The Universe Appears To Be Finely Tuned
A number of eminent scientists have commented that our universe appears to be finely tuned for life as we know it. Physicist Paul Davies, for example, has asserted that "There is now broad agreement among physicists and cosmologists that the universe is in several respects 'fine-tuned' for life".
Professor Stephen Hawking noted: "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."
Six Fundamental Physical Constants
Martin Rees, the British Astronomer Royal, has posited the fine-tuning of the Universe in terms of six physical constants.
- N, the ratio of the strength of electromagnetism to the strength of gravity for a pair of protons, is approximately 1036. According to Rees, if it were significantly smaller, only a small and short-lived universe could exist
- 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, 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 metric expansion, the universe would have collapsed before life could have evolved. On the other side, 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 positing that dark energy density is a constant. In terms of Planck units, and as a natural dimensionless value, the cosmological constant, 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. If the cosmological constant were not extremely small, 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 dimensions of spacetime nor if any other than 1 time dimension existed in spacetime
Arguments Against Fine-tuning
A number of scientists have objected to the idea of the universe being finely tuned for human existence as being an example of the anthropic principle.
The anthropic principle (in its strong form) holds that "The Universe must have those properties which allow life to develop within it at some stage in its history."
Some scientists argue that there may be other universes where conditions for life exist that are not based on anthropic (ie human) assumptions.