Above and Below

Above
and
Below
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All of the observable universe is filled with large numbers of photons, the so-called cosmic background radiation, and quite likely a correspondingly large number of neutrinos.

The current temperature of this radiation is about 3 K, or -270 degrees Celsius.

A vacuum is a volume of space (empty space) that is essentially empty of matter, so that gaseous pressure is much less than standard atmospheric pressure.

A perfect vacuum with a gaseous pressure of absolute zero is a philosophical concept that is never observed in practice, not least because quantum theory predicts that no volume of space is perfectly empty in this way.

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Physicists often use the term "vacuum" slightly differently, to discuss ideal test results that would occur in a perfect vacuum, which is simply called "vacuum" or "free space" in this context, and use the term partial vacuum to refer to the imperfect vacua realized in practice.

The quality of a vacuum is measured by how closely it approaches a perfect vacuum. The residual gas pressure is the primary indicator of quality, and it is most commonly measured in units of torr, even in metric contexts. Lower pressures indicate higher quality, although other variables must also be taken into account. Quantum mechanics sets limits on the best possible quality of vacuum. Outer space is a natural high quality vacuum, mostly of much higher quality than what can be created artificially with current technology.

Much of outer space has the density and pressure of an almost perfect vacuum. It has effectively no friction, which allows stars, planets and moons to move freely along ideal gravitational trajectories. But no vacuum is perfect, not even in interstellar space, where there are a few hydrogen atoms per cubic centimeter at 10 fPa (10 to the minus 16 Torr). The deep vacuum of space could make it an attractive environment for certain processes, for instance those that require ultraclean surfaces, but for small scale applications it is much easier to create an equivalent vacuum on Earth than to leave the Earth's gravity well.

Einstein argued that physical objects are not located in space, but rather have a spatial extent. Seen this way, the concept of empty space loses its meaning. Rather, space is an abstraction, based on the relationships between local objects. Nevertheless, the general theory of relativity admits a pervasive gravitational field, with properties that vary from one location to another. Only, one must take care not to ascribe to it material properties like velocity, and so on.

Stars, planets and moons keep their atmosphere by gravitational attraction, so atmospheres have no clearly delineated boundary. The density of atmospheric gas simply decreases with distance from the object. In Low Earth Orbit (about 300 km altitude) the atmospheric density is about 100 nPa, (10 to the minus 9 Torr,) still sufficient to produce significant drag on satellites. Most artificial satellites operate in this region, and they need to fire their engines every few days to maintain orbit.

Beyond planetary atmospheres, the pressure from photons and other particles from the sun becomes significant. Spacecraft can be buffeted by solar winds, but planets are too massive to be affected. The idea of using this wind with a solar sail has been proposed for interplanetary travel.

In 1913, Norwegian explorer and physicist Kristian Birkeland may have been the first to predict that space is not only a plasma, but also contains "dark matter". He wrote: "It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. We have assumed that each stellar system in evolutions throws off electric corpuscles into space. It does not seem unreasonable therefore to think that the greater part of the material masses in the universe is found, not in the solar systems or nebulae, but in "empty" space.

In 1930, Paul Dirac proposed a model of vacuum as an infinite sea of particles possessing negative energy, called the Dirac sea. This theory helped refine the predictions of his earlier formulated Dirac equation and successfully predicted the existence of the positron, which was discovered two years later in 1932. Despite this early success, the idea was soon abandoned in favour of the more elegant quantum field theory.

The development of quantum mechanics has complicated the modern interpretation of vacuum by requiring indeterminacy. Niels Bohr and Werner Heisenberg's uncertainty principle and Copenhagen interpretation, formulated in 1927, predict a fundamental uncertainty in the position of any particle, which, not unlike the gravitational field, questions the emptiness of space between particles. In the late 20th century, this principle was understood to also predict a fundamental uncertainty in the number of particles in a region of space, leading to predictions of virtual particles arising spontaneously out of the void. In other words, there is a lower bound on vacuum which is dictated by the lowest possible energy state of the quantized fields in any region of space. Ironically, Plato was right, if only by chance.

Even an ideal vacuum, thought of as the complete absence of anything, will not in practice remain empty. One reason is that the walls of a vacuum chamber emit light in the form of black-body radiation: visible light if they are at a temperature of thousands of degrees, infrared light if they are cooler. If this soup of photons is in thermodynamic equilibrium with the walls, it can be said to have a particular temperature, as well as a pressure. Another reason that perfect vacuum is impossible is the Heisenberg uncertainty principle which states that no particle can ever have an exact position. Each atom exists as a probability function of space, which has a certain non-zero value everywhere in a given volume. Even the space between molecules is not a perfect vacuum.

More fundamentally, quantum mechanics predicts that vacuum energy can never be exactly zero. The lowest possible energy state is called the zero-point energy and consists of a seething mass of virtual particles that have a brief existence. This is called vacuum fluctuation. While most agree that this represents a significant part of particle physics, it is a concept that would benefit from a deeper understanding than currently available. Vacuum fluctuations may also be related to the so-called cosmological constant in the theory of gravitation, if indeed this entity were to be observed in nature on a macroscopic scale. The best evidence for vacuum fluctuations is the Casimir effect and the Lamb shift.

In quantum field theory and string theory, the term "vacuum" is used to represent the ground state in the Hilbert space, that is, the state with the lowest possible energy. In free (non-interacting) quantum field theories, this state is analogous to the ground state of a quantum harmonic oscillator. If the theory is obtained by quantization of a classical theory, each stationary point of the energy in the configuration space gives rise to a single vacuum. String theory is believed to be analogous to quantum field theory but one with a huge number of vacua - with the so-called anthropic landscape.

Deep space is generally much more empty than any artificial vacuum that we can create, although many laboratories can reach lower vacuum than that of low earth orbit. In interplanetary and interstellar space, isotropic gas pressure is insignificant when compared to solar pressure, solar wind, and dynamic pressure, so the definition of pressure becomes difficult to interpret. Astrophysicists prefer to use number density to describe these environments, in units of particles per cubic centimetre. The average density of interstellar gas is about 1 atom per cubic centimeter.

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Still begs the question what is outside the observable universe.
Laser-cooling Brings Large Object Near Absolute Zero from MIT
LHC cooler than outer space by John @ Cosmic Variance
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