The definition depends on the speaker, because there are no definitions
Instead of individual galaxies, we see huge walls and tendrils containing thousands of galaxies; filaments of galaxies connect in nodes. These structures are huge; hundreds of millions of light-years across, containing thousands of galaxies. But the voids between these clusters can be even larger.
Astronomers first started thinking about these voids back in the 1970s, when the first large-scale surveys of the Universe were being made. By measuring the redshift of galaxies, and determining how fast they were speeding away from us, astronomers started to realize that the distribution of galaxies wasn’t even.
Some galaxies were relatively close, but then there were huge gaps in distance, and then another cluster of galaxies collected together.
Over the last few decades, astronomers have built sophisticated 3-dimensional models that map out the Universe in the largest scales. The
Sloan Digital Sky Survey, updated in 2009, has provided the most accurate map so far. The
Large Synoptic Survey Telescope, destined for first light in a few years will take this to the next level.
The largest void that we currently know of is known as the Giant Void, and it’s located about 1.5 billion light-year away. It has a diameter of 1 billion to 1.3 billion light-years across.
To be fair, these regions aren’t really completely empty. They just have less density than the regions with galaxies. In general, they’ve got about a tenth the density of matter that’s average for the Universe.
Galaxy MCG+01-02-015 is so isolated that if our galaxy, the Milky Way, were to be situated in the same way, we would not have known of the existence of other galaxies until the 1960s. Credit: ESA/Hubble & NASA and N. Gorin (STScI). Acknowledgement: Judy Schmidt
Which means that there’s still gas and dust in these regions, as well as dark matter. There will still be stars and galaxies out in the middle of those voids. Even the Giant Void has 17 separate galaxy clusters inside it.
You might imagine continuing to scale outward. Maybe you’re wondering if the this spongy distribution of matter is actually just the next step to an even larger structure, and so on, and so on. In fact, astronomers call this “the End of Greatness”, because it doesn’t seem like there’s any larger structure in the Universe than these giant galactic structures and the associated galactic voids.
We are in this tiny corner of the Local Group, which is part of the Virgo Supercluster, which is perched on the precipice of vast cosmic voids.
The voids are enormous spans existing between the lattice work of galaxies, but they are not completely empty. Nothing is.
Voids are seen as vast spaces between filaments (the largest-scale structures in the
universe), which contain
very few or no
galaxies. Voids typically have a diameter of 10 to 100 megaparsecs; particularly large voids, defined by the absence of rich
superclusters, are sometimes called
supervoids. They have less than one tenth of the average density of matter abundance that is considered typical for the observable universe. They were first discovered in 1978 in a pioneering study by Stephen Gregory and Laird A. Thompon at the
Kitt Peak National Observatory.
Voids are believed to have been formed by
baryon acoustic oscillations in the
Big Bang, collapses of mass followed by implosions of the compressed
baryonic matter. Starting from initially small
anisotropies from
quantum fluctuations in the early universe, the anisotropies grew larger in scale over time. Regions of higher density collapsed more rapidly under gravity, eventually resulting in the large-scale, foam-like structure or "cosmic web" of voids and galaxy filaments seen today. Voids located in high-density environments are smaller than voids situated in low-density spaces of the universe.
Voids appear to correlate with the observed temperature of the
cosmic microwave background (CMB) because of the
Sachs–Wolfe effect*. Colder regions correlate with voids and hotter regions correlate with filaments because of
gravitational redshifting. As the Sachs–Wolfe effect is only significant if the universe is dominated by
radiation or
dark energy, the existence of voids is significant in providing physical evidence for dark energy.
* Sachs-Wolfe effect: On large angular scales, the most important of various physical processes by which the
primordial density fluctuations should have left their imprint on the
cosmic microwave background radiation in the form of small variations in the temperature of this radiation in different directions on the sky. It is named after Rainer Kurt Sachs (1932- ) and Arthur Michael Wolfe (1939- ). The effect is essentially gravitational in origin. Photons travelling from the
last scattering surface to an observer encounter variations in the
metric which correspond to variations in the gravitational potential in Newtonian
gravity. These fluctuations are caused by variations in the matter density
from place to place. A concentration of matter, in other words an upward fluctuation of the matter density, generates a gravitational potential well. According to
general relativity, photons climbing out of a potential well will suffer a gravitational redshift which tends to make the region from which they come appear colder. There is another effect, however, which arises because the perturbation to the metric also induces a time-dilation effect: we see the photon as coming from a different spatial
hypersurface (labelled by a different value of the cosmic scale factor
a(t) describing the
expansion of the Universe).
For a fluctuation
in the gravitational potential, the effect of gravitational redshift is to cause a fractional variation of the temperature
T/T =
/
c2, where
c is the speed of light. The time dilation effect contributes
T/T = -
a/a (i.e. the fractional perturbation to the scale factor). The relative contributions of these two terms depend on the behaviour of
a(t) for a particular cosmological model. In the simplest case of a
flat universe described by a matter-dominated
Friedmann model**, the second effect is just -2/3 times the first one. The net effect is therefore given by
T/T =
/3
c2. This relates the observed temperature anisotropy to the size of the fluctuations of the gravitational potential on the last scattering surface.
It is now generally accepted that the famous
ripples seen by the
Cosmic Background Explorer (COBE) satellite were caused by the Sachs-Wolfe effect. This has important consequences for theories of cosmological
structure formation, because it fixes the amplitude of the initial
power spectrum of the primordial density fluctuations that are needed to start off the gravitational
Jeans instability*** on which these theories are based.
Any kind of fluctuation of the metric, including gravitational waves of very long wavelength, will produce a Sachs-Wolfe effect. If the primordial density fluctuations were produced in the
inflationary Universe, we would expect at least part of the COBE signal to be due to the very-long-wavelength gravitational waves produced by quantum fluctuations in the
scalar field driving inflation.
FURTHER READING:
Sachs, R.K. and Wolfe, A.M., `Perturbations of a cosmological model and angular variations of the cosmic microwave background',
Astrophysical Journal, 1967,
147, 73.
** Friedmann Model: Friedmann model
universe was developed in 1922 by the Russian meteorologist and mathematician Aleksandr Friedmann (1888–1925). He believed that
Albert Einstein’s general theory of
relativity required a theory of the universe in motion, as opposed to the static universe that scientists until then had proposed. He hypothesized a
big bang followed by expansion, then contraction and an eventual big crunch. This model supposes a closed universe, but he also proposed similar solutions involving an open universe (which expands infinitely) or a flat universe (in which expansion continues infinitely but gradually approaches a rate of zero).
See also Edwin P. Hubble.
*** Jeans Instability: Is the instability of a self-gravitating, thermally supported interstellar cloud that is thought to be responsible for the collapse of parts of the cloud larger than a scale size which becomes unstable, eventually fragmenting and forming stars.
See:
https://phys.org/news/2019-02-neutrinos-clustering-galaxies.html
See:
https://www.universetoday.com/131508/what-are-cosmic-voids/
See:
https://en.wikipedia.org/wiki/Void_(astronomy)