Question

What do we mean by the observable universe? How big is it? Answer both in absolute terms (that is, a size in light-years) and by describing a way of putting its vast size into perspective.

Short answer

The observable universe is approximately 93 billion light-years in diameter, showing how vast it is by comparing the Milky Way to a grain of sand on a football field.

Step-by-step solution

Understanding the Observable Universe

The observable universe refers to the part of the universe that we can currently see or observe from Earth, due to the finite speed of light. Because light travels at a finite speed, we can only see objects whose light has had time to reach us since the beginning of the universe, approximately 13.8 billion years ago.

Calculating the Size in Light-years

Despite the universe being 13.8 billion years old, the observable universe is approximately 93 billion light-years in diameter. This is because space itself has been expanding since the Big Bang, making the distance further than just 13.8 billion light-years in each direction.

Putting the Size into Perspective

To put the size of the observable universe into perspective, consider this: if the entire observable universe were scaled down to the size of a football field, the Milky Way Galaxy would only be about the size of a grain of sand on that field. This illustrates how incredibly vast the universe is compared to our galaxy.

Key concepts

Speed of Light

The concept of the speed of light is fundamental in understanding the universe. Light travels at an incredibly fast pace — about 299,792 kilometers per second (approximately 186,282 miles per second). This speed is constant in the vacuum of space and is the ultimate speed limit in the universe. Everything we see in the night sky, from distant galaxies to the closest stars, is viewed as they were in the past. This is because the light takes time to travel from those objects to us. This effect is often described as looking "back in time." For instance, the light from the Sun takes about 8 minutes to reach Earth. This means we see the Sun as it was 8 minutes ago. Understanding the speed of light helps us grasp the scale and history of the universe.

Light-years

A light-year is a unit of distance that describes how far light can travel in one year. It measures about 9.46 trillion kilometers (or nearly 5.88 trillion miles). When astronomers talk about the distance between celestial objects, they often use light-years because the numbers are far more manageable than using kilometers or miles.
The concept of light-years allows us to communicate astronomical distances more easily. For example, Proxima Centauri, the closest star to our solar system, is about 4.24 light-years away. This means its light takes over four years to reach us. Light-years help us understand both the vastness of the universe and the distances between galaxies and other cosmic structures.

Expanding Universe

The universe is constantly expanding, a discovery that reshaped our understanding of cosmic dynamics. Historically, Edwin Hubble's observations revealed that galaxies are moving away from us, with those farther away moving faster. This led to the conclusion that the universe itself is stretching.
As the universe expands, the space between galaxies increases, causing the observable universe to be much larger than the 13.8 billion light-years we might expect based on its age. Even though light has been traveling since the Big Bang, the distance the light has covered appears more because space has expanded during that time. The expansion of the universe continues, which is why its true size is often measured as about 93 billion light-years in diameter.

Milky Way Galaxy

The Milky Way Galaxy is our cosmic neighborhood, a barred spiral galaxy that contains our Solar System. It spans about 100,000 light-years in diameter and is home to hundreds of billions of stars. Our Sun is situated in one of its spiral arms, approximately 27,000 light-years from the galactic center.
Despite its vast size, the Milky Way is just a tiny part of the observable universe. When scaling the universe down to relatable terms, like a football field, our galaxy is minuscule in comparison — about the size of a grain of sand. This metaphor helps emphasize the enormous size discrepancy between our galaxy and the vastness of the observable universe. Understanding our place in the Milky Way is crucial for comprehending both local and cosmic scales.