Einstein@Home: How Volunteer Computing Helps to Reveal Pulsars and Gravitational Waves

Introduction:
Since 2015, the LIGO and Virgo gravitational-wave detectors have been revolutionizing our view of the universe by detecting ripples in space-time caused by colliding massive objects. This began with the first observing run (O1, 2015–2016), when scientists впервые “heard” the merger of two black holes, confirming a major prediction of Einstein’s theory of General Relativity and opening an entirely new window on the cosmos. Each subsequent run — O2 (2016–2017), O3 (2019–2020), and the most recent O4 (2023–2025) — has increased in sensitivity and duration, producing an ever-growing catalog of discoveries that reveal how compact objects like black holes and neutron stars form, interact, and collide across the universe.

Timeline of Key Discoveries & Contributions

1967 – Pulsars discovered
Radio astronomers discover the first pulsars, revealing the existence of neutron stars and opening an entirely new field of astrophysics.

1974 – First binary pulsar
Arecibo observations lead to the discovery of the first binary pulsar (PSR B1913+16), providing the first indirect evidence for gravitational waves and earning the 1993 Nobel Prize.

1990s – Major pulsar surveys begin
Arecibo and other radio telescopes begin large-scale pulsar surveys, dramatically increasing the known pulsar population and identifying many binary systems.

2000 – Green Bank Telescope (GBT) begins operations
The GBT becomes a major tool for pulsar astronomy, contributing discoveries of millisecond and relativistic binary pulsars throughout the 2000s and 2010s.

2004 – PALFA Survey starts (Arecibo)
The PALFA Survey begins, using Arecibo’s ALFA receiver to find faint and distant binary and millisecond pulsars, many later analyzed by Einstein@Home.

2008 – Fermi Gamma-ray Space Telescope launched
Fermi opens the gamma-ray sky, enabling the discovery of radio-quiet and binary gamma-ray pulsars that cannot be found with radio telescopes alone.

2010s – Einstein@Home pulsar discoveries accelerate
Volunteer computing leads to the discovery of dozens of new radio and gamma-ray pulsars, including exotic binary systems, using Arecibo, GBT, and Fermi data.

2015–2016 – O1: First gravitational waves detected
LIGO detects gravitational waves from merging black holes for the first time, confirming a key prediction of General Relativity.

2016–2017 – O2: First neutron-star merger
LIGO and Virgo detect GW170817, the first neutron-star merger, observed in both gravitational waves and light.

2018 – MeerKAT inaugurated
The MeerKAT radio array in South Africa comes online, offering exceptional sensitivity and access to the southern sky.

2019–2020 – O3: New classes of mergers
Dozens of gravitational-wave detections are made, including the first black hole–neutron star mergers.

2020 – Collapse of Arecibo Observatory
Arecibo ceases operations, ending new observations but leaving behind a rich legacy dataset still yielding discoveries.

~2022 – MeerKAT data joins Einstein@Home
MeerKAT becomes a primary source of binary radio pulsar discoveries, continuing Arecibo’s scientific legacy.

2023–2025 – O4: Hundreds of gravitational-wave signals
LIGO, Virgo, and KAGRA conduct the longest and most sensitive observing run to date, detecting hundreds of compact-object mergers.

Key points:

• Radio telescopes (Arecibo, GBT, MeerKAT) revealed neutron stars as precise cosmic clocks.

• Gamma-ray observations (Fermi) uncovered pulsars invisible in radio light.

• Gravitational-wave detectors (LIGO/Virgo/KAGRA) exposed the violent mergers of those same objects.

   

The Contributing Telescopes and Instruments

The Arecibo Observatory in Puerto Rico was one of the world’s most sensitive radio telescopes and played a central role in discovering binary pulsars. Beginning major pulsar surveys in the 1990s and notably conducting the PALFA Survey starting in 2004, Arecibo detected hundreds of pulsars, including many millisecond pulsars and systems in tight orbits. These discoveries have been crucial for studying extreme physics, testing Einstein’s theory of General Relativity, and providing targets for continued observation by projects like Einstein@Home. Even after Arecibo’s collapse in December 2020, its legacy data continue to be analyzed by volunteers to uncover rare pulsars.

The Green Bank Telescope (GBT) in West Virginia, USA, officially opened in 2000 and has since contributed significantly to Einstein@Home’s pulsar searches. Its sensitive observations focus on binary pulsars, including millisecond and relativistic systems, using advanced receivers to detect faint signals. Volunteer computers analyze GBT data to identify pulsars whose signals are Doppler-shifted by orbital motion. The GBT has helped expand our understanding of pulsar populations in the northern sky, contributing to discoveries throughout the 2000s and 2010s, including several millisecond and binary pulsars used in precision timing experiments.

The MeerKAT telescope in South Africa was completed and inaugurated around 2018, and its data began being used by Einstein@Home for pulsar searches around 2022, after Arecibo ceased operations. With its 64-dish array and high sensitivity, MeerKAT provides high-quality southern-sky data that enable the search for binary pulsars, including very faint and distant systems. Volunteers analyze the data to detect pulsars often in tight binary orbits, where the signals shift in frequency due to orbital motion. MeerKAT has rapidly become a major source of discoveries in the southern hemisphere, complementing the legacy of Arecibo and GBT.

The Fermi Gamma-ray Space Telescope was launched in June 2008 and has since surveyed the entire sky in gamma rays, detecting pulsars that are sometimes invisible in radio waves. Einstein@Home uses volunteer computers to search Fermi data for both isolated gamma-ray pulsars and pulsars in binary systems, analyzing the timing of gamma-ray photons for periodic signals. This collaboration has led to the discovery of several dozen new gamma-ray pulsars, including exotic systems, with significant discoveries reported in the 2010s and 2020s, expanding our knowledge of neutron stars that emit primarily in high-energy gamma rays rather than radio waves.

O1 (First Observing Run) ran from September 2015 to January 2016 and marked the beginning of the modern era of gravitational-wave astronomy. During this run, LIGO made the first-ever direct detection of gravitational waves, observing ripples in space-time produced by the merger of two black holes more than a billion light-years away. This groundbreaking discovery confirmed a key prediction of Einstein’s theory of General Relativity and opened a completely new way of observing the universe, allowing scientists to “listen” to cosmic events that were invisible to traditional telescopes.

O2 (Second Observing Run) ran from November 2016 to August 2017, following the first LIGO run, O1. O2 built on that success and made eight confident detections, including seven black hole mergers and the first neutron-star merger ever observed (GW170817). That neutron-star event was especially remarkable because it was seen not only in gravitational waves but also with telescopes across the electromagnetic spectrum, allowing scientists to study the explosion in unprecedented detail. O2 confirmed that gravitational-wave astronomy could reveal entirely new cosmic phenomena, from black-hole collisions to neutron-star mergers, changing our understanding of the universe.

O3 (Third Observing Run) ran from April 2019 to March 2020 and greatly expanded the number of gravitational-wave detections compared to previous runs. During O3, scientists detected dozens of events, mostly colliding black holes, but also including mergers involving neutron stars. For the first time, black hole–neutron star mergers were observed, revealing a new type of cosmic collision. This run showed that detecting gravitational waves had become routine and opened a new window on how massive stars and compact objects interact in the universe.

O4 (Fourth Observing Run) ran from May 2023 to November 2025 and was the longest and most sensitive run yet. Using the upgraded LIGO, Virgo, and KAGRA detectors, scientists recorded hundreds of gravitational-wave signals, mostly from binary black hole collisions, along with some involving neutron stars. While none of the O4 events were as dramatic as the first neutron-star merger seen in O2, the sheer number of detections is helping astronomers understand the populations of black holes and neutron stars across the universe, making gravitational-wave astronomy a powerful tool for exploring the cosmos.

Summary:
Across the period from 2015 to 2025, the LIGO, Virgo, and KAGRA detectors have transformed our understanding of the universe by detecting gravitational waves from colliding compact objects. The observing runs O1 through O4 revealed hundreds of cosmic mergers, beginning with the first detection of merging black holes in O1 (2015–2016), followed by the first neutron-star merger in O2 (2016–2017), the discovery of black hole–neutron star mergers in O3 (2019–2020), and hundreds of additional signals during the highly sensitive and long O4 run (2023–2025). Together, these discoveries turned gravitational-wave astronomy from a bold prediction into a mature observational science, providing a direct way to study the most violent events in the universe.

Complementing these dramatic collisions, Einstein@Home has revealed the quieter but equally informative signals from binary pulsars, using radio and gamma-ray data spanning more than two decades. Searches using the Arecibo Observatory (notably since the PALFA Survey began in 2004), the Green Bank Telescope (operational since 2000), the MeerKAT radio array in South Africa (contributing since around 2022), and NASA’s Fermi Gamma-ray Space Telescope (launched in 2008) have led to the discovery of dozens of new pulsars, including millisecond and binary systems. Together, gravitational-wave detections and pulsar discoveries provide complementary views of neutron stars and black holes, linking steady cosmic “clocks” with catastrophic mergers to deepen our understanding of extreme astrophysics.