Extra Dimensions: What Caused Compactification?

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Discussion Overview

The discussion centers on the causes of compactification of extra dimensions in the early universe, exploring theoretical implications and the relevance of string theory and quantum cosmology. It includes considerations of cosmological models and the status of string theory in contemporary research.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • Some participants question the necessity of explaining the compactification of extra dimensions, arguing that string theory is not widely accepted in cosmology and that ordinary cosmological models do not assume compactified dimensions.
  • One participant notes that string theorists have not provided a clear explanation for the existence of compactified dimensions, suggesting that the various models are ad hoc and lead to many versions of string theory without a definitive selection process.
  • Another participant references a string theory book, stating that all dimensions were curled up until the big bang, raising questions about why only four dimensions expanded and why those specific dimensions were chosen.
  • A further point raised is that for string theorists, the compactification of dimensions may be essential for the characteristics of strings, implying that without this geometry, particles and the cosmos as understood would not exist.

Areas of Agreement / Disagreement

Participants express differing views on the relevance and acceptance of string theory in cosmology, with some arguing against its necessity while others defend its implications. The discussion remains unresolved regarding the reasons behind compactification and the status of string theory versus alternative models like Loop quantum cosmology.

Contextual Notes

Participants highlight the lack of consensus on the existence and implications of compactified dimensions, as well as the evolving landscape of quantum cosmology research, which has shifted focus from string theory to Loop quantum cosmology in recent years.

keepitmoving
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what caused compactification of the extra dimensions in the early universe?
 
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keepitmoving said:
what caused compactification of the extra dimensions in the early universe?

I don't think one can say. String, with it's extra dimensions, is not widely accepted in cosmology. In ordinary workaday cosmology one doesn't assume that there are any compactified extra dimensions.

No explanation for their existence is therefore needed. And in any case none has been offered by the string theorists themselves---imagining different dimensionalities with dimensions rolled up various ways are simply ad hoc choices, which lead to millions of different versions of string, with no clear way to select one corresponding to our reality.

For practical purposes, in ordinary working cosmology one can forget about that extra dimension stuff.
=================

There is however a field called "quantum cosmology" which some years ago had a fair amount of string input. The prospects for string and for string-inspired cosmology looked more promising back in the 1990s.
You can get an idea of the current state of research in quantum cosmology (QC) by doing a keyword search in the Spires research archive
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+DK+QUANTUM+COSMOLOGY+AND+DATE+%3E+2006&FORMAT=www&SEQUENCE=citecount%28d%29

The field of quantum cosmology has been largely taken over (if one looks at the topcited recent research papers) by an alternative to string: Loop QC.

Most of the papers in the top 25, whether or not they say so in the title, are use the Loop QC approach to explaining the big bang. Public media has not caught up with this shift.
 
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Well, according to a string theory book that I read, all dimensions were curled up like that until the big bang when four of them expanded. So another question would be why did only four expand and why these specific four.
 
And that of course begs an additional question: why are they compacitified now? For string theorists, one answer would be that it's those extra dimensions that control string characteristics and without that additional geometry, all particles would cease to exist...there would be no cosmos.
 

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