## Tension in Cosmology?

I noticed this abstract (of a paper by Rest et al.) on the arXiv the other day:

We present griz light curves of 146 spectroscopically confirmed Type Ia Supernovae (0.03<z<0.65) discovered during the first 1.5 years of the Pan-STARRS1 Medium Deep Survey. The Pan-STARRS1 natural photometric system is determined by a combination of on-site measurements of the instrument response function and observations of spectrophotometric standard stars. We have investigated spatial and time variations in the photometry, and we find that the systematic uncertainties in the photometric system are currently 1.2% without accounting for the uncertainty in the HST Calspec definition of the AB system. We discuss our efforts to minimize the systematic uncertainties in the photometry. A Hubble diagram is constructed with a subset of 112 SNe Ia (out of the 146) that pass our light curve quality cuts. The cosmological fit to 313 SNe Ia (112 PS1 SNe Ia + 201 low-z SNe Ia), using only SNe and assuming a constant dark energy equation of state and flatness, yields w = -1.015^{+0.319}_{-0.201}(Stat)+{0.164}_{-0.122}(Sys). When combined with BAO+CMB(Planck)+H0, the analysis yields \Omega_M = 0.277^{+0.010}_{-0.012} and w = -1.186^{+0.076}_{-0.065} including all identified systematics, as spelled out in the companion paper by Scolnic et al. (2013a). The value of w is inconsistent with the cosmological constant value of -1 at the 2.4 sigma level. This tension has been seen in other high-z SN surveys and endures after removing either the BAO or the H0 constraint. If we include WMAP9 CMB constraints instead of those from Planck, we find w = -1.142^{+0.076}_{-0.087}, which diminishes the discord to <2 sigma. We cannot conclude whether the tension with flat CDM is a feature of dark energy, new physics, or a combination of chance and systematic errors. The full Pan-STARRS1 supernova sample will be 3 times as large as this initial sample, which should provide more conclusive results.

The mysterious Pan-STARRS stands for the Panoramic Survey Telescope and Rapid Response System, a set of telescopes cameras and related computing hardware that monitors the sky from its base in Hawaii. One of the many things this system can do is detect and measure distant supernovae, hence the particular application to cosmology described in the paper. The abstract mentions a preliminary measurement of the parameter w, which for those of you who are not experts in cosmology is usually called the “equation of state” parameter for the dark energy component involved in the standard model. What it describes is the relationship between the pressure P and the energy density ρc^{2} of this mysterious stuff, via the relation P=wρc^{2}. The particularly interesting case is w=-1 which corresponds to a cosmological constant term; see here for a technical discussion. However, we don’t know how to explain this dark energy from first principles so really w is a parameter that describes our ignorance of what is actually going on. In other words, the cosmological constant provides the simplest model of dark energy but even in that case we don’t know where it comes from so it might well be something different; estimating w from surveys can therefore tell us whether we’re on the right track or not.

The abstract explains that, within the errors, the Pan-STARRS data on their own are consistent with w=-1. More interestingly, though, combining the supernovae observations with others, the best-fit value of w shifts towards a value a bit less than -1 (although still with quite a large uncertainty). Incidentally value of w less than -1 is generally described as a “phantom” dark energy component. I’ve never really understood why…

So far estimates of cosmological parameters from different data sets have broadly agreed with each other, hence the application of the word “concordance” to the standard cosmological model. However, it does seem to be the case that supernova measurements do generally seem to push cosmological parameter estimates away from the comfort zone established by other types of observation. Could this apparent discordance be signalling that our ideas are wrong?

That’s the line pursued by a Scientific American article on this paper entitled “Leading Dark Energy Theory Incompatible with New Measurement”. This could be true, but I think it’s a bit early to be taking this line when there are still questions to be answered about the photometric accuracy of the Pan-Starrs survey. The headline I would have picked would be more like “New Measurement (Possibly) Incompatible With Other Measurements of Dark Energy”.

But that would have been boring…

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October 24, 2013 at 1:16 pm

Hints at phantom behaviour also in this very recent paper using multiple DE EOS

arxiv 1310.6161

October 25, 2013 at 3:51 pm

The real issue is the nature of any systematic error…

October 25, 2013 at 6:18 pm

…that’s “cosmologists” for you – you always worry that they know the answer they want before they start.

October 26, 2013 at 2:17 pm

To Phillip: re your claim about Sandage, it is interesting that he got H_0 ~ 75 in 1958, href=http://adsabs.harvard.edu/abs/1958ApJ…127..513S , then spent the next 30+ years pushing for lower values around 50. He also pioneered the globular cluster age estimates, and tended to get high values >~ 15 Gyr. Is it known whether he had a strong belief for Omega_m = 1 and wanted low H_0 to make H_0 t_0 = 2/3 ?

Peter: I think the “phantom” term for w < -1 originates from Rob Caldwell's paper in 1999, http://arxiv.org/abs/9908168/ , and the phrase comes from the Star Wars prequel of 1999… this theme re the "dark side" has continued, e.g. pic of Darth Vader on the cover of the Kolb 2006 dark energy task force.

Ian : I'm not quite sure what your point is re "knowing the answer". Most cosmologists probably would "prefer" to prove w not equal to -1 (or GR wrong) which gets a strong chance of a Nobel Prize, while just getting closer and closer to -1 probably does not. There was considerable discussion at the Durham Ripples meeting about the need for "blinding" in future cosmological analyses to avoid "confirmation bias".

October 26, 2013 at 3:02 pm

I think it would be great if w turned out to be less than -1, because then we’d all know for sure what we currently only suspect, ie that we don’t understand anything!