Using a diagram, explain why the expansion of the universe as seen from a distant galaxy would look the same as seen from our Galaxy.

The expansion of the universe is a fundamental concept that can be better understood through the cosmological principle. This principle asserts that on a large scale, the universe is both homogeneous and isotropic, meaning it appears the same in all directions and at all locations.

The principle forms the foundation of modern cosmological models because it suggests that the laws of physics are uniform throughout the universe. Essentially, the cosmological principle means that there is no 'special' place in the universe; each part of the universe looks like any other part when viewed on a sufficiently large scale.

Given the cosmological principle, it's easier to comprehend why the universe's expansion appears to be uniform regardless of the observer's location, just like how the inflation of a balloon will appear the same whether you're examining it up close or from a distance.

Let's delve into the concept of the 'observable universe.' This term refers to the portion of the entire universe that we can potentially observe from Earth or any other vantage point. This region is bound by the most distant light that has had time to reach us since the beginning of the cosmic expansion (the Big Bang).

Due to the finite speed of light and the finite age of the universe, there are regions of space whose light has not yet had enough time to reach us. Even though these areas are part of the universe, they lie outside our observable universe.

For any observer, located on Earth or in a distant galaxy, the observable universe may look different because it's centered on the observer's point in space. However, because of the cosmological principle, the properties and laws observed within each observable universe would essentially be identical.

Homogeneity and isotropy are two characteristics that describe the structure of the universe. Homogeneity implies that on a grand scale, the universe has a uniform composition and structure; this means that no matter where you look, the distribution of galaxies, stars, and other cosmic entities is relatively uniform.

Isotropy, on the other hand, means that the universe looks the same in all directions. It does not favour any direction over the others. When we observe the night sky from Earth or even from a far-off galaxy, there is a uniform distribution of galaxies across the vast cosmic landscape.

This understanding is crucial for explaining why the expansion of the universe would appear similar from different locations, upholding the notion that every point in the universe is equivalent and reinforcing the cosmological principle.

The Big Bang theory is the leading explanation about how the universe began. It posits that approximately 13.8 billion years ago, the universe started as a tiny, dense fireball that suddenly and rapidly expanded. This extraordinary event set into motion the expansive cosmos we observe today.

The theory implies that the universe has been expanding ever since the Big Bang. The evidence for this expansion comes from observations showing that galaxies are moving away from each other. This receding motion can be observed as a redshift in the light emitted by galaxies – analogous to the Doppler effect with sound, wherein sounds stretch out as a source moves away from an observer.

Under the Big Bang framework, the universe's ongoing expansion would look identical from any galaxy because this expansion began uniformly everywhere. The inflation of space between galaxies is not due to them moving through space but rather the fabric of space itself stretching.