# Homework 11. Hubble's Law

 Name: ________________________ DUE 11/21 ID number: ________________________

Hubble's law states that the recession velocity of a distant galaxy is directly proportional to its distance:

v = H0 × d   .

Here v is the galaxy's recession velocity, and d is its distance. The quantity H0 is Hubble's constant; its value is roughly

H0 = 72 (km/s) / Mpc   .

1. The peculiar galaxy NGC 7252 has a distance d = 47 Mpc . What is its recession velocity?
(Note: show your work to receive full credit! Make sure to give the units of your result.)

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2. The galaxy cluster Abel 2218 has a recession velocity v = 54,000 km/s . What is its distance?

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3. Suppose you know that Hubble's law is valid but you don't know the value of Hubble's constant H0 . What observations would you need to estimate the value of H0 ?

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4. Suppose you didn't even know if Hubble's law is valid (this also implies that you don't know what value H0 might have). What observations would you need to test Hubble's law?

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