vendredi 26 février 2016

Oh, gravitational waves, jingle, jingle in all the bands!

Dashing through space and time...
We show that the black hole binary (BHB) coalescence rates inferred from the advanced LIGO (aLIGO) detection of GW150914 imply an unexpectedly loud GW sky at milli-Hz frequencies accessible to the evolving Laser Interferometer Space Antenna (eLISA), with several outstanding consequences. First, up to thousands of BHB will be individually resolvable by eLISA; second, millions of non resolvable BHBs will build a confusion noise detectable with signal-to-noise ratio of few to hundreds; third – and perhaps most importantly – up to hundreds of BHBs individually resolvable by eLISA will coalesce in the aLIGO band within ten years. eLISA observations will tell aLIGO and all electromagnetic probes weeks in advance when and where these BHB coalescences are going to occur, with uncertainties of <10s and <1deg2 . This will allow the pre-pointing of telescopes to realize coincident GW and multi-wavelength electromagnetic observations of BHB mergers. Time coincidence is critical because prompt emission associated to a BHB merger will likely have a duration comparable to the dynamical time-scale of the systems, and is only possible with low frequency GW alerts
The multi-band GW astronomy concept. The violet lines are the total sensitivity curves (assuming two Michelson) of three eLISA configurations; from top to bottom N2A1, N2A2, N2A5 (from [11]). The orange lines are the current (dashed) and design (solid) aLIGO sensitivity curves. The lines in different blue flavours represent characteristic amplitude tracks of BHB sources for a realization of the flat population model (see main text) seen with S/N> 1 in the N2A2 configuration (highlighted as the thick eLISA middle curve), integrated assuming a five year mission lifetime. The light turquoise lines clustering around 0.01Hz are sources seen in eLISA with S/N< 5 (for clarity, we down-sampled them by a factor of 20 and we removed sources extending to the aLIGO band); the light and dark blue curves crossing to the aLIGO band are sources with S/N> 5 and S/N> 8 respectively in eLISA; the dark blue marks in the upper left corner are other sources with S/N> 8 in eLISA but not crossing to the aLIGO band within the mission lifetime. For comparison, the characteristic amplitude track completed by GW150914 is shown as a black solid line, and the chart at the top of the figure indicates the frequency progression of this particular source in the last 10 years before coalescence. The shaded area at the bottom left marks the expected confusion noise level produced by the same population model (median, 68% and 95% intervals are shown). The waveforms shown are second order post-Newtonian inspirals phenomenologically adjusted with a Lorentzian function to describe the ringdown.

...  in a three-arm LISA!
The observation of GW150914 brings unexpected prospects in multi-band GW astronomy, providing even more compelling evidence that a milli-Hz GW observatory will not only open a new window on the Universe, but will also naturally complete and enhance the payouts of the high frequency window probed by aLIGO. The scientific potential of multi-band GW astronomy is enormous, ranging from multimessenger astronomy, cosmology and ultra precise gravity tests with BHBs, to the study of the cosmological BHB merger rate, and to the mutual validation of the calibration of the two GW instruments. This is a unique new opportunity for the future of GW astronomy, and how much of this potential will be realized in practice, depends on the choice of the eLISA baseline. Should an extremely de-scoped design like the New Gravitational Observatory (NGO) [27] be adopted, all the spectacular scientific prospects outlined above will likely be lost. Re-introducing the third arm (i.e. six laser links) and increasing the arm-length to at least two million kilometres (A2) will allow observation of more than 50 resolved BHB with both eLISA and aLIGO, and the detection of the unresolved confusion noise with S/N> 30. We also stress that the most interesting systems emit at f > 10−2Hz, a band essentially ’clean’ from other sources. There, the eLISA sensitivity critically depends on the shot noise, which is determined by the number of photons collected at the detector mirrors. It is therefore important to reconsider the designed mirror size and laser power under the novel appealing prospect of observing more of these BHBs and with an higher S/N.
(Submitted on 22 Feb 2016)

Given its tremendous potential for fundamental physics and astrophysics, the European Space Agency (ESA) has selected the observation of the Universe at GW frequencies around one mHz as one of the three main science themes of the “Cosmic Vision Program” [42]. Indeed, a call for mission proposals for the “Gravitational Universe” science theme is expected for late 2016, and the L3 launch slot in 2034 has been reserved for the selected mission. The main candidate mission for this call (for which a decision will be made by 2018-19, so as to allow sufficient time for industrial production before the nominal 2034 launch date) is the evolving Laser Interferometer Space Antenna (eLISA) [43], named after the “classic LISA” concept of the late 90’s and early 2000s [44]. The eLISA mission concept consists of a constellation of three spacecraft, trailing the Earth around the Sun at a distance of about fifteen degrees. Each spacecraft will contain one or two test masses in almost perfect free fall, and laser transponders which will allow measurements of the relative proper distances of the test masses in different spacecraft via laser interferometry. This will allow the detection of the effect of possible GW signals (which would change the distance between the test masses). The most technically challenging aspect of the mission will be to maintain the test masses in almost perfect free fall. For this reason, a scaled-down version of one of eLISA’s laser links will be tested by the “LISA Pathfinder” mission. Pathfinder was launched by ESA in December 2015, and it will provide crucial tests of how well eLISA’s low frequency acceleration noise can be suppressed. 
... 
There are, however, other aspects to the eLISA mission that are yet to be evaluated and decided upon by ESA, within the constraints imposed by the allocated budget for the “Gravitational Universe” science theme. A “Gravitational Observatory Advisory Team” (GOAT) [45] has been established by ESA to advise on the scientific and technological issues pertaining to an eLISA-like mission. Variables that affect the cost of the mission include: (i) the already mentioned low-frequency acceleration noise; (ii) the mission lifetime, which is expected to range between one and several years, with longer durations involving higher costs because each component has to be thoroughly tested for the minimum duration of the mission, and may also require higher fuel consumption, since the orbital stability of the triangular constellation sets an upper limit on the mission duration and therefore achieving a longer mission may require the constellation to be further from the Earth; (iii) the length L of the constellation arms, which may range from one to several million km, with longer arms involving higher costs to put the constellation into place and to maintain a stable orbit and slowly varying distances between the spacecraft; (iv) the number of laser links between the spacecraft, i.e., the number of “arms” of the interferometer (with four links corresponding to two arms, i.e., only one interferometer, and six links to three arms, i.e., two independent interferometers at low frequencies [46]): giving up the third arm would cut costs (mainly laser power, industrial production costs), while possibly hurting science capabilities (especially source localization) and allowing for no redundancy in case of technical faults in one of the laser links.

 All the black-hole mergers we take?
Gravitational waves penetrate all of cosmic history, which allows eLISA to explore scales, epochs, and new physical effects not accessible in any other way (see figure {below}). Indeed a detectable gravitational wave background in the eLISA band is predicted by a number of new physical ideas for early cosmological evolution (Hogan, 2006, Maggiore, 2000). Two important mechanisms for generating stochastic backgrounds are phase transitions in the early Universe and cosmic strings. 
... the eLISA frequency band of about 0.1 mHz to 100 mHz today corresponds to the horizon at and beyond the Terascale frontier of fundamental physics. This allows eLISA to probe bulk motions at times about 3×10-18 – 3×10-10 seconds after the Big Bang, a period not directly accessible with any other technique. Taking a typical broad spectrum into account, eLISA has the sensitivity to detect cosmological backgrounds caused by new physics active in the range of energy from 0.1 TeV to 1000 TeV, if more than a modest fraction ΩGW of about 10-5 of the energy density is converted to gravitational radiation at the time of production.  
Various sources of gravitational wave background of cosmological origin are presented in detail in Binétruy et al.(2012)...

The observed (redshifted) frequency of wave-generating phenomena is shown as a function of cosmic scale factor a, with the present epoch at the right. The redshifted Hubble rate (horizon scale) is shown in black for a standard Grand Unified Theory (GUT) and a lower temperature Terascale (TeV) inflationary cosmology. Blue regions are accessible to electromagnetic (EM) observations: the Universe since recombination (right box) and cosmic microwave background (CMB) fluctuations (left box). The red bar shows the range of cosmic history accessible through eLISA from processes within the horizon up to about 1000 TeV.

One of the most promising science goals of the mission are supermassive black holes, which appear to be a key component of galaxies. They are ubiquitous in near bright galaxies and share a common evolution. The intense accretion phase that supermassive black holes experience when shining as quasi-stellar objects and active galactic nuclei erases information on how and when the black holes formed. eLISA will unravel precisely this information. Very massive black holes are expected to transit into the mass interval to which eLISA is sensitive along the course of their cosmic evolution. eLISA will then map and mark the loci where galaxies form and cluster, using black holes as clean tracers of their assembly by capturing gravitational waves emitted during their coalescence, that travelled undisturbed from the sites where they originated. On the other hand, middleweight black holes of 105M⊙ are observed in the near universe, but our knowledge of these systems is rather incomplete. eLISA will investigate a mass interval that is not accessible to current electromagnetic techniques, and this is fundamental to understand the origin and growth of supermassive black holes. Due to the transparency of the universe to gravitational waves at any redshift, eLISA will explore black holes of 105M⊙ – 107M⊙ out to a redshift z ≤ 20, tracing the growth of the black hole population.  
eLISa will also shed light on the path of black holes to coalescence in a galaxy merger. This is a complex process, as various physical mechanisms involving the interaction of the black holes with stars and gas need to be at play and work effectively, acting on different scales (from kpc down to 10-3 pc). Only at the smallest scales gravitational waves are the dominant dissipative process driving the binary to coalescence. eLISA will trace the last phase of this evolution. Dual active galactic nuclei (AGN), i.e. active black holes observed during their pairing phase, offer the view of what we may call the galactic precursors of black hole binary coalescences. They are now discovered in increasing numbers, in large surveys. By contrast, evidence of binary and recoiling AGN is poor, as the true nature of a number of candidates is not yet fully established. eLISA only will offer the unique view of an imminent binary merger by capturing its loud gravitational wave signal...  
Current electromagnetic observations are probing only the tip of the massive black hole distribution in the universe, targeting black holes with large masses, between 107M⊙ – 109M⊙ . Conversely, eLISA will be able to detect the gravitational waves emitted by black hole binaries with total mass (in the source rest frame) as small as 104 M and up to 107 M , out to a redshift as remote as z ∼ 20. eLISA will detect fiducial sources out to redshift z ≤10 with SNR ≥ 10 and so it will explore almost all the mass-redshift parameter space relevant for addressing scientific questions on the evolution of the black hole population. Redshifted masses will be measured to an unprecedented accuracy, up to the 0.1–1% level, whereas absolute errors in the spin determination are expected to be in the range 0.01–0.1, allowing us to reconstruct the cosmic evolution of massive black holes. eLISA observations hence have the potential of constraining the astrophysics of massive black holes along their entire cosmic history, in a mass and redshift range inaccessible to conventional electromagnetic observations.  
On smaller scales, eLISA will also bring a new revolutionary perspective, in this case relative to the study of galactic nuclei. eLISA will offer the deepest view of galactic nuclei, exploring regions to which we are blind using current electromagnetic techniques and probing the dynamics of stars in the space-time of a Kerr black hole, by capturing the gravitational waves emitted by stellar black holes orbiting the massive black hole. Extreme mass ratio inspirals (EMRI) detections will allow us to infer properties of the stellar environment around a massive black hole, so that our understanding of stellar dynamics in galactic nuclei will be greatly improved. Detection of EMRIs from black holes in the eLISA mass range, that includes black holes similar to the Milky Way’s, will enable us to probe the population of central black holes in an interval of masses where electromagnetic observations are challenging... 
General Relativity has been extensively tested in the weak field regime both in the solar system and by using binary pulsars. eLISA will provide a unique opportunity of confronting GR in the highly dynamical strong field regime of massive black holes. eLISA will be capable of detecting inspiral and/or merger plus ring-down parts of the gravitational wave signal from coalescing massive black holes binaries of comparable mass. For the nearby events (z ∼ 1) the last several hours of the gravitational wave signal will be clearly seen in the data, allowing direct comparison with the waveforms predicted by GR. The inspiral phase could be observed by eLISA up to a year before the final merger with relatively large SNR. Comparison of the observed inspiral rate with the predictions of GR will provide a valuable test of the theory in the regime of strong, dynamical gravitational fields. 
The merger of two black holes could be observed by eLISA throughout the Universe if it falls into the detector band. 
Pau Amaro-Seoane et al.
(Submitted on 17 Jan 2012)
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mercredi 24 février 2016

Taking (some) fun out of {go fast} scientific [reports]

Wet blanket policy...
Disclaimer: this first paragraph is just cheap entertaining and self-deprecating of the blogger, no offence to any real scientist of course!
The playful science popularizer Bob Burstard is looking for a freshsucker to swindle. Looking off in the distance, Bob sees a happy-go-lucky science enthusiast Alice Arxivpecker, minding the recent discovery of gravitational waves (GW) while whistling through internet. The cunning Burstard quickly types a draft report and polishes a catchy headline, causing Alice to web surf directly through his front page. He then tries to show her his theory to explain the Gamma-Ray Burst serendipitously detected 0.4 seconds after the GW event...

Then comes some serious information.

... on a Gamma ray burst associated with GW150914

The Fermi/GBM team reported a possible hard X-ray transient on 2015-09-14 at 09:50:45.8 UTC, about 0.4 s after the reported LIGO/Virgo burst trigger time, and lasting for about one second (Blackburn et al. 2015; Connaughton, V. et al 2016). The light travel time can introduce a time difference between INTEGRAL and Fermi detections of up to ±0.5 s, depending on the source position within the LVC error region. We do not observe any excess within a -0.5 s to +0.5 s window around the Fermi/GBM trigger (Figure 1), and set a 3-sigma upper limit of 1.5×10-7erg cm-2 in one second, assuming a typical short hard GRB, characterized by Band model with parameters α = −0.5, β = −2.5, Epeak = 1000 keV. A substantial part of the candidate event in the GBM comes from the high-energy BGO detector, above 100 keV (Blackburn et al. 2015), where the Fermi/GBM effective area is about a factor 30-40 smaller than that of the INTEGRAL/SPI-ACS... Finally, we stress that to compare the GBM and SPI-ACS sensitivities, it is inappropriate to use a soft spectral model as in the computation of our early flux upper limits (Ferrigno et al. 2015), since the spectral properties of the GBM candidate are very different.
V. Savchenko et al., (Submitted on 12 Feb 2016)



... a search of the INTEGRAL-ACS data revealed a detection rate of only 55% of GBM-detected weak short GRBs (Briggs et al., in preparation). We do not consider, therefore, the non-detection of GW150914-GBM by INTEGRAL-ACS, a sufficient reason to reject our candidate
V. Connaughton, et al. 
(Submitted on 11 Feb 2016 (v1), last revised 16 Feb 2016 (this version, v3))
... and the first evidence for nearly neutral black holes?
I have shown that for black hole - black hole (BH-BH) mergers, if at least one of the BHs carry even a small amount of charge Q, the inspiral process generates a loop circuit, which induces a magnetic dipole. The system behaves like a giant pulsar with an increasing wind power. If q can be as large as ∼ (10-5 − 10-4), the magnetospheric wind right before the coalescence would make a short-duration GRB. The GRB is expected to be delayed with respect to the GW chirp signal. The putative short GRB signal associated with GW 150914 (Connaughton et al. 2016) can be well interpreted with this theory. 
The nearly isotropic nature of this wind pulse suggests that every BH-BH merger could be associated with a short electromagnetic transient if Q is not strictly zero. The question is whether q can be large enough to make it observable. Given the large event rate of BH-BH mergers revealed by the detection of GW 150914 (Abbott et al. 2016b), the non-detection of very bright short GRBs already suggests that for the majority of BHs, q cannot significantly exceed 10-5. This is consistent with the general expectation that BHs are essentially not charged. Pessimistically, the weak short GRB signal detected by the Fermi/GBM team ... is a chance coincidence and non-physical. If so, the upper limit of a signal (Savchenko et al. 2016) would constrain q to be below a few 10-5 .
Bing Zhang (UNLV)
(Submitted on 15 Feb 2016 (v1), last revised 16 Feb 2016 (this version, v2))

//update February 26 2016
The physical constraints required by the association of the Fermi GBM signal contemporaneous with GW150914 - radiative power of 1049 erg.s-1 , and corresponding magnetic fields on the black hole of the order of 1012 Gauss - are astrophysically highly implausible. Combined with the relatively high random probability of coincidence of 0.22 percents, we conclude that the electromagnetic signal is likely unrelated to the BH merger. 
... requiring that the magnetic field is produced by electric charge on one of the black holes (Zhang 2016) would imply a charge Q = GM√(LEM)c5/2 = 5×1016 coulombs. This horrific amount of electric charge would have produced the electric field near the horizon E = √(LEM)c3/2/(GM)=2×1012 in cgs units (statvolts per centimeter), amounting to nearly 5% of the quantum Schwinger field EQ = me2c3/(ℏe).
Maxim Lyutikov (Purdue University)
(Submitted on 23 Feb 2016)
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mardi 23 février 2016

Plan(et) 9 from ou[te]r Spaceolar System

The man who killed planet nine ...
For those of a certain age, it will always be tough to accept that Pluto isn't a planet anymore, just as no adult Chicagoan can call the Sears Tower the "Willis Tower" without queasiness. We were taught from childhood to rank tiny Pluto, which circles the sun at an average distance of 3.5 billion miles, as every bit the equal of Neptune, Uranus, or even Jupiter. Thus the news, in 2006, that a bunch of killjoy astronomers had demoted it seemed like an insult to underdogs everywhere.  
Mike Brown, a professor of astronomy at Caltech, was the man who inadvertently caused this interplanetary crisis when he discovered Eris, an object even farther from the sun than Pluto, but considerably more massive. Briefly ballyhooed as the 10th planet, it soon provoked chaos within astronomy circles, calling into question the very meaning of the word "planet"—never precisely defined before—and finally taking Pluto down with it. Not for nothing did Mr. Brown name his discovery after the goddess of strife and discord.
By JAMES KENNEDY Updated Nov. 26, 2010 12:01 a.m.

... may have helped to revive it


Caltech researchers have found evidence of a giant planet tracing a bizarre, highly elongated orbit in the outer solar system. The object, which the researchers have nicknamed Planet Nine, has a mass about 10 times that of Earth and orbits about 20 times farther from the sun on average than does Neptune (which orbits the sun at an average distance of 2.8 billion miles). In fact, it would take this new planet between 10,000 and 20,000 years to make just one full orbit around the sun.
The researchers, Konstantin Batygin and Mike Brown, discovered the planet's existence through mathematical modeling and computer simulations but have not yet observed the object directly.
01/20/2016 Written by Kimm Fesenmaier

CREDITS: (DATA) JPL; BATYGIN AND BROWN/CALTECH; (DIAGRAM) A. CUADRA/SCIENCE

Let's call it Telisto ...
As far as the denomination of such a still undiscovered major body of the our planetary system, several names have become more or less popular over the years in either the specialized literature and in popular accounts; given the remarkable distance envisaged by Batygin & Brown (2016) for it, we reiterate the name Telisto [from τηλιστος ´ : farthest, most remote]. Nonetheless, in the following we will dub it PX or X for simplicity.
(Submitted on 16 Dec 2015 (v1), last revised 31 Jan 2016 (this version, v3))


... if it is located far enough?

The hunt for a planet X started in 1915 Lowell (1915), and some important limitations were provided by different approaches. The direct imaging survey of the far solar system by infrared space telescopes IRAS and WIZE did not detect any massive objects of Jupiter size inside 25000 AU and of Saturn size inside 10000 UA Luhman (2014). In 1993, Standish (1993) demonstrated that no anomalous residuals can be seen in the residual of the outer planet orbits. However since this work, no direct confrontation has been performed between the perturbations induced by a planet X on the orbits of the main planets of our solar system and the best fitted planetary ephemerides. The only estimates were made indirectly, based on the results of the ephemerides, but without refitting the whole parameters of these ephemerides (Iorio 2012, 2016). 
Since 1993, very accurate observations of Saturn orbit were obtained thanks to the tracking of the Cassini spacecraft during its exploration of the Saturnian system. As described in (Folkner et al. 2014), (Hees et al. 2014) and (Fienga et al. 2016), the ten year positions of Saturn allow significant improvement in our knowledge of Saturn’s orbit, as well as of Jupiter, Uranus, and Neptune orbits. These Cassini data have already been used very successfully to put some strict limits on the possibility of an anomalous Pioneer acceleration (Anderson et al. 2002; Fienga et al. 2010). Furthermore, thanks to the Cassini flyby of Jupiter, a supplementary accurate position of Jupiter is also used to build the ephemerides. Finally, the flyby of the New Horizons spacecraft of the Pluto-Charon system should bring some supplementary information and constraints for the existence of a super-Earth. 
We use here the dynamical model of the INPOP planetary ephemerides (Fienga et al. 2008, 2009, 2010, 2011, 2016) for testing the possibility of an additional planet, focussing on the proposed nominal planet P9 of Batygin & Brown (2016). In the dynamical model of INPOP planetary ephemerides, we add a super-Earth object of 10 M⊕ with different orbital characteristics, in agreement with the proposed orbit of P9. We then build new ephemerides including these objects and perform a global fit of the perturbed planet orbits to the whole data set that is used to construct the INPOP and DE430 JPL ephemerides (Folkner et al. 2014). 
Allowed zone for P9. The red zone (C14) is excluded by the analysis of the Cassini data up to 2014 (Sec.4). The pink zone (C20) is how much this zone can be enlarged by extending the Cassini data to 2020 (Sec.4). The green zone is the most probable zone for P9 (v ∈ [108◦ : 129◦ ]), with maximum reduction of the residuals at v = 117.8 ◦ (blue dot P9). The white zone is the uncertainty zone where the P9 perturbation is too faint to be detected.
The Cassini data provides an exceptional set of measures that acts as a very sensitive device for testing the possibility of an additional massive body in the solar system. With the data accumulated until 2014.4, we can exclude the possibility that P9 is in the section of the orbit depicted in red in Fig[ure above], with a true anomaly v in [−130◦;−100◦]∪[−65◦; 85◦]. We thus contradict the affirmation of Iorio (2016), who states that a body of 10 M⊕ is excluded if it resides closer to 1000 AU of the Sun. Iorio (2016) does not properly consider how much the presence of an additional body can be absorbed by the fit of all the other parameters in the solar system ephemerides. The global fit that we present here avoids this drawback. Moreover, we found that the presence of P9 could significantly decrease the Cassini residuals if v is in the interval [108◦;129◦], with a most probable position at v = 117.8◦+11-10. 
Since the Cassini data is at present the most precise sensor for testing the possibility of an additional body in the solar system, it is essential that Cassini continues to provide ranging data, since there will not be very soon an additional spacecraft around one of the planets beyond Saturn. Extending the Cassini data up to 2020 will already allow to state for the existence of P9 for v ∈ [−132◦;106.5◦]. Juno will soon arrive around Jupiter and will thus allow us to improve the orbit of Jupiter. This may not directly improve the constraints on P9, because Jupiter is less sensitive than Saturn to the perturbations of P9. Nevertheless, constraining Jupiter more tightly will allow us to improve the determination of P9, because less flexibility will exist for absorbing the perturbations due to P9.
(Submitted on 19 Feb 2016 (v1), last revised 22 Feb 2016 (this version, v2))

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samedi 13 février 2016

The real, the true and the plausible (or false, fake and not graduated)

//the title of this post has been slightly changed after the update on Feb. 22 2016

 A real blind-injection and fake signal of a direct detection of gravitational waves in 2010...

A rather strong signal was observed on September 16, 2010, within a minute or so of its apparent arrival at the detectors. The scientists on duty at the detector sites immediately recognized the tell-tale chirp signal expected from the merger of two black holes and/or neutron stars, and sprang into action. They knew that it could be a blind injection, but they also knew to act like it was the real thing. The event was beautifully consistent with the expected signal from such a merger. The figures below show the strength of the signal (redder colors indicate more signal power) in time (horizontal axis) and frequency (vertical axis). The signal sweeps upwards in frequency ("chirp") as the stars spiral into one another, approaching merger. The first plot is what was seen in the LIGO Hanford detector, and the second is what was seen at the same time in the LIGO Livingston detector. Despite apparent differences, the two signals are completely consistent with one another. The dark and light blue regions are typical of fluctuating noise in the detectors. 
The loudness of the signal was consistent with it coming from a galaxy at a distance between 60 and 180 million light-years from ours. The detector network is capable of locating the source in the sky only crudely; it seemed to be coming from the constellation Canis Major (the "Big Dog") in the southern hemisphere (the event was dubbed "the Big Dog" shortly thereafter). They sent alerts to partners operating robotic optical telescopes in the southern hemisphere (ROTSE, TAROT, Skymapper, Zadko) and the Swift X-ray space telescope, all of which took images of the sky on that and/or subsequent days in the hope of capturing an optical or X-ray "afterglow".


Blind Injection" Stress-Tests LIGO and VIRGO's Search for Gravitational Waves 2010

Versus the first true signal or false blind-injection five years later...
On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0×10-21. It matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203 000 years, equivalent to a significance greater than 5.1σ. The source lies at a luminosity distance of 410+160-180 Mpc corresponding to a redshift z=0.09+0.03-0.04. In the source frame, the initial black hole masses are 36+5-4M⊙ and 29±4M⊙, and the final black hole mass is 62±4 M⊙, with 3.0±0.5M⊙ c2 radiated in gravitational waves. All uncertainties define 90% credible intervals. These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first direct detection of gravitational waves and the first observation of a binary black hole merger.
                      Signal at LIGO Hanford Observatory            Signal at LIGO Livingston Observatory
B. P. Abbott et al.* 
(LIGO Scientific Collaboration and Virgo Collaboration)
(Received 21 January 2016; published 11 February 2016)

Gravitational waves (GW) ground-based instruments are all-sky monitors with no intrinsic spatial resolution capability for transient signals. A network of instruments is needed to reconstruct the location of a GW in the sky, via time-of-arrival, and amplitude and phase consistency across the network [102]. The observed time-delay of GW150914 between the Livingston and Hanford observatories was 6.9+0.5-0.4 ms. With only the two LIGO instruments in observational mode, GW150914’s source location can only be reconstructed to approximately an annulus set to first approximation by this time-delay [103, 104]. .. the sky map for GW150914 ... corresponds to a projected 2- dimensional credible region of 140 deg2 (50% probability) and 590 deg2 (90% probablity). The associated 3-dimensional comoving volume probability region is ∼10-2 Gpc3; for comparison the comoving density of Milky Way-equivalent galaxies is ∼107 Gpc-3. This area of the sky was targeted by follow-up observations covering radio, optical, near infra-red, X-ray, and gamma-ray wavelengths that are discussed in [105]; searches for coincident neutrinos are discussed in [106].


(Submitted on 11 Feb 2016)

//Update February 22 2016
Compared to a not graduated but plausible second astrophysical signal

Sixteen days of coincident data were used in the analysis of GW150914. This event was by far the most significant found in all transient searches performed. The compact binary coalescences search identified the second most interesting event on the 12th of October 2015. This trigger [designated Ligo-Virgo Trigger 151012] most closely matched the waveform of a binary black hole system with masses 23+18-5M⊙ and 13+4-5M⊙, producing a trigger with a false-alarm rate of 1 event per 2.3 years; far too high to be a strong detection candidate...  
We performed similar in-depth checks of potential noise sources for this trigger. For LIGO-Livingston data, LVT151012 is in coincidence with significant excess power at 10Hz lasting roughly three seconds, a portion of which can be seen in {the} figure {above}. There is no obvious indication of upconversion to the frequency range analyzed by the transient searches, so the low frequency noise is not thought to have caused the signal associated with LVT151012 in the Livingston detector. The data around this event were found to be significantly more non-stationary than those around GW150914. The noise transient rate in the hours around LVT151012 was significantly higher than usual at both LIGO detectors... This was likely due to increased low frequency ground motion associated with ocean waves [55]. The elevated noise transient rate at both sites induced a higher rate of background triggers around the time of LVT151012.

(Submitted on 11 Feb 2016 (v1), last revised 16 Feb 2016 (this version, v2))

... based on two different searches, with different models for the rates at which both noise triggers and astrophysical signals appear in the LIGO detectors, we find posterior probabilities 0.84 and 0.91 that LVT151012 is of astrophysical origin; this is the only other trigger from either search that has probability greater than 50% of being of astrophysical origin. Farr et al. (2015) presented a method by which a set of triggers of uncertain origin like this can be used to produce a rate estimate that is more accurate than that produced by considering only highly significant events.

B. P. Abbott, et al.
(Submitted on 11 Feb 2016)
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Who was (were) the first scientist(s) to [fore]see {the direct detection of} genuine gravitational waves?

Henri Poincaré?
...j’ai d’abord été conduit à supposer que la propagation de la gravitation n’est pas instantanée, mais se fait avec la vitesse de la lumière. Cela semble en contradiction avec un résultat obtenu par Laplace qui annonce que cette propagation est, sinon instantanée, du moins beaucoup plus rapide que celle de la lumière. Mais, en réalité, la question que s’était posée Laplace diffère considérablement de celle dont nous nous occupons ici. Pour Laplace, l’introduction d’une vitesse finie de propagation était la seule modification qu’il apportait à la loi de Newton. Ici, au contraire, cette modification est accompagnée de plusieurs autres ; il est donc possible, et il arrive en effet, qu’il se produisent entre elles une compensation partielle.
Quand nous parlerons donc de la position ou de la vitesse du corps attirant, il s’agira de cette position ou de cette vitesse à l’instant où l’onde gravifique est partie de ce corps ; quand nous parlerons ou de la vitesse du corps attiré, il s’agira de cette position ou de cette vitesse à l’instant où ce corps attiré a été atteint par l’onde gravifique émanée de l’autre corps ; il est clair que le premier instant est antérieur au second.
Note de M. H. Poincaré. 5 Juin1905


Albert Einstein?
In 1918 Einstein published the paper ÜBER GRAVITATIONSWELLEN [1] in which, for the first time, the effect of gravitational waves was calculated, resulting in his famous “quadrupole formula” (QF). Einstein was forced to this publication due to a serious error in his 1916 paper [2], where he had developed the linear approximation (“weak- field”) scheme to solve the field equations of general relativity (GR). In analogy to electrodynamics, where accelerated charges emit electromagnetic waves, the linearized theory creates gravitational waves, propagating with the speed of light in the (background) Minkowski space-time. A major difference: Instead of a dipole moment, now a quadrupole moment is needed. Thus sources of gravitational waves are objects like a “rotating dumbbell”, e. g. realized by a binary star system. As there was no chance for detecting gravitational waves, due to their extreme weakness of the order (v/c)5, the theory advanced slow in the first decades. The existence of gravitational waves was always a matter of controversy. Curiously Einstein himself was not convinced in 1936. In a paper with Nathan Rosen he came tothe conclusion, that gravitational waves do not exist! Curiously too is the story of its publication. Einstein’s manuscript, titled DO GRAVITATIONAL WAVES EXIST?, was rejected by the “Physical Review”. In an angry reply he withdrawed the paper, to appear later in the “Journal of the Franklin Institute” (choosing a less provoking headline [3]). To clear the situation, various approximation schemes were developed. One of the first, introduced by Einstein, Infeld and Hoffmann in 1938 [4], led to the famous EIH equations. This “post-Newtonian” treatment describes slow moving bodies in a weak field (“bounded systems”). In the EIH approximation there is no radiation up to the order (v/c)4 , the energy remains constant. The QF appears in the next order, as demonstrated by Hu in 1947 [5]. What’s about fast moving particles? This problem had to wait until the early 1960’s, when the Lorentz-invariant perturbation methods (“fast-motion approximation”), describing “unbounded systems”, were developed. The question of an analogy to the QF (“radiation damping”) was strongly discussed. In 1975 a major boost was caused by the discovery of the binary pulsar PSR 1913 + 16 by Hulse and Taylor [6]. Over the next years their data showed a decrease of the period of revolution – as predicted by the QF! But this (indirect) proof – in the “bounded” case – did not stop the controversy: On the contrary, the fight gets even stronger. The different approximation formalisms were criticized by Ehlers, Havas and others [7]. The basic difficulties are: (1) In contrast to electrodynamics, the equations of motion in GR are not a separate part of the theory, but already inherent in the field equations. (2) GR is an essential non-linear theory. Any approximation must treat these facts carefully. After a phase of clarification, introducing new methods (e. g. asymptotic field conditions, post-linear approximations), the believe in gravitational waves, and especially in Einstein’s quadrupole formula, is now stronger than ever – eventually visible in expensive terrestrial and space experiments.
WOLFGANG STEINICKE 

//Update February 20 2016:
There is an interesting presentation of Daniel Kennefick providing more information about the circumstances surrounding the 1936 Einstein and Rosen withdrawal of their article about the nonexistence of gravitational waves and the first encounter of the father of general relativity with anonymous peer review!

Joseph Weber?
There appears to have been little interest in the experimental detection of gravitational radiation for fortyfive years after their prediction. However in the late 1950s this changed with Joseph Weber of the University of Maryland suggesting the design of some relatively simple apparatus for their detection [8, 9]. This apparatus in its later stages consisted of an aluminium bar of mass approximately one ton with piezoelectric transducers bonded around its centre line. The bar was suspended from anti-vibration mountings in a vacuum tank. By means of the amplified electrical signals from the transducers Weber monitored the amplitude of oscillation of the fundamental mode of the bar. A gravitational wave signal of suitable strength would be expected to change the amplitude or phase of the oscillations in the bar. In the 1969/70 period Weber operated two such systems one at the University of Maryland and one at the Argonne National Laboratory and observed coincident excitations of the bars at a rate of one event per day [10, 11]. These events he claimed to be gravitational wave signals. 
However other experiments – at Moscow State University [12], Yorktown heights [13], Rochester [14], Bell Labs [15], Munich [16] and Glasgow [17] - failed to confirm Weber's detections... Several years of lively debate about the interpretation of Weber's results followed, the outcome being a somewhat predictable standoff between Weber and the rest of the community. An analysis of detector sensitivity of the Weber bar design suggested that the sensitivity was approximately 10-16 for millisecond pulses. However an event rate of one per day resulting from events at the centre of the galaxy - as claimed by Weber - corresponded to a very high loss of energy, and thus mass, from the galaxy, so high in fact that changes in the position of the outermost stars should have been visible due to a reduction in gravitational force towards the galactic centre [18]. A solution suggested for this – beaming of the energy in a narrow cone so that each detected event implied much less overall energy loss - was discussed by many authors but did not receive wide acceptance.
(Submitted on 4 Jan 2005)

M. E. Gertsenshtein and V. I Putsovoit?
Almost as soon as Weber had begun work on the first gravitational wave detector or the resonant-mass style, the idea arose to use interferometry to sense the motions induced by a gravitational wave. Weber and a student, Robert Forward, considered the idea in 1964. We will discuss below how Forward later went about implementing the idea. But the first discussion of the idea is actually due to two Soviet physicists, M.E. Gertsenshtein and V.I. Pustovoit. They wrote in 1962 a criticism of Weber’s 1960 Physical Review article, claiming (incorrectly) that resonant gravitational wave detectors would be very insensitive. Then, they make a remarkable statement justified only by intuition, that “Since the reception of gravitational waves is a relativistic effect, one should expect that the use of an ultrarelativistic body — light — can lead to a more effective indication of the field of the gravitational wave.” 
Gertsenshtein and Pustovoit followed up this imaginative leap by noting that a Michelson interferometer has the appropriate symmetry to be sensitive to the strain pattern produced by gravitational waves. They give a simple and clear derivation of the arm length difference caused by a wave of amplitude h. Next, they note that L.L. Bernshtein had with ordinary light measured a path length differences of 10-11 cm in a 1sec integration time. The newly invented laser, they claim, would “make it possible to decrease this factor by at least three orders of magnitude.” (The concept of shot noise never appears explicitly here, so it is not clear what power levels are being anticipated.) They assume that one might make an interferometer with arm length of 10 m, thus leading to a sensitivity estimate of 10-14√Hz for “ordinary” light, or as good as 10--17√Hz for a laser-illuminated interferometer. This, Gertsenshtein and Pustovoit claim, is 107 to 1010 times better (it isn’t clear whether they mean in amplitude or in power) than what would be possible with Weber-style detector. Putting aside their unjustified pessimism about resonant-mass detectors, their arguments about interferometric sensing are right on the mark, even conservative. 
For improvements beyond the quoted level, they make suggestions that are somewhat misguided. They say that observation time could be lengthened beyond 1 sec, which would be obvious for some sources (such as “monochromatic sinusoidal signals” or signals of long period) and hopeless for short bursts. Their other suggestion is to use “known methods for the separation of a weak signal from the noise background”; this suggestion is curious because known methods appear to be already built into their estimates that are referenced to a specific observing time. The other lack that is obvious in hindsight is any mention of mechanical noise sources. Still, the gist of the idea of interferometric detection of gravitational waves is clearly present, as is a demonstration that the idea can have interesting sensitivity.
For a variety of reasons, not least of which must have been the fact that it was written too early (before Weber’s work had progressed beyond design studies), the proposal of Gertsenshtein and Pustovoit had little influence. The activity that began the by-now flourishing field of interferometric gravitational wave detection started independently in the West. In fact, it began semi-independently at several places in the United States at around the same time. The roots of this work can be seen in a pair of papers, written in 1971-2, by two teams linked in an unusual collaboration that is acknowledged in the bodies of the papers, although not in the author lists. The first to be published was that of the Hughes Research Lab team, whose most committed member was Robert L. Forward, the former Weber student mentioned above. Later to appear, and not in a refereed journal, was the work of Rainer Weiss, an MIT physicist who had spent an influential postdoctoral stint with Robert H. Dicke at Princeton. Linking the two groups was someone who never published anything on the subject under his own name, but whose activity is mentioned in both papers — Philip K. Chapman, who had earned a doctorate in Instrumentation at MIT’s Department of Aeronautics and Astronautics before joining NASA as a scientist-astronaut.
Peter R. Saulson 1998
//Update February 20 2016


(I thank the JETP editorial office in general and Natalia Tserevitinova in particular to have made this article recently available online).



Marco Drago?
...on September 14, 2015, at just before eleven in the morning, Central European Time, the waves reached Earth. Marco Drago, a thirty-two-year-old Italian postdoctoral student and a member of the LIGO Scientific Collaboration, was the first person to notice them. He was sitting in front of his computer at the Albert Einstein Institute, in Hannover, Germany, viewing the LIGO data remotely. The waves appeared on his screen as a compressed squiggle, but the most exquisite ears in the universe, attuned to vibrations of less than a trillionth of an inch, would have heard what astronomers call a chirp—a faint whooping from low to high.
... 

The LIGO team includes a small group of people whose job is to create blind injections—bogus evidence of a gravitational wave—as a way of keeping the scientists on their toes. Although everyone knew who the four people in that group were, “we didn’t know what, when, or whether,” Gabriela González, the collaboration’s spokeswoman, said. During Initial LIGO’s final run, in 2010, the detectors picked up what appeared to be a strong signal. The scientists analyzed it intensively for six months, concluding that it was a gravitational wave from somewhere in the constellation of Canis Major. Just before they submitted their results for publication, however, they learned that the signal was a fake.



This time through, the blind-injection group swore that they had nothing to do with the signal. Marco Drago thought that their denials might also be part of the test

BY NICOLA TWILLEY (FEBRUARY 11, 2016)

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