The Gravitational Universe (1305.5720v1)

Abstract: The last century has seen enormous progress in our understanding of the Universe. We know the life cycles of stars, the structure of galaxies, the remnants of the big bang, and have a general understanding of how the Universe evolved. We have come remarkably far using electromagnetic radiation as our tool for observing the Universe. However, gravity is the engine behind many of the processes in the Universe, and much of its action is dark. Opening a gravitational window on the Universe will let us go further than any alternative. Gravity has its own messenger: Gravitational waves, ripples in the fabric of spacetime. They travel essentially undisturbed and let us peer deep into the formation of the first seed black holes, exploring redshifts as large as z ~ 20, prior to the epoch of cosmic re-ionisation. Exquisite and unprecedented measurements of black hole masses and spins will make it possible to trace the history of black holes across all stages of galaxy evolution, and at the same time constrain any deviation from the Kerr metric of General Relativity. eLISA will be the first ever mission to study the entire Universe with gravitational waves. eLISA is an all-sky monitor and will offer a wide view of a dynamic cosmos using gravitational waves as new and unique messengers to unveil The Gravitational Universe. It provides the closest ever view of the early processes at TeV energies, has guaranteed sources in the form of verification binaries in the Milky Way, and can probe the entire Universe, from its smallest scales around singularities and black holes, all the way to cosmological dimensions.

The Gravitational Universe: Exploring the Cosmos with Gravitational Waves

The paper "The Gravitational Universe" outlines the ambitious scientific objectives of the eLISA mission, a space-based gravitational wave observatory envisioned to probe the universe in unprecedented detail. The paper discusses the transition from electromagnetic-based astronomy to gravitational wave observation, paving the way to explore cosmic phenomena invisible to traditional methods.

Gravitational waves, as proposed by Einstein, are perturbations in spacetime caused by massive accelerating bodies. In contrast to electromagnetic radiation, gravitational waves traverse cosmic distances with little disturbance, making them invaluable in studying phenomena such as black hole mergers, the formation of stars, and the evolution of galaxies. The eLISA mission is poised to capitalize on this capability, aiming to offer a complete survey of the universe by detecting and interpreting gravitational waves.

Core Scientific Objectives

Probing Massive Black Holes Across Cosmic History

eLISA will focus on massive black holes, promising insights into their formation, growth, and the dynamics of their mergers. These observations are critical for understanding galaxy formation and evolution. Black holes, ranging from intermediate masses to supermassive giants, trace the life cycles of galaxies from cosmic dawn to the present, providing empirical data to test and refine gravitational theories.

Testing General Relativity and Exploring Strong Gravity

The detection of gravitational waves serves as a direct test of General Relativity in the strong-field regime. Insights will be gleaned from observing the inspiral, merger, and ringdown phases of binary black hole coalescences. eLISA’s precision could potentially reveal deviations from the Kerr metric, offering clues on quantum gravity and alternative theories.

Investigating Ultra-Compact Binaries

The Milky Way hosts numerous ultra-compact binaries, typically involving white dwarfs, neutron stars, or stellar-mass black holes. These binaries emit gravitational waves predominantly at mHz frequencies, a range uniquely accessible to eLISA. Observing these binaries will contribute to understanding their population dynamics and their evolutionary paths, potentially resolving puzzles regarding their coalescence rates and explosive phenomena like Type Ia supernovae.

Cosmological Implications

Gravitational waves also afford opportunities to explore cosmological phenomena at energy scales beyond the reach of terrestrial accelerators, such as TeV scale physics. eLISA could detect gravitational waves from the early universe, revealing processes like cosmic phase transitions and inflationary reheating.

Technological and Methodological Framework

The paper describes the technological blueprint for eLISA, including a triangular constellation of spacecraft engaging in laser interferometry to measure spacetime distortions. By employing drag-free control, these spacecraft will counteract external forces, preserving the integrity of ultra-sensitive measurements. Additionally, advanced data analysis frameworks developed through mock challenges will empower scientists to sift through the vast array of signals expected during operation.

Observational Strategy and Impact

By the launch target of 2028, eLISA will integrate into a rapidly evolving scientific landscape. Ground-based detectors like LIGO and VIRGO will have paved the way with high-frequency observations, while PTAs will offer insights at nHz levels. eLISA will bridge these gaps, expanding scientific horizons with its all-sky, continuous monitoring capability. Notably, it may identify counterparts to these signals in various electromagnetic bands, facilitating multimodal astronomical studies.

Concluding Remarks

The eLISA mission aims to transform our understanding of the universe, leveraging gravitational waves to explore the unseen depths of cosmic phenomena. Its contributions will span astrophysics, fundamental physics, and cosmology, ensuring its pivotal role in the scientific progression well into 2028 and beyond. By elucidating the gravitational universe, eLISA will enable researchers to test, refine, and potentially remodel our conception of the cosmos, harnessing spacetime’s subtle vibrations as a novel cosmic probe.