# Do U wana see Dark Matter ???

Discussion in 'Astronomy, Exobiology, & Cosmology' started by RawThinkTank, Oct 29, 2004.

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## So What do U think is that seen in the Sombrero Galaxy Picture ?

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1. ### blobranaRegistered Senior Member

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@Starthane Xyzth
hum,
tnx for correction.

Yeah, that was what i meant, er, stars <i>& dark dusty stuff/hydrogen</i>, which as you pointed out we can map fairly easily.

@Vortexx
Yeah, i believe ultimately that is what we will find.
Any gravity map will just show the position of any space/time distortion (be that from darkmatter of normal matter).
Like a density map of books will show the location of any library/bookstore, it will not show where the authors live.

3. ### RawThinkTankBannedBanned

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Well done but I am back. U said massive stars are rare ;

thats because all of them have now become dark matter due to their short lives.

5. ### Starthane Xyzthreturns occasionally...Valued Senior Member

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The rarity of large stars is also because few of them are ever formed, compared to the small ones. Collapsing molecular clouds have a tendancy to fragment into smaller and smaller clumps as they contract, until those clumps become dense enough to consolidate into protostars. There will always be fewer larger objects than smaller ones: compare the number of elephants to the number of mice...

7. ### blobranaRegistered Senior Member

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2,214
Hum,
ok, we'll say there are two possible dark matter candidates ...
<b>Wimps</b> (Weakly Interacting Massive Particles) - Exotic subatomic particles, such as axions, massive neutrinos, photinos
and <b>Machos</b> (Massive Compact Halo Objects) - Neutron stars, black holes, white dwarfs, brown dwarfs, your burnt out stars.
Ok, some of the missing mass is probably normal, but for some reason cannot be picked up by our telescopes, it will only account for 3% (there are some who put a figure as high as 35%!) of the missing mass.
To get a mental picture of this, that means to account for all of the mass required to explain how our Galaxy rotates, that would be a cool five thousand billion of these objects in the halo of our Galaxy alone, compared with just one or two hundred billion bright stars. that's a lot of dead stars...

Perhaps i didn't explain how we know the amount of baryonic matter (that is normal matter) cannot account for all the mass in the universe.
(see other post)
(Hum, this is the technical bit, look away if you are not an info-maniac)
When the universe was about 3 minutes old,
thats after big-bang,
protons and neutrons could fuse to make stable deuterium nuclei (a hydrogen isotope with one proton and one neutron).

Most of the helium in the universe was created from the primordial neutrons and protons (Although stars do produce some of the helium visible today), by the time the nucleosynthesis epoch ended. Stars fuse hydrogen nuclei to make a helium nucleus (fusion).
The fusion chain process in the early universe was slightly different than what occurs in stars because of the abundant free neutrons in the early universe. However, the general process is the same: protons react to produce deuterium (heavy hydrogen), deuterium nuclei react to make Helium-3 nuclei, and Helium-3 nuclei react to make the stable Helium-4 nucleus.
The amount of the final Helium-4 product is not as sensitive to the density of the early universe ...
<i>The deuterium nucleus is the weak link of the chain process, so the fusion chain reactions could not take place until the universe had cooled enough. The exact temperature depends sensitively on the density at that time. </i>
Extremely small amounts of Lithium-7 were also produced during the early universe nucleosynthesis process. Lithium-7 and deuterium density depends sensitively upon the density of protons (2 up + 1 down quarks) and neutrons during this time.

<b> If the universe were too dense, then most of the deuterium would have fused into helium. </b>
The more neutrons that decay before combining with protons, the smaller the abundances of heavier elements. Only in a low-density universe can the deuterium survive. A denser universe would have had more deuterium fused to form helium, so the amount of the remaining deuterium seen today is used as a probe of the early density because of the sensitivity of its production to the density of the protons and neutrons and temperature in the early universe.
After about 15 minutes the universe was too cold for fusion. Free neutrons and protons were synthesised into the light elements: deuterium (D), helium-3, and helium-4. The universe consisted of 10% helium and 90% hydrogen, (25% helium and 75% hydrogen, by mass).
There were also extremely small amounts of the Lithium-7 produced.
The elements heavier than helium were produced in the cores of stars.
The number of deuterium nuclei that do not later undergo fusion reaction to make Helium-3 nuclei also depends sensitively on the temperature and density of the protons and neutrons. A less dense universe would have had more deuterium remaining. Therefore, measurement of the primordial deuterium can show if there is enough matter to make the universe positively-curved and eventually stop the expansion. ...
The answer turns out that the fate of the Universe is, it will expand forever...

(The observed densities of the primordial isotopes to those computed from models and translating the results into Omega, (the density parameter), gives Omega = 0.015/h2 where h is the Hubble parameter divided by 100 km/sec/Mpc. The smaller Ho, the larger Omega; if Ho=50, Omega is approximately 0.03, whereas Ho=100 gives Omega of only 0.015.
This range is still much less than Omega=1, but nucleosynthesis limits can indicate only the density of baryons, because only baryons participate in nuclear reactions. Hence we must conclude that the universe contains less than the critical density of baryons. )

But,
The details results on the age, geometry and composition of the universe have been released by the WMAP probe (a follow up from cobe) has given a better range/measurement.
The Universe is 13.7 billion years old with an only a 1-% margin error.
With the first stars igniting 200 million years after the Big Bang.
Content of the Universe today: 4% Atoms, 23% Cold Dark Matter, 73% Dark energy. Expansion rate (Hubble constant) value: Ho= 71 km/sec/Mpc (with a margin of error of about 5%)
(using the above formula, Omega => 0.015/71 = 0.02)

New evidence for Inflation (in polarized signal) Fast moving neutrinos do not play any major role in the evolution of structure in the universe.
They would have prevented the early clumping of gas in the universe, delaying the emergence of the first stars, in conflict with the new WMAP data.
Fate of the Universe: it will expand forever...

The Finding have been long suspected. The value of omega as been mathematically show to be within 1 to 10 <sup> 60</sup> , of being flat.
And some would say that it <b>has to be</b>, just aesthetically wise...
Omega = 1

The evolution of the universe is determined by a struggle between the momentum of expansion (or something strange) and the pull of gravity.
The rate of expansion is determined by the Hubble Constant, Ho ,
while the strength of gravity depends on the density and pressure of the matter in the universe.
If the pressure of the matter is low, as is the case with most forms of matter we know of, then the fate of the universe is governed by the density.
If the density of the universe is less than the critical density'' which is proportional to the square of the Hubble constant, then the universe will expand forever.
If the density of the universe is greater than the critical density, then gravity would win (Big Crunch).

However, the recent observations of distant supernova have suggested that the expansion of the universe is actually accelerating which implies the existence of a form of matter with a strong negative pressure, such as the cosmological constant.

This is the strange form of matter that is referred to as the "dark energy" which is something even stanger than dark matter...

< edited because it didnt make any sense >

8. ### RawThinkTankBannedBanned

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I am damn sure that U have never heard about HyperNova and their origins.

9. ### Starthane Xyzthreturns occasionally...Valued Senior Member

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Of course I have heard of hypernovae. They are believed to be the dying display of very massive stars, and the most likely cause of gamma-ray bursts. And, yes, I know that the bursts happen all the time - several every day on average - but they can be detected from billions of light years away. There have surely been enough giant stars in the Universe over the last few gigayears to account for them, and still make up only a tiny percentage of all the stars that ever formed.

10. ### RawThinkTankBannedBanned

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Now I am certainly sure that U r trying to avoid the origins of these massive stars like a plague ?

11. ### Starthane Xyzthreturns occasionally...Valued Senior Member

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Why would I be trying to avoid them, or deny that massive stars do form? All I'm saying is that they are the exception, rather than the norm. If you look at a list of the 26 nearest stars, you will see that all but 3 of them are less luminous than our Sun - most of them, only a tiny pecentage of its luminosity. And the brightest (Sirius A) is "only" 26 times as bright as the Sun; the kind of really powerful stars which produce hypernovae shine with tens of thousands of Sunpower. None exist within hundreds of lightyears of Earth.

12. ### RawThinkTankBannedBanned

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The fact that we see Hypernovae in far distant space is because when universe was denser there were many more of these Massive stars. And sorry your story that "Collapsing molecular clouds have a tendancy to fragment into smaller and smaller clumps as they contract, until those clumps become dense enough to consolidate into protostars" is nothing but your invention that is not what is observed. Hence I conclude that most of the matter is now dark stars as they had formed into massive stars long time back.

13. ### RawThinkTankBannedBanned

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Blobrana , Why is everthing U say so much complex, try to give a link to each of what U say for its all new to everybody.

14. ### blobranaRegistered Senior Member

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@RawThinkTank
whoops,
Yeah, a bit over the top for beginners.
But , my first post was the main point, (the follow-up post was just an outline of the <i>technical bits</i> supporting it)

The conclusion is that normal matter (be it burnt out stars or dust clouds) makes up only a tiny fraction of the universe.

15. ### apoloRegistered Senior Member

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I must extend a big THANK YOU to Blobrana for canceling the blinking feature of his signature heart. I find it exstremely annoying to try and read a post, when there is a blinking or moving object on the screen at the same time.

REGARDS APOLO

16. ### Starthane Xyzthreturns occasionally...Valued Senior Member

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Looks like he didn't cancel it for long...

@RawThinkTank: if giant molecular clouds (GMCs) didn't break up into smaller clumps as they contract, then they could not form stars as we know them: most GMCs hold tens of thousands to millions of Solar masses. If a GMC formed a single aggregate object, it would be an unimaginable hyperstar with a luminosity exceeding half the galaxy and a lifespan of literally only a few weeks. You don't see any of them...