If dark matter is a type of particle that is subject to gravitation but not to electricity and magnetism, as some physicists have speculated, then such particles should accumulate in the cores of stars. That would mean that a star can achieve the density necessary to kick off nuclear fusion with only about 1/6 the amount of normal matter previously assumed to be necessary, the remaining 5/6 of the star’s total mass being non-interacting dark matter. That would further mean that a star should live no more than about 1/6 as long as would a star of the same total mass comprised of normal matter only. If dark matter does indeed congregate in the cores of stars and if it is truly incapable of participating in nuclear interactions, then it shouldn’t impede the nuclear interactions of normal matter. Additionally, dark matter of this type should not take up much volume. In a neutron star, the neutrons are packed very closely together since the volume of a collection of matter particles is based on electricity & magnetism, particle spin, and the Pauli Exclusion Principle– and neutrons are unaffected by electricity and magnetism. And yet the neutrons still can’t collapse into one another because neutrons have a spin of 1/2 and they are therefore subject to the Pauli Exclusion Principle. In any case stars that consist predominantly of dark matter particles should be much smaller in volume than stars comprised solely of normal matter of the same mass.
The only possible escape I can think of for the above reasoning is the possibility that dark matter particles have no mass. But my assumption has always been that a particle can only exert a gravitational force on other particles if it has mass. If that is not true, then everything that I thought I knew about gravitation goes out the window.
The presence of dark matter in the ratio of 5:1 would have an enormous impact on the composition of the universe. The theory of stellar evolution says that high mass stars burn progressively hotter as they age, and they burn successively higher atomic number atoms, in the following order:
- Hydrogen (atomic number 1)
- Helium (atomic number 2)
- Carbon (atomic number 6)
- Oxygen (atomic number 8)
- Neon (atomic number 10)
- Silicon (atomic number 14)
The burning of silicon results in iron (atomic number 26), and the burning of iron results in a net energy loss to the star– thereby allowing the force of gravitation to overwhelm the outward pressure of radiation, inevitably resulting in the collapse of the star.
If the core of a high mass star is comprised of 83% inert matter, then the result will be a far lower percentage of high atomic number atoms throughout the universe. And that would produce a universe with fewer rocky planets.
Billions of dollars and euros have been spent trying to find dark matter particles. So far all such attempts have failed. Maybe they have failed because dark matter isn’t a type of particle.
We should test the hypothesis that dark matter accumulates in the cores of stars. That wouldn’t necessarily tell us whether dark matter is a type of particle, but it would impose a severe constraint on the type of particle it might be. Unfortunately I don’t know that there’s a way to test for the presence of a lower-than-expected percentage of atoms with an atomic number greater than 2. It seems that would require a survey of the total mass of rocky planets throughout the universe as compared to the mass of the atoms which were present immediately after the period of recombination. That distribution is approximately as follows:
|Hydrogen (H)||~ 75%|
|4Helium (He)||~ 25%|
|Deuterium (2H)||~ 0.01%|
(A Universe From Nothing, Lawrence M. Krauss, pg. 111)
Such a survey, so far as I am aware, is beyond our present technology.
An analogy often invoked by physicists to explain the expansion of the universe is that of a loaf of raisin bread baking in the oven. As the bread bakes, it rises– and the raisins in the bread move farther apart from each other. Similarly, the matter of the universe, clumped as it is into galaxies, moves apart as the universe expands not because there was a massive explosion that blew matter out in all directions, but because space itself is expanding and the matter of the universe is simply being dragged apart with it as it expands. The raisins represent galaxies, or clusters of galaxies, and all of them are moving away from each other. So from the perspective of an observer in any one galaxy, all other galaxies are moving away.
This analogy raises several questions. First question: What it is that space is expanding into? The space/time of General Relativity has 3 physical and 1 temporal dimensions. If this space/time is expanding into a dimensionless void– a void that has no intrinsic structure of any kind– how is it that our familiar world of space/time gets imposed on that unstructured void? It would seem that there must be some mechanism that enables the creation of the structure of space/time itself.
Exactly what types of structure are able to be imposed on an unstructured void? Would it be possible to have a universe of 47 physical and 17 temporal dimensions? What about fractional dimensions? Or negative dimensions? Could a universe have physical dimensions only and no temporal dimensions?
If instead there are only a very few options for the numbers and types of dimensions in the resulting universe, that would seem to impose a significant constraint on the concept of an unstructured void. And one would have to ask: If an unstructured void has such significant constraints, is it really unstructured?
Second question: Alternatively, if the structure of space/time is expanding into a void that already has the same space/time structure as General Relativity, then what exactly is expanding? Clearly it is not the 3 physical / 1 temporal structure of space/time. Is it simply the outer boundary of our universe? If so, why would the expansion of an outer boundary pull all of the matter of the universe along with it?
Einstein famously added the Cosmological Constant to the equations of General Relativity because he wanted a steady state universe– one that was neither expanding nor contracting. Now that we know that the rate of expansion is increasing, some cosmologists have taken to considering the Cosmological Constant as the source of what is termed “Dark Energy.” But Einstein’s original concept of the Cosmological Constant wasn’t about expanding or contracting the boundary of the universe itself. Rather his thought was that the Cosmological Constant acts as a kind of anti-gravity that pushes the physical masses of the universe apart– within the boundaries of the universe.
Perhaps the solution is to re-imagine the Cosmological Constant as a force that propagates the structure of space/time into an undifferentiated void. But wouldn’t that be something different from a force that propels galaxies away from each other? Besides, there is nothing in General Relativity that specifically ties the Cosmological Constant to the propagation of space/time alone.
Third question: If we are explaining the expansion of the universe by an expansion of space/time itself, then wouldn’t that expansion extend all the way down to the atomic and subatomic levels? If so, then the expansion of the universe would be ripping apart every atom– and indeed every composite particle. That type of expansion would have enormous consequences for the evolution of the universe. A star born in the early stages of the universe would live for a much shorter time than a star of the same composition and size created today. That’s because the particles in the core of the star would be much closer together and therefore chemical and nuclear reactions would happen at a much faster rate.
The simplest answer to questions 1 and 2 above is that the 3 physical / 1 temporal structure of the space/time of General Relativity was already imprinted on the void into which the universe expanded at the time of the Big Bang. Of course we have no guarantee that the simplest explanation is the one that is correct, but if true that would mean that the raisin bread metaphor is very misleading. In the raisin bread metaphor, the bread itself (without the raisins) represents the structure of space/time. As the bread expands, space/time expands. But if the void into which the bread is expanding already has the same space/time structure, then it is only the material substance contained within space/time that is expanding, not space/time itself. And if the void already contained the space/time structure of our universe at the time of the Big Bang, then the void should have no boundary at all.
The only test I can think of that would give us a definitive answer to questions 1 and 2 is to actually find a universe with something other than a 3 physical / 1 temporal structure– or to somehow show that no other such structure is possible. But I’m pretty sure that no one has come up with a way to produce either such result.
As for question 3, it seems to me at least possible that it could be submitted to a test. The higher rate expected for nuclear and chemical reactions should show up as higher temperatures for first generation stars. Higher temperature means higher luminosity, and higher luminosity means shorter wavelength. Therefore the spectra of stars from the first generation should be much bluer if indeed space/time itself is what is expanding. Of course these bluer spectra would be red-shifted. So we should be looking for a population of stars with very high red shifts– meaning much older– that have spectra which are unexpectedly bluer than expected. Is such a survey possible? I’m not qualified to answer that question, but my guess is that if it is possible, it would be very hard to do.
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