An international team of astronomers
has created the largest ever three-dimensional map of distant galaxies in a bid
to help them understand one of the most mysterious forces in the universe.
Scientists including a team led by
Dr Florian Beutler at the University of Portsmouth's Institute of Cosmology and
Gravitation have spent a decade collecting measurements of 1.2 million galaxies
as part of the Sloan Digital Sky Survey III (SDSS-III).
This will allow them to make the
most precise measurements to date of 'dark energy'
– the force that is driving the accelerated expansion of the universe.
Dr Beutler said: "This
extremely detailed three-dimensional map represents a colossal amount of work.
The University of Portsmouth has worked with partner institutions for ten
years, helping to gather measurements of galaxies making up a quarter of the
sky.
"Using this map we will now be
able to make the most accurate possible measurements of dark energy, and the
part it plays in the expansion of the universe."
The new measurements were carried
out by the Baryon Oscillation Spectroscopic Survey (BOSS) program of SDSS-III.
Shaped by a continuous tug-of-war
between dark matter and dark energy, the map revealed by BOSS allows
astronomers to measure the expansion rate of the universe and thus determine
the amount of matter and dark energy that make up the present-day universe.
A collection of papers describing
these results was submitted this week to the Monthly Notices of the Royal
Astronomical Society.
BOSS measures the expansion rate of
the Universe by determining the size of the baryonic acoustic oscillations
(BAO) in the three-dimensional distribution of galaxies.
The original BAO size is determined
by pressure waves that travelled through the young universe up to when it was
only 400,000 years old (the Universe is presently 13.8 billion years old), at
which point they became frozen in the matter distribution of the Universe.
Measuring the distribution of
galaxies since that time allows astronomers to measure how dark matter and dark
energy have competed to govern the rate of expansion of the Universe.
To measure the size of these ancient
giant waves to such sharp precision, BOSS had to make an unprecedented and
ambitious galaxy map, many times larger than previous surveys.
At the time the BOSS program was
planned, dark energy had been previously determined to significantly influence
the expansion of the Universe starting about 5 billion years ago. BOSS was thus
designed to measure the BAO feature from before this point (7 billion years
ago) out to near the present day (2 billion years ago).
Dr Beutler said: "If dark
energy has been driving the expansion of the Universe over that time, our maps
tells us that it is evolving very slowly, if at all. The change is at most 20
per cent over the past seven billion years."
Dr Rita Tojeiro, of the University
of St Andrews, a partner in the project, added: "We see a dramatic
connection between the sound wave imprints seen in the cosmic microwave background 400,000 years after
the Big Bang to the clustering of galaxies 7-12 billion years later.
"The ability to observe a
single well-modelled physical effect from recombination until today is a great
boon for cosmology."
The map also reveals the distinctive
signature of the coherent movement of galaxies toward regions of the universe
with more matter, due to the attractive force of gravity. Crucially, the
observed amount of infall is explained well by the predictions of general
relativity. This agreement supports the idea that the acceleration of the
expansion rate is driven by a phenomenon at the largest cosmic scales, such as
dark energy, rather than a breakdown of gravitational theory.
Dr Jeremy Tinker, of New York
University, added: "BOSS has marked an important cosmological milestone,
combining precise clustering measurements of an enormous volume with extensive
observations of the primary cosmic microwave background to produce a firm
platform for the search for extensions to the standard cosmological model.
"We look forward to seeing this
programme extended with the coming decade of large spectroscopic surveys."
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