Ancient Egyptians lived inside of a great celestial sphere. To them, the sun, the stars, the moon, were all points of light plastered on the under-surface of this sphere, born each night out of the horizon as the sphere rotated east to west. All these things: stars, sun, moon, horizon, held deep wonder and spiritual meaning for the civilization. And life revolved delicately around these beliefs.
But no, the truth is more beautiful. We are points of light floating in deep space; we are one of many. I’d like to think they’d be amazed to hear it.
I remember from cosmology class the Night Sky Problem. It’s a great illustration of the many things we take for granted. In 1826 Olber asked a simple question, why is the night sky dark?, and astonished everyone. You see, back then, we believed the Universe was infinitely large and infinitely old, filled with an infinite number of stars at some finite density and luminosity. This means, intuitively (also rigorously, but that won’t be necessary here), every line of sight should end on a star. The sky should be, on average, everywhere as bright as our sun.
It was Edgar Allen Poe who preempted all the scientists by providing the first hint at a solution to the problem. Finite time. He wrote, in the rather controversial “Eureka”:
“Were the succession of stars endless, then the background of the sky would present us an uniform density… since there could be absolutely no point, in all that background, at which would not exist a star. The only mode, therefore, in which, under such a state of affairs, we could comprehend the voids which our telescopes find in innumerable directions, would be by supposing the distance of the invisible background so immense that no ray from it has yet been able to reach us at all.”
No one would be sure whether or not he was joking, (“because nothing was, therefore all things are.”), but on this topic, he would be right. The universe (that we know and live in) has a finite age. Current estimates place it at approximately 13.5 billion years. Light also travels at a finite speed. As we look deeper into space, we look further back in time. In fact, the whole history and evolution of the universe is painted right into the sky, it’s there for us to read if we know how, and where, to look. Imagine being able to look at a person, or an object, and see its whole history. This is one of the most profound things about cosmology.
The horizon distance is the distance to the edge of the current, observable universe. The distance from which light emitted immediately after the Big Bang is just now reaching the earth. This distance is very large. But it is not infinite. This is the main reason why the sky is full of holes.
But not the only. Light from the most distant sources are also redshifted dramatically as it travels to earth, by the continuing expansion of the universe. For example, the sky is nearly isotropically covered in a faint glow of microwaves from the last scattering of photons by the plasma which pervaded the early universe (at redshift 1100). Since our eyes cannot see in radio, or microwaves, or infrared, the sky appears dark to our eyes where it doesn’t, say, to a powerful radio telescope.
On the other hand, if you were to climb to Glacier Point in Yosemite and look up into the night sky there, you might not think it dark at all…
(This all goes back to the importance of asking questions. Easy questions. Obvious questions. Those are usually the ones most difficult to answer.)
Meanwhile, back on earth, my to-do list hasn’t changed in 2 weeks:
- finish cosmology problem sets
- read papers
- order b&w film
The last time I worked with film was high school. But then we had to develop our own negatives and enlarge our own prints in the dark room. For a small(?) fee of $7 here, Fuji will do all the work for you. I got my first 2 rolls of slide film back, shot with a borrowed Nikon FE film camera with a 35 mm lens and UV filter. The film is Velvia ISO 50 slide film, which is famous for its warm tones and almost offensively vivid colors.
Slides are such dainty little things.
A few I’ve scanned into the computer using AJ’s scanner. All shot during the last 2 weeks in Santa Cruz, CA.
My brain’s telling me there’s some really satisfying quality about these pictures that I can’t achieve with digital. But maybe it’s just the placebo effect. In any case, I really need to go back to Wilder Ranch.