2025/11/13
Summary of the observed signals from the O4a period
On August 25, 2025, the LIGO-Virgo-KAGRA collaboration released the Gravitational-Wave Transient Catalog GWTC-4.0 (Paper [1][2][3]), which includes all gravitational-wave signals detected to date, including those observed during the first approximately eight months of the fourth observing run (O4a: May 24, 2023 – January 16, 2024).
During the O4a period, 128 new candidate gravitational-wave events from compact binary mergers were added. Most of these events are believed to be binary black hole (BBH) mergers, but neutron star–black hole (NSBH) mergers—such as GW230518_125908 and GW230529_181500—are also included.
The component black hole masses in these binaries range from 5.79 solar masses (GW230627_015337) to 137 solar masses (GW231123_135430), the latter possibly being the most massive binary merger detected so far. Additionally, GW230814_230901 and GW231226_101520 were detected with a network signal-to-noise ratio (SNR) exceeding 30, providing high-quality data valuable for waveform analysis and astrophysical studies.
With the inclusion of these new events, the total number of candidates in the catalog has reached 218.
In Paper [4], scientists analyzed the gravitational-wave events listed in the GWTC-4.0 catalog to better understand how often different types of compact objects—like binary black holes (BBH), binary neutron stars (BNS), and neutron star–black hole (NSBH) pairs—merge in the universe.
By studying the observed gravitational waves, they estimated how frequently these cosmic collisions happen in the universe and examined the properties of the merging objects, such as their masses, spins, and mass ratios. Interestingly, the study found that black hole masses tend to cluster around several different values, suggesting that black holes may form through multiple different processes. The researchers also looked at how the merger rates change over time, as the universe evolves.
In Paper [5] (arXiv:2509.04348), researchers used 142 gravitational-wave events from the GWTC-4.0 catalog to study the expansion rate of the universe and to test whether Einstein’s theory of general relativity still holds true over cosmic distances.
Gravitational waves allow scientists to measure how far away an event occurred (called the luminosity distance) and how massive the objects were when they merged. However, to understand how fast the universe is expanding, they also need to know the redshift—a measure of how much the universe has stretched since the event happened.
To estimate redshift, the researchers used two methods:
They looked at patterns in the masses and how often mergers happen at different times in the universe’s history.
They compared the sky locations of the gravitational-wave events with catalogs of galaxies to find likely host galaxies and their redshifts.
By combining these methods, they estimated the Hubble constant—a key number that tells us how fast the universe is expanding—to be about 76.6+13.0-9.5 km/s/Mpc. When they also included data from GW170817, a gravitational-wave event with a known light signal, they were able to make even more precise measurements.
For more details, please refer to the explanations provided by the LVK collaboration linked below.
Papers
GWTC-4.0 Introduction: https://arxiv.org/abs/2508.18080
GWTC-4.0 Methods: https://arxiv.org/abs/2508.18081
GWTC-4.0 Results: https://arxiv.org/abs/2508.18082
GWTC-4.0 Rate and population: https://arxiv.org/abs/2508.18083
GWTC-4.0 Cosmology: https://arxiv.org/abs/2509.04348
Science Summary
English
GWTC-4.0 Catalog:https://ligo.org/wp-content/uploads/2025/08/O4a_catalog_omnibus.pdf
GWTC-4.0 Rate and population: https://ligo.org/wp-content/uploads/2025/08/O4a_Astro_Distrib.pdf
GWTC-4.0 Cosmology: https://ligo.org/wp-content/uploads/2025/09/O4a_Cosmology.pdf
Other languages: https://ligo.org/science-summaries/
Announcement by LIGO
GWTC-4.0 Catalog: https://ligo.org/science-summaries/O4a_catalog_omnibus/
GWTC-4.0 Rate and population: https://ligo.org/science-summaries/O4a_Astro_Distrib/
GWTC-4.0 Cosmology: https://ligo.org/science-summaries/O4a_Cosmology/