If the sun quit shining right now, we would not know it for more than eight minutes. It takes light (or any other form of information) that long to travel from the sun to Earth. Light covers the distance from the moon in about a second. Light from Andromeda, the closest large galaxy, travels for 2.5 million years before reaching Earth. The more distant the object, the longer light must travel. Since the universe is only 14 billion years old, this principle means that astronomers can define a boundary containing all the observable stuff of the universe. Beyond that boundary, the universe is not old enough for light to traverse the distance to Earth. By any human measure, that boundary is an incredible distance away, but it does mean that our observable universe has a definite size. This raises an interesting question: What, if anything, exists outside the observable universe?
Anything existing outside of our observable universe would reside in a separate universe, thus defining a multiverse. Now, you might object that this definition seems arbitrary since the simplest assumption of what might exist beyond the boundary is a whole lot more of the same stuff seen in the observable universe. However, this definition brings one major benefit: it quantifies the universe in a defensible way while providing direction for scientific investigation of anything that might exist beyond the universe.
Problems with Conventional Big Bang Cosmology
To understand why scientists think a multiverse exists requires a general understanding of big bang cosmology. During the earliest moments, the universe experienced unimaginably hot temperatures (so extreme that hell would seem cool by comparison), incredible densities, and remarkable uniformity. As the universe transitions from this initial state to one filled with galaxies, stars, and planets, three important changes occur.
Cosmic Microwave Background Radiation
Around 400,000 years old, the universe cooled to 3,000°C. Below this temperature, electrons can combine with protons to form neutral hydrogen atoms. This happens everywhere in the universe at basically the same time. The light emitted from the formation of the neutral hydrogen atoms records the temperature of the universe. The ensuing expansion of the universe during the last 13-plus billion years has stretched that light so that scientists detect it as microwaves. The uniformity of this cosmic microwave background radiation (CMB) provides strong evidence that we live in a big bang universe.
However, the uniformity does pose a problem. Calculating the expansion of the universe as far back as possible, the region that emits the CMB that we see in one direction would never be in contact with the region emitting the CMB from the opposite direction. Consequently, there is no reason for those two regions to have the same temperature, but they do. Additionally, although many measurements confirm the uniformity of the CMB, scientists have measured tiny ripples in the CMB that result in the formation of galaxies, stars, and planets. Standard big bang cosmology has no explanation for these two facts.
Geometry and Magnetic Monopoles
In the first fraction of a second (less than a trillionth of a trillionth of a second), the universe cooled enough for some important changes. Though an incredibly brief period of time, this period shaped the future of the universe. The temperature dropped such that the strong nuclear interaction separated from the weak nuclear and electromagnetic interactions. This transition laid the foundations for the amount of normal and dark matter the universe would contain. The amount of dark energy was determined even earlier.
One interesting consequence of this transition is the production of magnetic monopoles. Electric charge comes in monopole form—an electron is an electric monopole with negative charge, and a positron is an electric monopole with positive charge. Magnets only exist in dipole form—every north pole is paired with a south pole. Theoretical modeling shows that the separation of the strong nuclear and electroweak interactions produces an abundance of magnetic monopoles, but scientists find no evidence that they exist. Big bang cosmology has no explanation for this discrepancy.
The universe has expanded continuously over the last 14 billion years. The incredible amount of mass in the universe should cause the expansion to slow down. If the universe contained enough mass, the expansion would eventually stop and switch to contraction. With too little mass, the expansion would continue forever. With the just-right amount of mass, the expansion would gradually slow down and eventually stop (although it would take forever to do so). In scientific terms, the three scenarios correspond to closed, open, or flat geometries for the universe. If the universe were two-dimensional, these geometries would look like the surface of a ball, the surface of a saddle, or a piece of paper. Even without understanding dark matter or discovering dark energy, scientists knew that the geometry of the universe was remarkably close to flat. A universe that supports life must be close to flat, but flat is unstable. If the early universe were slightly open or slightly closed, it would be nowhere near flat today. Based on scientists’ calculations, in order for the universe to appear flat today, the mass density needed to vary no more than one part in 1024 (although some calculations put the number at one part in 1060)! Again, big bang cosmology offers no explanation for this incredible degree of fine-tuning.
Inflation to the Rescue
In the 1970s and 1980s, many scientists were working to resolve these problems with big bang cosmology. One group working on the magnetic monopole problem recognized that a specific kind of “phase transition” in the early universe could suppress the number of magnetic monopoles produced in the observable universe. Detailed studies of this phase transition showed that it also caused the universe to rapidly expand (exponentially, not linearly) in a way that solved the fine-tuning of the universe’s geometry and naturally produced the uniformity of the CMB. Additionally, the exponential expansion amplified subtle quantum fluctuations to reproduce the tiny temperature ripples found in the CMB. (Phil Halper produced a video describing the historical development of inflationary models and how they lead to a multiverse.)
Inflation, the moniker given to the period of rapid expansion, solved the four large problems associated with the big bang model. This fact convinced many scientists of inflation’s validity, and ongoing observations of the CMB and the clustering of galaxies in the universe continue to buttress the model. So, how does inflation relate to the multiverse? It does in two ways.
First, the exponential expansion of the universe means that our observable universe composes just a small fraction of the amount of stuff that exists. As scientists calculate the size of this multiverse, they get numbers ranging from 1,000 times the size of the observable universe to something that is spatially infinite. If inflation happened, a Level I multiverse exists. While this type of multiverse is non-controversial (in my opinion), all it really says is that a lot more of the same stuff exists beyond what scientists can measure.
Second, the simplest theoretical mechanisms that produce inflation inevitably lead to a Level II multiverse. It may be that all the proposed mechanisms are incorrect, but it certainly seems reasonable to conclude that if inflation happened, we live in a Level II multiverse.
Good scientific evidence makes the existence of a multiverse a reasonable conclusion (although not a sure thing). The real issue surrounding the multiverse is whether it fits more comfortably within a theistic worldview or a strictly naturalist worldview. As I have studied the multiverse, it has led me to think that one can make a strong case that the theistic worldview provides the best explanation of all the issues raised by the existence of a multiverse.