11 Modern Cosmology
227
required energy/matter density. Even including the dark matter required to
explain the dynamical behaviour of star clusters, galaxies and galaxy clusters,
which is estimated to form 24% of the energy density, the total “explained”
density is only 29%. That leaves 71% of the energy/matter density in the
universe unaccounted for. To add to the puzzle, this missing component must
be something that does not gravitate, otherwise the accelerated expansion
of the universe observed by astronomers would not be possible. Indeed, the
missing component must exert a form of “negative pressure” to induce this
acceleration. Since it produces physical effects, without being “matter”, this
mysterious ingredient has been named “dark energy”.
We see therefore that from the end of the 20th Century, the picture of the
universe accepted among astronomers has evolved, and converged towards
what is now called the Cosmic Concordance Model (a.k.a. the Cosmic Standard Model). It is also known as the CDM model. Here (lambda) is
the cosmological constant that Einstein dubbed as the biggest blunder of his
life (see Chap. 10). Cosmologists have arbitrarily reintroduced that constant
into the equations of General Relativity, and adjusted its value to account for
the accelerated expansion of the universe. The mathematics is thus satisfied;
however physics is left with the burden of finding a suitable interpretation for
.
As we have seen, because of the observed accelerating expansion of the
universe, dark energy must function as a type of anti-gravity. We may think
of it as being a fluid permeating the universe, but its density always stays
the same. Normally we would expect that in an expanding space any fluid
progressively dilutes, but this is not the case for dark energy, which dilutes
much more slowly than the universe expands. Reintroducing the cosmological constant of Einstein to GR provides the simplest formalism to describe
dark energy, since it is constant in both space and time.
The introduction of an ad hoc arbitrary constant into a physical theory is
an indicator that one’s physical understanding is far from complete. Ideally
the value of the constant should be derivable from other principles. However,
attempts to derive the value of from quantum field theories have been spectacularly unsuccessful, with the measured value of being only 10 –120 of the
theoretically predicted value. This discrepancy of 120 orders of magnitude has
been described as “the worst theoretical prediction in the history of physics”
[6]. In summary, we do not know what dark energy is, but it seems to work,
and provides a good fit to many cosmological observations. Of course, there
is no guarantee that sometime in the future it will not be replaced by an
altogether different theory. Such is the nature of science.
227
required energy/matter density. Even including the dark matter required to
explain the dynamical behaviour of star clusters, galaxies and galaxy clusters,
which is estimated to form 24% of the energy density, the total “explained”
density is only 29%. That leaves 71% of the energy/matter density in the
universe unaccounted for. To add to the puzzle, this missing component must
be something that does not gravitate, otherwise the accelerated expansion
of the universe observed by astronomers would not be possible. Indeed, the
missing component must exert a form of “negative pressure” to induce this
acceleration. Since it produces physical effects, without being “matter”, this
mysterious ingredient has been named “dark energy”.
We see therefore that from the end of the 20th Century, the picture of the
universe accepted among astronomers has evolved, and converged towards
what is now called the Cosmic Concordance Model (a.k.a. the Cosmic Standard Model). It is also known as the CDM model. Here (lambda) is
the cosmological constant that Einstein dubbed as the biggest blunder of his
life (see Chap. 10). Cosmologists have arbitrarily reintroduced that constant
into the equations of General Relativity, and adjusted its value to account for
the accelerated expansion of the universe. The mathematics is thus satisfied;
however physics is left with the burden of finding a suitable interpretation for
.
As we have seen, because of the observed accelerating expansion of the
universe, dark energy must function as a type of anti-gravity. We may think
of it as being a fluid permeating the universe, but its density always stays
the same. Normally we would expect that in an expanding space any fluid
progressively dilutes, but this is not the case for dark energy, which dilutes
much more slowly than the universe expands. Reintroducing the cosmological constant of Einstein to GR provides the simplest formalism to describe
dark energy, since it is constant in both space and time.
The introduction of an ad hoc arbitrary constant into a physical theory is
an indicator that one’s physical understanding is far from complete. Ideally
the value of the constant should be derivable from other principles. However,
attempts to derive the value of from quantum field theories have been spectacularly unsuccessful, with the measured value of being only 10 –120 of the
theoretically predicted value. This discrepancy of 120 orders of magnitude has
been described as “the worst theoretical prediction in the history of physics”
[6]. In summary, we do not know what dark energy is, but it seems to work,
and provides a good fit to many cosmological observations. Of course, there
is no guarantee that sometime in the future it will not be replaced by an
altogether different theory. Such is the nature of science.