In 1720, the famous British astronomer Edmond Halley posed a question: Given the countless stars in the universe, with the Milky Way alone containing hundreds of billions of stars, the sky in any direction should be bright. So why is the sky dark at night?
Later, the Frenchman Olbers and the German Olbers revisited this question and provided their own answers.
Because Olbers conducted the most in-depth research on this question, the paradox was named after him as Olbers' Paradox.
Olbers' proposed solution was the existence of light-absorbing material in interstellar space. This is, of course, incorrect because energy is conserved.
No matter the material, once it absorbs energy, it must eventually release this energy. Absorbing energy leads to an increase in temperature, which then radiates out.
In fact, if one assumes that the universe is infinite and homogeneously static, by integration, one would conclude that the sky should indeed be bright, which is easy to understand. Strictly speaking (mathematically speaking), Olbers' Paradox requires four conditions:
- The universe is homogeneous, meaning the distribution of celestial bodies and the energy they emit over time is roughly the same throughout the universe;
- The universe is infinite;
- The average energy emitted by celestial bodies does not change over time;
- The universe is static and does not change over time.
However, our current explanation for Olbers' Paradox is based on the Big Bang theory. We've discovered that the universe is expanding and has undergone rapid inflation in the past, meaning today's universe is larger than yesterday's. This implies that, tracing back in time, the universe has a starting point, a singularity from which the universe began and expanded rapidly after the explosion until today.
The Big Bang theory presents two issues. First, the universe is not infinite; the observable universe we can see is not infinite, with a radius of about 46 billion light-years. The derivation of the size of the observable universe is very complex and won't be expanded here.
Due to cosmic expansion, celestial bodies at the edge of the observable universe are moving away from us at the speed of light, making their light forever unreachable to us, hence why we cannot see the light from these celestial bodies.
Further explanation: the maximum speed of light applies locally, meaning your speed relative to surrounding objects cannot exceed the speed of light.
However, the expansion of the universe is an intrinsic property of all space, unrelated to relativity, allowing celestial bodies at the edge of the observable universe to have an additional velocity increment from cosmic expansion without violating relativity due to their vast distance from us.
In summary, it's consistent with special relativity that bodies at or beyond the edge of the observable universe are moving away from us at or faster than the speed of light, and the universe we can observe is finite, as are the celestial bodies we can see.
Second is the redshift of radiation. As the universe expands, celestial bodies moving away from us accelerate, causing photons emitted by distant bodies to shift towards longer wavelengths due to the Doppler effect, meaning the observed wavelength of photons from distant bodies increases, and their frequency decreases, i.e., their energy decreases.
Therefore, we can only observe a limited number of celestial bodies, receive light from a limited number of sources, and the energy of this light is decreasing. Thus, our night sky is not bright but dark, based on the explanation from the Big Bang theory, which itself has some unresolved issues.