Fundamental Physics with Pulsars

We summarize the current state of probing "fundamental physics" using radio pulsars, rotating neutron stars that emit beamed radiation observed as pulses at radio frequencies, as well as key opportunities for Canadian leadership within the next decade. In the context of pulsar astronomy, fundamental physics mainly concerns open questions in gravitational and nuclear astrophysics, such as the validity of general relativity and nature of nuclear physics in extreme-density environments. (Pulsar astronomy and its applications for the detection of gravitational waves is the subject of the standalone White Paper E055.) Several overarching questions that define the field, each of which are subject to active debate in the global radio-astrophysical community, are summarized as follows: What is the equation of state of ultra-dense matter found within neutron stars? What are the minimum and maximum masses of neutron stars? How fast can neutron stars spin? And in what ways do the distributions of neutron star masses and spin periods reflect the physics of compact stars and different formation channels? What is the correct theory of gravitation in the strong-field regime? How can multi-messenger measurements of neutron stars be used to test gravitation and further elucidate the mystery of their structure? As we discuss in this White Paper for the 2020-2030 Long Range Plan, Canadian scientists are poised to make foundational contributions to the field of pulsar astronomy and its application in studies of fundamental physics. Current and forthcoming radio interferometers developed and built by Canadian researchers are expected to discover and study thousands of pulsars in the Northern sky, most likely including sources in exotic orbital systems. Among these discoveries could be the first pulsars orbiting black holes, whether these black holes are of stellar mass or reside at the center of the Milky Way galaxy, that have long been sought after by observers and theorists alike. The discovery of neutron stars possessing the smallest and largest masses known will place the strongest constraints on viable interior-physics models that are unattainable from laboratory experiment. In order to enable these discoveries, we make the following recommendations: Canada should remain active in the construction, correlator design and implementation for the SKA in a manner that allows for Canadian researchers to take leadership in pulsar science. Funding should be found and allocated to extend the operation of CHIME past its nominal 5-yr observing run. In the context of pulsar science, the extension would provide additional integration times that directly benefit both the pulsar-search and pulsar-timing backends. Broad Canadian support should be provided for the CHORD initiative, which is being designed as a worthy successor to CHIME and its science directives, including pulsar astronomy and its applications to fundamental physics. Canada should consider providing tangible support for the 300-m Arecibo Observatory and 100-m Green Bank Telescope, two premier facilities that both overcame recent funding struggles. As discussed in the White Paper E055 by I. H. Stairs et al., such support may come in the form of either on-site personnel with scientific and/or technical expertise, instrumentation that further enable breakthrough science at both facilities, or the purchasing of telescope time for measurements related to this topic.

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