Sky and Telescope - April 2016 - 24

Seeing Cosmic Sound

E.M. HUFF / SDSS-III TEAM / SOUTH POLE TELESCOPE TEAM / ZOSIA ROSTOMIAN

expanding spherical shell would have a radius of 500
million light-years today.
In 1970 Jim Peebles and Jer Yu, both working in the
United States, and Rashid Sunyaev and Yakov Zel'dovich,
both working in the Soviet Union, predicted that primordial sound waves would produce observable eﬀects
in today's universe, aﬀecting the large-scale distribution
of galaxies. But astronomers had to wait three decades
for data collection methods to reach the extreme level of
sensitivity required to see these eﬀects.
Now, by precisely mapping the universe's galaxies
and examining their arrangement in space, astronomers
can ﬁnally study the imprint that primeval waves left
behind. Galaxy maps hence serve as a time machine to
study the earliest epochs of the universe.

Seeing Sound Waves

Galaxy Maps

3.8 billion years ago

5.5 billion years ago 13.7 billion years ago

BARYON ACOUSTIC OSCILLATIONS Today, a pair of
galaxies is most likely to lie 500 million light-years apart. This
preferred distance (white rings, left) can be traced back in time
to the cosmic microwave background (right), which immortalizes
the sound waves that sloshed through the early universe.

We observe primordial sound waves in two ways. The ﬁrst
method relies on sky maps that chart the temperature of
the cosmic microwave background. Though the background looks nearly the same in all directions, precise
maps show minuscule variations in temperature of a few
parts in 100,000 due to the inﬂation-enhanced density
ﬂuctuations. These variations reveal a strong signature
from primordial sound waves: just as ocean waves ripple
sand, baryon acoustic oscillations resulted in hot and cold
spots with a typical angular size of about 1 degree.
The radius of these sound waves' spherically expanding shells, 500 million light-years in today's universe,
represents their fundamental length. As for a violin
string, this length sets a fundamental tone as well as
a series of higher-frequency harmonic overtones. The
Planck satellite has by now produced exquisite observations of the harmonies present in the temperatures it
measures, providing a beautiful and compelling validation of the theory of the early universe.
In the second method, astronomers measure sound
waves' eﬀect on galaxy clustering in the SDSS maps.
Sound waves enhance the abundance of galaxies in a
speciﬁc pattern: any given pair of galaxies is slightly
more likely to be separated by 500 million light-years,
rather than 400 or 600 million light-years. Though this
can't be observed for an individual galaxy pair, the eﬀect
is detectable in large modern surveys with pairs from
hundreds of thousands of galaxies. The detection of this
statistical eﬀect serves as a "standard ruler."

DAVID KIRKBY

Mapping the Known Universe

FROM DUSK UNTIL DAWN The Sloan Digital Sky Survey
Telescope is a 2.5-meter f/5 Ritchey-Chrétien telescope with a
3-degree ﬁeld of view. Rather than sitting enclosed in a dome,
the telescope is protected by a wind baﬄe.

380,000 YEARS OLD The Planck satellite's full-sky image
of the cosmic microwave background shows the universe as it
existed during the age of recombination, when protons matched
up with electrons and photons ﬂew free. The inﬂation-enhanced
temperature variations seen here (colder is blue, hotter is yellow
and red) seeded the cosmic web of galaxies we see today.

April 2016 sky & telescope

To measure the standard ruler of baryon acoustic oscillations, astronomers must construct vast maps of the galaxies that are strewn across the universe like so many grains
of sand. So how does SDSS actually make these maps?
The survey uses a custom-built 2.5-meter f/5
Ritchey-Chrétien telescope. A large secondary mir-

Sky and Telescope - April 2016

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