Question 31.5: Recession of a Quasar A quasar is an object that appears sim...
Recession of a Quasar
A quasar is an object that appears similar to a star and that is very distant from the Earth. Its speed can be measured from Doppler shift measurements in the light it emits.
A certain quasar recedes from the Earth at a speed of 0.55c. How far away is it?
B Suppose we assume that the quasar has moved at this speed ever since the Big Bang. With this assumption, estimate the age of the Universe.
A:
We can find the distance from Hubble’s law:
R=\frac{v}{H}=\frac{(0.55)\left(3.00 \times 10^{8} m / s \right)}{17 \times 10^{-3} m /( s \cdot ly )}=9.7 \times 10^{9} ly
B:
We approximate the distance from the Earth to the quasar as the distance that the quasar has moved from the singularity since the Big Bang. We can then find the time interval from a calculation as performed in Chapter 2: \Delta t=\Delta x / v=R / v=1 / H \approx 18 billion years, which is in approximate agreement with other calculations.