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\subsection{Hubble's Constant}  Observations of distant galaxies made by Edwin Hubble in the 1920s turned up an interesting fact:  that every galaxy is moving away from every other galaxy, and the further apart two galaxies are,   the faster they are receding from each other.   This observation is explained by realising that the velocity is due to the expansion of the space   between galaxies, rather than relative motion between the galaxies, i.e. the Universe is expanding.   A common analogy is how the distance between two points drawn on an uninflated balloon increases  as the balloon is blown up.  This is known as Hubble's Law, and it can be expressed as  \begin{equation}  v = H_0 d  \label{eq:hubble}  \end{equation}  \noindent  where $v$ is the apparent recessional velocity of a galaxy at a distance $d$   from us, and the constant $H_0$ is known as Hubble's constant.  $H_0$ has the interesting units of km/s/Mpc (provided $v$ and $d$ are given in  km/s and Mpc respectively), which implies that the further away an object  is from us the faster its recessional velocity.  This means that the Hubble constant tells us how quickly the space in between galaxies is expanding.  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 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.