Stephen Hawking & Thomas Hertog's Top-Down Approach to Universe Theory

[alexsok]

Not sure which category to post this in (if moderators deem it necessary, then by all means, move it ;)))

Anyway, so i read about this new approach from Stephen Hawking so quote time! ;)

Cambridge physicist Stephen Hawking and his CERN colleague Thomas Hertog have proposed a radical new approach to understanding the universe that studies it from the "top down" rather than the "bottom up" as in traditional models. The approach acknowledges that the universe did not have just one unique beginning and history but a multitude of different beginnings and histories, and that it has experienced them all. But because most of these other alternative histories disappeared very early after the Big Bang to leave behind the universe we observe today, the best way to understand the past, they say, is to trace our knowledge back from the present (Phys. Rev. D 73 123527).

Most models of the universe are bottom-up, that is, you start from the well-defined initial conditions of the Big Bang and work forward. However, Hawking and Hertog say that this method is flawed because we do not and cannot know the initial conditions present at the beginning of the universe and that we only know the final state -- the one we are in now. Their idea is therefore to start with the conditions we observe today -- like the universe is 3D, nearly flat and expanding at an accelerating rate -- and work backwards in time to determine what the initial conditions might have looked like.

The new theory aims to get round a fundamental problem of string theory -- the most popular candidate for a "theory of everything" -- which is that it allows the existence of a multitude of different types of universes as well as our own. Each possible universe in this "landscape" has its own fundamental constants and even different numbers of space-time dimensions. Moreover, string theory does not favour any particular universe over another, which is not a good state of affairs as we clearly live in a universe with a particular set of physical properties.

To address this, Hawking and Hertog say that all these alternative universes of string theory may have actually existed together in the first few instants after the Big Bang. At this time, the universe was in a "superposition" of all these possible worlds. However, most of these universes then quickly faded away to leave behind our present-day universe. By tracing our universe back from the present to the past, we can ignore most of other branches that the universe took because they are too different from the current universe.

While this idea sounds fantastic, it is based on Richard Feynman’s "sum over paths" formulation in quantum theory, which says that the probability that a photon, say, arrives at a particular place can be calculated by summing up over all the different possible trajectories of the photon. Although the photon could follow lots of different paths, the straight-line path dominates over all the others so this is the one we see. In the same way, Hawking and Hertog say that the universe did not take just one path through time to arrive in its present state, but took a multitude of paths, or histories. The "sum over all histories" is therefore the universe we observe today.

The new top-down theory could also explain why some constants of nature seem to have finely tuned values that have allowed life to evolve in our universe. For example, the cosmological constant, Λ -- the force that appears to be causing the expansion of the universe to accelerate, or the density of dark energy -- has a small positive value; if it were any smaller or bigger then life would not exist. According to the new theory, the current universe must have "chosen" those histories that led to the "correct" value of Λ otherwise we would simply not be here to experience it -- a theory also known as the "anthropic principle".

Hawking and Hertog also say their model could be tested by comparing observations of the patterns of minute intensity variations in the cosmic microwave background (CMB) radiation with those calculated by their theory when it is more developed. The CMB is the radiation left over from the Big Bang and should contain "imprints" of some of the very early alternative histories within it.

So what do you think guys? Is it a novel approach or just a retooling of something already existing?

 

[CarlB]

Well I think it is a novel approach, but is also very likely to be as much a waste of time as what has been tried for the last 25 years. There may be some interesting mathematics come out of it, but they won't make any real progress.

The problem is that the method still reeks of the same problems that have dogged quantum mechanics. A real advance in physics will require a new ontology for QM, one that does not require physics students to believe what amount to mathematical fairy tales.

What we have now is just a calculational tool, not a true understanding of how the world works. What we have now is just an "effective" theory, it does not extrapolate. Since it is just a calculational tool, it cannot be reliably extrapolated to conditions far far beyond the ones in which it was developed.

To extrapolate that far is like extrapolating the lifetime of the hydrogen atom when the only tool you have is Maxwell's equations. You can crank the mathematics (maybe). You may get an answer. But your answer will have little to do with the way the world works.

When we get a theory of everything, you will recognize it because it will allow us to calculate all the parameters of the standard model in the same way (i.e. with great effort, but definitively and from very simple assumptions) that QM now allows us to calculate the properties of the chemical elements.

BWDIK.

Carl

 

[Kea]

Is it a novel approach or just a retooling of something already existing?

That's a very good question, alexsok. Some of us would be tempted to say no, not really to the former question, but perhaps some of the cosmologists here might like to express their opinion.

 

[marcus]

here are some references
http://arxiv.org/abs/astro-ph/0305562
Cosmology from the Top Down
Stephen Hawking
Talk presented at Davis Inflation Meeting, 2003

"... the best way to understand the past, they say, is to trace our knowledge back from the present (Phys. Rev. D 73 123527)."

that Physical Review Series D 73 article is available online as:

http://arxiv.org/abs/hep-th/0602091
Populating the Landscape: A Top Down Approach
S.W. Hawking, Thomas Hertog
22 pages, 1 figure
"We put forward a framework for cosmology that combines the string landscape with no boundary initial conditions. In this framework, amplitudes for alternative histories for the universe are calculated with final boundary conditions only. This leads to a top down approach to cosmology, in which the histories of the universe depend on the precise question asked. We study the observational consequences of no boundary initial conditions on the landscape, and outline a scheme to test the theory. This is illustrated in a simple model landscape that admits several alternative inflationary histories for the universe. Only a few of the possible vacua in the landscape will be populated. We also discuss in what respect the top down approach differs from other approaches to cosmology in the string landscape, like eternal inflation."

... might like to express their opinion.

 

[marcus]

alexok, I want to compliment you on finding some interesting articles
(not this one in particular, but several over time)
I especially remember your coming up with this Max Tegmark thing about the steadiness of Lambda, reported in Physicsweb

http://physicsweb.org/articles/news/8/6/14

and starting a thread about it
https://www.physicsforums.com/showthread.php?t=32424

but in this case, at least for the time being, I have nothing to add to what Kea and Carl already said.

 

[setAI]

wow! this resonates strongly with some of my thinking of late- specifically the idea that the 'past' is a sum-over-histories of many different rule-systems and universes running in parallel superposition-

 

[JohnnyTheFox]

Very refreshing read and I agree a lot with what carlB said. As a physics student I often feel some of it turns into a mathematical game. With things as elegant and clean as relativity and bits of astronomy ect some of the particle physics and QM seems like it's been made up on the spot.

Well time to leave work but I'll be reading more on this later!