The dynamics of the Universe

The present-day Universe appears to be expanding in all directions, as shown by the fact that all distant galaxies and clusters of galaxies appear to be receding from us. This was the first and most obvious piece of evidence indicating that our Universe was initially much denser, leading to the hypothesis of an origin in an initial “Big Bang”.

The recession velocities of the galaxies are measured by analyzing the light they emit, which in a spectrograph is seen to contain not only a continuum of frequencies but also discrete frequencies, due to electronic transitions between energy levels of atoms in these galaxies. Such lines have a well-determined laboratory frequency, and when we observe such well-known atomic lines but we see that their frequency is lower (or their wavelength is longer, since wavelength equals speed of light divided by frequency), we infer that the atoms and the galaxy are moving away from us. This effect is called the Doppler shift. A simple everyday acoustic analogy of this Doppler shift is provided by the pitch of an ambulance's siren, which gets lower as the ambulance speeds away from us: the motion away from us “stretches” out the wavelength.

The expansion velocities increase with the distance away from us at a rate which is proportional to the distance, as long as the galaxies are not too far away.