SCIENCE & TECHNOLOGY

Dark Energy, Dark Matter, and the Bible

The field of modern cosmology is the study of the universe and its most important properties, answering questions such as, What is our universe made of? How does it behave? and How did it get to its present state? Two concepts which come up regularly in helping cosmologists to answer these questions are known as dark matter and dark energy. And although we often hear about these ideas in the news or in popular science fiction, it's reasonable to ask: What do we actually know about dark matter and dark energy? How should a Christian think about these concepts, and do they conflict with the description of God's creation of the universe described in the opening chapters of Genesis? Before I try to answer these questions, let me briefly describe what dark matter and dark energy are, and then present some of the primary evidence for their existence.

Dark matter was first predicted in 1933 by a cosmologist named Fritz Zwicky who was trying to understand the motions of galaxies through space. Since the force of gravity causes things to move (e.g., think of dropping an apple), an object's motion through space can be related to the gravitational force that it feels: the stronger the gravitational force, the stronger the effects on the object's motion. In studying how galaxies move in groups, Zwicky found that there was not enough gravitational force between the individual galaxies in the group to explain the overall motions of each galaxy. In order to explain the motions he actually observed, Zwicky needed to find a way to make the gravitational force between the galaxies stronger than it initially appeared to be. To do this, he first noticed that the gravitational force could be made stronger if he made the galaxies "heavier" by imagining that each galaxy contained more material than just the stars which were visible to his telescopes. This extra material would therefore have to be invisible — meaning that it could not emit visible light the way stars do — and so it came to be known as "dark matter." This dark matter, Zwicky suggested, would make the galaxies heavier, and thereby add the extra gravitational force that was needed to explain the galactic motions he had observed. Since then, Zwicky's conclusions have come to be regarded as a standard part of how we understand our universe: dark matter is generally believed to be present in and around almost every galaxy, even though today we still do not actually know what dark matter is.

The presence of dark matter can also be inferred from the properties of our own galaxy's motions. If one measures the orbital speed of stars in our galaxy, and how this orbital speed depends on the stars' distances from the galaxy's center, it is possible to use law of gravity to predict how much mass must be contained within those stars' orbits. Interestingly, once scientists actually calculated the predicted mass, and compared it with the total mass of all visible stars in the same region, they again found that the predictions were much larger than the total visible mass, implying that there must be a lot more mass in the same region which is not visible. Precise numerical calculations based on the most recent cosmological data have estimated that this dark matter is about 5 times as heavy as "normal," visible matter, like stars. Thus, although we still do not know what dark matter is, it nevertheless has a good basis in observation and is very likely (in my opinion) to exist.

Despite having a similar name, dark energy is quite different from dark matter. To understand the primary evidence for dark energy, one first needs to understand something about how astronomers measure distances to cosmological objects, like galaxies. One of the most popular methods uses the concept of a "standard candle," which is an object which has a known brightness that we can calculate. If we can calculate how bright an object really is, and measure how bright it appears to be in the night sky, we can use this to estimate how far away the object is. Standard candles are therefore extremely useful for estimating distances in cosmology.

The other bit of background one needs in order to appreciate the significance of dark energy is the realization that the universe is expanding. This means that almost all galaxies are receding from our own galaxy, and the farther away a galaxy is, the faster it is receding from us. All of this can be written down in precise mathematical form, and it is possible to determine (after working through the math) whether the universe's expansion has been constant throughout its lifetime, or whether it has dramatically sped up (or slowed down) at some point in the past.

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Published 3-13-17