Alex Vilenkin discussed what lies beyond the cosmic horizon in the fourth-ever Dean's Faculty Forum last night.
"Until recently, physicists thought that the answer to this question was quite boring," said Vilenkin, a physics and astronomy professor and director of the Tufts Institute of Cosmology. "[But] the new world view suggests that these remote regions ... are strikingly different from what we see now around here."
To open the talk, which was organized by Dean of Arts and Sciences Robert Sternberg, Vilenkin described the Big Bang theory, which argues that the universe was created after a great explosion.
"At that time, there were no galaxies or stars or even atoms," he said. There was just a dense, expanding fireball.
According to Vilenkin, about three minutes after the bang, temperatures had cooled enough for atomic nuclei to form. After about a million years, atoms formed. Denser regions within the fireball grew and evolved into galaxies over the course of 10 billion years.
The Big Bang theory is "supported by a wealth of evidence," Vilenkin said. "Obviously, we have a very successful theory, but it leaves some very intriguing questions."
According to Vilenkin, these questions are addressed in the theory of cosmic inflation. The theory was developed about a quarter of a century ago by Alan Guth and suggests that during its early history, the universe was in the state of false vacuum. Due to the nature of a false vacuum, which has high repulsive gravity, the universe was made to expand with acceleration.
"In a small fraction of a second, a region smaller than an atom is blown up larger than the observable universe," Vilenkin said.
Eventually, the vacuum decays, producing a hot fireball with many of the qualities of the one described in the Big Bang theory.
"The theory is in good agreement with the data," he said. "Once you have the fireball ... standard cosmology theory takes over."
When Guth first introduced his theory 26 years ago, it was only a hypothesis.
"Now it has very impressive support from observational data," Vilenkin said.
The theory highlights a never-ending process called eternal inflation. While inflation ends in some regions, each as large as a universe, with big bangs and hot fireballs, in other places it still continues.
Vilenkin described each universe in this process as a bubble. While the bubbles expand, the space between them expands even faster, making room for more bubbles to form.
"Unfortunately, we cannot travel to other bubbles," he said. "We cannot catch up."
Because of this, we can only ever have indirect evidence of the existence of other bubbles, or universes, outside of our own.
Vilenkin then addressed a "bizarre and somewhat disturbing consequence" of this. Each region has its own history of "everything that happens down to minute events, like collisions of two atoms."
Still, the number of histories that can occur is finite, albeit an unimaginably large number.
If these finite number of histories play out in an infinite number of regions, "any history that has an above-zero probability will happen an infinite number of times," Vilenkin said. As a result, there are an infinite number of regions with the same history, allowing for the possibility that universes exist that are more or less identical to our own.
"I should tell you that scores of your duplicates are now listening to this same lecture," Vilenkin joked.
Still, most of the regions will be nothing like our universe, as "there are many more ways for things to be different than there are [for things to be] the same," according to Vilenkin.
"Only a tiny fraction of all these bubbles will be bio-friendly, and we live in one of them," he said. "Most of the bubbles are barren, but there is nobody there to complain about it."
Vilenkin acknowledged that while it was impossible to test this theory by taking measurements from other universes, scientists can use measurements from our observable region to make strategic predictions about the constants of nature.
If these predictions are confirmed, they may lend evidence to the idea of a multiverse.
Vilenkin then turned to his last topic, the question of whether or not our universe ever had a beginning, or if it was always undergoing eternal inflation.
Although "it would be an attractive solution," he thinks that inflation had to have had a beginning. "Even though inflation is eternal to the future, it cannot be eternal to the past," he said.
As for what happened before inflation, then, Vilenkin can't say definitively.
"I'm not going to give you a satisfactory answer," he said. "Because you'll keep asking what happened before that."
Vilenkin did propose that perhaps closed universes with zero energy and zero charge were, in fact, created from nothing.
"What this seems to suggest is that the laws are there in some sense even before the universe," he said. "If the laws of physics describe the creation of the universe from nothing, it appears they are not mere descriptions of the universe, but they have some ... existence before the universe."
After Vilenkin's lecture, Professor of Physics and Astronomy Lawrence Ford spoke briefly about what controversy existed over the cosmic inflation theory.
While the idea of inflation is fairly widely accepted, the concept of a multiverse is still more debated, he said.
The most controversial part, however, "basically is the use of the reasoning of the multiverse to try to draw conclusions about our own universe," Ford said. "That excites very heated debates."
