The Cosmic Microwave Background - 2.2.3.1
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More than a hundred years ago Albert Einstein wrote down his postulates of special relativity. This reshaped the world of physics and we got closer to understanding the nature of light. The second of the two postulates explicitly states that the speed of light in vacuum is constant. This changed our perception of the world. If light is constant, that means the light we see has traveled to us for some specific fixed time, before it reaches our eyes or telescopes. This also means that you are always looking into the past, because what you see is limited by the speed of light in how fast you can know about your surroundings. As an example, it takes a bit more than 8 minutes for light from the sun to reach earth. Thus, if you look at the sun, then you are not looking at how it looks now, but rather how it looked around 8 minutes ago. If you look further away to the nearest star outside our solar system, alpha Centauri, then this light is around 4 years old. Now what if you looked as far away as possible? Well, you wouldn't see much because our eyes can only see specific wavelengths of light. But what a telescope would see would be the Cosmic Microwave Background or CMB, and that is the topic of today!
Before we move on let us recap what light is. In everyday life we use the word light to describe the visible light that we see. However, visible light is nothing but electromagnetic radiation or waves. For a physicist, light is generally the entire spectra of electromagnetic waves. Thus, it is not only visible light, but also gamma rays, x-rays, UV-light, infrared light, microwaves, and radio waves. What determines the classification of light is energy or frequency as described my Max Planck with the equation.