In a Universe that comes to be dominated by dark energy, there are four regions: one where everything within it is reachable and observable, one where everything is observable but unreachable, one where things will someday be observable, and one where things will never be observable. The numbers correspond to our consensus cosmology as of early 2023. (Credit: Andrew Z. Colvin/Wikimedia Commons; annotations: E. Siegel)

The big theoretical problem of dark energy

The zero-point energy of empty space is not zero. Even with all the physics we know, we have no idea how to calculate what it ought to be.

One of the most fundamental questions we can ask about our Universe itself is “what makes it up?” For a long time, the answer seemed obvious: matter and radiation. We observe them in great abundances, everywhere and at all times throughout our cosmic history. For some ~100 years, we’ve recognized that — consistent with General Relativity — our Universe is expanding, and the way that the Universe expands is determined by all the forms of matter and radiation within it. Ever since we realized this, we’ve strived to measure how fast the Universe is expanding and how that expansion has changed over our cosmic history, as knowing both would determine the contents of our Universe.

In the 1990s, observations finally became good enough to reveal the answer: yes, the Universe contains matter and radiation, as about 30% of the Universe is made of matter (normal and dark, combined) and about ~0.01% is radiation, today. But surprisingly, about 70% of the Universe is neither of these, but rather a form of energy that behaves as though it’s inherent to space: dark energy. The way this dark energy behaves is identical to how we’d…