Anaya Nolan
Hubble's law, also known as the Hubble-Lemaitre law, is a fundamental concept in physical cosmology that describes the relationship between the velocity of a galaxy and its distance from Earth. It was formulated by Edwin Hubble in 1929 and states that the velocity of a galaxy moving away from Earth is directly proportional to its distance from us. This means that more distant galaxies recede faster than closer ones. Hubble's law is considered a cornerstone of observational cosmology, providing evidence for the Big Bang theory and revealing the existence of dark matter and dark energy. The law is expressed mathematically using the Hubble constant (H0), which represents the slope of the line relating distance to velocity. The discovery of Hubble's law was a pivotal moment in our understanding of the universe, indicating its ongoing expansion and revealing that the most distant galaxies move away from us at the fastest rates.
| Characteristics | Values |
|---|---|
| Galaxies are moving away from Earth | The farther a galaxy is from Earth, the faster it moves away |
| Velocity and distance | Velocity and distance are directly correlated |
| Redshift | A shift in the frequency of light emitted by the galaxy |
| Recession velocity | Recession velocity is v = dD/dt |
| Hubble constant | H0, a value that is time-dependent |
| Hubble parameter | A value that changes over time |
| Age of the universe | Older than 1/H |
| Expansion of the universe | Uniform expansion |
| Big Bang | The universe began with an explosion |
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What You'll Learn
Galaxies are moving away from Earth at speeds proportional to their distance
Hubble's law, also known as the Hubble-Lemaître law, is an observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In simpler terms, the further a galaxy is from Earth, the faster it moves away. This law is considered the first observational basis for the expansion of the universe, which is one of the pieces of evidence most often cited in support of the Big Bang model.
The discovery of Hubble's law is attributed to Edwin Hubble, who published his findings in 1929. However, the notion of the universe expanding at a calculable rate was first derived from general relativity equations in 1922 by Alexander Friedmann. Friedmann's equations showed that the universe might be expanding and presented the expansion speed if that were the case. Hubble's law can be formulated as v = H0D, where H0 is the Hubble constant and D is the proper distance from the galaxy to the observer. The Hubble constant is a constant in the sense that it does not depend on the magnitude or direction of the vector, but it may depend on time. The Hubble constant is most frequently quoted in km/s/Mpc, which gives the speed of a galaxy 1 megaparsec away as 70 km/s.
To determine a galaxy's recessional velocity, its redshift, or the shift in the frequency of light emitted by the galaxy, is measured. In 1923, Hubble observed Cepheids in Andromeda and used the period-luminosity relation to reveal that Andromeda lies far outside the Milky Way, providing the first proof that other galaxies exist. Hubble used Cepheid variable stars to measure the distances to a sample of galaxies and plotted the velocity-distance relationship for these galaxies. This plot demonstrated that the two quantities are directly correlated.
Hubble's law is considered a fundamental relation between recessional velocity and distance. It implies that the universe is expanding, as if there were a correlation between distance and velocity, which would not be the case if the universe were static and unchanging. This idea of an expanding spacetime led to the Big Bang and Steady State theories of cosmology. According to Hubble's law, the observer travelling with each galaxy sees all the other distant galaxies as receding from them. This observation does not indicate that Earth is near a centre from which the expansion is occurring but rather that every observer in an expanding universe will see objects receding from them.
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The universe is expanding
Edwin Hubble's 1929 paper on the relationship between distance and recession velocity of galaxies, unveiled the concept of an expanding universe. This discovery changed our understanding of the cosmos and inaugurated the field of observational cosmology. Hubble's law, also known as the Hubble-Lemaitre law, is the observation in physical cosmology that galaxies move away from Earth at speeds proportional to their distance. In other words, the further a galaxy is from Earth, the faster it moves away.
The discovery of Hubble's law is attributed to Hubble's work published in 1929. However, the notion of the universe expanding at a calculable rate was first derived from general relativity equations in 1922 by Alexander Friedmann. The Friedmann equations showed that the universe might be expanding and presented the expansion speed if that were the case. Hubble's law is considered a fundamental relation between recessional velocity and distance. The velocity of distant galaxies increases linearly with their distance, and this proportionality is known as Hubble's law.
Hubble's law can be formulated using simple Euclidean geometry, where the quantity H is called the Hubble constant. The Hubble constant is a constant in the sense that it does not depend on the magnitude or direction of the vector, but it may depend on time. According to Hubble's law, the universe is in uniform expansion, meaning there are no privileged positions in the universe. An observer travelling with any galaxy will see the surrounding galaxies receding from them. This is because the space between galaxies is expanding, and the galaxies are at specific positions in space.
The discovery of the linear relationship between redshift and distance, coupled with a supposed linear relation between recessional velocity and redshift, yields a straightforward mathematical expression for Hubble's law. The Hubble constant is used for determining the exact age of the universe, and it also helps in understanding dark matter and dark energy.
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The universe is in uniform expansion
The universe is expanding, and this expansion is uniform, meaning it is happening at the same rate in all directions. This is one of the key inferences that can be made from Hubble's Law. Edwin Hubble, in the 1920s, discovered that distant galaxies were moving away from Earth, and the speed at which they moved away increased with their distance. This became known as Hubble's Law, and it provided strong evidence for the Big Bang theory.
Hubble's Law states that the further an object is from us, the faster it is receding. This is often referred to as "Hubble flow" and indicates a universal expansion rate. By measuring the redshift of light from distant galaxies, scientists can determine how fast these galaxies are moving away from us, and this relationship between distance and speed has been found to hold true across the universe. This implies that space itself is expanding, causing everything in it to move apart.
The uniform expansion of the universe is a fundamental concept in modern cosmology. It suggests that the early universe was extremely dense and hot, and it expanded and cooled over billions of years. This expansion is not like an explosion, where fragments move outwards from a central point; instead, it is a continuous stretching of space-time itself. This means that distant galaxies are not simply moving through space but are carried by the expansion of space itself.
This idea of a uniformly expanding universe has been supported by various observations and studies over the years. For instance, the discovery and study of the Cosmic Microwave Background (CMB) radiation, which is highly uniform across the sky, provide strong evidence for a "hot, dense early universe." Additionally, the Large-Scale Structure of the universe, which refers to the distribution of galaxies and galaxy clusters, is consistent with the model of uniform expansion.
While the universe's expansion is uniform on a broad scale, there are local variations due to the gravitational interaction between nearby objects, leading to the formation of galaxy clusters, superclusters, and larger structures. Nonetheless, on a cosmic scale, the universe continues to expand uniformly, with distant galaxies moving away from us in accordance with Hubble's Law. The implications of this expansion, and the related concept of dark energy, remain active areas of research and discussion in cosmology.
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The age of the universe
Edwin Hubble's 1929 paper on the relation between distance and recession velocity of galaxies unveiled the expanding universe and changed our understanding of the cosmos. Hubble's law, also known as the Hubble-Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. This means that the farther a galaxy is from Earth, the faster it moves away. This discovery led to the Big Bang theory of cosmology, suggesting that the universe began in an explosion.
The discovery of Hubble's law was preceded by Alexander Friedmann, who derived his Friedmann equations from Einstein's field equations in 1922. These equations showed that the universe might be expanding at a calculable rate. In 1927, Georges Lemaître independently derived similar results, which would later become known as Hubble's law. Hubble's work built upon these findings, as well as the measurements of redshifts associated with galaxies made by Vesto Slipher and Milton Humason.
Hubble used Cepheid variable stars to measure the distances to a sample of galaxies. By plotting the distance to a galaxy against its velocity, he discovered a direct correlation between the two quantities. This relationship is known as the Hubble law and can be expressed mathematically. The Hubble constant (H0) is a key parameter in this equation, representing the slope of the line relating distance to velocity. The value of the Hubble constant has been determined by Planck to be equal to 46,200 mph per million light-years.
Hubble's law has important implications for our understanding of the universe. Firstly, it suggests that the universe is expanding. If the universe were static and unchanging, there would be no correlation between distance and velocity. However, the existence of a correlation indicates that the universe is expanding, supporting the Big Bang theory. Additionally, Hubble's law helps us understand dark matter and dark energy, contributing to our knowledge of the composition of the universe.
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The discovery of other galaxies
Edwin Hubble's work on the discovery of other galaxies began in 1923, when he observed the Andromeda galaxy, a fuzzy spiral region in the night sky. At the time, the prevailing view was that Andromeda was located inside our Milky Way galaxy and was assumed to be a nebula—a region of gaseous space where stars are made.
Hubble suspected otherwise. Using the Hooker Telescope at Mount Wilson, he identified Cepheid variables, a standard candle discovered by Henrietta Swan Leavitt in 1908. By comparing their apparent luminosity to their intrinsic luminosity, he could determine their distance from Earth. Hubble found Cepheids in several nebulae, including the Andromeda Nebula and Triangulum Nebula. His observations in 1924 proved conclusively that these nebulae were much too distant to be part of the Milky Way and were, in fact, entire galaxies outside our galaxy.
Hubble's discovery of nebulae outside our galaxy helped pave the way for future astronomers. He went on to estimate the distances to 24 extra-galactic nebulae, using a variety of methods. In 1929, he examined the relationship between these distances and their radial velocities, as determined from their redshifts. This work led to the discovery that the recessional velocity of a galaxy increases with its distance from Earth, a behaviour that became known as Hubble's law.
Hubble's law, also known as the Hubble-Lemaître law, is the observation in physical cosmology that galaxies are moving 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 implies that the universe is expanding, a finding that shook even Albert Einstein.
Hubble's work provided the first evidence that the universe contained far more than just our Milky Way galaxy. Astronomers now believe there are some 2 trillion galaxies stretching over 90 billion light-years. Hubble's observations have given us insight into how galaxies form, grow, and evolve over time.
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