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

Microwave background radiation and the no boundary proposal
If the no boundary proposal might be central to understanding these
developments, we should re-examine how it holds up in light of our rapidly
improving observational handle on the early cosmos. In particular we can
now understand the origins of structure in our universe using measurements
of cosmic microwave background radiation (p.134).
As the name suggests, this is made up of microwaves – as used by your
microwave oven, only much less powerful. They would heat your pizza to
only −270.4°C, which isn’t much good for defrosting, let alone cooking.
But these ultra-weak microwaves are spectacularly valuable, because there
is only one reasonable explanation for their presence: they are radiation left
over from an early time when the universe was very hot and dense. As the
universe expanded, the radiation cooled until it is just the faint remnant we
can detect today.

---

The existence of the background radiation was established in 1965.
Immediately upon its detection, it was seen as powerful direct evidence for
predictions based on Einstein’s general relativity. Part of my own PhD
thesis work, finished just months before, had been to show that the early
hot, dense phase was unavoidable in Einstein’s picture.
But the value of measuring the radiation has become greater still. At first
the microwaves seemed to have an identical intensity in every direction.
This led to ideas like inflation (p.144), which in its initial formulation was
intended to explain how the early universe came to be so uniform. On
closer inspection, it actually predicted there would be very slight variations
from place to place. The deviations from uniformity come about through
quantum mechanical uncertainty, which imposes a minimum level of
fluctuations.
As successive generations of space telescopes have measured the
microwave background radiation with increasing precision – first COBE in
1992 (p.49), then WMAP in 2001, and most recently Planck in 2013 – this
prediction has proved to be correct. There are indeed changes in the
intensity of the radiation, at the level of about one part in 100,000. More
significantly, we have now determined that the precise pattern of variations
agrees with the specific predictions I and others made by combining
inflation with the no boundary proposal.
To describe the physical conditions at the big bang, the no boundary
proposal combines Einstein’s relativity with quantum theory. It says that
when we go back towards the beginning of our universe space and time
become fuzzy and ‘cap off ’, somewhat like the North Pole on the surface
of the earth. Asking what came before the big bang is meaningless
according to the no boundary proposal, because there is no notion of time
available to refer to. It would be like asking what lies north of the North
Pole.
With my colleagues James Hartle (with whom I first put forward the no
boundary proposal more than thirty years ago) and Thomas Hertog I have
put all this to the test. We calculated what kind of universe would emerge
from the big bang according to the no boundary proposal, and compared
this prediction with our observations. This confirms that our universe
should have come into existence with a burst of inflation.
So the features now measured in the microwave background radiation
appear to confirm inflation and the no boundary proposal. But there is one

---

key prediction of the theory which has yet to be verified. According to
inflation, a small part of the fluctuations in the microwave radiation can be
traced to gravitational waves generated during the phase of rapid expansion.
This primordial gravitational radiation is the analogue of the quantum
radiation from black holes and can be regarded as coming from the event
horizon of the early inflationary stages of the universe. Its detection would
confirm that black holes emit quantum radiation, something almost
impossible to confirm directly. I will say more about detection of
gravitational waves below, but those generated in the early universe show
up most clearly in the polarization of the radiation. We are only in the early
stages of measuring this polarization, and there is real hope that it will
provide firm and convincing evidence for our theory of the big bang.
Even without a clear view of the polarization, the cosmic microwave
background data are so good that we can now start to fill in some of the
blanks. Inflation and the no boundary proposal leave a number of details
unspecified: the precise energies involved, for example, and the link to the
underlying particle physics. These details subtly change the expected
patterns; by carefully studying what is seen, we are now beginning to
understand physics near the grand unification energy. To put that in context,
it is a million million times higher than can be probed by the very best
experimental facility on earth, the Large Hadron Collider.