What Caused the ‘Bang’ of the Big Bang?
We are living in “the golden age of cosmology” as scientists and engineers continue to learn more about the universe’s origin that led to us being here today.
Published January 1, 2002
By Fred Moreno, Dan Van Atta, Jill Stolarik, and Jennifer Tang
Academy Contributors
We’re all familiar with the Big Bang theory –– the most widely known model explaining the evolution of the universe. According to this standard model, the universe began some 1,010 to 1,515 billion years ago in a hot, dense state where particles were rapidly expanding and cooling. As the universe cooled, matter congealed to forms stars, galaxies and clusters of galaxies. Today the universe continues to expand, and at an accelerating rate.
But what sparked the “bang” of the Big Bang? What circumstances existed just prior to this nascent event to trigger the birth of the modern universe? The answers to these questions may lie in another scenario about the origins of the universe –– the inflationary model –– proposed by Alan H. Guth.
Guth, the Victor F. Weisskopf Professor of Physics at the Massachusetts Institute of Technology, described his model at The New York Academy of Sciences (the Academy) in October. The event, “Inflationary Cosmology and the Accelerating Universe,” was jointly hosted by the Academy and the M.I.T. Alumni Club.
An Inflating Cosmos
The notion of an inflating cosmos, which has received substantial support in the last two decades, may explain many of the mysteries of the universe: its enormity, its uniformity, why it began so extraordinarily close to its critical density and why it is considered geometrically “flat.” It even offers a possible explanation for the origin of essentially all matter and energy in the observable universe –– no small feat.
Guth noted that the Big Bang model does not explain the “bang” itself, but rather its aftermath. “Inflation provides a prehistory, a possible explanation for what happened before the Big Bang. Moreover, the same force that was responsible for triggering inflation billions of years ago is still at work, causing our universe to continue to swell in size at a rate faster than ever before.”
According to the inflationary model, the initial matter of the universe could have been a billion times smaller than a single proton. This patch of matter grew exponentially, doubling and redoubling in size every 10-37 seconds, but its density remained the same and energy was conserved. At this point, Guth explained, gravity was “turned on its head.” A repulsive gravitational field arose, the opposite of what we know as gravity here on Earth — a force that repelled, rather than attracted matter. This initial inflationary period was blindingly fast, lasting only a tiny fraction of a second.
The repulsive gravitational field was highly unstable, however, and decayed much like a radioactive substance. It then erupted, releasing energy and creating the hot primordial soup of particles that is thought to have existed at the moment of the Big Bang.
Cooled Too Quickly
According to the Big Bang model, the universe cooled too quickly to explain current uniformity, the even distribution of stars and galaxies. The theory of inflation gives us a way to understand it, since the universe during inflation was small enough to distribute its contents uniformly. Cosmic radiation is also remarkably uniform, the same intensity to about 1 part in 100,000. The inflationary theory received a further boost in 1992 when the Cosmic Background Explorer (COBE) found enough tiny variations, or “ripples,” in this uniformity to explain how, despite inflation and overall uniformity, there could still be local distribution of matter into stars, galaxies, and galaxy clusters, interspersed with patches of empty space.
The inflationary model may also explain the geometric “flatness” of the universe, a universe critically balanced between eternal expansion and eventual collapse. At one second after the Big Bang the critical mass density was apparently very close to a value of one, which would be an inexplicable coincidence without inflation. Guth’s theory shows that unlimited inflation can take any curved surface and make it appear flat, thus providing a general principle for explaining a phenomenon that is at the same time consistent with astronomical observations.
Recent observations of distant supernovae lend further support to Guth’s inflationary model. Astronomers measure changes in the expansion rate of the universe by using supernovae type Ia explosions as “standard candles.” By observing that these supernovae are appearing dimmer — and therefore moving farther away — they’ve determined to their great surprise that the rate of expansion in the universe today is actually larger than it was five billion years ago.
Repulsive Gravity
Guth attributes this once again to repulsive gravity. “So the universe today is not slowing down under the influence of gravity, which is what everybody had thought previously,” he said, “but in fact is actually speeding up in its expansion rate.”
Inflation is certainly not the answer to all of the questions about the origins and future of the universe. For one thing, some of the tenets of the model may be at odds with the uncertainty principles of quantum physics. But the coming together of cosmology and particle physics, coupled with new data generated from recording devices such as COBE, give astrophysicists great reason for excitement. Concluded Guth, “We are living in the golden age of cosmology.”
