# Dark Energy

In some ways, the argument supporting dark energy only really started to be seriously debated in 1998, when scientists discovered the possible accelerated expansion of the universe. This idea was based on the observation of a type of exploding star, i.e. a type-1a supernova, whose distance can be inferred from its apparent brightness. Basically, what scientists had found was that the more distant supernovae were dimmer than expected, implying that the recessional distance, due to the expansion of the universe, was larger than expected, i.e. the universe was accelerating with respect to some earlier time.

To put this situation into some historical perspective, at the beginning
of the 20^{th} century, most scientists would have probably
shared Einstein’s view of a static universe. Only after Hubble’s discovery
did most scientists become *'convinced*' of the idea of an expanding
universe, which was then assumed to be slowing under the effects of
gravity. So, the idea that the universe could actually be accelerating
required yet another fundamental shift in thinking, because it appears
to require an unknown source of energy to explain the acceleration.
Although the issue as to what caused the expansion prior to the acceptance
of the idea of dark energy might, yet again, be tabled as an open question

In order to account for the observed acceleration, the hypothesis
of dark energy was forwarded along with the idea of negative pressure.
Given the amount of acceleration required, it was estimated that the
energy-density of dark energy would have to be 6.2E-10 joule/m^{3}.
What was subsequently said to add weight to this hypothesis was the
fact that the estimated energy density also explained the spatial flatness
of the universe in terms of the sum of all the component energy densities
adding up to the critical density, i.e.
*8.53E-10
joule/m*^{3}. However, this said, the exact nature of
dark energy is still very speculative. Current ideas suggest that dark
energy would have to be homogeneous in distribution and have a net negative
pressure in excess of any gravitational effects associated with its
energy density. However, possibly the most perplexing attribute is its
apparent unchanging energy density under expansion. All other forms
of energy density change as a function of expansion, while dark energy
remains constant per unit volume. Therefore, while the energy density
of dark energy was virtually zero in comparison to all the other energy
densities of the earlier universe, its constant energy density grows
relative to all other energy densities as a function of the expanding
volume - see *density
graph*. Today, dark energy is thought to account for 73% of
the critical density. There is also a suggestion in this characteristic
of dark energy that the total energy per unit volume of the universe
must be increasing in apparent contradiction to the conservation of
energy - see *energy
graph*.

Given the level of speculation surrounding dark energy, it should
be no surprise that there are still many competing ideas. However, given
the somewhat unusual attributes of this form of energy, there is a tendency
to look towards quantum theory for an explanation. In physics, zero-point
energy is the lowest possible energy that a quantum system may possess
as it is considered to be the ground state energy of a system. This
idea is backed up by some experimental evidence in the form of the
*Casimir
effect*, which can be observed in nano-scale devices. However, in quantum
theory,
*zero-point energy* is often thought to be synonymous with the
idea of *vacuum energy*, i.e. the energy within a unit volume of empty
space; while in cosmology, this energy might also be discussed in terms
of a
*cosmological constant* [Λ]. However, in many respects, dark energy
is an idea on which general relativity and quantum theory don’t always
see eye to eye. While the actual details behind these opposing `*worldviews*`
is too complex to consider in detail, some insight into the issues may
be of some value.

- If we put cosmology into the general relativity camp, the energy
density of the vacuum is essentially determined by the idea of spatial
curvature. However, this simply translates into the assertion supporting
a spatially flat universe, i.e. k=0, where all known energy density
components must add up to the critical density. On the basis of
the ΛCDM model, the present-day dark energy or vacuum energy
density corresponds to 73% of the critical density, i.e. 6.2E-10
joule/m
^{3}. As stated, this also seems to account for the observed acceleration. - However, various flavours of quantum theory can come up
with very different answers. For example, if we define the zero-point
energy in terms of harmonic oscillators with energy E=hf/2 and simply
add up all the possible harmonic oscillations based on the assumption
that spacetime is a continuum; the vacuum energy density would be
infinite. Even if we modify the previous assumption to only include
those oscillations with a wavelength greater than the Planck length,
i.e. ~10
^{-35}meters, the answer is finite, but still enormous in comparison to the observed dark energy estimates.

There are several other permutations along this line, but the bottom
line, at present, seems to be that only the energy density estimate
associated with the cosmological definition of dark energy makes any
sense, although acknowledging that lack of support from quantum theory,
at this time. However, within the confines of the
*concordance model*, we shall
assume that dark energy does have negative pressure as defined by the
equation of state [ω=-1]. While we might initially perceive negative
pressure in terms of a region of lower pressure, i.e. a vacuum that
would suck things from a region of higher pressure, dark energy is said
to act more like anti-gravity. However, the justification for some
of these statements will be deferred to the discussion entitled ‘*Interpreting
**Friedmann*’ in which the equation of state for each
energy density will be derived.