Importance of supernovae at z > 1.5 to probe dark energy Page: 3 of 6
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is subject to bias because of ignoring other parameters
. -----w)----- -- -- --- --
- --a(.)=0.01, no sys
0 0.5 1 1.5 2
FIG. 3: Uncertainty in determination of the dark energy
equation of state today as a function of survey depth
Zmax; w denotes assuming a priori that there is no time
variation while wo allows the possibility. The red, dot-
ted arrows denote the difference; ignoring the possibility
that w varies with time grossly underestimates the error,
especially for shallow surveys. The blue, solid arrows
show the effect of ignoring systematic errors. Precisely
(and accurately) determining the equation of state re-
quires supernovae at z > 1.5.
The mere possibility of time variation also carries im-
portant implications for error estimation. An a priori
assumption of constant behavior not only biases the con-
clusions on cosmology and dark energy, but gives strongly
deviant estimations of the associated errors, illustrated in
Fig. 3. That is, one gets inaccurate results extremely pre-
cisely! The error a(w) - assuming a constant equation
of state - disagrees with a(wo) - merely allowing for the
possibility of time variation - by a factor 3 for a survey
observing 2000 (plus 300 low z) SNe out to Zmax = 0.5.
Another virtue of a deep survey to z > 1.5 is that this
disagreement is only 25% at Zmax = 1.7. This is shown
by the red dotted arrows.
The necessity for a long baseline survey is even more
evident in Fig. 4, which shows the uncertainty a (w').
2Rather than calling these families of models degenerate, it is
more evocative to call them congeneric: resembling in nature or
action. This has the connotation in chemistry of a molecule that
acts analogously but yields a very different taste.
0 0.5 1 1.5 2
FIG. 4: Uncertainty in determination of the time vari-
ation of the dark energy equation of state as a function
of survey depth zma. Even in the idealized case of no
systematic error the uncertainty rises steeply as Zmax de-
creases. One needs a survey extending to Zmax ? 1.5 to
detect this key discriminator of fundamental theories.
The error sensitivity curve steepens dramatically as the
depth decreases below Zmax = 1.5, rapidly worsening to
Along with the uncertainty in dark energy properties
is that in our cosmological knowledge. So rather than
fixing the dimensionless matter density QM, we take as a
realistic case a gaussian prior a(2M) = 0.03, i.e. QM =
0.3 + 0.03.
IV. HERESY BY DEED: IGNORING
Uncertainties in source, propagation, or detector im-
pose a floor on our ability to reduce errors merely by
gathering large numbers of supernovae. While the great
advantages of supernovae as a probe are the long his-
tory of supernova studies, the rich data stream and cross-
checks they provide in their lightcurves and spectra, and
their underlying physical simplicity, we still cannot ignore
the impact of astrophysics on our attempts to measure
In Fig. 3 we see the huge discrepancy between the pre-
cision claimed in the ideal situation (actually with a prior
a(QM) = 0.01, not fixed QM) and in the presence of sys-
tematics (see blue solid arrows). The systematic error
essentially represents imperfect knowledge of all the as-
-- (Om)=0.03, sys
_ a(m)=0.01. sys
--a(9.)=0.01, no sys
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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/m1/3/: accessed November 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.