Does Big Bang Cosmology Prove the Universe Had a Beginning? (2000)
There are a number of arguments for the existence of God which depend, at least in part, upon the notion that the universe had a beginning. Sometimes apologists appeal to Big Bang cosmology to support that notion. For instance, according to Christian apologist Phil Fernandes:
The big bang model also teaches that the universe had a beginning. In 1929, astronomer Edwin Hubble discovered that the universe is expanding at the same rate in all directions. As time moves forward, the universe is growing apart. This means that if one goes back in time the universe would be getting smaller and smaller. Eventually, if one goes back far enough into the past, the entire universe would be what scientists call “a point of infinite density.” This marks the beginning of the universe, the big bang.
It is perfectly understandable that Fernandes and many others would make such an argument–as far as I can tell, the argument was commonplace among scientists and science writers long before religious apologists picked it up. However, the argument is incorrect. In saying so, I am not saying anything novel or unorthodox–rather, I am just repeating what cosmologists have long known yet somehow failed to communicate adequately to the public, as much as they may have tried. I will try to explain myself in the rest of this paper. The details may sound technical on a first reading, since I will be making reference to general relativity and quantum mechanics, but the main point is actually quite schematic, and I believe it should be understandable to anyone.
So what does Big Bang cosmology tell us?
As Fernandes correctly notes, the universe is expanding. Using the general theory of relativity, we can therefore infer from this data that the universe should be smaller and smaller as one looks back into the past. However, this works only up to a point. There is a point in time called the “Planck time” (after the late physicist Max Planck, one of the pioneers of quantum mechanics) before which our ability to infer the behavior of the universe on the basis of general relativity alone is destroyed. The problem is that prior to the Planck time, the universe is so small that quantum mechanical effects become very important. Therefore, a correct description of the behavior of the universe prior to the Planck time requires a synthesis of quantum mechanics and general relativity–a theory of quantum gravity, in other words. And to this date, no full theory of quantum gravity has been developed, much less attained the consensus status that post-Planck-time Big Bang theory enjoys. Without such a theory, we cannot draw from cosmology any conclusions about whether the universe had a beginning or not.
University of Chicago physicist Robert M. Wald made this point nicely as early as 1977, although I imagine it must have been understood well before then:
Do we expect the theory of general relativity to break down in the extreme conditions near a spacetime singularity? The answer is yes. We know that on a microscopic scale, nature is governed by the laws of quantum theory. However, the principles of quantum mechanics are not incorporated into general relativity. Hence, we do not believe that general relativity can be a true, final theory of nature. Classical mechanics (that is, Newton’s laws of motion) provides us with an accurate description of the motion of macroscopic bodies, but it breaks down when we attempt to apply it on atomic distance scales. In a similar manner, we believe that general relativity provides an accurate description of our universe under all but the most extreme circumstances. However, near the big bang singularity when the scale factor a goes to zero and the density and curvature become infinite, we expect general relativity to break down. What is the new, fundamental theory of nature which incorporates the principles of both general relativity and quantum theory? What does this theory say about spacetime singularities? Even the most optimistic theorist can only hope for the beginning of an answer to these questions within the foreseeable future.
Advances have certainly been made since Wald wrote the above passage, but as of yet, there is no definitive theory of quantum gravity. Nevertheless, we can still ask what quantum gravity might say about the origin of the universe. According to Penn State physicist Lee Smolin, there are three possible scenarios:
- [A] There is still a first moment in time, even when quantum mechanics is taken into consideration.
- [B] The singularity is eliminated by some quantum mechanical effect. As a result, when we run the clock back, the universe does not reach a state of infinite density. Something else happens when the universe reaches some very high density that allows time to continue indefinitely into the past.
- [C] Something new and strange and quantum mechanical happens to time, which is neither possibility A or B. For example, perhaps we reach a state where it is no longer appropriate to think that reality is composed of a series of moments that follow each other in a progression, one after another. In this case there is perhaps no singularity, but it may also not make sense to ask what happened before the universe was extremely dense. 
So it remains possible that once a theory of quantum gravity is generated and established, Fernandes and like-minded apologists will be able to refer to cosmology as scientific proof that the universe had a beginning. In the absence of such a theory, however, that particular line of argument must be suspended.
 P. Fernandes. 1997. The God Who Sits Enthroned: Evidence for God’s Existence. Bremerton, WA: IBD Press. p. 96.
 Of course, if that turns out not to be the case for you, please send feedback and indicate what I haven’t explained clearly, so I can make revisions as necessary.
 For anyone trying to anticipate my argument, I would like to stress that the objection I am making does not hinge upon the adequacy of such notions as “the past” in a relativistic framework.
 I believe one source of the public’s confusion about what Big Bang theory says about the origin of the universe is the fact that the Planck time is typically referred to as something like “the first 10-43 seconds of the universe.” If it is the first 10-43 seconds of the universe, then wouldn’t it seem that there must have been a beginning to the universe? I presume that what cosmologists mean when they talk about Planck time that way is that if you were to ignore quantum mechanical effects, and thus predict a beginning of the universe from general relativity alone, then the Planck time would be 10-43 seconds after that hypothetical beginning. This provides a convenient way to assign dates to everything, but tells us nothing about whether there was really a beginning to the universe.
 R. M. Wald. 1977. Space, Time, and Gravity: The Theory of the Big Bang and Black Holes. Chicago: University of Chicago Press. p. 53.
 L. Smolin. 1997. The Life of the Cosmos. Oxford: Oxford University Press. p. 82. Format altered slightly here.
 As Nicholas Huggett (University of Illinois at Chicago philosopher of physics) informs me, there is a fourth scenario. It relies upon technical consderations, but if they are too confusing you may safely ignore them for the purposes of this paper, since the viability of scenarios [B] and [C] alone are enough to establish the thesis of this paper. In any case, the fourth scenario is that [D] there is an initial singularity, but that it is cut out of the timeline, so that while the universe has a finite age, there is no first instant and hence no beginning–i.e. the set of times is a half-open set, like the set (0, 1] on the real number line. Surprisingly, this scenario is viable even in the pure general relativistic picture of the Big Bang–a point made, for instance, by Adolf Grünbaum (see A. Grünbaum. 1989. “The Pseudo-Problem of Creation.” Philosophy of Science 56(3): 391). So even if quantum gravity did not come into play, Big Bang cosmology apparently would not demonstrate with certainty that the universe had a beginning.
 I would like to thank Matt Lund, Nick Huggett, and Jon Jarrett for reviewing this paper.