Monday, March 24, 2014

GWPAW 2013: Impressions from India

In the latest issue of the LIGO Magazine I have a short article on my (relatively) recent trip to India to attend the Gravitational Wave Physics & Astronomy Workshop. Below I reproduce (a partially un-edited version of [apologies to the editors for reverting some of their changes here]) the article, with added links!
Family constraints have meant I’ve been off the conference circuit for a bit, so the 3rd Gravitational Wave Physics & Astronomy Workshop (GWPAW, formerly the Gravitational Wave Data Analysis Workshop, GWDAW, which ran on 14 occasions) seemed like a good opportunity to get back into the swing of conference attendance. Plus, its location at the Inter-University Centre for Astronomy & Astrophysics (IUCAA) in Pune, India presented the chance to visit a new country. Due to the location of the meeting, many of the other non-local attendees were able to experience a bit of India, including a group that organised a tour round Mumbai (and subsequent train journey to Pune), a couple who started their trip with a holiday in the backwaters of Kerala, and others visiting family or friends. While it would have been a great opportunity for me to see India, I was unable to bookend my trip with any site-seeing, so my experience of India outside of the confines of IUCAA mainly came from my taxi ride from Mumbai to Pune. The taxi ride itself was an interesting insight into travel in India - the first half of the approximately three and a half hour ride (it’s about a 170km journey) was just in leaving Mumbai, where the roads that are about as chaotic as they come. The system seems to be to spot a gap in the traffic, even if it looks too small for the mode of transport you are in, and then squeeze into it. Astonishingly this method (accompanied by liberal application of the horn) got us through the traffic unscathed. The freeway between Mumbai and Pune is apparently one of the best roads in India, and can supposedly offer great views as you climb up into the rocky hills, but a combination of jet lag and low clouds/smog meant that I couldn’t appreciate the trip/views fully (from the plane on my flight back from Pune to Mumbai I was able to see the views I'd previously missed). 
In Pune I stayed at the very pleasant Seasons Apartment Hotel, which as the name suggests offered large apartments with a lounge and kitchenette (and free bottled water, which is a must for travellers there). Not feeling very adventurous on my arrival I just opted for dinner at the hotel, but it was definitely worthwhile as the open air rooftop bar/restaurant offered great views of the city. The hotel was just about within walking distance of IUCAA, where the meeting was held (which I had briefly considered as a travel option), but the organisers had put on a taxi service to and from the hotel every day. On travelling to IUCAA I was thankful for this as negotiating the roads, many of which lacked pavements, may have proved daunting. IUCAA itself is situated on the Pune University campus, but is fairly self-contained with its own “housing colony” for guests, students and postdocs to stay. During the meeting we didn’t have to go far between talks in the Chandrasekhar auditorium, coffee breaks (which consisted of strong black tea really) breaks and meals. 
As well as our taxi service the organisers provided breakfast, lunch and dinner within IUCAA under a large marquee. The food was great, although you may have been hard-pressed if you didn’t like curry - not a problem for me though! Some of the dishes were pretty spicy, but I suspect they were they were probably still toned down from their usual standard heat levels. We also had freshly made roti cooked in a tandoor oven by the side of the marquee. 
Kathak dance recital
On the first evening we had entertainment put on in the form of a Kathak Dance Recital in the meeting auditorium. The singing and musical accompaniment was mesmerising. Afterwards Sathya presented the dancers and musicians with houseplants, which I can only assume is the standard thank-you gift.
And what about the science? The meeting was weighted towards compact binary coalescences (CBC) and electromagnetic follow-up, but that’s not surprising given that these are the most likely sources of the first advanced detector observations. In fact it was good to have a GWPAW where many of talks were about things that could be done in the near future, rather than having to look ahead decades, further cementing the idea that gravitational wave detections are on the horizon! A couple of standout talks were Parameswaran Ajith's overview of the status and prospects for modelling CBC waveforms and Jocelyn Read’s talk on the potential for measuring neutron star equations of state with advanced detectors. Most sessions had lively discussions following the talks, with one particular participant always ready to provide some vigorous questioning. 
The breaks and poster sessions in the grounds of the auditorium (which amongst other things contained a giant sundial and a set of swings connected as a coupled harmonic oscillator) were always buzzing with conversation, which for me yielded a potential future collaboration with an IUCAA postdoc. There were many interesting posters, but I particularly liked a couple: one was Chris Messenger’s describing a method to extract redshift information from neutron star mergers by observing modes of a potentially short-lived post-merger hyper-massive neutron star; and another was Shaon Ghosh’s on electromagnetic follow-up of CBC signals. During the meeting my own poster was upgraded to a talk (due to passport related issues for one of the invited speakers causing him to miss the meeting), so I had to quickly put together my own slides. 
The meeting turned out to be incredibly productive and fascinating, as well as welcoming and well-organised. The organisers and IUCAA staff were really friendly and helpful. It was a great chance for many Indian students and postdocs to attend the meeting and share their work, and for people from the LVC to interact with them. This was particularly useful because the distance means many collaborators in the USA and Europe got to discuss topics in person, and allowed us to develop these relationships in the run-up to LIGO India. This will be good for bringing through new local people into the field in the run up to LIGO India. There was a great deal of enthusiasm from the IUCAA director Ajit Kembhavi to keep up the efforts with the suggestion that IUCAA and other Indian institutions host summer school-type events in the future. The next GWPAW to look forward to will be in Osaka, Japan in June 2015, closely followed by Amaldi in South Korea. 
It’s a shame I didn’t get to experience more of the country, but I did I get to discover a taste for the Indian Coca-Cola equivalent, “Thums-Up”, while discussing exciting science halfway around the world.

Friday, March 21, 2014

Direct or indirect?

This week has seen the potentially momentous result from the BICEP2 experiment indicating the detection of gravitational waves from the inflationary era of the Universe, just a tiny fraction of a second after the Big Bang. It's a fantastic result, and if/when confirmed by other experiments (e.g., Planck) will be huge leap in developing our understanding of the beginnings of the Universe. Many other people have discussed the background (that's just a scattering of a few of the many links to some scientific and more general descriptions of the results) and potential implications of the results, and a few areas for some considered scepticism, but I wanted to briefly talk about whether this classes as a direct or indirect detection of gravitational waves. I'm mainly interested in this because, to be clear up front, I'm part of a large scientific collaboration (the LIGO Scientific Collaboration [LSC]) that is currently trying for direct gravitational wave detection using a set of specially designed detectors/observatories (LIGO, Virgo and GEO600) here on Earth. I should also point out the views I'm giving are entirely my own and definitely not those of the LSC.

I should note that on BICEP2's FAQ the word "direct" gets used in the answer to the question "Have you detected a gravitational wave?" to which they answer "The frequency of the cosmic gravitational waves is very low, so we are not able to follow the temporal modulation. However, we are indeed directly observing a snapshot of gravitational waves through their imprints on matter and radiation over space." Whether this fits into my description of direct or indirect below is another question!

What do I mean by indirect or direct detection? Well in 1993 Hulse and Taylor won the Nobel Prize in Physics for their earlier observation of a pulsar in a neutron star binary system, which was losing energy exactly as predicted through the emission of gravitational waves. This has always been said to be an indirect detection of gravitational waves, i.e., it wasn't physically measuring the waves themselves, but was inferring their presence through the energy they carry away as observed by the binary system's evolution (since their original observations this effect has been measured in many other binary neutron star systems, which also provide other tests of general relativity). With the gravitational wave detectors (such as the aforementioned LIGO, Virgo and GEO600) they aim to directly detect the waves by actually seeing their effect in stretching and squeezing the distance between parts of the detectors. So, the former uses some observations to measure the properties of a source (the orbital evolution of a binary system) and from that infer the presence of gravitational waves, whilst the later directly measures their effect within a detector system. [On a slight aside there could be much discussion on the semantics of "direct" observation/detection - in pretty much all observations (including a persons senses) you could say that you're variously removed/abstracted by a number stages from directly measuring/experiencing the effect of something. In scientific observations it's pretty much always the case that you're having to use proxies to convey some information to you. In most astronomy photons are counted by a CCD, processed by a computer and then displayed, whilst in particle physics you're often measuring the decay of one particle through the products it produces, which themselves are relayed to you through tracks left on silicon detectors, or energy deposited in calorimeters. However, in most cases using "direct" observation/detection is probably a fair term.] 

So, in the case of the BICEP2 results, where they're measured the imprint of gravitational waves in the cosmic microwave background (CMB), where does that fit on the scale (if there is some scale in between!) of direct or indirect detection? Initially I was biased against calling this a direct detection. As mentioned above this is mainly due to working as part of a collaboration hoping to soon directly detect gravitational waves with ground-based detectors. I (not wanting to speak for the rest of the collaboration) would like us to be the first to claim a direct detection, so there's a level of guardianship (or unjustified feeling of ownership!) over that claim. However, I think (obviously I'm not the sole arbiter) the CMB measurements deserve the right to be called more than an indirect detection, so for now I'll go with the compromise of semi-direct detection (as used by Andrew Jaffe here).

So, why not indirect? Well, the gravitational waves that are observed in the CMB have (redshifted) frequencies of order 10-17 Hz, which corresponds to wavelengths of ~1 Gigaparsec. To measure such waves you'd need a detector about the size of the Universe. There's obviously no way you could build a physical detector to measure that, so using the CMB's the only way to do it - it is the only "detector" you could have available. In this sense they don't seem to fit with the indirect pulsar binary system paradigm above. [Note that there are also efforts to measure gravitational waves with frequencies around 10-9 Hz using astrophysical objects (in this case pulsars) as the components of a "detector".]

But, why only semi-direct then? This is maybe a technicality that could be argued over, but I suppose it comes down to the basic fact that despite the CMB being the only way to perform the measurement of ultra-low frequency waves you still aren't physically measuring the wave in a detector on Earth (another example might be dark matter, who's effects are imprinted in various astronomical observations, but you still want to see them in a detector on Earth to claim detection). You're also having to use the effect of the gravitational waves on density perturbations, which in turn affect the light intensity, which then affects the CMB polarisation signal received; in a laser interferometric detector the gravitational wave affects the position of mirrors, which in turn effect the phase of reflected and detected light, which you could argue (an I may be pushing it here) is a step less removed than the case with the CMB. There's also the case (which may not be entirely relevant in a direct/indirect argument) that given that the CMB polarisation signal (by the very nature of how it had to be formed during a short period in recombination when photons could diffuse far enough that they would encounter different temperature regions, but that there were still enough free electrons to scatter off and give a polarisation signal) was imprinted within a short space of time, it is just a single snapshot of the gravitational wave signal. Gravitational wave detectors on the other hand (including those using pulsars) are able to measure the variations as the waves pass them, so give a complete time series of the signal. My hand wavy analogy (also implied on the BICEP2 FAQ) is that the CMB measurement is like seeing a photograph of the shadows of water waves on a ripple tank, whereas gravitational wave detectors are like continuously measuring the position of a cork floating on top of the tank.
Shadows of waves on a ripple tank. Analogous to the imprints of gravitational waves in the CMB polarisation? [Credit]
Whether the BICEP2 result is indirect, direct or semi-direct detection of gravitational waves it doesn't take away from the fantastic work they've done and it's still an amazing feat of observation and analysis.

Anyway, that's my view. What do you think?

Thursday, January 30, 2014

The origin of carbon

Last summer I was asked to write an article on the origin of carbon for The Geographer, which is the quarterly newsletter of the Royal Scottish Geographical Society. The original article can be found here (see page 8), but I've been given permission to reproduce it here (any comments/corrections are welcome):

Carbon is the fourth most abundant element in the Universe (after hydrogen, helium and oxygen) and is the sixth lightest element. To understand it's origins and relative abundance we first have to go back to the origin of the Universe itself.

By the mid-20th Century Edwin Hubble's observations of an expanding Universe suggested that it had started out from an extremely dense and hot initial state: a "cosmic fireball" produced by the Big Bang. However, a question for the Big Bang model was how it produced the known elements in their currently observed abundances (called Big Bang nucleosynthesis). In 1948 a PhD student called Ralph Alpher, working with the renowned physicist George Gamow, published a paper called "The Origin of Chemical Elements" claiming to solve this problem. But, the title slightly overstated the outcome of their work. It was ground-breaking and correctly predicted that in this "comsic fireball" the three lightest elements (hydrogen, helium and lithium) would be made in the abundances that are observed today. However, their work couldn't produce any heavier elements and it was in fact the problem of making carbon that was the stumbling block. The basic process of forming elements is that you take nucleons (protons and neutrons) and fuse them together to create heavier atomic nuclei. You can then fuse further nucleons, or atomic nuclei, together to produce heavier and heavier elements. This is complicated by several facts: the rates that fusion reactions take place can differ enormously for different nuclei; the rates depend very strongly on temperature and density; and, certain nuclei are unstable to radioactive decay and are very short-lived. To create carbon you require six protons and six neutrons, so it can be made by fusing two helium nuclei (two protons and two neutrons) to give a beryllium nucleus and then sticking on another helium nucleus to give carbon. However, Alpher and Gamow found that because the beryllium nuclei only has a lifetime of ~10-16 seconds there wasn't enough time during the hot and dense early stages of the Universe for it to fuse with another helium nucleus and produce carbon. They were therefore left with a Universe containing only the three lightest elements, which was contrary to all observational evidence!

This problem with Big Bang nucleosynthesis was jumped upon by opponents of the Big Bang as a failure of the model. One such person was Sir Fred Hoyle, a forthright theoretical astrophysicist at Cambridge, who, along with others, put forward Steady State models of the Universe (i.e. an infinite Universe with no beginning). However, his models still required that there was some way that elements could be produced, so the problem of creating carbon from lighter nuclei still needed to be solved. In the calculations for trying to fuse three helium nuclei (called the triple alpha process, since helium nuclei are also known as alpha particles) he still found that only insignificant amounts of normal carbon could not be produced during the short life of beryllium, but the production rate would dramatically increase if carbon nuclei were created in an "excited" state i.e. a nucleus with additional potential energy in it. There was no theoretical reason why such an "excited" state should exist (in fact it is still unknown [sorry for the non-open access article link] why this state exists!), but Hoyle argued that because we exist and we require carbon for our existence, then if this is the only way significant amounts of carbon can be produced then this state must be possible. His calculations gave him a precise number for the amount of energy in this state, but he had to convince someone to run an experiment to see if it was true. While visiting the California Institute of Technology in 1953 he persuaded the nuclear experimental groups led by Willy Fowler and Ward Whaling to look for this excited state and soon after it was confirmed that it did indeed exist1.

This didn't mean that Big Bang nucleosynthesis could now produce carbon and the heavier elements as the process was still far too slow given the expansion of the Universe, but there were other environments where it could take place - the cores of massive stars. Hoyle and Fowler, along with the married couple of Margaret and Geoffrey Burbidge, were able to show how all the elements from beryllium up to iron were synthesised in the cores of stars (called stellar nucleosynthesis). In these massive stellar cores there is a high enough temperature and density of helium nuclei so that even though the beryllium produced from fusing two helium nuclei is extremely short-lived there is enough of it that some will fuse with another helium nuclei to form the excited state of carbon. Since carbon was required as the starting point for production of all the heavier elements this allows the large variety we see today. The deaths of these massive stars in supernova explosions has since seeded the Universe we the huge quantities of carbon we see today.

The evidence now shows that the lightest elements were indeed produced during the Big Bang and the Universe has had enough time to produce all other elements (including Carbon) in their observed abundances, via processing in stars.

1A more detailed account of this and the many other people actually involved in the work can be found in H. Kragh, (2010) When is a prediction anthropic? Fred Hoyle and the 7.65 MeV carbon resonance.