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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