Big Bang Theory

Welcome from the page on Hubble's discovery of an expanding Universe. What does this mean? Here is a short presentation on the best current theory that attempts to explain the origin of the Universe ... the Big Bang. 10 Easy Steps

The Big Bang Theory simply states that the entire known Universe began 12-15 billion years ago when a tiny point of infinite energy that was compacted into a space of zero volume began to expand for unknown reasons or causes. It is as if a singularity were to reverse its course. Some pretty good websites that will teach this subject perhaps far better than I are found below. These websites are included so you can have access to a diverse set of opinions on the subject and draw you own conclusion after reviewing some of this additional information.

NASA Cosmology Homepage

Cambridge Cosmology Homepage

Big Bang from a religious perspective

Big Bang from atheist perspective

Big Bang potential errors

Be thorough and open-minded when you look at the opposing viewpoints as you develop your own opinion

From a theoretical standpoint, the notion of the Universe coming from a point of infinite density has a problem for me. If a singularity were to collapse to such a point of zero volume, then by mathematical definition it would achieve infinite density and infinite energy, The singularity would have begun with a limited supply of mass and collapsed to a point where energy becomes infinite. If mass and energy are interchangeable, than this generation of infinite energy from finite mass seems to defy laws of Physics. If the Big Bang is the result of a sudden expansion of such a singularity, why have singularities not blown apart since then, during the 11-16 billion year history of the Universe. If gravity is this relentless force that will allow nothing to escape from a Black Hole, then why did the initial point expand in defiance of that force?

I am not an opponent of Big Bang Theory for the sheer observable expansion of the Universe supports the model, but it is a theory that does not yet answer all of the questions posed to it. Additionally, strict creationism is not possible to test scientifically because as we stated earlier, science seeks to test and retest until either the same answer always pops up, resulting in a theory or even a law, or the answer is different one time and then the hypothesis must be restated. This falsifiability is a key feature to good science, and creationism cannot be falsified, so it cannot really be tested with good science. To believe in a created Universe from absolutely nothing requires as much faith as does the Big Bang which contends that the Universe came from absolutely nothing. It simply becomes a matter of you making a decision and then seeking answers or evidence for your decision.

Back to a study of Astronomy

The subject of Cosmology should be offered as a stand-alone course, but perhaps not yet for high school students who are still concerned more with the events of tomorrow or the weekend than they are with philosophical and theological dilemmas.

Let's Look at What We Observe and Believe to be Happening in the Universe

Universe Is Expanding

Edwin Hubble and Milton Humason discovered in the 1920's that the universe is not static---it is expanding. This is enough to resolve Olber's Paradox. As the universe expands, the light waves are stretched out and the energy is reduced. Also, the time to receive the light is also lengthened over the time it took to emit the photon. Because the luminosity = the energy/time, the apparent brightness will be reduced enough by the expansion to make the sky dark.

The expansion of the universe means that galaxies were much closer together long ago. This implies that there is a finite age to the universe, it is not eternal. Even if the universe is infinite, the light from places very far away will not have had enough time to reach us. This will make the sky dark. The Universe is finite and also big, with the farthest objects moving away the fastest. Some of these objects are so far away and moving so fast that their light has been stretched into non-visible forms like infrared, micro, and even radio waves! This is seen in the pair of diagrams below:

The Universe is expanding, and from the red-shifting of radiation from distant objects, a few questions arise:

How did the Universe begin?

What will happen to the Universe in the future?

How old is the Universe?

What is out there at the edge, and can we ever go there?

How much stuff is in the Universe and is it different in different places?

The Big Bang

The best model for the beginning of the Universe is the "Big Bang" model. Tested as a scientific theory, many aspects of this model seem in agreement with the experimentations and observations done to date. The idea is really quite simple: If the Universe is expanding today, then it must have been smaller in the past. The farther you go back in time, then the small the Universe should have been. It is reasonable, along this line of thinking, to conclude that the Universe was very, very small a very, very long time ago.

Furthermore, since the farther out we look in space, the more stretched the radiation reaching us becomes, it is reasonable to conclude that any visible light we sense from extremely distant places must be coming from highly energetic sources. The farther out in space the sources are, the more energetic they appear to be. Therefore, the most distant light sources is also the most energetic, but its energy is stretched by the expansion that we observe to presently low forms. It is reasonable, along this line of thinking, to conclude that the Universe was very, very hot a very, very long time ago.

Take these two lines of thinking together, the Universe was essentially infinitely hot and infinitely small a measurable finite amount of time ago. Based on the observed red shifts of distant galaxies, Belgian priest Georges Lemaitra proposed that the entire Universe began with the explosion of some primeaval atom. Somehow, this tiny point in space suddenly expanded. Somehow this seemingly infinite amount of energy expanded and cooled. As the energy expanded, it cooled to form protons, electrons, and neutrons which combine in a predictable manner to make large amounts of Hydrogen and lesser amounts of Helium. These two gases are all that the Universe made in the beginning since the ongoing expansion lacked sufficient energy to synthesize heavier elements.

The expanding cloud (for now the Universe has actual material and not just energy) clumped into galaxies whose interiors clumped into stars. It is these collections of galaxies that continue their relentless expansion outward. Hubble's work on red-shifted galaxies furthered the explosion hypothesis in 1929. The discovery of the cosmic background radiation (seen below) by Arno Penzias and Robert Wilson in 1964 gave stronger support for the model.

The notion of such a beginning seemed ludicrous to English astronomer Fred Hoyle in the 1950 when the theory was first proposed. In fact, it seemed so ridiculous that he asked essentially, "Do you mean to tell me the Universe just expanded in some sort of Big Bang." While he hated the term that he himself coined, it stuck with the astronomy community, and has done so ever since.

Please avoid the following common misconceptions about the Big Bang:

The Big Bang did not occur at a single point in space as an "explosion." It is better thought of as the simultaneous appearance of space everywhere in the universe. That region of space that is within our present horizon was indeed no bigger than a point in the past. Nevertheless, if all of space both inside and outside our horizon is infinite now, it was born infinite. If it is closed and finite, then it was born with zero volume and grew from that. In neither case is there a "center of expansion" - a point from which the universe is expanding away from. In the ball analogy, the radius of the ball grows as the universe expands, but all points on the surface of the ball (the universe) recede from each other in an identical fashion. The interior of the ball should not be regarded as part of the universe in this analogy.
By definition, the universe encompasses all of space and time as we know it, so it is beyond the realm of the Big Bang model to postulate what the universe is expanding into. In either the open or closed universe, the only "edge" to space-time occurs at the Big Bang (and perhaps its counterpart the Big Crunch), so it is not logically necessary (or sensible) to consider this question.


It is beyond the realm of the Big Bang Model to say what gave rise to the Big Bang. There are a number of speculative theories about this topic, but none of them make realistically testable predictions as of yet. Below are links to a protagonist and antagonist view of the Big Bang Model. Both sites use good science in their arguments, and you are strongly encouraged to navigate there.

Tests for Big Bang Cosmology from the WMAP Cosmology Pages is a great place to peruse for additional information.

Why the Big Bang is Wrong is a series of arguments presented by John Kierein and might also be of interest to you.

Perhaps we can determine when the Big Bang took place be measuring how old the Universe appears to be.

The age of the universe can be easily estimated from the simple relation of Time = Distance/Speed. The Hubble law, Speed = Ho × Distance, can be rewritten 1/Ho = Distance/Speed, where Ho is the Hubble constant. Notice that the expansion time interval = 1/Ho. The Hubble constant tells us the age of the universe, i.e., how long the galaxies have been expanding away from each other: Age = 1/Ho. This value for the age is an upper limit since the expansion has been slowing down due to gravity. Taking the expansion slowdown into account, we get an age closer to 2/(3 Ho). Still, the age looks like a number × (1/Ho), so if the Hubble constant is large, the age of the universe will be small.

Today, we have three measurements for the age of the Universe. If the Universe is Open, as all evidence seems to indicate, then the measure of age becomes either 17 billion years or 13 billion years (if it is flat). If we fully understand stellar evoution, then the measurements of some globular clusters place the age at 15-20 billion years. Clearly we have a problem in determining the age of the Universe. Furthermore, we have trouble pinning down a value for the Hubble Constant. The small value of Ho = .5 indicates a Universe that is on the order of 15-20 billion years of age. A larger value of Ho = 1.0 puts the Universe at 10 billion years. Astronomers who measure the expansion of the Universe to determine the Hubble Constant have found values of Ho that vary greatly, and thus constrain the Universe to be quite variable in age. To see where these numbers come from, move to Age of the Universe Question.

Recent efforts have pointed to agreement among most astronomers toward a Hubble Constant value of Ho = .8. This places the age of the Universe at 13-14 billion years, and this is in agreement with the Flat Universe model that is discussed in the Inflation Theory page of this course, but a good deal of speculative disagreement continues. Here is where the mind becomes challenged to think along realms where scientific theory and multiple philosophies butt against each other, but such an exercise in thought as you may be experiencing is good. Keep asking questions and searching for answers.

Time to move on to Cosmological Principles, or back to Red-Shifting, Expansion, or to Cosmology Introduction, or go to the Syllabus .


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