As pretty and as awe-inspiring our universe might seem, it surely beholds secrets and mysteries far beyond something our insignificant human mind can possibly comprehend. From Unseen Dark Matter to Dark energy to faster-than-light expansion, the universe has and will always possess secrets that will intrigue even the generations to come. One such thing of a mystery is a Void.
Void – What Is It?
Speaking conventionally in Astronomical terms, Voids are vast spaces between large-scale structures in the Universe which contain very few or even no galaxies, nebulas, and other deep space objects.
In astronomy and cosmology, such voids in space have a diameter ranging anywhere from 10 to 100 Megaparsecs (1 Megaparsecs = 3.26 * 10^6 Lightyears = 3.08*10^19 Kilometers). And as evident from the scale conversion, even 1 Megaparsec is an unfathomable distance for the human mind to even put into perspective!
Now, these Voids in the universe, were not just there when the Big Bang happened, in-fact they are formed due to something called, “Baryon Acoustic Oscillations (BAO)” (In simple language, BAO is nothing but the fluctuations in the density of normal matter (like atoms) caused by the waves from plasma in the early stages of the universe).
These voids have less than 1/10th of the Average Density of Matter present elsewhere in the universe.
The Importance of Voids
Presence of such voids in the universe is not entirely for the fun of astronomers, In-fact, they prove, evidence of the physical existence of Dark Energy. Now, you might be wondering, how does the existence of Voids can be an evidence of Dark energy, and there is a deep astronomical reason behind this, which I am sure most you will be quite interested to read about!
So, as we know from the above section, the Voids in the universe have an initial cause of Baryon Acoustic Oscillations (BAOs). Now, this is not precisely the end of the story right away. These large voids are formed due to the collapse of mass followed by the implosion of compressed Baryonic Matter (Normal Matter like atoms, etc).
Now initially, the anisotropies (property of an object being directionally dependent) arising from Quantum Fluctuations (temporary change in the amount of energy at a point in space) were small in the early universe, but as the universe grew larger and larger, the anisotropies in-universe grew over time as well. As an effect, Regions in space with higher density collapsed rapidly under themselves due to gravity (the same logic, how a Star collapses under its own gravity to form a Blackhole). This resulted in large-scale structures called Cosmic Web of voids (a foam-like structure of many different voids inter-wound like a Web). This is also the reason why voids located in high-density regions in space are smaller than those located in low-density regions in space. This is also why the voids that are very huge (like 50-100 Megaparsecs across) are generally in low-density regions in space.
But where does Dark energy come in here?
Now, to explain the role of our old (very old) friend Dark energy here, we must for a moment; turn our focus to yet another effect called Sachs-Wolfe effect. Named after Rainer K. Sachs and Arthur M. Wolfe, this effect is essentially a property of Cosmic Microwave Background Radiation (CMB). In this, the Photons from CMB are gravitationally redshifted which causes the CMB spectrum to appear very uneven.
Now, enough of that, but the point I wish to make here, is that Wolf-Sachs effect is only significant in a universe dominated by radiation or Dark Energy. So, the existence of such voids is significant in providing the physical evidence of Dark Energy.
The Bootes Void
Also, called the Great Nothing, Bootes Void is an enormous, region in space that most likely resembles the shape to that of a sphere. It contains very few galaxies in comparison to other regions of the universe. Since it lies in the Vicinity of the Bootes Constellation, it gets the name.
The Bootes Void stretches for around 330 Million Light Years in diameter (remember, 1 Light Year = 9.4 * 10^12 Km!), which makes it to occupy about 0.27% of the whole Observable Universe. It lies about 700 Million Light Years from the earth and is one of the largest (if not the largest) voids in the Universe, which is also the reason why it is called a Supervoid.
As evident from the picture above, one can note that the Bootes void’s Galaxy density is lesser than that of its neighbors.
Now, most people actually confuse the Bootes Void with Barnard 68, the Dark Nebula, which is actually the main reason for this whole articles.
Barnard 68 – The Elephant in the Room
Now, finally, after addressing all the necessary pre-requisites that will enable you to understand Voids, we now arrive at the very Core of this Article itself, the Barnard 68 – Dark Nebula.
As horrifying as its name might seem, you will be left shocked once you see it’s images!
Barnard-68 is a type of Dark Nebula (A Dark Nebula is a type of Interstellar Cloud of molecules which is so Dense, that it obstructs the flow of light from objects beyond itself, which gives it an impression of a Massive-dark-object) located near the Constellation Ophiuchus and is actually inside our galaxy Milky Way at a distance of about 400 Light Years from Earth.
The Barnard 68 is so close to the earth, that NO Star can be seen between it and our sun.
The Interior of the Dark Nebula is very cold (~16 K or -257 C). The mass of this Dark nebula is almost twice to that of our Sun and it stretches about ½ Light Years across. It is thus located about 500 Light Years from earth making itself, one of the closest (if not the closest) Dark Nebulas to planet earth.
In order for this Molecular Dark Cloud to become a Star, it’s internal gravity must be dominant long enough to cause the internal collapse of the cloud and thus reach a temperature and density where the Fusion (Nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atom nuclei and subatomic particles. This process releases an immense amount of energy and our Sun generates its energy by nuclear fusion itself) can be sustained.
The Future of Barnard 68
The very definite edges, opacity and other features of Barnard-68 point out that the Dark nebula is very well on the brink of a Gravitational Collapse followed by becoming a Star within the next 200,000 Years, which considering in cosmic time scale, is a moment if not a jiffy. As such, the Barnard-68 we are seeing is a pre-collapse view of the Dark Nebula, so enjoy the view of it while it is there.
For Astronomy geeks like us, we are thus providing you the Right Ascension and Declination of this Dark Nebula, so that you can view it from your own Telescope!
Right Ascension: 17h 22m 38.2s
Declination: −23° 49′ 34″