200
R. N. Mohapatra
26.3 Other Examples
There are other examples where the anthropic principle seemed to provide
useful insight into the ways of nature (for a description of some examples
see [8, 19, 28]). An early and famous one, already mentioned, is the Hoyle
state in a carbon nucleus. Its presence is essential for three alpha particles to
fuse and form carbon in the stellar core and go beyond the carbon nucleus
to other heavier nuclei of the periodic table. If this state did not exist, the
universe as we know it would be impossible, since three helium nuclei cannot
fuse in the stellar core and there would be no heavier element than beryllium
in the universe. The state was not known when Hoyle postulated it from
what we could call the anthropic arguments in 1954. It was experimentally
discovered 4 years later by a group of scientists at Cal. Tech., Pasadena, in the
beta decay of boron-12. Scientists, since that time are still trying to understand
this state from the underlying principle of nuclear physics and may finally have
succeeded [43].
There are many other cases where the anthropic principle has been invoked.
A well known one is the value of the cosmological constant. What is a
cosmological constant? In the past two decades, it has been found from
astrophysical observations involving a class of supernovae, known as type Ia
supernovae, that the universe is now accelerating at a small rate, instead of
slowing down as the Hubble expansion would have predicted. This could
have been caused by the cosmological constant. It has been estimated that the
cosmological constant amounts to dark energy, which accounts for about 70%
of the total energy budget of the universe. It started dominating the universe’s
energy density since the time when the universe was about 11 billion years old
(Fig. 26.1).
The origin of this cosmological acceleration and the amount of dark energy
associated with it that dominates the energy budget of the universe are
not understood yet. One explanation is that there is a constant called the
cosmological constant present in Einstein’s equation for the behavior of matter
in general relativity, which has led to the accelerated expansion of the universe.
When scientists fit the observations with Einstein’s equation, they find that the
value of the cosmological constant required is (2 × 10
−4 eV)
4 . This is a very
small number indeed, although its role in the universe is huge. In an attempt
to understand this small number, Steven Weinberg showed that there may be
an anthropic reason for this value. He showed that if the value of cosmological
constant was more than 200 times larger, the universe as we know it would
not exist; the galaxies would not form, and as a result, life would not exist.
R. N. Mohapatra
26.3 Other Examples
There are other examples where the anthropic principle seemed to provide
useful insight into the ways of nature (for a description of some examples
see [8, 19, 28]). An early and famous one, already mentioned, is the Hoyle
state in a carbon nucleus. Its presence is essential for three alpha particles to
fuse and form carbon in the stellar core and go beyond the carbon nucleus
to other heavier nuclei of the periodic table. If this state did not exist, the
universe as we know it would be impossible, since three helium nuclei cannot
fuse in the stellar core and there would be no heavier element than beryllium
in the universe. The state was not known when Hoyle postulated it from
what we could call the anthropic arguments in 1954. It was experimentally
discovered 4 years later by a group of scientists at Cal. Tech., Pasadena, in the
beta decay of boron-12. Scientists, since that time are still trying to understand
this state from the underlying principle of nuclear physics and may finally have
succeeded [43].
There are many other cases where the anthropic principle has been invoked.
A well known one is the value of the cosmological constant. What is a
cosmological constant? In the past two decades, it has been found from
astrophysical observations involving a class of supernovae, known as type Ia
supernovae, that the universe is now accelerating at a small rate, instead of
slowing down as the Hubble expansion would have predicted. This could
have been caused by the cosmological constant. It has been estimated that the
cosmological constant amounts to dark energy, which accounts for about 70%
of the total energy budget of the universe. It started dominating the universe’s
energy density since the time when the universe was about 11 billion years old
(Fig. 26.1).
The origin of this cosmological acceleration and the amount of dark energy
associated with it that dominates the energy budget of the universe are
not understood yet. One explanation is that there is a constant called the
cosmological constant present in Einstein’s equation for the behavior of matter
in general relativity, which has led to the accelerated expansion of the universe.
When scientists fit the observations with Einstein’s equation, they find that the
value of the cosmological constant required is (2 × 10
−4 eV)
4 . This is a very
small number indeed, although its role in the universe is huge. In an attempt
to understand this small number, Steven Weinberg showed that there may be
an anthropic reason for this value. He showed that if the value of cosmological
constant was more than 200 times larger, the universe as we know it would
not exist; the galaxies would not form, and as a result, life would not exist.