Astronomy Study Guide

The red-shift of light from distant galaxies provides crucial evidence for an expanding universe. Red-shift occurs when the light observed from a celestial object shifts towards the red end of the electromagnetic spectrum. This happens because the source of the light is moving away from the observer; the further away the galaxy, the greater its red-shift. This phenomenon occurs due to the stretching of light waves as the space between the galaxy and the observer expands. If galaxies were moving towards us, we would observe a blue-shift. The fact that nearly all distant galaxies exhibit red-shift indicates that they are moving away from us, and the universe as a whole is expanding.

Red-shift, the observed shift of spectral lines from distant galaxies towards longer, redder wavelengths, is a crucial piece of evidence in cosmology. Both the Big Bang theory and the Steady State theory account for red-shift, but they interpret its cause differently. In the Big Bang theory, red-shift is understood as a consequence of the expansion of space itself. As the universe expands, it stretches the light waves travelling through it, increasing their wavelength. This increase is proportional to the distance of the galaxy, meaning more distant galaxies show greater red-shift, indicating they are receding faster. This supports a universe that originated from a singular point and has been expanding ever since. The Steady State theory, on the other hand, proposed that the universe is eternally expanding but maintains a constant average density through the continuous creation of new matter. In this model, red-shift would also be observed because galaxies are indeed moving away from each other. However, the Steady State theory attributed this recession to galaxies moving through space, rather than space itself expanding. It aimed to explain the observed red-shift while maintaining a universe that is unchanging in its overall appearance over time.