Physics of the Cosmos
Exploring fundamental questions regarding the physical forces of the universe

Gravitational Wave Science Interest Group
(GW SIG)

Decadal Survey White Paper Projects

Gravitational Wave Science White Papers

Submitted by community members to the GW SIG and Astro2020

  1. Cosmology with a Space-Based Gravitational Wave Observatory
  2. Details

    Lead Author Robert Caldwell
    Lead Author email robert.r.caldwell@dartmouth.edu
    Coauthors Mustafa Amin, Craig Hogan, Kelly Holley-Bockelmann, Daniel Holz, Philippe Jetzer, Ely Kovetz, Priya Natarajan, David Shoemaker, Tristan Smith, Nicola Tamanini
    Key words
    Link to white paper / draft [PDF]
    Abstract There are two big questions cosmologists would like to answer—How does the Universe work, and what are its origin and destiny? A long wavelength gravitational wave detector—with million km interferometer arms, achievable only from space—gives a unique opportunity to address both of these questions. A sensitive, mHz frequency observatory could use the inspiral and merger of massive black hole binaries as standard sirens, extending our ability to characterize the expansion history of the Universe from the onset of dark energy-domination out to a redshift z ~ 10. A low-frequency detector, furthermore, offers the best chance for discovery of exotic gravitational wave sources, including a primordial stochastic background, that could reveal clues to the origin of our Universe.

  3. Where are the Intermediate Mass Black Holes?
  4. Details

    Lead Author Jillian Bellovary
    Lead Author email jbellovary@amnh.org
    Coauthors  
    Key words intermediate mass black holes; black holes; gravitational waves; intermediate mass ratio inspirals
    Link to white paper / draft [PDF]
    Abstract Observational evidence has been mounting for the existence of intermediate mass black holes (IMBHs), but observing them at all, much less constraining their masses, is very challenging. One theorized formation channel for IMBHs is as the seeds for supermassive black holes in the early universe. As a result, IMBHs are predicted to exist in the local universe in dwarf galaxies, as well as wandering in more massive galaxy halos. However, these environments are not conducive to the accretion events or dynamical signatures which allow us to detect IMBHs. The Laser Interferometer Space Antenna (LISA) will demystify IMBHs by detecting the mergers of these objects out to extremely high redshifts, while measuring their masses with extremely high precision. These observations of merging IMBHs will allow us to constrain the formation mechanism and subsequent evolution of massive black holes, from the 'dark ages' to the present day, and reveal the role that IMBHs play in hierarchical galaxy evolution.

  5. The gravitational wave view of massive black holes
  6. Details

    Lead Author M. Colpi
    Lead Author email monica.colpi@mib.infn.it
    Coauthors M. Colpi, K. Holley-Bockelmann, T. Bogdanovic, P. Natarajan, J. Bellovary, A. Sesana, M. Tremmel, J. Schnittman, J. Comerford, E. Barausse, E. Berti, M. Volonteri, F. M. Khan, S. T. McWilliams, S. Burke-Spolaor, J. S. Hazboun, J. Conklin, G. Mueller, S. Larson
    Key words  
    Link to white paper / draft PDF
    Abstract Coalescing, massive black-hole (MBH) binaries are the most powerful sources of gravitational waves (GWs) in the Universe, which makes MBH science a prime focus for ongoing and upcoming GW observatories. The Laser Interferometer Space Antenna (LISA)—a gigameter scale space-based GW observatory—will grant us access to an immense cosmological volume, revealing MBHs merging when the first cosmic structures assembled in the Dark Ages. LISA will unveil the yet unknown origin of the first quasars, and detect the teeming population of MBHs of 104-7 Msun forming within protogalactic halos. The Pulsay Timing Array, a galactic-scale GW survey, can access the largest MBHs the Universe, detecting the cosmic GW foreground from inspiraling MBH binaries of ~ 109 Msun. LISA can measure MBH spins and masses with precision far exceeding that from electromagnetic (EM) probes, and together, both GW observatories will provide the first full census of binary MBHs, and their orbital dynamics, across cosmic time. Detecting the loud gravitational signal of these MBH binaries will also trigger alerts for EM counterpart searches, from decades (PTAs) to hours (LISA) prior to the final merger. By witnessing both the GW and EM signals of MBH mergers, precious information will be gathered about the rich and complex environment in the aftermath of a galaxy collision. The unique GW characterization of MBHs will shed light on the deep link between MBHs of 104–1010 Msun and the grand design of galaxy assembly, as well as on the complex dynamics that drive MBHs to coalescence

  7. Tests of General Relativity and Fundamental Physics with Space-borne Gravitational Wave Detectors
  8. Details

    Lead Author Emanuele Berti
    Lead Author email berti@jhu.edu
    Coauthors Enrico Barausse, Ilias Cholis, Juan Garcia-Bellido, Kelly Holley-Bockelmann, Scott A. Hughes, Bernard Kelly, Ely D. Kovetz, Tyson B. Littenberg, Jeffrey Livas, Guido Mueller, Priya Natarajan, David H. Shoemaker, Deirdre Shoemaker, Jeremy D. Schnittman, Michele Vallisneri, and Nicolás Yunes
    Key words General relativity; fundamental physics; black holes
    Link to white paper / draft PDF
    Abstract Low-frequency gravitational-wave astronomy can perform precision tests of general relativity and probe fundamental physics in a regime previously inaccessible. A space-based detector will be a formidable tool to explore gravity's role in the cosmos, potentially telling us if and where Einstein's theory fails and providing clues about some of the greatest mysteries in physics and astronomy, such as dark matter and the origin of the Universe.

  9. Gravitational Wave Survey of Galactic Ultra Compact Binaries
  10. Details

    Lead Author Tyson Littenberg, Warren Brown
    Lead Author email tyson.b.littenberg@nasa.gov
    Coauthors Katelyn Breivik, Warren R. Brown, Michael Eracleous, J. J. Hermes, Kelly Holley-Bockelmann, Kyle Kremer, Thomas Kupfer, and Shane L. Larson
    Key words Ultra compact binaries; Multimessenger sources
    Link to white paper / draft PDF
    Abstract Ultra-compact binaries (UCBs) are systems containing compact or degenerate stars with orbital periods less than one hour. Tens of millions of UCBs are predicted to exist within the Galaxy emitting gravitational waves (GWs) at mHz frequencies. Combining GW searches with electromagnetic (EM) surveys like Gaia and LSST will yield a comprehensive, multimes- senger catalog of UCBs in the galaxy. Joint EM and GW observations enable measurements of masses, radii, and orbital dynamics far beyond what can be achieved by independent EM or GW studies. GW+EM surveys of UCBs in the galaxy will yield a trove of unique insights into the nature of white dwarfs, the formation of compact objects, dynamical interactions in binaries, and energetic, accretion-driven phenomena like Type Ia superonovae.

  11. What we can learn from multi-band observations of black hole binaries
  12. Details

    Lead Author Curt Cutler
    Lead Author email cjcutler@jpl.nasa.gov
    Coauthors Curt Cutler, Emanuele Berti, Karan Jani, Ely D. Kovetz, Lisa Randall, Salvatore Vitale, Kaze W.K. Wong, Kelly Holley-Bockelmann, Shane L. Larson, Tyson Littenberg, Sean T. McWilliams, Guido Mueller, Jeremy D. Schnittman, David H. Shoemaker, and Michele Vallisneri
    Key words LIGO; IMBH
    Link to white paper / draft PDF
    Abstract The LIGO/Virgo gravitational-wave (GW) interferometers have to-date detected ten merging black hole (BH) binaries, some with masses considerably larger than had been anticipated. Stellar-mass BH binaries at the high end of the observed mass range (with "chirp mass" M ≳ 25 Msun) should be detectable by a space-based GW observatory years before those binaries become visible to ground-based GW detectors. This white paper discusses some of the synergies that result when the same binaries are observed by instruments in space and on the ground. We consider intermediate-mass black hole binaries (with total mass M ~ 102–104 Msun) as well as stellar-mass black hole binaries. We illustrate how combining space-based and ground-based datasets can break degeneracies and thereby improve our understanding of the binary's physical parameters. While early work focused on how space-based observatories can forecast precisely when some mergers will be observed on the ground, the reverse is also important: ground-based detections will allow us to "dig deeper" into archived, space-based data to con´Čüdently identify black hole inspirals whose signal-to-noise ratios were originally sub-threshold, increasing the number of binaries observed in both bands by a factor of ~ 4–7

  13. The unique potential of extreme mass-ratio inspirals for gravitational-wave astronomy
  14. Details

    Lead Author Christopher P. L. Berry
    Lead Author email christopher.berry @northwestern.edu
    Coauthors Scott A. Hughes, Carlos F. Sopuerta, Alvin J. K. Chua, Anna Heffernan, Kelly Holley- Bockelmann, Deyan P. Mihaylov, M. Coleman Miller, and Alberto Sesana
    Key words EMRIs; extreme mass-ratio inspirals; massive black holes; nuclear star clusters
    Link to white paper / draft [PDF]
    Abstract The inspiral of a stellar-mass compact object into a massive (~ 104 – 107 Msun) black hole produces an intricate gravitational-wave signal. Due to the extreme-mass ratios involved, these systems complete ~ 104 – 105 orbits, most of them in the strong-field region of the massive black hole, emitting in the frequency range ~ 10-4 –1 Hz. This makes them prime sources for the space-based observatory LISA (Laser Interferometer Space Antenna). LISA observations will enable high-precision measurements of the physical characteristics of these extreme-mass-ratio inspirals (EMRIs): redshifted masses, massive black hole spin and orbital eccentricity can be determined with fractional errors ~ 10-4 – 10-6, the luminosity distance with better than ~ 10% precision, and the sky localization to within a few square degrees. EMRIs will provide valuable information about stellar dynamics in galactic nuclei, as well as precise data about massive black hole populations, including the distribution of masses and spins. They will enable percent-level measurements of the multipolar structure of massive black holes, precisely testing the strong-gravity properties of their spacetimes. EMRIs may also provide cosmographical data regarding the expansion of the Universe if inferred source locations can be correlated with galaxy catalogs.
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