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Updated: 2026-01-08
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.
Figure 1: Astrophysical horizon of current and future
gravitational-wave detectors. This diagram compares the reach of
current detectors (like Advanced LIGO A+ and Advanced Virgo) with
proposed next-generation detectors such as Cosmic Explorer (CE)
and Einstein Telescope (ET). The radial axis indicates redshift,
showing how far into the universe different detectors can see
binary black hole (BH+BH) and neutron star (NS+NS) mergers.
GW150914 and GW170817 represent the first observed binary black
hole and binary neutron star mergers, respectively, serving as
benchmarks. Next-generation detectors like the Einstein Telescope
are planned to have a cosmological reach, capable of detecting
sources across the entire universe.
Credit:
Cosmic Explorer Project
Figure 2: Astrophysical sensitivity (amplitude spectrum of the
detector noise) as a function of frequency for Cosmic Explorer,
the third observing run (O3) and upgraded (A+) sensitivities of
Advanced LIGO, LIGO Voyager, NEMO, and the Einstein Telescope.
Credit:
Cosmic Explorer Project
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)