New Hubble Space Telescope Discoveries of Type Ia Supernovae at
z > 1: Narrowing Constraints on the Early Behavior of Dark Energy

http://arxiv.org/abs/astro-ph/0611572

We have discovered 21 new Type Ia supernovae (SNe Ia) with the Hubble
Space Telescope (HST) and have used them to trace the history of cosmic
expansion over the last 10 billion years. These objects, which include
13 spectroscopically confirmed SNe Ia at z > 1, were discovered during
14 epochs of reimaging of the GOODS fields North and South over two
years with the Advanced Camera for Surveys on HST. Together with a
recalibration of our previous HST-discovered SNe Ia, the full sample of
23 SNe Ia at z > 1 provides the highest-redshift sample known. Combined
with previous SN Ia datasets, we measured H(z) at discrete,
uncorrelated epochs, reducing the uncertainty of H(z>1) from 50% to
under 20%, strengthening the evidence for a cosmic jerk--the transition
from deceleration in the past to acceleration in the present. The
unique leverage of the HST high-redshift SNe Ia provides the first
meaningful constraint on the dark energy equation-of-state parameter at
z >1.

The result remains consistent with a cosmological constant (w(z)=-1),
and rules out rapidly evolving dark energy (dw/dz >>1). The defining
property of dark energy, its negative pressure, appears to be present
at z>1, in the epoch preceding acceleration, with ~98% confidence in
our primary fit. Moreover, the z>1 sample-averaged spectral energy
distribution is consistent with that of the typical SN Ia over the last
10 Gyr, indicating that any spectral evolution of the properties of SNe
Ia with redshift is still below our detection threshold.

See: http://arxiv.org/pdf/astro-ph/0611572


NASA fails to produce new data on dark energy
http://www.astro.ucla.edu/~wright/cosmolog.htm#News

16 Nov 06 - NASA held a press telecon today about dark energy, but
neither the press release nor the images accompanying it contained
any useful information. There was no paper about the data on the
preprint server either. UPDATE: 20 Nov 06 - the Riess et al. paper
has reached the preprint server. Unfortunately the link to access the
data is broken so I can't give an independent assessment of the
analysis. The paper assumes that the Universe is flat when trying to
find the properties of the dark energy, and bases this assumption on
the fact that a flat Universe with a cosmological constant fits all
the available data. Therefore the conclusion of this paper, which is
that the dark energy looks like a cosmological constant, is built
into the assumptions. This analysis is not sound logic. But the data
are sound, and everything remains consistent with a flat Universe
with a cosmological constant.

New Hubble Space Telescope Discoveries of Type Ia Supernovae at
z > 1: Narrowing Constraints on the Early Behavior of Dark Energy

http://arxiv.org/abs/astro-ph/0611572

We have discovered 21 new Type Ia supernovae (SNe Ia) with the Hubble
Space Telescope (HST) and have used them to trace the history of cosmic
expansion over the last 10 billion years. These objects, which include
13 spectroscopically confirmed SNe Ia at z > 1, were discovered during
14 epochs of reimaging of the GOODS fields North and South over two
years with the Advanced Camera for Surveys on HST. Together with a
recalibration of our previous HST-discovered SNe Ia, the full sample of
23 SNe Ia at z > 1 provides the highest-redshift sample known. Combined
with previous SN Ia datasets, we measured H(z) at discrete,
uncorrelated epochs, reducing the uncertainty of H(z>1) from 50% to
under 20%, strengthening the evidence for a cosmic jerk--the transition
from deceleration in the past to acceleration in the present. The
unique leverage of the HST high-redshift SNe Ia provides the first
meaningful constraint on the dark energy equation-of-state parameter at
z >1.

The result remains consistent with a cosmological constant (w(z)=-1),
and rules out rapidly evolving dark energy (dw/dz >>1). The defining
property of dark energy, its negative pressure, appears to be present
at z>1, in the epoch preceding acceleration, with ~98% confidence in
our primary fit. Moreover, the z>1 sample-averaged spectral energy
distribution is consistent with that of the typical SN Ia over the last
10 Gyr, indicating that any spectral evolution of the properties of SNe
Ia with redshift is still below our detection threshold.

See: http://arxiv.org/pdf/astro-ph/0611572


NASA fails to produce new data on dark energy
http://www.astro.ucla.edu/~wright/cosmolog.htm#News

16 Nov 06 - NASA held a press telecon today about dark energy, but
neither the press release nor the images accompanying it contained
any useful information. There was no paper about the data on the
preprint server either. UPDATE: 20 Nov 06 - the Riess et al. paper
has reached the preprint server. Unfortunately the link to access the
data is broken so I can't give an independent assessment of the
analysis. The paper assumes that the Universe is flat when trying to
find the properties of the dark energy, and bases this assumption on
the fact that a flat Universe with a cosmological constant fits all
the available data. Therefore the conclusion of this paper, which is
that the dark energy looks like a cosmological constant, is built
into the assumptions. This analysis is not sound logic. But the data
are sound, and everything remains consistent with a flat Universe
with a cosmological constant.