This animation explains how BAOs arose in the early universe and how astronomers can study the faint imprint they made on galaxy distribution to probe dark energy’s effects over time. In the beginning, the cosmos was filled with a hot, dense fluid called plasma. Tiny variations in density excited sound waves that rippled through the fluid. When the universe was about 400,000 years old, the waves froze where they were. Slightly more galaxies formed along the ripples. These frozen ripples stretched as the universe expanded, increasing the distance between galaxies. Astronomers can study this preferred distance between galaxies in different cosmic ages to understand the expansion history of the universe.
Credit: NASA's Goddard Space Flight Center
Music: "Pulse and Glow" from Adrift in Time. Written and Produced by Lars Leonhard.
Sound waves from the nascent universe, called baryon acoustic oscillations (BAOs), left their imprint on the cosmos by influencing galaxy distribution. Researchers have explored this imprint back to when the universe was three billion years old, or roughly 20% of its current age of 13.8 billion years.
For most of its first half-million years, the universe looked extremely different than it does today. Instead of being speckled with stars and galaxies, the cosmos was filled with a sea of plasma – charged particles – that formed a dense, almost uniform fluid.
There were tiny fluctuations of about one part in 100,000. What few variations there were took the form of slightly denser kernels of matter, like a single ounce of cinnamon sprinkled into about 13,000 cups of cookie dough. Since the clumps had more mass, their gravity attracted additional material.
It was so hot that particles couldn’t stick together when they collided – they just bounced off each other. Alternating between the pull of gravity and this repelling effect created waves of pressure – sound – that propagated through the plasma.
Over time, the universe cooled and particles combined to form neutral atoms. Because the particles stopped repelling each other, the waves ceased. Their traces, however, still linger, etched on the cosmos.
When atoms formed, the ripples essentially froze in place, carrying within them a bit more matter than the average across the universe. With the repulsive pressure of the plasma gone, gravity became the dominant force.
Over the course of hundreds of millions of years, clumps from the plasma that once filled the universe slurped up more material to become stars. Their mutual gravity pulled stars together into groups, ultimately forming the galaxies we see today. And slightly more galaxies formed along the ripples than elsewhere.
While the waves no longer propagated, the frozen ripples stretched as the universe expanded, increasing the distance between galaxies. By looking at how galaxies are spread out in different cosmic epochs, we can explore how the universe has expanded over time.
Scientists have noticed a pattern in the way galaxies cluster together from measurements of the nearby universe. For any galaxy today, we are more likely to find another galaxy about 500 million light-years away than slightly nearer or farther.
The small peak near the center of the graph in this video shows how BAOs subtly influenced galaxy distribution. Today, there is a slight bump in the probability of finding galaxies about 500 million light-years away from each other. This distance shrinks as we look farther out into space, to earlier cosmic times.
Galaxies speckle the sky in this illustration. Astronomers have observed the faint imprint of BAOs in the way galaxies cluster together. This pattern, barely visible in this image, can only be studied using an enormous sample of galaxies.
This illustration highlights the impact BAOs had on galaxy clustering. Slightly more galaxies formed along the ripples of the primordial sound waves than elsewhere. Then the rings of galaxies stretched with the expansion of the universe. Other galaxies are dimmed in this image to make the effect easier to see.