Why is the Night Sky Dark?
Perhaps you have found your way by starlight on a fine moonless night, although it is no longer possible in urban areas. A person once considered why the night sky was dark. If the universe was an infinite spread, he thought that stellar surfaces should be visible in every direction, and therefore the night sky should be as bright as the solar surface. This idea is called “Olbers’ Paradox” after its inventor. The paradox has now been solved, since it is known that the universe is not infinite. Still, it is a paradox that should not be excluded from the history of cosmology.
Although the night sky is dark, it is not perfect darkness. Astronomically, radiation shining all over the sky is called “background radiation,” an established field of space observation. There are many kinds of background radiation at various wavelengths and, in some cases, since radiation sources cannot be separated according to each celestial body, radiation is observed as background radiation. The most famous background radiation is Cosmic Microwave Background (CMB). CMB is believed to have been present when high-temperature plasma was cooled and neutralized as the Universe was expanding (i.e., about 400,000 years after the beginning of the universe), and thus serves as direct evidence of the Big Bang.
How Far has the History of the Universe Been Clarified?
How did the universe evolve, and how were galaxies, stars, and planets formed after the Big Bang? It can be said that “the history of the Universe” is the biggest astronomical subject to excite mankind’s intellectual curiosity. One interesting point about astronomical observation is that knowledge about the Universe in the past can be acquired by observing far-distant space. Since the speed of light is limited, light from distant celestial bodies has been emitted a long time ago. It is also well known that the universe is still expanding. The farther away the celestial body is, the faster it flies away from us, and its light wavelength is extended by the Doppler Effect (redshift: z = rate of extension of wavelength). CMB light that we now observe has traveled through space for more than ten billion years and its wavelength has been extended 1000 times (z1000) by the redshift. Since CMB was discovered, it has been observed closely. The CMB spectrum is almost perfectly uniform and is nearly identical with that of black-body radiation at an absolute temperature of 2.7 K. It fluctuates at a level of around 1/100,000, however, and analysis of it suggests that the universe is flat, consists mostly of unknown dark matter and will continue expanding.
Observations of far-distant galaxies are essential to clarify space history. Thanks to huge telescopes like “SUBARU” that allow us to observe even more-distant galaxies, it was revealed that galaxies already existed 1 billion years (corresponding to z6) after the universe was born. The evolution of the Universe illustrated in Figure 1 shows that an unknown area lies between the CMB era and the era of the most distant galaxies observed. This area, known as the “Universe’s dark ages,” is of great astronomical importance since the first stars and galaxies probably formed here.