Stephen Hawking and Thomas Hertog recently published an article that should be of interest to many of us. When the normal “bottom-up approach” is applied to cosmology, one ends up with a finely tuned universe as we all know. Hawking has apparently been busy trying to find a way around that “problem” with a “top down approach”.
In fact if one does adopt a bottom-up approach to cosmology, one is immediately led to an essentially classical framework, in which one loses all ability to explain cosmologyÃ¢â‚¬â„¢s central question – why our universe is the way it is. In particular a bottom-up approach to cosmology either requires one to postulate an initial state of the universe that is carefully fine-tuned  – as if prescribed by an outside agency – or it requires one to invoke the notion of eternal inflation , which prevents one from predicting what a typical observer would see.
Here we put forward a different approach to cosmology in the string landscape, based not on the classical idea of a single history for the universe but on the quantum sum over histories . We argue that the quantum origin of the universe naturally leads to a framework for cosmology where amplitudes for alternative histories of the universe are computed with boundary conditions at late times only. We thus envision a set of alternative universes in the landscape, with amplitudes given by the no boundary path integral .
The measure on the landscape provided by no boundary initial conditions allows one to derive predictions for observations. This is done by evaluating probabilities for alternative histories that obey a set of constraints at late times. The constraints provide information that is supplementary to the fundamental laws and act as a selection principle. In particular, they select the subclass of histories that contribute to the amplitude of interest. One then identifies alternatives within this subclass that have probabilities near one. These include, in particular, predictions of future observations. The framework we propose is thus more like a top down approach to cosmology, where the histories of the universe depend on the precise question asked.
4 Replies to “Populating the Landscape: A Top Down Approach”
…and soon we shall be able to calculate the number of angels dancing on the heads of pins in an infinite number of universes.
cheesman – my sentiments exactly
I think this is the interesting upshot of the paper:
When one extends these considerations to a potential that depends on a multidimensional
moduli space, one finds that only a few of the minima of the potential will be populated, i.e. will have significant amplitudes.
The top down approach we have described leads to a profoundly different view of cosmology, and the relation between cause and effect. Top down cosmology is a framework in which one essentially traces the histories backwards, from a spacelike surface at the present time.
Amplitude is simply the square root of probability, so the terms can be used somewhat interchangeably. I find it interesting, then, that only a ,b>few of the ‘multidimensional moduli space’ will have ‘signifcant amplitudes (probability)’. I like 3-dimensional, flat space. Makes me feel at home.
What Hawking is saying is current observations may affect past histories, thus only histories consistent with conscious being are allowed, hence fine tuning.
But Paul Davies rightly points out, if such a circular feedback loop exists (the future affecting the past, or even multi-universes) to create fine tuning, why should it exist in the first place! Thus, Hawking’s strong anthropic principle only describes a mechanism of fine tuning, not an ultimate cause of fine tuning. Further, the Strong Anthropic mechanism could be incomplete if not wrong, if the ultimate cause of the fine Tuning is the Ultimate Observer (God) not human observers.
Barrow and Tipler make a good case that there must be an Ultimate Observer (much like a First Cause in Aquinas and Aristotle’s writings). However, Barrow and Tipler have physical law and high-powered math to argue their case rather than philosophy or theology.
See my take: