A brief History of Time: From Big Bang to Black Holes

Gravitational waves
Using polarization of the cosmic microwave background to show that
gravitational waves were produced in the early universe, as mentioned
earlier, would be one way to very directly confirm the high energies
involved in inflation. I hope that we will not have to wait too long for this
development; in the meantime, we have recently seen direct confirmation
that gravitational waves (p.101) can be produced in the modern-day
universe. Exactly a century after Einstein first predicted their existence, a
worldwide consortium of scientists known as the LIGO collaboration

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announced in 2016 that gravitational waves had been detected for the first
time.
The first sixty years were the hardest. During this time there was
confusion over the status of the waves: should they exist in practice or are
they just a mathematical artefact, unconnected with reality? Even Einstein
seemed uncertain, and came close to publishing an erroneous disproof of
their physicality in the 1930s. But over time the physics community settled
on the view that the waves should be real. One consequence was that energy
would be very slowly lost from orbiting bodies. Until recently, such energy
loss was our only evidence for the existence of the waves (p.102). This was
very convincing, but still indirect.
Actually measuring gravitational waves as they pass through the earth is
far more technologically challenging, which is why it took until 2016. But
the decades of technological development have proved worthwhile, because
we now have a completely new way to study the universe. Even the first
events that LIGO detected – waves resulting from the collision and merging
of two black holes – allowed us to confirm our understanding of a process
that no traditional telescope has ever been, or will ever be, able to probe.
For me, it was really exciting to see observations of colliding black holes.
LIGO will observe many such events in the near future. These observations
will, I believe, confirm a prediction I made in 1970 – that the surface area
of the final black hole was greater than the sum of the initial holes’ areas.
This ‘area theorem’, which led to my slightly later realization that black
holes will gradually lose their mass over time, was secure on mathematical
grounds. But one can never be too sure of an idea until it is tested against
nature.
There is a bright future for LIGO and other gravitational wave
observatories. We can expect to build up a large catalogue of detections,
providing detailed insight into the populations of black holes in our
universe. That in turn will allow us to search for even slight deviations from
predictions based on Einstein’s theory. As we continue our search for a full
quantum theory of gravity, this treasure trove of information about extreme
regions of space-time will be immensely valuable.