A new theory suggests that the current expansion rate of the universe is not fixed, but was quite different in the early universe, suggesting that the universe could start contracting at some point. This could completely upend our current understanding that a universe will end with a great expansion that pushes atoms apart. The theory could encompass the entire expansion history of our universe within one unified model. It uses a scalar field, which acts like a cosmic thermostat that can flip the universe's fundamental vacuum energy density from a negative value in the very early universe to the positive one we observe today. The discovery could help resolve persistent issues in our cosmic story, such as the difference between how fast we think the universe expanding based on early cosmos data and how fast it measures it expanding locally (a discrepancy, sometimes called Hubble tension).