Don Lincoln is a senior scientist at the U.S. Department of Energy's Fermilab, America's largest Large Hadron Collider research institution. He also writes about science for the public, including his recent "The Large Hadron Collider: The Extraordinary Story of the Higgs Boson and Other Things That Will Blow Your Mind" (Johns Hopkins University Press, 2014). You can follow him on Facebook. Lincoln contributed this article to Space.com's Expert Voices: Op-Ed & Insights.
It has been a century since Albert Einstein published his first papers laying out his crowning intellectual achievement, the theory of general relativity. This theory showed that space was malleable and could twist and distort under the influence of matter. Since a) the shape of space is affected by the distribution of matter and energy, and b) matter moves around, this further means that the shape of space is dynamic — twisting and bending and changing with time. This idea was truly revolutionary.
In the early days, the implications of this theory were not completely obvious and necessary data were missing. This led to some miscues and changes to the theory asscientists of the time developed greater understanding. One such incident is particularly interesting.
The repulsive cosmological constant
While Einstein wrote the first papers describing relativity in November of 1915 as a basic theory of gravity, he and others soon applied the theory to the behavior of the universe as a whole. One of the early implications is that because all matter attracts all other matter, a static universe would not long remain static. The gravitational attraction would cause all matter to collapse into a single point. And even if one did not start with a static universe, the mass distribution of the universe should evolve.
It was a prejudice of the time that the universe was constant and eternal, forever unchanging — at least on the largest levels. This led Einstein to add a term to his initial equations in 1917. While the original formulation of general relativity included only the attractive form of gravity, this new term, called the cosmological constant, was a repulsive term. The attractive and repulsive forms of gravity could be tuned to balance one another, resulting in a stationary and unchanging cosmos.
The situation changed with the construction of a powerful telescope — the Hooker Telescope on Mt. Wilson — shortly after Einstein wrote down his gravitational equations. Although Edwin Hubble is often credited with determining that galaxies are moving away from the Milky Way, the actual story is far more complicated. Astronomers were already discussing the situation in the early 1920s.
However, in 1929, Hubble published a paper in which he established that not only were galaxies moving away from the Milky Way, but that more-distant galaxies were also receding more quickly. That is, the universe was not static. It was expanding. This observation (and those preceding Hubble's paper) led Belgian priest Georges Lemaître to propose in 1931 that the universe originated from a small and compact state, what he called a "Cosmic Egg" and what is now called the Big Bang.
With the realization that his earlier prejudice for an unchanging cosmos was wrong, Einstein removed the cosmological constant from his equations. He was reported by physicist George Gamow as having called it his "biggest blunder." There is some dispute, however, on whether this term originated from Einstein or from Gamow, a notorious wag.
But whether Einstein actually uttered the famous catchphrase, he certainly regretted adding the term and felt that removing it was the right thing to do. Without a static universe, there was simply no need for the addition to his original equations. And this was considered the state of the art for nearly 70 years: The universe was expanding.
A sharper eye on the universe
During the middle years of the 20th century, astronomers discussed the fate of the universe. If the universe began in the denser state suggested by the Big Bang theory and was expanding, then gravity should slow the expansion. It was an open question whether gravity would overcome the expansion (resulting in a "Big Crunch") or expand forever — but the fact that the expansion should slow seemed inarguable.
Although there were earlier hints, it was in 1998 that two experiments studying the question announced the answer, and it was quite a surprise. The expansion of the universe was not slowing. It was speeding up! For this observation, Brian Schmidt of the Australian National University’s Stromolo Observatory, Adam Riess of the University of California, Berkeley, and Saul Perlmutter at Lawrence Berkeley National Laboratory shared the 2011 Nobel Prize in physics. The accelerating expansion has been confirmed and is now regarded as well-established fact.
But that observation led to an obvious question: If, according to Einstein's theory of general relativity, gravity is an attractive force, what could account for the accelerating expansion? What could be pushing the matter of the universe apart? Was it time for Einstein's cosmological constant to experience a renaissance?
Well the short answer is yes. And no. And maybe.
To explain the observation, scientists now say that something like the cosmological constant is necessary. But it may or may not be the actual cosmological constant that Einstein proposed.
Astronomers use the term "dark energy" to describe an energy field in the universe that is effectively a repulsive form of gravity. Telescope observations show this energy field is currently overpowering the more familiar form of attractive gravity throughout the universe, leading to the accelerating expansion.
So what do scientists know about this dark energy? One question is whether it is constant or changing. Einstein's cosmological constant is a field of constant energy density. This is a bit counterintuitive, as a constant density and the expanding volume of the universe means an increasing energy, but this is allowed within the rules of general relativity. On the other hand, there is no reason a priori that dark energy should be constant. Thus, another form of dark energy has been proposed, called "quintessence." Quintessence is a type of dark energy that can change with time.
While scientists are confident that dark energy must exist, there is an open debate as to its nature. Is it constant, or does it vary with time — is it the cosmological constant or quintessence? And, of course, with a question of this magnitude, researchers have undertaken a vigorous experimental program to find the answers.
The Dark Energy Survey is an ambitious effort to understand the nature of dark energy. By measuring the velocity of extremely distant galaxies and supernovae, these scientists will hopefully be able to answer this important mystery. And the answer has truly cosmic implications.
Depending on whether quintessence or the cosmological constant is the right answer, there could be different stories of how the universe will end.
Dark energy almost certainly exists and was anticipated by Einstein nearly a century before its discovery. The story of how he added it and then removed it from his theory is an oft-told one, and it is said that he regretted its temporary inclusion. Over 80 years after Einstein decided that the cosmological constant was an error, it is clear that something like it must exist — the removal was the real error.
In an ironic twist, it might well be that removing this innovative energy term from his equation was Einstein's actual greatest blunder.
Read more of Lincoln's work on his Expert Voices landing page.
Follow all of the Expert Voices issues and debates — and become part of the discussion — on Facebook, Twitter and Google+. The views expressed are those of the author and do not necessarily reflect the views of the publisher. This version of the article was originally published on Space.com.
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