If you follow much of what happens in the modern field of cosmology, you have probably encountered references on a regular basis to concepts like the expansion of the universe and inflation. However, although we tend to use the terms "expansion" and "inflation" somewhat interchangeably in every-day parlance, the two terms actually mean very different things in the context of cosmology. My goal here is to briefly explain the difference between the technical usage of these two concepts, and to show that they are not both equally confirmed by the available evidence. I then want to use this subject as an opportunity to illustrate a chain of reasoning which has become quite common in the natural sciences, one which precludes legitimate scientific explanations from containing explicit references to a Creator. I'm not going to aim for a comprehensive treatment of these issues; I only want to provide a short discussion to give a flavor for what makes these ideas so fascinating, as well as what kinds of conclusions they should lead Christians to accept or reject, based on the truth of Scripture and what the available evidence actually requires them to believe.
Let me begin by discussing what cosmologists have in mind when they talk about the expansion of the universe. There is little question that the universe is expanding. How do we know this? The idea is relatively simple: in the same way that a policewoman might use her radar gun to measure the speeds of passing cars on a highway, astronomers have long used their version of a radar gun to measure the "speeds" of galaxies in outer space. The surprising result of these measurements is that almost all galaxies are moving away from us and that the speed of each galaxy depends on how far away it is, with the galaxies near us receding at relatively low speeds, and the galaxies much further away receding at relatively large speeds.
This is similar to the situation which one encounters in baking a loaf of bread with chocolate chips interspersed throughout it: from the perspective of each chocolate chip, every other chocolate chip is moving away from it as the loaf of bread rises and expands. Moreover, the chocolate chips which are relatively close together will not move apart too much, while the chocolate chips which start out far apart will move much more during the expansion of the bread (because there is more dough in between them to expand). From the perspective of each chocolate chip, therefore, the chips which are nearby do not appear to move too quickly, while the chips which are far away do appear to move rather quickly. This is exactly the sort of thing we see when we look at other galaxies spread throughout the universe, and the fact that each galaxy (or "chocolate chip") is moving away from us in the manner we observe gives us good reason to think that we live inside a universe (or "loaf of bread") which is expanding as time goes on.
There's one thing I should clarify before we go any further: the fact that galaxies appear to be receding from us at various speeds doesn't mean that those galaxies are actually moving at those speeds; it means that the universe itself is expanding, and that the galaxies are just "along for the ride." The same thing is true in the analogy of the loaf of bread: the chocolate chips are not actually moving inside the bread; they are just being carried along as the bread itself expands. This is an important distinction to keep in mind when trying to understand what cosmologists today actually believe about the universe and how it began.
The most widely accepted view among scientists today of how the universe came into being is the so-called "Big Bang theory," which states that the universe resulted from something like a colossal explosion out of a space-time singularity. The idea of a singularity can be a little confusing, so let me describe it in terms of an analogy. It is best not to think of the singularity as some little "egg" with the whole universe jammed up inside of it, which is how it is often visualized. Rather, the singularity is much more like an old video tape that (if one is polite) one rewinds before returning to the video rental store: the singularity is like the point at which the video stops rewinding altogether. It's not that the singularity is what existed before the universe, or that the singularity is some strange object which contained the whole universe and then blew up into the universe. Rather, the idea is that when we finish "rewinding" the universe according to the physics we know about, we eventually hit a point where we can't go back any further, and this is what we call the singularity.
Before I talk about how all of this factors into what scientists today actually believe, let me throw one more complication into the mix. As I've already argued, scientists have good reason for thinking that the universe truly is expanding. It turns out that scientists also have good reason to think that the universe itself — even the emptiest regions of pure, outer space — is very close to being the same temperature everywhere. This is quite surprising, if you think about it. In fact, the only way to really make sense of this observation (if you don't have the option of thinking God may have created it that way) is to suppose that, at some point long ago, the universe must have been extremely small, allowing it all to naturally exist at the same temperature. The only way to make this idea consistent with what we observe today is to tack on another phase of the universe's evolution just prior to the earliest moments of the universe's lifetime, during which the universe underwent an extremely rapid expansion, increasing in size by several orders of magnitude. This idea has come to be known as the inflationary hypothesis (or just "inflation"). The key difference between inflation and the "normal" expansion of the universe that we observe today is that the former is thought to have been a relatively short period of extremely rapid expansion, while the latter is now known to be a much slower process. So, to use the standard jargon, the universe is no longer inflating, but it is certainly expanding.
The other major difference between inflation and expansion in cosmology is the role that these two concepts play in the theory itself. Expansion is something which we have good reason to think we can observe: subject to some very reasonable assumptions, we can read off the "speeds" of the other galaxies (relative to ours) in the same way that our hypothetical policewoman can read the speeds of other cars (relative to her own). Inflation, on the other hand, is something which has not been observed directly. Rather, the only "evidence" which currently supports the inflationary hypothesis is the fact that it ostensibly resolves the "constant-temperature" problem I mentioned a bit ago (as well as a few similar problems that I haven't mentioned). It would therefore be inaccurate to characterize both expansion and inflation as being equally confirmed by the available evidence: although the expansion of the universe observed today is virtually certain, the inflationary hypothesis is not at all so well justified. The expansion of the universe is an example of "observational science," which depends to a minimal extent on background assumptions for interpretation of the observation in question. The inflationary hypothesis, on the other hand, is an example of "historical science," which depends on background assumptions (like the truth of the Big Bang theory), and not merely on direct observations alone. Clearly, modern cosmology is a mix of both historical and observational science, and Christians seeking to understand the significance of new results in this field should pay close attention to the distinction between these two concepts.
The Big Bang theory (and its companion, the inflationary hypothesis) originated in part from the kind of historical reasoning that we have been considering: take the observed expansion of the universe and rewind it as far back as it will go. Of course, in order for this to make sense, you need to assume that the universe could have existed for that long. This assumption seems plausible if one assumes, in turn, that the laws of nature provide us with an exhaustive description of reality, and that there is no need to believe in a Creator to understand the universe we inhabit. It's crucial to realize the direction that the logic here is flowing: science has in no way required us to stop believing in a Creator. Rather, the Big Bang Theory itself has been essentially constructed on the foundation that God is at least unnecessary for explaining the history of the universe, if He exists at all. Science has not disproved God's existence; it has simply been constructed on the implicit assumption that God cannot play an operative role in any true, scientifically legitimate description of the universe's history.
From a Christian perspective, we ought to recognize that the universe requires an explanation for its existence. The singularity I mentioned earlier is actually a major problem for physicists: it is where physics stops working altogether (or, at least, the rules we know of cease to apply or make sense). As I explained above, the singularity is the point we eventually hit if we try to "rewind" the expansion of the universe as far back as we can: the further back in time we go, the smaller the universe becomes, and the higher the temperatures go. Eventually, the temperatures get so high that even our best knowledge of physics is unable to predict what should have happened. Either something in our knowledge of physics must change before we reach that point, or the universe must simply have had a beginning which was caused by something (or Someone) nonphysical.
Moreover, there are some cosmologists who have suggested that even the theories which do involve some fundamental change to our understanding of physics cannot sustain a universe which is infinitely old. A paper by Mithani and Vilenkin (2012) has pointed out that, in three of the most popular models which purport to explain what happened "before" the Big Bang, none of the three can be extended infinitely far into the past, but must eventually hit a point where they also must stop "rewinding," or collapse before they have gotten very far at all. So even the proposed modifications to physics which would help cosmologists "get around" the Big Bang's apparent requirement of a beginning to the universe would ultimately seem to encounter the same problems as the Big Bang model itself.
There are many technical details here, of course, and I haven't even attempted to address all of them. Nevertheless, what we appear to be seeing in all of this is an instance of the basic principle that we find in Scripture: in Romans 1:19-20 that "that which is known about God is evident within them; for God made it evident to them. For since the creation of the world His invisible attributes, His eternal power and divine nature, have been clearly seen, being understood through what has been made, so that they are without excuse" (NASB). In particular, creation screams that it was, in fact, created, in no small part because its very existence requires Someone to have brought its existence about. And this is exactly what we read in Genesis 1:1: "[i]n the beginning, God created the heavens and the earth" (NASB). Cosmologists appear to be bumping up against what Christians have known for centuries: that the universe cannot have existed from eternity past, but that it must have had a beginning, and that there must have been a Creator to bring it about.