Olbers’s Paradox, often called the “dark night sky paradox,” questions a seemingly obvious observation: why is the night sky dark if the universe is infinite and uniformly filled with stars? The paradox suggests that in such a universe, every line of sight should eventually end on the surface of a star, making the sky uniformly bright, like the surface of the sun. Modern cosmology resolves this puzzle by demonstrating that the universe is neither infinitely old nor static, relying on two primary factors derived from the Big Bang theory.
The Paradox’s Flawed Premise: A Finite Universe
The central flaw in the paradox lies in the assumption of a universe that has existed forever. If the universe were infinitely old, light from every star, no matter how distant, would have had sufficient time to reach Earth. However, the universe has a finite age, estimated to be approximately 13.8 billion years, a time span established by the Big Bang model.
Because light travels at a finite speed, we can only see objects whose light has traveled to us within this time limit. This distance defines our “Cosmic Horizon,” which is a boundary of time, not a physical edge. Any star or galaxy located farther away than about 13.8 billion light-years has not yet had its light reach us.
The number of stars contributing to the night sky’s brightness is finite, not infinite, as the paradox assumes. The observable universe is limited in size and content, preventing the sky from being saturated with starlight. The darkness we observe is a direct consequence of the universe’s beginning and the speed of light.
The Effect of Cosmic Expansion and Redshift
The second resolution is the dynamic nature of the cosmos, specifically its expansion. Edwin Hubble’s observations showed that galaxies are moving away from us, and the farther away a galaxy is, the faster it recedes. This expansion of space stretches the light waves traveling through it, a phenomenon called cosmological redshift.
As light waves stretch, their wavelength increases and shifts toward the red end of the spectrum, eventually moving into the infrared and microwave regions. This stretching causes the photons to lose energy, significantly dimming the light we receive. Light from the most distant galaxies is so redshifted that it becomes invisible to the human eye, having shifted far outside the visible spectrum.
The Cosmic Microwave Background (CMB) is a prime example of this redshift. Originating from the universe’s earliest, hot phase, this light has been stretched over billions of years into a faint, cool microwave glow. This energy loss means that even if light from all stars could reach us, the light from the most distant sources would be too low-energy to visibly illuminate the night sky.
Why Interstellar Dust is Not the Answer
An early explanation for the dark sky was that interstellar dust and gas absorbed the light from distant stars, blocking our view. While space does contain absorbing material, this mechanism cannot be the resolution to the paradox, according to the laws of thermodynamics.
If absorbing matter were the only factor, the dust would eventually heat up from continually absorbing the infinite starlight energy the paradox predicts. Once heated, the dust would begin to re-radiate that energy, reaching a temperature comparable to the surface of the stars and re-emitting the energy as its own light.
This re-radiation would eventually make the sky bright again, though at a different wavelength, such as infrared. Therefore, the dust would only temporarily absorb the energy before reradiating it, failing to maintain the darkness of the night sky over long timescales. The finite age and the expansion of the universe are the necessary conditions for the darkness we observe.