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Do you know why the oldest light in the universe is so uniform?
In the vast universe, there is a kind of microwave radiation called cosmic microwave background radiation (CMB). This radiation is everywhere, permeating every corner of the observable universe. Although the background often appears dark when we observe the space between stars and galaxies with an ordinary optical telescope, a faint, almost uniform background light can be detected using a sensitive radio telescope. The existence of this light is crucial to our understanding of the origin of the universe because it proves the Big Bang theory is correct.
The cosmic microwave background radiation provides us with a wealth of information about the early state of the universe.
In the Big Bang model, the universe in its earliest days was filled with a dense, hot plasma. As the universe expanded, these plasmas cooled to the point where neutral hydrogen could be formed. At this point, the universe was no longer opaque, but became transparent, allowing photons to travel freely through the vast space. This process is called the epoch of recombination, and it is the massive release of photons that allows us to detect this ancient light today.
Although the cosmic microwave background radiation appears uniform, it is not completely smooth. Highly sensitive detectors can detect weak anisotropies caused by the interaction between matter and photons. The distribution of these anisotropic structures across the sky can also be represented by a power spectrum, showing a series of peaks and valleys that capture the physics of the early Universe.
The first peak reveals the overall curvature of the Universe, while the second and third peaks detail the densities of normal and dark matter.
When astronomers examine these temperature inhomogeneities using ground-based and space-based experiments like COBE, WMAP, and Planck, they discover that the structure and evolutionary history of the universe are not random but are profoundly influenced by the early conditions of the universe. In fact, the data obtained from these experiments allow us to better understand what the universe looks like today.
Since the 1920s, many scientists have begun to speculate and study this cosmic background radiation. In 1964, the increasingly mature radio technology enabled two American astronomers, Arno Penzias and Robert Wilson, to accidentally discover CMB. This discovery not only successfully confirmed the predictions of the Big Bang model, but also won them the 1978 Nobel Prize in Physics.
The color temperature of this radiation is around 2.725 K, which is consistent with the characteristics of ideal blackbody radiation.
The discovery of the CMB was a milestone in physics. Not only because of its high measurement accuracy, but also because these data can be verified by various theoretical models, thus providing strong evidence for our understanding of the evolution of the universe. In the following decades, detection results from multiple detectors continued to correct our understanding of the cosmic microwave background radiation. These experiments, both on the ground and in space, demonstrate increasingly rigorous testing methods and approaches.
In the evolution of the universe, the existence of these early photons brings us many questions and thoughts. Its uniformity reflects the special characteristics of the early state of the universe. How is this state reflected in today's galaxy layout and matter distribution? Does this mean that future research will usher in another new era of understanding the universe?
