What does it mean when astronomers say that we live in an expanding universe? What is actually expanding?

Question: Explain the concept of an expanding universe and what is actually expanding. Discuss the observational evidence and the underlying theory supporting this idea. Answer: The concept of an expanding universe refers to the observation that galaxies are moving away from each other, suggesting that the fabric of space itself is stretching and getting larger over time. This was first observed by astronomer Edwin Hubble who discovered the redshift of galaxies, indicating their recession from us. This empirical evidence, known as Hubble's law, is a key component supporting the concept of an expanding universe. The Big Bang Theory, which posits that the universe started its expansion roughly 13.8 billion years ago from a singularity, fundamentally supports this idea as well. It's important to note that it's not the galaxies or other objects that are expanding, but rather the space-time continuum itself. The implications of this expansion are critical for understanding the universe's eventual fate, as it influences factors like the total amount of energy, the balance between matter and dark energy, and the universe's overall geometry.