NASA’s Nancy Grace Roman Space Telescope will detect vestiges of sound waves that once rippled through the primordial cosmic sea. According to new simulations, Roman’s observations could extend these measurements into an unprobed epoch between the universe’s infancy and the present day. Studying the echoes from this era will help us trace the evolution of the universe and solve pressing cosmic conundrums.
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 – the same epoch Roman’s BAO studies are optimized to investigate. Now a team of scientists has demonstrated that the mission could peer even farther back in time to explore impressions left by BAOs.
“This isn’t something you can study in a lab, so we created mock universes and ran simulations,” said Siddharth Satpathy, who led the study. Now a machine-learning engineer at Cisco in San Francisco, he conducted this research while earning a doctorate in computational astrophysics from Carnegie Mellon University in Pittsburgh. “We were excited to find that Roman will be powerful enough to study BAO remnants in the universe’s youth,” he added.
The team’s findings were published Sept. 11 in the Monthly Notices of the Royal Astronomical Society.
Hot plasma soup
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.
“BAOs have left their mark on the cosmos, but we haven’t fully examined their traces,” said co-author Rupert Croft, a professor of physics at Carnegie Mellon University. “By studying BAO impressions in an unprobed region, we can excavate cosmic fossils, which will allow us to unearth new information about the forces that have shaped the universe.”
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. But looking farther out into space, to earlier cosmic times, means that this distance – the vestige of the frozen BAO ripples – will decrease.
Roman will extend previous research by mapping the expansion of the universe to unprecedented detail. Satpathy and his team showed that Roman’s surveys will be able to probe for BAO remnants five times farther than originally planned, back to when the universe was only about 600 million years old – just 4% of its current age.
Learning more about the way the cosmos has expanded over time will allow scientists to explore dark energy – a mysterious pressure that is accelerating the expansion of the universe. Roman is optimized to survey the cosmos for BAO impressions across the middle of its current age because that’s when scientists think dark energy transitioned from being a minor contributor to the contents of the universe to the most dominant force.
But some theories hypothesize about of dark energy activity when the universe was much younger. Peering farther into the universe’s past will help add pieces to the puzzle.
“We haven’t extensively explored BAO imprints from when the universe was very young because we need an enormous sample of galaxies to do so,” said Jason Rhodes, a senior research scientist at NASA’s Jet Propulsion Lab in Pasadena, California. “That’s where Roman comes in. The mission has such a wide field of view that observations like this will become possible.”
Roman’s High Latitude Spectroscopic Survey will measure accurate distances and positions for millions of galaxies. Scientists plan to analyze how their distribution varies with distance by creating a 3D map of the universe, which will help us decipher how dark energy has shaped the cosmos over time.
Two additional Roman surveys will also study dark energy, and each technique will cross-check the others. The mission will provide important data to help scientists investigate, and possibly even foresee, the universe’s fate.