Most of us take sight for granted. The farthest-ranging of our five senses, sight enables remote observation of events, and shapes our perception of time and causality. Sight depends on visible light, a form of electromagnetic radiation with a natural velocity of 186,282 miles/sec (called c in physics). On everyday distance scales, c is so fast that observation is practically instantaneous. But c is not infinitely fast, and this dramatically changes the nature of seeing through the cosmos.
Einstein’s General Relativity theory states that neither mass nor energy can exceed light speed (c might as well stand for cosmic speed limit). Thus, events can neither transpire nor convey information faster than c. Since c is finite, light (and its information) requires time to travel. Given a sufficiently long distance, light travel is far from instantaneous.
Astronomers use the light-year as a cosmic yardstick. It is equal to the distance light travels in one year, about 5.9 trillion miles. For example, the brilliant star Sirius is 8.6 light-years from Earth. This means that when we see Sirius, the light spent 8.6 years to reach our eyes. It also means that we see Sirius as it existed 8.6 years ago, not in its current state. Thus, through space we also peer into the past.
Space is expanding— has been since its origin in the Big Bang event 13.9 billion years ago. Under the influence of dark energy, the expansion rate has intensified over the last 9.8 billion years. Objects at intergalactic (and greater) distances are gradually accelerating apart, like chocolate chips in rising cookie batter. Their separation can be measured by calculating the comoving distance to account for expansion over time. On smaller scales, objects are spared expansion effects due to mutual gravitational attraction (the fortunate reason we are not torn apart by surrounding space).
How far can we see? This is defined by the diameter of the observable universe, based on the earliest possible light currently reaching us— it is 93.2 billion light-years. Outside this, the universe is invisible until its light can arrive. However, continuing expansion will ensure that beyond 62 billion light-years, currently emitted light will never arrive. At closer distances, arriving light will progressively attenuate through redshifting. Over billions of years, the observable universe will steadily grow darker and sparser.
The ancient Greeks saw the heavens as static and eternal, a notion rooted in the word firmament. Today, we know a dynamically evolving realm surrounds us. Ironically, its light constrains what we can see and understand about it. On a starry night, perhaps you will remember the Latin phrase tempus fugit. Time flies, even for the cosmos.
Western Slope Skies is produced by the Black Canyon Astronomical Society and KVNF Community Radio. This feature was written and voiced by Michael T. Williams.
