Group items tagged

As soon as you move away from a perfectly homogeneous and isotropic cosmological model, what is meant by the "acceleration" of spacetime becomes ambiguous. It's important to be clear on what type of acceleration is being considered in any given observational or theoretical study, since different types can have very different cosmological implications.

A recent-historical analysis of cosmological parameter estimation. "Of the 28 measurements of Omega_Lambda in our sample published since 2003, only 2 are more than 1 sigma from the WMAP results. Wider use of blind analyses in cosmology could help to avoid this."

Their detection of confirmation bias (aka unconscious experimenter bias, or groupthink) may not be so significant: most if not all of those Omega_Lambda measurements will have used WMAP CMB priors. Next step would be to try and correct for that. Their warning for future analyses is spot on though: parameter estimation needs to be done blind.

A neat paper demonstrating constraints on faster than light neutrinos using cosmological bounds on the number of effective relativistic species at BBN and at the CMB. For a simple Lorentzian tachyon there is an imaginary mass \mu, and an anergy dependent speed v(\mu,E)>1 (where c=1). The bounds on N_eff translate to bounds on the mass, and therefore bounds on the speed at different energies for this type of super-luminal neutrino. From the CMB the speed at OPERA energies (\sim GeV) is bounded to be v-1<10^{-23}ish, whereas OPERA claimed v-1\sim 10^{-5}. The constraint at \sim MeV is also tighter than SNe1987A constraints. These constarints further rule out explanation of the OPERA results with this type of neutrino. Even though I would not think OPERA is explained by anything like this anyway, I still think it is a simple and neat way to use cosmology. It would probably make a good problem sheet/exam question!

Modern MCMC method for cosmological parameter estimation. "While Metropolis-Hastings is constrained by overheads, CosmoHammer is able to accelerate the sampling process from a wall time of 30 hours on a single machine to 16 minutes by the efficient use of 2048 cores. Such short wall times for complex data sets opens possibilities for extensive model testing and control of systematics."

This paper is by David Wallace (a philosopher in Oxford, not the novelist). The idea seems to be to talk through some of the statements that are made about the treatment of entropy in gravitation. I found this to be a useful exercise, and there are some interesting thoughts in here, even if the cosmology is a bit hit and miss. In particular he points out that the formation of structure in the Universe does not necessarily imply that gravitational fields in the Universe have to carry large amounts of entropy at late times.

Good question. So far the only compelling definitions of gravitational entropy have been in stationary space-times (those that admit a time-like Killing vector). There are various suggestions for how to define entropy in other situations, most notably Penrose's Weyl curvature hypothesis, but nothing concrete has yet emerged.

Is there not a definition of gravitational entropy from the holographic principle?

A long paper on measures in Cosmology, which I haven't read in its entireity yet. However, I found the final comment in the abstract quite provocative and interesting: "In a universe where the second law of thermodynamics holds, one cannot make use of our knowledge of the present state of the universe to "retrodict" the likelihood of past conditions." This is due to laws being time symmetric, while in practice we have the second law. In practice we *must* resort to Occam's razor and/or beauty arguments. Later: "if one wishes to make an argument in favor of inflation having occurred in the early universe, this argument must be based upon its being a simple and/or elegant hypothesis that accounts for observed phenomena. Any argument about the "likelihood" of inflation based upon the Liouville (or other) measure on phase space will require a justification for the use of this measure." Sometimes, beauty and Occam's razor are incompatible, or rather scale dependent (as in the case of the string landscape), and from a purely philosophical point of view I don't feel entriely comfortable with either being used as a criterion of truth when trying to discover things about the universe. However, this paper makes this seem inevitable. Certainly food for thought.

"We argue using simple models that all successful practical uses of probabilities originate in quantum fluctuations in the microscopic physical world around us, often propagated to macroscopic scales. Thus we claim there is no physically verified fully classical theory of probability. We comment on the general implications of this view, and specifically question the application of classical probability theory to cosmology in cases where key questions are known to have no quantum answer."

Discovery of a group of quasars which appear to form a structure over 1 Gpc across. This is much larger than any previously-suggested homogeneity scale in LambdaCDM.

There's been a growing awareness that the 'free' functions used to parameterise modified gravity aren't really as arbitrary as one might first think. This methods paper suggests how to use these ideas - although the forecast isn't all that encouraging. Still, the proof of the pudding is yet to come.

Abstract: In the standard cosmological framework, the Hubble diagram is interpreted by assuming that the light emitted by standard candles propagates in a spatially homogeneous and isotropic spacetime. However, the light from "point sources" -- such as supernovae -- probes the universe on scales where the homogeneity principle is no longer valid.

Celia visited us for several months last summer - this paper is the outcome of her work here. In EBI gravity there is an 'Eddington-dominated' epoch in the universe prior to radiation domination, which can avoid the need for a big bang singularity. However, it turns out that tensor perturbations in this early phase are unstable. It's particularly interesting that the instability only shows up at the perturbative level, whilst the background cosmology remains non-singular.

They claim a ~100x speed increase when using GPUs over a single processor. Execution time on the GPUs is comparable with a 128-processor MPI implementation.

Massive Gravity is this year's must-have theory of modified gravity. The concept is simple - what if the graviton had a (very small) mass? However, building a consistent and viable theory from this idea has proved very difficult. It has now been achieved for the background-level cosmology, and can fit the accelerated expansion (with the usual fine-tuning problems, of course!) This paper takes the first steps towards the perturbation theory that needs to be developed if we are to test Massive Gravity with measurements of structure growth, etc.

This is a dramatic illustration of the fact that any amount of observational data, necessarily restricted to the past lightcone and necessarily with non-zero errors, cannot predict anything mathematically about the future even one hour hence without further assumptions. It is also a display of the difference between mathematics and physics: the physicist necessarily employs intuition about the real world.

I'm confused by the final statement that we can't predict anything without further assumptions. It's a function of the complexity of the model, surely. If we have a simple model so that all the relevant constants are fixed by the observations, then they uniquely predict the future. However, if your model is complicated and has parameters unconstrained by experiment, then you can choose them to give "ripping" cosmologies within the hour. This is why we like to choose models that make sense (which is our intuition, as they say, for example to not have silly things like phantom fields, and why we choose to work within rigorous frameworks and seek to embed or motivate models within them), and also why we do model comparison. It may be possible to have some rip within the hour, but we can quantify how unlikely that is given the data, or how unlikely it is within the landscape. The statement they give is very deep sounding, but I think it has very little content.

Epic paper by Sluse et al, on high precision astrometry in lensed quasar systems. Attention optical astronomers! They deconvolve their images! And get very small error bars as a result. Interesting claim about being able to quantify the lens environment (the dreaded "external covergence"). This is the biggest systematic error in H0/w determination - great if we can reduce it further through improved lens modelling.

From N-body simulation, the authors find that concentration-mass relation displays a turnover for group scale dark matter haloes, for the case of WDM particles with masses of the order ~0.25 keV. This may be interpreted as a hint for top-down structure formation on small scales. Is there any reionization mechanism in this scenario?

1)Because the size of the simulated boxes were ten times smaller in previous studies. 2)Weak lensing at scale below ~1arcsec could work. Their results might be helpful for estimating non-linear power spectrum based on a certain halo model.

Cooool. Was Malin there this morning? This could be right up her street, with her flexion stuff! Also, weak lensing at < 1" sounds a bit like strong lensing to me - I'll read the paper in detail and see if there's anything we can already say from our (admittedly modest) SDSS samples. Thanks!

This group continue Steinhardt and Turok's on going interest in cyclic cosmology by using an anti-gravity phase to resolve a crunch/bang transition.

A good overview of the current status of weak lensing shape measurement software.

Intended for the next lensing lunch by the way.

Maybe an ACT insider could tell us more...