Why is the Night Sky Dark?

When we observe the sky from a location free of light pollution, the sheer number of visible stars makes them nearly impossible to count. Yet, despite the abundance of stars overhead, most of the sky remains dark, and images captured in space confirm that the cosmos is predominantly dark. Given the vast number of stars in the universe, why doesn’t their collective light brighten the night sky? This question, which has intrigued researchers since the 16th century, is known as 'Olbers' Paradox' named after the German astronomer Heinrich Wilhelm Olbers, who wrote about it in the early 19th century. While the question appears straightforward and goes back a long way, the explanation involves insights into the structure of the universe obtained predominantly during the last century.

Despite the trillions of stars in the universe, they do not illuminate the dark night sky. A field of wheat late at night | Source: NASA

Understanding the Paradox

If you were to ask an astronomer from the early 18th century about the structure of the universe, they would describe it as infinite, static, and homogeneous. By infinite, they mean the universe has no boundaries and extends endlessly in all directions. Static implies that the universe is unchanging over time; the number and distribution of stars we see today are much the way they were in the past and will be in the future. Homogeneity suggests uniformity; regardless of where you look, every region of the universe contains the same basic components, such as stars that make up galaxies and galaxy clusters. Additionally, stars throughout the universe are composed of similar materials. Imagine the universe as a salad bowl: though each spoonful might vary slightly in the mix and cut of vegetables, on average, the overall composition remains consistent.

Let's assume we observe the sky and sum up all the light coming from the stars within our field of view. The shape of our field of view is conical, similar to an ice cream cone, with us at the narrow end.  As this cone extends into space, it encompasses an increasingly larger area, and if stars are uniformly distributed across the universe, the number of stars within our field of view increases the farther out we look. However, stars emit light in all directions, dispersing it across space. This dispersion causes the intensity of the light to decrease as it travels, reducing the amount of light that directly reaches us directly from distant stars. Mathematically, these two factors—the increasing number of stars with distance and the diminishing intensity of light—appear to counterbalance each other. According to this reasoning, each "layer" of our conical field of view should contribute equally to the total light, suggesting an infinitely bright sky, contrary to what we actually see at night. Why then is the night sky dark?

Our field of view expands the farther an object is from our eyes. The boundaries of the human field of view | Source: Codexserafinius, Shutterstock

A Paradox of Incorrect Assumptions

The resolution to Olbers' Paradox hinges on reevaluating our foundational assumptions about the universe - that it is infinite, static, and homogeneous. If we find that these assumptions are incorrect, we can revise our conclusions and resolve the paradox. Over the past century, astrophysicists have found that each of these assumptions is incorrect to some degree.

Let's begin with the assumption that the universe is static. In 1929, Edwin Hubble discovered that the universe is expanding and that the farther away a galaxy is from us, the faster it is moving away. Hubble's discovery allowed scientists to estimate that the universe is about 13 billion years old. Over time, further research demonstrated that not only is the universe expanding, but also that the rate of its expansion is changing. In 2011, the Nobel Prize in Physics was awarded to Saul Perlmutter, Brian Schmidt, and Adam Riess for their discovery that the universe's expansion rate is currently accelerating. Therefore, the universe is far from static: it was "born" 13 billion years ago and has been expanding in all directions ever since.

Next, let's examine the assumption that the universe is infinite. Current measurements and observations have not detected any edge or boundary to the universe. Consequently, two primary models emerge: one proposes that the universe is flat and infinite, while the other suggests it is curved and closed, resembling the surface of a sphere where traveling far enough in one direction would eventually lead back to the starting point. Advanced measurements, including those conducted by the Hubble Telescope, indicate that the universe’s curvature is minimal, leading to the current prevailing consensus that the universe is likely flat and infinite. Given this, why might there still be reasons to question this assumption?

While the universe may indeed be infinite, the observable universe—the portion from which we can receive signals—is finite. This limitation arises because the speed of light is finite and the universe has only existed for a finite amount of time. Therefore, we cannot perceive light from stars that are too far away. At first glance, one might estimate the radius of the observable universe to be 13 billion light-years corresponding to the age of the universe times the speed of light. However, the actual radius is approximately 46.5 billion light-years due to the universe’s expansion. We can observe light from stars that are now very distant, but when their light began its journey to us, those stars were much closer. As time progresses and light from increasingly distant stars reaches us, the radius of the observable universe continues to increase.

The sky appears in vibrant colors when viewed through a radio-sensitive telescope. Simulation of the cosmic microwave background radiation | Source: Declman Hillman, Shutterstock

Adressing Incorrect Assumption Number Three

Lastly, let's confront the assumption that the universe is homogeneous. Current astrophysical research suggests that inhomogeneities in the early universe led to the formation of stars, galaxies, and galaxy clusters. In 2006 John Mather and George Smoot were awarded the Nobel Prize in Physics for their contributions to this understanding. While it’s true that on average, similar numbers of stars populate every volume of space in the universe, the light we receive from distant stars differs from that of nearby stars for several reasons:

First, as previously mentioned, distant galaxies are moving away from us at high speeds. This recession causes a shift in the wavelength of light actually emitted by these objects compared to the wavelength that reaches us, a phenomenon known as the Doppler effect. As a result, the light emitted by distant stars shifts towards the infrared or microwave radiation spectrum – types of radiation which are invisible to the naked eye.

Secondly, the speed of light is finite, so the light reaching us from distant stars was emitted a long time ago. When observing a star one light-year away, we see it as it was a year ago. For example, for stars billions of light-years away, we see them as they were when they were very young stars, at a time when they emitted less light.

This is the solution to Olbers' Paradox: the universe is not infinite, static, or perfectly homogeneous. These factors cause the light from distant stars to be less intense than the light from nearby stars. As a result, the amount of light reaching us is not infinite, which is why the night sky is dark.