(A) Redshift: If a star moves away from Earth at a high speed, the light it emits stretches in wavelength, making it appear redder than it actually is; conversely, if a star approaches Earth quickly, its light's wavelength shortens, making it appear bluer. This is similar to how the sound of a train’s horn changes as it approaches (the pitch increases due to shorter wavelengths) and moves away (the pitch decreases due to longer wavelengths). This phenomenon is known as the Doppler effect.

 

(B) Event Horizon: In 1929, Edwin Hubble plotted the redshifts of 46 galaxies against their distances and discovered that the speed at which galaxies recede is proportional to their distance. That is, the relationship between the distance 'd' and the recession speed 'v' of two galaxies is proportional, expressed as 'v ~ d', and formulated into the equation (v=H*d) by introducing a constant known as Hubble's constant. Hubble's constant is about 70 (Km/s)/Mpc, indicating that galaxies recede at a speed of 70 km/s per million parsecs. Since one parsec is approximately 3.26 light-years, this translates to a speed of 2,100 km/s per 100 million light-years. It revealed that the universe is expanding.

 

Further observations in 1931 on more distant galaxies astonishingly revealed that Galaxies farther away recede faster, suggesting that eventually, some might recede faster than the speed of light, 300,000 km/s. Such galaxies, moving faster than light, become unobservable to us. This defines the limit of the observable universe, known as the cosmic event horizon.

The calculation of the speed of light at 300,000 km/s divided by the expansion rate of 2,100 km/s per 100 million light-years yields approximately 14.3 billion years, which correlates closely to the actual estimated age of the universe, around 13.37 billion years. This expansion from a tiny beginning signifies the inception of the Big Bang theory.

 

(C) Cosmic Expansion:

As illustrated below, the distance between two points in space increases over time.

 

Our universe is expanding under the influence of dark matter (an unknown substance that causes the opposite effect of gravity), and this expansion has no center; regions farther from me expand faster relative to nearer regions. In the past, it was simply thought that galaxies were moving apart, but it is now understood that space itself is expanding.

 

(D) Observable Universe: The range of the universe observable from Earth at this moment (46.5 billion light-years)

Galaxies outside the event horizon are observable because, when the light started its journey, those galaxies were within the horizon. However, as the universe continues to expand, those galaxies move beyond the horizon, and the light emitted currently will never reach Earth. Thus, we only see the past images of these galaxies, making the size of the observable universe much smaller than that of the event horizon.

 

(E) Size of the Universe:

It is impossible to accurately measure the size of the universe, and it is even unknown whether the universe is finite or infinite. We can only speculate that it might be spherical with a diameter of about 93 billion light-years based on the limit of 46.5 billion light-years that we can measure. The farthest light we currently observe left its source 13.8 billion years ago, and since the universe has continued to expand, the distance between its starting point and Earth is significantly more than 13.8 billion light-years, calculated to be 46.5 billion light-years.

 

(F) Universe Expansion and Entropy: (Han Joo-hwan's inference, June 26, 2025)

The expansion of the universe, as discussed in (C), involves the increasing distance between galaxies and stars, not the expansion of the stars themselves. This is akin to dropping an ink drop in clear water, where the ink molecules spread out over time. The overall space where the ink molecules can exist increases, but the size of the molecules themselves does not; rather, the distance between the molecules increases, enlarging the space (region) they occupy. (This phenomenon is identical to the diffusion of gases.)

As the space available for the molecules increases, the number of ways (Ω) the molecules can arrange themselves also increases, thereby increasing the system's entropy (S=k*ln(Ω)). This aligns with the thermodynamic principle that our world evolves spontaneously in a direction that maximizes entropy.

 

Given that the same physical laws must apply, this providence should also operate in our universe, suggesting that the increase in the distance between stars, and thus the expansion of the universe, should be viewed as a process that increases the entropy (or the arrangement options of the stars) within the universe space. However, in diffusion, the space where the ink molecules distribute themselves enlarges as the molecules move, whereas in the expansion of the universe, it is the space itself that expands, which is a different concept. Of course, these explanations still do not account for why regions farther away expand faster...