Classroom Aid - Hubble's Law

Summary

In 1923, Hubble discovered that Andromeda was a galaxy, leading to the understanding that spiral and elliptical nebulae were galaxies too. He mapped their radial velocities against their distances, finding a linear relationship between a galaxy's velocity and distance, known as Hubble's Law. This law, with its Hubble constant, allows us to measure cosmic distances and infer the universe's age, currently estimated at 13.4 billion years.

Takeaways

• 🌌 In 1923, Hubble discovered that Andromeda was a galaxy, not just a nebula, and was over 2 million light years away from the Milky Way.
• 🔭 Hubble's subsequent studies revealed that other spiral and elliptical nebulae were also galaxies.
• 📊 Hubble mapped the radial velocity of galaxies against their distance from Earth, observing a correlation between the two.
• 🌀 He found that galaxies were moving away from us, with the velocity of recession increasing with distance.
• 📉 The spectra of galaxies were predominantly redshifted, indicating their motion away from us.
• ⚖️ Hubble's Law was established, showing a linear relationship between a galaxy's recession velocity and its distance from us.
• 🔑 The Hubble constant was introduced, a key value in determining the distance of galaxies based on their redshift.
• 🔍 The accuracy of distance measurements in astronomy and cosmology hinges on the precise value of the Hubble constant.
• 📈 Over time, the Hubble constant has been refined, with the current best value being 22.4 kilometers per second per million light years.
• 🌠 The Hubble flow describes the steady movement of galaxies away from each other, implying an expanding universe.
• 🕰 By using the Hubble constant, the age of the universe can be estimated to be around 13.4 billion years.
• 🔮 Modern cosmology recognizes that the Hubble constant is not truly constant and refers to it as the Hubble parameter, which provides a slightly older universe age of about 13.8 billion years.

Q & A

What significant discovery did Hubble make about Andromeda in 1923?

Hubble discovered that Andromeda was not just a nebula but an entire galaxy, over 2 million light years away from our own Milky Way.

What did Hubble observe about other spiral and elliptical nebulae after his study of Andromeda?

Hubble found that other spiral and elliptical nebulae were also galaxies, not just part of the Milky Way.

What method did Hubble use to determine the radial velocity of galaxies?

Hubble used the shift in spectral lines to map the radial velocity of galaxies.

What is the relationship between a galaxy's distance and its receding velocity according to Hubble's observations?

Hubble found a linear relationship where the further away a galaxy is, the faster it is moving away from us.

What is Hubble's Law and how is it used to determine the distance of a galaxy?

Hubble's Law states that the receding velocity of a galaxy is equal to the Hubble constant times the distance of the galaxy from us. It allows us to calculate the distance of a galaxy if we know its redshift.

What is the Hubble constant and what does it represent?

The Hubble constant is a value that describes the rate at which galaxies are receding from us. It is used in Hubble's Law to calculate distances in the universe.

What is the current best value for the Hubble constant as mentioned in the script?

The current best value for the Hubble constant is 22.4 kilometers per second per million light years, with an uncertainty of plus or minus 3.2.

What is the Hubble flow and what does it imply about the universe's past?

The Hubble flow refers to the steady movement of galaxies away from each other. It implies that in the past, galaxies were closer together, suggesting an expanding universe.

How can the Hubble constant be used to estimate the age of the universe?

By using the Hubble constant, one can estimate the age of the universe by calculating the time it would take for a galaxy to reach its current distance from us at its current velocity, which is the inverse of the Hubble constant.

What is the estimated age of the universe based on the Hubble constant?

Using the Hubble constant, the estimated age of the universe is approximately 13.4 billion years.

How has the understanding of the Hubble constant evolved in modern cosmology?

In modern cosmology, the Hubble constant has been refined and is now referred to as the Hubble parameter. It gives a slightly larger age for the universe, around 13.8 billion years.