Searches for Binary Mergers with Sub-solar Mass Components in Data from the First Part of LIGO--Virgo--KAGRA's Fourth Observing Run
This paper reports on a null search for sub-solar mass compact binary mergers using LIGO-Virgo-KAGRA data from the first part of the fourth observing run, establishing stringent upper limits on their merger rates and consequently constraining primordial black hole and dissipative dark matter models as potential dark matter candidates.
Original authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, A. Adam, C. Adamcewicz, S. Adhicary, D. Adhikari, N. 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, S. Albanesi, L. Albers, W. Ali, S. Al-Kershi, C. Alléné, A. Allocca, S. Al-Shammari, P. A. Altin, S. Alvarez-Lopez, W. Amar, O. Amarasinghe, A. Amato, F. Amicucci, C. Amra, C. Anand, A. Ananyeva, S. B. Anderson, W. G. Anderson, M. Andia, M. Ando, M. Andrés-Carcasona, J. L. Andrey, T. Andrić, J. Anglin, J. Anna, S. Ansoldi, J. M. Antelis, S. Antier, M. Aoumi, E. Z. Appavuravther, 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, F. Attadio, F. Aubin, K. AultONeal, G. Avallone, E. A. Avila, S. Babak, C. Badger, S. Bae, S. Bagnasco, L. Baiotti, R. Bajpai, T. Baka, K. A. Baker, T. Baker, G. Balbi, G. Baldi, N. Baldicchi, M. Ball, G. Ballardin, 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, N. Barman, P. Barneo, 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, P. Baxi, J. C. Bayley, A. C. Baylor, P. A. Baynard II, M. Bazzan, V. M. Bedakihale, F. Beirnaert, M. Bejger, D. Belardinelli, A. S. Bell, C. Bellani, L. Bellizzi, D. Beltran-Martinez, W. Benoit, I. Bentara, M. Ben Yaala, S. Bera, F. Bergamin, B. K. Berger, S. Bernuzzi, M. Beroiz, C. P. L. Berry, I. Berry, D. Bersanetti, T. Bertheas, A. Bertolini, J. Betzwieser, D. Beveridge, G. Bevilacqua, N. Bevins, R. Bhandare, R. Bhatt, A. Bhattacharjee, D. Bhattacharjee, S. Bhattacharyya, S. Bhaumik, V. Biancalana, A. Bianchi, F. Bianchi, I. A. Bilenko, G. Billingsley, A. Binetti, S. Bini, C. Binu, S. Biot, O. Birnholtz, S. Biscoveanu, A. Bisht, M. Bitossi, M. -A. Bizouard, S. Blaber, J. K. Blackburn, L. A. Blagg, C. D. Blair, D. G. Blair, N. Bode, N. Boettner, P. Bogdan, G. Boileau, M. Boldrini, G. N. Bolingbroke, A. Bolliand, L. D. Bonavena, R. Bondarescu, F. Bondu, 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, 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, B. C. Brown, D. D. Brown, M. L. Brozzetti, S. Brunett, G. Bruno, R. Bruntz, J. Bryant, Y. Bu, F. Bucci, J. Buchanan, O. Bulashenko, T. Bulik, H. J. Bulten, A. Buonanno, K. Burtnyk, R. Buscicchio, D. Buskulic, C. Buy, R. L. Byer, R. Cabrita, V. Cáceres-Barbosa, L. Cadonati, G. Cagnoli, C. Cahillane, A. Calafat, T. A. Callister, E. Calloni, S. R. Callos, G. Caneva Santoro, K. C. Cannon, H. Cao, L. A. Capistran, E. Capocasa, G. Capoccia, E. Capote, G. Capurri, G. Carapella, F. Carbognani, K. J. Cardona-Martínez, M. Carlassara, J. B. Carlin, T. K. Carlson, M. F. Carney, M. Carpinelli, G. Carrillo, J. J. Carter, G. Carullo, A. Casallas-Lagos, J. Casanueva Diaz, C. Casentini, S. Caudill, M. Cavaglià, R. Cavalieri, G. Cella, S. Cepic, P. Cerdá-Durán, E. Cesarini, N. Chabbra, W. Chaibi, A. Chakraborty, P. Chakraborty, S. Chakraborty, S. Chalathadka Subrahmanya, R. Chalmers, C. Chan, J. C. L. Chan, M. Chan, K. Chang, P. Charlton, E. Chassande-Mottin, C. Chatterjee, Debarati Chatterjee, Deep Chatterjee, M. Chaturvedi, S. Chaty, A. Chen, A. H. -Y. Chen, D. Chen, H. Chen, H. Y. Chen, S. Chen, Y. Chen, G. Cheng, H. P. Cheng, P. Chessa, T. Cheunchitra, H. T. Cheung, S. Y. Cheung, F. Chiadini, G. Chiarini, A. Chiba, A. Chincarini, D. 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Davenport, M. Davier, T. F. Davies, D. Davis, L. Davis, M. C. Davis, P. Davis, E. J. Daw, M. Dax, J. De Bolle, M. Deenadayalan, J. Degallaix, M. De Laurentis, C. J. Delgado Mendez, F. De Lillo, S. Della Torre, W. Del Pozzo, O. M. del Rio, A. Demagny, F. De Marco, G. Demasi, F. De Matteis, N. Demos, T. Dent, A. Depasse, N. DePergola, R. De Pietri, R. De Rosa, C. De Rossi, M. Desai, V. Deshmukh, R. De Simone, S. Determan, A. Dhani, R. Dhurkunde, R. Diab, C. Diaz, M. C. Díaz, M. Di Cesare, G. Dideron, T. Dietrich, L. Di Fiore, C. Di Fronzo, M. Di Giovanni, T. Di Girolamo, D. Diksha, J. Ding, S. Di Pace, I. Di Palma, D. Di Piero, F. Di Renzo, Divyajyoti, A. Dmitriev, J. P. Docherty, Z. Doctor, N. Doerksen, E. Dohmen, A. Doke, A. Domiciano De Souza, L. D'Onofrio, F. Donovan, K. L. Dooley, T. Dooney, S. Doravari, O. Dorosh, F. Dosopoulou, W. J. D. Doyle, M. Drago, J. C. Driggers, M. Dubois, R. R. Dumbreck, L. Dunn, U. Dupletsa, D. D'Urso, P. Dutta Roy, H. Duval, P. -A. Duverne, S. E. 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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
Imagine the universe is a giant, dark ocean. For years, scientists have been using giant "ears" (called LIGO, Virgo, and KAGRA) to listen for the splashes made when heavy objects, like black holes or neutron stars, crash into each other. Usually, these objects are as heavy as our Sun or even heavier.
But what if there are tiny, invisible "pebbles" in this ocean? Specifically, what if there are black holes or neutron stars that weigh less than the Sun? These are called "sub-solar mass" objects. Finding them would be a huge deal because, in our current understanding of how stars die, nature shouldn't be able to make black holes this small. If we find them, it would mean they weren't born from dying stars, but perhaps from the very beginning of the universe or from some strange, invisible "dark matter."
This paper is the report card from the first part of the fourth "listening season" (called O4a) of these gravitational wave detectors. Here is what the scientists found, explained simply:
1. The Great Hunt for Tiny Black Holes
The team used three different computer programs (think of them as three different detectives: GstLAL, MBTA, and PyCBC) to sift through the data collected between May 2023 and January 2024. They were looking for the specific "chirp" sound that would be made if two tiny, sub-solar mass objects merged.
The Result: They found nothing.
No tiny black holes. No tiny neutron stars.
The only "loud" sound they heard was a known event (GW230529_181500), but when they weighed it, they realized it was made of heavy objects (heavier than the Sun), so it didn't count as a "tiny" discovery.
2. How Good Was the Search? (The "Net" Analogy)
Even though they didn't find anything, the paper is very important because it tells us how good their net was.
Imagine you are fishing in a lake. If you catch no fish, you can't just say "there are no fish." You have to ask, "Was my net big enough? Did I fish in the right spot?"
- The Net: The scientists calculated exactly how much of the universe they "swept" with their search. They call this the "sensitive space-time hypervolume."
- The Catch: They found that their net was good enough to catch tiny black holes if they were common. Because they didn't catch any, they can now say with 90% confidence: "If these tiny black holes exist, they are very rare."
3. What This Means for "Dark Matter"
Since they didn't find these objects, the paper uses this "empty net" to test two popular theories about Dark Matter (the invisible stuff that holds galaxies together).
Theory A: Primordial Black Holes (PBHs)
- The Idea: Some scientists think dark matter is made of tiny black holes formed right after the Big Bang.
- The Verdict: The search results say that if these tiny black holes exist, they can't make up 100% of the dark matter. In fact, for black holes around the size of the Sun, they can make up at most 7% of the dark matter. For slightly smaller ones, the limit is even tighter. It's like saying, "Dark matter might be made of these pebbles, but it can't be all pebbles; there must be something else too."
Theory B: Dissipative Dark Matter (Dark Black Holes)
- The Idea: Another theory suggests dark matter can cool down and clump together to form "Dark Black Holes," similar to how gas clouds form stars.
- The Verdict: The search puts a very strict limit on this. If these Dark Black Holes exist, they can only make up about 0.001% (or 1 in 100,000) of the total dark matter. This is a very small number, effectively ruling out many versions of this theory.
4. A New Kind of Listening
For the first time, this paper also checked if their search could hear low-mass neutron stars (stars made of super-dense matter, but lighter than usual).
- The Challenge: Tiny neutron stars are weird. They are so squishy that they stretch and squeeze each other before crashing, which changes the sound they make. The standard "listening templates" usually ignore this.
- The Result: The team showed that their search is sensitive enough to hear these squishy, tiny stars if they exist, even though they didn't find any this time. This sets the stage for future searches to listen for these specific "squishy" sounds.
The Bottom Line
The scientists listened very carefully for tiny, exotic black holes and neutron stars using the world's most sensitive ears. They didn't hear a single one.
Why is this good news?
In science, a "null result" (finding nothing) is often a victory. It means they have successfully ruled out many wild ideas about what the universe is made of. They have tightened the noose around the theories of "Primordial Black Holes" and "Dark Black Holes," telling us that if these objects exist, they are far rarer than we hoped. The search continues, and with more listening time coming up, the net will only get bigger and better.
Technical Summary: Searches for Binary Mergers with Sub-solar Mass Components in Data from the First Part of LIGO–Virgo–KAGRA's Fourth Observing Run
Problem and Motivation
Gravitational wave (GW) detectors have primarily detected compact binary coalescences (CBCs) involving stellar-mass black holes (BHs) and neutron stars (NSs) with component masses ≥1M⊙. However, the existence of sub-solar mass (SSM) compact objects (0.2M⊙≤m≤1M⊙) would imply exotic formation channels beyond standard stellar evolution. Key theoretical motivations include Primordial Black Holes (PBHs), a candidate for dark matter, and Dark Black Holes (DBHs) formed via dissipative dark matter models. While previous searches in LIGO–Virgo–KAGRA (LVK) data (O1–O3) have constrained these populations, no confident SSM candidates have been identified. This paper addresses the search for SSM binaries in the first part of the fourth observing run (O4a), aiming to improve constraints on dark matter models and, for the first time, quantify sensitivity to low-mass neutron star binaries.
Methodology
The analysis utilizes strain data collected by the LIGO detectors (Hanford and Livingston) between May 24, 2023, and January 16, 2024. The search employs three independent matched-filtering pipelines: GstLAL, MBTA, and PyCBC.
- Search Parameters: The template banks target CBCs with redshifted primary masses (1+z)m1 between 0.2M⊙ and 10.0M⊙, and secondary masses (1+z)m2 between 0.2M⊙ and 1.0M⊙. The mass ratio q=m2/m1 is restricted to 0.1≤q≤1.0. Spin effects are included, with dimensionless spin magnitudes χ≤0.9 for masses ≥0.5M⊙ and χ≤0.1 for masses ≤0.5M⊙.
- Pipeline Specifics:
- GstLAL: Uses a geometric placement method for the template bank (>3.0 million templates) with a minimum match of 0.965. It employs IMRPhenomD waveforms with a low-frequency cutoff of 45 Hz.
- MBTA: Utilizes a multi-band template analysis approach with >2.5 million templates based on TaylorF2 and SpinTaylorT4 approximants. It includes single-detector triggers in its archival analysis.
- PyCBC: Uses a geometric placement algorithm for ∼2.0 million templates. It lowers the low-frequency cutoff to 30 Hz for filtering (though bank generation starts at 45 Hz to manage computational cost) and uses SEOBNRv4ROM for total masses >1.0M⊙.
- Sensitivity Estimation: The sensitive space-time hypervolume (⟨VT⟩) is determined empirically by injecting simulated signals into the O4a data. Two fiducial populations are used:
- SSM BBH: At least one component <1M⊙.
- SSM BNS: Includes tidal interaction effects modeled using IMRPhenomXP_NRTidalv3 with the GPPVA+DD2 equation of state, extending into the sub-solar regime (down to 0.5M⊙).
- Rate and Dark Matter Constraints: Upper limits on merger rates are derived using the "loudest event" statistic formalism at 90% confidence. These limits are applied to constrain the fraction of dark matter in PBHs (fPBH) for both early- and late-forming binary scenarios, and the fraction of dark matter in DBHs (fDBH) within a dissipative dark matter framework.
Key Results
- Candidate Events: No statistically significant GW transients from SSM candidates were identified. All triggers were consistent with the expected noise background, with the exception of the previously published event GW230529_181500. This event was detected by all pipelines but is excluded from the SSM candidate list because its inferred secondary mass exceeds 1M⊙.
- Sensitivity: The sensitive hypervolume ⟨VT⟩ for O4a is comparable to O3 results, with a mean ratio of ⟨VT⟩O4a/⟨VT⟩O3 of approximately 1.0–1.4 depending on the pipeline and mass bin. The slight gains from improved detector sensitivity are offset by the shorter observing duration of O4a (0.76 yr) compared to O3 (0.9 yr).
- Merger Rate Limits: The 90% confidence level upper limits on the merger rate of SSM BBHs range from ∼110 to 10,000Gpc−3yr−1, depending on the chirp mass and pipeline. For a simple, fixed population of SSM BNSs, the upper limit is estimated at ∼86Gpc−3yr−1.
- Dark Matter Constraints:
- Primordial Black Holes (PBHs): For late-forming PBH binaries, the search excludes fPBH=1 for masses above 0.9M⊙. For early-forming binaries, the upper limits on fPBH range from ≤7% at 1M⊙ to ≤40% at 0.35M⊙.
- Dissipative Dark Matter (DBHs): The analysis constrains the fraction of dark matter in DBHs (fDBH) to be ≤(1.2 to 1.3)×10−5 when the minimum DBH mass is 1M⊙. This excludes a region in the parameter space of dark matter fraction and minimum mass.
Significance and Claims
The paper claims several firsts and updates to the field:
- First SSM BNS Sensitivity: This work reports the sensitivity of SSM searches to binaries containing low-mass neutron stars for the first time, estimating a sensitive hypervolume of ∼10−3Gpc3yr for such populations.
- Updated PBH Constraints: It provides the first constraints on the PBH fraction for late-forming binaries using O4a data and presents the tightest constraints to date from GW observations for early-forming PBH binaries.
- Dissipative Dark Matter Limits: The results place strict limits on the formation of DBHs in dissipative dark matter models, excluding specific regions of the fDBH–Mmin parameter space.
- Operational Milestone: The paper highlights the successful operation of independent low-latency SSM searches during O4, enabling prompt follow-up, though this paper focuses on the archival results.
The authors note that O4a data alone provides only marginal improvements over previous O3 limits due to the shorter observation time. However, they project that including data from the remainder of the fourth observing run (O4b and O4c) will increase search sensitivity by a factor of 2–3, leading to significantly more stringent constraints or potential detections. The absence of detections continues to rule out standard astrophysical channels for SSM black holes, reinforcing the need for exotic formation scenarios if such objects are discovered.
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