[“Models of the quantum vacuum” series]
A theoretical physicist walks into a bar. The bartender says, “What can I get you?” The physicist says, “Nothing.” The bartender gives the physicist an empty glass. The physicist says, “Thanks, that’s plenty!”
Physicists take emptiness quite seriously. So-called empty space is an important area of study and research. The future of the cosmos, eh.
When I worked at Hughes Space & Communications, visiting the High Bay was a rare opportunity. The scale of the place, … the reflective surfaces of satellites and thermal wraps. Then there was the “shake & bake” area, which included a large thermal vacuum chamber in which satellites were tested in a simulated space environment — vacuum, radiation, and thermal cycling. I’m not sure what vacuum quality that facility achieved (high or ultra high vacuum, for example) — how close the pressure was to that in outer space. The chamber still contained matter particles (other than from any outgassing).
The lowest pressures currently achievable in laboratory are about 10−13 torr (13 pPa). However, pressures as low as 5×10−17 Torr (6.7 fPa) have been indirectly measured in a 4 K cryogenic vacuum system. This corresponds to ≈100 particles/cm3.
Is null outer space truly empty? How about a “perfect” vacuum? Quantum mechanics says not really. There still is energy in the state “(that is, the solution to the equations of the theory) with the lowest possible energy (the ground state of the Hilbert space).”
While that may seem strange — that a complete void is not really empty, there’s something even more puzzling, namely, what physicists call the vacuum energy problem and the “non-zero expectation value.”
I finished reading Sean Carroll’s book about the Higgs boson recently. In chapter 12 “Beyond this horizon,” he talks about the problem with vacuum energy . It has to do with cosmic acceleration, as determined by astronomical measurements in the last 20 years.
To explain the astronomers’ observations, we don’t need very much vacuum energy; only about one ten-thousandth of an electron volt per cubic centimeter. Just as we did for the Higgs field value, we can also perform a back-of-the-envelope estimate of how big the vacuum energy should be. The answer is about 10^116 electron volts per cubic centimeter. That’s larger than the observed value by a factor of 10^120, a number so big we haven’t even tried to invent a word for it. … Understanding the vacuum energy is one of the leading unsolved problems of contemporary physics. — Carroll, Sean (2012-11-13). The Particle at the End of the Universe: How the Hunt for the Higgs Boson Leads Us to the Edge of a New World (Kindle Locations 3603-3608). Penguin Publishing Group. Kindle Edition.
So, astronomers say that we see the universe expanding in a certain way. Theoretical physicists say that we know the density of the universe. Carroll says that the problem is deeper than the energy contributed by any Higgs field. Assuming that we’re not in a GIGO state, how do we model the universe being “pushed apart?”
Wiki: In many situations, the vacuum state can be defined to have zero energy, although the actual situation is considerably more subtle. The vacuum state is associated with a zero-point energy, and this zero-point energy has measurable effects. In the laboratory, it may be detected as the Casimir effect. In physical cosmology, the energy of the cosmological vacuum appears as the cosmological constant. In fact, the energy of a cubic centimeter of empty space has been calculated figuratively to be one trillionth of an erg (or 0.6 eV). An outstanding requirement imposed on a potential Theory of Everything is that the energy of the quantum vacuum state must explain the physically observed cosmological constant. 
Quantum field theory states that all fundamental fields, such as the electromagnetic field, must be quantized at each and every point in space. A field in physics may be envisioned as if space were filled with interconnected vibrating balls and springs, and the strength of the field were like the displacement of a ball from its rest position. The theory requires “vibrations” in, or more accurately changes in the strength of, such a field to propagate as per the appropriate wave equation for the particular field in question. The second quantization of quantum field theory requires that each such ball-spring combination be quantized, that is, that the strength of the field be quantized at each point in space. Canonically, if the field at each point in space is a simple harmonic oscillator, its quantization places a quantum harmonic oscillator at each point. Excitations of the field correspond to the elementary particles of particle physics. Thus, according to the theory, even the vacuum has a vastly complex structure and all calculations of quantum field theory must be made in relation to this model of the vacuum.
The theory considers vacuum to implicitly have the same properties as a particle, such as spin or polarization in the case of light, energy, and so on. According to the theory, most of these properties cancel out on average leaving the vacuum empty in the literal sense of the word. One important exception, however, is the vacuum energy or the vacuum expectation value of the energy. The quantization of a simple harmonic oscillator requires the lowest possible energy, or zero-point energy of such an oscillator to be:
E = hν/2
Summing over all possible oscillators at all points in space gives an infinite quantity. To remove this infinity, one may argue that only differences in energy are physically measurable, much as the concept of potential energy has been treated in classical mechanics for centuries. This argument is the underpinning of the theory of renormalization. In all practical calculations, this is how the infinity is handled.
Vacuum energy can also be thought of in terms of virtual particles (also known as vacuum fluctuations) which are created and destroyed out of the vacuum. These particles are always created out of the vacuum in particle-antiparticle pairs, which in most cases shortly annihilate each other and disappear. However, these particles and antiparticles may interact with others before disappearing, a process which can be mapped using Feynman diagrams. Note that this method of computing vacuum energy is mathematically equivalent to having a quantum harmonic oscillator at each point and, therefore, suffers the same renormalization problems.
The Casimir attraction between uncharged conductive plates is often proposed as an example of an effect of the vacuum electromagnetic field. Schwinger, DeRaad, and Milton (1978) are cited by Milonni (1994) as validly, though unconventionally, explaining the Casimir effect with a model in which “the vacuum is regarded as truly a state with all physical properties equal to zero.” In this model, the observed phenomena are explained as the effects of the electron motions on the electromagnetic field, called the source field effect. … This point of view is also stated by Jaffe (2005): “The Casimir force can be calculated without reference to vacuum fluctuations, and like all other observable effects in QED, it vanishes as the fine structure constant, α, goes to zero.”