a point at which time and space were infinitely shrunk. Hawking and Penrose, though, have argued that there is no singularity. A singularity would imply a definite point at which time begins, but this is inconsistent with the emergence of temporal metric from quantum fluctuations that would have obtained prior to 10-43 sec. of the Universe’ existence, just prior to the Big Bang. Singularities get smeared out at such small scales when the Universe would have been 10-33 centimeters in length. Singularities would be replaced by very dense concentrations or matter-energy.
Nucleosynthesis: This refers to the production of elements under extremely high temperatures. Big bang nucleosynthesis is the process whereby the lightest chemical elements (hydrogen, helium, and lithium) were produced in the extreme temperatures that prevailed the first second after the big bang. Heavier elements (e.g., carbon, iron) are produced by similar thermonuclear processes in stars and strewn into space by a variety of processes, especially the death-explosion of massive stars (called a supernova). Nucleosynthesis provides further evidence for big bang cosmology. Stellar processes cannot account for all the elements in the Universe. Stars are formed in dense clouds of hydrogen gas in space, so stars do not explain the existence of hydrogen itself. And even though stars convert hydrogen to helium, there is too much helium in the Universe to have originated solely from stars. (Most stars actually convert helium into heavier elements, so the helium they produce is processed into elements other than helium). There must be or have been some other process that produced these chemical elements. Big bang cosmology can explain the origin of these elements.
Homogeneity, Isotropy, and Cosmic Irregularities: The Universe is homogenous in the sense that matter is evenly distributed throughout it when viewed on large scales. Similarly, our Universe exhibits a large-scale isotropy. It looks the same in all directions. It is symmetric. This is evidenced strikingly in the cosmic background radiation that pervades the cosmos, a left over relic from a time when the Universe was about 300,000 years old. The fact that the Universe is homogenous and isotropic implies a good deal of fine-tuning at the very beginning because any deviations from uniformity then would be exponentially increased as time passed and the Universe expanded. There are of course smaller-scale irregularities imposed on the over all smoothness of the Universe, and these give rise to more interesting structural features of the Universe, such as stars, galaxies, and galaxy clusters. These are much like waves or ripples on an otherwise smooth ocean surface. The size of the cosmic ripples is 10-5 (often referred to as Q). Q gives the Universe the requisite roughness to develop an interesting array of cosmic structures. Big Bang cosmology assumes but does not explain why the Universe started out in such a synchronized fashion, with a near perfect uniformity but just enough roughness to produce galaxies and other cosmic structures.
Flatness and Critical Density: If the average mass-density of the Universe exceeds about five atoms per cubic meter, then the Universe has a density high enough to eventually halt its expansion (gravity will overcome expansion energy) and the Universe will collapse. Otherwise expansion energy will have the upper hand and the universe will expand for eternity. Critical density refers to the mass-density that places the Universe on the border of eventual collapse and eternal expansion (and gives it a flat or Euclidean geometry). The ratio between the actual and critical density is called Omega. Full critical density is an Omega = 1.
Since most of the matter of the Universe is unseen (so called dark matter) it is difficult to determine what the actual density is. The matter that makes all the stars in the Universe (not just our galaxy, but every galaxy) contributes only a fiftieth of the critical density (roughly 1/10 of atom per cubic meter). Gases between galaxies contribute about as much. So dismantling all the visible matter in the cosmos would yield only 0.2 atoms per cubic meter, far less than is needed to exceed the critical density. There is also dark matter, but it probably does not contribute more than a fifth of the critical density. So the average density today is at least 0.3. But if omega today is at least 0.3, it must not have been too far off from 1 a second or two after the big bang. Deviations from unity or 1 would have been amplified during expansion. Thus if omega began much higher than 1, gravity would have quickly overcome expansion energy. If omega were much less than 1, expansion energy would have overcome gravity, which would have thrust omega toward zero. For instance, a beginning omega of .5 would have dropped to .25 after the universe expanded to a factor of two. So because omega is at least 0.3 today, it could not have been too far from 1 immediately after the big bang. Furthermore, this is suggested by more recent ideas of accounting for the missing mass of the Universe. The current view is that a cosmological constant (or dark energy) accounts for the rest of the critical density of the Universe, bringing it to just about 1. (The cosmological constant also explains why expansion is accelerating). Thus the Universe likely has a flat geometry, not like a piece of paper or table, but rather the Universe lacks the kind of curvature that would give rise to geometric distortions over long distances. As Euclid maintained, parallel lines will never meet. The flatness, of course, is an indication of the important balance between expansion and gravity.
For recent information confirming the flatness of the Universe, see NASA article SPIDER-WEB SENSOR REVEALS A FLAT UNIVERSE.
Fine-Tuning: Fine-tuning generally refers to the unique values assigned to important cosmic numbers at the dawn of time, immediately after the big bang. Among other things, the universe has just the right sort of ripples or roughness imposed on just the right sort of smoothness to produce a uniform universe that also has interesting cosmic structures like stars and galaxies. The universe also has just the right expansion rate to allow for the formation of stable stars. All of these are crucial to making the Universe a life conducive-system. Life requires a narrow range of chemical elements (such as carbon), and these elements depend on fundamental laws and initial conditions of the Universe having just the right sort of values and ratios to each other. Hence, more specifically, the fine-tuning of the Universe refers to that fact that the Universe's physical laws and initial conditions (at the big bang) are calibrated within a very narrow range so as to make the Universe conducive to life. This fine-tuning is a priori very unlikely. There appear to be only two main candidates to explain fine-tuning: (1) there are a sufficiently large number of Universes that exhaust (or nearly so) all possible states of energy-matter or (2) our Universe was designed by an intelligent being.
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References:
Joseph Silk, The Big Bang, 3rd edition
(New York: W.H. Freeman and Company, 2001)
Robert M. Wald, Space, Time, and Gravity: The
Theory of the Big Bang and Black Holes, 2nd edition (University
Press of Chicago, 1992)
Martin Rees, Just Six Numbers (Basic Books,
1999)
Alan Guth, The Inflationary Universe (Perseus Publishing, 1998)
The following are some good on-line resources for
astronomy and cosmology:
John
Hawley’s History of Cosmology Text (1998)
Sten Odenwald’s Ask the
Astronomer Site
Cosmos: National
Cosmology Supercomputer
The
Ever Expanding Universe in Modern Cosmology, David R. Gilson
