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3 Questions: Alan Guth on new insights into the ‘Big Bang’

MIT physicist explains how new results bolster his 1980 theory of cosmic inflation.
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Alan Guth
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Earlier this week, scientists announced that a telescope observing faint echoes of the so-called “Big Bang” had found evidence of the universe’s nearly instantaneous expansion from a mere dot into a dense ball containing more than 1090 particles. This discovery, using the BICEP2 telescope at the South Pole, provides the first strong evidence of “cosmic inflation” at the birth of our universe, when it expanded billions of times over. 

The theory of cosmic inflation was first proposed in 1980 by Alan Guth, now the Victor F. Weisskopf Professor of Physics at MIT. He discussed the significance of the new BICEP2 results with MIT News.

Q: Can you explain the theory of cosmic inflation that you first put forth in 1980?

A: I usually describe inflation as a theory of the “bang” of the Big Bang: It describes the propulsion mechanism that drove the universe into the period of tremendous expansion that we call the Big Bang. In its original form, the Big Bang theory never was a theory of the bang. It said nothing about what banged, why it banged, or what happened before it banged.

The original Big Bang theory was really a theory of the aftermath of the bang. The universe was already hot and dense, and already expanding at a fantastic rate. The theory described how the universe was cooled by the expansion, and how the expansion was slowed by the attractive force of gravity.

Inflation proposes that the expansion of the universe was driven by a repulsive form of gravity. According to Newton, gravity is a purely attractive force, but this changed with Einstein and the discovery of general relativity. General relativity describes gravity as a distortion of spacetime, and allows for the possibility of repulsive gravity.

Modern particle theories strongly suggest that at very high energies, there should exist forms of matter that create repulsive gravity. Inflation, in turn, proposes that at least a very small patch of the early universe was filled with this repulsive-gravity material. The initial patch could have been incredibly small, perhaps as small as 10-24 centimeter, about 100 billion times smaller than a single proton. The small patch would then start to exponentially expand under the influence of the repulsive gravity, doubling in size approximately every 10-37 second. To successfully describe our visible universe, the region would need to undergo at least 80 doublings, increasing its size to about 1 centimeter. It could have undergone significantly more doublings, but at least this number is needed.

During the period of exponential expansion, any ordinary material would thin out, with the density diminishing to almost nothing. The behavior in this case, however, is very different: The repulsive-gravity material actually maintains a constant density as it expands, no matter how much it expands! While this appears to be a blatant violation of the principle of the conservation of energy, it is actually perfectly consistent.

This loophole hinges on a peculiar feature of gravity: The energy of a gravitational field is negative. As the patch expands at constant density, more and more energy, in the form of matter, is created. But at the same time, more and more negative energy appears in the form of the gravitational field that is filling the region. The total energy remains constant, as it must, and therefore remains very small.

It is possible that the total energy of the entire universe is exactly zero, with the positive energy of matter completely canceled by the negative energy of gravity. I often say that the universe is the ultimate free lunch, since it actually requires no energy to produce a universe.

At some point the inflation ends because the repulsive-gravity material becomes metastable. The repulsive-gravity material decays into ordinary particles, producing a very hot soup of particles that form the starting point of the conventional Big Bang. At this point the repulsive gravity turns off, but the region continues to expand in a coasting pattern for billions of years to come. Thus, inflation is a prequel to the era that cosmologists call the Big Bang, although it of course occurred after the origin of the universe, which is often also called the Big Bang.

Q: What is the new result announced this week, and how does it provide critical support for your theory?

A: The stretching effect caused by the fantastic expansion of inflation tends to smooth things out — which is great for cosmology, because an ordinary explosion would presumably have left the universe very splotchy and irregular. The early universe, as we can see from the afterglow of the cosmic microwave background (CMB) radiation, was incredibly uniform, with a mass density that was constant to about one part in 100,000.

The tiny nonuniformities that did exist were then amplified by gravity: In places where the mass density was slightly higher than average, a stronger-than-average gravitational field was created, which pulled in still more matter, creating a yet stronger gravitational field. But to have structure form at all, there needed to be small nonuniformities at the end of inflation.

In inflationary models, these nonuniformities — which later produce stars, galaxies, and all the structure of the universe — are attributed to quantum theory. Quantum field theory implies that, on very short distance scales, everything is in a state of constant agitation. If we observed empty space with a hypothetical, and powerful, magnifying glass, we would see the electric and magnetic fields undergoing wild oscillations, with even electrons and positrons popping out of the vacuum and then rapidly disappearing. The effect of inflation, with its fantastic expansion, is to stretch these quantum fluctuations to macroscopic proportions.

The temperature nonuniformities in the cosmic microwave background were first measured in 1992 by the COBE satellite, and have since been measured with greater and greater precision by a long and spectacular series of ground-based, balloon-based, and satellite experiments. They have agreed very well with the predictions of inflation. These results, however, have not generally been seen as proof of inflation, in part because it is not clear that inflation is the only possible way that these fluctuations could have been produced.

The stretching effect of inflation, however, also acts on the geometry of space itself, which according to general relativity is flexible. Space can be compressed, stretched, or even twisted. The geometry of space also fluctuates on small scales, due to the physics of quantum theory, and inflation also stretches these fluctuations, producing gravity waves in the early universe.

The new result, by John Kovac and the BICEP2 collaboration, is a measurement of these gravity waves, at a very high level of confidence. They do not see the gravity waves directly, but instead they have constructed a very detailed map of the polarization of the CMB in a patch of the sky. They have observed a swirling pattern in the polarization (called “B modes”) that can be created only by gravity waves in the early universe, or by the gravitational lensing effect of matter in the late universe.

But the primordial gravity waves can be separated, because they tend to be on larger angular scales, so the BICEP2 team has decisively isolated their contribution. This is the first time that even a hint of these primordial gravity waves has been detected, and it is also the first time that any quantum properties of gravity have been directly observed.

Q: How would you describe the significance of these new findings, and your reaction to them?

A: The significance of these new findings is enormous. First of all, they help tremendously in confirming the picture of inflation. As far as we know, there is nothing other than inflation that can produce these gravity waves. Second, it tells us a lot about the details of inflation that we did not already know. In particular, it determines the energy density of the universe at the time of inflation, which is something that previously had a wide range of possibilities.

By determining the energy density of the universe at the time of inflation, the new result also tells us a lot about which detailed versions of inflation are still viable, and which are no longer viable. The current result is not by itself conclusive, but it points in the direction of the very simplest inflationary models that can be constructed.

Finally, and perhaps most importantly, the new result is not the final story, but is more like the opening of a new window. Now that these B modes have been found, the BICEP2 collaboration and many other groups will continue to study them. They provide a new tool to study the behavior of the early universe, including the process of inflation.

When I (and others) started working on the effect of quantum fluctuations in the early 1980s, I never thought that anybody would ever be able to measure these effects. To me it was really just a game, to see if my colleagues and I could agree on what the fluctuations would theoretically look like. So I am just astounded by the progress that astronomers have made in measuring these minute effects, and particularly by the new result of the BICEP2 team. Like all experimental results, we should wait for it to be confirmed by other groups before taking it as truth, but the group seems to have been very careful, and the result is very clean, so I think it is very likely that it will hold up.

Press Mentions

Science Friday

Prof. Alan Guth speaks with Christina Couch of Science Friday about his career and the cosmos. Of what inspired him to pursue a career in science, Guth recalls conducting experiments with a friend and being “very excited about the idea that we can really calculate things, and they actually do reflect the way the real world works.”

WBUR

Asma Khalid profiles Professor Alan Guth for WBUR's “Visionaries” series, which features experts in a variety of fields. Guth reminisces about how a high school teacher fostered his interest in physics, his time as a student at MIT and his development of the theory behind why the universe expanded so quickly after the Big Bang. 

National Geographic

Dan Vergano of National Geographic profiles Professor Alan Guth’s career in physics. "What always fascinated me about science was the desire to understand what underlies it all, and I think physics is basically the study of that," Guth explains. 

Boston Globe

Neil Swidey profiles Professor Alan Guth and his work developing the theory of cosmic inflation in a piece for The Boston Globe Magazine. “Perhaps you went to school with someone like Alan Guth, a child so preternaturally gifted that the teachers didn’t know what to do with him,” Swidey writes.

New York Times

Professor Max Tegmark writes for The New York Times about recent research that appears to support the theory of cosmic inflation, and the implications of this discovery for the study of physics and the origins of the universe.

Bloomberg

Professor Alan Guth discusses new research that supports his 1980 theory of cosmic inflation with John Lauerman of Bloomberg News.

Salon.com

“Most inflationary models, almost all, predict that inflation should become eternal,” said Professor Alan Guth in an article published by Salon. New research has found evidence to support the theory of inflation, which Guth hypothesized in 1980.

New Scientist

New Scientist reporter Stuart Clark explores the origins of the theory of cosmic inflation, which explains the rapid expansion of the universe. Clark explains that while MIT Professor Alan Guth is widely credited as the father of inflation, his work sparked widespread interest and a plethora of different theories about how the universe could have expanded.

New York Times

The New York Times’ Dennis Overbye explores Professor Alan Guth’s theory of inflation in the wake of the discovery of gravitational waves from the seconds after the Big Bang occurred. The discovery appears to confirm Guth’s findings.

NPR

MIT Professor Alan Guth speaks with Here and Now’s Robin Young about recent findings that shed light on the origins of the universe and Guth’s work developing the theory of cosmic inflation.

The Guardian

The Guardian explores the theory of cosmic inflation, pioneered by MIT Professor Alan Guth in 1979. This week, scientists announced that they had spotted gravitational waves from the seconds after the formation of the Universe. The findings appear to confirm Guth’s seminal work.

Los Angeles Times

Los Angeles Times reporter Amina Khan explores the new findings that appear to confirm MIT Professor Alan Guth’s theory of cosmic inflation.  “Guth's inflation theory became a cornerstone of our understanding of the early universe — but scientists had thought it would be difficult, if not impossible, to prove,” writes Khan.

Nature

Nature reporter Ron Cowen explores Professor Alan Guth’s theory of cosmic inflation, which scientists may have confirmed by searching for gravitational waves from the seconds after the Universe’s formation.  “Guth’s idea was that the cosmos expanded at an exponential rate for a few tens of trillionths of trillionths of trillionths of seconds after the Big Bang, ballooning from subatomic to football size.”

BBC News

MIT Professor Alan Guth talks to BBC News about the new scientific evidence that appears to support a Big Bang Theory for the origin of the universe. Guth was one of the first physicists to propose the theory of cosmic inflation.

New York Times

New York Times reporter Dennis Overbye profiles Professor Alan Guth, one of the first physicists to set forth the theory of cosmic inflation. This week astronomers presented evidence that could confirm Guth’s work.

Forbes

“Cosmic inflation, meanwhile, was proposed by MIT’s Alan Guth (who attended the CfA press conference) in 1979 and explains why the universe appears to be bigger than its age suggests,” writes Paul Rodgers in Forbes of the announcement this week that scientists had discovered evidence that confirmed Guth’s seminal work.

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