Arecibo Puts Limits on Gravitational Wave Models | Smart
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The expected gravitational wave spectrum at nanohertz frequencies from various supermassive black-hole merger models (color) along with upper limits of the spectrum
measured from the NANOGrav nine-year data set (black). The black-dashed line represents the experimental upper limit of the gravitational wave strength when assuming
that the signal is entirely due to super massive black hole binary mergers (i.e., power-law); the solid line represents the upper limit when allowing for the derived
spectrum to have any shape. The colored areas correspond predictions of three different models. At large frequencies, the free-shape spectrum is dominated by white-
noise (i.e. non-astrophysical) signals due to pulsars with small data sets.
Until this year, astronomers have only been able to indirectly determine the presence of gravitational waves -- tiny, wave-like shifts of space and time -- through
the measurements of decaying orbits of neutron stars. In January 2016, the LIGO collaboration announced the first direct detection of gravitational waves from a system
of black holes orbiting and colliding together. The discovery by LIGO has ushered in the era of gravitational-wave astronomy, showing that direct measurements of
spacetime ripples are possible.