Gavin Howe
Edwin Hubble's work in the field of astronomy has been instrumental in shaping our understanding of the universe. Hubble's law, also known as the Hubble-Lemaitre law, states that the velocity of a galaxy is directly proportional to its distance from Earth. This means that the further a galaxy is from Earth, the faster it moves away. This discovery has provided the first observational basis for the expansion of the universe, and Hubble's work has also been used to calculate the Hubble constant, which reflects the rate of this expansion. However, the exact value of the Hubble constant is still a topic of debate among scientists. This is because the velocities and distances in Hubble's law are not directly measured but inferred from redshift and brightness. Nevertheless, Hubble's law has significant implications for our understanding of the universe and has been used to support the Big Bang model.
| Characteristics | Values |
|---|---|
| Basis | First observational basis for the expansion of the universe |
| Discovery | Published by Edwin Hubble in 1929 |
| Calculation | If you know H0 and can calculate the velocity, v, from the spectrum, then you can use the equation to calculate the distance, d, to that galaxy |
| Velocity | Measured using the Doppler shift |
| Distance | Inferred from brightness |
| Hubble Constant | The constant of proportionality between the "proper distance" D to a galaxy and its speed of separation v |
| Hubble Constant Value | Between 21.2 and 22.0 (km/s)/Mly, which is between 69.1 and 71.7 (km/s)/Mpc |
| Hubble Flow | The motion of astronomical objects due to the expansion of the universe |
| Hubble Diagram | A plot of the velocity (or redshift) of an object with respect to its distance from the observer |
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What You'll Learn
Galaxies moving away from Earth
Edwin Hubble's work in the late 1920s led to the discovery that galaxies are moving away from Earth. This discovery, now known as Hubble's Law, states that a galaxy's velocity is directly proportional to its distance from Earth. In other words, the farther a galaxy is from Earth, the faster it moves away. This proportionality is known as the Hubble constant.
Hubble's observations of Cepheid variable stars in "spiral nebulae" enabled him to calculate the distances to these objects. He discovered that these objects were well outside the Milky Way and were, in fact, galaxies. By combining his measurements of galaxy distances with Vesto Slipher and Milton Humason's measurements of the redshifts associated with the galaxies, Hubble discovered a rough proportionality between an object's redshift and its distance. This redshift refers to the shift in the frequency of light emitted by a galaxy as it moves away from Earth, causing the wavelength of light to stretch and shift towards the red end of the spectrum.
Hubble's Law is considered the first observational basis for the expansion of the universe, and it is one of the most often-cited pieces of evidence in support of the Big Bang model. The ongoing expansion of the universe means that everything is moving away from everything else, and the universe may be infinite in scope.
While Hubble's Law applies to the majority of galaxies, there are rare exceptions. For example, Messier 90 is a spiral galaxy that is moving towards the Milky Way. Scientists can tell that this galaxy is moving closer due to the light it emits. Messier 90 exhibits a phenomenon called "blueshift," where the wavelength of light is compressed and appears bluer as the galaxy moves towards Earth.
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The universe expanding
Edwin Hubble's 1929 paper on the relation between distance and recession velocity of galaxies, now known as Hubble's Law, revealed the expanding nature of the universe. Hubble's Law states that the recessional velocity of a galaxy is proportional to its distance from Earth. In other words, the farther a galaxy is from Earth, the faster it moves away. This discovery upended the long-standing view that everything in the universe was static.
Hubble's Law can be depicted in a ""Hubble diagram"" that plots the velocity of an object with respect to its distance from the observer. The diagram shows a clear trend of increasing velocity with distance, indicating that the universe is expanding in all directions. This expansion is believed to have been caused by the stretching of spacetime itself, with the rate of expansion remaining constant in all directions at any given time. However, this rate has changed over the lifespan of the universe.
The Hubble Constant, a critical number in cosmology, represents the constant rate of cosmic expansion. It is calculated by finding the slope of the line in the Hubble diagram. Despite numerous measurements and calculations, astronomers have not agreed on the exact value of the Hubble Constant. The latest measurements of distant, exploding stars suggest a value of 69.8 km/s/Mpc, while other reports place it as high as 74 km/s/Mpc.
Determining the precise value of the Hubble Constant is challenging due to the difficulty in measuring the distances to and velocities of distant galaxies. Astronomers rely on indirect methods, such as assuming that all galaxies of the same type have the same physical size, to estimate these values. The Hubble Constant is crucial because it can reveal missing pieces in our understanding of physics and help determine the age of the universe and its history.
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The Big Bang model
The Big Bang is a widely accepted scientific model for the origin and evolution of the universe. It posits that the universe was once extremely hot and dense, and has since expanded and cooled over time. This expansion is a key component of the model, and it is supported by Hubble's Law, also known as the Hubble-Lemaître law.
Hubble's Law, discovered by Edwin Hubble in 1929, states 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 is moving away. The discovery of this law built upon the work of astronomers like Carl Wilhelm Wirtz, Vesto Slipher, and Henrietta Swan Leavitt, who made measurements and calculations of galactic distances and velocities. Hubble's observations of Cepheid variable stars allowed him to calculate the distances to these objects and determine that they were well outside the Milky Way. Combining these distance calculations with measurements of galactic redshifts, Hubble discovered a correlation between the redshift of an object and its distance, indicating that the universe is expanding.
The concept of an expanding universe was first derived from Einstein's equations of general relativity by Alexander Friedmann in 1922. This was further supported by Georges Lemaître in 1931, who proposed that the universe emerged from a "primeval atom," introducing the modern notion of the Big Bang. The Big Bang model explains a broad range of phenomena, including the abundance of light elements, the cosmic microwave background (CMB) radiation, and large-scale structure. The uniformity of the universe, known as the horizon and flatness problems, is explained through cosmic inflation, a phase of accelerated expansion during the earliest stages of the universe.
While the Big Bang model is widely accepted, there are still aspects of the observed universe that are not fully explained by it. These include the unequal abundances of matter and antimatter, known as baryon asymmetry, the nature of dark matter, and the origin of dark energy. Scientists continue to refine their understanding of the universe and work to resolve these outstanding questions.
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Hubble's observations of Cepheid variable stars
Edwin Hubble's observations of Cepheid variable stars in "spiral nebulae" enabled him to calculate the distances to these objects. These stars have very predictable patterns of brightness, which can be used to derive a light curve, making them reliable distance markers. Hubble used the work of fellow astronomer Henrietta Leavitt to predict the brightness of these stars, which enabled him to calculate their distances from Earth.
The discovery of Cepheid variable stars as reliable distance markers had far-reaching implications for our understanding of the cosmos. Prior to Hubble's work, it was widely accepted that the Milky Way was the only galaxy, and objects such as Andromeda were believed to be 'spiral nebulae' within our own galaxy. However, Hubble's calculations revealed that these 'spiral nebulae' were, in fact, distant galaxies, shattering the notion that the Milky Way was the extent of the universe.
In summary, Edwin Hubble's observations of Cepheid variable stars were pivotal in shaping our understanding of the universe. These stars served as reliable distance markers, allowing Hubble to establish the correlation between distance and velocity, which became known as Hubble's Law. Furthermore, the study of Cepheid variable stars contributed to the refinement of the Hubble constant and challenged the notion of a static universe, paving the way for the acceptance of the expanding nature of the cosmos.
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Hubble's constant
Hubble's law, also known as the Hubble-Lemaître law, states 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 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 Hubble constant, denoted as H0, is the constant of proportionality in Hubble's law. It represents the ratio between the "proper distance" D to a galaxy and its speed of separation v. In other words, it tells us how fast the universe is expanding. The Hubble constant is given by the equation: v = H0D. While H0 is constant at any given moment, the Hubble parameter H, of which H0 is the current value, varies over time. Thus, the term "constant" can be misleading.
The Hubble constant is typically quoted in km/s/Mpc, with a value of around 70 km/s/Mpc. This implies that a galaxy 1 megaparsec (3.09 x 10^19 km) away would be moving away at a speed of 70 km/s. However, the true value of the Hubble constant is still a subject of debate among scientists, and different methods of measurement yield different results. The value of the Hubble constant is crucial in cosmology as it can be used to determine the age of the universe and its history.
To calibrate the Hubble constant, it is necessary to plot the distances of multiple galaxies using independent methods. This task is challenging, and astronomers have been arguing over the precise value of the Hubble constant for decades. The Hubble Space Telescope was launched with a primary objective to measure the Hubble constant accurately.
In summary, the Hubble constant is a critical value in cosmology that quantifies the rate of expansion of the universe. While the concept of the Hubble constant was pioneered by Edwin Hubble in 1929, the exact value remains a subject of ongoing research in modern astronomy.
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Frequently asked questions
Hubble's Law, also known as the Hubble-Lemaitre 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.
The distance of an object can be calculated by measuring its recessional velocity and then computing its distance based on a value of the Hubble constant. The Hubble constant is the rate at which the universe is expanding. The Hubble constant is given by H0, the constant of proportionality between the "proper distance" D to a galaxy and its speed of separation v.
The primary limitation of using Hubble's Law to determine the distance of an object is the uncertainty in the value of the Hubble constant. While the Hubble constant is generally believed to be around 65 kilometers per second for every megaparsec in distance, scientists still cannot agree on its exact value. Additionally, some of the recessional velocity may not be due to Hubble expansion, and the method assumes that Hubble's Law is correct.
