How high are those mountains now?

A few years ago I wrote about searches undertaken (by myself, others in the LIGO Scientific Collaboration, the Virgo Collaboration, and a selection of pulsar astronomers) looking for gravitational waves from a selection of pulsars using data from the initial science runs of the LIGO and Virgo detectors. Earlier this week we released the latest search for gravitational waves from 200 pulsars using data from LIGO's first observing run (O1) during the advanced detector era (after a complete upgrade of the detectors). The main result is that no compelling evidence for gravitational waves was found from any of the pulsars, but the results still provided some interesting highlights. A summary of the paper, and main results, can be found here, and I reproduce it below:

The recent detection of gravitational waves from merging binary black hole systems (GW150914 and GW151226) has now opened up the exciting new field of gravitational wave astronomy. However, the signals from these black hole binaries were transient and were only observable in our LIGO detectors for the last second, or less, before they merged. The sources were also more than a billion light years' distant, and so lay far beyond our own Milky Way Galaxy. The signals displayed a characteristic "chirping" form, in which the frequency, or pitch, and amplitude increased over the short duration of the signal. But, we are also searching for signals that could look quite different from that chirp pattern - for example sources that are constantly "on" (i.e. continuous) and emitting at an almost fixed (or monochromatic) frequency, like a pure audio tone.

One possible source of gravitational waves that could be much more local and would provide a continuous signal is a rapidly-rotating compact object known as a neutron star. We have known these objects exist ever since they were first observed as pulsars. They are the collapsed cores of massive stars that have run out of fuel and undergone a supernova explosion. With a mass of slightly more than the Sun's (∼2.8×10³⁰ kg) packed into a sphere of radius ∼10 km, neutron stars are about 40 trillion times denser than lead. A teaspoon of neutron star material would weigh about 10 million metric tons, roughly equivalent to the weight of a small mountain on Earth. Neutron stars are also spinning very rapidly, and for some their surfaces are rotating at up to ∼10% of the speed of light. So, these stars are very extreme objects! Our best understanding is that their pulsed emission comes from beams of radiation emanating from the magnetic poles of the stars acting like a lighthouse. If the magnetic and rotation axes of the neutron star are not aligned then a pulse is observed as the radiation beam sweeps across the Earth once per rotation.

To generate gravitational waves a pulsar must have some non-symmetric distortion in its shape - e.g. a "mountain" - that is not along its rotation axis. There are various ideas about how such a distorted star could form. The distortion could be "frozen" into the crust or core of the star after it was born in the supernova, or formed from material falling onto the star, or be produced and maintained through extremely large internal magnetic fields. However, due to the huge gravitational pull at the star's surface, the material forming the "mountain" needs to be really strong so as not to be flattened out. A mountain on Earth made of jello, for example, could not get very big before collapsing under its own weight, but one made of solid rock can become as large as, or larger than, Everest. For a pulsar with a crust made up of "normal" neutron star material (highly distorted atomic nuclei, free electrons and neutrons) the maximum deformation that could be sustained before collapsing is about 10 cm, so not very high for a "mountain" (scaling up the relative dimensions this would be equivalent to a ∼50 m hill on Earth). If the star was made up from more exotic materials, e.g. if it were a solid quark star, then it could possibly sustain a "mountain" up to ∼10 m in height. The "mountain" size can also be expressed in terms of the star's ellipticity (ε), which is a rough measure of the distortion's size as a fraction of the star's radius.

Making a few reasonable assumptions we can estimate the maximum amplitude of gravitational waves being emitted by most pulsars. To do this we use the law of conservation of energy. Pulsars are seen to slow down (known as spin-down) over time. This spin-down takes a very long time, and even the most rapidly spinning-down objects only decrease in frequency by less than a hundredth of a Hertz (or equivalently, increase their periods by less than ten microseconds) over a year. But, given the huge moment of inertia that the stars possess, this still represents a very large loss in rotational energy, corresponding to a power of ∼10³¹ Watts, or well over ten thousand times the Sun's luminosity. If we assume that all of this energy is being lost by emission of gravitational waves we can calculate the amplitude with which we would observe those waves at the Earth. This is called the "spin-down limit". When our searches are sensitive enough to reach below this limit we start probing interesting new territory, where gravitational wave signals could be detectable. We do, however, know that the spin-down limit is a naive upper limit, in that a large part of the spin-down can also be attributed to other mechanisms, such as magnetic dipole radiation.

Just as with the "lighthouse" model for their electromagnetic emission, gravitational wave signals are expected at a frequency related to the rotation rate, typically at twice this value. There are just over 430 known pulsars (see the Australia Telescope National Facility pulsar catalogue) spinning fast enough for their gravitational wave emission to be in the sensitive frequency band of the current Advanced LIGO detectors (∼20 to 2000 Hz). Our previous searches in LIGO and Virgo gravitational wave observatory data looked for gravitational wave signals from 195 pulsars, with the spin-down limit being surpassed for two of them (the Crab and Vela pulsars).

In this new analysis we have searched for a total of 200 of these pulsars using data from the first observing run of the Advanced LIGO detectors. To help reach the best sensitivity we have used information about these pulsars obtained through radio and gamma-ray observations; these have provided very precise knowledge of the pulsars' positions, rotational frequencies, and how their frequencies change over time. This information has allowed us to accurately track any potential gravitational wave signal in our data over the whole length of the three-month science run (a search method called "coherent integration").

From these searches we were not able to detect evidence for gravitational radiation from any of the pulsars. But, we have produced the most sensitive upper limits yet, and for eight pulsars our observations have produced limits (using three largely independent statistical methods) on the gravitational wave amplitude that are below the spin-down limits. Two of the pulsars that have long been of interest for gravitational wave searches, due to their large spin-downs, are the Crab and Vela pulsars. These are the pulsars for which we have now surpassed the spin-down limit by a factor of 20 and 10 respectively. From this we can say that, respectively, less than ∼0.2% and 1% of their spin-down energy loss is due to gravitational radiation. We can also limit the ellipticity (roughly speaking the relative deformation, or "mountain" size, compared to the star's total size) of the stars, and say that there are no "mountains" on the Crab pulsar greater than ∼10 cm in height, and none on Vela greater than ∼50 cm. Among the other pulsars, we found 32 more that are within a factor of ten of the spin-down limit. From the gravitational wave data alone we can limit the "mountain" size for some of these to less than ∼0.1 mm, although the spin-down limit is more stringent for those pulsars.

Future science runs of the Advanced LIGO and Advanced Virgo gravitational wave detectors will provide even greater sensitivity for known pulsar searches, and, at the very least, should allow us to surpass the spin-down limits for ten-or-more sources. Searches are also underway for continuous signals that are not associated with any currently known pulsars. These have to search the whole sky and a broad range of frequencies and spin-down values, making the searches computationally intensive, but opening up the possibility of observing previously unknown objects.

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?

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.

How high are pulsar "mountains"?

Just a quick post to highlight a paper that I (and others in the LIGO Scientific Collaboration and Virgo Collaboration, and a selection of radio, X-ray and gamma-ray pulsar astronomers) have been working on recently. The paper gives the most recent results from the search for gravitational waves from pulsars using data from the LIGO and Virgo gravitational wave detectors. A summary of the results from this search can be found here, but I also reproduce it below (see the link for the result plots from the paper):

Einstein's General Theory of Relativity predicts that the motion of masses can lead to the emission of gravitational radiation, commonly called gravitational waves. These waves, which are distortions in the fabric of space-time, ripple out from their sources at the speed of light. Far away from the source their effect is tiny. The distortions from even the strongest sources (which are some of the most violent events in the Universe) stretch and squeeze the distance between any objects they pass by a fractional amount (called the strain) of order 10^-23. That is equivalent to a change in distance between the Earth and the Sun of just a few times an atomic radius! However, scientists have built detectors, based on laser interferometry, to perform very high precision distance measurements that are capable of measuring these extremely small distortions. In the US there are two such detectors called the Laser Interferometer Gravitational-wave Observatory (LIGO), in Italy there is the Virgo detector and in Germany there is the GEO600 detector. These are operated, and their data analyzed, by hundreds of scientists from across the world as part of the LIGO Scientific Collaboration and Virgo Collaboration.

An artist's impression of a pulsar. Image credit: Michael Kramer (JBCA, Unversity of Manchester).

One of the ways we are taking advantage of the fantastic sensitivity of these detectors is to search for continuous gravitational waves from pulsars. Pulsars were first observed in 1967 at the University of Cambridge by the radio astronomers Jocelyn Bell and Antony Hewish. They are neutron stars, which are the collapsed cores of massive stars that have run out of fuel and gone supernova (up until this discovery they had just been theoretical objects first proposed by Walter Baade and Fritz Zwicky in 1934). They are very rapidly spinning, with rotation periods ranging from a few seconds to a few milliseconds, so their surfaces are rotating at up to ∼10% of the speed of light! With a mass of slightly more than the Sun (∼2.8×10³⁰ kg) packed into a sphere of radius ∼10 km, they are about 40 000 billion times denser than lead (this is equivalent to squashing the entire population of the Earth into a thimble). They also have magnetic fields a billion to a few thousand billion times that of the Earth. So, these are very extreme objects! The pulsed emission comes from beams of radiation emanating from the magnetic poles of the stars acting like a lighthouse. If the magnetic and rotation axes are not aligned then pulses are observed as the radiation beam sweeps across the Earth once per rotation.

To generate gravitational waves a pulsar must have some non-symmetric distortion that is not along its rotation axis, i.e. a "mountain". This distortion could have been: frozen into the crust or core of the star after it was born in the supernova; formed from material falling onto the star; or, be produced and maintained though extremely large internal magnetic fields (larger even than the external fields described above). However, due to the huge gravitational field at the star's surface the material forming the "mountain" needs to be really strong to not be flattened out (a mountain on Earth made of jello would not get very big before collapsing under its own weight, but one made of solid rock can become as large as, or larger than, Everest). For a pulsar with a crust made up of normal neutron star material the maximum deformation that could be sustained before collapsing is about 10 cm, so not very high for a "mountain" (scaling up in height only this would be equivalent to a ∼50 m hill on Earth). If the star was made up from more exotic materials, e.g. if it were a solid quark star, then it could possibly sustain a "mountain" up to ∼10 m in height. The "mountain" size can also be expressed in terms of the star's ellipticity (ε), which is a measure of its size as a fraction of the star's radius.

Making a few reasonable assumptions we can estimate the maximum amplitude of gravitational waves being emitted by most pulsars. To do this we use the law of conservation of energy. Pulsars are seen to slow down (spin-down) over time. This spin-down takes a very long time, and even the most rapidly spinning-down objects only decrease in frequency by less than a hundredth of a Hertz (or equivalently, increase their periods by less than ten microseconds) over a year. But, given the huge moment of inertia of the stars this still represents a very large loss in rotational energy, corresponding to a power of ∼10³¹ Watts, or well over ten thousand times the Sun's luminosity. If we assume that all of this energy is being lost by emission of gravitational waves we can calculate the amplitude with which we would observe them at Earth. This is called the "spin-down limit". If we can achieve detector sensitivities that allow searches to reach below this limit then we are probing interesting new territory, where gravitational wave signals could be detectable.

There are just over 350 pulsars (see the Australia Telescope National Facility catalog) spinning fast enough for their gravitational wave emission to be in the sensitive frequency band of the current detectors (∼20 to 2000 Hz). We have searched for a total of 195 of these pulsars using data from the LIGO, Virgo and GEO600 science runs, with the most up-to-date results for 179 of them coming from the most recent LIGO S6 and Virgo VSR2 and VSR4 runs. To help reach the best sensitivity we have used information about these pulsars obtained through radio, X-ray and gamma-ray observations; these have provided very precise knowledge of the pulsars' frequencies, positions and how their frequencies change over time. This information has allowed us to accurately track any potential signal in our data over the whole length of the science run (called coherent integration).

From these searches we were not able to detect evidence for gravitational radiation from any of the pulsars. But, we have produced the most sensitive upper limits yet, and for seven pulsars we are starting to probe an interesting regime within a factor of five of the spin-down limit. For the Crab pulsar and Vela pulsar we have surpassed the spin-down limit. From this we can say that, respectively, less than ∼1% and 10% of their spin-down energy loss is due to gravitational radiation. We can also say that there are no "mountains" on the Crab pulsar greater than ∼1 meter, and none on Vela greater than ∼10 meters. Among the other pulsars, we found eight more within a factor of ten of the spin-down limit. From the gravitational wave observations alone we can limit the "mountain" size for some of these to less than ∼1 mm, although the spin-down limit is more stringent for those pulsars.

When the current upgrades to the LIGO and Virgo detectors are complete we expect to be able to beat the spin-down limit for many more pulsars. This includes pulsars where we could limit the maximum mountain size to less than a few tenths of a millimeter! It also means we will be in a regime where we can make the first direct detections of gravitational waves from pulsars.

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

Newly discovered pulsar

Today it was announced that Einstein@home has made it's first discovery (a public version of the paper can be found here) - unfortunately not a gravitational wave signal (as it was initial designed to solely search for), but a radio pulsar spinning at 41Hz with the snappy title of J2007+2722. This is still very exciting news as it's "...the first time an astronomical object has been discovered by this kind of distributed-computing project,". In brief pulsars are neutron stars, which are the ultra dense, rapidly rotating, remnants of stars several times more massive than our Sun leftover after they have ended their normal life via a supernova explosion.

The Einstein@home project was set up in 2005 as a distributed computing effort (like the more well known SETI@home, which has a screen saver that searches for extraterrestrial life in radio data) to make use of the public's spare compute cycles to search for gravitational waves from pulsars using data from the LIGO gravitational wave detectors. It's since become one of the largest distributed computing projects there is. The sensitivity of data from the LIGO detectors is currently such that the chances of Einstein@home finding a gravitational waves from an unknown pulsar are quite slim (although more sensitive data in the next few years will give far higher chances), so it was decided a couple of years ago to turn some of Einstein@home's computing power towards searching radio data for pulsars.

Surveys with large radio telescopes are the prime way of finding pulsars (although some can also be seen in other parts of the electromagnetic spectrum) - radio data from these surveys is searched to look for regularly spaced pulses, although these can be weak and the pulse time of arrivals at the telescope will be dispersed over different observation radio frequencies. The spacing of the pulses will also change due to the Doppler effect as the Earth revolves and orbits the Sun, or also if the pulsar is itself in orbit around another star in a binary system. For pulsars in extreme orbits, where the objects are very close together and circling each other with periods of minutes to hours (the current smallest binary orbital period for a pulsar is about 2 hours), the Doppler effect can be very large and cause the pulse spacing to change rapidly. Standard radio pulsar search techniques, which assume that the pulse spacing is only slowly varying, have a hard time time finding these objects. The Arecibo radio telescope has been conducting many surveys over the last few years, but there hasn't been the computing power to exhaustively search this data for these extreme binary pulsar systems. It is data from these surveys that has now been passed to Einstein@home, which is able to use it's large computing power to search for many different sizes of changing pulse spacing, included the rapid changes caused by the extreme systems.

Since starting searches for radio pulsars in Arecibo data with Einstein@home it has been able to find almost 120 pulsars that had previously been known about (although none are in extreme binary systems). However, this new announcement is for a pulsar that had not previously been known about - Einstein@home was the first search to find it! Using this initial discovery they were then able to get follow-up observations using the Green Bank radio telescope to confirm the pulsar signal and further study it. The pulsar itself isn't the most exciting object - it's not in a binary system, but it is reasonably rare as it's an isolated recycled pulsar. A recycled pulsar is one that has been "spun-up" from a slow spin-rate (probably about 1Hz, or one rotation per second) to a much faster rate by accreting material from a companion star (gravitationally pulling material from the other star onto itself). The only way for a pulsar to have a rotation rate as fast as this newly discovered pulsar is either for it to have been "recycled", or for it to still be spinning fast after it was born - this pulsar is slowing down it's rotation rate very slowly indicating it has a weak magnetic field, which generally is expected to not be the case for young, newborn, pulsar i.e. it must be an old, and therefore recycled, pulsar. However, as we saw above for a star to be recycled it must have had a binary companion, which from looking at the pulse spacings we know is not the case for this pulsar - so what's up? There are other known recycled pulsars that are isolated, i.e. not in a binary system, and it is thought that the system must have been disrupted due to the companion star going supernova and kicking it's own remnant out of the systm.

Anyway, this is the first discovery and it's a great boost to the Einstein@home project. Hopefully this will get more people to sign up. It should lead to more pulsar discoveries (maybe at the rate of a couple per year) and possibly some in extremely fast orbits. Ultimately we obviously hope that Einstein@home will also give us some gravitational wave discoveries.

Happy Birthday Hubble

Today is the 20th birthday (of sorts) of the Hubble Space Telescope. It was launched on 24th April 1990 in the Discovery Space Shuttle, although it wasn't actually deployed until the 25th and it's first light image was released on 20th May 1990. It had some teething problems to start with (a flawed mirror that blurred images meaning that the telescope could not be used at it's maximum resolution), but was specifically designed so that it could be serviced by astronauts and fixes applied. Since the fix it has been one of the most iconic and most importantly scientifically productive instruments ever (a "top 10" of its major discoveries can be seen here, although there are many others). Several servicing missions have gone on to improve its scientific capabilities by adding new detectors, and also greatly extending its lifetime. It also takes a great picture or two (certainly prettier than anything we'll be able to produce by observing gravitational waves), which has helped endear it to the public and make it some famous.

These images have been released to celebrate it's birthday, and hopefully they'll be a lot more to come before it finally closes its eye.


Source: Hubblesite.org

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.]