I was talking with Chuck Greenlee the other day — he’s my roommate when he’s not editing columns for The Post — and he asked what happened to light that gets shined into the sky. That’s easy, I replied, as light travels infinitely far until it come into contact with something that absorbs it. It effectively propagates forever. But then I got to thinking, and that’s not quite accurate — light does travel forever, until it doesn’t.
Extragalactic astronomers peer as deeply into the cosmos as science allows, though the deeper they look, the younger the universe appears. All electromagnetic waves, light included, travel at a velocity of exactly 299,792,458 meters every second, though most people are familiar with 3 x 108 meters per second (a value that actually breaks the laws of physics, but I digress). The speed is specifically how quickly a photon, or a discrete “packet” of light, moves in a vacuum. The most readily available vacuum is that of space, and light moves the same there as in Earthly laboratories.
Space is unimaginably vast, about 2.5 trillion quadrillion cubic miles. To make this number a little less unwieldy, astronomers decided upon a standard which is the distance light travels in one year, 5.88 trillion miles. While this doesn’t help much with the total observable volume of universe, it does help with much nearer distances. The Alpha Centauri system (closest star system to ours) goes from 25 trillion miles to 4.37 light years. Since Alpha Centauri is 4.37 light years away, the light took 4.37 years to travel to us and thus we are seeing Alpha Centauri 4.37 years in the past. Closer to home, sunlight covers the 93 million miles to Earth in 8 minutes, so we are always seeing the Sun as it was 8 minutes ago.
Taking the speed of light to its limits brings us near to the edge of the universe. As it is now, the observable universe is 46.5 billion light years in radius. It would appear then that light from a galaxy on the very edge would have taken 46.5 billion years to travel to us, but that is not so. The Cosmic Microwave Background, the first light to be released after the Big Bang, shows a universe as it was 13.7 billion years ago, and its light took 13.7 billion years to reach us. In the intervening time, the universe has expanded as the light moved across it to more than three times its observed radius.
Everything you can interact with is normal, or baryonic, matter, which comprises just less than 5 percent of the observable universe. Another 27 percent is dark matter, which as the name implies doesn’t react with electromagnetic wave though it is detectable via gravitational analysis. The remaining 68 percent of the universe is comprised of something called dark energy. Scientists know little about dark energy, but they do agree on one thing: the universe is expanding and dark energy is the cause.
Edwin Hubble spent much of his life studying, as he called them in the 1920s, extragalactic nebulæ. In his research, he found galaxies recede from us more quickly the further from us they are. More than that, the universe’s expansion is accelerating, which poses some problems. Most notably is that eventually, the galaxies at the edge of the universe will move away so quickly, their photons will be unable to reach us. The cosmological horizon will slowly begin to shrink as more and more galaxies move away. Eventually, galaxies in our Local Group will recede away, then the stars in our galaxy, then even the atoms that make up Earth and smaller. There will come a time that a single particle is all that exists in the universe, because for the universe to exist, there must be observation of it and all the rest of the universe had expanded away.
Now back to the main point. If Chuck had a strong enough light source to not be scattered within our atmosphere, the photons would propagate in space forever. However, at a certain point, the universe will have so expanded to make the cosmological horizon just large enough for the remaining photons. As the photons will make up the entirety of the universe, they can’t move any further because there is no more distance to travel. For non-relativistic movers like you or I, this whole propagation and expansion process took trillions and trillions of years. However, to a photon moving at the speed of light, the whole trip is instantaneous.
