Distance Scale: RR Lyrae vs. Cepheids
Primordial Helium Abundance
Further verification of Big Bang Model
The Distance Scale: Lingering Ambiguity
In addition to the above references, there are a couple of other noteworthy publications that remind us that we have not yet determined Ho to an unambiguous accuracy of 10%.
Finally, there have been two new studies which do a reasonable job in showing the presence of systematic errors in use the of the SZ effect to determine Ho. These references are:
So, it would seem that Ho is still in the likely range of 60 -90 km/s and everyone and their dog will write a paper that recovers this result.
The Primoridal Helium Abundance
Ge etal modelled the excitation that could come from other sources such as collisions and/or UV pumping (either from the metagalactic UV flux or from the distant QSO). When these other sources of excitation are taking into account, the derived CMB temperature at z = 1.97 is 7.9 +/1 1 K. Using a CMD temperature at z = 0 of 2.74 K predicts Tz=1.97 = 8.1 K, well within the error bars. This good agreement is a strong verification that the MWB we observe today is indeed relic radiation.
More Evidence for a Positive Cosmological Constant?
The evidence comes from determining the curvature in the classical Hubble diagram which is a plot of redshift vs apparent magnitude. If there is no luminosity evolution in the source, such a diagram will reveal the deceleration parameter of the Universe, qo. The method works as follows:
The relationship between the luminosity distance, dL and the redshift of a galaxy, z , can be expressed as a power series where only the first two terms are important:
While this is a classic test of cosmology (and therefore is discussed in Peebles, Weinberg, etc) it was not included in this book primarily because any astrophysical source will have luminosity evolution and hence attempts to determine qo in this manner are subject to model dependent evolutionary corrections. Historically this was first done assuming the brightest galaxy in a cluster of galaxies had a fixed L. As more data were acquired, it became apparent that this could not be the case and the evolutionary corrections to L became difficult to accurately model. I don't consider the Hubble diagram to be a clean laboratory for measuring qo.
From the form of equation we can see that values of qo > 1 means that objects of a given apparent flux, will be located at a higher redshift. For values of qo < 1, they will be located at a lower redshift. In essence, qo is a measure of the value of the expansion parameter, Ho, at some particular redshift. If the universe is matter dominated, then Ho is being lowered as gravity slows down the expansion of the Universe. The slope of the line in the redshift-distance relationship is Ho. Thus, at some distant redshift, Ho has a higher value than it does now, and this would produce a non-constant slope (i.e. curvature) in the redshift-distance relation if it goes out to sufficiently high redshift to see the effect. If the universe is dominated by vacuum energy, then, qo will be negative. This means the universe is accelerating and Ho is higher now than it was in the past. In this scenario, the Universe is then older than Ho-1 as discussed in Chapter 2 and elsewhere in the book.
A schematic illustration of this is shown below.
Here we see that the various deflections away from the qo = 1 linear relation. The deflections first become noticeable at a redshift of about 1/2 and are obvious by a redshift of 2. Of course, astrophysical sources at redshift = 2 have quite low fluxes and are difficult to measure accurately.
The Reiss etal 1998 dataset uses the Multicolor Light Curve correction scheme for Supernova to correct them to a common luminosity. This is discussed on pages 59--64. Their sample consists of 16 supernova with redshift between z = 0.16 -- 0.97. The determined distances are 10--15% farther than would be expected in a matter dominated Universe. This indicates that qo is less than zero. Their Hubble diagram is shown below (note the Y-axis is distance modulus, m-M).
The nearby and distant samples have been combined into one diagram. While formally the fit with a positive CC is better than the fit to an open Universe model (Omegamatter = 0.2), the error bars combined with the small sample size suggest that this diagram and its associated fits will evolve in the future. However, the observation that virtually all the distant Supernova sit above what would be expected for the OMEGA = 1 case, is perhaps the most significant result of this study.
The diagram above used the distance as determined by the MCLS method. Alternatively, one can use the luminosity-decline calibration of Phillips (1993) to derive the distance. The results are similar:
Finally, the preferred value of the CC that results from this study leads to a dynamical age of the Universe of about 14.5 Gyr.
While this data is encouraging, I think claims that we have observational
evidence for a positive CC are premature. However, the technique is
quite promising, particularly if SN can be discovered at z ~ 1. On
the other hand, as pointed out by von Hippel etal (1997), there are
substantial areas of concern about the calibration of intrinsic SN Ia
luminosity at moderate look-back times.
While this data is encouraging, I think claims that we have observational evidence for a positive CC are premature. However, the technique is quite promising, particularly if SN can be discovered at z ~ 1. On the other hand, as pointed out by von Hippel etal (1997), there are substantial areas of concern about the calibration of intrinsic SN Ia luminosity at moderate look-back times.