The Big Bang time paradox

[Ned]

Originally posted by KrazyHorse


Most of the dark matter must have low interaction with light and regular matter. Otherwise the evolution of structure would have followed a different path. Dark matter collapses into clumps earlier than regular matter does because the dark matter is not in thermal equilibrium with the regular matter, and thus is not held from collapsing by radiation pressure.

As for what it actually is, we don't know. It's demonstrably not simply frozen out neutrinos. That accounts for less than 2% of it.

The bulk of it must be in an unknown field.

KH, could to "universe" at one time been composed only of dark matter?

[TheStinger]

Its made up mostly of odd socks

[Impaler[WrG]]

We can see "normal" matter with our telescopes as far back as they are able to see, and I belive prior to that the models predict normal matteras far back as they go. So as far as I can tell the chances are very slim that could be the case. From what KH says it sounds like the dark matter has been around for a long time but as we cant say what it is it cant realy be in the model proper so the model must be flawed in that regard. Also if their was at one time only Dark-Matter (the weird unknown particle kind) their would have to be some new explination for ware regular matter came from.

KH: I heard about the nutrino have been shown to have some tiny mass but your saying that all of them put together only chipped away 2% of the unkown dark-matter. Likewise the cold but normal matter cant be anything more then a trivial portion of the total. Are their any other major now-know components of Dark matter that were originaly un-known when theh whole Dark Matter total was first nailed down?

[KrazyHorse]

KH, could to "universe" at one time been composed only of dark matter?

Dark matter is assumed to have the same "equation of state" as regular matter. What the equation of state tells you is how much "pressure" is exerted by the component in question. Matter exerts 0 pressure. This is basically equivalent to saying that the "total amount "of matter is independent of the scale factor (except insofar as matter is annihilated or pair-created). More properly, it tells you that the density of matter goes down as the cube of the scale factor. Think of the Universe as a box. If you double the scale factor you increase its volume by 2*2*2 = 8. If you had 1kg of matter per meter cubed before doubling then after doubling you will have 1/8 kg per meter cubed. Both dark matter and regular matter are hit by the same scale factor cubed dependence, so to a first approximation the amount of dark and regular matter in the universe remain in the same proportion.

Radiation (light) and hot matter (stuff that's moving ultrarelativistically) are hit by an extra scale factor (from redshift), so they drop off quicker than matter. They also exert negative pressure on the Universe's expansion.

Finally, if dark energy is a true cosmological constant, its density remains a constant independent of the scale factor. It thus exerts positive pressure on the Universe's expansion (tending to force it to accelerate)

The thermal history of the Universe goes as follows, therefore: at early time, the Universe was hot and radiation dominated. Dark matter and neutrinos were so pressed together that they were in thermal equilibrium with the visible Universe. They behaved just like regular matter. Then the Universe expanded and cooled off. The dark matter stuff "froze out" (i.e. dropped out of contact with regular matter). It became real dark matter. The Universe expanded more and more. The radiation density dropped faster than the matter density. The Universe thus became matter dominated for a time. It kept expanding. The matter density kept dropping. The radiation density kept dropping faster, to the point where nowadays radiation density is negligible. The dark energy density, however, remained constant (if it's a true cosmo constant). Just a little while ago it caught up to the matter density and passed it. We now live in a dark energy dominated Universe. The pressure from this dark energy forces the Universe to accelerate.

[KrazyHorse]

Are their any other major now-know components of Dark matter that were originaly un-known when theh whole Dark Matter total was first nailed down?

No. Nobody knows what the bulk of DM is. It's probably an undiscovered type of particle. Or else it's something really weird.

[KrazyHorse]

Neutrinos would be an ideal explanation for DM except that they don't work.

We know too much about their cross-section and we've put too good a bound on their mass for them to work.

We need something else.

[Diadem]

Is it not possible to explain Dark Matter in another way?

I've once read something about that the observations supporting dark matter can also be explained by making some adjustments in GR. Was some time ago though, not sure if it's still the same for additional observations.

[KrazyHorse]

All the explanations using modified gravity I've seen are hokey.

[Impaler[WrG]]

I've heard of this alternative means of explaining dark-matter (aka dark-matter is not realy their), some modification of gravity or was it Newtons F=ma. In any case its kind of "radical" and as KH says its not excepted by the main-stream. I also heard some resent observations shed doubt on the idea and supported the dark-matter exists camp.

KH: The dark-energy as you say is not very well understod in that we dont know if it is as you say a true cosmological constant (inwhich case is has the property of not disipating as the universe expands). If that were the case and combined with what you were saying about it being dominant now will force the universe to expand forever baring anything weird we dont know about, correct? On the other hand if its not a true constant what is the alternative/s and how do they affect the universe? What observations can be made or are being made to figure this out?

[snoopy369]

Originally posted by KrazyHorse
All the explanations using modified gravity I've seen are hokey.

I'd use "hokey" to describe most of astrophysics to be honest

/me runs

[Kuciwalker]

KH would probably agree with you, actually

[Kuciwalker]

KH: doesn't increasing the amount of dark matter/energy increase the amount of energy in the universe, so if the density is constant it would make the universe try to contract?

[Addled Platypus]

Cosmogony has three paradoxes

1. There can be no effect without a cause, What ever transpired near the outset of time, each must have been caused by some prior event. So we can never attain an account of the very beggining

(I.E. what came first the chicken od the egg)

2. You cant get something from nothing-or -for nothing. The "origin" of the universe, if that concept is to have any meaning, must create the universe out of nothing, Therefore there can be no logical explanation of genesis

3. Regardless of its energy, the universe muat have originated from another sysytem, and that sysytem must in turn have had an origin of some sort, And so we are caught in infinite regress

Timothy Ferris - " The Whole Shebang"

[snoopy369]

Originally posted by Kuciwalker
KH: doesn't increasing the amount of dark matter/energy increase the amount of energy in the universe, so if the density is constant it would make the universe try to contract?

If density is constant then the universe isn't contracting, is my understanding of the word 'density' ... well, if density and mass are constant, anyway.

If you increase the mass while keeping density constant, which is my admittedly moronic understanding of your statement here, doesn't that by definition increase the volume, leading to an expanded universe?

[Kuciwalker]

No we're talking about dark matter/energy. We apparently (according to KH) don't know the rate at which they decrease wrt the volume of the universe. Radiation goes faster than 1/x^3, mass at 1/x^3, and if the density of dark matter is constant, then it doesn't decrease as the universe expands.

KH, please correct me if I misunderstood your post