Since 2015, the discoveries of black-hole and neutron-star mergers through the direct detection of gravitational waves (GW) with the LIGO and Virgo detectors have revolutionized studies of compact objects and have opened up the new field of GW Astrophysics, while enabling uniquely exciting multi- messenger astronomy studies as well. Even with just one or several GW detections we have answered important, long standing questions and made a number of surprising discoveries. The now ten black-hole detections: (i) confirmed theoretical predictions that pairs of binary black holes form in nature and merge within a Hubble time; (ii) surprised us with the existence of “heavy” black holes with masses greater than those found in X-ray binaries; (iii) tested general relativity in the strong-field regime, for the first time ever. The one neutron-star detection:(i) proved the association of neutron-star mergers with short gamma-ray bursts, as hypothesized for decades; (ii) confirmed theoretical predictions for the production of heavy elements through detection of “kilonova” emission; (iii) confirmed predictions of neutron-star merger rates based on binary pulsars in the Milky Way and further constrained these rates. At the same time, these discoveries raised a set of new questions, for example: (i) how do “heavy” black holes actually form from stars?; (ii) are the known high-mass X-ray binaries progenitors of merging binary black holes? If not, what are their immediate progenitors and why have we not discover them as X- ray sources? (iii) Is the brightness of the one kilonova associated with potentially more asymmetric mass ratios in double neutron star systems compared to those in the Milky Way? (iv) Is there a new sub-group of faint short gamma-ray bursts and do the neutron-star masses and spins play a role? At present the gravitational-wave detectors are undergoing upgrades with the goal of increasing their sensitivity significantly. For the year-long 3rd observing run (O3 starting in April 2019), the LIGO detectors are expected to reach out to 120 Mega-parsecs for neutron-star mergers and out to just over 1,000 Mega-parsecs for a pair of 30 solar-mass black holes. The early detections provided us with merger rate constraints which we can use to calculate expected detection samples by the end of O3: We expect BH-BH detection rates of about 30-100 per year and NS-NS rates of 1-10 per year. It is evident that very soon the nascent field of GW astronomy will transition from the observational stage of a handful of exciting, unique discoveries to a mature stage with observational samples of significant population sizes. Such data sets will enable us to ask new types of questions, questions regarding population characteristics: (i) What are the underlying mass and spin distributions of black holes? (ii) are mass gaps and cut-offs predicted by models supported by the data? (iii) what are the robust model predictions for the properties of merging compact objects and how to GW observations constrain them? (iv) what are the dominant formation channels for binary compact objects? In less than two years we will have the most advanced GW observations ever providing us with observational samples of significant sizes and with BH and NS property measurements of accuracies comparable or better than from electromagnetic observations. Such observations will be ripe for constraining model predictions of massive star evolution, compact formation, binary star interactions, and properties of compact objects in binaries. Constrained models will also hel
|Effective start/end date||5/1/19 → 8/31/22|
- Gordon E. and Betty I. Moore Foundation (8477)
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