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Updated: 2025-11-20
A gravitational wave is a ripple in the fabric of space and time, created when massive objects such as black holes or neutron stars accelerate or collide. These waves travel across the Universe at the speed of light, stretching and squeezing space itself as they pass. Predicted by Albert Einstein in 1916 as part of his General Theory of Relativity, gravitational waves were first directly detected a century later, in 2015, by the LIGO observatory. Their discovery opened a new way of observing the cosmos — not through light, but through the vibrations of spacetime — allowing scientists to study some of the most energetic and mysterious events in the Universe.
The Einstein Telescope (ET) is a planned European third-generation gravitational-wave observatory that will take this new field of astronomy to the next level. It will consist either of a triangular-shape geometry or two separated L-shape arms geometry placed in underground tunnels, each about 10 kilometres long, housing ultra-sensitive laser interferometers. By operating deep underground and at cryogenic temperatures, ET will be able to detect much weaker signals than current instruments, observing black hole and neutron star mergers from the farthest reaches of the cosmos.
The Einstein Telescope will consist of a triangular underground observatory with sides 10 kilometres long. Each side will host two laser interferometers arranged in an “X” configuration. By measuring minute changes in the distance between suspended mirrors at the ends of these arms — as small as one thousandth of the diameter of a proton — ET will detect passing gravitational waves. The design includes two sets of interferometers: one optimized for low frequencies (using cryogenically cooled mirrors to reduce thermal noise) and another for high frequencies, allowing the observatory to cover a broad range of sources and timescales.
The observatory will be built deep underground to minimize the effects of seismic vibrations and environmental noise. Three potential sites are currently under study: one in Sardinia (Italy), another one in the Euregio Meuse-Rhine region, located at the border of Belgium, the Netherlands, and Germany whereas the third one is Lusatia in Saxony, Germany. The final choice will depend on detailed geological, environmental, and logistical evaluations.
The Einstein Telescope will be about ten times more sensitive than current detectors such as LIGO, Virgo, and KAGRA. This higher sensitivity will allow scientists to observe thousands of gravitational-wave events every year, reaching back to the time when the first stars and galaxies were forming. It will make it possible to perform precise measurement of neutron star properties, tests of Einstein’s theory of General Relativity under extreme conditions, and the search for primordial gravitational waves that carry information from the earliest moments after the Big Bang.
The Einstein Telescope will also work in close connection with other astronomical observatories to form part of the global multimessenger network, linking gravitational-wave detections with observations of light, neutrinos, and cosmic rays. Such combined observations will help identify the origins of the most energetic phenomena in the Universe, from gamma-ray bursts and magnetar flares to the merging of compact stellar remnants.
A recent ET Collaboration white paper (see
The Science of the Einstein Telescope, arXiv:2503.12263) outlines in detail the scientific reach of ET: across
astrophysics, cosmology, fundamental physics and nuclear matter
under extreme conditions. The document studies both the proposed
triangular single-site and dual L-shaped geometries, and
emphasises how ET will form a key node in the global
multimessenger network — enabling multi-band gravitational-wave
observations and combining them with electromagnetic, neutrino and
cosmic-ray signals. It also highlights the major data-analysis
challenges ahead, given the high event rates and precision
expected for a third-generation detector.
In addition, a proposed ET U.S. counterpart third generation ground-based gravitational wave observatory, the Cosmic Explorer, will have a considerable detection frequency overlap. It will effectively act as a "Super-LIGO" that scales up current technology, its arms will be 10 times larger than LIGO’s, i.e., 40 km long. As a result, the two gravitational wave detectors will act as two distinct halves of a single global observatory. The Einstein Telescope will play the role of a Woofer because it will be underground and will use cryogenic (super-cooled) mirrors, it will be immune to the surface seismic noise that plagues other detectors. It excels at low frequencies (below 10 Hz). The Cosmic Explorer will play the role of a Tweeter because it will be placed on the surface with massive 40 km arms, it will sacrifice low-frequency sensitivity for incredible high-frequency (above 100 Hz) sensitivity.
The primary research purpose of the Einstein Telescope is to serve as a time machine for the universe, dramatically extending our observational horizon to the very edge of the cosmos. Its special low-frequency sensitivity is crucial because it allows the detector to observe much heavier objects, such as intermediate-mass black holes, and to track binary systems for hours or days before they merge. Scientifically, this enables a complete census of black holes throughout cosmic history, allowing astronomers to observe the birth of the very first black holes formed in the early universe and understand how they evolved into the supermassive giants found at the centers of galaxies today.
Beyond mapping the history of the universe, the Einstein Telescope acts as an extreme physics laboratory designed to test the fundamental laws of nature under conditions that cannot be replicated on Earth. One of its key scientific goals is to study the behavior of matter at supranuclear densities by analyzing the precise properties of neutron stars during a merger thereby revealing the equation of state for the densest matter in the universe. For the same purpose, neutron star oscillations, the formation of stellar black holes, and supernova explosions will be targeted by the ET. Furthermore, the telescope will perform high-precision tests of General Relativity in the strong-field regime, essentially checking if Einstein's theory holds up near the event horizons of black holes, while simultaneously providing independent measurements of the universe's expansion rate to help resolve current tensions in cosmological models. The ET will aim at detecting the cosmological gravitational wave background created by phase transitions, cosmic strings or quantum gravity effects occurring through the evolution of the early universe. In addition, it shall probe the possible creation of primordial black holes following the aforementioned phase transitions, which could have impacted the stellar population dynamics and evolution of galaxies.
The observatory will be built deep underground to shield it from
surface vibrations and environmental noise. Two possible locations
are currently under study: the Sos Enattos site in Sardinia
(Italy) and the Euregio Meuse-Rhine region on the
Belgium–Netherlands–Germany border. Both offer exceptional
geological stability and low-seismic conditions needed for the
precise measurement of spacetime distortions.
ET will employ innovative technologies such as cryogenically
cooled mirrors, high-power laser systems, and advanced
vibration-isolation systems. These developments will allow an
unprecedented sensitivity, enabling the detection of signals from
the most distant and ancient events in the Universe. More
information is available on the official Einstein Telescope
website.
Researchers from the IFJ PAN, NZ 15 will take part in the scientific programme of the Einstein Telescope (ET), contributing to theoretical studies, numerical relativity simulations, gravitational wave inputs, data analysis methods, and the development of the multimessenger astrophysics framework that connects gravitational-wave, neutrino, and cosmic-ray observations.
Within IFJ PAN, the group currently includes researchers who focus on neutron star physics and their role as gravitational-wave sources, exploring the connection between dense-matter equations of state and observable waveforms either from compact star oscillations or from their mergers. Complementarly, the modeling of early universe phase transitions, formation of cosmic strings and primordial black holes as well as quantum gravity effects are studied in order to derive associated gravitational wave signals. Work is also done on gravitational waves from eccentric binary systems, the influence of spin-induced effects, and advanced data analysis and Fisher matrix techniques used to assess parameter estimation accuracy for future ET observations.
In the coming years, the team plans to expand its activities by including an experimental physicist to strengthen the link between detector technology and astrophysical data analysis.
The opportunities of the Einstein Telescope (EN)