# What is the Cosmological Constant?

In the early part of the 20th century, a young scientist named Albert Einstein was considering the properties of light and mass, and how they are related to each other. The result of his deep thinking was a theory of relativity. His work changed modern physics and astronomy in ways that are still being felt. Every science student learns his famous equation E=MC2 as a way of understanding how mass and light are related. It's one of the fundamental facts of existence in the cosmos.

### Constant Problems

As profound as Einstein's equations for the general theory of relativity were, they posed a problem. He was aiming to explain how mass and light in the universe and their interaction could still result in a static (that is, non-expanding) universe. Unfortunately, his equations predicted the universe should be either contracting or expanding. Either it would expand forever, or it would reach a point where it could no longer expand and it would start to contract.

This didn't feel right to him, so Einstein needed to account for a way to keep gravity at bay to explain a static universe. After all, most physicists and astronomers of his time simply assumed that the universe WAS static. So, Einstein invented a fudge factor called the "cosmological constant" that tidied up the equations and resulted in a lovely, non-expanding, non-contracting universe. He came up with a term called Lambda (greek letter), to denote the density of energy in a given vacuum of space. Energy drives expansion and lack of energy stops expansion. So he needed a factor to account for that.

### Galaxies and the Expanding Universe

The cosmological constant didn't fix things the way he expected. Actually, it did seem to work... for a while. That was until another young scientist, named Edwin Hubble, made a profound observation of variable stars in distant galaxies. The flickering of those stars revealed the distances of those galaxies, and something more. Hubble's work demonstrated not only that the universe included many other galaxies, but, as it turns out, the universe was expanding after all and we now know that the rate of expansion has changed over time.

That pretty much reduced Einstein's cosmological constant to a value of zero and the great scientist had to rethink his assumptions. Scientists didn't discard the cosmological constant. However, Einstein would later refer to his addition of a cosmological constant to general relativity as the greatest blunder of his life. But was it?

### A New Cosmological Constant

In 1998, a team of scientists working with the Hubble Space Telescope was studying distant supernovae and noticed something quite unexpected: the expansion of the universe is accelerating. Moreover, the rate of expansion isn't what they expected and was different in the past.

Given that the universe is filled with mass, it seems logical that the expansion should be slowing down, even if it was doing so ever so slightly. So this discovery seemed to run contrary to what Einstein's equations would predict. Astronomers had nothing they currently understood to explain the apparent acceleration of expansion. It's as if an expanding balloon changed its rate of expansion. Why?  No one is quite sure.

In order to account for this acceleration, scientists have gone back to the idea of a cosmological constant. Their latest thinking involves something called dark energy. It's something cannot be seen or felt, but its effects can be measured. This is the same as dark matter: its effects can be determined by what it does to light and visible matter. Astronomers may now know what dark energy is, just yet. However, they do know that it's affecting the expansion of the universe. Understanding what it is and why it's doing that is going to require a great deal more observation and analysis.

Maybe the idea of a cosmological term wasn't such a bad idea, after all, assuming dark energy is real. It apparently is, and it poses new challenges for scientists as they seek out further explanations.

Edited and updated by Carolyn Collins Petersen.