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Is it possible?



 
 
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  #1  
Old April 22nd 14, 08:14 AM posted to sci.astro.research
David Staup[_2_]
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Default Is it possible?

Is it possible that Dark Matter is repulsive to itself (by whatever
mechanism) while being attracted (by gravity) to normal matter?

It seems to me that that might explain inflation and the current
acceleration of expansion and a whole host of other observations.
  #2  
Old April 23rd 14, 08:43 AM posted to sci.astro.research
Eric Flesch
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Posts: 321
Default Is it possible?

On Tue, 22 Apr 14, David Staup wrote:
Is it possible that Dark Matter is repulsive to itself (by whatever
mechanism) while being attracted (by gravity) to normal matter?


No, because if dark matter were attracted to normal matter, then
normal matter would (ipso facto) be attracted to dark matter. The
dark matter would settle into shells around the matter (not being able
to come to close because of repulsion to other similarly-attracted
dark matter), and thus the consequence would be a modification in the
inverse-square law. We know the inverse square law has not been
modified because it is precisely inverse square. Therefore there are
no such shells of dark matter and so the hypothesis is disproved.

Eric
  #3  
Old April 23rd 14, 08:44 AM posted to sci.astro.research
Phillip Helbig---undress to reply
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Default Is it possible?

In article , David Staup
writes:

Is it possible that Dark Matter is repulsive to itself (by whatever
mechanism) while being attracted (by gravity) to normal matter?


Perhaps it is theoretically possible. For example, all DM particles
could be positively charged. (Of course, in this case, they would
interact electromagnetically and hence wouldn't be dark, but perhaps
there is some unknown repulsive force.)

It seems to me that that might explain inflation and the current
acceleration of expansion and a whole host of other observations.


No. The problem is that ORDINARY MATTER is observed in accelerated
expansion etc. If it were somehow being pushed apart by dark matter,
then this dark matter would have to couple rather strongly to ordinary
matter. However, this is not the case; the interaction cross section is
quite low, which is why there have been no (convincing) direct
detections yet.

It also wouldn't explain inflation in any real sense. Inflation has the
universe expand by several orders of magnitude in size. Any sort of
repulsive but otherwise normal matter would be thinned out too quickly
to have much of an effect.

In general, the idea doesn't solve anything: accelerated expansion
(whether now or during inflation) remains, but something unknown (dark
matter which is gravitationally attractive but self-repulsive) is
postulated for which there is otherwise no sort of evidence at all.

In general, there is too much emphasis on the "need for an explanation".
Not all of the universe is made of the same stuff we are. Big deal.
Isn't this what the basic expectation should be? Suppose it turned out
that most of the universe is made of the stuff we are (baryons); why
should this be? Wouldn't this be much more in need of an explanation?
As for accelerated expansion, the cosmological constant appears in the
equations of general relativity and has a value to be determined by
observation, just as the value of the gravitational constant is
determined by observation. (The fact that Einstein introduced the
cosmological constant for the wrong reason and later didn't like it is
historically interesting but not something the universe cares about.)
Note also that while GR can explain the equivalence of inertial mass and
passive gravitational mass, it can't explain the reason why an inertial
mass should have an active gravitational mass. It just does. Either
this demands an explanation as well (but hardly anyone seems to think
so), or one accepts the cosmological constant for what it is.
  #4  
Old April 23rd 14, 08:48 AM posted to sci.astro.research
Steve Willner
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Posts: 1,172
Default Is it possible?

In article ,
David Staup writes:
Is it possible that Dark Matter is repulsive to itself (by whatever
mechanism) while being attracted (by gravity) to normal matter?


That would be inconsistent with general relativity, but that's no
reason not to consider the possibility. I don't see how repulsive
dark matter could clump up, though, as seems to be needed for galaxy
clusters and to explain CMB fluctuations. It would also lead to a
time dependence of the expansion contrary to observations, which are
consistent with a cosmological constant that does not vary in time.

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  #5  
Old April 23rd 14, 09:38 PM posted to sci.astro.research
David Staup[_2_]
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Posts: 347
Default Is it possible?

On 4/23/2014 2:48 AM, Steve Willner wrote:
It would also lead to a
time dependence of the expansion contrary to observations,


I thought we have seen a rate of expansion that is increasing over
time...no?
  #6  
Old April 26th 14, 09:18 AM posted to sci.astro.research
Phillip Helbig---undress to reply
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Default Is it possible?

In article , David Staup
writes:

On 4/23/2014 2:48 AM, Steve Willner wrote:
It would also lead to a
time dependence of the expansion contrary to observations,


I thought we have seen a rate of expansion that is increasing over
time...no?


Yes. The OP's idea, though, would lead to one decreasing with time.
  #7  
Old April 26th 14, 09:31 AM posted to sci.astro.research
Steve Willner
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Posts: 1,172
Default Is it possible?

It would also lead to a
time dependence of the expansion contrary to observations,


In article ,
David Staup writes:
I thought we have seen a rate of expansion that is increasing over
time...no?


The observed change in expansion rate with time is consistent with a
cosmological constant, which I believe means that acceleration is
constant with time. The technical description is that parameter w
has a value very close to -1.

If I understand the original suggestion of dark matter particles that
repel each other, their average distance apart would increase with
time, and the simple expectation would be that the repulsive force
would change, giving w different from -1. One could presumably
overcome this by making the force-distance law just what is needed,
but I don't see how to overcome the other problems with the idea.

--
Help keep our newsgroup healthy; please don't feed the trolls.
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  #8  
Old April 27th 14, 03:53 AM posted to sci.astro.research
Phillip Helbig---undress to reply
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Posts: 629
Default Is it possible?

In article , Steve Willner
writes:

It would also lead to a
time dependence of the expansion contrary to observations,


In article ,
David Staup writes:
I thought we have seen a rate of expansion that is increasing over
time...no?


The observed change in expansion rate with time is consistent with a
cosmological constant, which I believe means that acceleration is
constant with time.


It's more complicated. At first, there is deceleration, then there is
acceleration, so neither is the rate of expansion constant with time,
but neither is the acceleration. However, the universe (and all
universes with a positive cosmological constant which don't collapse in
the future) will asymptotically approach the de Sitter universe, in
which the Hubble constant is constant in time (it is called the Hubble
constant not because it is generally constant in time, but because it is
a constant when fitting data points; the cosmological constant, however,
IS constant in time). However, the Hubble constant is the rate of
increase of the scale factor divided by the scale factor, so a constant
Hubble constant means an increasing acceleration in absolute terms
(exponential, in fact).

The technical description is that parameter w
has a value very close to -1.


Right. A pure cosmological constant has w=-1 exactly and there is no
observational evidence that this is not the case, and there is
observational evidence that it is quite close to -1.
  #9  
Old May 1st 14, 01:18 PM posted to sci.astro.research
Steve Willner
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Posts: 1,172
Default Is it possible?

In article ,
Phillip Helbig---undress to reply writes:
At first, there is deceleration, then there is
acceleration,


What I meant was that acceleration _owing to a cosmological constant_
is constant in time. At least I believe that's the case, but I've
never had a proper course in modern cosmology. If the dark energy is
something other than (or in addition to) a cosmological constant, the
time dependence of its acceleration is likely to be different.

There is also deceleration due to normal gravitational attraction,
important only at early epochs. The net acceleration is the sum of
these.

A pure cosmological constant has w=-1 exactly and there is no
observational evidence that this is not the case, and there is
observational evidence that it is quite close to -1.


Agreed.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
  #10  
Old May 1st 14, 05:33 PM posted to sci.astro.research
Phillip Helbig---undress to reply
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Posts: 629
Default Is it possible?

In article , Steve Willner
writes:

In article ,
Phillip Helbig---undress to reply writes:
At first, there is deceleration, then there is
acceleration,


What I meant was that acceleration _owing to a cosmological constant_
is constant in time.


OK, there is an overlap between deceleration due to matter and
acceleration due to the cosmological constant. Matter thins out with
time so acceleration takes over.

At least I believe that's the case,


It's not. Consider the de Sitter universe, which has a cosmological
constant and no matter. The expansion law is exponential, i.e. the
acceleration increases with time. Since the Hubble constant is
dr/dt*1/R, it is constant in time. The parameter q is constant at -1.
It is defined as \frac{-\ddot R R}{\dot R^{2}} or
\frac{-\ddot R}{RH^{2}}. Since H is constant in time, \ddot R must
increase with time in proportion to R. So, of course, as with any
exponential, all derivatives are exponential.

but I've
never had a proper course in modern cosmology.


I still recommend three books: Edward Harrison's COSMOLOGY: THE SCIENCE
OF THE UNIVERSE, Bondi's COSMOLOGY and Berry's COSMOLOGY AND
GRAVITATION. Of course, these won't contain details of the CMB etc, but
partially for that reason they are good on the foundations. A good
historical overview, starting in the mists of time but with an emphasis
on the twentieth century, is Barrow's THE BOOK OF UNIVERSES.

If the dark energy is
something other than (or in addition to) a cosmological constant, the
time dependence of its acceleration is likely to be different.


Right. That's pretty much the definition of "dark energy".
 




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