12 research outputs found
Anthropic predictions for vacuum energy and neutrino masses
It is argued that the observed vacuum energy density and the small values of
the neutrino masses could be due to anthropic selection effects. Until now,
these two quantities have been treated separately from each other and, in
particular, anthropic predictions for the vacuum energy were made under the
assumption of zero neutrino masses. Here we consider two cases. In the first,
we calculate predictions for the vacuum energy for a fixed (generally non-zero)
value of the neutrino mass. In the second we allow both quantities to vary from
one part of the universe to another. We find that the anthropic predictions for
the vacuum energy density are in a better agreement with observations when one
allows for non-zero neutrino masses. We also find that the individual
distributions for the vacuum energy and the neutrino masses are reasonably
robust and do not change drastically when one adds the other variable.Comment: 9 pages, 4 figure
Multiple universes, cosmic coincidences, and other dark matters
Even when completely and consistently formulated, a fundamental theory of
physics and cosmological boundary conditions may not give unambiguous and
unique predictions for the universe we observe; indeed inflation, string/M
theory, and quantum cosmology all arguably suggest that we can observe only one
member of an ensemble with diverse properties. How, then, can such theories be
tested? It has been variously asserted that in a future measurement we should
observe the a priori most probable set of predicted properties (the
``bottom-up'' approach), or the most probable set compatible with all current
observations (the ``top-down'' approach), or the most probable set consistent
with the existence of observers (the ``anthropic'' approach). These inhabit a
spectrum of levels of conditionalization and can lead to qualitatively
different predictions. For example, in a context in which the densities of
various species of dark matter vary among members of an ensemble of otherwise
similar regions, from the top-down or anthropic viewpoints -- but not the
bottom-up -- it would be natural for us to observe multiple types of dark
matter with similar contributions to the observed dark matter density. In the
anthropic approach it is also possible in principle to strengthen this argument
and the limit the number of likely dark matter sub-components. In both cases
the argument may be extendible to dark energy or primordial density
perturbations. This implies that the anthropic approach to cosmology,
introduced in part to explain "coincidences" between unrelated constituents of
our universe, predicts that more, as-yet-unobserved coincidences should come to
light.Comment: 18 JCAP-style pages, accepted by JCAP. Revised version adds
references and some clarification
Anthropic prediction in a large toy landscape
The successful anthropic prediction of the cosmological constant depends
crucially on the assumption of a flat prior distribution. However, previous
calculations in simplified landscape models showed that the prior distribution
is staggered, suggesting a conflict with anthropic predictions. Here we
analytically calculate the full distribution, including the prior and anthropic
selection effects, in a toy landscape model with a realistic number of vacua,
. We show that it is possible for the fractal prior
distribution we find to behave as an effectively flat distribution in a wide
class of landscapes, depending on the regime of parameter space. Whether or not
this possibility is realized depends on presently unknown details of the
landscape.Comment: 13 page
Is Our Universe Natural?
It goes without saying that we are stuck with the universe we have.
Nevertheless, we would like to go beyond simply describing our observed
universe, and try to understand why it is that way rather than some other way.
Physicists and cosmologists have been exploring increasingly ambitious ideas
that attempt to explain why certain features of our universe aren't as
surprising as they might first appear.Comment: Invited review for Nature, 11 page
Anthropic predictions for vacuum energy and neutrino masses in the light of WMAP-3
Anthropic probability distributions for the cosmological constant and for the
sum of neutrino masses are updated using the WMAP-3 data release. The new
distribution for Lambda is in a better agreement with observation than the
earlier one. The typicality of the observed value, defined as the combined
probability of all values less likely than the observed, is no less than 22%.
We discuss the dependence of our results on the simplifying assumptions used in
deriving the distribution for Lambda and show that the agreement of the
anthropic prediction with the data is rather robust. The distribution for the
sum of the neutrino masses is peaked at 1 eV, suggesting degenerate masses, but
a hierarchical mass pattern is still marginally allowed at a 2 sigma level.Comment: References added; 11 pages, 6 figure
Anthropic prediction for a large multi-jump landscape
The assumption of a flat prior distribution plays a critical role in the
anthropic prediction of the cosmological constant. In a previous paper we
analytically calculated the distribution for the cosmological constant,
including the prior and anthropic selection effects, in a large toy
``single-jump'' landscape model. We showed that it is possible for the fractal
prior distribution we found to behave as an effectively flat distribution in a
wide class of landscapes, but only if the single jump size is large enough. We
extend this work here by investigating a large () toy
``multi-jump'' landscape model. The jump sizes range over three orders of
magnitude and an overall free parameter determines the absolute size of the
jumps. We will show that for ``large'' the distribution of probabilities of
vacua in the anthropic range is effectively flat, and thus the successful
anthropic prediction is validated. However, we argue that for small , the
distribution may not be smooth.Comment: 33 pages, 7 figures Minor revisions made and references added. arXiv
admin note: substantial text overlap with arXiv:0705.256
What does inflation really predict?
If the inflaton potential has multiple minima, as may be expected in, e.g.,
the string theory "landscape", inflation predicts a probability distribution
for the cosmological parameters describing spatial curvature (Omega_tot), dark
energy (rho_Lambda, w, etc.), the primordial density fluctuations (Omega_tot,
dark energy (rho_Lambda, w, etc.). We compute this multivariate probability
distribution for various classes of single-field slow-roll models, exploring
its dependence on the characteristic inflationary energy scales, the shape of
the potential V and and the choice of measure underlying the calculation. We
find that unless the characteristic scale Delta-phi on which V varies happens
to be near the Planck scale, the only aspect of V that matters observationally
is the statistical distribution of its peaks and troughs. For all energy scales
and plausible measures considered, we obtain the predictions Omega_tot ~
1+-0.00001, w=-1 and rho_Lambda in the observed ballpark but uncomfortably
high. The high energy limit predicts n_s ~ 0.96, dn_s/dlnk ~ -0.0006, r ~ 0.15
and n_t ~ -0.02, consistent with observational data and indistinguishable from
eternal phi^2-inflation. The low-energy limit predicts 5 parameters but prefers
larger Q and redder n_s than observed. We discuss the coolness problem, the
smoothness problem and the pothole paradox, which severely limit the viable
class of models and measures. Our findings bode well for detecting an
inflationary gravitational wave signature with future CMB polarization
experiments, with the arguably best-motivated single-field models favoring the
detectable level r ~ 0.03. (Abridged)Comment: Replaced to match accepted JCAP version. Improved discussion,
references. 42 pages, 17 fig
The Mathematical Universe
I explore physics implications of the External Reality Hypothesis (ERH) that
there exists an external physical reality completely independent of us humans.
I argue that with a sufficiently broad definition of mathematics, it implies
the Mathematical Universe Hypothesis (MUH) that our physical world is an
abstract mathematical structure. I discuss various implications of the ERH and
MUH, ranging from standard physics topics like symmetries, irreducible
representations, units, free parameters, randomness and initial conditions to
broader issues like consciousness, parallel universes and Godel incompleteness.
I hypothesize that only computable and decidable (in Godel's sense) structures
exist, which alleviates the cosmological measure problem and help explain why
our physical laws appear so simple. I also comment on the intimate relation
between mathematical structures, computations, simulations and physical
systems.Comment: Replaced to match accepted Found. Phys. version, 31 pages, 5 figs;
more details at http://space.mit.edu/home/tegmark/toe.htm
Testing the Cosmological Constant as a Candidate for Dark Energy
It may be difficult to single out the best model of dark energy on the basis
of the existing and planned cosmological observations, because many different
models can lead to similar observational consequences. However, each particular
model can be studied and either found consistent with observations or ruled
out. In this paper, we concentrate on the possibility to test and rule out the
simplest and by far the most popular of the models of dark energy, the theory
described by general relativity with positive vacuum energy (the cosmological
constant). We evaluate the conditions under which this model could be ruled out
by the future observations made by the Supernova/Acceleration Probe SNAP (both
for supernovae and weak lensing) and by the Planck Surveyor cosmic microwave
background satellite.Comment: 6 pages, 2 figures, revtex
Sinks in the Landscape, Boltzmann Brains, and the Cosmological Constant Problem
This paper extends the recent investigation of the string theory landscape in
hep-th/0605266, where it was found that the decay rate of dS vacua to a
collapsing space with a negative vacuum energy can be quite large. The parts of
space that experience a decay to a collapsing space, or to a Minkowski vacuum,
never return back to dS space. The channels of irreversible vacuum decay serve
as sinks for the probability flow. The existence of such sinks is a
distinguishing feature of the string theory landscape. We describe relations
between several different probability measures for eternal inflation taking
into account the existence of the sinks. The local (comoving) description of
the inflationary multiverse suffers from the so-called Boltzmann brain (BB)
problem unless the probability of the decay to the sinks is sufficiently large.
We show that some versions of the global (volume-weighted) description do not
have this problem even if one ignores the existence of the sinks. We argue that
if the number of different vacua in the landscape is large enough, the
anthropic solution of the cosmological constant problem in the string landscape
scenario should be valid for a broad class of the probability measures which
solve the BB problem. If this is correct, the solution of the cosmological
constant problem may be essentially measure-independent. Finally, we describe a
simplified approach to the calculations of anthropic probabilities in the
landscape, which is less ambitious but also less ambiguous than other methods.Comment: 42 pages, 5 figures, the paper is substantially extended, a section
on the cosmological constant is addeed; the version published in JCA