Importance of supernovae at z > 1.5 to probe dark energy Page: 1 of 6
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Importance of Supernovae at z > 1.5 to Probe Dark Energy
Eric V. Linderl and Dragan Huterer2
'Physics Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720
2Department of Physics, Case Western Reserve University, Cleveland, OH 44106
The accelerating expansion of the universe suggests that an unknown component with strongly
negative pressure, called dark energy, currently dominates the dynamics of the universe. Such a
component makes up ~ 70% of the energy density of the universe yet has not been predicted by
the standard model of particle physics. The best method for exploring the nature of this dark
energy is to map the recent expansion history, at which Type Ia supernovae have proved adept.
We examine here the depth of survey necessary to provide a precise and qualitatively complete
description of dark energy. Realistic analysis of parameter degeneracies, allowance for natural
time variation of the dark energy equation of state, and systematic errors in astrophysical
observations all demonstrate the importance of a survey covering the full range 0 < z < 2 for
revealing the nature of dark energy.
The discovery of the acceleration of the expansion of
the universe through the Type Ia supernova distance-
redshift relation is a major development in cosmol-
ogy [1, 2]. Exploring the expansion history of the uni-
verse is a key aim of cosmology, producing literally a text-
book picture of the universe. Furthermore, such a map
provides key clues to the underlying physics, independent
of whether this is dark energy, higher dimensions, or an
altered theory of gravitation .
In its interpretation as arising from a universal vac-
uum, or dark, energy, such a component would comprise
some 70% of the critical density, be unclustered on sub-
horizon scales, and possess a substantially negative equa-
tion of state (EOS) w = p/p $1 -0.6 . While these
properties are unexpected from the standard model of
particle physics, it has been suggested that they can be
motivated by a number of fundamental theories [5, 6].
Dark energy thus poses a crucial mystery to unravel for
the fields of high energy physics, cosmology, and gravita-
Supernovae studies, which first provided the evidence
for the acceleration, are well suited for elucidating the
nature of the dark energy [7, 8]. One experiment being
designed specifically to probe the accelerating universe
using supernovae is the Supernova/Acceleration Probe
(SNAP ). At an initial theoretical glance, the red-
shift range over which this exploration is most easily
done seems simple to understand: the energy density
dominance and dynamical influence (accelerating power)
of dark energy enters at redshifts z $1 0.7 (see Fig. 1).
Moreover, an idealized perturbative, or Fisher matrix,
calculation shows that the "sweet spot" of sensitivity to
the equation of state w lies at z 0.3 [5, 8, 10]. So why
are observations at z > 1 necessary for characterizing the
The answer lies in the breakdown of the ideal case:
0 0.2 0.4 0.6 0.8
FIG. 1: The epochs of equality between the dark energy
density and matter and of transition from acceleration to
deceleration are plotted vs. dark energy equation of state.
The positively slanted hatching denotes the accelerating
phase; the negatively slanted hatching shows when the
dark energy density dominates over the matter density.
Despite these both occurring below redshift z 0.7, dark
energy can be probed to much higher redshift.
* Cosmological degeneracies
* Dark energy model degeneracies
* Systematic errors
The required survey depth depends on the rigor of our
scientific investigation, how much we are willing to as-
decel era ting
iii : "-. i i I
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Linder, Eric V. & Huterer, Dragan. Importance of supernovae at z > 1.5 to probe dark energy, article, August 8, 2002; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc786981/m1/1/: accessed November 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.