Large-scale Structure of the Universe

1Astronomy, cosmology and astrophysics all study the universe in order to better understand the formation of the large-scale structure of the cosmos. In contrast with early historical perceptions, the universe does appear to support many types of large-scale structures, which are not yet fully understood,

  • Planets plus a star form solar systems
  • Solar systems in turn form galaxies
  • Galaxies form clusters and super-clusters
  • Galactic clusters are separated by voids connected by filaments

Since the invention of the telescope, scientists have been able to peer further into space and observations expanded to include the 'fixed` stars. Although, historically, the initial focus was primarily constrained to explaining the relative motion of the planets within our own solar system, the realization that the Earth and the solar system were part of the structure of a galaxy only came at a later stage. Initially, the power of the telescopes only allowed the observation of fuzzy spiral objects that were generally grouped together under the heading of `nebulae`. However, by the end of the 18th century, the German philosopher, Immanuel Kant, had suggested that the Milky Way might be one of many `island universes`. As telescopes continued to improved, it emerged that certain nebulae had a spiral shape and by the 19th century the idea that the Milky Way itself might be a spiral nebula was beginning to be suggested. Finally, in 1918, the American astronomer Harlow showed that the Sun is positioned some 30,000 lightyears from the centre of the Milky Way, which has an overall radius of some 50,000 light-years. Subsequently, Edwin Hubble demonstrated that most `nebulae` were objects outside our galaxy and ultimately concluded that these object were, in fact, other galaxies.

Initially, the spiral structure of many galaxies was difficult to explain. When modelled as a gravitational system with angular velocity, the period of rotation is calculated to be of the order 200-300 million years. However, given the age of the galaxy, the visible spiral arms should have been lost, as they became increasing wound up after multiple rotations. This problem was known as the `winding dilemma` and one solution forwarded is the concept of a `density wave`. This concept can be introduced, by analogy, as being similar to traffic on a motorway, where overtaking trucks can cause a build up of other traffic behind them. A similar explanation is proposed for the density of stars in the spiral arms of many observed galaxies, i.e. they exist because the arms exert a gravitational influence on stars and gas that orbit the galaxy. As a result, gas clouds orbit more slowly in the spiral arms and, as a consequence, the density increases in this region. Therefore, spiral arms do not end up getting wound tighter on each rotation, because they are not solid objects, but rather density patterns that shift like cars in traffic. This explanation also goes some way to explaining why there are so many young stars in the spiral arms. The higher density of gas means more gas clouds and cloud collisions. This, in-turn, triggers more star formation and as the young stars age, they drift out of the spiral pattern.

There are basically three types of star populations in our galaxy, which reflect the nature of the structure of the galaxy itself, i.e. the disk, bulge and halo populations. The disk population inhabits the rotating, flattened region of our galaxy, while the bulge population is restricted to the rounded, central region of the galaxy that also rotates, and finally the halo population that inhabits the far outer regions of the galaxy. The nature of the stars in these regions tends to vary by composition. The disk contains most of the gas and young stars, although old stars can also be found there. In contrast, old stars are predominately in the bulge, while the halo only contains very old stars and globular clusters. However, more importantly, the reason for this difference provides a clue to how the galaxy itself was formed. Stars in the disk and bulge regions tend to be rich in heavy elements, while halo stars tend to be very poor in the heavy elements. It is believed that the heavy elements are mostly produced by supernova explosions so, as time passes, the gas clouds within the galaxy will see an increase in the heavier elements, such as carbon, iron. As a consequence, the more recent star will be richer in heavy elements and can be used as a guideline to date the stars by region

Prior to 1989, it was commonly assumed that super-clusters were the largest structures in existence, and that they were distributed more or less uniformly throughout the universe in every direction. However, based on redshift survey data, the `Great Wall` structure was discovered, comprising of a sheet of galaxies more than 500 million light years long and 200 million light years wide, but only 15 million light years thick. The existence of this structure had escaped notice because it requires locating the position of galaxies, in three dimensions, derived from its distance as determined by its spectral redshift. Subsequent studies allude to a universe with large-scale structures in conjunction with a collection of giant bubble-like voids separated by sheets and filaments of galaxies in which a super-cluster appears as an occasional relatively dense nodes. At the centre of the local super-cluster there is a gravitational anomaly, known as `The Great Attractor`, which affects the motion of galaxies over a region hundreds of millions of light years. These galaxies are all redshifted, in accordance with Hubble's law, which can be interpreted as a recession velocity, indicating that the galaxies are receding from us and from each other. However, the redshift variance is sufficient to suggest the existence of a collective mass of the order of some tens of thousands of galaxies. Discovered in 1986, the Great Attractor is located some 150-250 million light years in the direction of the Hydra and Centaurus constellations. Another anomaly that has been interpreted as an indicator of a large-scale structure is called the 'Lyman Alpha Forest ', which is a collection of absorption lines associated with quasars. This effect seems to suggest the existence of huge thin sheets of intergalactic gas and it is believed that these sheets are associated with the formation of new galaxies.

Note: It is simply highlighted that while many simply reject all of the ideas being forwarded under the general heading of Plasma Cosmology, the reader might wish to consider some of these ideas. Other issues of possibly interest are discussed under the heading of Cosmic Speculation and The Plasma Model.

While not refuting any of the interpretations of the standard model, a note of caution may be necessary at this stage. Most cosmologists are well aware that that many of their interpretations are still speculative and could have an alternative explanation. For example, there has been some observation of a redshift quantisation, which alludes to an unexplained discrete jump in recession velocity with distance, which it is claimed cannot be explained within the accepted interpretation of redshift. While supporters of the standard model may dismiss these anomalies, they remain a subject of controversy. Therefore, while accepting the current tenets of the model, it may be prudent to retain an open mind, as some structures on a cosmic scale may not actually conform to our initial model. The duty of inquiry still exists.

It has long been accepted that gravitation may effect the path of light and, in cosmology, this has led to the idea of gravitational lensing in which a distant object may be located at a point that is actually different to its real origin. In cosmology, a foreground object, such as a galaxy, can distort the space around itself, as predicted by general relativity, which then deflects light passing nearby. In some circumstances, a strong gravitational lens can magnify a distant object, which would otherwise be very difficult to observe. Such techniques have been important additions to the cosmologist toolbox and have allowed cosmologist to peer ever deeper into the universe. Based on the increasing weight of observation, in conjunction with advances in physics, astrophysicists have attempted to further model the large-scale structures of the universe based on the underlying assumptions of the Big Bang model.

Today, there is now much speculation about the type and nature of matter that makes up the universe, which has led to further speculation about the expected distribution of matter, which might support or refute certain cosmological theories. For example, based on gravitational anomalies that cannot be explained by conventional matter, it has been speculated that much of the universe must consist of cold dark matter.  Clearly, the existence of such exotic matter could profoundly change the evolutionary model of the universe. However, today, cosmologists have, by general consensus, interpreted the irregularities in the cosmic microwave background radiation and presence of high redshift into a model that suggests we are living in an expanding universe of finite age, in the order of 13.7 billion years.