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Malin Velander

Results of the GREAT08 Challenge: an image analysis competition for cosmological lensing - 4 views

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    A good overview of the current status of weak lensing shape measurement software.
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    Intended for the next lensing lunch by the way.
David Marsh

Tachyonic Neutrinos and Cosmology - 1 views

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    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!
David Marsh

Tunneling and Rolling to False Vacua - 0 views

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    The authors construct exact instanton solutions for tunneling over very small barriers in the presence of gravity, and demonstrate matching between previous results, and with the flat potential and no gravity case.

    Confusingly, it seems that for consistency one should include the tunneling effect along side the rolling of a field on a flat potential, even when there is no barrier. I'm not sure quite what this means operationally, but I think it may have effects for models of quintessence where the asymptotic future is a big crunch. Here it seems we may not be able to consider simple scalar field rolling, but may also have to include the instanton effects.

    More excuses to go back and read Coleman-DeLuccia again are always good.
Tessa Baker

Lensing of 21cm Fluctuations by Primordial Gravitational Waves - 4 views

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    The current upper limit on the tensor-to-scalar ratio r is ~0.2, and should improve by an order of magnitude with future experiments. These authors claim that measuring the curl component of the lensing power spectrum of 21cm emission could yield incredible constraints r~10^(-9) (though this headline figure corresponds to their most optimistic case). The angular resolution of our planned 21cm telescopes is the crucial quantity in determining these constraints.
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    This is a very thought-provoking paper. At z=50 - the redshifted wavelength of HI is 21 cm * 51 which is 21m [about the length of a cricket wicket]. To image fluctuations you would need to space antennae
    by about half a wavelength, or by ~10 m. So far, so good, LOFAR is trying this already, filling many cricket pitches worth of land with antennae in Northern Holland. The UK even has its own little version at
    Chilbolton near Winchester. So far, so good.

    However, to get to r~10^-9 you need (apparently) to get to l_ max ~ 10^7, or an angular resolution of about 0.01
    arcsec (Better than Hubble Space Telescope resolution). This means that, according to lambda/D, the total size of your 21-cm instrument has to have a diameter of ~ [21 m] / (10^-7), or ~10-times larger than the Earth.

    Of course, the atmosphere is getting close to opaque at these wavelengths, and the radio frequency
    interference is so bad that you'd want to put such an instrument on the back side of the moon.
    Unfortunately, the moon isn't large enough either, so you'd have to launch (or remotely deploy) something ~10-times the size of the Earth into deep space.

    This might be quite expensive, but in the SKA project we have most of the machinery to simulate such an instrument if any of you theorists out there are interested in collaboration.
Celia Escamilla

The Pseudo-Rip - 2 views

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    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.
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    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.
Tessa Baker

Seeing in the dark - II. Cosmic shear in the Sloan Digital Sky Survey - 0 views

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    Recommended by Jo.
Phil Bull

On the measurement of cosmological parameters - 3 views

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    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."
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    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.
David Marsh

Antigravity and the big crunch/big bang transition - 1 views

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    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.
Timothy Clifton

Gravity, Entropy, and Cosmology: In Search of Clarity - 3 views

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    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.
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    Potentially simple question alert: what is the entropy associated with a gravitational field? Is there a statistical physics (ie, about disorder vs order) interpretation to whatever thermodynamic integral is implied by Eintein's equations?
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    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.
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    Is there not a definition of gravitational entropy from the holographic principle?
Kaiki Inoue

Nonlinear Evolution of Cosmological Structures in Warm Dark Matter Models - 3 views

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    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?
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    Sorry I can't be there today - I am really interested in this paper though! I have some questions:
    1) Why hasn't this been seen before? Is their resolution or volume only now high enough? Could it be an artifact?
    2) What do they suggest we measure, and how well? M? c? Inner profile slope? In how many systems?
    Thanks!
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    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.
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    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!
Kaiki Inoue

Measuring our Peculiar Velocity by "Pre-deboosting" the CMB - 1 views

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    Our peculiar velocity with respect to the CMB rest frame can be measured through off-diagonal correlations in harmonic expansion coefficients due to aberration. The author shows that a new technique called "pre-deboosting"
    will work for detecting such correlations up to l~10000. If the measured residual peculiar velocity is different from the expected value, it would be a hint of some anomalies on superhorizon scales.
Phil Marshall

COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses X. Modeling based on hi... - 1 views

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    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.
Phil Bull

Test for anisotropy in the mean of the CMB temperature fluctuation in spherical harmoni... - 1 views

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    The statistical anisotropy of the mean of the CMB temperature fluctuations is tested. The naive inflationary prediction is that the mean a_lm's are zero, but the authors find a deviation from this expectation for l=221 - 240.
Phil Bull

Stacking catalog sources in WMAP data - 1 views

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    Point sources from the WMAP 7 catalogue are stacked, and the results are found to be consistent with the WMAP beam models. The correctness of the beam models used had been questioned by Shanks and others, who found that inaccurate models could introduce a significant bias into the measured CMB power spectrum. The authors here also find evidence for spectral steepening above 61GHz. This changes the estimates for the spectrum of unresolved point sources. Accounting for this effect, the primordial power spectrum seems to be closer to scale invariant than first thought.
Phil Bull

[1111.6585] The Milky Way has no thick disk - 2 views

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    Rather than being split up into two components of different aged stellar populations with distinct scale heights, a thin and thick disc, the Milky Way seems to have "a continuous and monotonic distribution of disk thicknesses".
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    That's not what Lucio Meyer said!
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