Echoes of ECOs: gravitational-wave signatures of exotic compact objects and of quantum corrections at the horizon scale (1608.08637v2)

Abstract: Gravitational waves from binary coalescences provide one of the cleanest signatures of the nature of compact objects. It has been recently argued that the post-merger ringdown waveform of exotic ultracompact objects is initially identical to that of a black-hole, and that putative corrections at the horizon scale will appear as secondary pulses after the main burst of radiation. Here we extend this analysis in three important directions: (i) we show that this result applies to a large class of exotic compact objects with a photon sphere for generic orbits in the test-particle limit; (ii) we investigate the late-time ringdown in more detail, showing that it is universally characterized by a modulated and distorted train of "echoes" of the modes of vibration associated with the photon sphere; (iii) we study for the first time equal-mass, head-on collisions of two ultracompact boson stars and compare their gravitational-wave signal to that produced by a pair of black-holes. If the initial objects are compact enough as to mimic a binary black-hole collision up to the merger, the final object exceeds the maximum mass for boson stars and collapses to a black-hole. This suggests that - in some configurations - the coalescence of compact boson stars might be almost indistinguishable from that of black-holes. On the other hand, generic configurations display peculiar signatures that can be searched for in gravitational-wave data as smoking guns of exotic compact objects.

• The paper demonstrates that ECOs with photon spheres produce distinctive gravitational-wave echoes following a black hole-like ringdown.
• It explores head-on collisions of boson stars, showing that these signals can mimic black hole mergers until collapse occurs post-merger.
• The study quantifies how quantum corrections at horizon scales create delayed echo patterns, challenging conventional black hole models.

Echoes of Exotic Compact Objects in Gravitational-wave Astronomy

The paper "Echoes of ECOs: gravitational-wave signatures of exotic compact objects and of quantum corrections at the horizon scale" focuses on analyzing gravitational-wave (GW) signatures resulting from exotic compact objects (ECOs) and potential quantum corrections near the horizon scale. The authors, Vitor Cardoso et al., provide substantial insights into the nature of these signals and their implications, leveraging the context of advances in GW detection technology.

Overview

One major emphasis of this work is the exploration of gravitational-wave signals emanating from the merger of binary compact objects. The analysis extends previous arguments suggesting that the post-merger ringdown waveform of these ECOs closely resembles that of black holes initially but diverges with distinct secondary pulses, or "echoes", at later times. The paper broadens this inquiry by considering ECOs with photon spheres under varied conditions.

Key Contributions

1. Universality of ECO Echoes: The research highlights that a broad set of ECOs which possess a photon sphere demonstrate an initial ringdown waveform akin to black holes. The key identifier of these ECOs is a universal pattern of echoes, modulated and distorted, associated with the modes of vibration of the photon sphere, detectable at late times in the GW signature.
2. Head-on Collisions of Boson Stars: The paper uniquely discusses the gravitational-wave signal generated from the head-on collision of equal-mass ultracompact boson stars. These ECOs, specifically boson stars characterized by a solitonic potential, can mimic the signature of a white hole-black hole collision up to the merger event, beyond which they exceed the maximum boson star mass and collapse into a black hole. This demonstrates potential difficulty in distinguishing between ultra-compact boson stars and traditional black holes solely from observational data.
3. Microscopic Corrections at Horizon Scales: A central hypothesis is that any quantum-scale corrections at the horizon of an ECO manifest as delayed echoes following the main burst of radiation. This is based on theoretical models and numerical results that suggest the appearance of these distinct echo patterns due to reflection and interference effects in the gravitational potential.

Implications

Practically, the findings from this paper underscore the necessity for precise and sensitive GW observations to potentially detect the minor yet telling echoes indicative of ECOs. Theoretically, the paper provides fertile ground for questioning long-standing assumptions about the nature of black holes and challenges researchers to rigorously explore quantum gravity implications at horizon scales.

These investigations can further illuminate the boundaries of general relativity and potentially highlight new physics, particularly concerning strong-gravity phenomena. For future developments, the methodology could serve as a precursor to diversified approaches in detecting and interpreting GW signals emanating from beyond-standard astrophysical objects.

Conclusion

In sum, this paper presents an analytical and numerical dissection of GW signals from ECOs, highlighting the potential of these objects to masquerade as black holes. The insights into gravitational echoes could play a crucial role in advancing observational astrophysics and theoretical physics. Verification of these concepts through next-generation GW detectors could deliver crucial insights into the quantum properties entwined within gravitational interactions, potentially unveiling new paradigms in the physics governing the cosmos.