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The Big Bang theory remains the leading scientific explanation for the origin of our Universe. It has withstood numerous challengers, and has survived virtually unscathed. Compelling evidence for the Big Bang began to surface in the 1920s, and the observational support continues to grow stronger. But by the late 1970s, the Big Bang had developed a few chinks in its armor, and the concept of inflation would come to the rescue.
By the late 1970s, observations had accumulated showing that the Universe’s overall geometry was nearly "flat." This means that on large scales, parallel lines would remain perfectly parallel, instead of diverging or converging. If one used the numeral 1 to specify the average density of matter and energy needed in a given volume of space to make the Universe flat (designated by the Greek letter omega, W), the measured density was about 0.3. This might not seem very close to 1, but according to the Big Bang, any slight deviation from 1 in the early universe would quickly diverge from 1 as the Universe expanded. If the initial value of W were 1.01, today W would be many orders of magnitude larger than 1. If the initial value were 0.99, today W would be an incredibly small number. There was no reason why the Big Bang should produce a universe so finely tuned to have W exactly equal to 1 at the very beginning, a seeming coincidence that troubled cosmologists.
In addition, measurements of the cosmic microwave background (CMB) showed that its temperature was extraordinarily uniform in all directions. But why should this be the case? Shouldn't some regions of the early universe been slightly warmer or cooler than others? This conundrum came to be known as the horizon problem.
Third, calculations showed that the hot, dense conditions of the early universe should have allowed heavy particles to form with just one magnetic pole. The primordial soup should have produced these so-called monopoles in numbers so enormous that physicists should observe them to be extremely abundant in nature. But no monopole has ever been found.
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| Dr. Alan Guth |
In a stroke of brilliance, a postdoctoral researcher at Stanford University named Alan Guth realized in 1979 that one process could solve all of these problems in one fell swoop: inflation. Guth showed that as the early universe expanded and cooled, it might suddenly transition into a new state, called a "false vacuum." Under these conditions, the universe could suddenly expand at an exponential rate for as long as the false vacuum remained stable.
In just a tiny fraction of a second, the Universe could have expanded many, many, many orders of magnitude in size. Two points in space would have moved away from each other faster than it would take light to travel between them. This faster-than-light expansion does not violate Einstein's Special Relativity, since that theory applies to the motion of light within space, not to the expansion of space itself.
Inflation solves the flatness problem since inflating space automatically drives W to a value nearly equal to 1, just as inflating a balloon to an enormous size makes its surface nearly flat. Inflation solves the horizon problem by kicking in at the very earliest moments of cosmic history, when the Universe was still in thermal equilibrium (meaning every region had the same temperature). Inflation would greatly magnify these uniform initial conditions, insuring that all regions of the Universe at later times would have nearly equal temperatures, as seen in the CMB. Finally, inflation solves the monopole problem by spreading monopoles over an enormous volume of space, insuring that their spatial density is so low that it’s unlikely we will ever see one.
If inflation actually happened, the Universe visible to our telescopes is just a minuscule portion of the entire Universe. Distant regions will forever remain out of view, since their emitted light will never reach us. More modern variants of inflation, proposed by theorists Andrei Linde, Alexander Vilenkin, and others, posit that inflating space is an eternal process that continues to spawn new universes—distinct regions of space that are physically separated from and inaccessible to one another. Each universe could, in principle, have its own distinct laws of physics (many of which will be incompatible with life). In recent years, these theories have gained acceptance in the cosmological community, and the idea of multiple universes has entered the mainstream of scientific thought.
Cosmologists find inflation to be an extremely attractive idea because it solves many problems at once. Moreover, inflation makes specific predictions that have been confirmed by observations. The discovery of the accelerating universe in 1998 shows that W is almost exactly equal to 1 today, which is easily explained by inflation. Inflation also predicts that the dominant warm and cool spots in the CMB should be about 1 degree across on the sky, and that’s exactly the value measured by NASA's Wilkinson Microwave Anisotropy Probe (WMAP). Still, cosmologists have yet to explain exactly how or why the Universe would have inflated, and to date, no satellites have had sufficient sensitivity to detect the imprint on the CMB of gravitational waves produced by inflating space-time. So inflation remains a widely accepted but unconfirmed modification to the Big Bang theory.
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