einstein

Back in 1917, when Einstein had analyzed the “cosmological considerations” arising from his general theory of relativity, most astronomers
thought that the universe consisted only of our Milky Way, floating with its 100 billion or so stars in a void of empty space. Moreover, it seemed a
rather stable universe, with stars meandering around but not expanding outward or collapsing inward in a noticeable way.
All of this led Einstein to add to his field equations a cosmological constant that represented a “repulsive” force (see page 254). It was invented
to counteract the gravitational attraction that would, if the stars were not flying away from one another with enough momentum, pull all of them
together.
Then came a series of wondrous discoveries, beginning in 1924, by Edwin Hubble, a colorful and engaging astronomer working with the 100inch
reflector telescope at the Mount Wilson Observatory in the mountains above Pasadena, California. The first was that the blur known as the
Andromeda nebula was actually another galaxy, about the size of our own, close to a million light years away (we now know it’s more than twice that
far). Soon he was able to find at least two dozen even more distant galaxies (we now believe that there are more than 100 billion of them).
Hubble then made an even more amazing discovery. By measuring the red shift of the stars’ spectra (which is the light wave counterpart to the
Doppler effect for sound waves), he realized that the galaxies were moving away from us. There were at least two possible explanations for the fact
that distant stars in all directions seemed to be flying away from us: (1) because we are the center of the universe, something that since the time of
Copernicus only our teenage children believe; (2) because the entire metric of the universe was expanding, which meant that everything was
stretching out in all directions so that all galaxies were getting farther away from one another.
It became clear that the second explanation was the case when Hubble confirmed that, in general, the galaxies were moving away from us at a
speed that was proportional to their distance from us. Those twice as far moved away twice as fast, and those three times as far moved away three
times as fast.
One way to understand this is to imagine a grid of dots that are each spaced an inch apart on the elastic surface of a balloon. Then assume that
the balloon is inflated so that the surface expands to twice its original dimensions. The dots are now two inches away from each other. So during
the expansion, a dot that was originally one inch away moved another one inch away. And during that same time period, a dot that was originally
two inches away moved another two inches away, one that was three inches away moved another three inches away, and one that was ten inches
away moved another ten inches away. The farther away each dot was originally, the faster it receded from our dot. And that would be true from the
vantage point of each and every dot on the balloon.
All of which is a simple way to say that the galaxies are not merely flying away from us, but instead, the entire metric of space, or the fabric of the
cosmos, is expanding. To envision this in 3-D, imagine that the dots are raisins in a cake that is baking and expanding in all directions.
On his second visit to America in January 1931, Einstein decided to go to Mount Wilson (conveniently up the road from Caltech, where he was
visiting) to see for himself. He and Edwin Hubble rode in a sleek Pierce-Arrow touring car up the winding road. There at the top to meet him was
the aging and ailing Albert Michelson, of ether-drift experiment fame.
It was a sunny day, and Einstein merrily played with the telescope’s dials and instruments. Elsa came along as well, and it was explained to her
that the equipment was used to determine the scope and shape of the universe. She reportedly replied, “Well, my husband does that on the back of
an old envelope.” 49
The evidence that the universe was expanding was presented in the popular press as a challenge to Einstein’s theories. It was a scientific drama
that captured the public imagination. “Great stellar systems,” an Associated Press story began, “rushing away from the earth at 7,300 miles a
second, offer a problem to Dr. Albert Einstein.” 50
But Einstein welcomed the news. “The people at the Mt. Wilson observatory are outstanding,” he wrote Besso. “They have recently found that the
spiral nebulae are distributed approximately uniformly in space, and they show a strong Doppler effect, proportional to their distances, that one can
readily deduce from general relativity theory without the ‘cosmological’ term.”
In other words, the cosmological constant, which he had reluctantly concocted to account for a static universe, was apparently not necessary, for
the universe was in fact expanding.* “The situation is truly exciting,” he exulted to Besso. 51
Of course, it would have been even more exciting if Einstein had trusted his original equations and simply announced that his general theory of
relativity predicted that the universe is expanding. If he had done that, then Hubble’s confirmation of the expansion more than a decade later would
have had as great an impact as when Eddington confirmed his prediction of how the sun’s gravity would bend rays of light. The Big Bang might
have been named the Einstein Bang, and it would have gone down in history, as well as in the popular imagination, as one of the most fascinating
theoretical discoveries of modern physics. 52
As it was, Einstein merely had the pleasure of renouncing the cosmological constant, which he had never liked. 53 In a new edition of his popular
book on relativity published in 1931, he added an appendix explaining why the term he had pasted into his field equations was, thankfully, no longer
necessary. 54 “When I was discussing cosmological problems with Einstein,” George Gamow later recalled, “he remarked that the introduction of
the cosmological term was the biggest blunder he ever made in his life.” 55
In fact, Einstein’s blunders were more fascinating and complex than even the triumphs of lesser scientists. It was hard simply to banish the term
from the field equations. “Unfortunately,” says Nobel laureate Steven Weinberg, “it was not so easy just to drop the cosmological constant, because
anything that contributes to the energy density of the vacuum acts just like a cosmological constant.” 56
It turns out that the cosmological constant not only was difficult to banish but is still needed by cosmologists, who use it today to explain the
accelerating expansion of the universe. 57 The mysterious dark energy that seems to cause this expansion behaves as if it were a manifestation of
Einstein’s constant. As a result, two or three times each year fresh observations produce reports that lead with sentences along the lines of this one
from November 2005: “The genius of Albert Einstein, who added a ‘cosmological constant’ to his equation for the expansion of the universe but
then retracted it, may be vindicated by new research.” 58