The fireball then continues to expand by inertia, but now it consists of normal matter, its gravity is attractive, and the expansion gradually slows down. The decay of the antigravity material marks the end of inflation and plays the role of the big bang in this theory.
The beauty of the idea was that in a single shot inflation explained why the universe is so big, why it is expanding, and why it was so hot at the beginning. A huge expanding universe was produced from almost nothing. All that was needed was a microscopic chunk of repulsive gravity material. Guth admitted he did not know where the initial chunk came from, but that detail could be worked out later. “It’s often said that you cannot get something for nothing,” he said, “but the universe may be the ultimate free lunch.”
Many Worlds in One: The Search for Other Universes
by Alex Vilenkin
PART I
GENESIS
・ 1 ・
What Banged, How It Banged, and What Caused It to Bang
In the context of inflationary cosmology, it is fair to say that the universe is the ultimate free lunch.
–ALAN GUTH
On a Wednesday afternoon, in the winter of 1980, I was sitting in a fully packed Harvard auditorium, listening to the most fascinating talk I had heard in many years. The speaker was Alan Guth, a young physicist from Stanford, and the topic was a new theory for the origin of the universe. I had not met Guth before, but I had heard of his spectacular rise from obscurity to stardom. Only a month before, he belonged to the nomadic tribe of postdocs-young researchers traveling from one temporary contract to another, in the hope of distinguishing themselves and landing a permanent job at some university. Things were looking bleak for Guth: at age thirty-two he was getting a bit old for the youthful tribe, and the contract offers were beginning to dry out. But then he was blessed with a happy thought that changed everything.
Guth turned out to be a short, bouncy fellow, full of boyish enthusiasm, apparently untarnished by his long wanderings as a postdoc. From the outset, he made it clear that he was not trying to overthrow the big bangtheory. There was no need to. The evidence for the big bang was very persuasive, and the theory was in good shape.
The most convincing evidence is the expansion of the universe, discovered by Edwin Hubble in 1929. Hubble found that distant galaxies are moving away from us at very high speeds. If the motion of the galaxies is traced backward in time, they all merge together at some moment in the past, pointing to an explosive beginning of the universe.
Another major piece of evidence in favor of the big bang is the cosmic background radiation. Space is filled with microwaves of about the same frequency as we use in microwave ovens. The intensity of this radiation dwindles as the universe expands; hence what we now observe is the faint afterglow of the hot primeval fireball.
Cosmologists used the big bang theory to study how the fireball expanded and cooled, how atomic nuclei formed, and how the grand spirals of galaxies emerged from featureless gas clouds. The results of these studies were in excellent agreement with astronomical observations, so there was little doubt that the theory was on the right track. What it described, however, was only the aftermath of the big bang; the theory said nothing about the bang itself. In Guth’s own words, it did not say “what ‘banged,’ how it ‘banged,’ or what caused it to ‘bang.'”
To compound the mystery, on closer examination the big bang appeared to be a very peculiar kind of explosion. Just imagine a pin balancing on its point. Nudge it slightly in any direction and it will fall. So it is with the big bang. A large universe sprinkled with galaxies, like the one we see around us, is produced only if the power of the primordial blast is fine-tuned with an incredible precision. A tiny deviation from the required power results in a cosmological disaster, such as the fireball collapsing under its own weight or the universe being nearly empty.
The big bang cosmology simply postulated that the fireball had the required properties. The prevailing attitude among physicists was that physics can describe how the universe evolved from a given initial state, but it is beyond physics to explain why the universe happened to start in that particular configuration. Asking questions about the initial state was regarded as “philosophy,” which, coming from a physicist, translates as a waste of time. This attitude, however, did not make the big bang any less enigmatic.
Now Guth was telling us that the veil of mystery surrounding the bigbang could be lifted. His new theory would suddenly silent. Everybody was intrigued.
The explanation the new theory gave for the big bang was remarkably simple: the universe was blown up by repulsive gravity! The leading role in this theory is played by a hypothetical, superdense material with some highly unusual properties. Its most important characteristic is that it produces a strong repulsive gravitational force. Guth assumed that there was some amount of this material in the early universe. He did not need much: a tiny chunk would be sufficient.
The internal gravitational repulsion would cause the chunk to expand very rapidly. If it were made of normal matter, its density would be diluted as it expanded, but this antigravity stuff behaves completely differently: the second key feature of the strange material is that its density always remains the same, so its total mass is proportional to the volume it occupies. As the chunk grows in size, it also grows in mass, so its repulsive gravity becomes stronger and it expands even faster. A brief period of such accelerated expansion, which Guth called inflation, can enlarge a minuscule initial chunk to enormous dimensions, far exceeding the size of the presently observable universe.
The dramatic increase in mass during inflation may at first appear to contradict one of the most fundamental laws of physics, the law of energy conservation. By Einstein’s famous relation, E = mc2, energy is proportional to mass. (Here, E is energy, m is mass, and c is the speed of light.) Sothe energy of the inflating chunk must also have grown by a colossal factor, while energy conservation requires that it should remain constant. The paradox disappears if one remembers to include the contribution to the energy due to gravity. It has long been known that gravitational energy is always negative. This fact did not appear very important, but now it suddenly acquired a cosmic significance. As the positive energy of matter grows, it is balanced by the growing negative gravitational energy. The total energy remains constant, as demanded by the conservation law.
In order to provide an ending for the period of inflation, Guth required that the repulsive gravity stuff should be unstable. As it decays, its energy is released to produce a hot fireball of elementary particles. The fireball then continues to expand by inertia, but now it consists of normal matter, its gravity is attractive, and the expansion gradually slows down. The decay of the antigravity material marks the end of inflation and plays the role of the big bang in this theory.
The beauty of the idea was that in a single shot inflation explained why the universe is so big, why it is expanding, and why it was so hot at the beginning. A huge expanding universe was produced from almost nothing. All that was needed was a microscopic chunk of repulsive gravity material. Guth admitted he did not know where the initial chunk came from, but that detail could be worked out later. “It’s often said that you cannot get something for nothing,” he said, “but the universe may be the ultimate free lunch.”
All this assumes, of course, that the repulsive gravity stuff really existed. There was no shortage of it in science fiction novels, where it had been used in all sorts of flying machines, from combat vehicles to antigravity shoes. But could professional physicists seriously consider the possibility that gravity might be repulsive?
They sure could. And the first to do that was none other than Albert Einstein.
Across the Universe
For Tufts cosmologist Alexander Vilenkin, the toughest questions are his favorite ones to try and answer.
http://www.tufts.edu/home/feature/?p=vilenkin
When the sign on the map says “You Are Here,” what does “here” mean? How did you get there? And how many “heres” are there, anyway? These are the types of questions that Tufts cosmology expert Alexander Vilenkin, professor in the Department of Physics and Astronomy and head of the Tufts Institute of Cosmology, explores on a daily basis.
Last year, in his book “Many Worlds in One,” Vilenkin detailed his theories on the explosive process of “inflation” that sparked the Big Bang and the notion that multiple regions identical to ours exist in remote parts of the universe. Last week, Vilenkin discussed these topics and more at the Dean’s Faculty Forum, an event organized by School of Arts & Sciences Dean Robert Sternberg to highlight faculty research and ideas.
We spoke with Vilenkin about his background, his theories and his motivation for studying the mysteries of the universe.
On his interest in cosmology:
I was interested in physics when I was in high school. I read some books on the general theory of relativity and some papers by Einstein, and doing cosmology was my dream ever since. Cosmology studies the history of the universe, how the universe came about and how it evolved. I thought these were the most intriguing questions of all, and if you can study them, why do anything else?
On the nature of his work:
I think it’s like any job. Most of it is not very glamorous. Much of it is just doing hard calculations. Like any scientist, to be successful you really have to be obsessed with what you’re doing; otherwise you wouldn’t spend all these hours banging your head against the wall and doing the calculations. But when you really understand something, and you finally see the thing in a different light and you learn something about the universe, it’s extremely exciting.
On coming to the United States and Tufts:
I came to the U.S. [from the Ukraine] in ’76. I was really surprised that I didn’t experience any disadvantage as a foreigner. I got to graduate school right away and managed to get my Ph.D. in one year. I was in a hurry because I was a little old for the game. I was 26 and usually you get to graduate school a little earlier than that; I wasted some time going to the army and then waiting for permission to emigrate from the Soviet Union.
I got my degree, but it was a time when getting a job in physics was not easy. On top of that, very few people worked in cosmology and it was pretty hopeless to get a job in that field. So my Ph.D. thesis was not on cosmology, it was on the physics of biopolymers, like DNA. In the course of my emigration I met a famous scientist-refusenik Mark Azbel, who was working on DNA and was kind enough to work with me. That was my initiation into the craft of physics and it was very useful. I continued with biopolymers in my Ph.D. research, and after a year of postdoc I was hired at Tufts [in 1978] as a condensed matter physicist. All along I was doing research in cosmology in my spare time.
That first appointment at Tufts was a visiting professor position for one year, and then I applied for a regular job. At that time, I ‘came out of the closet’ and said ‘Okay, I’m interested in cosmology,’ so the talk I gave here when I applied was on my cosmological research. That was a risky move, but luckily I did get the job.
On the Tufts Institute of Cosmology, founded in 1989:
There was this eccentric guy, Roger Babson, who made a lot of money in the stock market. I think he was the first to start collecting statistical information about businesses and publishing a newsletter. He claimed that he made much of his money applying Newton’s law of gravity to the stock market謡hat goes up, comes down. He later decided to give some money to universities for research in antigravity because he thought that if antigravity was discovered, that would help prevent aircraft accidents. The gift [in 1961] came with the ‘Antigravity Stone’, which now stands in the middle of the campus, near the library and University chapel. A couple of years after I joined Tufts, someone came to me and said, ‘Are you working on antigravity?’ I said, ‘No, I’m not working on antigravity, I’m working on gravity.’ In the end the Babson money went to establish the Tufts Institute of Cosmology.
The funny thing is that we do work on antigravity now, because in the new theories of cosmology, the key concept is the so-called inflation and inflation is the very rapid accelerated expansion of the universe propelled by a repulsive gravitational force. So we do work on antigravity, as Babson wanted us to. We did not get to preventing airplane accidents though.
When someone gets a Ph.D. in cosmology, our group goes to the antigravity stone for a little ceremony. The student kneels in front of the stone and the adviser drops an apple on the student’s head.
On his theories about the origin of the universe:
We now have good reasons to believe that most of the universe is in the state of explosive, accelerated expansion, called inflation. The expansion is so fast that in a tiny fraction of a second a region the size of an atom is blown to dimensions much greater than the entire currently observable universe. In our local neighborhood, inflation ended about 14 billion years ago, and the energy that drove the expansion went to ignite a hot fireball of particles and radiation. This is what we call the big bang. As the fireball continues to expand, it cools down, galaxies are gradually pulled together by gravity, and cosmic space lights up with stars.
While all this is happening in our local region, inflation still continues in remote parts of the universe, and other regions like ours are constantly being produced. This never ending process is called eternal inflation. The big bang in this scenario is no longer a one-time event in our past: multiple bangs went off before it in distant parts of the universe, and countless other bangs will erupt in the future.
This new worldview has a somewhat bizarre consequence. The process of eternal inflation creates an infinite number of regions like ours. Each of these regions has its own history, from the big bang till present, but it follows from quantum mechanics that the number of possible histories is finite. It is an unimaginably large number, 10 to the power 10 to the power 150, but the important point is that it is finite. Now, if you have a finite number of histories that are unfolding in an infinite number of regions, it follows that every history that can possibly happen will happen and it will happen an infinite number of times. So there are an infinite number of regions where we have exactly the same planet, like our Earth, with exactly the same people and we are having exactly the same conversation. This is not something that would affect your daily life, but it’s something to ponder.
I, for one, was very depressed by this conclusion for a while, because I thought it robs us of our uniqueness. I thought, you can think of our civilization as being good or bad, but it is unique and for that reason alone you would treasure it as a work of art, right? But now this picture of the universe tells us that there are an infinite number of civilizations which are identical to ours scattered in the universe. There are also all possible variations. You have regions where Elvis is still alive and regions where dinosaurs still roam the earth.
On why the study of the origin of the universe is important:
If you look at different ancient civilizations and ask what we remember them for, the philosophy and art and the useless things like pyramids are the things that strike us most. Satellites exploring the cosmos and huge particle accelerators are the pyramids of our age. I think this is what our civilization will be remembered for, our technology and scientific achievement.
On whether there is a final answer to questions concerning the origin of the universe:
I wish I could answer that. I don’t know. It’s possible that underlying the universe is just one equation. You just have to figure it out. But even then, from knowing the equation and being able to work out its consequences is a huge distance. There would still be a practically infinite number of phenomena that need to be studied. So I think there is no danger that physicists will be out of work any time soon.
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