Global challenges Study Guide

All objects with a temperature above absolute zero emit thermal radiation, which is a form of electromagnetic radiation. This emission is not dependent on whether the object is hot enough to glow visibly; even objects at room temperature emit infrared radiation. The key characteristics of this emitted radiation depend significantly on the object's temperature. As an object's temperature increases, two main changes occur: first, the total intensity of the emitted radiation increases. This means hotter objects radiate more energy per second. Second, the distribution of wavelengths within the emitted radiation shifts. Specifically, hotter objects emit radiation with a shorter peak wavelength. For example, a warm human body primarily emits infrared radiation, which we cannot see. An electric hob at medium temperature might emit infrared radiation and some dull red light. A very hot filament in a light bulb emits a wide spectrum of light, including visible light and significant amounts of infrared radiation, appearing bright white. This relationship between temperature and radiation is fundamental to understanding energy transfer in various systems, from everyday objects to stars.

The 'red-shift' of light from distant galaxies provides crucial evidence for the expanding universe. When an object that emits light moves away from an observer, the wavelengths of the light waves observed are stretched, causing them to appear longer. For visible light, longer wavelengths are at the red end of the electromagnetic spectrum. Therefore, light from galaxies moving away from us is 'shifted' towards the red end of the spectrum, hence the term red-shift. The greater the speed at which a galaxy is receding, the greater the observed red-shift.

Astronomers observe the light spectra from galaxies. When they compare these spectra to spectra from elements on Earth, they find that the characteristic absorption or emission lines in the galactic spectra are shifted towards the red end. This means the galaxies are moving away. Furthermore, observations have shown that the vast majority of galaxies exhibit red-shift, with more distant galaxies showing a greater red-shift. This correlation implies that more distant galaxies are receding from us at a higher speed. This relationship between a galaxy's distance and its recession speed is a key piece of evidence supporting the Big Bang theory and the idea that the universe is continually expanding from a common point in time.