The fine-tuned universe is the hypothesis that, because "life as we know it" could not exist if the constants of nature—such as the electron charge or the gravitational constant—had been even slightly different, the universe must be tuned specifically for life. In practice, this hypothesis is formulated in relation to dimensionless physical constants. These physical constants, parameters, and initial conditions of the universe at or around the Big Bang played a key role in allowing life to emerge and evolve. There are about 31 physical constants in the Standard Model of particle physics and the standard model of cosmology combined, and they seem to have unusual precision.
History
In 1913, chemist Lawrence Joseph Henderson wrote The Fitness of the Environment, one of the first books to explore fine tuning in the universe. Henderson discusses the importance of water and the environment to living things, pointing out that life as it exists on Earth depends entirely on Earth's very specific environmental conditions, especially the prevalence and properties of water.
In 1961, physicist Robert H. Dicke argued that certain forces in physics, such as gravity and electromagnetism, must be perfectly fine-tuned for life to exist in the universe.
Astronomer Fred Hoyle argued for a fine-tuned universe: "From 1953 onward, Willy Fowler and I have always been intrigued by the remarkable relation of [...] and your fixing would have to be just where these levels are actually found to be. [...] A common sense interpretation of the facts suggests that a superintellect has monkeyed with physics, as well as with chemistry and biology, and that there are no blind forces worth speaking about in nature." In his 1983 book The Intelligent Universe, Hoyle wrote, "The list of anthropic properties, apparent accidents of a non-biological nature without which carbon-based and hence human life could not exist, is large and impressive."
The desire to resolve the fine-tuning problem led to the expectation that the Large Hadron Collider would produce evidence of physics beyond the Standard Model, such as supersymmetry, but by 2012 it had not produced evidence for supersymmetry at the energy scales it was able to probe. This absence leaves the fine-tuning problem unresolved, leading some physicists to suggest that the Standard Model may fundamentally require fine-tuning rather than a natural explanation.
Background and arguments
Physicist Paul Davies said: "There is now broad agreement among physicists and cosmologists that the Universe is in several respects 'fine-tuned' for life. But the conclusion is not so much that the Universe is fine-tuned for life; rather it is fine-tuned for the building blocks and environments that life requires". Astrophysicist Luke Barnes performed a literature review of scientific research on fine tuning and found that fine tuning is generally acknowledged by physicists of secular and non-secular worldviews. Davies also said that "'anthropic' reasoning fails to distinguish between minimally biophilic universes, in which life is permitted, but only marginally possible, and optimally biophilic universes, in which life flourishes because biogenesis occurs frequently". Among scientists who find the evidence persuasive, a variety of natural explanations have been proposed, such as the existence of multiple universes introducing a survivorship bias under the anthropic principle.
The premise of the fine-tuned universe assertion is that physical constants are so highly ordered and exact (by orders of magnitude with many zeroes—indicating non-random or finely adjusted values) that a small change in several of the constants would make the universe radically different. Stephen Hawking observed: "The laws of science, as we know them at present, contain many fundamental numbers, like the size of the electric charge of the electron and the ratio of the masses of the proton and the electron. ... The remarkable fact is that the values of these numbers seem to have been very finely adjusted to make possible the development of life". Physicists have tried to find underlying theories or calculations that would give rise or explain the values of these physical constants.
For example, if the strong nuclear force were 2% stronger than it is (i.e. if the coupling constant representing its strength were 2% larger) while the other constants were left unchanged, diprotons would be stable; according to Davies, hydrogen would fuse into them instead of deuterium and helium. This would drastically alter the physics of stars, and presumably preclude the existence of life similar to what we observe on Earth. The diproton's existence would short-circuit the slow fusion of hydrogen into deuterium. Hydrogen would fuse so easily that it is likely that all the universe's hydrogen would be consumed in the first few minutes after the Big Bang. This "diproton argument" is disputed by other physicists, who calculate that as long as the increase in strength is less than 50%, stellar fusion could occur despite the existence of stable diprotons.
The precise formulation of the idea is made difficult by the fact that it is not yet known how many independent physical constants there are. The Standard Model of particle physics has 25 freely adjustable parameters and general relativity has one more, the cosmological constant, which is known to be nonzero but profoundly small in value. Because physicists have not developed an empirically successful theory of quantum gravity, there is no known way to combine quantum mechanics, on which the standard model depends, and general relativity.
Without knowledge of this more complete theory suspected to underlie the standard model, it is impossible to definitively count the number of truly independent physical constants. In some candidate theories, the number of independent physical constants may be as small as one. For example, the cosmological constant may be a fundamental constant but attempts have also been made to calculate it from other constants, and according to the author of one such calculation, "the small value of the cosmological constant is telling us that a remarkably precise and totally unexpected relation exists among all the parameters of the Standard Model of particle physics, the bare cosmological constant and unknown physics".
Examples
Physical constants
There are about 31 constants in the standard model of particle physics and the standard model of cosmology combined. Some widely acknowledged constants by physicists of diverse backgrounds, that were fixed (many zeroes and orders of magnitude—indicating non-random or finely adjusted values) at or around the time of Big Bang include:
- The force of gravity - G = 6.67418 × 10−11 m3kg−1s−2 with a coupling constant αg = 10−39
- The electromagnetic force (fine structure constant) - α = 0.007297
- The strong force (cohesion of atomic nuclei and nucleons) - αs = 1
- The weak force (within atomic nucleus) - αw = 10−6
- Speed of light - c = 299,792,458 m/s
- Planck constant - h = 6.62607015×10−34 J⋅Hz−1
- Boltzmann constant - k = 1.380649×10−23 J⋅K−1
- Charge of a proton and electron - e = 1.602176634×10−19 C
- Mass of proton - 1.6726219 × 10−27 kg
- Mass of neutron - 1.6749274 × 10−27 kg (.14% greater than mass of proton)
- Mass of electron - 9.109383 × 10−31 kg
- Mass energy density of the universe at the origin
- Velocity of expansion of the universe
- Cosmological constant - 1.289 × 10−52 m−2
- Density and mass of the universe after the Big Bang
- Speed of expansion of the universe after the Big Bang
Martin Rees formulates the fine-tuning of the universe in terms of the following six dimensionless physical constants.
- N, the ratio of the electromagnetic force to the gravitational force between a pair of protons, is approximately 1036. According to Rees, if it were significantly smaller, only a small and short-lived universe could exist. If it were large enough, they would repel them so violently that larger atoms would never be generated.
- Epsilon (ε), a measure of the nuclear efficiency of fusion from hydrogen to helium, is 0.007: when four nucleons fuse into helium, 0.007 (0.7%) of their mass is converted to energy. The value of ε is in part determined by the strength of the strong nuclear force. If ε were 0.006, a proton could not bond to a neutron, and only hydrogen could exist, and complex chemistry would be impossible. According to Rees, if it were above 0.008, no hydrogen would exist, as all the hydrogen would have been fused shortly after the Big Bang. Other physicists disagree, calculating that substantial hydrogen remains as long as the strong force coupling constant increases by less than about 50%.
- Omega (Ω), commonly known as the density parameter, is the relative importance of gravity and expansion energy in the universe. It is the ratio of the mass density of the universe to the "critical density" and is approximately 1. If gravity were too strong compared with dark energy and the initial cosmic expansion rate, the universe would have collapsed before life could have evolved. If gravity were too weak, no stars would have formed.
- Lambda (Λ), commonly known as the cosmological constant, describes the ratio of the density of dark energy to the critical energy density of the universe, given certain reasonable assumptions such as that dark energy density is a constant. In terms of Planck units, and as a natural dimensionless value, Λ is on the order of 10−122. This is so small that it has no significant effect on cosmic structures that are smaller than a billion light-years across. A slightly larger value of the cosmological constant would have caused space to expand rapidly enough that stars and other astronomical structures would not be able to form.
- Q, the ratio of the gravitational energy required to pull a large galaxy apart to the energy equivalent of its mass, is around 10−5. If it is too small, no stars can form. If it is too large, no stars can survive because the universe is too violent, according to Rees.
- D, the number of spatial dimensions in spacetime, is 3. Rees claims that life could not exist if there were 2 or 4 spatial dimensions. Rees argues this does not preclude the existence of ten-dimensional strings.
Max Tegmark argued that if there is more than one time dimension, then physical systems' behavior could not be predicted reliably from knowledge of the relevant partial differential equations. In such a universe, intelligent life capable of manipulating technology could not emerge. Moreover, protons and electrons would be unstable and could decay into particles having greater mass than themselves. This is not a problem if the particles have a sufficiently low temperature.
Carbon and oxygen
An older example is the Hoyle state, the third-lowest energy state of the carbon-12 nucleus, with an energy of 7.656 MeV above the ground level. According to one calculation, if the state's energy level were lower than 7.3 or greater than 7.9 MeV, insufficient carbon would exist to support life. To explain the universe's abundance of carbon, the Hoyle state must be further tuned to a value between 7.596 and 7.716 MeV. A similar calculation, focusing on the underlying fundamental constants that give rise to various energy levels, concludes that the strong force must be tuned to a precision of at least 0.5%, and the electromagnetic force to a precision of at least 4%, to prevent either carbon production or oxygen production from dropping significantly.
Explanations
Explanations for the observed fine tuning have been proposed including naturalistic and non-naturalistic arguments.
Multiverse
If the universe is just one of many (possibly infinitely many) universes, each with different physical phenomena and constants, it is unsurprising that there is a universe hospitable to intelligent life. Some versions of the multiverse hypothesis therefore provide a simple explanation for any fine-tuning, while the analysis of Wang and Braunstein challenges the view that this universe is unique in its ability to support life.