Sky and Telescope - April 2016 - 23

Astronomers are combing through the largest map of galaxies
ever created to find the echoes of primordial sound waves.
Daniel Eisenstein In the Sacramento
Mountains near Cloudcroft, New Mexico,
the Sloan Digital Sky Survey (SDSS) has
been charting the cosmic web of galaxies
in the largest map ever created. Conceived
of in the mid-1980s, the SDSS saw ﬁrst
light in 1998 and soon brought astronomy to massproduction mode. The collaboration now includes some
1,000 astronomers from more than 50 institutions worldwide, and they're tackling a grand goal: measuring the
faint imprint of cosmic sound waves that reverberated
throughout the universe during its ﬁrst 380,000 years.
In 2014, SDSS announced a new map that charts
the locations of 1.5 million galaxies within a volume
equivalent to that of a cube 7.5 billion light-years on a
side. This project is known as the Baryon Oscillation
Spectroscopic Survey (BOSS), and it aims to shed light
on the nature of dark matter and dark energy and the
fate of the universe itself.

Sloshing in the Primordial Soup
Today's universe is ﬁlled with structure at cosmic scales -
giant clusters, ﬁlaments, and walls of galaxies separated
by voids as large as 200 million light-years across. This cosmic web doesn't come from recent interactions between
galaxies - it arose in tiny density variations in the young,
hot, and plasma-ﬁlled universe. By studying the arrangement of relatively nearby galaxies, cosmologists can reveal
how those small, early seeds evolved over time.
In the theory of cosmic inﬂation, the universe
expanded at a gigantic rate for a fraction of a second
almost instantly after the Big Bang. The simplest model
says the observable universe grew from a size 100 billion
times smaller than a proton to roughly a meter across.
But due to quantum uncertainties within inﬂation, some
regions grew slightly diﬀerently than others. Portions
that expanded a little less emerged from the inﬂationary
period denser than average, while other parts came out
somewhat sparser.
These initial density variations created the main actor
of this story - sound waves. Even as inﬂation enhanced
gravity-exerting clumps, the photons within those clumps
produced enormous outward pressure. The opposing
forces set particles and photons into motion in the form
of sound waves. The end of inﬂation is similar to the popping of a balloon: the balloon's surface enforces a pressure
diﬀerence, but once it breaks, the air inside expands and
creates a sound wave that travels spherically outward.

Each region of the early universe created such a sound
wave. Initially, those sound waves rushed about in the
plasma-ﬁlled universe. There were no stars or galaxies
yet in that dense, hot place. Electrons and nuclei couldn't
separate far because of their electrostatic attraction, but it
was too hot for them to settle down into atoms. Meanwhile, photons within the soup couldn't move far before
scattering oﬀ an unbound electron.
In this tightly coupled system, pressure ﬂuctuations
(aka sound waves) carried electrons and nuclei along
with them, leaving the dark matter unaﬀected. The primordial soup was so hot that the speed of sound reached
57% of the speed of light. Because scientists refer to protons and neutrons as baryons, these early sound waves
are called baryon acoustic oscillations.
But as the universe expanded, it cooled. Eventually,
380,000 years after the Big Bang, atoms began to form.
With no free electrons to scatter them about, the photons ﬁnally ﬂew free; the neutral cosmic gas, no longer
interacting with the photons, could no longer feel their
pressure. The sound speed plummeted and the spherically outgoing waves stalled.
Over the ﬁrst 380,000 years, the waves had traveled
an enormous distance, and even after stalling, their
eﬀects expanded along with the universe. One such

D. E

ISE

NS T

E IN

VISUALIZING SOUND WAVES A single disturbance (left) initiates a sound wave, like a rock dropped in a pond. And like a ripple
on the pond's surface, that disturbance propagates outward (center). In the early universe, each region of the universe initiated its
own sound wave, and the resulting ripples overlapped (right). Each
ripple has a radius of 500 million light-years in today's universe, and
overlapping ripples generate smaller (higher-frequency) overtones.
To watch the animation, go to http://is.gd/cosmicsoundwaves.

MAPPING THE UNIVERSE (Facing page) This artist's conception shows the Sloan telescope, which charted more than a
million galaxies, represented by colored dots, to reveal the imprint left behind by sloshing primordial plasma.

SkyandTelescope.com April 2016

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http://www.is.gd/cosmicsoundwaves http://www.SkyandTelescope.com

Sky and Telescope - April 2016

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