## What kind of thing is GW190814?

Posted in The Universe and Stuff with tags , , , , , , , on June 24, 2020 by telescoper

There’s been a lot of interest in the past day or two over an event that occurred in the LIGO detectors last August, entitled GW190814. A paper has appeared declaring this to be “the observation of a compact binary coalescence involving a 22.2–24.3 M  black hole and a compact object with a mass of 2.50–2.67 M “. That would be interesting of course because the smaller object is smaller than the black holes involved in previous detections and its mass suggests the possibility that it may be a neutron star, although no electromagnetic counterpart has yet been detected.  It’s a mystery.

I was quite excited when I saw the announcement about this yesterday but my enthusiasm was dampened a bit when I saw the data from the two LIGO detectors at Hanford and Livingston in the USA and the Virgo detector in Italy.

Visually, the Livingston detection seems reasonably firm, but the paper notes that there were thunderstorms in the area at the time of  GW190814 which affected the low-frequency data. There doesn’t look like anything at all but noise in the Virgo channel. The Hanford data may show something but, according to the paper, the detector was “not in nominal observing mode at the time of GW190814” so the data from this detector require special treatment. What you see in the Hanford channel looks rather similar to the two (presumably noise) features seen to the left in the Livingston plot.

I know that – not for the first time – I’m probably going to incur the wrath of my colleagues in the gravitational waves community but I have to sound a note of caution. Before asking whether the event involves a black hole or a neutron star you have to be convinced that the event is an event at all.  Fortunately, at least some of the data relating to this have been released and will no doubt be subjected to independent scrutiny.

Now I’m going to retreat into my bunker and hide from the inevitable comments…

## Results from the Event Horizon Telescope

Posted in Astrohype, The Universe and Stuff with tags , , on April 10, 2019 by telescoper

Following yesterday’s little teaser, let me point out that there is a press conference taking place today (at 2pm Irish Summer Time, that’s 3pm Brussels) to announce a new result from the Event Horizon Telescope. The announcement will be streamed live here.

Sadly, I’m teaching at the time of the press conference so I won’t be able to watch, but that doesn’t mean that you shouldn’t!

I’ll post pictures and comments when I get back. Watch this space. Or you could watch this video..

UPDATE: Well, there we are. Here is the image of the shadow’ of the event horizon around the black hole in M87:

The image is about 42 micro arcseconds across. I guess to people brought up on science fiction movies with fancy special effects the image is probably a little underwhelming, but it really is an excellent achievement to get that resolution. Above all, it’s a great example of scientific cooperation – 8 different telescopes all round the world. The sizeable European involvement received a substantial injection of funding from the European Union too!

Other parameters are here:

The accompanying EU press release is here. Further information can be found here. The six publications relating to this result can be found here:

## Gravitational Redshift around the Black Hole at the Centre of the Milky Way

Posted in The Universe and Stuff with tags , , , , , , on July 26, 2018 by telescoper

I’ve just been catching up on the arXiv, and found this very exciting paper by the GRAVITY collaboration (see herefor background on the relevant instrumentation). The abstract of the new paper reads:

The highly elliptical, 16-year-period orbit of the star S2 around the massive black hole candidate Sgr A* is a sensitive probe of the gravitational field in the Galactic centre. Near pericentre at 120 AU, ~1400 Schwarzschild radii, the star has an orbital speed of ~7650 km/s, such that the first-order effects of Special and General Relativity have now become detectable with current capabilities. Over the past 26 years, we have monitored the radial velocity and motion on the sky of S2, mainly with the SINFONI and NACO adaptive optics instruments on the ESO Very Large Telescope, and since 2016 and leading up to the pericentre approach in May 2018, with the four-telescope interferometric beam-combiner instrument GRAVITY. From data up to and including pericentre, we robustly detect the combined gravitational redshift and relativistic transverse Doppler effect for S2 of z ~ 200 km/s / c with different statistical analysis methods. When parameterising the post-Newtonian contribution from these effects by a factor f, with f = 0 and f = 1 corresponding to the Newtonian and general relativistic limits, respectively, we find from posterior fitting with different weighting schemes f = 0.90 +/- 0.09 (stat) +\- 0.15 (sys). The S2 data are inconsistent with pure Newtonian dynamics.

Note the sentence beginning Over the past 26 years…’!. Anyway, this remarkable study seems to have demonstrated that, although the star S2 has a perihelion over a thousand times the Schwarzschild radius of the central black hole, the extremely accurate measurements demonstrate departures from Newtonian gravity.

The European Southern Observatory has called a press conference at 14.00 CEST (13.00 in Ireland and UK) today to discuss this result.

## Newsflash: another LIGO detection!

Posted in The Universe and Stuff with tags , , , on June 1, 2017 by telescoper

I’ve just heard the news that  LIGO has just announced the detection of another gravitational-wave signal, which has been given the identifier GW170104; it was detected on 4th January 2017.

The event was the merger of a black-hole binary system a redshift z=0.2, which is a proper distance of about 800 Mpc in the standard cosmological model, the most distant event yet detected. There are also tantalising hints that at least one of the black holes had spin opposite the orbital angular momentum, which implies it may have originated in a globular cluster. For more details please see the refereed paper.

If you’d rather just look at the plot here is the evidence for the event, in the form of coincident signals at the two components of LIGO:

I reckon there’s a good chance of seeing members of the Cardiff University Gravitational Physics group celebrating in the pub later this evening!

It’s also a reasonable inference given the rate of detection of these events so far that we’re going to see many more in the very near future!

## Formation of black holes in the dark [HEAP]

Posted in The Universe and Stuff with tags , on September 28, 2016 by telescoper

Given the title of my website I could hardly resist reblogging this arXiver post. I’m not an expert on Black Hole (BH) formation, so would be interested to hear opinions on how plausible is this idea that massive BHs might form via implosion rather than following a Supernova explosion.

http://arxiv.org/abs/1609.08411

A binary black hole (BBH) with components of 30-40 solar masses as the source of gravitational waves GW150914 can be formed from a relatively isolated binary of massive stars if both BHs are formed by implosion, namely, by complete or almost complete collapse of massive stars with no energetic SNe accompanied by a sudden mass loss that would significantly reduce the mass of the compact objects, and in most cases unbind the binary system. BBHs can also be formed by dynamical interactions in globular clusters, if the BHs are formed with no energetic SNe that would kick the BHs out from the cluster before BBH formation. Besides, if BHs of ~10 solar masses as in the source GW151226 are formed by implosion, the formation of BBHs would be prolific, and their fusion would make an important contribution to a stochastic gravitational wave background. Theoretical models set mass ranges for…

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## Making Massive Black Hole Binaries Merge

Posted in The Universe and Stuff with tags , , , , , on February 16, 2016 by telescoper

Many fascinating questions remain unanswered by last week’s detection of gravitational waves produced by a coalescing binary black hole system (GW150914) by LIGO. One of these is whether the fact that the similarity of the component masses (29 and 36 times the mass of the Sun respectively) is significant.

An interesting paper appeared on the arXiv last week by Marchant et al. that touches on this. Here is the abstract (you can click on it to make it larger):

Although there is some technical jargon, the point is relatively clear. It appears that very masssive, very low metallicity binary stars can evolve into black hole binary systems via supernova explosions without disrupting their orbit. The term ‘low metallicity’ characteristises stars that form from primordial material (i.e. basically hydrogen and helium) early in the cycle of stellar evolution. Such material has very different opacity properties from material with significant quantities of heavier elements in it, which alters the dynamical evolution considerably.

(Remember that to an astrophysicist, chemistry is extremely simple. Hydrogen and helium make up most of the atomic matter in the Universe; all the rest is called “metals” including carbon, nitrogen, and oxygen…. )

Anyway, this theoretical paper is relevant because the mass ratios produced by this mechanism are expected to be of order unity, as is the case of GW150914.  One observation doesn’t prove much, but it’s definitely Quite Interesting…

Incidentally, it has been reported that another gravitational wave source may have been detected by LIGO, in October last year. This isn’t as clean a signal as the first, so it will require further analysis before a definitive result is claimed, but it too seems to be a black hole binary system with a mass ratio of order unity…

You wait forty years for a gravitational wave signal from a binary black hole merger and then two come along in quick succession…

## LIGO: Live Reaction Blog

Posted in The Universe and Stuff with tags , , , , on February 11, 2016 by telescoper

So the eagerly awaited press conference happened this afternoon. It started in unequivocal fashion.

“We detected gravitational gravitational waves. We did it!”

As rumoured, the signal corresponds to the coalescence of two black holes, of masses 29 and 36 times the mass of the Sun.

The signal arrived in September 2015, very shortly after Advanced LIGO was switched on. There’s synchronicity for you! The LIGO collaboration have done wondrous things getting their sensitivity down to such a level that they can measure such a tiny effect, but there still has to be an event producing a signal to measure. Collisions of two such massive black holes are probably extremely rare so it’s a bit of good fortune that one happened just at the right time. Actually it was during an engineering test!

Here are the key results:

Excellent signal to noise! I’m convinced! Many congratulations to everyone involved in LIGO! This has been a heroic effort that has taken many years of hard slog. They deserve the highest praise, as do the funding agencies who have been prepared to cover the costs of this experiment over such a long time. Physics of this kind is a slow burner, but it delivers spectacularly in the end!

You can find the paper here, although the server seems to be struggling to cope! One part of the rumour was wrong, however, the result is not in Nature, but in Physical Review Letters. There will no doubt be many more!

And right on cue here is the first batch of science papers!

No prizes for guessing where the 2016 Nobel Prize for Physics is heading, but in a collaboration of over 1000 people across the world which few will receive the award?

So, as usual, I had a day filled with lectures, workshops and other meetings so I was thinking I would miss the press conference entirely, but in the end I couldn’t resist interrupting a meeting with the Head of the Department of Mathematics to watch the live stream…

P.S. A quick shout out the UK teams involved in this work, including many old friends in the Gravitational Physics Group at Cardiff University (see BBC News item here) and Jim Hough and Sheila Rowan from Glasgow. If any of them are reading this, enjoy your trip to Stockholm!

## That Big Black Hole Story

Posted in The Universe and Stuff with tags , , , , , , , , on February 28, 2015 by telescoper

There’s been a lot of news coverage this week about a very big black hole, so I thought I’d post a little bit of background.  The paper describing the discovery of the object concerned appeared in Nature this week, but basically it’s a quasar at a redshift z=6.30. That’s not the record for such an object. Not long ago I posted an item about the discovery of a quasar at redshift 7.085, for example. But what’s interesting about this beastie is that it’s a very big beastie, with a central black hole estimated to have a mass of around 12 billion times the mass of the Sun, which is a factor of ten or more larger than other objects found at high redshift.

Anyway, I thought perhaps it might be useful to explain a little bit about what difficulties this observation might pose for the standard “Big Bang” cosmological model. Our general understanding of galaxies form is that gravity gathers cold non-baryonic matter into clumps  into which “ordinary” baryonic material subsequently falls, eventually forming a luminous galaxy forms surrounded by a “halo” of (invisible) dark matter.  Quasars are galaxies in which enough baryonic matter has collected in the centre of the halo to build a supermassive black hole, which powers a short-lived phase of extremely high luminosity.

The key idea behind this picture is that the haloes form by hierarchical clustering: the first to form are small but  merge rapidly  into objects of increasing mass as time goes on. We have a fairly well-established theory of what happens with these haloes – called the Press-Schechter formalism – which allows us to calculate the number-density $N(M,z)$ of objects of a given mass $M$ as a function of redshift $z$. As an aside, it’s interesting to remark that the paper largely responsible for establishing the efficacy of this theory was written by George Efstathiou and Martin Rees in 1988, on the topic of high redshift quasars.

Anyway, this is how the mass function of haloes is predicted to evolve in the standard cosmological model; the different lines show the distribution as a function of redshift for redshifts from 0 (red) to 9 (violet):

Note   that the typical size of a halo increases with decreasing redshift, but it’s only at really high masses where you see a really dramatic effect. The plot is logarithmic, so the number density large mass haloes falls off by several orders of magnitude over the range of redshifts shown. The mass of the black hole responsible for the recently-detected high-redshift quasar is estimated to be about $1.2 \times 10^{10} M_{\odot}$. But how does that relate to the mass of the halo within which it resides? Clearly the dark matter halo has to be more massive than the baryonic material it collects, and therefore more massive than the central black hole, but by how much?

This question is very difficult to answer, as it depends on how luminous the quasar is, how long it lives, what fraction of the baryons in the halo fall into the centre, what efficiency is involved in generating the quasar luminosity, etc.   Efstathiou and Rees argued that to power a quasar with luminosity of order $10^{13} L_{\odot}$ for a time order $10^{8}$ years requires a parent halo of mass about $2\times 10^{11} M_{\odot}$.  Generally, i’s a reasonable back-of-an-envelope estimate that the halo mass would be about a hundred times larger than that of the central black hole so the halo housing this one could be around $10^{12} M_{\odot}$.

You can see from the abundance of such haloes is down by quite a factor at redshift 7 compared to redshift 0 (the present epoch), but the fall-off is even more precipitous for haloes of larger mass than this. We really need to know how abundant such objects are before drawing definitive conclusions, and one object isn’t enough to put a reliable estimate on the general abundance, but with the discovery of this object  it’s certainly getting interesting. Haloes the size of a galaxy cluster, i.e.  $10^{14} M_{\odot}$, are rarer by many orders of magnitude at redshift 7 than at redshift 0 so if anyone ever finds one at this redshift that would really be a shock to many a cosmologist’s  system, as would be the discovery of quasars with such a high mass  at  redshifts significantly higher than seven.

Another thing worth mentioning is that, although there might be a sufficient number of potential haloes to serve as hosts for a quasar, there remains the difficult issue of understanding precisely how the black hole forms and especially how long it takes to do so. This aspect of the process of quasar formation is much more complicated than the halo distribution, so it’s probably on detailed models of  black-hole  growth that this discovery will have the greatest impact in the short term.

## From Darkness to Green

Posted in History, The Universe and Stuff with tags , , , , , , , , , , on March 7, 2014 by telescoper

On Wednesday this week I spent a very enjoyable few hours in London attending the Inaugural Lecture of Professor Alan Heavens at South Kensington Technical College Imperial College, London. It was a very good lecture indeed, not only for its scientific content but also for  the plentiful touches of droll humour in which Alan specialises. It was also followed by a nice drinks reception and buffet. The talk was entitled Cosmology in the Dark, so naturally I had to mention it on this blog!

At the end of the lecture, the vote of thanks was delivered in typically effervescent style by the ebullient Prof. Malcolm Longair who actually supervised Alan’s undergraduate project at the Cavendish laboratory way back in 1980, if I recall the date correctly. In his speech, Malcolm referred to the following quote from History of the Theories of the Aether and Electricity (Whittaker, 1951) which he was kind enough to send me when I asked by email:

The century which elapsed between the death of Newton and the scientific activity of Green was the darkest in the history of (Cambridge) University. It is true that (Henry) Cavendish and (Thomas) Young were educated at Cambridge; but they, after taking their undergraduate courses, removed to London. In the entire period the only natural philosopher of distinction was (John) Michell; and for some reason which at this distance of time it is difficult to understand fully, Michell’s researches seem to have attracted little or no attention among his collegiate contemporaries and successors, who silently acquiesced when his discoveries were attributed to others, and allowed his name to perish entirely from the Cambridge tradition.

I wasn’t aware of this analysis previously, but it re-iterates something I have posted about before. It stresses the enormous historical importance of British mathematician and physicist George Green, who lived from 1793 until 1841, and who left a substantial legacy for modern theoretical physicists, in Green’s theorems and Green’s functions; he is also credited as being the first person to use the word “potential” in electrostatics.

Green was the son of a Nottingham miller who, amazingly, taught himself mathematics and did most of his best work, especially his remarkable Essay on the Application of mathematical Analysis to the theories of Electricity and Magnetism (1828) before starting his studies as an undergraduate at the University of Cambridge which he did at the age of 30. Lacking independent finance, Green could not go to University until his father died, whereupon he leased out the mill he inherited to pay for his studies.

Extremely unusually for English mathematicians of his time, Green taught himself from books that were published in France. This gave him a huge advantage over his national contemporaries in that he learned the form of differential calculus that originated with Leibniz, which was far more elegant than that devised by Isaac Newton (which was called the method of fluxions). Whittaker remarks upon this:

Green undoubtedly received his own early inspiration from . . . (the great French analysts), chiefly from Poisson; but in clearness of physical insight and conciseness of exposition he far excelled his masters; and the slight volume of his collected papers has to this day a charm which is wanting in their voluminous writings.

Great scientist though he was, Newton’s influence on the development of physics in Britain was not entirely positive, as the above quote makes clear. Newton was held in such awe, especially in Cambridge, that his inferior mathematical approach was deemed to be the “right” way to do calculus and generations of scholars were forced to use it. This held back British science until the use of fluxions was phased out. Green himself was forced to learn fluxions when he went as an undergraduate to Cambridge despite having already learned the better method.

Unfortunately, Green’s great pre-Cambridge work on mathematical physics didn’t reach wide circulation in the United Kingdom until after his death. William Thomson, later Lord Kelvin, found a copy of Green’s Essay in 1845 and promoted it widely as a work of fundamental importance. This contributed to the eventual emergence of British theoretical physics from the shadow cast by Isaac Newton which reached one of its heights just a few years later with the publication a fully unified theory of electricity and magnetism by James Clerk Maxwell.

But as to the possible reason for the lack of recognition for John Michell who was clearly an important figure in his own right (he was the person who first developed the concept of a black hole, for example) you’ll have to read Malcolm Longair’s forthcoming book on the History of the Cavendish Laboratory!

## Black Hole Firewalls, etc.

Posted in Biographical, The Universe and Stuff with tags , , on September 2, 2013 by telescoper

Well, just back to the office after taking a restful weekend in Cardiff to recover from the trials and tribulations of the meeting of the Astronomy Grants Panel of the Science and Technology Facilities Council in Swindon last week. I’ve got a lot to catch up on, so I’ll just post this video which explains all about the issue of Black Hole Paywalls Fire Sales Firewalls about which there’s a not inconsiderable to-do and hoo-ha going on in the world of Physics. Pity they couldn’t put a firewall around Swindon, that’s all I can say…