# What's the universe telling us post-LIGO?

Since the LIGO Scientific Collaboration announced the first direct detection of gravitational waves on February 11, 2016, there have been at least 51 scientific papers written up on the topic discussing a variety of possibilities. The earliest papers parallel the announcement’s two ostensible achievements:

- Albert Einstein was right when he postulated the existence of gravitational waves in 1915, in his theory of general relativity.
- LIGO’s working principle is valid – in other words, the observatory
*works*.

The third achievement was more of a signal: that the era of gravitational astronomy has begun, an era in which humankind will be able to study objects in the universe based on the gravitational effects they have on their surroundings, on the spacetime continuum. And in keeping with this new possibility, many of the 51 papers explore what else we can figure about the two blackholes that merged and caused the waves that LIGO detected.

Here’s a categorised list of their (informed) hypotheses along with brief descriptions.

**Okay, was it a legit detection? Does it fit the theory? And is LIGO awesome yet?**

- http://arxiv.org/abs/1602.08492 – “We summarise the follow-up observations reported by 25 teams via private Gamma-ray Coordinates Network Circulars, giving an overview of the participating facilities, the gravitational wave sky localisation coverage, the timeline and depth of the observations”
- http://arxiv.org/abs/1602.06833 – “total-variation denoising techniques may thus offer an additional viable approach for waveform reconstruction”
- http://arxiv.org/abs/1602.04782 – “The chirp signal from the gravitational-wave event GW150914 is used to place numerous first constraints on gravitational Lorentz violation”
- http://arxiv.org/abs/1602.04779 – “We point out that GW150914 experienced a Shapiro delay due to the gravitational potential of the mass distribution along the line of sight of about 1800 days”
- http://arxiv.org/abs/1602.04666 – “… we can design activities that directly involve the detection of GW150914, the designation of the Gravitation Wave signal detected on September 14, 2015, thereby engage the students in this exciting discovery directly. The activities naturally do not include the construction of a detector or the detection of gravitational waves. Instead, we design it to include analysis of the data from GW150914, which includes some interesting analysis activities for students of the introductory course.”
- http://arxiv.org/abs/1602.04531 – “We find that the existence of GW150914 does not require enhanced double black hole formation in dense stellar clusters or via exotic evolutionary channels. … We predict that BH-BH mergers with total mass of 20-80 Msun are to be detected next.”
- http://arxiv.org/abs/1602.04199 – “Based on our observations, we conclude that it is unlikely that GW150914 was caused by the core collapse of a supergiant in the LMC, consistent with the LIGO Collaboration analyses of the gravitational wave form as best described by a binary black hole merger”
- http://arxiv.org/abs/1602.04198 – “We report initial results of a deep search for an optical counterpart to the gravitational wave event GW150914, the first trigger from the Advanced LIGO gravitational wave detectors”
- http://arxiv.org/abs/1602.03847 – “The stochastic gravitational-wave background from binary black holes, created from the incoherent superposition of all the merging binaries in the Universe, could be higher than previously expected. Using the properties of GW150914, we estimate the energy density of such a background from binary black holes. … We conclude that this background is potentially measurable by the Advanced LIGO/Virgo detectors operating at their projected final sensitivity.”
- http://arxiv.org/abs/1602.03845 – “In Advanced LIGO, detection and astrophysical source parameter estimation of the binary black hole merger GW150914 requires a calibrated estimate of the gravitational-wave strain sensed by the detectors”
- http://arxiv.org/abs/1602.03844 – “This paper describes the transient noise backgrounds used to determine the significance of the event (designated GW150914) and presents the results of investigations into potential correlated or uncorrelated sources of transient noise in the detectors around the time of the event”
- http://arxiv.org/abs/1602.03843 – “We find that the reconstructed waveform is consistent with the signal from a binary black-hole merger with a chirp mass of ∼30M⊙ and a total mass before merger of ∼70M⊙ in the detector frame”
- http://arxiv.org/abs/1602.03841 – “Within our statistical uncertainties, we find no evidence for violations of general relativity in the genuinely strong-field regime of gravity”
- http://arxiv.org/abs/1602.03840 – Discusses the properties of the merger
- http://arxiv.org/abs/1602.03839 – “GW150914 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 5.1 {sigma}”
- http://arxiv.org/abs/1602.03838 – “At full sensitivity, the Advanced LIGO detectors are designed to deliver another factor of three improvement in the signal-to-noise ratio for binary black hole systems similar in masses to GW150914”

**What were the particulate or energetic effects of the blackhole merger?**

- http://arxiv.org/abs/1602.08764 – “The intermediate Palomar Transient Factory (iPTF) autonomously responded to and promptly tiled the error region of the first gravitational wave event GW150914 to search for an optical counterpart. We obtained radio data with the Very Large Array and X-ray follow-up with the Swift satellite for this transient. None of our candidates appear to be associated with the gravitational wave trigger, which is unsurprising given that GW150914 came from the merger of two stellar-mass black holes.”
- http://arxiv.org/abs/1602.08436 – “We discuss [high-energy neutrinos] emission in connection with the … event GW150914 which could be associated with a short gamma-ray burst detected by the Fermi Gamma-ray Burst Monitor (GBM) 0.4 s after the GW event and within localisation uncertainty of the GW event”
- http://arxiv.org/abs/1602.07352 – “We argue that the physical constraints required by the association of the Fermi GBM signal contemporaneous with GW150914 are astrophysical highly implausible”
- http://arxiv.org/abs/1602.06961 – “The recent detection of the gravitational wave source GW150914 by the LIGO collaboration motivates a speculative source for the origin of ultrahigh energy cosmic rays as a possible byproduct of the immense energies achieved in black hole mergers, provided that the black holes have spin … and there are relic magnetic fields and disk debris remaining from the formation of the black holes or from their accretion history”
- http://arxiv.org/abs/1602.05529 – “We model the afterglow of the Fermi GBM event associated with LIGO detection GW150914, under the assumption that the gamma-ray are produced by a short GRB-like relativistic outflow”
- http://arxiv.org/abs/1602.05411 – “We search for coincident neutrino candidates within the data recorded by the IceCube and ANTARES neutrino detectors. A possible joint detection could be used in targeted electromagnetic follow-up observations, given the significantly better angular resolution of neutrino events compared to gravitational waves.”
- http://arxiv.org/abs/1602.05140 – “The presence of at least one neutron star has long been thought to be an essential element of the model: its tidal disruption provides the needed baryonic material whose rapid accretion onto the post-merger black hole powers the burst. The recent tentative detection by the Fermi satellite of a short GRB in association with the gravitational wave signal GW150914 produced by the merger of two black holes has shaken this standard paradigm.”
- http://arxiv.org/abs/1602.05050 – “We find that the 1.4 GHz radio flux peaks at ∼1E5 sec after the burst trigger. The radio afterglow is detectable if the ambient matter is dense enough with density larger than ∼10E−2 cm^−3.”
- http://arxiv.org/abs/1602.04764 – “The observation of gravitational waves from the Laser Interferometer Gravitational-Wave Observatory event GW150914 may be used to constrain the possibility of Lorentz violation in graviton propagation”
- http://arxiv.org/abs/1602.04735 – “Mergers of stellar-mass black holes are not expected to have electromagnetic counterparts. However, the Fermi GBM detector identified a gamma-ray transient 0.4 s after the gravitational wave (GW) signal GW150914 with consistent sky localisation”
- http://arxiv.org/abs/1602.04337 – “We briefly show how the very recent LIGO gravitational wave observation GW150914, emitted by a binary black hole merger distant ∼1.3 [billion] ly from the Earth, tightens the phenomenological bound on a massive graviton or on the screening of gravity”
- http://arxiv.org/abs/1602.04180 – “Our results constrain the ratio of the energy promptly released in gamma-rays in the direction of the observer to the gravitational wave energy”
- http://arxiv.org/abs/1602.03846 – ‘Astrophysical Implications of the Binary Black-Hole Merger GW150914’

**How fast did the gravitational waves move through spacetime?**

- http://arxiv.org/abs/1602.05882 – “Connaughton et al. report the discovery of a possible electromagnetic counterpart to the gravitational wave event GW150914 discovered by LIGO. Assuming that the EM and GW are emitted at the same instant, a constraint is placed on the ratio of the speeds of light and gravitational waves at the level of 1E-17.”
- http://arxiv.org/abs/1602.04188 – “We point out that the observed time delay between the detection of the signal at the Hanford and Livingston LIGO sites from the gravitational wave event GW150914 places an upper bound on the speed of propagation of gravitational waves, c_gw ≲ 1.7 in the units of speed of light”
- http://arxiv.org/abs/1602.04460 – “The difference between the gravitational wave velocity and the speed of the light is found to be smaller than a factor of 1E-17, nicely in agreement with the prediction of general relativity theory”

**LIGO can tell us how other observatories could spot gravitational waves (and perform follow-ups checks of the merger LIGO picked up on)**

- http://arxiv.org/abs/1602.06951 – “We show that the black hole binary (BHB) coalescence rates inferred from the advanced LIGO 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”
- http://arxiv.org/abs/1602.04715 – “We discuss the prospects of eLISA for detecting gravitational waves from Galactic binary black holes similar to GW150914”
- http://arxiv.org/abs/1602.04488 – “… the LAT observed the entire LIGO localisation region within ~70 minutes of the trigger, and thus enabled a comprehensive search for a gamma-ray counterpart to GW150904. The study of the LAT data presented here did not find any potential counterparts to GW150904”
- http://arxiv.org/abs/1602.04156 – “We have searched for an optical counterpart to the first gravitational wave source discovered by the LIGO experiment, GW150914, using a combination of the Pan-STARRS1 wide-field telescope and the PESSTO spectroscopic follow-up programme”
- http://arxiv.org/abs/1602.03920 – Probing “the connection between compact binary mergers and short Gamma-ray bursts”
- http://arxiv.org/abs/1602.03868 – “We report on observations taken with the Swift satellite two days after the GW trigger. No new X-ray, optical, UV or hard X-ray sources were detected in our observations, which were focussed on nearby galaxies in the gravitational wave error region and covered 4.7 square degrees.”

**Any other blackhole mergers out there?**

- http://arxiv.org/abs/1603.00884 – “… we systematically vary model assumptions within existing uncertainties and study their effects on the evolution of blackholes in globular clusters and the final structural properties of [the clusters]”
- http://arxiv.org/abs/1602.08767 – “We consider a system composed of ten black holes with initial mass of 30 M⊙. As a result, we show that mergers of accreting stellar-mass blackholes are classified into four types.”
- http://arxiv.org/abs/1602.05554 – “Here we derived the binary black hole merger rate for isolated binary systems based on the nearby ultra-luminous X-ray source (ULX) luminosity function (LF)”
- http://arxiv.org/abs/1602.04226 – “We explore the evolution of stellar mass black hole binaries which are formed in self-gravitating active galactic nuclei disks”
- http://arxiv.org/abs/1602.03842 – “Here we report on the constraints these observations place on the rate of binary blackhole coalescences. Considering only GW150914, assuming that all BBHs (BBH) in the universe have the same masses and spins as this event, imposing a false alarm threshold of 1 per 100 years, and assuming that the BBH merger rate is constant in the comoving frame, we infer a 90% credible range of 2−53/Gpc^3/year (comoving frame)”
- http://arxiv.org/abs/1602.03790 – “The masses inferred for the black holes in the binary progenitor of GW150914 are amongst the most massive expected at anything but the lowest metallicities in our models. We discuss the implications of our analysis for the electromagnetic follow-up of future LIGO event detections.”

**We still know nothing about dark matter and dark energy… right?**

- http://arxiv.org/abs/1603.00699 – Asks what the LIGO find can tell us about the nature and strength of dark energy
- http://arxiv.org/abs/1603.00464 – “We consider the possibility that the black-hole binary detected by LIGO may be a signature of dark matter. Interestingly enough, there remains a window for masses 10M⊙ ≲ M_bh ≲ 100M⊙ where primordial black holes may constitute the dark matter.”
- http://arxiv.org/abs/1602.07670 – “We describe the minimal modification required for self-acceleration and show that its maximum likelihood yields a 2.4-sigma poorer fit to cosmological observations compared to a cosmological constant, which, although marginally still possible, questions the concept of cosmic acceleration”
- http://arxiv.org/abs/1509.08458 – “… gravitational-wave cosmology breaks the dark degeneracy in observations of the large-scale structure between two fundamentally different explanations of cosmic acceleration – a cosmological constant and a scalar-tensor modification of gravity”

**Could the blackhole merger have done anything strange?**

- http://arxiv.org/abs/1602.08759 – “After comparing the real and imaginary parts of the ringdown signal of GW150914 with the corresponding quantities for a variety of gravastars, and notwithstanding the very limited knowledge of the perturbative response of rotating gravastars, we conclude it is unlikely that GW150914 produced a rotating gravastar unless its surface is infinitesimally close to the event horizon”
- http://arxiv.org/abs/1602.08086 – “The magnetospheric activity just before the merger made the FRB, and subsequently an undetected short GRB. The gravitational wave (GW) event GW150914 would be a sister of FRB 150418 in this second scenario. In both cases, one expects an exciting prospect of a GW/FRB/GRB associations.”
- http://arxiv.org/abs/1602.06526 – “We apply the delay in timing of FERMI GMB transient occurred in coincidence with gravitational waves event GW150914 observed by LIGO to constrain the size of the spherical brane-universe expanding in multi-dimensional space-time”

Obviously some papers belong in more than one category; I’ve binned them according to which categories the unanswered questions in them would best belong in. And why did I draw up this list? Boredom had a bit of a role to begin with but as I picked up more papers, it became harder to keep track of the different avenues of research. And as even more papers crop up, I’ll probably return to – and update – this list, but until then I think there’s fodder here enough for dozens of blog posts.