Does the Milky Way galaxy exist in a void?
If you recall, in a couple of articles a few weeks ago, I wrote about the structure of the universe. In those articles, I used the analogy of the universe as an immense room filled with countless, criss-crossing cobwebs, stretching between the walls, floor and ceiling.
I referenced the webs as being wide in some areas, thin in others, and, at times, forming relatively dense clumps where they intersect one another. The "cobwebs" in the article were analogous to the massive streams, threads, walls and sheets of galactic filaments stretching throughout the universe, giving it its structure and form. I also spoke of "voids" between the filaments, walls, and sheets of galaxies.
When we think of a void, we rightly envision an empty area encased by some form of matter, i.e., the surrounding matter serves as a boundary to the void. However, in reality, a true void (an area devoid of anything and everything) only exists within a laboratory vacuum chamber, in which nothing, not even air molecules, are present.
When astronomers and astrophysicists talk about voids in relation to the structure of the universe, they are talking about the huge spaces, which they refer to as "cosmic voids'', between the galactic filaments that comprise the structure of the known universe.
COSMIC VOIDS
These cosmic voids may contain only a few or no galaxies, as most galaxies are gravitationally bound together into massive cosmic structures that are, as previously noted, identified as galactic filaments. Although these voids contain approximately 15 per cent of the mean matter density of the universe — a ratio of the number of galaxies per unit of galactic volume, versus the total mass of the matter within a unit of volume — they, for all intensive purposes, appear empty to the common observer.
Voids were first studied during the 1960s and 70s, when astronomers began identifying superclusters and voids in their research into the distribution of galaxies throughout the observable universe. Since then, astronomers have identified a huge number of voids in the surrounding cosmic neighbourhood.
If you would like to see a detailed list of the larger voids identified to date, go to https://en.wikipdia.org/ wiki/list_of_largest_cosmic_structures#list_of_largest_voids. Cosmic voids are believed to have formed during the early stages of the universe's expansion (i.e., "the Big Bang"), when fluctuations in the density of the universe's visible baryonic matter (i.e., composite subatomic particles), caused by acoustic density waves in the primordial plasma present in the early universe, created baryon acoustic oscillations. When the regions of higher plasma density collapsed faster due to the effect of gravity, the end result was the large-scale, cosmic web-like formations of voids and galactic filaments that astronomers see today.
LOCAL VOID
In 1987, astronomers R. Brent Tully (b. 1943) from the University of Hawaii's Institute of Astronomy, and J. Richard Fisher (b. 1943) at the National Radio Astronomy Observatory in Charlottesville, Virginia, U.S., discovered that, while our Milky Way galaxy is surrounded by other galaxies and cosmic structures, the local universe surrounding the Milky Way galaxy is approximately 15-50 per cent less dense than other surrounding areas. They found that the Milky Way Galaxy actually sits at the edge of an extensive, nearly empty region of space, an area they named the "Local Void".
They also discovered that this Local Void is composed of three separate sections separated by galactic filament bridges.
As mentioned above, though they may appear "empty" to most observers, voids are not typically totally empty of galaxies, though the actual number present may be low. There are, in fact, five galaxies within the Local Void:
- the two dwarf galaxies Pisces A and Pisces B, located 18.4 million light years (Mly) and 30 Mly away, respectively, within the constellation of Pisces - the Fish;
- NGC 7077, a compact dwarf galaxy, located 56 Mly away in the constellation of Aquarius - the Water-bearer;
- NGC 6503, located 17 Mly away in the constellation of Draco - the Dragon;
- NGC 6789, located 12 Mly away, also in Draco.
LOCAL SHEET
Our Milky Way galaxy occupies a spot within a large, flat array of galaxies known as the "Local Sheet" — a region of space wherein the Milky Way galaxy, the other members of the Local Group, and a number of other galaxies occupy an area approximately 23 Mly in radius x 1.5 Mly in thickness, all sharing a similar velocity.
The Local Group is composed of the Milky Way galaxy, the Andromeda galaxy, and a number of smaller dwarf galaxies which are gravitationally bound by the Milky Way galaxy and the Andromeda galaxy. The Local Group is itself part of the much larger Laniakea Supercluster.
The Local Sheet is part of what is referred to as the "Local Volume" - a collection of more than 500 galaxies located in a section of the observable universe in the Virgo Supercluster; the Local Sheet constitutes one galactic wall (a region of space with a typical mean density of matter abundance) of the Local Void.
UNIVERSE EXPANSION
As astrophysics has now established, our universe is expanding at an increasing rate. While the exact velocity rate of the universe's expansion is hotly debated (it is a very complicated computation), the most recent indication is that, for every 3.3 Mly/1 megaparsec (Mpc)/ 3 billion trillion km that a galaxy moves away from us, it appears to be moving at approximately 74 km/sec (266,000 km/hr) faster, give or take 2.5km/sec/mpc.
As the universe expands, the cosmic structures which give form to the universe, and the voids within the cosmic structures also expand, and will continue to do so until the massive galactic filaments that constitute the overall cosmic form are stretched to the point where they break apart into smaller structures.
It is now known that the speed of the Milky Way galaxy, its larger galactic neighbour the Andromeda galaxy, and the smaller surrounding dwarf galaxies in the Local Group deviate from the overall speed of the universe's expansion by as much as
600 km/sec (2,160,000 km/ hr).
Tully and Fisher theorized that at least half of this deviation is the result of a combination of the gravitational pull of the nearby Virgo Supercluster of galaxies, as well as the innate expansion of the Local Void itself, as it grows larger due to the expanding universe.
In 2007, based on his studies of the dwarf galaxy ESO 461-36 situated within the Local Void, Tully discovered that the Local Sheet was moving away from the Local Void at approximately 260 km/sec (936,000 km/hr), indicating that the Local Void was not only huge, but was itself expanding.
While the exact dimensions of the Local Void are not known, the current estimate puts it at approximately 45 Mpc or 150 Mly across, and possibly as large as 150300 Mpc or 495-990 Mly in size, beginning at the edge of the Local Group. While our Milky Way galaxy isn't situated in the depths of the Local Void, it certainly occupies a spot in its suburbs; who knows, perhaps in a few billion years, as the universe continues its unrelenting expansion, it may move further downtown.
THIS WEEK'S SKY
Jupiter (mag. -2.3, in Aries the Ram) remains, yet again this coming week, the sole bright planet readily visible in the night sky, becoming visible shortly after 6 p.m., 50 degrees above the southwest horizon as darkness falls, before sinking towards the horizon and setting just before 11:30 p.m. Jupiter is joined by Uranus (mag. +5.8, just above Jupiter in Aries) by about 7 p.m., before it, too, drops towards the horizon and disappears from view by about 12:15 a.m.
Mercury (mag. -1.1, in Aquarius - the Water-bearer), at superior solar conjunction on Feb. 28, is too close to the Sun to be observed.
Venus (mag. -3.9, in Capricorn - the Sea Goat) and Mars (mag. +1.3, in Capricorn) are also not visible, with Venus no higher than 5 degrees above the southeast horizon at dawn, and Mars only 1 degree above the horizon at dawn this coming week.
Saturn (mag. +1.0, in Aquarius), recently passed behind the Sun (superior solar conjunction), and is only 2 degrees from the Sun at dawn. Likewise, Neptune (mag. +8.0, in Pisces - the Fish), moving towards solar conjunction, is currently 19 degrees from the Sun, and not visible.
Until next week, clear skies.