The cosmic beginning and quantum cosmology

4 problems

• Can a nonsingular bounce reproduce all observed primordial spectra?

  Open

  Instead of inflation, the universe might have contracted and then bounced into expansion. Such models must avoid unstable or unphysical fields at the bounce while matching observations.

• Did time have a beginning, and what came before the hot Big Bang?

  Open

  A theorem shows that an expanding universe like ours cannot be extended infinitely into the past using ordinary spacetime. Whether quantum gravity removes that edge or describes a true beginning is unknown.

• What probability measure applies to observations in eternal inflation?

  Open

  In many inflation models inflation never ends everywhere and keeps producing infinitely many regions with different properties. Predicting what an observer should see then needs a rule for comparing infinities, and different rules give contradictory answers.

• Is the Lorentzian no-boundary wave function well defined and stable?

  Contested

  James Hartle and Stephen Hawking, two theoretical physicists, proposed that the universe began without any boundary in time, in the way the surface of a sphere has no edge at its pole. Recent calculations disagree on whether this proposal gives a sensible universe or one with uncontrollably large ripples.