INTRO BY DEB: While much of our work at BioLogos is about presenting the case for evolutionary creation, we also take the time to analyze scientific proposals made by Christians who oppose evolution and an ancient universe. Today we continue a blog series focusing on a proposal from young-earth creationist scientist Jason Lisle to explain how distant starlight could have reached Earth if the universe were created roughly 6,000 years ago. Our guide through the topic is Casper Hesp, a graduate student in astrophysics and a gifted science writer. This series is intended for readers without any background in astronomy who want to learn more about God’s creation and how to think carefully about issues of science and faith.
When we look up at the night sky, we marvel at the countless lights specking the darkness. These lights testify to God’s glory (Psalm 19:1), but they also testify to the universe’s unfathomable size and age. Light from the most distant stars needs billions of years to reach us. In our previous posts, we discussed the attempt of astrophysicist Jason Lisle (a young-earth creationist) to solve this problem for the young-earth creationist paradigm. See our first installment for more details on his proposal, known as the ASC model. As we have seen in the past five posts, the proposal suffers from serious issues, both in theory and in practice. However, it is not fair merely to point out faults in a model, because its value can only be understood in comparison to the available alternatives. That’s why, along our way we made sure to make a comparison with the cosmological “standard model”: an expanding universe (Big Bang) with a cosmic history of about 13 billion years. In every case the ASC model has shown major problems while the Big Bang model is an excellent fit to the data.
Before we can conclude our series, there’s still one elephant in the room which needs to be dealt with. In his original article, Lisle not only put forward his proposal for solving the distant starlight problem, but also made the charge that the “standard” cosmological paradigm suffers from a similar issue with light-travel time: the so-called “horizon” problem. This accusation has been endorsed and repeated by other young-earth ministries in subsequent years as a way to play down the distant starlight problem. The horizon problem does indeed exist, and it does have a conceptual link with the distant starlight problem. However, that does not mean it is comparable to the distant starlight problem in terms of severity. If that were true, our whole discussion of distant starlight might be reduced to a case of “the pot calling the kettle black”. Even when we take into account the horizon problem, the standard model integrates the available evidence much better than any model proposed by young-earth creationists.
A problem on the horizon
What exactly is the horizon problem, and how big is it? The current Big Bang model (i.e. the standard model) describes the universe as ever-expanding. If we had a way to reverse (and speed up) the course of time, we could see the universe shrinking. By going over 13 billion years back in time, we would be able to witness the moment when light started traveling freely for the first time. That light is still visible nowadays as the Cosmic Microwave Background (CMB; see our previous post for more details). Travelling another 300,000 years back in time from that point, we would arrive at the point where standard physics breaks down completely: the initial singularity. This is a huge mystery which we will not discuss here.
The observation relevant to our discussion is that the temperature of the CMB is almost exactly the same everywhere. This is true even if we look at opposite sides of the observable universe. If we look eastward on earth, there is a certain limit to how far your eye can see: the horizon. In a similar way, astronomers use the word “horizon” when they talk about the regions in the universe that lie as far away from us as we can possibly observe. The horizon is determined by the distance that light could have traversed since the universe began. If we look at regions on opposite ends of our horizon (cf., east and west), these are located on opposite ends of our observable universe today. If those areas used to be in contact with each other, their temperature could have evened out through physical processes. However, the 300,000-year time-window between the initial singularity and the emission of the CMB is not large enough to “connect” those regions of our baby universe. In other words, those regions were not located within each other’s horizon (hence the name of the problem, see this link for an illustration).
This leaves Big Bang cosmology with two options. Either (1) the universe started out almost perfectly uniform without causal contact between those regions, or (2) the physical picture of Big Bang cosmology is incomplete. The first option closes the door on physical explanations of the uniformity. That’s why cosmologists have generally favored the second option (and have made impressive progress in that, see below). They are interested in understanding the cosmos in physical terms. In a way, the horizon problem is not about the Big Bang model, but about understanding what gave rise to the initial conditions of that model.
Making a mountain out of a molehill
First of all, it’s important to realize that even the least favorable solution to the horizon problem is still much less dramatic than the proposed solutions for the lack of time in young universe models. This can be illustrated by talking about it from a theological perspective. In case of the Big Bang model, we might solve the horizon problem by assuming that God initially created an almost perfectly uniform universe with tiny density differences from which all the galaxies sprang. A bit ad hoc scientifically, but nothing theologically objectionable here. Now compare this with young universe models. These require a special workaround to explain distant starlight and associated time issues. At the very least, a young universe requires the assumption that God initially created it being “mature”. Unfortunately, this maturity includes an impressive collection of phenomena that indicate event histories spanning billions of years. Consider, for example, the presence of fully formed galaxies, many of them still showing the scars of past collisions, relativistic jets, and differences between distant and nearby galaxies. In that case, God would have loaded the initial conditions of Creation with evidence of events that never happened to begin with. Such deception appears to contradict God’s character as revealed to us in the Scriptures. This provides a stark contrast with the most ad-hoc solution to the horizon problem, initial uniformity, which does not require the assumption that God is lying to us through nature. See the table below for an overview.
Towards a physical solution: an inflatable universe
The set of equations that govern the expansion of the universe is fairly straightforward. They describe how the growth of space occurred gradually for the largest part of cosmic history. However, in the very early universe, circumstances could have been different. Respectable theorists think that the universe went through a period of extremely rapid expansion in the very beginning (around the first 10-32 seconds): inflation. The exact particle mechanism is unknown, but postulating such an inflation era solves a number of questions concerning the initial conditions of the Big Bang.2 One of those is the uniformity of the universe. The period of time before inflation could allow for wrinkles in the universe to be straightened out. Inflation is a relatively simple construct that speaks to several problems at the same time. On top of that, some of its predictions regarding the CMB were confirmed observationally. For these reasons, many cosmologists accept it as an elegant solution that outperforms alternatives. Note here that a solution was only needed because the initial conditions of the Big Bang were otherwise considered to be too “special”. There is also a respectable group of theorists who disagree with inflation theory in its current form. It’s work in progress. For the causality problems of young-universe models, on the other hand, there is no working physical solution that explains the initial conditions.
Table 1. An overview of the comparison between young-universe models and Big Bang cosmology in terms of their causality issues.
The merit of a model…
At this point, let me emphasize the distinction between the Big Bang model and its initial conditions. While cosmologists are still working on physical explanations of the starting point of the Big Bang (such as inflation theory), that is not the crux of the matter if we’re discussing its merit as a scientific model. The merit of the cosmological standard model (i.e., Big Bang cosmology) is based on its success in describing the development of our universe, given a relatively small set of initial conditions. Completely independent observations have repeatedly favored the Big Bang over alternatives. Further scientific work is still being done to understand the specifics of its starting point.
An interesting parallel might be drawn with evolutionary theory, which is a powerful explanatory tool for understanding how present-day species emerged from past populations. It already integrates the findings of diverse fields such as paleontology and genetics in an elegant manner. The scientific merit of evolutionary theory is derived from its ability to do exactly that. It does not hinge on theories concerning the development of the first life-form (abiogenesis), which is still largely beyond the reach of empirical research. In a similar way, the value of the Big Bang model does not depend on a complete explanation of how its initial conditions arose. Instead, its achievements are understood by appreciating how it manages to describe the development of the universe after the starting point.
We can now formulate an answer to the question posed in the title of this post. Does Big Bang cosmology have a big problem, comparable to the distant starlight problem of young-universe models? Definitely not. It is misguided to place the horizon problem and the distant starlight problem in the same box, despite their conceptual connection. Our limited understanding of the very early universe does not make or break the Big Bang model. The horizon problem results from minor theoretical concerns. Contrastingly, distant starlight is a devastating problem because it directly contradicts the central claim of young-universe models (i.e., that the universe is less than 10,000 years old). On top of that, we have seen in this series that the universe is filled with evidence of event histories stretching across millions and billions of years. Many times over, such evidence gives our universe the appearance of great age; and indeed, that is the conclusion to take home from this series of posts. When we talk about the age of the universe, ignoring this evidence is no light matter.