Bubblecar

2004-May-11, 02:35 PM

Max Tegmark's recent publications on the Multiverse (Levels 1 to 4) have attracted some controversy. In my "local" science forum, the following extract from the Level 1 multiverse came in for some criticism (Level 1 is just our "normal" universe, assuming that it's flat & homogeneous, as the CMBR evidence seems to indicate):

If space is infinite and the distribution of matter is sufficiently uniform

on large scales, then even the most unlikely events must

take place somewhere. In particular, there are infinitely

many other inhabited planets, including not just one but

infinitely many with people with the same appearance,

name and memories as you. Indeed, there are infinitely

many other regions the size of our observable universe,

where every possible cosmic history is played out. This

is the Level I multiverse. (From Max Tegmark, Parallel Universes, downloadable here - http://arxiv.org/abs/astro-ph/0302131)

Several critics argued that infinite space & a uniform distribution of matter aren't sufficient to ensure these infinitely many duplicates, or to ensure that "every possible cosmic history is played out". I didn't fully understand these criticisms, but Tegmark now seems to have modified the story somewhat - this scenario is now also dependent on statistical properties derived from quantum fluctuations in the inflation phase -

"...infinite space alone guarantees only that SOME Hubble volume will have a duplicate, not that our own will. However, if (as in the current cosmological standard model) the cosmic density fluctuations originate from quantum fluctuations during inflation, their statistical properties DO guarantee that our (and indeed every) Hubble volume has a duplicate." (From his new Multiverse FAQ, downloadable here - http://www.hep.upenn.edu/~max/multiverse.html)

Since it does appear that the flat, homgeneous universe corresponding to the Level 1 multiverse may well be the REAL universe, how confident can we really be about these infinitely many copies of ourselves, & "every possible cosmic history" being assured of "probability 1" ?

If space is infinite and the distribution of matter is sufficiently uniform

on large scales, then even the most unlikely events must

take place somewhere. In particular, there are infinitely

many other inhabited planets, including not just one but

infinitely many with people with the same appearance,

name and memories as you. Indeed, there are infinitely

many other regions the size of our observable universe,

where every possible cosmic history is played out. This

is the Level I multiverse. (From Max Tegmark, Parallel Universes, downloadable here - http://arxiv.org/abs/astro-ph/0302131)

Several critics argued that infinite space & a uniform distribution of matter aren't sufficient to ensure these infinitely many duplicates, or to ensure that "every possible cosmic history is played out". I didn't fully understand these criticisms, but Tegmark now seems to have modified the story somewhat - this scenario is now also dependent on statistical properties derived from quantum fluctuations in the inflation phase -

"...infinite space alone guarantees only that SOME Hubble volume will have a duplicate, not that our own will. However, if (as in the current cosmological standard model) the cosmic density fluctuations originate from quantum fluctuations during inflation, their statistical properties DO guarantee that our (and indeed every) Hubble volume has a duplicate." (From his new Multiverse FAQ, downloadable here - http://www.hep.upenn.edu/~max/multiverse.html)

Since it does appear that the flat, homgeneous universe corresponding to the Level 1 multiverse may well be the REAL universe, how confident can we really be about these infinitely many copies of ourselves, & "every possible cosmic history" being assured of "probability 1" ?