30-04-2003 21:05									#1
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quote: Brits seek missing WIMPs of universe LONDON (Reuters) --British scientists equipped with state-of-the-art detectors deep underground in northern England have begun a search for one of the most tantalizing secrets of the universe -- Dark Matter. "If we are successful in our quest then we are looking at a place in the history books," Neil Spooner of Sheffield University said Tuesday. "This will be one of the great discoveries of our time." Scientists around the world are racing to be the first to discover the truth about Dark Matter, which cannot be seen because it does not emit light. They believe it makes up the vast majority of the universe. Scientists say stars account for less than 1 percent of the mass of the universe, with gas clouds and other objects accounting for close to another 5 percent. No one is quite sure what makes up the missing remainder, which has been dubbed Dark Matter. In a bid to identify the prime suspect known as Weakly Interacting Massive Particles or WIMPs, British scientists have installed detectors 3,600 feet down a salt mine at Boulby on the North Yorkshire moors. They are buried deep underground in an area of low natural radioactivity where intervening rock should shield them from interference and filter out cosmic bombardment. "This is an outstanding research facility equipped with some of the world's most sensitive Dark Matter detectors," Ian Halliday, chief executive of the Particle Physics and Astronomy Research Council, said in a statement. "It is a crucial addition to the UK's resources in a research field where British scientists are playing a world-leading role -- the race by physicists around the globe to discover these exotic, as yet undetected, Dark Matter particles," he added. The theory is that although billions of sub-atomic particles called WIMPs are passing through the atmosphere and the Earth every second they only rarely encounter the nucleus of an atom, making it shudder slightly. The detectors are designed to be able to detect these tiny collisions that are so rare that scientists calculate that in a 2.2-pound block of material, less than one WIMP a day will strike the nucleus of an atom and make it move. -------------------------------------------------------------------------------- Copyright 2003 Reuters. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed. Find this article at: http://www.cnn.com/2003/TECH/space/...reut/index.html

Interesting. Thoughts? I have a question for smarter 'Poly members. If the universe is infinite, how can scientists say that stars are 1% of the universe? Isn't that an infinite number as well? Or are they talking about proportions within our detectable range?

	30-04-2003 21:12									#2
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Is the universe infinite ? I thought it was expanding, hence finite

	30-04-2003 21:17									#3
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If it constantly expands, how is it finite?

	30-04-2003 21:17									#4
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I am sorry, but I am a strong believer in MACHOs. I haven't seen substancial proof for WIMPs.

	30-04-2003 21:17									#5
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I've always been taught it is infinite.

	30-04-2003 21:20									#6
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It may be infinite, but the causal universe, the parts of the universe which we can have any cause and effect from and so for all intents and purposes is the size of the universe, is 14 billion light years.

	30-04-2003 21:21									#7
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It is, even by Spiffor's definition.

	30-04-2003 21:24									#8
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erm, if it would be infinite, I don't think that they would be looking for WIMPs or MACHOs in the first place. both of these were hypotheses drawn to explain gravitational forces that were much stronger than the visible bodies would suggest.


Hmmm. Dark Matter.

	30-04-2003 21:25									#9
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I suggest you all read up on Olber's paradox.

	30-04-2003 21:26									#10
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Okay, once we find out that "WIMP's" exist; what then?

	30-04-2003 21:29									#11
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Olber's Paradox is, at best, a theory.

	30-04-2003 21:32									#12
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quote: Originally posted by Sava If the universe is infinite, how can scientists say that stars are 1% of the universe?

You can look at gravitational effects in the bit of the universe we can see. Since we think we understand gravity (at large distances) pretty well, this predicts that there is a lot more 'stuff' in the universe. In other words, gravity looks to strong - this would be explained by there being lots of non-luminous matter.

quote: Originally posted by Spiffor Is the universe infinite ? I thought it was expanding, hence finite

Something that is infinite can still expand. Imagine having an infinite rubber sheet with a chequer pattern drawn on it. If the sheet 'expands' the squares will get bigger.

	30-04-2003 21:34									#13
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But, RJ, The amount of matter is finite, right? ( That's what I meant what I said 'universe' in my previous post. )

	30-04-2003 21:41									#14
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If the universe started with the Big Bang a finite amount of time ago, how could it be infinite in size, unless it at least expanded for part of the time at an infinite speed?

	30-04-2003 21:42									#15
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quote: Originally posted by Azazel But, RJ, The amount of matter is finite, right? ( That's what I meant what I said 'universe' in my previous post. )

You will start to get into metaphysical difficulties here. As BC said, the amount of matter that we can see is finite (14 billion years worth) so in a way the universe if finite (if you can't see it it isn't there!). But presumably someone on Alpha Centauri would see beyond our horizon (in certain directions), therefore seeing more 'stuff', so in that sense, it is infinite.

Also note that the universe being infinite and the amount of matter being infinite are not the same thing.

	30-04-2003 21:44									#16
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quote: Originally posted by Urban Ranger If the universe started with the Big Bang a finite amount of time ago, how could it be infinite in size, unless it at least expanded for part of the time at an infinite speed?

I tend to think that the "Big Bang" was a local event and doesn't represent all matter in the universe.

	30-04-2003 21:44									#17
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quote: Originally posted by Urban Ranger If the universe started with the Big Bang a finite amount of time ago, how could it be infinite in size, unless it at least expanded for part of the time at an infinite speed?

It was infinite at the start.

(This is all supposition of course - the universe could be spherical (and therefore not infinite) for all we know.: travel in one direction long enough and come back to where you started.)

	30-04-2003 21:51									#18
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As I understand it the current theory is that the universe will expand forever; that is the force of gravity will never be able to over come the outward momentum of the big bang and pull the universe back into a singularity.

If dark matter (our so called anti-matter) does exist and if it does have an equal gravitational pull, as Einstein predicts it would, then does that mean the universe will began contracting at some future time? If the universe begans to contract then that would pull anti-matter into contact with regular matter. What happens then?

	30-04-2003 21:54									#19
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actually, dark matter is not likely entirely antimatter

Jon Miller

	30-04-2003 21:55									#20
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quote: travel in one direction long enough and come back to where you started.)

I've always failed to understand this. ( obviously, it's understandable on a surface of a ball, like earth, but in space? )

	30-04-2003 21:58									#21
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quote: Originally posted by Rogan Josh You will start to get into metaphysical difficulties here. As BC said, the amount of matter that we can see is finite (14 billion years worth) so in a way the universe if finite (if you can't see it it isn't there!). But presumably someone on Alpha Centauri would see beyond our horizon (in certain directions), therefore seeing more 'stuff', so in that sense, it is infinite. Also note that the universe being infinite and the amount of matter being infinite are not the same thing.

Well, not to quibble, but since Alpah Centauri is so close to us, I doubt someone there would see anything more than we would when it came to the far reaches of the known universe.

If the Big Bang is indeed the source of all matter in the universe, then it's reasonable to believe what we see is the limit of the universe.

	30-04-2003 22:02									#22
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Oerdin: No-one really knows (whether there is enough matter to close the universe) but it looks like there isn't. The current evidence points to the universe expanding forever. Recent data even seems to show that the rate of expansion is accelerating.

JM: Anti-matter is a much more intereting question imho. If everything were created at the Big Bang, there should be as much matter as anti-matter (since particles would be produced in particle/aniparticle pairs). But everything we see around us is matter - not anti-matter. So where is all the anti-matter.

	30-04-2003 22:29									#23
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quote: Originally posted by Rogan Josh Oerdin: No-one really knows (whether there is enough matter to close the universe) but it looks like there isn't. The current evidence points to the universe expanding forever. Recent data even seems to show that the rate of expansion is accelerating.

Which if I understand the postulated theory correctly results in the causal universe shrinking. Distant supernovae are supposedly fading away due to red shift, indeed everything at the edge of the visible universe is 'fading away'. If this expansion of space accelerates then the fading process becomes more pronounced and nearer objects fade into nothingness too. Eventually the visible/causal/for all intents and purposes actual universe will be forever shrinking until everthing is 'frozen' and never interacts with anything.

	30-04-2003 22:36									#24
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"Which if I understand it correctly results in the causal universe shrinking. Distant supernovae are fading away due to red shift indeed everything at the edge of the visible universe is 'fading away'. If the expansion of space accelerates then the fading process becomes more pronounced and nearer objects fade into nothingness. Eventually the visible/causal/for all intents and purposes actual universe will be forever shrinking until everthing is 'frozen' and never interacts with anything."



Trying to grasp...

So you're saying that the universe, once all these far away objects become too far away to see, will have shrunk, to our perception?

However not in fact, right? Those objects are still out there, just at too great a distance to percieve...yes?

	30-04-2003 22:37									#25
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quote: Originally posted by Boris Godunov Well, not to quibble, but since Alpah Centauri is so close to us, I doubt someone there would see anything more than we would when it came to the far reaches of the known universe. If the Big Bang is indeed the source of all matter in the universe, then it's reasonable to believe what we see is the limit of the universe.

The cosmological principle (CP) would disagree with that.

We are uniformly surrounded by galaxies, stars and planets etc. A person on a planet 14 billion light years away, on th edge of our visible space should have the same type of view based on the CP. If he looks around and he only sees what we see, then he will only see stars and galaxies in one direction - in the direction of us. This contradicts the CP and is thus not a palatable scenario.

	30-04-2003 22:38									#26
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quote: Originally posted by Lancer So you're saying that the universe, once all these far away objects become too far away to see, will have shrunk, to our perception? However not in fact, right? Those objects are still out there, just at too great a distance to percieve...yes?

That's the theory as I understand it.

	30-04-2003 22:41									#27
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quote: Originally posted by Rogan Josh JM: Anti-matter is a much more intereting question imho. If everything were created at the Big Bang, there should be as much matter as anti-matter (since particles would be produced in particle/aniparticle pairs). But everything we see around us is matter - not anti-matter. So where is all the anti-matter.

RJ: what determines the path of matter and antimatter particles when they appear from energy? is it purely random?

	30-04-2003 23:43									#28
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quote: Originally posted by Big Crunch The cosmological principle (CP) would disagree with that. We are uniformly surrounded by galaxies, stars and planets etc. A person on a planet 14 billion light years away, on th edge of our visible space should have the same type of view based on the CP. If he looks around and he only sees what we see, then he will only see stars and galaxies in one direction - in the direction of us. This contradicts the CP and is thus not a palatable scenario.

That's not true either. If the universe had expanded at the speed of light after the Big Bang then there would be no stars 14 billion light years away because we would see the Big Bang singularity itself 14 billion light years away (in all directions). Then, what you just said would be true - everyone, everywhere would have the same horizon, bounded by the Big bang singularity.

As it happens though, cosmologists think that the universe was expanding considerably faster than the speed of light for some time (called inflation), so parts of the universe which were in causal contact, then went out of causal contact, then back in again (if you see what I mean. So our horizon isn't the Big Bang, but is a point in space X Billion light years away (I have no idea of the actual number). So someone on a different star will have a different horizon.

	30-04-2003 23:51									#29
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faster than light?

	30-04-2003 23:57									#30
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quote: Originally posted by Azazel RJ: what determines the path of matter and antimatter particles when they appear from energy? is it purely random?

Well, what they tell you on these popular science programs on TV isn't true. Partilce, anti-particle pairs do not just spontaneously appear out of the a vacuum. There has to be something there for them to come. You kind of implicitly know this since you said 'from energy' - energy has to be in some form, eg. as a photon.

So to answer your question, it depends on what the object which gives rise to the particle anti-particle pair is, and more specifically, if it has any angular momentum.

If it doesn't (eg. a Higgs boson) then there is nothing to define any direction as 'special' so the direction the particle anti-particle will go in will be random (in the rest from of the emitting particle). Obviously the particle and its anti-paerticle will go in opposite directions (momentum conservation).

If it does (eg. a photon) then there is a special direction defined by the axis of its spin, so they might preferentially go either in that direction or perpendicular to it (depends on the particle). There will still be a rotation symmetry around the axis of spin though.