8.3. Supernova 1997ff - serendipity and careful planning
In the spirit of "extraordinary claims require extraordinary proof," the
magnitude of the discovery of an accelerating universe requires an
extremely careful elimination of potential contaminating astrophysical
effects. A pervasive screen of "gray" dust, for example, that can dim
the light while leaving little imprint on the spectral energy
distribution, has been suggested as an alternative explanation for the
unexpected faintness of high-z SNe Ia
(Aguirre 1999).
While gray dust, e.g., quasispherical grains larger then
0.1 µm, could be made to reproduce some of the
observations, some measurements do disfavor a 30% visual opacity due to
gray dust at the ~ 2.5
level
(Riess et
al. 2000).
Nevertheless, both the possibility of gray dust and, in particular, the
ever-existing challenge of luminosity evolution, did leave the case for
an accelerating universe somewhat short of compelling by 2001. The
strongest evidence so far against alternatives to an accelerating
universe came from SN 1997ff.
In 1997, Ron Gilliland and Mark Phillips reobserved the Hubble Deep
Field North with HST, with the goal of detecting high-z
supernovae
(Gilliland,
Nugent and Phillips 1999).
They discovered two supernovae: SN 1997fg, at a redshift of z = 0.95 was
found in a late-type galaxy, while SN 1997ff was found to be hosted by an
elliptical galaxy. The initial, photometrically determined redshift,
placed the latter galaxy at z ~ 0.95-1.32 with considerable
uncertainty. Serendipitously, the Guaranteed Time Observer (GTO) program
of Thompson et
al. (1999)
included imaging the host of SN 1977ff with the near infrared
camera, NICMOS, on board HST. One of these exposures, amazingly enough,
was actually taken within hours of the discovery of the SN. Furthermore,
six months after the GTO program,
Dickinson (2003, in preparation; GO 7817) reobserved the field with
NICMOS. The combination of all the HST and ground-based observations (in
the U, B, V, I, J, H, and
K bands) allowed for an improved photometric redshift
determination for the host of z = 1.65 ± 0.15
(Budavári
et al. 2000),
making SN 1997ff the farthest known supernova to date. A
probability density function based on the partial supernovae light curve
in three bands also gave z
1.7, as did a tentative
determination of the redshift based on the spectrum of the host taken
with the Keck telescopes (see
Riess et al. 2001,
for a full description).
The importance of detecting a SN Ia at such a high redshift cannot
be overemphasized. Even with the suggested value of the vacuum energy
density of
0.7, at
z
1
attractive gravity would still have dominated over the cosmic repulsion,
resulting in a decelerated expansion. Consequently, SNe Ia
in the redshift range z ~ 1-2 should appear brighter
relative to SNe in a coasting universe. On the other hand, any effect
that increases monotonically with redshift, as would be expected from
dust extinction and simple evolutionary effects, would predict that the
SNe Ia would be dimmer with increasing
redshift. SN 1997ff was found to be brighter by ~ 1.1 mag (at
the > 99.99% confidence level) than expected for an alternative
source of dimming (e.g., dust or evolution) beyond z ~
0.5. Figure 29 shows a
redshift-distance relation in which the points are redshift-binned data
from
Perlmutter et
al. (1999)
and Riess et
al. (1998),
together with a family of curves for flat cosmological models. The
transition from a decelerating to an accelerating phase occurs (if the
"dark energy" is represented by a cosmological constant; see
Section VIII D) at ztr =
(2
/
M)1/3 - 1. As can be seen from the
figure, the only cosmological model that is consistent with all
the data (including SN 1997ff) is one in which
M
0.35,
0.65.
![]() |
Figure 29. Hubble diagram of SNe Ia minus
an empty (i.e.,
|
Measurements of the power spectrum of the cosmic microwave background
(e.g.,
de
Bernardiset al. 2002,
Abroe et al. 2002,
Hu et al. 2001,
Netterfield
et al. 2002)
provided strong evidence for a geometrically flat
(M +
1) universe. When
combined with several estimates of
M from
mass to light ratios, x-ray temperature of intracluster gas, numbers,
and dynamics of clusters of galaxies, all giving
M
0.2-0.3
(e.g.,
Carlberg et
al. 1996,
Bahcall, Fan and
Cen 1997,
Bahcall et
al. 2000,
Strauss and
Willick 1995),
the inescapable conclusion was that there is a dark energy component of
0.7 - consistent with
the value found from
high-redshift supernovae. Most recently, the best fit cosmological model
has been determined by the Wilkinson Microwave Anisotropy Probe
(WMAP). In combination with other large-scale structure work, the WMAP
results imply
tot = 1.02
± 0.02,
0.73
(Bennett et
al. 2003,
Spergel et
al. 2003).