Archive for Antony Lewis

Three Cheers for Three Chairs!

Posted in Uncategorized with tags , , , on November 2, 2016 by telescoper

Just a quick post to say public congratulations to three of my former colleagues in the Department of Physics and Astronomy at the University of Sussex.

My spies tell me that the following have recently been promoted to Professorial positions:

  • Kathy Romer (now Professor of Astrophysics) – Kathy is principal investigator of the XMM Cluster Survey collaboration and is coordinating cluster research for the Dark Energy Survey project.
  • Antony Lewis (now Professor of Cosmology) – Antony works on theoretical models of the early universe, as well as comparing observations with cosmological models, and is part of the core team analysing data from the Planck satellite.
  • Jacob Dunningham (now Professor of Physics)  – Jacob is Head of the Atomic Molecular & Optical (AMO) Physics group at Sussex  and works on quantum mechanical entanglement spans the fields of quantum information, quantum optics, Bose-Einstein condensation, and metrology.

As former  Head of School  I knew these were in the system but I left before the somewhat laborious promotions process was completed, so it’s very nice to receive confirmation that they all went through.

P.S. Extra-special congratulations to Kathy, because she was born on Tyneside (i.e. not in the Midlands).

 

Another take on cosmic anisotropy

Posted in Cosmic Anomalies, The Universe and Stuff with tags , , , on October 22, 2009 by telescoper

Yesterday we had a nice seminar here by Antony Lewis who is currently at Cambridge, but will be on his way to Sussex in the New Year to take up a lectureship there. I thought I’d put a brief post up here so I can add it to my collection of items concerning cosmic anomalies. I admit that I had missed the paper he talked about (by himself and Duncan Hanson) when it came out on the ArXiv last month, so I’m very glad his visit drew this to my attention.

What Hanson & Lewis did was to think of a number of simple models in which the pattern of fluctuations in the temperature of the cosmic microwave background radiation across the sky might have a preferred direction. They then construct optimal estimators for the parameters in these models (assuming the underlying fluctuations are Gaussian) and then apply these estimators to the data from the Wilkinson Microwave Anisotropy Probe (WMAP). Their subsequent analysis attempts to answer the question whether the data prefer these anisotropic models to the bog-standard cosmology which is statistically isotropic.

I strongly suggest you read their paper in detail because it contains a lot of interesting things, but I wanted to pick out one result for special mention. One of their models involves a primordial power spectrum that is intrinsically anisotropic. The model is of the form

P(\vec{ k})=P(k) [1+a(k)g(\vec{k})]

compared to the standard P(k), which does not depend on the direction of the wavevector. They find that the WMAP measurements strongly prefer this model to the standard one. Great! A departure from the standard cosmological model! New Physics! Re-write your textbooks!

Well, not really. The direction revealed by the best-choice parameter fit to the data is shown in the smoothed picture  (top). Underneath it are simulations of the sky predicted by their  model decomposed into an isoptropic part (in the middle) and an anisotropic part (at the bottom).

lewis2

You can see immediately that the asymmetry axis is extremely close to the scan axis of the WMAP satellite, i.e. at right angles to the Ecliptic plane.

This immediately suggests that it might not be a primordial effect at all but either (a) a signal that is aligned with the Ecliptic plane (i.e. something emanating from the Solar System) or (b) something arising from the WMAP scanning strategy. Antony went on to give strong evidence that it wasn’t primordial and it wasn’t from the Solar System. The WMAP satellite has a number of independent differencing assemblies. Anything external to the satellite should produce the same signal in all of them, but the observed signal varies markedly from one to another. The conclusion, then, is that this particular anomaly is largely generated by an instrumental systematic.

The best candidate for such an effect is that it is an artefact of a asymmetry in the beams of the two telescopes on the satellite. Since the scan pattern has a preferred direction, the beam profile may introduce a direction-dependent signal into the data. No attempt has been made to correct for this effect in the published maps so far, and it seems to me to be very likely that this is the root of this particular anomaly.

We will have to see the extent to which beam systematics will limit the ability of Planck to shed further light on this issue.