Scientists have put together a map of the universe, a 3D behemoth representing 650 billion cubic light years, home to 1.2 million galaxies. But of course, being scientists, they didn’t send out an intern with a measuring wheel, a notepad, and enough sandwiches to last a few billion years. They used science to do the measuring.
The map was actually created to measure the amount of dark matter in the universe, in order to see if it was responsible for the fact that the expansion of the universe is accelerating, despite the fact that gravity should be slowing it. Which means that possibly the most impressive map ever made was the by-product of a hunt for something else.
The map was assembled over a decade, pulling data from various sources which are themselves quite astonishing. For instance, the Baryon Oscillation Spectroscopic Survey (BOSS) measures acoustic waves left over from the beginnings of the universe. Imagine dropping a rock into an ocean, waiting for a few billion years, and then measuring the effects of the ripples caused by that rock, and you have something similar. These cosmic ripples actually affected the way matter was distributed in the universe, and therefore the positioning of galaxies.
"Measuring the acoustic scale across cosmic history gives a direct ruler with which to measure the Universe’s expansion rate," said the Max Planck Institute’s Ariel Sanchez, as reported in the Christian Science Monitor.
These measurements then had to be rejiggered to account for the acceleration of the galaxies as they continue to speed away from the original of the universe. You know how a police siren rises in pitch, then drops agains as it passes you? That’s the Doppler effect, which occurs because the waves are effectively squeezed closer together as the cop car approaches, then stretched apart as it recedes. The same thing happens to light, if the objects are moving fast enough, as they are on the scale of the universe. Only with light, the stretching or squashing of the light waves shifts their color, so you can compare the "red-shift" of a star to the color it would be at rest, and calculate its speed.
The result of these mind-bending long-distance measurements is a map that tells us that the universe consists of about 70% dark matter—stuff that cannot be seen or measured, but which affects it nonetheless. The map also ties together the history of the universe itself, stretching from 7 billion years ago to 2 billion years ago.
The map isn't done yet. It still only covers a quarter of the sky. But with better surveys and instruments every few years, the creators are sure to continue expanding it.
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