Yes, we've found another one

After the excitement of February when we (the LIGO Scientific Collaboration & Virgo Collaboration, or LVC for short) announced the first direct detection of a gravitational wave signal a lot of people having understandably been asking "Well, did you see any more?". The analysis performed for the announcement of the first detection (of the source called GW150914) used just over a month of data from the start of a longer observing run (that we called O1), which ran from the 12^th September 2015 until the 19^th January 2016. So we did have more data "in the can". And, as it happened that additional data did indeed provide us with another highly significant detection^*. This new signal was observed in what was the early hours of Boxing Day in the UK, although it was still Christmas Day in the US when it hit the two LIGO detectors - so we can call it a late Christmas present. As we work in Coordinated Universal Time (UTC), which follows Greenwich Mean Time (GMT), the signal has been given the title GW151226 (i.e. it arrived on 26^th December 2015), but internally has generally been called "The Boxing Day Event".

So, is this signal different from the first one? Well, like GW150914 it appears to be the result of two black holes inspiralling in to each other and merging, although the two merging black holes are smaller at roughly 14 and 8 times the mass of the Sun. A nice illustration of where these black holes sit, in terms of mass and radius, compared to other known black holes is shown here. However, unlike GW150914 we can pretty definitively say that at one of the merging black holes is spinning. Another thing to note is that if you look at Figure 1 from our detection paper for GW151226 you can't really see the signal in the data time series (whereas GW150914 stuck out like a sore thumb!) and it pretty much looks like noise.  As the system was less massive (but at a similar distance to) GW150914 the amplitude of the signal was intrinsically smaller. The saviour to this was that it also lasts longer in the detector's sensitive frequency bands (see Figure 1 in this paper that discusses all the detection's together) which means that you can integrate (basically sum together) over the longer signal and still "see" it in the noise.

Given that we'd already announced the detection you may be wondering what's important about this new one. The main thing is that we are now starting to reveal a population of objects rather than a single one. From looking at the population you can start to understand the distribution of source properties and investigate how they form. Admittedly with just two (and a bit) sources you really can't say much - it would be hard to work out the distribution of everyone's height by measuring just two people, but you at least get a rough idea of the likely range. It also allows us to be sure that the first signal wasn't a fluke, and suggests that we'll see many more of these objects in our upcoming observing runs (the next one, O2, should start this autumn with slightly better sensitivity than O1, and hopefully include the Virgo detector).

We often say that these gravitational waves are opening a new astronomical window on the Universe. And they really are! Imagine that the sky had always been covered in cloud, so you'd never been able to see the Moon, planets or stars (although in this scenario assume that you had a pretty good theory that the diffuse light coming through the clouds was being emitted by very distant objects called "stars".). Then, imagine that one night there's a slight chink in the clouds and through that you see a black sky with a single shining point of light in it. Wow! Your theory about "stars" was right! As the nights go on the clouds clear even more and reveal even more stars and other astronomical objects and the wonders of the Universe (and the exciting physics they reveal(!)) open up to you. It's a slightly tortured analogy, but you can kind of see that we're just seeing the first few points of light as the clouds are just starting to clear.

Some further information/reading:

• The science summary of these results can be found here.
• The GW151226 detection paper is here and a paper detailing all the binary black hole events detected in O1 is here.
• The data, and tutorials on using it (with ipython/jupyter notebooks), for GW151226 can be found here.
• A (probably non-exhaustive) list of blog posts by other LVC members about this detection (in no particular order):
• The Wave that Stole Christmas by Daniel Williams
• Merry Christmas, LIGO: Another Gravitational Wave! by Amber Stuver
• The Cosmic Classroom on Boxing Day by Shane Larson
• The Boxing Day Event by Christopher Berry
• Gravitational waves found again: here’s how they could whisper the universe's secret by Graham Woan
• Black holes rule! by Mark Hannam

^*The eagle-eyed of you may have know that amongst all the papers produced about our detection announcement there was also mention of a candidate gravitational event that was within the originally analysed dataset. We've estimated that this candidate, dubbed LVT151012 (for LIGO-Virgo Trigger), has a roughly 90% chance of being a real astrophysical signal, but we like to be far more certain than that to claim it as a definite signal.

The wait is over

Did you feel anything odd at around 09.50am GMT on 14th September 2015 (I'll let you do the time zone conversion)? Did you notice a disturbance in the force? Did you feel a tingle down your spine? Did you have butterflies in your stomach? Or, did you just feel a little bit wibbly?

No!? Well, at around that time a gravitational wave slammed into you at the speed of light, tried to rip your component atoms apart and then pull them together again¹, and then passed out the other side of you. But you didn't even notice, did you! It's not particularly surprising you didn't feel anything as the disturbance the wave produced was spectacular mainly in its minuscule effect - the waves would have attempted¹ to 'wibble' you from head-to-toe by only about 0.000000000000000000001 m (see Fig. 1 for an illustration of the effects of such a wave!).

Fig. 1. The effect of a passing gravitational wave on an alligator (as found around the LIGO Livingston observatory), a tumbleweed (as found in plentiful supply around the LIGO Hanford observatory) and myself (as found in the School of Physics & Astronomy at the University of Glasgow). Note that there is no discernible effect on any of these, except maybe as slight noticeable increase in my happiness at the prospect of the last 13 years of my working life having not been futile!

However, we (humanity in general, but large teams of scientists - many within the LIGO Scientific Collaboration [LSC] and Virgo Collaboration, including me - more specifically) have managed to build instruments that did indeed feel something on this date and time - a signal which we've given the catchy name GW150914 (or "The Event" as it was known for a while within the collaboration). These instruments, in this case the two US-based LIGO observatories (now entering their advanced phase), one in Hanford, Washington and the other in Livingston, Louisiana², both felt the waves' passing and saw a very consistent signal (see Fig. 2) - other than it looking like exactly what we'd expect from a gravitational wave source, our confidence that this was a real signal came from empirically estimating how often such a consistent and strong signal would have be seen by chance (i.e., from random [generally non-Gaussian] noise fluctuations in the detectors), which we work out as being less than once per 200,000 years. So, we're pretty sure (greater than 5.1σ in annoyingly frequentist statistical terminology), i.e. certain, that the signal was real. And, what's more, we've been able to use the pattern of wibbles the instruments felt to work out that this gravitational wave was emitted by two black holes, both tens of times more massive than the Sun, whacking into each other at about half the speed of light, to form the largest small(!) (~solar mass) black hole we know of. The amount of energy this event emitted was a whopping 5×10⁴⁷ Joules (quite possibly the most luminous event we've ever observed), equivalent to three times the mass of the Sun being converted directly to energy (remember E=mc²). Or, if you're into some "fun" energy conversions this is apparently equivalent to "the number of kilocalories in 2×10²⁸ cubic kilometres of butter (that's the volume of 14 billion Suns of pure butter!)" or "3 quadrillion times the energy required to destroy the planet Earth"!

Fig. 2. The GW150914 gravitational wave signal observed in the two LIGO detectors (this is figure 1 from the discovery paper, Abbott et al., Phys. Rev. Lett., 116, 061102 (2016)) (Credit: The LIGO Scientific Collaboration and the Virgo Collaboration)

This is the first ever detection³ we've had and it is quite a big deal, both scientifically (there's a whole load of awesome astrophysics that has been done using the signal, and it opens up a whole new area of astronomy) and personally. Some people have been in the gravitational wave detection game for almost five decades, but we've all had to wait patiently without seeing any definite sign of them in our detectors⁴ (this has occasionally been to the amusement of colleagues in other areas of physics and astronomy). As a member of the LSC myself since starting studying for my PhD at the University of Glasgow in the autumn of 2002 (when what is now known as the "initial" LIGO detectors had just started taking data) I've only been waiting 13 years, but that's still my entire working life. For some additional context here's what I wrote in my thesis acknowledgements section back in 2005:

I was attracted to the field of gravitational wave research due to the promise that we would be entering exciting times with several large scale projects bringing in unprecedented amounts of new data. Given this the discovery of gravitational waves would be just round the corner, opening up gravitational wave astronomy for real. Little did I know that this has been exactly what’s been said for around 30 years! Despite this I do actually believe that I’ve entered the field at a prime time and finding gravitational waves is just around, if not the first corner, then the next one.

So, ten years later, we have now turned that "next corner" and gravitational wave astronomy is finally with us! Many more detections should now be forthcoming in our future observation runs, hopefully including other exciting sources as well as more merging pairs of black holes.

Finally, the other rather cool, and timely, thing is that the signal arrived a century after Einstein published his General Theory of Relativity from which the prediction of gravitational waves arises. Einstein's prediction of gravitational waves and also Schwarzschild's solution to Einstein's equations from which predictions of black holes would arise, were both published a century ago in 1916.

The paper describing the detection and analysis of GW150914 has been peer reviewed and is now published (Abbott et al, Phys. Rev. Lett., 116, 061102, 2016) and further papers detailing the detectors, analyses and science results can be found here. Also, summaries (at a less detailed level) of the main science we've obtained from the signal can be found here. The data containing the signal and some example codes showing how to view, and hear(!), it are available here, so you should go ahead and take a look yourself.

More information, reactions and opinions about this amazing discovery (and the time-line of how the detection happened⁵) can be found in blog posts by several fellow collaboration members linked below and within a special edition of the LIGO Magazine:

• Amber Stuver (Post-doc, LIGO Livingston Observatory)
• Andrew Williamson (PhD student, Cardiff University)
• Bangalore Sathyaprakash (Professor, Cardiff University)
• Brynley Pearlstone (PhD student, University of Glasgow)
• Christopher Berry (Post-doc, University of Birmingham)
• Chris North (Lecturer, Cardiff University)
• Daniel Hoak (Post-doc, Virgo)
• Daniel Williams (PhD student, University of Glasgow)
• Mark Hannam (Professor, Cardiff University)
• Rebecca Douglas (PhD student, University of Glasgow)
• Roy Williams (Research Scientist, Caltech)
• Sean Leavey (PhD student, University of Glasgow)
• Shane Larson (Research Associate Professor, Northwestern University)

Also, be sure follow @ligo and @ego_virgo on twitter along with the hastags #GravitationalWaves, #EinsteinWasRight, #BinaryBlackHole and #AdvancedLIGO.

P.S. If you want to know what I was doing when GW150914 passed by it probably involved nappies (diapers for those in the US), feeding a child/cleaning bottles, or doing laundry, as I was on the final day of paternity leave following the birth of my second child (here's my first child one simulating a gravitational wave chirp). I didn't see the growing emails about the signal until early that afternoon when I thought I should try clearing out my inbox before returning to work the following day. It definitely made going back to work that bit more exciting. However, all the work to get the analyses of this event checked has slightly eaten into my main job, which is to search for gravitational waves from pulsars.

¹ Don't worry, gravity is very weak. The forces keeping your various constituent bits and bobs together are far more than enough to overcome any piddling gravitational wave that passes through you. Any displacement (stretching or squeezing) is only noticeable (if you have an exquisitely sensitive gravitational wave detector at least) between freely falling objects in the same local frame, i.e. if there are effectively no other external forces acting on the objects.
² Note that another detector called Virgo is also due to start taking data later this year, but wasn't operational at the time, and that a smaller (and unfortunately less sensitive, but still important) detector called GEO600 was operational, but not observing when the event passed by.
³ Prior to this direct detection (some quibbles over the "directness" of detection can be found here) many gravitational waves have obviously continuously been impinging on the Earth and passing through us, but this is the first time we've had the technology to catch one in the act.
⁴ There are very good reasons why they've not been seen until now (basically boiling down to us not having been able to build sensitive enough detectors), but that hasn't made us any less impatient.
⁵ The signal was first "detected" about three minutes after it arrived by online analysis software that looked for generic transient (short-duration) coherent signals, i.e. blips in the data that appeared at the same time in both detectors and looked similar. The first reasonably detailed estimates of the source parameters (i.e. that is was two black holes merging) were with us within about a day. After a couple of days we release our estimate of the location of the source in the sky and released it to selected astronomy groups to point their telescopes at. Following that full and proper detailed studies of the signal, and very careful checks on the performance of the detectors, have taken many months of painstaking work and placed great strain⁶ on the collaboration. But, given the general inertia that you get within a large collaboration (which we've experienced releasing results that didn't contain any signals) we've actually turned around a finished detection paper (after 14 draft iterations), and twelve(!) companion papers in remarkably short time.
⁶ This is a hilarious gravitational wave pun.

Disclaimer: everything on this blog is my own personal opinion and any mistakes are my own.

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?

Stepping off the ladder

After a long break from posts I thought I return with a proper science-based post. A few days ago a couple of friends of mine posted this very interesting paper on the pre-print arXiv:

Measuring a cosmological distance-redshift relationship using only gravitational wave observations of binary neutron star coalescences

Chris Messenger¹, Jocelyn Read

Detection of gravitational waves from the inspiral phase of binary neutron star coalescence will allow us to measure the effects of the tidal coupling in such systems. These effects will be measurable using 3rd generation gravitational wave detectors, e.g. the Einstein Telescope, which will be capable of detecting inspiralling binary neutron star systems out to redshift z=4. Tidal effects provide additional contributions to the phase evolution of the gravitational wave signal that break a degeneracy between the system's mass parameters and redshift and thereby allowing for the simultaneous measurement of both the effective distance and the redshift for individual sources. Using the population of O(10³-10⁷) detectable binary neutron star systems predicted for the Einstein Telescope the luminosity distance--redshift relation can be probed independently of the cosmological distance ladder and independently of electromagnetic observations. We present the results of a Fisher information analysis applied to waveforms assuming a subset of possible neutron equations of state. We conclude that the redshift of such systems can be determined to O(10%) for z>1 and in the most optimistic case accuracies of 2% can be achieved.

It's a very interesting paper as it could provide a way to get round a reliance that has been at the heart of various aspects of cosmology - the cosmic distance ladder (you might also want to read Phil Plait's description of this). If you want to know the distance of an object that is very far away you need to be able to measure some aspect of that object that has a well know relation to how far away it is. For example, if you have a 100 W light bulb that's 10m away you know that the flux (energy per second per unit area, or basically how bright it looks) that reaches you will be 100 W / (4π x (10 m)²) = 0.08 W/m². The same light bulb at twice the distance will be four times dimmer as you know that the flux falls off as the square of the distance - the famous inverse square law. In this case you know the actual power output of the bulb, so by measuring the flux that you receive you can easily work out the distance. Things are a bit more difficult for objects in space as unlike for a light bulb, which tells you its power on the box, they don't come with instructions! So a bit more inference is required to make some so called "standard candles" for astronomical distance measurements.

For the Sun, we know its distance very well and we can also measure its brightness, so if we see other stars (much further away) that look a lot like the Sun (things like their spectral type/colour can tell us this) then by measuring their brightness and assuming they emit the same power as the Sun we can infer their distance. There is another way of telling the distance to nearby stars (within a few hundred light years of us) called parallax, which involves geometry - this uses the fact that as the Earth orbits the Sun the position you observe a star at will shift with respect to distant background objects (like how a close object will appear to shift if you close one eye then the other). The size in the shift and the well know size of the Earth's orbit gives you the distance to the star via simple trigonometry. So for these stars you can measure their distance and brightness (giving their intrinsic luminosity or power ouput) and their spectral type. Again you can use these now well defined stars to calibrate a distance scale for further away stars of the same spectral type that are too far away to get a parallax.

These distance measurements work relatively nearby e.g. in our galaxy, but as you get further away it gets harder to resolve individual stars to get their brightness and spectral type. There is a special type of star called Cepheid variables that allow you to measure distances a bit further. These stars are intrinsically bright (you can see them at further distances) and they have a special property - they pulsate - and the period of the pulsation is related to their overall brightness. So, if you can measure the period of some Cepheids nearby where their distance can be measured by looking at other close stars then you can calibrate the relationship and then stretch the distance ladder out further. A type of event that is even brighter and can be seen out to even greater distances is a Type Ia supernova. These events occur when a white dwarf star (the Earth-sized remnant of a star like the Sun after it has run out of fuel to burn) accretes (gravitationally pulls) material off a companion star. White dwarfs are only stable if their mass is below about 1.4 times that of the Sun and if enough material is accreted to exceed this limit they will explode! As all these stars are the same mass when they explode it is assumed that they will all emit the same amount of power. So again, if you can measure some of these events nearby, where other distance measurements are valid, then they can be calibrated to use as a cosmological distance measurement.

This is called the distance ladder because you can see it relies on several steps. There are other things that need to be taken into account, for example extinction (the dimming of light as it passes through the extremely tenuous, but nevertheless present interstellar/intergalactic medium between us and a source), which makes things appear dimmer than the should be and could lead to their distance being overestimated; or the small possibility that the physics of Type Ia supernovae is be different over the course of the universe's history, so they won't all have the same intrinsic luminosity.

So, why is this important for cosmology? Well cosmology is all about working out the history and geometry of the universe, so how the universe evolves over time is essential to this. Edwin Hubble showed that the universe is expanding and the further away you look (from Cepheid distance measurements) the faster that expansion (as given by a redshift) is happening - know as Hubble's Law. [Redshift is causes by the Doppler effect - light, or any wave, will appear to have a longer wavelength (i.e. be redder) if its source is moving away from the observer than if they were stationary with respect to each other.] In Hubble's Law the relationship between the velocity and distance is constant, but working out the Hubble constant's value (or indeed whether it is constant) requires well known distances and redshifts. [Redshift can be measured by looking for certain features in the spectrum of a star and seeing how much they are shifted by, or by seeing how much the overall spectrum is shifted to the red.] This also relates to the geometry of the universe - is it flat, or curved on the largest scales? One of the major recent(ish - well late 90s) discoveries, using observations of Type Ia supernovae, showed that distant supernova appeared dimmer (e.g. further away) that would be expected from the simple Hubble relation, suggesting that the expansion of the universe has started accelerating in its relatively recent history (the last few billion years!)

The above discussion has talked about using electromagnetic radiation to work out the redshift vs. distance relation, but now we'll switch to the crux of the paper I mentioned which instead uses gravitational waves. The paper is trying to provide a way to get the distance vs. redshift relation without relying on the standard distance ladder of Cepheids and supernova. This could either independently confirm the current cosmological models, or show up systematic errors in the distance scale, both of which would be very important to know. The basis is that certain sources of gravitational waves, namely the inspiral of two neutron stars (the ultra dense remnants of massive stars the cores of which collapse during a supernova [described in a bit more detail here]), are "standard sirens" - like standard candles, but a sound-based analogy is generally thought more apt for these gravitational wave sources as they are in the audio frequency range of around 10-1000 Hz. In these systems the pair of neutron stars will orbit around each other gradually losing energy (and shrinking their orbit) via emission of gravitational waves - this was demonstrated to be true by the Nobel prize winning discovery by Hulse and Taylor of a pair of neutron stars behaving in exactly the way predicted. The amount of energy released via gravitational waves increases as the orbit decays, but still would only be observable with planned ground-based gravitational wave detectors in the very final stages of their evolution - when their orbits have decayed so much that they're about to collide and merge together. This signal is useful, because (as shown by Bernard Schutz) the strength and evolution of the signal gives the source distance directly, independently of the distance ladder - the way the frequency evolves gives the system's redshifted mass, whilst the amplitude contains a combination of the redshifted mass and distance, so the former can be used to get the systems distance from the latter.

Unfortunately the signal doesn't directly enode the source's redshift, it gives a combination the the source mass and redshift that cannot be unentangled, so independent measurements of the redshift are required using a regular telescope. Unlike regular telescopes gravitational wave telescopes are omnidirectional, which means they see the whole sky all the time (although their sensitivity is not equal over the whole sky due to their antenna pattern), but on their own they cannot pinpoint a source's position. You need multiple detectors to get positional information by working out the differences in a signal's arrival time between detectors and triangulating the source's position - and many detectors have been built to give such an array. However, even with several detectors the positional information is not great (even if you can pinpoint a small patch of sky, say the angular size of the moon (or about half a degree), it contains a lot of galaxies that could have been the home of the gravitational wave signal), so finding the home of the signal and measuring its redshift is difficult. In some cases if the gravitational wave signal is coincident with a gamma-ray burst then that helps localising the host with follow-up observations, but this will not be the case for all events.

The holy grail is therefore to find a way to get the distance and redshift information from the gravitational wave signal alone, and that is what the paper provides. How you do this comes about by including some extra physics in the how the orbits of the binary neutron stars decay. For the majority of the binary lifetime you can calculate the gravitational wave signal by assuming that the neutron stars are both point particles i.e. all their mass is in a single point. However, neutron stars are not point particles and there will come a point when they get so close that their very strong gravitational fields will start to distort each other and pull the stars apart. This distortion will be encoded in the gravitational wave signal and give information about the stars' masses. As noted before the combined mass and redshift can already be measured from the signal, so this other mass measurement can unentangle these and give you a redshift. So, voila! Distance and redshift can be measured all in one go.

Unfortunately (again!) there are some caveats. For example: the accuracy that you can measure the redshift depends on redshift - due to how the strength of the signal changes, and how the gravitational wave detectors work, there's a sweet spot at about redshift 2; the accuracy depends on what exactly the neutron star is made of (the star might be stiff or soft and therefore harder or easier to distort), and what a typical neutron star is made of needs to be know beforehand (currently there are many theoretical models, but no conclusive evidence for which is/are correct); and finally these measurements require a gravitational wave detector called the Einstein Telescope, which is currently only a design concept although hopefully will be built some time in the 2020s. It's still very interesting work though and I think the above caveats are by no means insurmountable in the not-to-distant future.

[Update: another similar interesting paper has also been recently posted.]

[Update: I should just note that the method works because the known neutron star radius can be used as a reference length in the waveform, which is kind of analogous to knowing the rest wavelength of a particular spectral line and therefore working out the redshift by measuring the difference from the observed wavelength.]

¹Chris was the instigator and lead guitarist in our famous bands Corpse Full of Bees and Look Up for Danger

Waiting for the envelop (sic)

Read this if you want to find out what's being keeping us gravitational wave types occupied for the last few months. At some point I might elaborate a bit more in this. (The title of this post will become clearer once you've read the link)

Science case

On Saturday night I got involved in a drunken discussion/argument with another astrophysicist over facilities for future astronomy. As I said it was a drunken argument (from my side at least - the other guy could have been completely sober) and I was probably not too coherent, got slightly tetchy and aggressive (sorry) and took it a bit more seriously than I should have, but I thought I'd go over the main thrust of the disagreement as I vaguely remember it.

In general I thought that funding the two major planned astronomical mega-observatories, the optical/infrared European Extremely Large Telescope (E-ELT) and the radio observatory the Square Kilometre Array (SKA), was a very good thing for advancing science. Whereas the other guy (TOG) thought that funding these two projects would drain too much money from smaller current telescope facilities, and they would be produce insufficient new science to be able to justify us losing access to these smaller class of telescopes. This is particularly prevalent in light of the recent funding crisis in which the UK astronomy funding agency (STFC) has been squeezing funding for various projects, in particular saying it maybe withdrawing UK membership/funding from things like UKIRT and Gemini. TOG was also worried (I think) that all ESO resources would be put into the E-ELT at the expense of their other facilities.

Science-wise I think TOG was being overly pessimistic. His basic premise was that these new telescopes wouldn't really be opening up any major new discovery space and they'd just allow us to confirm what we already know, but maybe with a bit more precision. A summary of the science case laid out for the E-ELT can be found here, and that for the SKA can be found here (with each chapter available for free on arXiv, e.g. here). These cases do obviously say a lot about what we can do to expand upon current knowledge (it's far easier to write a case based on what we currently know), and in fact I think their ability to do this gives a sufficient jump in sensitivity that the increase in science in these areas is very worthwhile [I'm going to cop out here and not give any specific examples, but see the above links]. However, I also think that there is a major new discovery space that will be opened up and lots of unknown stuff to find out.

The science case wasn't really the main thrust of TOGs argument thought - it was the funding side of things that was the main concern (you might be interested in telescoper's post about different people's ideas about what should be funded, and I think in this comment he show's similar concerns that TOG has). He said (if I remember correctly) he'd love to have access to an E-ELT/SKA for his research, but not necessarily at the cost of not having access to other facilities. No astronomer is going to have access to unlimited time on the E-ELT/SKA, so they'd obviously like to have access to other telescopes and be able to carry on doing productive science and there's a worry that this may not be possible in the future. I again thought that this was a bit too pessimistic, and maybe I'm being a bit naive, but I can't see how such a decimation of smaller facilities would be allowed - smaller class telescopes may have to find more novel ways of funding though (e.g. the LSST and ATA), or by partnership with smaller countries, or groups of universities. Importantly, I think that developing the E-ELT/SKA (via its pathfinders and precursors) is very important for providing technical innovations and pushing boundaries of observational techniques, which will feed back into making smaller class telescopes cheaper [maybe I have this the wrong way round, so someone can correct me] and able to do better science themselves.

I do have a skewed view of this kind of issue though. As a person working on gravitational wave observations we have a few (relatively expensive) detectors that create one data set for use by everyone (or for the moment at least those inside the collaborations that built and maintain the detectors), and don't have to compete for observing time on individual telescopes. I'm mainly making my decision based on my view that these new mega-observatories will produce far more novel science than the current technology is able to (and in small part that they look so cool). I might have different ideas if I felt I'd be unable to get new data myself due to limited observational chances on fewer telescopes, and consequently produce fewer papers and probably therefore have a diminished competitive edge in the academic jobs market. We do have some vested interest in having smaller class telescopes though in that when we see gravitational waves it's important to do optical follow-ups to get the most information about the sources. We're unlikely to be able to get the E-ELT and SKA to go quickly into a follow-up mode, but the smaller class telescopes will be vital.

Anyway, maybe I'm talking rubbish, what are your views?

[Update - on a related note this paper briefly reviews big versus small science/instrumentation in physics and astronomy.]

Can we make... an inappropriate cut scene?

I wasn't really planning on watching tonight's episode of Horizon, but I was sat in front of the TV and it came on after MasterChef, so I stayed seated and gave it a look. Now the reason I hesitate to watch Horizon is that I (like many people I've talked to) have got more and more frustrated with it over the last few years. It's gone from a respectable science program that was informative and inspiring, without being too stuffy and impenetrable, to a gimiky show that is more about fancy computer graphics, wierd camera work, controversial titles, and very little (and also badly presented) science content - this may be just the way you have to do things in TV these days (and I probably have a rose-tinted view of some of the way it used to be - in fact I generally preferred Channel 4's Equinox), but I doesn't seem a good method to present science to the public (and actually get them to be interested and learn something) to me.

Tonight's episode was entitled "Can We Make a Star on Earth?" and was presented by Prof. Brian Cox - the BBC's current go-to guy for physics related programming (he's already got at least a couple of other Horizon's under his belt). The program was essentially about investigations into using nuclear fusion as a future power source, as well as a fair amount of the background science leading up to fusion and our energy needs. I actually was rather surprised as I thought the program was pretty good, but with some reservations. The science content of the program covered a good deal of topics at a nice level and was generally well presented. The scientists that were interviewed were well chosen and did a good job explaining and enthusing about their subject. The experiments were impressively presented and you got a good idea of the challenges that they face. So the general things that I liked about how Horizon used to be were there. The problems came when they did pieces to camera - they just didn't work or add to the program. We don't need the bizarre camera angles and weirdly positioned shots (including strange almost subliminal close-ups of his food) and cut scences. We don't need the opening minutes of random pointless musings, which almost made me switch off. When Cox was just there as a narrator, or interviewing others, all was rather good and he can do it well, but when he's on screen they decide to take it too far. In reality I don't really think Horizon requires having a presenter - a good narrator can convey all the information as well as adding awe, excitement and perspective about the science being presented.

Seeing as I know exactly how the show should be I think the BBC should ask me to take over. I'm sure it would be easy... ;)

Beam me up

The LHC turns on tomorrow for its first major test run with proton's circling around one of the two rings. As there's only going to be one beam there should be actually be no collisions just yet. This is still quite exciting as it should test out a lot of the systems needed to ramp the whole thing up to a proper operational level. I don't want to pander to the sensationalist media that seems to have been getting the public unduly concerned about it destroying us all, but I do want to add another word or two as to why it won't do that - particle physicists are not maniacal Bond villians! They do not want to destroy the world or participate in its destruction through anything they do. They really like living (and skiing - CERN is pretty near the Alps you see), and anyway discovering something cool and new really isn't worth shit if you've not there to see that you've discovered it (or if you can't go skiing anymore!) If they (the thousands of physicists who've worked on the project, and even more of their colleagues at universities and labs around the world) thought there was any chance of the LHC doing anything bad then they wouldn't do it. So let me reiterate the LHC is perfectly safe and there is no chance at all of it so much as harming a fly (well a fly could get irradiated if it was stupid enough to go near the beam tube or one of the detectors while it running, but then it would be its own darn fault).

The main thing I'm worried about tomorrow is if something technical goes wrong like some of the superconducting magnets failing as that could set things back a while, and we've already been waiting ages for it to come online. But while your waiting to see if these first tests are successful and things are running smoothly you can go and play at being a particle physicist yourself here.

Changing priorities

A while back I briefly posted about the release of STFC's programmatic review in which it ranked the priorities of varies particle physics and astronomy research projects (much, much more can be found all over the internet e.g. here and here.) The programmatic review was based on the advice of the Physics, Astronomy & Nuclear Physics Science Committee (PPAN) which was made up of a relatively small group of physicists and astronomers. This process was highly criticised by the wider UK physics and astronomy community for not being informed by a much broader consultation of experts. So for the last few months the priorities have been reassessed by a variety of ad hoc panels with the aim of giving a more truthful representation of where the community believes the UK's priorities should lie (although even this process has not been entirely well received, due to it's untimely and hurried nature.) Obviously it would be nice, and most profitable scientifically, to see everything being given a high priority, but with the current climate some hard choices have to be made. For the projects receiving high to medium priorities I think things should be reasonably safe funding-wise at least until the government's next comprehensive spending review, however it does leaves the projects with the lowest priority facing the very harsh reality of having funding completely pulled. The reason I'm writing this post is that yesterday the reports from these ad hoc consultation panels were released. I've not read all of them, but from this BBC article it's seems that things are in pretty similar shape priority-wise to how they were after the programmatic review. However, various things like eMerlin and some solar projects seem to have been slightly bumped up the list (although I think it's mainly a result of another tier being added to the bottom of the priority scale and they've been moved off the bottom one.) The one report I have looked at is that for Astro-particle physics, in which gravitational wave research lives, and it seems to have been kept at the highest level of prioritisation. According to the report there had been some grumbling from the wider community that there were too many high-to-medium priority gravitational wave projects in the list, but I think the report answers the that criticism well. For the relatively small level of funding that we (the UK) put into gravitational wave research (a few million pounds) we get a disproportionately large amount back i.e. we have access to all the data from the current GEO600, LIGO (a billion dollar scale project!) and Virgo detectors, as well as that which will come from the next generation of detectors. Also the potential reward for being the gravitational wave game is very high. People may well disagree and cite the fact that nothing's been detected yet, but I think these are very valid reasons to keep up the funding of these projects. Of course I would say this as it's my field of research. These reports aren't yet the final outcome though and we'll have to wait until 8^th July until the final version of the programmatic review is published.

Budgetary constraints

Way back at the end of 2006 I wrote about a project I'd been working on about searching for gravitational waves from the Crab pulsar (see that post here for more links and details) and how that work had lead to an exciting new result i.e. that we beat pre-existing limits on the amount of energy that could be released via gravitational radiation from the Crab pulsar as it's rotation slows down. Since then we've added more to the search and firmed up the result and I (and others) have presented it at several meetings (e.g. at the AAS) , but we now finally have the paper submitted to the pre-print arXiv (you won't be able to see it until Sunday evening unless you have the secret password.) In my previous post I wasn't able to say what the result was (we'd embargoed the exact numbers until our collaborations internal review process was finished), but now I can reveal them. The main result is that we can say that less than 4% of the energy available from the slowing down of the Crab pulsar is radiated away via gravitational waves. This is a result that it's not currently possible to get via methods other than direct gravitational wave observations, because electromagnetic observations and models of the Crab pulsar and it's surrounding nebula (which is powered by the pulsar's slow-down) are full of too many uncertainties and assumptions that they can't give a good constraint on the energy budget i.e. you can't accurately add up all the possible mechanisms which could contribute to the Crab pulsar slowing down. There should be a press release about this on Monday, so keep your eyes peeled.

[Update: The University of Glasgow's press release can be found here.]

[More updates: The paper has been accepted for publication in The Astrophysical Journal Letters, which is pretty prestigious :)]

Top priority

Despite the current funding problems at STFC it seems that the future's looking pretty good for the UK gravitational wave community. Yesterday saw the release of STFC's programmatic review (found here), which informs how funding gets distributed. In it the ground-based gravitational wave detector GEO600, and the upgrade to the LIGO detectors, Advanced LIGO, (which are a very large part of what the IGR group at Glasgow works on) were given highest proirity, and the future space-based detector LISA and its precursor technology testing mission LISA Pathfinder (also a major part of the Glasgow work) were given medium-high priority. The review is now in a three week consultation period, in which the Particle Physics and Astronomy community will discuss whether this is the way they want to go forward. Inevitably there will be people trying to argue their projects position up the priority list, which means that they'll also be people trying to argue that some of the things in the high priority section shouldn't be there, but in general things are looking pretty rosy for gravitational waves.

[Update: I should point out that despite there appearing to be two gravitational wave projects in the "High priority" category (Advanced LIGO and GEO600), these in reality should be just one project under the heading of "Ground-based gravitational wave detectors". They are part and parcel of the same thing and not two separate funding entities.]

Funding press release

Today the STFC gave this press release on the current status of their funding crisis (see here and here for a detailed break-down of the past few months events with regards to the UK astronomy and particle physics communities respectively) following the discussion at the council meeting at the end of January. There's not really much new information in the press release and more info on the councils priorities probably won't be known until the end of February. It does seem however that current levels of funding are going to be safe until the outcome of the Wakeham review (a "fundamental review of the strength and role of UK physics") are known. The STFC chief's Keith Mason and Peter Warry still can't help but put a positive spin on the crisis though, which was annoying to start with, but becomes more and more frustrating each time you hear it.

Cut it out

Just in case you're someone that reads this blog and aren't one of my friends who've already posted the link on their own site, go here to sign a petition about the snafu at STFC. In the words of the petition creator:

"Due to cost overruns the UK's funding agency for particle physics and astronomy, STFC, is recouping £80M with deep cuts to UK physics operations in these areas. These include ending the UK's involvement in the International Linear Collider - the next generation of particle physics experiment. This risks relegating the UK to second tier involvement in future research and critically damaging the country's standing within the community. Furthermore UK Astronomy will be seriously hit with up to a 25% cut in grants. This is incompatible with the government's stated aim of making Britain a world leader in science. A review of this decision has recently been announced and we urge the Prime Minister to press for another solution to this problem before UK physics is set back by decades."

I won't add my own thoughts on this as the internet is already awash with opinion.

[Update - it seems things aren't all a bed of roses in the US either!]

Is it coincidence?

For those interested in gravitational wave detection you may be excited to note that as of this morning at 00:28 UTC the three LIGO detectors have taken one years worth of triple coincidence data. What this means is that there is now one years worth of overlapping data from all three detectors. Overlapping data is essential when hunting for unmodelled short duration bursts of gravitational waves (e.g. from a supernova or GRB), as seeing an event in multiple detectors gives you confidence that the event is real (one detector would never be enough for you to rule out that an event wasn't just some random instrumental glitch, or environmental contamination) and allows you to get some positional information about the event. The current LIGO science run (S5 - meaning the fifth run of data taking) had this one year target as its main goal, also the detectors have generally been at their design sensitivities, meaning there's lots of nice high quality data for us to search through.

Beyond funding

I've just got this news via cosmicvariance. The US National Academy of Science has just released its recommendations to NASA and the DoE for the Beyond Einstein program - see here for the press release. The basic conclusion they came to is that a Joint Dark Energy Mission should be first off, given that it will be using pretty much proven technologies and a mission could be easily put together, whereas the space-based gravitational wave detector LISA requires more technological testing before they could place it top of the list for recommending. It does, however say that LISA should become the flagship mission in the future. What all this means in practical terms for the US side of LISA funding I'm not sure? LISA has been pushed back with regards to a potential launch date due to technological and funding issues for ages, but this could lead to yet more delays (although not entirely unexpected.) This is obviously a bit of a disappointment for a gravitational wave researcher like me, and I honestly think that the science return from LISA would be more than one of the JDEM missions could give - I mean they're only trying to understand what potentially makes up 70% of the universe, so who cares about that? ;) But seriously LISA has the potential to do very good cosmology as well as provide loads of other fundamental astrophysics with possible completely new discoveries being made - it just has a little bit more risk attached to pull off the mission, but many people are working very hard to make it as sound as possible. As Sean at cv notes maybe ESA should consider finding a more reliable partner for LISA - China, Japan, India what are your thoughts?

[Update: Apparently, according to my boss, the US recommendations are pretty much a match for the timescale that ESA was planning for LISA anyway, so they shouldn't really cause any significant extra delays. Not so bad really then.]

Do you feel lucky

I'm not quite sure why it was BBC website front page news (although I'm glad it was and is pretty cool especially if you're astronomiclly inclined), but there's an interesting story about a couple of research groups produing high resolution images from ground based telescopes. The reason that this is cool is that normally from the ground the resolution of the images you can produce (i.e. the ability the distinguish between separate objects - like stars - or pick out detail) is limit by the turbulent motions of the atmosphere. Patches of air with different denisties along the line of sight to the object will bend (refract) the light (like light passing through a glass prism) by different amount leading to the image at the telescope jittering around and varying in intensity (basically the same as the twinkling of stars when you look at them with you eye). To image faint objects you need fairly long camera exposure times; these variations in the atmosphere happen on far shorter timescales, so the jittering/brightness variations of the source will smear it out on the final image. In general there's a rule which says that the bigger a telescope you have the better resolution you can get, so an 8.2m telescope like one of those at the VLT should be able to tell two objects seperated by ~0.02 arcseconds apart (i.e. two objects seperated by about 30 metre on the moon), but is in practice limited to a resolution of ~0.5 acrseonds by the atmosphere - that doesn't mean that bigger telescopes on the ground aren't better than smaller ones as they still collect more light and can therefore see fainter objects.) To get round the effects of the atmosphere the Hubble Space Telescope was built, which was able to achieve its full theoretical resolution of ~0.5 acrseonds with its 2.4m mirror, however Hubble was very expensive and is hard to maintain - being in space and all - so people have been trying to think of way to get around the atmospheric effect with ground based telescopes.

The article above talks of two ways of doing this, which have both seperately been around for a few years (the adaptive optics idea for longer), but seem to have finally been used together. The first idea is that of adaptive optics, which basically monitor the effect of the atmospheric distortions on the image and then corrects for these by applying and opposite distortion to one of the secondary mirrors in the telescope thereby correcting for the atmospheric effects. The monitoring and corrections have to be performed on a millisecond timescale. The second idea, which is now also making use of very efficient CCD cameras, is Lucky imaging. This basically comprises of taking lots and lots of photos of the object with short exposure times (hence the need for the very efficient cameras, so as to catch as much light as possible in a short time.) Some of these lucky images will have been taken when the atmospheric distotions were small, so you keep these and thow away the bad ones. You can then stack up the lucky images to help build up a stronger image. It's actually rather simple!

Anyway that was a rather unexpected astronomy post and the main reason I started it was so that I could show this cool movie of the Crab pulsar (which I do research on) taken using Lucky Imaging - you can see the flashing star with a bright pulse and then a fainter interpulse as radiation beamed from the star's poles intercept Earth once per rotation - the image is slowed down from the actual rotation rate of 30 times per second.

Show it like it is!

This autumn the US TV channel CBS will be airing a new sitcom called "The Big Bang Theory" based on geeky male physics types trying to deal with matters relating to the opposite sex:-

"The Big Bang Theory" is a comedy from the Emmy Award nominated Co-Creator and Executive Producer of "Two and a Half Men" Chuck Lorre, about brainy best friends Leonard (Johnny Galecki, "Roseanne") and Sheldon (Jim Parsons "Judging Amy"), who can tell you anything you want to know about quantum physics, but when it comes to dealing with everyday life here on earth they're lost in the cosmos. Neither fully understands that scientific principles don't always apply in matters of the heart - until they meet their sexy new neighbor Penny (Kaley Cuoco, "8 Simple Rules...") on "The Big Bang Theory." Penny is a friendly screenwriter/waitress from the midwest who also happens to be newly single. She quickly makes an impression on the other members of Leonard and Sheldon's geek squad on "The Big Bang Theory:" Howard Wolowitz (Simon Helberg, "Studio 60 on the Sunset Strip"), who portrays himself as the Casanova of Cal Tech, and fellow whiz kid Rajesh Koothrappali (Kunal Nayyar, "NCIS"), who is rendered speechless around anyone unprepared to converse about the Theory of Relativity or other scientific jargon.

The chemistry between this gaggle of geniuses and a delightful damsel is about to undergo a stimulating series of inter-personal experiments on "The Big Bang Theory."

Obviously much hilarity is bound to ensue from all the whacky situations that these nerds get themselves into as they try to woo their attractive neighbour (you can see a clip here.) The promo blurb for this show is critiqued here and doesn't exactly leave you anticipating great things from the show (however, to be fair, in the comments section of the linked blog post one of the shows producers does make a good effort at defending it, and no-one's actually seen it yet to really judge it.) I just wonder whether it can be as bad as Supernova!

The last post (from the APS)

This will be my last post from the APS meeting. I'll just take a moment comment on a few more of the talks I've seen.

One of the most entertaining talks of the meeting was on Selling Physics to Unwilling Buyers given by Lawrence Krauss (of The Physics of Star Trek fame). Krauss is well known for his public understanding of science work and is very good at it due to his enthusiasm and understanding of the problems that the general public have with science. He doesn't shy away from saying that the vast majority of people think that science is dull, difficult and that scientists are untrustworthy. But he thinks that the apprehensions are things that you can get past by showing science in its true light. We need to show that science isn't some abstract thing that has no impact on people's lives, but is in fact integral to pretty much every area of life, and basic science is in fact easy to demonstrate and understand. Also people need to be shown that by applying the basics of scientific, or experimental principles, they are able apply a better filtering of ideas or views that are obvious crap from those which have a weight of evidence. I can't do justice to all the things he said here, but the main new thing I came away with was that I should use more video clips in public talks.

At the moment I'm sitting in the morning plenary talks session and have heard a couple of very good talks. The first talk was by Jacqueline Hewitt from MIT and was about looking from a 21 cm background from neutral Hydrogen in the dark ages. The 21 cm line (the wavelength of the electromagnetic wave) is a radio frequency emission line from neutral Hydrogen (i.e. an electron bound to a proton), and the dark ages represent a period in the universe between the production of the CMBR (i.e. the time at which protons and electrons first are able to stay bound within Hydrogen atoms, without being disassociated by the radiation field) and the reionisation of that Hydrogen by radiation from the first stars after they form. I'd not really heard much on this subject before and was very interested to here about the methods for going about trying to detect this emission. I'll keep an eye on this stuff in the future as they should be able to learn a lot about structure formation in the early universe from such observations.

The next talk in the plenary session was about global warming by James Hansen - I think it's probably more useful to hear this stuff from a scientist who works on the subject than from Al Gore. I've never really been confronted (or looked out myself) the evidence for human induced global warming, so this talk was very enlightening. He showed how the Earth's climate (temperature, sea levels, greenhouse gas levels) has fluctuated by large amount over periods of hundred of thousands of years and the reasons for this. The main point being that these changes are very gradual and the feedback mechanisms of the Earth take a long time to respond to them, whereas now the man-made release of greenhouse gases is well over and above natural level and the planet can't deal with these as part of the natural long term trends. The idea that short term variations in the solar flux have lead to the current warming don't seem to hold up much weight after you've seen the long term trends. He proposed many of the main ideas to deal with energy consumption and emission problems, but obviously these need governments to introduce proactive policies or nothing will get done.

Anyway I need to make a move and head to the airport soon. Next post will be back from blighty.

More science news

Other interesting talks I saw yesterday involved dark matter and GR.

In this dark matter session I saw a talk on Noble liquid dark matter detectors and axion detection. The talk on Noble liquid dark matter detectors was by Richard Gaitskell of Brown University. This man is the most English man ever! He puts Hugh Grant to shame. The talk was littered with many utterances of "Damn!" and general foppish bumbling, in the endearingly charming way that only a true Englishman can pull off. It was an entertaining talk describing the how you can look for neutron recoils caused by WIMPs (a term for a variety of dark matter candidates) within various Noble gas detectors. The other talk in this session that I saw was about axion detection by Dave Tanner of the University of Florida (who some of us at Glasgow, and within the gravitational wave community will know). The axion is another dark matter candidate. It was proposed as a solution to the CP problem in various weak interactions (you expect interactions to be symmetric i.e. if you take a reaction and mirror it [swap the charges, parities and directions of the particles involved] then you expect things to look the same, but there are certain interaction involving the weak force where charge conjugation and parity are not symmetric [imagine walking towards a mirror, but your reflection moves away from the mirror - that would be a strongly broken symmetry]). You can't detect the axion itself, but it will decay into two photons although with a decay time much greater than the age of the universe. If you want to try and detect this decay you have to induce the decay. The talk was about the axion detector ADMX, which aims to detect axions forming a cold dark matter halo around the Milky Way. It does this by using a microwave cavity to try and induce the axions to decay. One of the most interesting ideas talked about was a future plan to generate and detect axions using optical cavities within large magnetic fields. You create axions from the photon field on one side of a wall, this axions flow through the wall and are then converted back to photons on the other side - you essentially are "shining light through a wall"! It was very interesting.

The final talk I saw yesterday was a nice historical perspective on the development and use of General Relativity. It was a very entertaining talk, with lots of anecdotes, which showed that even the great minds who worked on the foundations of GR could make mistakes.

Later today I give my own talk, so I should give it a run through at some time to make sure I can stay on time (I've only got 10 minutes).

Science report

In a shock to all my loyal reader's I'm going to talk about science and comment on some of the talks I've seen here at the APS meeting. Yesterday afternoon I went to a talk given by Geoff Burbidge of Burbidge, Burbidge, Fowler and Hoyle (or B²FH as it is commonly referred to in the astronomy community) fame - this was one of the seminal works on stellar nucleosynthesis i.e. the creation of heavy elements within stars. The paper was published in 1957 so Burbidge was giving a lecture in a session dedicated to the anniversary of it. As you'll probably gather from the paper being 50 years old Burbidge himself is getting on a bit. He's originally English, but has lived in the US for many years and has an accent that fluctuates between the two as he talks. The start of his talk was a good historical perspective of his field, which is always interesting to here from one of the people who was around at it's very start. However, for the majority of the talk he moved onto discussing his many problems with current cosmology. You see he is one is the dwindling minority (a fact he indeed joked about) who, like his more famous co-author Fred Hoyle, still hold onto some form of a steady state universe. Part of his theory relies on the CMB being explained by the synthesis of Helium from Hydrogen, rather than from the hot fireball Big Bang model. Although he makes his points well, some of his conclusions do seem to be rather clutching at straws to hold onto a model that pretty much all evidence points against. It was a very entertaining talk in all.

Today I have seen some very nice talks on the proposed Dark energy missions SNAP, Destiny and ADEPT. The first talk was by Saul Perlmutter, the lead author on the original Supernova cosmological acceleration results (supernova observations seemed to show that the universe is currently undergoing an accelerating expansion caused by an as yet unknown dark energy). He gave a very nice, and understandable, overview of the scientific objectives of SNAP (Supernova Acceleration Probe). Another of the talks was by Daniel Eisenstein of the University of Arizona describing the science objectives of ADEPT, which goes about the dark energy problem in a rather difference way that the supernova observations. This is instead looking for Baryon Acoustic Oscillations (BAO). This requires looking at the large scale distribution in space of galaxies at different redshifts. Now my knowledge of BAOs was very limited before this talk, but it was all very well explained and I came out feeling quite well informed on the subject. It basically says that there should be a naturally preferred distance scale between galaxies set down by the speed of sound propagation, and therefore matter density propagation, in the early universe. You look for these preferred scales by looking at the distribution of galaxies, and indeed they are seen. I shouldn't be too enthusiastic about these missions though as they're in direct competition against the gravitational wave mission LISA for being the first Beyond Einstein mission to be funded - of course it would be nice if they could all be done.

I also saw a talk this morning by a promising young physicist by the name of Jennifer Watson, of the University of Edinburgh, but currently working at SLAC. I learned all about the B → k^*ll decay process and how you can distinguish it from the charmonium decay. Fascinating stuff ;)

More news tomorrow maybe.