Keith Mack
Edwin Hubble is credited with the discovery of Hubble's Law, which states that galaxies are moving away from Earth at speeds proportional to their distance. Hubble's 1929 paper on the observed relation between distance and recession velocity of galaxies unveiled the expanding universe and revolutionized our understanding of the cosmos. However, some attribute the discovery to Belgian priest and astronomer Georges Lemaître, who in 1927, two years before Hubble's publication, derived what is now known as Hubble's Law. The law is considered the first observational basis for the expansion of the universe and plays a crucial role in cosmology, allowing scientists to calculate the age of the universe and its history. Despite this, there is still debate over the exact value of the Hubble constant, which would provide valuable insights into our understanding of physics.
| Characteristics | Values |
|---|---|
| Name of the Law | Hubble's Law, also known as the Hubble–Lemaître law |
| Who discovered it | Edwin Hubble, though Georges Lemaître derived the same solution in 1927, two years before Hubble's discovery |
| Year of discovery | 1929 |
| Field | Physical cosmology |
| What it states | Galaxies are moving away from Earth at speeds proportional to their distance |
| Calculation | v = H0D, with H0 being the Hubble constant and D being the proper distance to a galaxy |
| Hubble Constant | Between 50-74 km/s/Mpc |
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What You'll Learn
Edwin Hubble's 1929 paper
In his paper, Hubble re-examined the motion of the sun concerning extra-galactic nebulae, addressing a paradox involving a variable K term of several hundred kilometres. Previous attempts to explain this paradox through correlations between radial velocities and distances had not been convincing. Hubble's work focused on nebular distances believed to be reliable, determined by applying absolute-luminosity criteria to specific types of stars, including Cepheid variables, novae, and blue stars involved in emission nebulosity.
The significance of Hubble's 1929 paper lies in its conclusion that there was a linear relationship between the recessional velocity of galaxies and their distance from Earth. This discovery provided observational evidence for the expansion of the universe, which is now considered a fundamental principle in cosmology. The law is expressed mathematically as v = H0D, where v represents recessional velocity, H0 is the Hubble constant, and D denotes the proper distance to a galaxy.
Hubble's work built upon earlier contributions by astronomers such as Vesto Slipher, who reported redshifts for nebulae in 1912, and Carl Wilhelm Wirtz, who deduced in 1922 and 1924 that more distant galaxies recede faster. Additionally, Georges Lemaître, a Belgian priest and astronomer, independently derived what is now known as Hubble's Law in 1927, two years before Hubble's publication. However, Lemaître's work went unnoticed by Hubble, who made the same discovery independently.
Despite the contributions of these pioneers, Hubble's name is synonymous with the law due to his comprehensive and widely recognised work in the field. Hubble's Law has had a lasting impact on our understanding of the universe, providing a basis for calculating its age and supporting the Big Bang model.
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Georges Lemaître's 1927 paper
In 1927, Georges Lemaître, a Belgian priest, astronomer, and academic, published a paper in the Annales de la Société Scientifique de Bruxelles ("Annals of the Scientific Society of Brussels"). The paper, titled "Un Univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extragalactiques" ("A homogeneous Universe of constant mass and growing radius accounting for the radial velocity of extragalactic nebulae"), proposed a theory of the expansion of the universe.
Lemaître's work built upon Albert Einstein's theory of relativity, arguing that the equations of Einstein's theory implied that the universe was not static but rather expanding. He suggested that the well-known redshifts of spiral nebulae could be explained by the relativistic expansion of space. Lemaître also connected this prediction to a simple relation of proportionality between the average recessional velocity of galaxies and their distance from Earth. This idea of an expanding spacetime would eventually contribute to the Big Bang and Steady State theories of cosmology.
Lemaître's 1927 paper is considered a breakthrough, as it provided the first explanation of the observations on the recession velocities of galaxies as a natural consequence of dynamical cosmological solutions of Einstein's field equations. However, due to the relatively obscure nature of the Belgian journal in which it was published, his work initially gained little traction among astronomers and physicists outside Belgium. An English translation of Lemaître's paper was published in 1931 in the Monthly Notices of the Royal Astronomical Society, but it omitted his estimate of the "Hubble constant," leading to temporary confusion among historians.
Lemaître's work independently derived what is now known as Hubble's law, or the Hubble-Lemaître law, two years before Edwin Hubble published his own article in 1929. Hubble's work, which was based on better and more abundant data, showed that galaxies recede at a velocity proportional to their distance from the observer. Despite this, Lemaître declined to take a stand over priority, and the law carries Hubble's name today.
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Hubble's background and career
Edwin Hubble, the astronomer after whom Hubble's law is named, was born in 1889 in Marshfield, Missouri, and moved to Wheaton, Illinois, in 1900. In his youth, Hubble was known more for his athletic abilities than his intellectual prowess, although he did earn good grades across various subjects.
Hubble's intense interest in astronomy since his boyhood led him to pursue a degree in law, first at the University of Chicago and later at Oxford University, at his father's request. After his father's death in 1913, Hubble returned to the US Midwest and began teaching Spanish, physics, and mathematics at New Albany High School in New Albany, Indiana. During this time, he also coached the boys' basketball team.
A year later, Hubble entered graduate school to study astronomy at the University of Chicago's Yerkes Observatory, where he received his Ph.D. in 1921. His dissertation was titled "Photographic Investigations of Faint Nebulae." Hubble's early work included the discovery of a dozen Cepheid variables in the Andromeda galaxy between 1923 and 1924, which demonstrated the galaxy's significant distance from Earth.
In 1929, Hubble published work confirming that the recessional velocity of a galaxy increases with its distance from Earth, indicating that the universe is expanding. This behavior became known as Hubble's law. However, it is important to note that the concept of an expanding universe was first derived from general relativity equations formulated by Alexander Friedmann in 1922, and later proposed by Georges Lemaître in 1927, two years before Hubble's discovery.
Hubble's work greatly expanded the known stellar distance scale and contributed significantly to the field of astronomy. He authored two classic books, "The Realm of the Nebulae" (1936) and "The Observational Approach to Cosmology" (1937), and continued his research until he suffered a stroke in 1953.
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The Hubble constant
Hubble's Law can be represented by the equation v = H0D, where v is the recessional velocity or speed of separation, and D is the "proper distance" to a galaxy, which can vary over time. H0, the Hubble constant, is the constant of proportionality between these two values. Despite improvements in technology and nearly a century of measurements, the exact value of the Hubble constant remains uncertain. The current value is estimated to be around 70 km/s/Mpc, but there is no universal agreement on this number.
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Hubble's law today
Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies move away from Earth at speeds proportional to their distance. In other words, the farther a galaxy is from Earth, the faster it moves away. This law is considered the first observational basis for the expansion of the universe and is often cited in support of the Big Bang model. Hubble's law is considered a fundamental relation between recessional velocity and distance. The velocity of an object is plotted with respect to its distance from the observer, and a straight line of positive slope on this diagram is the visual depiction of Hubble's law.
Hubble's law is important because of the value of the Hubble constant, H, which is used to determine the exact age of the universe. It also helps in understanding dark matter and dark energy. The Hubble constant is defined as the unit of measurement used to describe the expansion of the universe. It is considered a constant because it remains the same at any given moment in time, but it is not truly constant since the Hubble parameter, of which the Hubble constant is the current value, varies with time.
The Hubble constant is determined by measuring the redshift, a shift in the frequency of light emitted by a galaxy, and the proper distance between the galaxy and the observer. This law only applies to distant galaxies, as nearby galaxies and objects in the Solar System have peculiar velocities that cause them to lie far from the line relating velocity to distance.
While Hubble's law is named after Edwin Hubble, who published his work in 1929, it should be noted that Belgian priest, astronomer, and academic Georges Lemaître derived the law in 1927, two years before Hubble. Alexander Friedmann also derived equations showing that the universe might be expanding in 1922, and before that, in 1912, astronomer Vesto Slipher reported redshifts for nebulae.
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Frequently asked questions
Hubble's Law is named after Edwin Hubble, who published his work on the law in 1929.
No. In 1927, two years before Hubble published his article, Georges Lemaître, a Belgian priest and astronomer, published research deriving what is now known as Hubble's Law.
Yes, Hubble served at the Mount Wilson Observatory in Southern California, where he made many key discoveries using the 100-inch Wilson telescope. He also authored two classic books: *The Realm of the Nebulae* (1936) and *The Observational Approach to Cosmology* (1937).
Hubble's Law, also known as the Hubble-Lemaître law, is the observation that galaxies are moving away from Earth at speeds proportional to their distance. This discovery provided the first observational basis for the expansion of the universe and is often cited as evidence for the Big Bang model.
