Fine-tuning evidence has raised compelling questions about the universe's beginning, evolution and eventual end.
Image Courtesy NASA/WMAP Science Team.
Every science student can recall having to memorize certain important numbers during their school years: the gravitational constant, the speed of light and Avogadro’s number, to name a few. In recent years, scientists have found extremely precise values for these fundamental constants. But what if the value of these constants were ever so slightly different? In some cases, life would not even exist.
In recent years, physicists have increasingly accepted the idea that our existence implies that certain properties of the universe could not have been different. In other words, some of these properties — particularly the fundamental constants — might be “finely-tuned” to support life. This “anthropic” reasoning has been around since the early 1970s but gained wider support in the 1980s when Steven Weinberg, a Nobel laureate in physics, used the idea successfully in the realm of cosmology.
“This idea has always been controversial,” said Dr. Alejandro Jenkins, a postdoctoral researcher in high-energy physics at Florida State University. “However, people have taken it more seriously since Weinberg.”
To understand the importance of Weinberg’s prediction, one must start with a history of what Albert Einstein considered to be his own biggest blunder: the cosmological constant.
Blunder or Brilliant?
Albert Einstein originally determined a cosmological constant to maintain a static universe. Edwin Hubble later showed that the universe is increasing in size, however, leading Einstein to consider the cosmological constant obsolete. Observations in the 1990s have shown that not only is the universe expanding, but it is also getting bigger at a faster rate. Although it appears that Einstein was wrong about the universe’s growth, his cosmological constant has become useful in explaining this acceleration.
Under the Big Bang theory, the universe started off extremely small and underwent rapid expansion for about a trillionth of a trillionth of a second. This tiny window of time is known as the inflationary period. “Inflation was a period of humongous, violent, exponential growth,” said Jenkins.
This short-lived period of growth was caused by vacuum energy. Also known as "dark energy," this energy pervades space even in the absence of matter. The amount of this energy in an area of space can be represented by the cosmological constant.
In 1987, Steven Weinberg used anthropic reasoning to predict a value for the cosmological constant that was verified experimentally almost 10 years later. Weinberg considered which values of the constant would support life and concluded that the life-supporting values must be correct. As it turns out, only a small range of values allow for life to exist in the universe.
“A large and positive cosmological constant would lead to everything being ripped apart: the universe would be cold and filled with radiation,” explained Jenkins. “If the constant was large and negative, the universe would immediately re-collapse on itself.”
Therefore, Weinberg concluded correctly that the cosmological constant must be small, or we wouldn’t be here. In a sense, the cosmological constant is finely tuned to a positive non-zero value, and large deviations from this value would lead to the demise of life in the universe.
Since the success of anthropic reasoning in cosmology, researchers have tried to apply the idea in diverse areas of physics, from thermodynamics to the microscopic world of high-energy physics. Research in these other areas, however, has had less success with fine-tuning arguments than in cosmology. For instance, many of the fundamental constants surrounding high-energy physics could vary significantly and still sustain life. Even one of the four fundamental forces, the weak nuclear force, may not be necessary to support life: galaxies and stars can theoretically still form without this force. Despite its shortfalls, fine-tuning evidence remains compelling within the realm of cosmology and raises deep questions about our universe’s beginning, evolution and eventual end.
God and the Multiverse
While anthropic reasoning gave rise to increased physics research, it also ignited a philosophical debate over the role of a supernatural creator in physics. If certain fundamental constants are finely tuned for life and they easily could have been different, then why do we happen to live in the universe that sustains life?
Philosophers and physicists are generally divided between two answers to this question. One group argues that God created the universe and intended for life to exist, hence the finely tuned values of certain constants. On the other hand, most physicists argue that our universe is only one of an infinite amount of others that exists. Within this “multiverse,” we are lucky enough to live in one of the universes that supports life.
“The fine-tuning evidence provides equal support for the multiple universe hypothesis and the God hypothesis,” said Brad Monton, a University of Colorado at Boulder philosopher who specializes in the philosophy of physics. “The probability that goes down when you consider this evidence is the hypothesis that there’s no God and one universe.”
Both Monton and Jenkins argue that physicists generally agree with this sentiment. Consequently, those who favor the God hypothesis have to rely on more than just fine-tuning evidence. Instead of simply using anthropic reasoning, these proponents of the God hypothesis draw from other areas of physics to argue for the existence of God. Perhaps the most well known proponent of the God hypothesis within the physics community is Frank Tipler, a mathematical physicist at Tulane University and co-author of The Anthropic Cosmological Principle.
In his book, Tipler distinguishes between the weak anthropic principle and the final anthropic principle. The weak version typically states that given that life is here, the universe must have properties that support life. The final anthropic principle is much stronger and states that intelligent beings must come into existence in the universe; once they come into existence, they cannot go out of existence.
“Most physicists are concerned with the weak anthropic principle,” said Tipler. “However, the final anthropic principle implies that humans are essential; it’s more controversial.”
Tipler uses fine-tuning evidence for his argument, but he also draws from thermodynamics, quantum mechanics and special relativity. Although the principle relies on a variety of physical theories for support, anthropic reasoning forms the crux of the argument. From this principle, Tipler argues that the universe was designed to have life-sustaining properties by a creator, namely God.
Most physicists, however, remain unconvinced by Tipler's argument. Instead, a large majority of physicists try to keep God out of their explanations because they think religion lies outside the realm of science. The multiverse reasoning doesn’t explicitly deny God’s existence but simply sidesteps the issue.
“A lot of physicists don’t endorse the fine-tuning argument because it’s so controversial,” said Monton.
Despite the small amount of support for these design arguments in physics and philosophy academic circles, political organizations supporting intelligent design, such as the Discovery Institute, have yet to use them.
“These groups really only focus on biology-based arguments, but the fine-tuning argument is really interesting,” said Monton. “People take it seriously, and I don’t know why [these groups] aren’t latching themselves onto it.”
Fine-tuning arguments suggest that science and religion may be more intertwined than previously thought. Although these arguments have yet to truly enter the public arena, scientists and philosophers alike continue to fervently debate the topic. Meanwhile, researchers continue to search for evidence of finely-tuned properties of our universe, reminding us that the smallest of particles can raise some of the universe’s biggest questions.
By Brian Jacobsmeyer