Evidence for an Expanding Universe

🚀 Evidence for an Expanding Universe

Figure 2: Hubble's law — recession velocity is proportional to distance. The gradient gives the Hubble constant H₀. Data point colours shift blue→red to show red-shift increases with distance.

Hubble's Key Observations

By measuring the red shift of hundreds of galaxies, Hubble established three key facts:

1. The light from almost all distant galaxies is red-shifted
2. This means almost all galaxies are moving away from us
3. The greater the distance to a galaxy, the greater its red shift — meaning more distant galaxies are moving away faster

This relationship is described by Hubble's Law: recession velocity is proportional to distance. The constant of proportionality is the Hubble constant, which can be used to estimate the age of the universe.

What the Expansion Really Means

It is important to understand that galaxies are not moving through space away from us. Space itself is expanding, carrying the galaxies with it. This is why every galaxy (beyond our local group) is receding, and why more distant ones appear to recede faster — there is simply more space between them and us that is expanding.