Cosmology and the Expanding Universe: Big Bang & Steady State
Cosmology and the Expanding Universe
Cosmology, a branch of astronomy, studies the structure, origins, and development of the entire universe. Astronomy is the science that studies the stars and components of the universe. Astrophysics studies the composition, structure, and evolution of celestial objects.
Cosmology describes the universe through the application of mathematical models, which are sets of equations used to describe physical systems. These models help to further study the properties of the system described and allow for predicting the properties of new states when modifying any of the variables.
The Evolution of Cosmological Models
In the early twentieth century, the accepted cosmological model was that of a static, eternal, and infinite universe, which had always existed and would exist forever, without a beginning or end. However, in 1929, Edwin Hubble demonstrated experimentally that galaxies are moving away from each other, indicating that the universe is expanding. This observation suggested that the universe had an origin.
The Big Bang Model
The Big Bang cosmological model proposes a dynamic and finite universe that originated from an infinitely dense, hot, and intangible point. It has continued to expand, driven by an energy called dark energy.
The Steady State Model
In 1948, an alternative to the Big Bang model emerged: the steady state cosmological model. This model admits the expansion of the universe but posits an infinite universe without a defined beginning, where matter is continuously generated by unknown mechanisms.
Key Figures in Cosmology
- Albert Einstein: In 1915, his theory of general relativity provided a mathematical description of the universe but initially gave an incorrect answer regarding the existence of an eternal, static universe.
- Edwin Hubble: In 1929, he demonstrated the expansion of the universe.
- George Gamow: In 1948, along with others, he developed the Big Bang cosmological model and presented an updated version that explained the creation of the first chemical elements.
The Expansion of the Universe
In 1924, Hubble discovered that what were thought to be nebulae were actually distant galaxies similar to our own. He measured the distances to several galaxies and found that they were receding.
Hubble’s Law
Hubble’s Law states that the recessional velocity of a galaxy is directly proportional to its distance from us. It can be expressed as:
v = H0D
where:
- v = recessional velocity (in km/s)
- D = distance between the galaxy and Earth (in megaparsecs: Mpc)
- H0 = Hubble constant, the constant of proportionality (approximately 70 km/s/Mpc)
Measuring the Speed of Recession of Galaxies
The light that reaches Earth from stars in galaxies contains a mixture of colors that can be separated by a spectroscope, giving rise to a spectrum. Each spectrum is made up of the seven colors of the rainbow (red light has the longest wavelength, and violet the shortest). These spectra are similar to bar codes.
Hubble measured the position of the spectral absorption bands of certain chemical elements present in several galaxies at different distances from Earth. He compared these spectra with those obtained in the laboratory for the same chemical elements. He discovered that the absorption bands experienced shifts toward longer wavelengths, and these shifts were more pronounced in more distant galaxies. This phenomenon, known as the redshift of spectral lines, is due to the Doppler effect and indicates that galaxies are moving away from each other.