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May 8, 2013

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 A multiverse review,

with special reference to anthropic constraints

on the cosmological constant


Supervised by Professor Bernard Carr
Submitted September 2013 in part fulfilment of the requirements for the degree of
Master of Science in Astrophysics of Queen Mary University of London


We review theories for the formation of our universe with its specific constraints
of physics from a particular set of parameters. We assess the claim that current
cosmology and string landscape scenarios suggest other universes in which the parametersvary, and suggest further conjectures not yet discussed in the multiverse arena.


In this dissertation I review the scientific and philosophical grounds for the multiverse
concept. Not all versions of the multiverse hypothesis include inflation and
the cosmological constant. Noting that, I examine the central role taken by the
cosmological constant with respect to the anthropic argument (§ 4.3) and physical
parameters and coupling constants in relation to the multiverse (§ 3.2.1).
String theory and inflationary cosmology have developed rapidly over the last 35
years to now credibly forward the existence on scientific grounds of an ensemble
of cosmic regions, or universes. Physical parameters, like the masses of elementary
particles and the coupling constants, could vary between regions. The anthropic
principle (Carter 1974)[C], (a hypothesis developed in § 3.2.1 and § 4.3.1), states
the observed physics of our cosmic region is due to the selection by the observer of that particular physics.
The parameters of particle physics in the standard model are not god given fundamental
constants of nature. The values may just be our specified quantum state in
our region. Energy landscape scenarios with countless local minima may exist with
these variables, each relating to a specific quantum vacuum state. The local energy
minimum is the value of that minimum itself, the cosmological constant parameter,
Lambda (Λ), generally referred to as the energy density of the quantum vacuum (see § 4.2.1).
The standard concordance1 model immediately hits a problem. From the initial
singularity, such a massive unit must surely immediately recollapse back to a singularity
under its own gravity2? Further, the basic idea is that inflation commences
10−36 second after the Big Bang, and the vacuum energy causes this exponential
expansion. In the course of this inflation, bubble universes, or their variants, create
the multiverse. Does inflation need to occur at all?
The multiverse (or meta-universe) is the hypothetical set of infinite or finite sized possible universes. It is everything that exists (or indeed, could be contrived to
exist), be it space, time, matter, energy and the laws and constants of physics. The
specific multiverse hypothesis considered dictates the structure of that multiverse.
According to the Oxford English Dictionary, the term was originally termed in 1895
by the American writer, philosopher and psychologist William James.3. For an
introduction to Multiverse, see (Carr 2009) or (Carr and Ellis 2008) [CE].
Though the cosmological constant (CC) is not necessary in all the multiverse
hypotheses, in the development of this dissertation, I shall attempt to show its role
in the evolution of any universe in which it is involved, and its required part in the
anthropic principle thesis.


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