|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
Galaxies without dark matter halos?
From: Ulf Torkelsson
You are actually forgetting one of the strongest arguments in favour of the big bang. The abundances of the light elements. It is tremendously difficult to make a universe consisting of 75% hydrogen and 25% helium without a hot big bang, and it is virtually impossible to make any deuterium in any other scenario. The big bang at the same time explains these abundances and the microwave background. Please correct me if I'm wrong: Wouldn't you get the same universe consisting of almost 75% H and 25% He, including some deuterium, so long as the model includes any hot large explosion that produces all the matter we observe? Is there something about the physics of that reaction that requires that it not be a part of a larger universe? -- Richard S. Sternberg [Mod. note: top-posting fixed. Please don't top-post -- mjh.] |
#2
|
|||
|
|||
Galaxies without dark matter halos?
Richard S. Sternberg wrote:
Please correct me if I'm wrong: Wouldn't you get the same universe consisting of almost 75% H and 25% He, including some deuterium, so long as the model includes any hot large explosion that produces all the matter we observe? Is there something about the physics of that reaction that requires that it not be a part of a larger universe? In order to be consistent with the high degree of isotropy of the microwave background and the fairly homogeneous distribution of mass on the largest scales, the nucleosynthesis must have taken place uniformly throughout the universe that we can observer today. Furthermore in order to get the observed element abundances the material that underwent the nucleosynthesis must have consisted of free protons and neutrons in the ratio expected a few minutes after the big bang. Ulf Torkelsson |
#3
|
|||
|
|||
Galaxies without dark matter halos?
Richard S. Sternberg wrote:
Please correct me if I'm wrong: Wouldn't you get the same universe consisting of almost 75% H and 25% He, including some deuterium, so long as the model includes any hot large explosion that produces all the matter we observe? Is there something about the physics of that reaction that requires that it not be a part of a larger universe? In order to be consistent with the high degree of isotropy of the microwave background and the fairly homogeneous distribution of mass on the largest scales, the nucleosynthesis must have taken place uniformly throughout the universe that we can observer today. Furthermore in order to get the observed element abundances the material that underwent the nucleosynthesis must have consisted of free protons and neutrons in the ratio expected a few minutes after the big bang. Ulf Torkelsson |
#4
|
|||
|
|||
Galaxies without dark matter halos?
On Wed, 27 Aug 2003, Ulf Torkelsson wrote:
Furthermore in order to get the observed element abundances the material that underwent the nucleosynthesis must have consisted of free protons and neutrons in the ratio expected a few minutes after the big bang. Ulf Torkelsson A bit stronger than that: If the expansion & cool down is too slow ("nuclear reactions stay in equalibrium") then there is time for larger atoms to built up and those with the most binding energy are favored --Lots of iron and little hydrogen results. But if it cools too fast there isn't a chance for helium to form and any neutrons tend to decay to more hydrogen rather than be captured --lots of hydrogen and little of anything else. Since the heavier elements tend to be built up in steps, how much of one element is made at one time is sensitive to how much of some particular lighter elements were made ealier, so the final abundances are sensitive to some details of the cool down. Folding in the binding energies vs. tempurature, densities etc. and running the cool down of the universe as predicted by the standard (Friedman-Robertsosn-Walker metric) you get good agreement with observed abunancies or the first few elements which are, admittedly, hard to measure. opinionBut it is impressive to get any agreement... /opinion Some of this is covered at a popular level in George Gamow's _The creation of the universe_ and in many (most?) cosmology texts, i.e. _The Early Universe_ by Kolb & Turner. Note that this happens well after the time (below the energy scale/temperature) of inflation but before recombination (CMB) 3ch |
#5
|
|||
|
|||
Galaxies without dark matter halos?
On Wed, 27 Aug 2003, Ulf Torkelsson wrote:
Furthermore in order to get the observed element abundances the material that underwent the nucleosynthesis must have consisted of free protons and neutrons in the ratio expected a few minutes after the big bang. Ulf Torkelsson A bit stronger than that: If the expansion & cool down is too slow ("nuclear reactions stay in equalibrium") then there is time for larger atoms to built up and those with the most binding energy are favored --Lots of iron and little hydrogen results. But if it cools too fast there isn't a chance for helium to form and any neutrons tend to decay to more hydrogen rather than be captured --lots of hydrogen and little of anything else. Since the heavier elements tend to be built up in steps, how much of one element is made at one time is sensitive to how much of some particular lighter elements were made ealier, so the final abundances are sensitive to some details of the cool down. Folding in the binding energies vs. tempurature, densities etc. and running the cool down of the universe as predicted by the standard (Friedman-Robertsosn-Walker metric) you get good agreement with observed abunancies or the first few elements which are, admittedly, hard to measure. opinionBut it is impressive to get any agreement... /opinion Some of this is covered at a popular level in George Gamow's _The creation of the universe_ and in many (most?) cosmology texts, i.e. _The Early Universe_ by Kolb & Turner. Note that this happens well after the time (below the energy scale/temperature) of inflation but before recombination (CMB) 3ch |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
"Dark matter" forms dense clumps in ghost universe (Forwarded) | Andrew Yee | Astronomy Misc | 0 | November 21st 03 04:41 PM |
Galaxies without dark matter halos? | greywolf42 | Astronomy Misc | 34 | November 5th 03 12:34 PM |
A Detailed Map of Dark Matter in a Galactic Cluster Reveals How Giant Cosmic Structures Formed | Ron Baalke | Astronomy Misc | 3 | August 5th 03 02:16 PM |
Galaxies without dark matter halos? | Ed Keane III | Research | 4 | August 4th 03 12:39 PM |
Hubble tracks down a galaxy cluster's dark matter (Forwarded) | Andrew Yee | Astronomy Misc | 0 | July 17th 03 01:42 PM |