We start at the end and using mathematical jargon. Corollary: The best way to understand how the universe formed and evolved is to study physics and music. Conclusion: the entire evolution of the universe is written in the harmonic melody of the cosmic radiation in the background, "only" you have to know how to transcribe it in a score to understand the cosmos. Now for the details.
What is sound? Sound is a vibration that travels like an (acoustic) wave through a medium. It can be created by vibrating the string of a musical instrument, a violin, for example, which transfers the oscillatory movement of its strings to air molecules, the wave travels to our ears, where our eardrum vibrates, which causes a nervous signal and Finally, our brain interprets this signal. The string can vibrate at what is known as its fundamental frequency, meaning the center of the string moves back and forth with its ends fixed. The string can vibrate in another way, for example with the center fixed in addition to the ends, and the 2 ends of the string located at a distance of a quarter of the length of the string moving periodically. It's what's known as harmonic, the second, to be more specific, the first was what we call the fundamental frequency. There are infinite harmonics, just divide the string into 3 equal parts, 4, 5, ... N parts, and create a vibration that leaves N + 1 fixed points while the other points on the string move up and down, N of them reaching maximum distances to the resting position.
What is music? The fundamental frequency of our violin string corresponds to a musical note. The second harmonic is a different note, one octave apart. The third harmonic is another one, a fifth above the previous one, the fourth is another eighth and so on. With different strings and different forms of vibration, all the musical notes that come to mind can be created. The normal thing, yes, is that a violin string does not vibrate following a single perfect harmonic, it is difficult to achieve that purity of movement, but several are combined, each with different intensities. The combination of harmonics is something that distinguishes some instruments from others, or some voices from others, is what is called timbre.
How do you build a melody? Each string with its set of harmonics, each instrument with its timbre and intensity, some playing louder and some playing softer, can be combined to form an ordered set of musical sounds. This set ends up having an entity of its own, a meaning and causes a pleasant sensation to the listener (and subjectively "understands").
What does this have to do with the universe? The universe is full of photons since its origins, when it was only 370,000 years old (equivalent to the first day of life for those who reach 100 years old), which have survived until today and witnessed everything that happened. This is what is called the cosmic background radiation, CMB for its acronym in English, and its discovery was the first major support for the Big Bang theory. This radiation was created by the universe itself, which behaves like any object, emitting photons with characteristics (number, frequencies, intensity) that depend on temperature. When the universe created this radiation, it was about 3,000 degrees. Today, due to the effect of spacetime expansion, the universe has cooled, so the temperature we measured by studying this cosmic background radiation is 2.7255 degrees Kelvin, equivalent to -270.4245 degrees Celsius, better take a small cardigan to traverse outer space!
And here comes the music of the cosmos. Similar to how the sound of a violin string can be divided into linear harmonics, each with a specific intensity, the temperature distribution of the cosmic background radiation in the sky follows a pattern (complicated, let's not deny it) and can be divided into spherical harmonics. In physics we usually call these multipolar moments harmonics, they are widely used, from quantum mechanics to the study of planetary gravity, and we represent them by the letter l (in a very calligraphic font Rubio), which assumes integer values between 0 and infinity. The value l = 0 is equivalent to a constant: on a first approximation, the temperature value of the universe today is 2.7255 K, wherever you look. The value l = 1 is equivalent to a sky divided by 2 with symmetry in some direction, and in fact the CMB shows a slightly warmer temperature, ten thousandths of a degree! Towards an orientation and cooler only in the opposite direction.
By studying all multipolar moments and building a power spectrum, which is a study of the intensities of all harmonics in the cosmic background radiation, what is known as precision cosmology can be done. So far, we have been able to know the spatial distribution in the sky (then it would be angular) of the CMB temperature with an accuracy of the order of tens of microdegree Kelvin and even multipolar moments of the order of l = 2500, spectacular! Most importantly, you can fit this distribution with a cosmological model that includes measurable parameters such as the rate at which the universe is expanding, its age, the amount of dark matter that exists, the amount of dark energy and its properties., the moment the cosmic background radiation was created, the speed at which the Sun moves relative to the entire universe, or what the universe was like before the formation of the CMB or even beyond our observable universe, the very geometry and topology of the universe. With all these parameters, we reproduce the power spectrum in an extremely meticulous way.
We conclude now: the CMB is the perfect melody of the universe, it has everything, it reaches the highest level of information thanks to its original timbre and the distortion it suffered throughout the history of the universe, influenced by everything that existed. Your score is the spectrum of powers built from studying your spherical harmonics. You just need powerful telescopes to transcribe the score and physical models of the universe to interpret it. So, all that remains is to enjoy the music provided by the knowledge of the cosmos.
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