宇宙は光より速い速度で膨張してるのになんで星の光が届くんや？
It's puzzling why we can see distant starlight if the universe is expanding faster than light. The key is that space itself is stretching, distinct from light's speed *through* space. Ancient light, emitted long ago, perseveres through this expanding space to reach Earth, allowing us to still witness the glow of distant stars.
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Cosmic Expansion
The expansion of the universe refers to the phenomenon where the fabric of space itself stretches over time, increasing the distance between galaxies, rather than celestial bodies like galaxies moving through space. This concept derives from Albert Einstein's general theory of relativity and was supported by Edwin Hubble's observations. Hubble discovered that more distant galaxies recede from us at a faster rate, naming this "Hubble's Law." Crucially, this "speed" is not the velocity at which objects move *within* space, but an apparent velocity due to the expansion of space itself. Therefore, from a certain perspective, regions receding faster than the speed of light can exist. One answer to the article's question, "Why can we still see starlight even though the universe is expanding faster than the speed of light?", is that even if the space between the light source and Earth greatly stretched during its journey due to this cosmic expansion, the light itself continuously travels through that space at the speed of light. Imagine dots drawn on a balloon moving further apart as the balloon inflates; similarly, galaxies move apart due to the expansion of space. The light we observe from stars was emitted from relatively nearby locations when the expansion began, and as the universe expanded during its travel to us, those stars now appear to be very far away.
Principle of the Constancy of the Speed of Light
The Principle of the Constancy of the Speed of Light is one of the two fundamental postulates of Albert Einstein's special theory of relativity, stating that "the speed of light in a vacuum is always constant, regardless of the motion of the observer or the motion of the light source." This speed is approximately 300,000 kilometers per second and is considered the maximum speed at which information can travel in the universe. This principle shows that the everyday rule of velocity addition (e.g., throwing a ball in a moving train adds the train's speed to the ball's speed) does not apply to light. So, why doesn't the possibility of cosmic expansion exceeding the speed of light contradict the principle of the constancy of the speed of light? The key lies in the concept of "expansion of space." The principle of the constancy of the speed of light describes the speed at which light *travels through space*. In contrast, the expansion of the universe is a phenomenon where *space itself stretches*, which is fundamentally different from the speed at which an object moves through space. For instance, the statement that a spaceship travels at 100,000 kilometers per second is entirely different from the statement that cosmic space itself stretches at a rate of 100,000 kilometers per second. Light always propagates locally at the speed of light within its own space, but as that entire space expands, a situation arises where the distance to the destination keeps increasing even as light travels. Consequently, light emitted from distant galaxies, while having its path stretched by the expansion of space, nonetheless continues to travel towards us at the speed of light, ensuring that some of it can eventually reach Earth.
Observable Universe
The observable universe refers to the region of the universe from which light has been able to reach us since the Big Bang, encompassing all celestial bodies observable from Earth. Although the age of the universe is approximately 13.8 billion years, implying a maximum observable distance of simply 13.8 billion light-years, its actual radius is estimated to be around 46 billion light-years. The reason for this discrepancy lies in cosmic expansion. While light traveled for 13.8 billion years to reach Earth, the cosmic space itself was continuously expanding. Thus, a celestial body that was relatively close to Earth when the light was emitted is now located much farther away by the time its light reaches us. Therefore, the light we see from the most distant objects was emitted 13.8 billion years ago, but those objects themselves are currently located over 46 billion light-years away. The concept of the observable universe is crucial for understanding the article's question, "Why does light still reach us even with expansion faster than light?" The starlight we can see originates from locations within this observable universe and had sufficient time to reach Earth. While the universe as a whole is speculated to be much vaster or even infinite, light from regions beyond the observable universe is thought to never reach Earth, even if traveling at the speed of light, due to the universe's accelerating expansion.