Emmanuel N. Saridakis

The universe is expanding, but at what rate is a question that has been troubling astronomers for decades. Two methods of measurement give different answers, and this discrepancy is called the Hubble tension. Some look at the early universe through the cosmic microwave background, others at nearby supernovae and Cepheid variables.

A balloon inflates on its own, even though no one is blowing it up. That's roughly how the accelerated expansion of the universe looks, and to explain it, scientists usually invoke dark energy, an invisible and poorly understood force. But what if the balloon expands because new air molecules are continuously appearing inside it?

In the 1920s, Edwin Hubble noticed galaxies were moving apart — thus discovering the expansion of the Universe.

A wormhole is a hypothetical tunnel in curved spacetime connecting two distant regions of the universe and potentially shortening the journey between them.