Expert: Courtney Seligman - 7/14/2010

Question
I am reading Brian Greene's The Fabric of the Cosmos & managed to get confused about his description of Einstein's Cosmological Constant (CC).  I think he said that Einstein amended his equations for general relativity in 1919, putting in the CC so that the universe would be static--neither contracting nor expanding.  Greene also says (I think) that if Einstein had Not put in the CC, his orig equations would have predicted Hubble's later discovery that the universe was expanding.  BUT surely since Einstein didn't know about "negative gravity," Einstein's orig equations would have predicted a Contracting--not an Expanding--universe.  Right?  Can you clarify?

Answer
Einstein's original equations predict that if the mass-density of the Universe is greater than a certain value per unit of empty space, then the Universe will contract. But they also predict that if the mass-density is less than that value, the Universe will expand. So barring a change in the equations, the Universe should either be getting steadily larger or smaller, depending upon the ratio of mass to empty space.

Einstein added "lambda", his "cosmological constant", because in 1919 there was no evidence that the Universe was either expanding or contracting, and since his equations said it MUST do one of those things, he had to "fix" the equations to match what he thought was reality. He did so by creating a number which canceled out whatever expansion or contraction would otherwise be predicted by the equations. However, once it was discovered that the Universe WAS expanding, Einstein called his creation of the "cosmological constant" the biggest mistake of his career, presumably because (1) pulling things out of the air like that is bad physics, and (2) it kept him from making a prediction which would have been proven spectacularly correct, just a few years later.

I might note that the term "negative gravity", although one way of expressing the tendency of absolutely empty space to expand at a certain rate, isn't usually considered to be gravity in the normal sense. When we talk of gravity as a force, we tend to assign it to whatever (large) mass bends the fabric of space-time to produce the curved motion which we associate with gravity. But the natural expansion of space cannot be assigned to any object, because it is due to the ABSENCE of objects of significant mass. Hence the tendency in recent years to assign the term "dark energy" to the phenomenon, thereby avoiding the use of the term "negative gravity". But no matter how you look at it, or what you call it, even though Einstein had no way of knowing that his original equations were correct, they did indeed include both "positive" and "negative" gravity.