How is the universe expanding?

The universe is expanding because space itself is stretching, causing galaxies to move away from each other.

The concept of the universe expanding is a fundamental aspect of cosmology, the study of the origin and evolution of the universe. This idea is based on the observations of Edwin Hubble in the 1920s, who discovered that galaxies are moving away from us in all directions. This phenomenon is known as the Hubble's Law. The further away a galaxy is, the faster it appears to be moving away from us. This is not because the galaxies are moving through space, but because space itself is expanding.

The expansion of the universe is often likened to a balloon being inflated. If you imagine galaxies as dots on the surface of the balloon, as the balloon inflates, the dots move further apart. However, the dots themselves aren't moving, it's the balloon (or space) that's expanding. This analogy helps to visualise the concept, but remember that the universe is three-dimensional, not two-dimensional like the balloon's surface.

The driving force behind the universe's expansion is not fully understood, but it's often attributed to a mysterious form of energy known as dark energy. Dark energy is thought to make up about 70% of the universe and its main property is that it creates a pressure that accelerates the universe's expansion.

The rate of expansion is described by the Hubble constant, although there is ongoing debate among astronomers about its exact value. This constant is crucial for determining the age and size of the universe. Despite the uncertainties, the expansion of the universe is a well-established fact in modern cosmology. It's a fascinating topic that continues to challenge our understanding of the universe we live in.