Hubble's law

Space is getting bigger.

This means galaxies move away from us. The farther a galaxy is, the faster it moves. This is a big rule of space.

Scientists find this by looking at light. Light from far away looks different. This helps them see how fast things go. Edwin Hubble helped show this rule. It shows our whole universe is growing. It is a very big idea!

The universe is growing. Galaxies are moving away from Earth. This happens because space itself is expanding.

There is a special rule for this movement. It is called Hubble's law. This rule says that distance matters. The farther a galaxy is from us, the faster it moves away.

Scientists find this speed using light. They look for redshift. Redshift is a shift in the color of light. It happens when an object moves away. This change tells us how fast the galaxy is going.

Edwin Hubble helped prove this rule in 1929. He used a powerful telescope. He studied stars called Cepheid variables. These stars helped him find distances to far places. He combined his work with other scientists. Vesto Slipher measured speeds of light. Henrietta Swan Leavitt helped find distances. Together, they showed the universe is not still. It is always growing. This discovery changed how we see the whole cosmos.

Hubble's law is a very important rule in science. It explains how galaxies move through space. This rule says that galaxies are moving away from Earth. The speed of this movement depends on distance. A galaxy that is far away moves faster than a nearby one.

This discovery shows that our universe is not staying the same size. Instead, the universe is actually expanding. This movement is often called the Hubble flow. It is a key piece of evidence for the Big Bang model.

Scientists use light to figure out how fast galaxies move. They look for something called redshift. Redshift is a shift in the frequency of light. When a galaxy moves away, its light changes in a specific way.

Astronomers can measure this shift to find the recessional velocity. This is the speed at which a galaxy moves away from us. They also need to know how far away the galaxy is. They often use the brightness of stars to find these distances. By combining speed and distance, they can see the pattern of expansion.

Many scientists helped build our understanding of this law. In 1912, Vesto Slipher measured the first Doppler shift in a spiral nebula. He found that almost all these objects were moving away from Earth. In 1922, Alexander Friedmann used math to show the universe could expand. Then, in 1927, Georges Lemaître published research about this expansion.

Edwin Hubble published his own work in 1929. He used a very powerful telescope at Mount Wilson Observatory. He combined his data with the work of many other researchers.

Edwin Hubble used special stars to find his measurements. These are called Cepheid variable stars. By studying these stars, he could calculate distances to objects outside our Milky Way galaxy.

Hubble studied 46 different galaxies to find his results. He found a relationship between distance and speed. This relationship is described by the Hubble constant. This constant tells us the rate of expansion. The units used for this constant are km/s/Mpc. This helps scientists measure how much space grows over time.

This discovery changed how everyone thought about the cosmos. Before this, many people thought the universe was static. This means they thought it never changed size. Even Albert Einstein once thought the universe was static. He even added a special term to his equations to make them fit that idea. After Hubble's work, Einstein called his old idea a mistake. Now, we know the universe is always growing. This law helps us understand the history and the future of everything in space.

Hubble's law, also known as the Hubble–Lemaître law, is a fundamental observation in physical cosmology. It describes how galaxies are moving away from Earth at speeds that are proportional to their distance. In simpler terms, the farther away a galaxy is, the faster it appears to recede from us. This phenomenon provides the first observational basis for the expansion of the universe. This movement of astronomical objects due solely to the expansion of space is called the Hubble flow. It serves as a primary piece of evidence for the Big Bang model.

To understand how scientists determine these speeds, we must look at the behavior of light. Astronomers measure a galaxy's recessional velocity by looking at its redshift. Redshift is a shift in the frequency of light emitted by a galaxy. When an object moves away, the light waves it emits are stretched. This stretching changes the light toward the red end of the spectrum. By measuring this fractional shift compared to a stationary reference, scientists can calculate how fast the object is moving.

Determining the distance to these galaxies is a separate but equally vital step. Edwin Hubble used Cepheid variable stars to calculate these distances. These are specific types of stars that change in brightness. By observing them in what were then called "spiral nebulae," Hubble found they were actually located far outside the Milky Way galaxy. He combined these distance measurements with the redshift velocities measured by Vesto Slipher and Milton Humason. This allowed him to see a clear relationship between how far a galaxy is and how fast it moves.

The relationship is expressed through the Hubble constant, which is the constant of proportionality in the equation. The formula relates the speed of separation to the proper distance of a galaxy. The units for this constant are typically expressed in km/s/Mpc, which stands for kilometers per second per megaparsec. While it is called a "constant," it is actually a Hubble parameter that can vary over time. One way to state this rate is as a relative expansion rate. For example, at the current rate, an unbound structure grows by 7% every billion years.

The discovery of this expansion was a gradual process involving many researchers. In 1912, Vesto Slipher measured the first Doppler shift in a spiral nebula and found most were receding. In 1922, Alexander Friedmann used Einstein's field equations of general relativity to show the universe might expand. In 1927, Georges Lemaître independently derived the law and estimated the expansion ratio. Although Hubble's 1929 work is most famous, Lemaître had published the idea earlier in French.

This discovery fundamentally changed our view of the cosmos and corrected previous scientific errors. Before these observations, many scientists believed in a static universe that did not change size. Even Albert Einstein originally included a "cosmological constant" in his equations to force a static solution. After Hubble's work proved the universe was expanding, Einstein called his assumption a "greatest mistake." He visited Mount Wilson Observatory in 1931 to thank Hubble for providing the evidence needed for modern cosmology.

Today, Hubble's law connects to much larger questions about the nature of space and time. The reciprocal of the Hubble constant is known as the Hubble time, which is approximately 14.4 billion years. Current evidence suggests that the expansion of the universe is actually accelerating. This means that for any given galaxy, the recession velocity is increasing over time as it moves further away. The study of this expansion continues to help scientists understand the history and the ultimate fate of the entire universe.