The cooler regions (bluer hint in Figure 3) correspond to collected (by our detectors) photons with less energy and longer wavelengths. The nearby region surrounding the electron that that “cold” photon last scattered off of has more matter present than the nearby regions surrounding the electron that a “hot” photon last scattered off of. The greater abundance of matter “robbed” the photon of more energy. Thus we can relate the non-uniformities in the CMBR to slight non-uniformities in matter density. The gravity exerted by regions with more matter density on nearby particles “overpowered” the gravity exerted by regions with lower matter density. Over very long periods of time this slight imbalance lead to the formation of galaxy superclusters and clusters. According to Newtonian gravity if the matter density was completely uniform galaxy clusters would have never formed. Three centuries ago, Isaac Newton explained (using his law of gravity) that if the distribution of matter in the Universe was completely uniform, all of the matter would condense into a “great spherical mass”:

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Therefore, according to Newtonian gravity, if the matter distribution was completely uniform structure (i.e. galaxy clusters, galaxies, stars, planets, etc.) would have never arisen.

The origin of the slight non-uniformity in matter density can be explained by quantum fluctuations in the beginning of the Universe. According to the time-energy uncertainty principle, there will always be particles randomly popping in and out of existence. Since the particles randomly pop in and out of existence, at any instant of time there will always be slight non-uniformities in the distribution of matter and energy. Cosmologists speculate that during the time interval when the Universe was $$t=10^{-37}$$ seconds old until it was $$10^{-35}$$ seconds old (called the inflationary era), the fabric of space and time stretched apart faster than the speed of light. From the time-energy uncertainty principle we know that at the instant when the age of the Universe was $$t=10^{-37}s$$ the distribution of matter and energy was slightly non-uniform. Then, during the inflationary era, the space between every particle expanded faster than the speed of light and thus every particle was causally disconnected during this short period of time. You could imagine that inflation “blew up” and enlarged these non-uniformities while keeping the proportions of their separation distances the same. In the words of the cosmologist Max Tegmark, “When inflation stretched a subatomic region [of space] into what became our entire observable Universe, the density fluctuations that quantum mechanics [and the Uncertainty Principle in particular] had imprinted were stretched as well, to sizes of galaxies and beyond. (p. 107, Our Mathematical Universe)”

The miniscule non-uniformities in temperature (and therefore energy) of the photons coming from the CMBR tells us about the miniscule non-uniformities in matter density when the Universe was only about 300,000 years old. For about the first 300,000 years of the Universe’s life, the Cosmos was too hot for electrons to be captured by hydrogen and helium nuclei. This “soup” of electrons and atomic nuclei acted like an electrical conductor and electrical conductors are opaque to light. Over the next (roughly) 100,000 years the Universe cooled enough for atomic nuclei to capture electrons allowing photons, for the first time, to travel across long distances without being scattered. Somewhere around this time period photons scattered off of electrons for the last time—they would not interact with matter again for another roughly 13.5 billion years. The last electrons that each photon scattered off of can be related to the strength of the gravitational field and thus the distribution of matter in nearby regions around each of those electrons. When our detectors collect photons from the time of last scattering, our detectors are “seeing” the CMBR. From the CMBR you’ll notice that some of the regions are cooler than others.

References

1. Singh, Simon. Big Bang: The Origin of the Universe. New York: Harper Perennial, 2004. Print.

2. Wikipedia contributors. "Cosmic Microwave Background." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 12 May. 2017. Web. 18 May. 2017.

3. Tegmark, Max. Our Mathematical Universe: My Quest for the Ultimate Nature of Reality. New York: Knopf, 2014. Print.