How Does the Universe Expand?

When observing distant celestial bodies, it becomes apparent that they are receding from us. The farther away an object is, the quicker it appears to be moving away. For instance, objects situated 13.8 billion light-years from us, within the Hubble sphere, recede at the speed of light, while even more distant objects seem to move away faster than light itself!

This observation may seem to contradict the theory of relativity, which prohibits faster-than-light (FTL) travel. However, the reality is different. Distant galaxies are not propelling away due to their own motion; instead, the fabric of space between us and them is expanding so rapidly that the distance increases at a rate that exceeds light speed.

Why Do Distant Galaxies Recede Faster?

The uniform expansion of space occurs everywhere. For example, if every meter of space expands by 1 centimeter per second, then objects situated 1 kilometer apart would be separated by an additional 10 meters within that same second. At a distance of 100 kilometers, the increase would be 1,000 meters, and at 1,000 kilometers, it would be 10,000 meters. Thus, the farther apart objects are, the more pronounced the increase in space between them over time.

This phenomenon can be illustrated by considering dots drawn on an expanding balloon's surface; as the balloon inflates, the dots that are farther apart move away from each other more rapidly than those that are closer together.

Are We at the Center of Expansion?

One might wonder if the apparent motion of all galaxies away from us implies that we occupy a central position in this expansion. The answer is no. The uniform expansion of space means that, regardless of the galaxy chosen as a reference point, it will seem as though everything is moving away from it. In reality, there is no central point of expansion.

The observable universe has a boundary approximately 46.5 billion light-years away, beyond which we cannot see. Photons emitted by objects outside this boundary will never reach us because the space between those objects and us is expanding faster than the light can travel.

Where Is the Universe Expanding?

A pertinent question arises: Where exactly is this expansion occurring? The straightforward answer is that the universe is not expanding into anything. The universe is infinite and has no edges, and it has always been infinite, even during the Big Bang. When scientists refer to the universe being compressed to a minuscule size at the moment of the Big Bang, they are discussing the size of the observable universe, not the entirety of it.

Thus, the universe's expansion is an internal process; distances among objects increase while the universe itself remains infinite. No external space is required for it to expand.

Why Do Galaxy Collisions Occur?

If the universe is expanding, why will our galaxy eventually collide with the Andromeda galaxy in over 4 billion years? The answer lies in proximity: when objects are closer together, the increase in space between them during expansion is less significant. Gravitational forces can draw nearby galaxies together more quickly than the expanding space can separate them.

Will Expansion Continue Indefinitely?

In general, yes, but three potential outcomes exist: the Big Crunch, the Big Rip, and a state of thermal equilibrium (also known as the heat death of the universe). If the Big Crunch theory holds true, gravitational forces could halt the expansion, causing the universe to contract back into a singularity, leading to another Big Bang and a cycle of expansion and contraction.

The Big Rip scenario suggests that accelerated expansion may eventually cause distances between fundamental particles to grow so vast that interaction becomes impossible. This could occur in around 22 billion years. Alternatively, gravitational forces may slow expansion without stopping it, leading to a gradual cooling and eventual thermal equilibrium.

Current observations indicate that the universe is likely headed towards the Big Rip scenario, though this is not yet definitive.

If you want to see more articles about space, please clap!

Subscribe to our channel and feel free to ask any questions; I will address them in future articles.

If you appreciate my work, consider supporting me by becoming a member of Medium for just $5 a month, which will help us produce even better content.