When is the last time you watched the sky revolve around us?
Earth rotates on its axis at 1,000 miles per hour (1600 kilometers per hour). At the same time, it flies around the sun at 67,000 m/h (110,000 km/h). And the Sun, with all its planets and rocks and dust in tow, makes its way around the center of the Galaxy, our Milky Way, at 520,000 m/h (830,000 km/h). And then, the Milky Way itself is hurtling toward the nearby Andromeda galaxy at 250,000 m/h (400,000 km/h).
The fastest space craft (and fastest man-made object in history), Juno, will slingshot around Earth on its way to Jupiter, eventually reaching a speed of 165,000 m/h. The NASA space shuttle reaches speeds of 17,000 m/h (27,000 km/h).
The average human walking speed is 3.1 m/h (5.0 km/h).
Though we sit in this coordinated maelstrom, we can still understand all of space and time on the largest scales. But, to do so, we must consider it statistically, on the whole, at great breadth and as a collection – not merely the sum of disconnected parts or separate events.
All across the universe, there are supernovae – exploding stars that blink in a cataclysmic, cosmically infinitesimal moment. Quasars are small regions that surround the supermassive black holes at the centers of galaxies that flash on and off on the timescales of hours to months. Each galaxy in the universe is creating some dimple in space-time due to its mass. Imagine a vast expanse of sand dunes: all light passing by these galaxies must traverse through it, resulting in distorted images by the time they get to us.
These are just some of the events that go on constantly around us, without regard for our existence, as we spin round and round, imagining a static quilt of stars turning about us. And they are just some of the celestial targets that will tell us more about how fast the universe is expanding.
To better understand these events, and the acceleration of spacetime, we wait for the targets to be at a place in the sky when we can see them – when the sun is down and this part of Earth is pointed in their direction. Our targets come from a large swath of sky, one-eighth of the celestial sphere. And across this expanse, we will obtain a uniform sample of targets. The uniformity – homogeneity or constancy – is crucial: we must observe all galaxies brighter than a certain amount, and within a certain distance to have a clean, uniform sample. Otherwise, variations in that information could be misconstrued, or at best they could muddy our measurement of dark energy.
Building the collection starts with amassing a set of deep images of the sky: these are but snapshots of long-gone eons, and they are the first step in our process of discovery. From the images, we distill vast catalogs of celestial bodies – galaxies, stars, motes and seas of hot gas and dust – an accounting of what the universe has so far created. This catalog can be further distilled when studied as a whole. The final concentrate is a small set of numbers that summarizes the fate of our universe: a measurement of the strength of dark energy.
Our spaceship Earth is a pebble in the swirling cosmic sea around us. We watch it as if we are separate, sometimes forgetting we come from it. As we look up from within our snowglobe on a mountaintop in the Chilean Andes, it becomes easier to remember that we are a conduit between the finite and the infinite.
Good night, and keep looking up.
Det. B. Nord