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\subsection{Hubble's Constant}
Reference: Zeilik, Gregory \& Smith, Introductory Astronomy \& Astrophysics,
Chapter 22.
The universe is expanding; every galaxy is moving away from every other
galaxy, and the further two galaxies are apart, the faster they are
receding from each other. This is known as Hubble's Law, after the
famous American astronomer who discovered the expansion of the Universe
back in the 1920's. It can be written as:
\begin{equation}
v = H_0 d
\label{eq:hubble}
\end{equation}
\noindent
where $v$ is the velocity at which something is moving away from us, $d$ its
distance from us, and the constant $H_0$ is known as Hubble's constant.
This constant is basically a measure of how fast the Universe is expanding.
If it is large, the universe is growing like crazy. 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 need the
velocity at 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 a spectrometer on a telescope,
we can measure the Doppler shift in these lines, and hence the velocity $v$.
These velocities are listed in Table \ref{tab:hydragalvecolity}, for several of the brightest
galaxies in Hydra I.