Search for Compact Binary Coalescences with Sub-solar Mass Components in Data from the Second Part of LIGO-Virgo-KAGRA's Fourth Observing Run
This paper reports a gravitational wave search for compact binary coalescences with sub-solar mass components using data from the second part of LIGO-Virgo-KAGRA's fourth observing run, finding no significant detections but establishing stringent upper limits on merger rates that constrain dark matter scenarios involving primordial and dissipative-model black holes.
Original authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, A. Abe, I. Abouelfettouh, F. Acernese, K. Ackley, A. Adam, S. Adhicary, D. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I. -L. Ahrend, L. Aiello, A. Ain, P. Ajith, T. Akutsu, L. Albers, W. Ali, S. Al-Kershi, C. Allan, C. Allene, A. Allocca, S. Al-Shammari, J. A. Alvarez, S. Alvarez-Lopez, W. Amar, O. Amarasinghe, A. Amato, F. Amicucci, C. Amra, A. B. Anand, C. Anand, A. Ananyeva, S. B. Anderson, W. G. Anderson, M. Andia, M. Ando, F. Andrade-Oliveira, M. Andrés-Carcasona, J. L. Andrey, T. Andrić, J. Anglin, J. Anna, J. M. Antelis, S. Antier, T. Aoki, M. Aoumi, E. Z. Appavuravther, E. A. Appelt, S. Appert, S. K. Apple, K. Arai, A. Araya, M. C. Araya, M. Arca Sedda, F. Arciprete, J. S. Areeda, N. Aritomi, F. Armato, S. Armstrong, N. Arnaud, M. Arogeti, S. M. Aronson, G. Ashton, Y. Aso, L. Asprea, M. Assiduo, S. Assis de Souza Melo, S. M. Aston, P. Astone, P. S. Aswathi, F. Attadio, F. Aubin, K. AultONeal, G. Avallone, N. Avdeev, E. A. Avila, S. Babak, C. Badger, S. Bae, S. Bagnasco, S. Baimukhametova, L. Baiotti, T. Baka, K. A. Baker, T. Baker, G. Balbi, G. Baldi, N. Baldicchi, M. Ball, G. Ballardin, M. Ballelli, S. W. Ballmer, S. Banagiri, B. Banerjee, D. Bankar, T. M. Baptiste, P. Baral, M. Baratti, J. C. Barayoga, K. Baric, B. C. Barish, D. Barker, D. Barman, N. Barman, F. Barone, B. Barr, M. Barrios, L. Barsotti, M. Barsuglia, D. Barta, M. A. Barton, I. Bartos, A. Basalaev, R. Bassiri, A. Basti, M. Bawaj, J. C. Bayley, A. C. Baylor, P. A. Baynard II, M. Bazzan, V. M. Bedakihale, F. Beirnaert, M. Bejger, A. S. Bell, C. Bellani, D. S. Bellie, D. Beltran-Martinez, E. Benedetti, W. Benoit, I. Bentara, M. Ben Yaala, S. Bera, F. Bergamin, B. K. Berger, M. Beroiz, C. P. L. Berry, I. Berry, D. Bersanetti, T. Bertheas, A. Bertolini, J. Betzwieser, D. Beveridge, N. Bevins, J. Bezerra-Sobrinho, R. Bhandare, R. Bhatt, A. Bhattacharjee, D. Bhattacharjee, S. Bhattacharyya, S. Bhaumik, V. Biancalana, F. Bianchi, I. A. Bilenko, M. Bilicki, G. Billingsley, A. Binetti, S. Bini, S. Biot, O. Birnholtz, S. Biscoveanu, A. Bisht, M. Bitossi, M. -A. Bizouard, S. Blaber, J. Black, J. K. Blackburn, L. A. Blagg, C. D. Blair, D. G. Blair, M. Bloch, N. Bode, N. Boettner, P. Bogdan, G. Boileau, M. Boldrini, G. N. Bolingbroke, L. D. Bonavena, V. A. Bonhomme, E. Bonilla, M. S. Bonilla, A. Bonino, R. Bonnand, A. Borchers, N. Borghi, V. Boschi, S. Bose, V. Bossilkov, Y. Bothra, A. Boudon, T. D. Boybeyi, M. Boyle, A. Bozzi, C. Bradaschia, M. J. Brady, P. R. Brady, A. Branch, M. Branchesi, T. Briant, A. Brillet, M. Brinkmann, P. Brockill, E. Brockmueller, A. F. Brooks, D. D. Brown, M. L. Brozzetti, S. Brunett, G. Bruno, R. Bruntz, J. Bryant, Y. Bu, F. Bucci, A. Buchicchio, A. Buggiani, O. Bulashenko, T. Bulik, H. J. Bulten, A. Buonanno, K. Burtnyk, R. Buscicchio, N. 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Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Gravity is the invisible thread that binds the universe, stretching across the vast emptiness between stars and galaxies. For decades, scientists have listened to the ripples this force creates when massive objects collide, a phenomenon known as gravitational waves. These ripples carry stories of the most violent events in the cosmos, such as the crash of black holes or the merger of neutron stars, the dense remnants of dead stars. While we have heard the deep, heavy thuds of stellar-mass black holes, there remains a quiet question about the existence of much lighter objects. Could there be black holes or neutron stars that weigh less than our Sun? Such "sub-solar" objects would be impossible to form through the normal death of a star, which typically leaves behind a heavy core. If they exist, they might be something entirely exotic, perhaps formed in the first moments after the Big Bang or created by a mysterious form of matter that does not interact with light. Finding them would rewrite our understanding of how the universe is built and what dark matter—the invisible substance holding galaxies together—might actually be.
To answer this question, a global team of scientists used the most sensitive ears in history: the LIGO, Virgo, and KAGRA detectors. These instruments, located in the United States, Italy, and Japan, are designed to sense the tiny distortions in space-time caused by passing gravitational waves. The researchers focused their search on a specific period of time, from April 2024 to January 2025, analyzing data collected during the second part of the collaboration's fourth major observing run. They looked for signals from binary systems—pairs of compact objects spiraling toward each other—where at least one of the objects weighed less than the Sun. Their search covered a wide range of possibilities, looking for pairs where the lighter object was between 0.2 and 1 times the mass of the Sun, and the heavier partner ranged from 0.2 up to 10 times the Sun's mass.
The team employed three different, independent computer programs to sift through the data, each acting like a distinct filter to catch any potential signal hidden in the background noise. These programs compared the detector data against millions of theoretical wave patterns, searching for a match that would indicate a real cosmic event. The result was a clean sweep: no confirmed detections of these light-weight mergers. While the computers did flag a few unusual candidates that stood out from the usual static, further investigation showed that these were not signals from space. Instead, they were consistent with random fluctuations in the detectors or noise from the environment. The most significant of these false alarms, identified by two of the programs, did not reach a level of certainty that would allow the scientists to claim a discovery. In the world of gravitational wave astronomy, a single event is not enough; the signal must be strong enough to rule out the possibility of it being a coincidence, and these candidates fell short of that threshold.
Although they did not find the objects they were looking for, the search was far from a failure. By not finding anything, the team was able to set strict limits on how common these objects could be. They calculated that if sub-solar mass black holes exist, they must be incredibly rare. Specifically, they determined that the rate of such mergers in the universe cannot exceed a few thousand events per cubic billion light-years per year, depending on the exact masses involved. This lack of detection allows them to place tight constraints on two leading theories about dark matter.
The first theory involves primordial black holes, which are hypothetical black holes that formed from the dense clumps of the early universe rather than from dying stars. If these objects made up a significant portion of dark matter, the detectors should have heard them colliding by now. The new data suggests that primordial black holes can account for no more than a small fraction of the dark matter in the universe, specifically less than about 3 percent for objects with a mass similar to the Sun, and even less for lighter ones. The second theory concerns "dissipative dark matter," a model where dark matter particles interact with each other in complex ways, similar to how gas cools and collapses to form stars. In this scenario, the dark matter could form its own version of black holes. The search results indicate that if such dark black holes exist and make up all of the dark matter, they must be heavier than a certain minimum mass, effectively ruling out the lightest versions of this theory.
The study also extended its reach to neutron stars, the ultra-dense cores of dead stars. While standard neutron stars are usually heavier than the Sun, some theories suggest that under specific conditions, lighter versions could exist. The researchers simulated how their detectors would respond to these lighter neutron stars and found that their search was sensitive enough to detect them if they were common. They concluded that the rate of mergers involving these light neutron stars is also very low, placing an upper limit of about 250 events per cubic billion light-years per year. This work not only tightens the net around exotic dark matter theories but also demonstrates the growing power of the global detector network. As the instruments become more sensitive and the observing runs continue, the window into the sub-solar mass universe will only get clearer, bringing scientists closer to either discovering these elusive objects or definitively closing the door on their existence.
Technical Summary: Search for Compact Binary Coalescences with Sub-solar Mass Components in Data from the Second Part of LIGO–Virgo–KAGRA's Fourth Observing Run
Problem and Motivation
Gravitational wave (GW) detectors are primarily sensitive to mergers of stellar-mass compact objects (black holes and neutron stars with masses ≥1M⊙). However, the detection of compact binary coalescences (CBCs) involving sub-solar mass (SSM) components (masses <1M⊙) would provide evidence for exotic formation channels beyond standard stellar evolution. Two primary motivations drive this search:
- Primordial Black Holes (PBHs): PBHs are a candidate for dark matter. While they can span a wide mass spectrum, the detection of an SSM black hole would conclusively distinguish them from stellar black holes, as no known astrophysical channel produces stellar-mass black holes below the Chandrasekhar limit.
- Dissipative Dark Matter: Models involving dark matter with complex particle interactions (dissipative dark matter) predict the formation of "dark black holes" (DBHs) in dense halos. The merger rates of these objects are inversely proportional to their mass, implying that SSM DBHs could have high merger rates, offering a probe into the microphysics of dark matter.
Additionally, SSM searches are sensitive to binary neutron star (BNS) systems containing low-mass neutron stars, which may form through specific astrophysical mechanisms (e.g., mass stripping in binaries or formation in neutron-rich disks).
Methodology
The analysis utilizes data from the second part of the fourth observing run (O4b) of the LIGO–Virgo–KAGRA (LVK) collaboration, spanning from April 10, 2024, to January 28, 2025. This period includes coincident operation of the two LIGO detectors and the Virgo detector.
- Search Pipelines: Three independent matched-filtering pipelines—GstLAL, MBTA, and PyCBC—analyzed the strain data.
- Parameter Space: The search targeted CBCs with a secondary mass (m2) between 0.2M⊙ and 1M⊙, and a primary mass (m1) between 0.2M⊙ and 10M⊙. The mass ratio q=m2/m1 was restricted to 0.1≤q≤1.
- Template Banks: Template banks were constructed to cover the specified mass range with spin magnitudes χ≤0.9 for masses ≥0.5M⊙ and χ≤0.1 for masses ≤0.5M⊙. Waveform models included IMRPhenomD, SEOBNRv4ROM, TaylorF2, and SpinTaylorT4, with low-frequency cutoffs typically set at 45 Hz (or 35 Hz for MBTA filtering) to manage computational costs.
- Sensitivity Estimation: Sensitivity was quantified via the "sensitive hypervolume" (⟨VT⟩) using an injection campaign. ∼3.7×107 simulated signals (BBH and BNS) were added to the O4b data. The analysis combined results from O4a and O4b using a mixture model to derive robust sensitivity estimates for the full observing period.
- Candidate Significance: Candidates were ranked by false-alarm rate (FAR). Significance was assessed against the background noise distribution, with trials factors considered for the cumulative significance across multiple pipelines and observing runs.
Key Results
- Detection Status: No significant GW candidates were identified. Two outlier candidates were reported by the GstLAL (S240422ed) and MBTA (S241229bq) pipelines. However, follow-up analyses indicated these were consistent with noise fluctuations. S240422ed was previously identified in low-latency searches but lost significance in the offline SSM search; S241229bq showed a FAR of 0.0043 yr−1 (approx. 2.8σ) in MBTA but was consistent with noise in other pipelines and failed significance thresholds when accounting for trials factors.
- Merger Rate Limits: Based on the null detection, upper limits on the merger rate (R90) for SSM BBHs were established at the 90% confidence level.
- For low-spin SSM BBHs, the most constraining limits range from 50 to 5000 Gpc−3yr−1, depending on the component mass combination.
- For SSM BNS systems (assuming a simple fixed population model), the upper limit is 249 Gpc−3yr−1.
- Dark Matter Constraints:
- Primordial Black Holes: Assuming a monochromatic mass distribution, the fraction of dark matter in PBHs (fPBH) is constrained to be ≤(0.018–0.032) at 1M⊙ and ≤(0.10–0.18) at 0.35M⊙ for early-forming binaries. For late-forming binaries, fPBH<1 is excluded for masses above (0.62–0.74)M⊙.
- Dissipative Dark Matter: For atomic dark matter models, the fraction of dark matter in DBHs (fDBH) is constrained to as low as (1.0–1.1)×10−5 for minimum masses around (0.65–0.85)M⊙. If DBHs constitute all dark matter (fDBH=1), minimum masses above (4.2–4.4)×10−2M⊙ are excluded.
Significance and Claims
The paper presents the most comprehensive results to date from the LVK's SSM search, updating previous constraints from the O4a run. The primary significance lies in the updated and tightened constraints on the abundance of sub-solar mass compact objects, which directly limits the viability of specific dark matter scenarios (PBHs and dissipative DBHs) in the sub-solar mass regime.
The authors emphasize that the absence of detections provides upper limits rather than evidence for existence. The constraints on the PBH fraction are complementary to those derived from the non-observation of a stochastic GW background. For dissipative dark matter, the results probe the microphysics of dark matter models by excluding specific ranges of minimum DBH masses and fractions of dark matter density.
The paper also highlights the first sensitivity estimates for SSM BNS systems including tidal effects in the injection waveforms, demonstrating that while sensitivity is slightly reduced compared to BBH searches due to template mismatch, the search remains effective for probing low-mass neutron star populations. The authors note that future observing runs (O4c and beyond) with increased sensitivity and observing time are expected to further improve these constraints or potentially detect a signal.
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