Quasars, Primeval Sound Waves Help Astronomers Make Largest Map Of The Universe Yet

A slice through largest-ever three-dimensional map of the Universe. Earth is at the left, and distances to galaxies and quasars are labeled by the lookback time to the objects. Lookback time means how long the light from an object has been traveling to reach us here on Earth. The locations of quasars  are shown by the red dots, and nearer galaxies in yellow. The right-hand edge of the map is the limit of the observable universe, from which we see the light left over from the Big Bang called the Cosmic Background Radiation. Most of the empty space between the quasars and the edge of the observable universe are from the “dark ages”, prior to the formation of most stars, galaxies, or quasars. Credit: Anand Raichoor and the SDSS collaboration

Astronomers with the Sloan Digital Sky Survey (SDSS) have created the first map of the large-scale structure of the universe based entirely on the positions of quasars. A quasar, certainly one of the coolest words in the astronomical lexicon, looks like an incredibly bright point of light in the center of a remote galaxy. Often, the galaxy itself is invisible because it’s so far away, but quasars are powered by supermassive black holes. Stars, cosmic clouds and even rogue planets that stray to close to the black hole’s edge release energy as they fall into a swirling disk around the hole before getting sucked in. Friction between the material causes it to heat up to more than 18,000,000°F (10,000,000°C) and emit light all across the spectrum from infrared to X-rays.

“Because quasars are so bright, we can see them all the way across the universe,” said Ashley Ross of the Ohio State University, the co-leader of the study. “That makes them the ideal objects to use to make the biggest map yet.”

Quasars reside in a variety of galaxies. When seen through ground-based telescopes, these compact, enigmatic light sources resemble stars, yet they’re billions of light-years away and several hundred billion times brighter than normal stars. Credit: John Bahcall, Mike Disney Hubble Space Telescope / NASA/ESA

The quasars in the map above are so far away that the light we see tonight left them long before the Earth even existed. To make the map, astronomers used the 98.4-inch (2.5-meter) Sloan Foundation Telescope to measure the positions for more than 147,000 quasars over two years.  The telescope’s observations gave the team the quasars’ distances, which they used to create a 3-D map of where the quasars are.

The team didn’t stop there. They also looked at “baryon acoustic oscillations” (BAOs). BAOs are the present-day imprint of sound waves that traveled through the early universe when it was much hotter and denser after its genesis in the Big Bang. If we could return to that time, we’d see the universe around us would be a blazing hot, swishy material called plasma resembling the center of the sun. Within this hellfire, any movement created sound waves that rippled through the plasma like waves on a pond.


A Flight Through the Universe. This animation shows close to 400,000 galaxies, with images of the actual galaxies in these positions derived from the Sloan Digital Sky Survey (SDSS) Data Release 7. Vast as this slice of the universe seems, it reaches only about 1.3 billion light years from Earth.

But when the universe reached the ripe old age of 380,000 years — a baby really — it cooled down enough for atoms to hold onto their electrons and become neutral. We live in mostly neutral territory to this day. A neutral universe allowed light, which had been trapped in the plasma, to stream across the light years. In a word, the universe became transparent to light. And those BAOs? The last ripples became frozen in place, leaving their imprint to this day on the large-scale, 3-D structure of the universe. Since then, the expansion of the universe has inflated and writ large those original waves. They’re a key reason astronomers observe galaxies arranged in long filaments and sheets like material between the cavities in a giant sponge.


Simulation of the effect of BAOs on the large-scale structure of the cosmos

The observed size of the BAO can be used as a “standard ruler” to measure distances. Just as you can estimate the length of a football field by measuring the apparent size of a ruler when viewed from the other side of the field, you can use the BAO “ruler” for measuring distances between galaxies and quasars.

The results of the new study confirm the current standard model of the universe: it’s expanding after its start in the Big Bang; invisible dark matter is present in great quantities and dark energy is causing the expansion to speed up. Scientists the world over are working to determine the nature of both dark matter and dark energy, the biggest astrophysical questions (or headaches!) of our time. As we do, our knowledge of the large-scale structure of the universe can only increase.