Why is Space Dark? 5 Surprising Facts About the Darkness of Space

A stunning view of the Milky Way galaxy, shining brightly against the pitch-black background of space. — Photo by Andrej Prelesnik on Pexels.com

“Space is big. Really big. You just won’t believe how vastly, hugely, mind-bogglingly big it is.”
Douglas Adams, The Hitchhiker’s Guide to the Galaxy

Space is also dark. Really dark. You might wonder why, given that there are trillions of stars in the observable universe, many of which are brighter than our sun. Why isn’t the night sky a dazzling spectacle of light?

This question, known as Olbers’ paradox, has puzzled astronomers for centuries. It seems to contradict the common sense notion that more stars should mean more light. However, as you will discover in this blog post, the answer is not so simple.

Here are five surprising facts about the darkness of space that will help you unravel this cosmic mystery.

1) The universe is finite but expanding.

One possible explanation for the darkness of space is that the universe is finite, meaning that there are only a limited number of stars in it. If that were the case, then the light from those stars would eventually fade away as it travels across the vast distances of space. However, this explanation is not satisfactory, because it implies that the universe has a definite edge, which is hard to imagine.

A more plausible explanation is that the universe is finite but expanding, meaning that it is getting bigger and bigger over time. This means that the stars are moving away from each other, and from us, at an increasing rate. The farther away a star is, the longer it takes for its light to reach us. Therefore, we can only see the stars that are within a certain distance from us, called the observable horizon. Stars beyond that horizon are too far away for their light to reach us, even if they exist. This limits the amount of starlight that we can see, and leaves some regions of the sky dark.

2) The universe is young.

Another factor that contributes to the darkness of space is the age of the universe. According to the best scientific estimates, the universe is about 13.8 billion years old. That might sound like a very long time, but it is actually quite short in cosmic terms. It is not enough time for the light from the most distant stars to reach us, even if they are within the observable horizon.

To illustrate this point, imagine that you are looking at a star that is 10 billion light-years away from you. This means that the light that you see from that star left it 10 billion years ago, when the universe was only 3.8 billion years old. At that time, the star was much closer to you than it is now, because the universe was smaller and less expanded. However, you cannot see the star as it is now, because its current light has not reached you yet. You can only see its past light, which shows you how it looked 10 billion years ago.

This means that the farther away a star is, the older it looks. And if a star is farther away than 13.8 billion light-years, then its light is older than the universe itself, which is impossible. Therefore, we cannot see any stars that are beyond that distance, even if they exist. This also limits the amount of starlight that we can see, and leaves some regions of the sky dark.

3) The universe is redshifted.

A third factor that affects the darkness of space is the redshift of the universe. This is a phenomenon that occurs when the light from a moving object changes its color as it travels through space. The light from an object that is moving away from us becomes redder, while the light from an object that is moving towards us becomes bluer. This is similar to how the sound of a siren changes its pitch as it passes by us.

Since the universe is expanding, most of the stars and galaxies are moving away from us at very high speeds. This means that their light is redshifted, or shifted towards the red end of the spectrum. As the light travels farther and farther, it becomes more and more redshifted, until it reaches a point where it is no longer visible to the human eye. It becomes infrared, then microwave, then radio, and so on. This means that we cannot see the light from the most distant stars and galaxies, even if they are within the observable horizon. Their light is too redshifted for us to detect. This also reduces the amount of starlight that we can see, and makes the sky darker.

4) The universe is cooling.

A fourth factor that influences the darkness of space is the temperature of the universe. The universe is not a uniform place, and it has different regions with different temperatures. However, on average, the universe is cooling down as it expands. This is because the expansion causes the energy density of the universe to decrease, and the energy density is related to the temperature.

The cooling of the universe has two effects on the starlight that we see. First, it means that the stars themselves are cooling down, and emitting less light. Second, it means that the background radiation of the universe, which is the leftover heat from the Big Bang, is also cooling down, and becoming less bright. The background radiation, also known as the cosmic microwave background, is the oldest light in the universe, and it fills the entire sky. However, as the universe cools, the background radiation becomes dimmer and dimmer, and less able to compete with the starlight. This also contributes to the darkness of space.

5) The universe is evolving.

A fifth and final factor that shapes the darkness of space is the evolution of the universe. The universe is not a static place, and it changes over time. The stars and galaxies that we see today are not the same as the ones that existed in the past, or the ones that will exist in the future. The universe has a history, and a destiny.

The evolution of the universe affects the starlight that we see in two ways. First, it means that the stars and galaxies are changing their properties, such as their size, shape, color, and brightness. Some stars are born, while others die. Some galaxies merge, while others split. These changes alter the amount and distribution of light that they emit, and create a dynamic and diverse night sky.

Second, it means that the stars and galaxies are changing their positions, relative to us and to each other. As the universe expands, the stars and galaxies drift apart, and their apparent locations in the sky shift. This creates a phenomenon called the proper motion of the stars, which is the apparent movement of the stars across the sky over time. The proper motion of the stars is very slow, and it takes thousands of years to notice any significant change. However, over the course of billions of years, the proper motion of the stars can rearrange the constellations, and create new patterns of light and dark in the sky.

The darkness of space is not a simple or straightforward phenomenon. It is the result of a complex interplay of various factors, such as the finiteness, expansion, age, redshift, temperature, and evolution of the universe. These factors limit the amount and visibility of the starlight that we can see, and create regions of darkness in the sky. However, these factors also make the night sky a fascinating and dynamic spectacle, filled with wonders and mysteries.

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