Since space itself is expanding, the wavelength of the traveling light also stretches. This means that while the light travels, it continues to redshift due to cosmic expansion.
The amazing thing is that all these values fit in this single curve. If the values were different the peaks would shift left or right, or be higher or lower. While the image of the CMB is wonderful, with its swirls of color, this graph is even more wonderful. It tells us that our understanding of the universe is on track.
There are actually two ways that the spectrum of microwave light can be affected. One is by changing the temperature of the source, but another is by moving relative to the source. The latter is commonly known as the Doppler effect. Just as the visible light from stars and galaxies can be shifted toward the blue when they are moving toward us and red when they are moving away from us, the CMB can be redshifted and blueshifted.
What’s truly amazing about this image is that it’s an observation of the first light of the universe. Within the first few minutes after the big bang, nuclei, electrons and photons formed, but for quite a while these were so hot that the light produced was quickly scattered or absorbed. Finally after about 380,000 years the electrons and nuclei formed atoms, and the light of the universe could finally travel freely through the universe.
Suppose you picked up a grain of sand and held it at arm’s length. If you held it up in the night sky, it would block a tiny fraction of the visible heavens. Now suppose instead of of a sand grain it were a tiny window, through which you could see even the faintest light. Finally, suppose you were to take your tiny window and point it at the darkest patch of night you could find. What would you see?
Evidence of an expanding universe doesn’t prove it began with a big bang. Claiming the observable universe was once smaller than an apple is a pretty crazy idea, so you’d be right to ask for more evidence before taking the idea seriously. Fortunately we happen to have a great deal of evidence. One of the earlier indications is known as the cosmic microwave background.
Hubble’s constant shows a relationship between the distance of a galaxy and the speed at which it moves away from us. Its discovery was the first evidence that the universe is expanding.