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Homework Help: Flat matter only universe

  1. Oct 4, 2006 #1
    I just finished a problem from ryden's intro to cosmology, but im not sure the answer makes sense here is the problem and my work:

    suppose you are in a flat, matter-only universe that has a hubble constant H0=70 km/s Mpc. you abserve a galaxy with z=1. how long will you have to keep observing the galaxy to see its redshift change by one part in 10^6?

    The equation to use is :

    dz/dt0 = H0(1+z) - H0(1+z)^[3(1+w)/2]

    we know:
    H0=70km/s Mpc
    plugging into the equation we get dt=-7.28E-13 s*Mpc/km
    multiplying (-7.28E-13 s*Mpc/km) * ( 3.1E19 km/Mpc) = -22.6E6 s.

    this seems a bit large and is negative.
    does this seem right?

  2. jcsd
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