Researching Concepts of Dark and Celestial Matter
The study of the celestial world predates written history. The first civilizations created complex myths about to explain the existence of such celestial matter as the sun, moon and stars. Greek and Roman scholars debated about the cosmos and its relationship to the earth. Such scholarly investigations and controversies about the cosmos continue unabated today. Even as technology becomes increasingly sophisticated, new questions arise. The latest discussions revolve around the existence of dark matter and, if it exists, where.
Scientists now know that outer space is not an empty vacuum, devoid of celestial matter. Among the stars and other stellar bodies it is not empty. Instead, these areas contain matter in very small amounts, primarily in the form of interstellar gas (Sparke and Gallagher). This gas makes up a significant part of the creation and destruction of larger structures, such as stars. Approximately 98 percent of interstellar matter is gaseous; 5 percent of all the universe's matter consists of this interstellar gas. Even if this is a significant amount of matter, the way that this gas is distributed can be extremely scattered and inconsistent. Larger areas of gas called giant molecular clouds form, which become so huge they start to combine. As they become increasingly heavy due to the gravity, the central area falls inward and disperses heat that lead to nuclear reactions and the formation of new stars. Eventually, some of these stars have a total collapse and then burst and release new interstellar gas and the cycle is repeated (Ehrenfreund and Charnley).
The majority of this interstellar gas consists of hydrogen and helium; when water is present, it becomes a gas cloud or visible cloud of gas called a nebula. These can be glowing nebulae that transmit wavelengths of light. Several hundred of these nebulae are already observed. These nebulae can also appear dark, becoming visible against a bright background and obstructing the light from stars (Snow).
At the end of the 1990s, the Hubble Space Telescope showed that the universe was expanding more slowly in the past than it is currently. This was contrary to what they believed. They thought that gravity would slow this current expansion, and they could not explain why it was going faster. Scientists thus theorized that there was something that fills up space, but cannot be seen. Contrary to the celestial matter that can be seen, this is "dark" matter, or it is not in the form of stars and plants that can be observed. Also, it does not emit any measurable electromagnetic radiation. It can be acknowledged only by its gravitational interaction with visible matter. Perhaps as much as 90 percent of a typical galaxy contains this dark matter. It is seen through the major effect it has on the universe, particularly in the creation of galaxies and clusters of galaxies. Formations smaller than stellar clusters may not consist of measurable amounts of this dark matter (Freeman and McNamara).
According to Freeman and McNamara, based on the gravitational laws, this dark matter needs to have mass in order to wield a gravitational force on other celestial objects. It may consist of normal baryons, or protons, neutrons and electrons. The baryonic dark matter may instead be made up of "massive compact halo objects" (MACHOs), for example large-mass splanets, cool brown or old dwarf stars, neutron stars, and very cold hydrogen gases. MACHOs account for about 20 percent of the dark matter in the Milky Way. Dark matter may also consist of non-baryonic matter, including what are called "weakly interacting massive particles (WIMPs). Scientists have suggested two possible types of non-baryonic dark matter, which may be determined by how they are difficult for gravitational energy to restrict. This hot dark matter should be located both within and between galaxies. The neutrino is considered the most possible form of hot dark matter. "Cold dark matter" does not have as much energy as the corresponding hot dark matter. It moves slower and its gravitational pull may be to galaxy clusters. Galaxy clusters are the largest gravitation-bound formations found in the universe, which are critical to research on dark matter and the evolution of the cosmos. Being able to use galaxy clusters as a means of determining the structure of dark matter is based on their three-dimensional formation and masses (Newman et al). Possibilities for cold dark matter may be sterile neutrinos, which only relate with one another through gravity.
In the 21st century, the concept of dark matter has become an important discussion of scientists due to its impact on theories of astrophysics and cosmology. Considerable efforts are being advanced to use the most sophisticated telescopes to map the distribution of dark matter. For example, two teams of astronomers recently used NASA's Chandra X-Ray Observatory to map the dark matter in the galaxy cluster designated Abell 383, located approximately 2.3 billion light years from Earth. These researchers were able to find dark matter in two dimensions of the sky and to determine the way that this dark matter is distributed across the line of sight (Newman et al). These recent studies of Abell 383 have given scientists the most detailed three-dimensional visuals ever taken of dark matter in a galaxy cluster. The teams say that the dark matter is stretched out similar to the shape of a huge American football, instead of being spherical like a basketball, with the football tip closely aligned to the line of sight.
Last year, Space Daily reported that researchers in Nagoya, Japan, used large-scale computer simulations and observational data of gravitational lensing to see if they could observe dark matter around these galaxies. The researchers found that galaxies do not have definitive edges, but long outskirts of dark matter, far beyond the area where stars exist. This dark matter is distributed in a very well-organized manner and extends to intergalactic space, unlike luminous celestial matter, such as stars, which are constrained by a limited region. In addition, the approximate total amount of dark matter in the peripheries of the galaxies provides an explanation for the gap that occurs between the global cosmic mass density and that coming from galaxy number counting weighted by their masses. Studies such as this, which are steadily increasing in the number being undertaken, shed considerable information on the long-standing mysteries about the existence of dark matter and its location. Scientists are now stipulating, contrary to beliefs just a few decades ago, that the universe does not have any empty space. It is filled with celestial matter and dark matter. They are now learning it is not only what they see that provides answers, but also what they do not see.
Sources Cited:
Ehrenfreund, Pascale and Steen Charnley. Organic molecules in the interstellar medium, comets and meteorites. Annual Review of Astronomy and Astrophysics 38(2000): 427-83.
Freeman, Ken and Geoff McNamara. In search of dark matter. New York: Springer Newman, A et al. Astrophysical Journal Letter 728 (2011): 39-45.
Snow, Theodore P. Composition of interstellar gas and dust. Journal of Geophysical Research 105.A5 (2000): 10239-10248 Space Daily. Missing dark matter located.
Spark, L., and J. Gallagher. Galaxies in the Universe. Cambridge: Cambridge University press, 2005.