Theoretical Astrophysics
Theoretical astrophysics is a specialized branch of astronomy focused on understanding the universe's fundamental mechanisms, including the physical properties of celestial bodies and phenomena like dark matter, dark energy, and the concept of wormholes. This discipline employs mathematical models and computer simulations to explore and analyze aspects of the universe that cannot be directly observed, such as the gravitational effects of invisible matter and the behaviors of light in space.
The field has evolved significantly since its emergence in the 19th century, with early contributions from scientists like Robert Emden, who created models of stellar structures, and Edwin Hubble, who revealed the existence of galaxies beyond the Milky Way and provided evidence for the universe's expansion. Theoretical advancements have also been shaped by prominent figures such as Albert Einstein, whose theory of general relativity transformed our understanding of gravity and space-time.
Astrophysicists continue to develop theories, such as cosmic inflation and the nature of dark energy, to address pressing questions about the universe's origins and its ultimate fate. Concepts like wormholes, while still hypothetical, are considered within the framework of established scientific theories. Overall, theoretical astrophysics plays a crucial role in expanding our knowledge of the cosmos and addressing some of the most profound mysteries of existence.
Theoretical Astrophysics
FIELDS OF STUDY: Astrophysics; Astronomy; Cosmology
ABSTRACT: Theoretical astrophysics involves the study of the physics behind celestial objects, such as stars and planets, and other phenomena, including black holes and dark energy. It includes observation and analysis of their structures, properties, processes, and behaviors. Theoretical astrophysics is also applied to the formation of galaxies and the expansion of the universe. Among the concepts that have come from theorists in this branch of science are the big bang theory, cosmic inflation, and relativity.
The Universe in Theory
Theoretical astrophysics is a branch of astronomy that is concerned with the study of the universe. It involves not only the physical aspects of stars and planets but also some of the mysteries of outer space, such as dark matter, dark energy, and possible wormholes. Astrophysicists try to understand planets’ movements and their effects on each other, how light travels in space, and the causes and effects of gravity. They rarely can make close-up examinations or measurements, and some of the phenomena they study are invisible. Using computers, astrophysicists construct models or simulations that allow them to analyze and test their theories.
Pioneers in Theoretical Astrophysics
While the science of astronomy dates back thousands of years, astrophysics began to emerge as a subfield only in the nineteenth century. Early astrophysicists included Swiss astronomer and meteorologist Robert Emden (1862–1940), who studied the relationship between the pressure and density within stars. He developed an early mathematical model called a polytrope. A polytrope is a solution to an equation known as the Lane-Emden equation, named for Emden and American astrophysicist Jonathan Homer Lane (1819–80). This equation describes the structure of a gaseous sphere whose gravity is balanced by its internal pressure. A polytrope can serve as a simplified model for analyzing the structure of stars.
More early contributions to the field of astrophysics came from American astronomer Edwin Hubble (1889–1953). In the 1920s, astronomers were still debating whether the Milky Way galaxy constituted the whole of the universe. Hubble was the first to demonstrate conclusively that what were previously thought to be nebulae within the Milky Way were in fact other galaxies outside of it. He also helped discover the expansion of the universe and calculate its age.
The work of German-born physicist Albert Einstein (1879–1955) in the early twentieth century closed some longstanding gaps in theoretical physics. Einstein was the first to determine that the speed of light is always constant. He also developed the theory of general relativity, which states that a large object, such as a planet, can warp space-time, just as a heavy ball placed on an outstretched rubber sheet will warp its shape. The indentation caused by the object creates a gravity well, resulting in gravitational pull, which in turn can cause light to bend.
Russian-born physicist George Gamow (1904–68) helped develop the big bang theory, which posits that the universe began with the explosive expansion of a singularity, or a single point of infinite density. Gamow believed that if this theory were correct, scientists should be able to find the radiation left over from this explosion throughout the universe. This radiation, later dubbed cosmic microwave background radiation (CMB), was discovered by American physicists Robert Woodrow Wilson (b. 1936) and Arno Allan Penzias (b. 1933) in 1965, nearly twenty years after Gamow predicted its existence. Wilson and Penzias shared the 1978 Nobel Prize in Physics for their discovery.
American astronomer Carl Sagan (1934–96) was another pioneer in astrophysics. Sagan was the director of the Laboratory for Planetary Studies at Cornell University. In addition to astrophysics, he specialized in many other subfields of astronomy, including cosmology, astrobiology, and planetary science. Sagan was well known both as a popular-science writer and for presenting the award-winning documentary series Cosmos: A Personal Voyage (1980). He also shared his fascination with exobiology, or the search for and study of extraterrestrial life, with many people who respected his ideas on the subject.
More recently, theoretical physicists such as Stephen Hawking (1942–2018) have brought more new ideas and evidence to the field. Hawking focused particularly on the study of black holes. In 1974, he proposed that, contrary to prior scientific understanding, some energy can in fact escape from black holes. Based on the nature of this hypothetical energy, later named Hawking radiation, Hawking suggested in 1981 that physical information can be irretrievably lost within a black hole. This suggestion came to be known as the "information paradox," so called because it violates the tenets of quantum theory. Since then, Hawking has revised his conclusions. In 2014, he proposed that instead of having an event horizon from which nothing can escape, a black hole has an "apparent horizon" that can change.
Theories in Astrophysics
Astrophysicists have continually developed and improved on important theories related to the creation of the universe. In some cases, new theories and knowledge support earlier ideas. In others, assumptions are proved wrong and new theories must be developed. For example, the theory of cosmic inflation was developed in the 1980s to resolve outstanding problems with the big bang theory. One such problem was the uniform temperature of the universe in all directions. Without a period of rapid inflation in the earliest moments of the universe, there simply would not have been enough time since the big bang for the universe to reach thermal equilibrium.
Another such problem also involves the expansion of the universe. Astrophysicists had long supposed that the rate of expansion was gradually slowing due to the effects of gravitational force. However, in 1998, data from the Hubble Space Telescope revealed that it is actually speeding up. Astrophysicists scrambled to explain how this could happen. While the cause has yet to be conclusively identified, the most widely accepted hypothesis is that some strange force in the universe is causing the acceleration. This force, dubbed dark energy, is an as-yet-unknown form of energy that is believed to account for about 68.3 percent of the total mass and energy of the universe. In an effort to learn more, the National Aeronautics and Space Administration (NASA) assisted the European Space Agency (ESA) in developing the Planck mission. In 2009, the ESA launched the Planck spacecraft, an unmanned observatory that carried instruments capable of measuring variations in the oldest light in the universe. NASA also helped analyze the data.
Planck collected evidence of both dark energy and dark matter, a hypothetical form of matter that makes up an estimated 84.5 percent of all matter in the universe, or 26.8 percent of the total mass-energy content. (Ordinary matter accounts for only 4.9 percent of the universe’s mass-energy.) Dark matter is so called because it does not emit, reflect, or absorb light and thus cannot be directly observed. Its existence was first proposed in the 1930s to account for the discrepancy between the observable masses of galaxies and calculations of their masses based on their gravitational effects. The percentage of dark matter in the universe was similarly calculated from the gravitational force exerted on observable matter.
The Planck mission lasted until 2013. At that point it had collected enough information for scientists to process the data through a supercomputer. It took scientists until 2018 and 2019 to fully examine the data from Planck and arrive at several conclusions. They were then able to revise prior theories on the age of the universe, which is now believed to be 13.8 billion years old, slightly older than originally believed. They also found the universe is relatively flat, with a minimal curvature. In the past, some astrophysicists theorized that the universe may one day stop expanding and began a contraction that would end in a “big crunch,” sort of the opposite to the big bang. However, data from Planck shows this theory to be unlikely.
Another popular subject in theoretical astrophysics is the possible existence of wormholes. Wormholes are hypothetical tunnels through time and space that could allow for shortcuts in space travel—and, in science-fiction stories, often do. Although there is no physical evidence that they exist, Einstein’s theory of general relativity means that they are mathematically possible. Some theorists believe that if large, stable examples of wormholes could be found, they might make space travel and even time travel possible.
PRINCIPAL TERMS
- big bang: the theoretical initial explosion that caused the universe to expand outward from a single, infinitely dense point.
- dark energy: energy of an unknown nature that is believed to be accelerating the expansion of the universe.
- dark matter: a hypothetical type of matter that cannot be directly observed but is believed to constitute most of the matter in the universe.
- general relativity: the theory, developed by Albert Einstein, that gravity is the result of matter causing space-time to curve.
- cosmic inflation: the theory that a sudden, exponential expansion of the universe occurred within the first second of the big bang.
- polytrope: a solution to a certain equation that can be used as an approximate model of a star.
- wormhole: a hypothetical tunnel through time and space.
Bibliography
"Astronomy." Astrophysical.org. Istituto Scientia, 2004–14. Web. 4 May 2015.
Clavin, Whitney. "Supercomputer Helps Planck Mission Expose Ancient Light." Planck. California Inst. of Technology, 21 Mar. 2013. Web. 4 May 2015.
Durham, Ian T. "Emden, Robert." Biographical Encyclopedia of Astronomers. Ed. Thomas Hockey et al. 2nd ed. Vol. 2. New York: Springer, 2014. Print.
Magrass, Yale R. "Stephen Hawking." Salem Press Biographical Encyclopedia (2014): n. pag. Research Starters. Web. 3 June 2015.
Redd, Nola Taylor. "What Is a Wormhole?" Space.com. Purch, 13 Apr. 2015. Web. 4 May 2015.
Siegel, Ethan. "How The Planck Satellite Forever Changed Our View Of The Universe." Forbes, 19 July 2018, www.forbes.com/sites/startswithabang/2018/07/19/how-the-planck-satellite-changed-our-view-of-the-universe/?sh=7eabd4f67ad2. 13 June 2022.
Tate, Karl. "Einstein’s Theory of Relativity Explained (Infographic)." Space.com. Purch, 5 Mar. 2015. Web. 4 May 2015.
Wanjek, Christopher. "Ringside Seat to the Universe’s First Split Second." NASA. NASA, 16 Mar. 2006. Web. 4 May 2015.
"What Is Dark Matter?" NASA Education. NASA, 22 Feb. 2012. Web. 4 May 2015.