Science Case for the Einstein Telescope (1912.02622v4)

Abstract: The Einstein Telescope (ET), a proposed European ground-based gravitational-wave detector of third-generation, is an evolution of second-generation detectors such as Advanced LIGO, Advanced Virgo, and KAGRA which could be operating in the mid 2030s. ET will explore the universe with gravitational waves up to cosmological distances. We discuss its main scientific objectives and its potential for discoveries in astrophysics, cosmology and fundamental physics.

• The paper establishes the scientific framework for the Einstein Telescope, detailing its potential to detect gravitational waves from cosmological distances.
• The paper demonstrates that the Einstein Telescope will observe diverse astrophysical events, including binary black hole and neutron star mergers across cosmic history.
• The paper highlights advanced design strategies, such as underground placement and cryogenic techniques, to minimize noise and enhance sensitivity.

Overview of the Einstein Telescope Science Case

The paper provides a comprehensive scientific framework for the proposed Einstein Telescope (ET), a third-generation ground-based gravitational-wave (GW) detector. Building upon the advancements made by second-generation detectors such as Advanced LIGO, Advanced Virgo, and KAGRA, the ET is poised to significantly enhance our capability to explore the Universe through gravitational waves. The paper delineates the diverse scientific objectives of the ET and underscores its potential impact across astrophysics, fundamental physics, and cosmology.

Scientific Objectives

The Einstein Telescope aims to achieve an unprecedented level of sensitivity in gravitational wave detection, extending observations to cosmological distances. Key scientific objectives of the ET include:

1. Astrophysical Discoveries:
   • The ET will enable a comprehensive census of binary black hole (BH) and neutron star (NS) populations across vast cosmic epochs, potentially observing BH-BH mergers up to redshifts $z \sim 20$.
   • One primary interest is detecting mergers in the early Universe, potentially unraveling the existence of primordial black holes.
2. Probing Fundamental Physics:
   • By observing the quasi-normal modes of BH mergers, the ET will provide stringent tests of General Relativity near the event horizon.
   • The ET is expected to detect deviations from GR predictions or identify exotic compact objects, should they exist.
3. Exploring Dark Matter and Energy:
   • The detector will offer insights into the nature of dark matter by studying BH and NS systems potentially influenced by dark matter interactions.
   • Through precise measurement of the luminosity distance, the ET will significantly advance our understanding of dark energy and modifications of gravity at cosmological scales.
4. Stochastic Backgrounds:
   • The ET's improved sensitivity could detect cosmological stochastic backgrounds of GWs, possibly arising from primordial phases of the Universe such as inflation or phase transitions.
   • Observing these backgrounds would open a new observational window into the physics of the very early Universe, far beyond the reach of electromagnetic observations.

Technical and Strategic Implementation

The ET design emphasizes a triangular configuration, featuring three nested interferometers with 10 km arms. This geometry enhances the detector's sensitivity and allows for a broad frequency coverage. Key design aspects also include placing the facility underground to minimize seismic noise and using advanced techniques such as cryogenic cooling and high optical power to minimize thermal and quantum noise contributions.

Implications and Future Prospects

The implications of the Einstein Telescope span various scientific disciplines. In astrophysics, the ET promises a transformative understanding of compact object populations, potentially identifying primordial BHs, pivotal in deciphering the Universe's evolutionary history. The detector's contributions to fundamental physics could test and expand the current framework of GR, possibly paving the way to insights into quantum gravity regimes.

In cosmology, the potential for significant progress in understanding dark energy and uncovering the processes of the early Universe's formative moments further underlines its importance. An interconnected ET infrastructure, potentially paired with other detectors like LISA, would extend these studies by enabling multi-band GW astronomy, enhancing observational accuracy and the scope of discoveries.

In summary, the Einstein Telescope stands as a cornerstone in the future of gravitational wave astronomy, with the potential to significantly advance our understanding of the cosmos by probing phenomena that remain inaccessible to existing electromagnetic observations.