Archive for November, 2012

SPT and the CMB

Posted in The Universe and Stuff with tags , , , , , , , , , on November 30, 2012 by telescoper

I’ve been remiss in not yet passing on news  from the South Pole Telescope, which has recently produced a number of breakthrough scientific results, including:  improved cosmological constraints from the SPT-SZ cluster survey (preprint here); a new catalogue of 224 SZ-selected cluster candidates from the first 720 square-degrees of the survey (preprint here); the first measurement of galaxy bias from the gravitational lensing of the CMB (preprint here); the first CMB-based constraint on the evolution of the ionized fraction during the epoch of reionization (preprint here); the most-significant detection of non-Gaussianity induced from the gravitational lensing of the CMB (preprint here); and the most precise measurement of the CMB damping tail and improved constraints on models of Inflation (preprint here).

Here’s the graph that drew my eye (from this paper). It shows the (angular) power spectrum of the cosmic microwave for very high (angular) frequency spherical harmonics; the resolution of SPT allows it to probe finer details of the spectrum that WMAP (also shown, at lower l).

Slide1

This is an amazing graph, especially for oldies like me who remember being so impressed by the emergence of the first “acoustic peak” at around l=200 way back in the days of Boomerang and Maxima and gobsmacked by WMAP’s revelation of the second and third. Now there are at least six acoustic peaks, although of progressively lower amplitude. The attenuation of the CMB fluctuations at high frequencies is the result of diffusion damping – similar to the way high-frequency sound waves are attenuated when they pass through a diffusive medium (e.g. a gas).  The phenomenon in this case is usually called Silk Damping, as it was first worked out back in the 1960s by Joe Damping Silk.

Anyway, there’ll be a lot more CMB news early (?) next year from Planck which will demonstrate yet again that cosmic microwave background physics has certainly come a long way from pigeon shit

Simulations and False Assumptions

Posted in The Universe and Stuff with tags , , , , on November 29, 2012 by telescoper

Just time for an afternoon quickie!

I saw this abstract by Smith et al. on the arXiv today:

Future large-scale structure surveys of the Universe will aim to constrain the cosmological model and the true nature of dark energy with unprecedented accuracy. In order for these surveys to achieve their designed goals, they will require predictions for the nonlinear matter power spectrum to sub-percent accuracy. Through the use of a large ensemble of cosmological N-body simulations, we demonstrate that if we do not understand the uncertainties associated with simulating structure formation, i.e. knowledge of the `true’ simulation parameters, and simply seek to marginalize over them, then the constraining power of such future surveys can be significantly reduced. However, for the parameters {n_s, h, Om_b, Om_m}, this effect can be largely mitigated by adding the information from a CMB experiment, like Planck. In contrast, for the amplitude of fluctuations sigma8 and the time-evolving equation of state of dark energy {w_0, w_a}, the mitigation is mild. On marginalizing over the simulation parameters, we find that the dark-energy figure of merit can be degraded by ~2. This is likely an optimistic assessment, since we do not take into account other important simulation parameters. A caveat is our assumption that the Hessian of the likelihood function does not vary significantly when moving from our adopted to the ‘true’ simulation parameter set. This paper therefore provides strong motivation for rigorous convergence testing of N-body codes to meet the future challenges of precision cosmology.

This paper asks an important question which I could paraphrase as “Do we trust N-body simulations too much?”.  The use of numerical codes in cosmology is widespread and there’s no question that they have driven the subject forward in many ways, not least because they can generate “mock” galaxy catalogues in order to help plan survey strategies. However, I’ve always worried that there is a tendency to trust these calculations too much. On the one hand there’s the question of small-scale resolution and on the other there’s the finite size of the computational volume. And there are other complications in between too. In other words, simulations are approximate. To some extent our ability to extract information from surveys will therefore be limited by the inaccuracy of our calculation of  the theoretical predictions.

Anyway,  the paper gives us quite a few things to think about and I think it might provoke a bit of discussion, which is why I mentioned it here – i.e. to encourage folk to read and give their opinions.

The use of the word “simulation” always makes me smile. Being a crossword nut I spend far too much time looking in dictionaries but one often finds quite amusing things there. This is how the Oxford English Dictionary defines SIMULATION:

1.

a. The action or practice of simulating, with intent to deceive; false pretence, deceitful profession.

b. Tendency to assume a form resembling that of something else; unconscious imitation.

2. A false assumption or display, a surface resemblance or imitation, of something.

3. The technique of imitating the behaviour of some situation or process (whether economic, military, mechanical, etc.) by means of a suitably analogous situation or apparatus, esp. for the purpose of study or personnel training.

So it’s only the third entry that gives the intended meaning. This is worth bearing in mind if you prefer old-fashioned analytical theory!

In football, of course, you can even get sent off for simulation…

Three Astronomy Jobs at Sussex – The Deadline Approaches!

Posted in The Universe and Stuff with tags , , , on November 29, 2012 by telescoper

I’m taking the liberty of repeating this advertisement in case anybody out there missed it. Here is an announcement of three (new, permanent) jobs in Astronomy at the University of Sussex. You can also find an advertisment in the November AAS Jobs Register. In fact this is it. The deadline is 30th November, i.e. tomorrow, so if you want to apply then you had better get your skates on!

Full details of the positions are in the above links, but the gist is that applications are invited for 3 permanent, full-time faculty positions within the Astronomy Centre.

The 8 existing faculty have research interests that span the observation, modelling/simulation and theory of extragalactic astronomy and cosmology.  We are seeking talented and ambitious colleagues whose research interests complement and extend our current activity.

I’ll be interested to see how many people apply as a result of seeing this here announcement, so if you do fill in an application form  be sure to answer the question “Where did you see this post advertised” with “In the Dark”!

Open Day and Subject Fair

Posted in Education with tags , , , , , on November 28, 2012 by telescoper

Today was the Postgraduate Open Day for Cardiff University, so I trooped off at lunchtime to man person the School of Physics & Astronomy stand in the Great Hall of the Students in the Students’ Union for the Subject Fair. It’s the first time I’ve ever been in the Great Hall, in fact, for no other reason than it’s for the students not the staff. Anyway, I have to say it didn’t look all that great, although at least it was warm. There’s no heating in my office right now, you see. And they provided coffee and biscuits.

Other than that it was just me, some leaflets and an uncountable infinity of Herschel Space Observatory souvenir pens sitting there for two hours. And there was lousy mobile coverage so I couldn’t even tweet. I got a bit bored, actually. I wish I’d taken my knitting. I did take a picture though…

It has to be said that a general Postgraduate Open Day like this isn’t a very effective method of recruiting postgraduate students, not in Physics and Astronomy at least. Most potential applicants come to apply by looking at web pages and/or listening to advice from people in the department where they are doing or did their first degree. People have already decided between Physics and, say, Astronomy and certainly between either of those and Sociology, so the idea of stalls competing for custom is a bit absurd.

Still, as Director of Postgraduate Studies I decided that it was good for Physics and Astronomy to show willing by maintaining a presence at such events, and if as a bonus we recruit even one promising PhD student then it’s probably worth the investment in time. The additional complication with this now is that I’m soon leaving to go to Sussex University, so I was tempted to tell visitors about opportunities there. I didn’t though. Mainly because I hardly spoke to anyone all afternoon…

The moral of this tale – if there is one – is that recruitment in different subjects is very different, so the “one size fits all” centralised approach isn’t always the right way to proceed. Schools and departments know their market better than anyone, so they need to be allowed to do their own thing at least part of the time.

I suspect this is an argument I’ll shortly be making elsewhere.

Diamond Lights

Posted in Football, Music, Science Politics with tags , , , on November 27, 2012 by telescoper

Apparently there’s been a posh do this evening at the Royal Society to celebrate the 10th Anniversary of the Diamond Light Source. In fact the Diamond Light Source has its own anniversary blog that’s been posting celebratory things for a while; the actual anniversary being celebrated was the signing of the agreement to set up the Diamond Light Source, which happened on March 27th 2002. Actual operations didn’t commence until 2007, at a total cost of £260m, which is when STFC was created and told to pick up the tab for running the facility which, together with a few other things, precipitated a financial crisis from which UK particle physics and astronomy are only just starting to recover.

I don’t be churlish about the good science the Diamond Light Sources is undoubtedly doing so I thought I’d mark the anniversary here. The blog I mentioned above has a video page but it sadly doesn’t contain the video I most expected to see. This, Diamond Lights, was released – or did it escape? – in 1987 and it “stars” Glen Hoddle and Chris Waddle who, as singers, were both excellent footballers. I’m surprised STFC Chief Executive John Womersley didn’t record a cover version of this as part of the anniversary celebrations…

The Problem of the Eiffel Tower

Posted in Cute Problems with tags , , , , on November 27, 2012 by telescoper

Too busy today (again) for anything else so I’m going to resort (again) to the Cavendish Problems in Classical Physics. I think I’ll eschew the multiple-choice format for this one, but will say that there is a small hint in the fact that the question is split into two parts:

The Eiffel Tower is 300m high and is situated at a latitude 49° N. What are the magnitude and direction of the deflection caused by the Earth’s rotation to:

  1. the bob of a plumb-line hung from the top of the Tower;
  2. the point of impact of a body dropped from the top?

Please give your answers, with reasons, through the comments box below. For legal reasons I should make it clear that you are not expected to perform either experiment.

To my own correspondents…

Posted in Biographical, The Universe and Stuff with tags , on November 26, 2012 by telescoper

So. Today I finally finished a stack of things I should have done weeks ago, including compiling the teaching timetable for next semester (when I won’t even be here) Anyway, that means I can now move onto the next stack of things that I should also have done weeks ago, after I’ve finished marking the batch of 100 second-year coursework scripts sitting on my desk. There’s no rest for the <insert appropriate adjective>.

Anyway, it’s now just a couple of months before I shuffle off the coil of Cardiff, and the enormity of the impending move becomes more apparent every time I go into my office and observe the quantity of books and papers filling the groaning shelves. Today, however, I made a decision that will make moving simpler: I decided to ditch all the drawer-loads of correspondence marked “Other”, i.e. all the unsolicited letters and manuscripts I’ve accumulated over the years about “alternative” theories of cosmology and whatnot. Here’s an example:

I can’t really make head nor tail of this one, but sometimes have a vague feeling that it might just be a sort of cosmic Rosetta Stone, offering up the Secrets of the Universe in diverse languages. Sadly, however, it’s more likely that the languages involved are Balderdash, Gibberish and Gobbledegook.

I regret to announce, therefore, that the plethora of papers telling me why Einstein was wrong, how the Universe is really in the shape of a spiral, how the Great Pyramid of Giza explains the Higgs Boson, and why the Big Bang couldn’t have happened, will have to go to the Great Shredder in the Sky (if that’s where it is).

Anyway, to all my correspondents all I can say is that I’ve enjoyed reading your letters – you must be very fond of your old typewriters – and I’m grateful for the time you took to draw the diagrams by hand in so many lovely colours. And I’m impressed by your qualifications as Electrical Engineers. Really. I’m sorry I didn’t reply to you all individually, but I just didn’t have the time. And now it pains me to realise I don’t have the space either…