The Cosmic Paradox of the Dark Fruity Center
by Joy Moses-Hall
Sunday, December 24, 2017
When gazing at the night sky, our eyes are drawn to the bright twinkling lights above. But between the stars, in the darkness, lurks paradox.
What looks like empty space up there may not be empty after all.
Instead, the invisible voids are where the universe pulls together, and at the same time pulls apart.
Gravity pulls the universe together. You can see it in the patterns of the orbits out there: the Moon orbits the Earth, the Earth orbits the Sun, the Sun orbits the galactic center. Orbits exist because the gravity of large objects prevents other objects from drifting away. We can see the patterns. Gravity is illuminated by the orbits of the things we see.
But there is a problem.
Measurements of star orbits show they go too fast. The stars in galaxies don’t follow the patterns of the Moon and Earth. There is too much gravity. Yet gravity math should be the same for galaxies. Maybe there is another, unlit substance out there, throwing gravity but not light. We call this dark, gravity-spewing substance Dark Matter. There is a lot of it. And it may be non-ordinary. All those bright things — mainly stars — are made up of ordinary protons and neutrons and electrons, like us. But dark matter doesn’t have to be.
Maybe that’s why we can’t see it. It is a gravity substance, not a light-and-gravity substance.
These clusters of darkness hold the galaxies together and suck in other galaxies to make them bigger and fewer.
This darkness pulls the universe together.
But there is another problem.
There was a time when scientists thought the overall universe acted like tennis balls: when you toss a tennis ball up, it zooms away from you, but then it comes back. Earth’s gravity tugs on it from the instant it leaves your hand, slowing it down until it can flee no more, when it turns around and falls back. The mathematics of the universe seemed like that, too: the Big Bang flung stars and galaxies like tennis balls, but eventually the gravity of all of the matter in the universe should slow them down until they could flee no more, and they would fall back together in a big crunch.
But measurements of distant galaxies don’t match up. They are farther away than they should be. Again, they are going too fast, and in the wrong direction. Somehow, they speed up as they flee.
This time the problem is too little gravity to pull things back, or, perhaps there is an antigravity called dark energy actively pushing galaxies away. This dark energy was flung out, like the galaxies, but unlike light-shedding and gravity-shedding substances, which tend to clog up on the way out, dark energy strands spread smoothly through the cosmos, outrunning gravity and dispersing an evenness of structure which pulls the size of the universe outward. The older the universe gets, the bigger the dark spaces get, and the farther the galaxies separate.
This darkness pulls the universe apart.
At the same time that the gravity matter is tightening, drawing in, pulling faster; the dark energy scaffolding is stretching, expanding out, extending faster.
Here on the ground, in a cookie universe just as paradoxical as the cosmos. What’s invisible is not gravity or energy, but agreement on what constitutes a thumb.
The cookie matter that holds relatives together and yet pushes us apart is the thumbprint, an almond cookie with a thumb-sized dollop of raspberry jam in the middle. One year, my brother, Tim, and I shared a kitchen and a batch of thumbprint dough to make our cookies.
When they came out of the oven, the cookies, and indeed our universe, clearly had split. My pan came out first: a young universe of hundreds of small, closely-spaced crumbs, each with a tiny depression -- think mouse-pinkie -- filled with jam.
Tim’s, on the other hand — a very disproportionate hand — were not thumbprints but fistprints. Each the size of a pie, one cookie would serve a family of 6. His half of the dough yielded 5 cookies. His pan resembled an old, empty universe with just a few large but wide-spread fruity centers.
In billions of years, our own galaxy could be one of those giant fruity centers, alone in the vastness of space.
Joy Moses-Hall teaches physics and astronomy at Pitt Community College. She has a PhD in oceanography and is the author of the novel Wretched Refuge.