According to Einstein's theory of relativity, the curvature
of spacetime was infinite at the big bang. In fact, at this point all
mathematical tools fail, and the theory breaks down. However, there remained
the notion that perhaps the beginning of the universe could be treated in a
simpler manner, and that the infinities of the big bang might be avoided. This
has indeed been the hope expressed since the 1980s by the well-known
cosmologists James Hartle and Stephen Hawking with their "no-boundary proposal",
and by Alexander Vilenkin with his "tunnelling proposal".

Now scientists at the Max Planck Institute for Gravitational
Physics (Albert Einstein Institute/AEI) in Potsdam and at the Perimeter
Institute in Canada have been able to use better mathematical methods to show
that these ideas cannot work. The big bang, in its complicated glory, retains
all its mystery.

"Hence the
"no-boundary proposal" does not imply a large universe like the one
we live in, but rather tiny curved universes that would collapse
immediately", says Jean-Luc Lehners, who leads the "theoretical
cosmology" group at the AEI.

One of the principal goals of cosmology is to understand the
beginning of our universe. Data from the Planck satellite mission shows that
13.8 billion years ago the universe consisted of a hot and dense soup of
particles. Since then the universe has been expanding. This is the main tenet
of the hot big bang theory, but the theory fails to describe the very first
stages themselves, as the conditions were too extreme. Indeed, as we approach
the big bang, the energy density and the curvature grow until we reach the
point where they become infinite.

As an alternative, the "no-boundary" and
"tunneling" proposals assume that the tiny early universe arose by
quantum tunnelling from nothing, and subsequently grew into the large universe
that we see. The curvature of spacetime would have been large, but finite in
this beginning stage, and the geometry would have been smooth - without
boundary (see Fig. 1, left panel). This initial configuration would replace the
standard big bang. However, for a long time the true consequences of this
hypothesis remained unclear.

Now, with the help of better mathematical methods, Jean-Luc
Lehners, group leader at the AEI, and his colleagues Job Feldbrugge and Neil
Turok at Perimeter Institute, managed to define the 35 year old theories in a
precise manner for the first time, and to calculate their implications. The
result of these investigations is that these alternatives to the big bang are
no true alternatives.

As a result of Heisenberg's uncertainty relation, these
models do not only imply that smooth universes can tunnel out of nothing, but
also irregular universes. In fact, the more irregular and crumpled they are,
the more likely (see Fig. 1, right panel).

Hence one cannot circumvent the big bang so easily. Lehners
and his colleagues are now trying to figure out what mechanism could have kept
those large quantum fluctuations in check under the most extreme circumstances,
allowing our large universe to unfold.

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