  Until recently, the only way to observe the Universe was from light received
by telescopes. But we are now able to measure gravitational waves, which are
ripples in the fabric of the Universe predicted by Albert Einstein. If two very
dense objects (like black holes) orbit each other closely, they warp space and
send out gravitational waves. For black holes that are similar in mass to the
Sun, scientists use the LIGO detector on Earth. But for the biggest black holes
in the Universe (billions of times more massive than the Sun), scientists
monitor a net of rapidly-spinning neutron stars (called pulsars) across the
Milky Way. Any gravitational wave passing by will change how long radio signals
from these pulsars take to get to Earth. The NANOGrav Collaboration monitored
34 of these pulsars over 11 years, in an attempt to detect gravitational waves
from giant black holes.
