deletions | additions       diff --git a/subsection_Hubble_s_Constant_Observations__.tex b/subsection_Hubble_s_Constant_Observations__.tex  index d8af754..be57e2f 100644  --- a/subsection_Hubble_s_Constant_Observations__.tex  +++ b/subsection_Hubble_s_Constant_Observations__.tex 
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If it is large, then the Universe is expanding rapidly.   We are going to use the distance we measured to the Hydra I cluster to estimate $H_0$.  We have already measured the distance $d$ to Hydra I. Now we We also  need the recessional  velocityat which it is rushing away from us  $v$. This is measured using the Doppler effect. The gas in most Hydra I galaxies emits emission-lines of Oxygen, Hydrogen and other elements. With The wavelength of these lines can be measured extremely precisely in a lab,  and by measuring their Doppler-shifted wavelengths with  a spectrometer on a telescope, we can measure obtain the velocity $v$ using the formula  \begin{equation}  \frac{v}{c} = \frac{\Delta \lambda}{\lambda}\, ,  \end{equation}  where $c = 2.998 \times 10^5$ km/s, $\Delta \lambda$ is  the Doppler shiftin these lines,  and hence $\lambda$ is   rest wavelength of  the velocity $v$. emission line.  These velocities are listed in Table \ref{tab:hydragalvecolity}, for several of the brightest  galaxies in Hydra I.