Accelerated expansion

[Oolon Colluphid]

I was going to post this under the '...marble' thread, but I decided to start a new one.

Before I begin, I should point out that I do, in fact, believe that the universe is expanding. That doesn't mean that I fully understand it, and the information I've read prompts me to have some questions. Oftentimes, what is theory in one sentence, becomes the factual basis of the next. I don't have any specific examples of this close to hand, but it frequently crops up when dark matter/energy is cited. Anyway.....

I ran across the following at Firstscience:

"In the very dense early universe, when matter was close together, gravitational attraction was strong and expansion was slowing. Today, because of continued expansion, matter is farther apart and the density of the universe is low - so low that it has apparently dropped below the density of some unidentified dark energy now causing it to expand ever faster."

http://www.firstscience.com/SITE/ARTICLES/dark.asp

As has been stated here (that was in the '..small as a marble' thread), the initial expansion took place in an almost negligible amount of time, during which the universe ballooned to a size larger than is visible at present. Now, I'm not going to get into an "I can't see it happening ergo it didn't" argument, but, from my limited understanding, there is at least the possibility of inconsistency. So if someone could answer my questions, or aim me in the direction of more detailed information, perhaps I could get a better handle on the whole 'inflation' thing.

When did the four forces diverge?
How soon after inflation began, did matter....materialise?
These two are related since, presumably, without matter, these four forces could not exist, as is my understanding.

What evidence is there to suggest that all the matter in the universe popped into existence in such close proximity as to have a significant impact on the expansion?
Is there even any relationship between the expansion of the space that contains matter, and the apparent motion of the matter contained therein?
Is there any way to tell whether space itself is still expanding?

At what point in time did 'dark energy' make it's appearance?
Besides the apparent motion of galaxies, what evidence is there to support the theory of 'dark energy/matter'?

Is it at all possible that so-called dark energy is simply the manifestation of some kind of macro-gravity that we are only now becoming aware of? It might be that gravity is not merely additive, but is multiplicative (I don't know if that's even a word, but I hope you know what I'm getting at!). The rotation of a galaxy might be analogous to a skater pulling his/her arms closer, and thus spinning faster (gravity substituting for the arm muscles, in this case).

The crux of the matter is, if the expansion of the universe is indeed accelerating, would it be possible to perform a backward extrapolation to see if there was a time when the universe was static? And if so, could we reasonably expect that time to coincide with current estimates of the age of the universe?

I realise this is all rather jumbled, but such is my mind as it currently exists!

 

[wollery]

I'm afraid that I'm not qualified to answer most of your questions, but there's a timeline on this talkorigins page

 

[JohnF_73]

I'll do what I can to answer, but let's start by saying that I think Inflation is just an hypothesis at this point. (Someone can correct me if I'm wrong.)

The idea of inflation is appealing because if it's true, it would account for many of the so-far-unaccountable things we've noticed about our universe.

When did the four forces diverge?

They didn't all diverge at the same time. As we've discovered through particle accelerator experiments, the forces combine at high energies. When the ambient energy levels in the universe dropped below certain points, the forces diverged.
e.g Force -> Strong and Electroweak
Electroweak -> Weak and Electromagnetism (10^-11 seconds old)
Electromagnetism -> Electricity and Magnetism

As far as I know, we don't yet know at what energies these forces combine into a single force. So I can't say when they all diverged.

How soon after inflation began, did matter....materialise?

Not sure about that either. It was probably something like 10^-35 seconds after the big bang.
That's 0.000000000000000000000000000000000001 seconds.
Since at that point, the universe had cooled to around 10^28 Kelvin. (Sufficiently less than maximum allowable temperature in this universe.)

We know particles such as quarks were presumably first.

These two are related since, presumably, without matter, these four forces could not exist, as is my understanding.

We don't know enough about this yet. It's possible, so far as I know, that the forces could exist without baryonic matter.

What evidence is there to suggest that all the matter in the universe popped into existence in such close proximity as to have a significant impact on the expansion?

This is a badly formed question.
When you say 'proximity' you are implying a large space into which matter could spontaneously appear. This isn't the case. The big bang was an outword expansion of spacetime and energy. The energy filled the entire spacetime region that existed. Thus, everything was 'in proximity' to everything else.

Is there even any relationship between the expansion of the space that contains matter, and the apparent motion of the matter contained therein?

I don't know what you mean.

Is there any way to tell whether space itself is still expanding?

Yes. When two points appear to be moving further apart faster than the speed of light, it is because the space between the objects is still expanding.

At what point in time did 'dark energy' make it's appearance?

We don't know enough about it yet. (Or if it even exists for sure.)

Besides the apparent motion of galaxies, what evidence is there to support the theory of 'dark energy/matter'?

Not much, actually.

Is it at all possible that so-called dark energy is simply the manifestation of some kind of macro-gravity that we are only now becoming aware of?

Yeah, I think it's possible. Cool story called ' The Xi effect' dealt with something like this, I believe.

It might be that gravity is not merely additive, but is multiplicative (I don't know if that's even a word, but I hope you know what I'm getting at!). The rotation of a galaxy might be analogous to a skater pulling his/her arms closer, and thus spinning faster (gravity substituting for the arm muscles, in this case).

I think the laws of angular momentum do not operate in this way. Someone can correct me if I'm wrong.

if the expansion of the universe is indeed accelerating, would it be possible to perform a backward extrapolation to see if there was a time when the universe was static? And if so, could we reasonably expect that time to coincide with current estimates of the age of the universe?

That's an interesting idea. I don't immediately see any problems with it.

 

[Unnamed]

When did the four forces diverge?

If I am reading it right, 10^-12s, right after the inflationary period.
http://hyperphysics.phy-astr.gsu.edu/Hbase/astro/planck.html#c6

How soon after inflation began, did matter....materialise?

It depends on what you call matter. At this time, there were already quarks, which are matter. Protons and Electrons came soon after, then nuclei, but atoms came "much" later at 700 000 years.
http://hyperphysics.phy-astr.gsu.edu/Hbase/astro/bbcloc.html#c1

These two are related since, presumably, without matter, these four forces could not exist, as is my understanding.

I am sure a physicist could clarify this for you, but that's not completely correct. I believe both gravity and electromagnetism could exist in a universe of pure energy.

At what point in time did 'dark energy' make it's appearance?
Besides the apparent motion of galaxies, what evidence is there to support the theory of 'dark energy/matter'?

I also would like to see answers to both. Right now, since people don't know what is dark matter/energy, it would be premature to say how they appeared.

Is it at all possible that so-called dark energy is simply the manifestation of some kind of macro-gravity that we are only now becoming aware of? It might be that gravity is not merely additive, but is multiplicative (I don't know if that's even a word, but I hope you know what I'm getting at!). The rotation of a galaxy might be analogous to a skater pulling his/her arms closer, and thus spinning faster (gravity substituting for the arm muscles, in this case).

These hypotheses are testable, if you provide an equation. There are a few modified gravity theories that seem to explain some of the observations, but never all of them at the same time.

The crux of the matter is, if the expansion of the universe is indeed accelerating, would it be possible to perform a backward extrapolation to see if there was a time when the universe was static? And if so, could we reasonably expect that time to coincide with current estimates of the age of the universe?

That's an interesting point, but I don't think it is reasonable to expect a constant acceleration.

I realise this is all rather jumbled, but such is my mind as it currently exists!

I am trying to give you something to read and clarify your ideas.

My own opinion is that dark matter and energy are the flogiston^WP/ether of our times. I'm hoping that I will live to see the new branch of physics that will arise to explain it.

 

[Oolon Colluphid]

Couple of 'Phews!', here! I was beginning to think my questions were either too stupid, or the answers to them so obvious that no-one was going to bother responding. I do appreciate the answers so far, and I have some additional ones (every answer raises more questions. That's the beauty of curiosity!) and I see I need to clarify one or two points from my OP. I have things to do this afternoon, but I'll get back to it later on.
Thanks again

ETA: Nearly forgot the other 'Phew!'. I'm relieved to see that I'm not the only one with doubts about dark energy/matter.

 

[emperorchaos]

Well at least in less than a decade we should have some more answers to these questions. With the help of The James Webb Space Telescope we'll be able to see the formation of some of the earliest stars and galaxies.

This is very interesting indeed. I look forward to the science mission of this telescope as much as I did to the TPF, which has now been "deferred indefinitely" by NASA (read "cancelled"). It was, perhaps, one of those most interesting endeavours NASA ever proposed. Our only hope now is that the ESA will launch its own version of a terrestrial planet finder (Darwin).

 

[SpaceFluffer]

ETA: Nearly forgot the other 'Phew!'. I'm relieved to see that I'm not the only one with doubts about dark energy/matter.

Well, clearly neither of you are aware of the masses of evidence that point to the existence of both. I certainly wouldn't claim that both dark energy and dark matter definitely exist, but no other theory has come close to accounting for the observations.

As has been pointed out, the rotation curves of galaxies (the thousands that have been measured) all point to there being a invisible halo of mass around each galaxy with several times the mass of the visible matter. Observations of galaxy clusters also show that only 10-20% of the matter in clusters is visible, and that the unseen matter stretches out to several galactic radii from each galaxy. Modifications to gravity have been proposed to deal with this (i.e. there is no invisible matter, but gravity is different). Such theories can reproduce the galactic rotation curves but have not adequately reproduced cluster and supercluster observations.

But there's far more evidence for dark matter:

The cosmic microwave background (CMB) consists of the photons left over from 300,000 years after the Big Bang when the matter and EM radiation decoupled. The CMB is extremely isotropic (1 part in 10^5) but the small anisotropies tell us a great deal about the early universe and it's evolution since 300,000 years.

The photons have been flying around the universe since then, and how they look to us depends upon many factors. Firstly, they tell us about density peturbations at 300,000 years - what the universe was like, how 'clumpy' the matter was, etc. They tell us how the universe has expanded since then, and what the curvature of the universe is like.

The CMB has been measured to extreme accuracy by the WMAP probe in the last few years, and the data overwhelmingly points to a universe that is close to flat, and is made up of about 4% baryonic ('regular', or standard model) matter, 22% non-baryonic dark matter and 74% dark energy.

But even without the CMB measurements, the result is the same. Surveys such as the Sloan Digital Sky Survey (SDSS), the Hubble Key Project and the 2 degree Field Redshift Survey (2DFGRS) have measured the redshifts of literally hundreds of thousands of galaxies and show the same predictions. Observations of Type 1a supernova alone show that the expansion of the universe is accelerating. Calculation and measurement of Big Bang Nucleosynthesis predictions - that is, the amount of the light elements created in the first few minutes after the Big Bang - also agree that the universe is ~4% baryonic matter.

One still finds substantial agreement between datasets even when any one of the datasets is removed from the analysis. All of these huge experimental undertakings have observations that overlap and they all point to the same conclusions.

We still do not know what the dark matter and dark energy are, but the evidence for their existence is extremely compelling.

 

[Oolon Colluphid]

Unfortunately I don't have as much time tonight as I thought I would, because I have to leave for work at 3am. However:

Well, clearly neither of you are aware of the masses of evidence that point to the existence of both. I certainly wouldn't claim that both dark energy and dark matter definitely exist, but no other theory has come close to accounting for the observations.

I am aware of some, but not all the evidence. That's why I asked about that in my OP. Still, most of what I have read has a tendency to treat DM/DE as fact, rather than theory.

As has been pointed out, the rotation curves of galaxies (the thousands that have been measured) all point to there being an invisible halo of mass around each galaxy with several times the mass of the visible matter. Observations of galaxy clusters also show that only 10-20% of the matter in clusters is visible, and that the unseen matter stretches out to several galactic radii from each galaxy. Modifications to gravity have been proposed to deal with this (i.e. there is no invisible matter, but gravity is different). Such theories can reproduce the galactic rotation curves but have not adequately reproduced cluster and supercluster observations.

What about super-massive black holes that supposedly dwell at the heart of most, if not all, galaxies? As for the rotation curves, I wonder if the short length of time (cosmologically speaking) we have been able to observe galaxies in sufficient detail is anywhere near long enough to accurately calculate their angular momentum, velocity, etc.

Several times the visible matter? As in any 3 (we'll ignore time, for the moment) dimensional object, the visible makes up only a small percentage of the whole. In fact, even from the outskirts, much of our own galaxy is obscured by dust clouds. As far as gravity is concerned, until a quantum theory of gravity is established (if, indeed, one even exists), it cannot be easily dismissed as the main, if not sole, architect of any anomolous celestial motions we might observe.

And where, if at all, do the notoriously elusive 'Higg's Boson' and 'Higg's Field' fit into all of this? Assuming that they do exist. Theory suggested a mass of <95kev, IIRC, but even after 135kev, no sign yet. Perhaps once the LHC comes on-line this 'God particle' will surface. Perhaps not....

But there's far more evidence for dark matter:

The cosmic microwave background (CMB) consists of the photons left over from 300,000 years after the Big Bang when the matter and EM radiation decoupled. The CMB is extremely isotropic (1 part in 10^5) but the small anisotropies tell us a great deal about the early universe and it's evolution since 300,000 years.

The photons have been flying around the universe since then, and how they look to us depends upon many factors. Firstly, they tell us about density peturbations at 300,000 years - what the universe was like, how 'clumpy' the matter was, etc. They tell us how the universe has expanded since then, and what the curvature of the universe is like.

One thing that has always bothered me about the CMB is, where is it coming from? AFIK, it reaches Earth from all directions at once, rather than from a specific direction that might point to 'ground zero', as it were (should I say 'space zero'?). And even if the CMB is radiation left over from the BB, why has it taken so long to reach us? Should it not have gone screaming past us at 'C' billions of years ago? I realise my questions are probably incredibly naiive, but everything I read seems to assume a certain level of knowledge which I am not in possession of!

The CMB has been measured to extreme accuracy by the WMAP probe in the last few years, and the data overwhelmingly points to a universe that is close to flat, and is made up of about 4% baryonic ('regular', or standard model) matter, 22% non-baryonic dark matter and 74% dark energy.

I'm not in a position to question the data, but do tend to wonder about the conclusions that have been drawn from it. We can see so little of the universe that it seems presumptious to me for anyone to extrapolate what little is known and apply it to the entire universe.

But even without the CMB measurements, the result is the same. Surveys such as the Sloan Digital Sky Survey (SDSS), the Hubble Key Project and the 2 degree Field Redshift Survey (2DFGRS) have measured the redshifts of literally hundreds of thousands of galaxies and show the same predictions. Observations of Type 1a supernova alone show that the expansion of the universe is accelerating. Calculation and measurement of Big Bang Nucleosynthesis predictions - that is, the amount of the light elements created in the first few minutes after the Big Bang - also agree that the universe is ~4% baryonic matter.

Red shifts are another pet peeve of mine! lol How can there be a doppler shift if electromagnetic radiation travels at 'C' regardless of the relative motion of the observer?? Never mind, I think I just visualised it. It's because light can only travel at 'C' that receding objects appear red-shifted, correct? At least all these questions are forcing me to look for possible answers, rather than just scratch my head and say "I dunno...sounds kinda fishy to me..." Trouble is, without someone more knowledgable (as if that would be difficult!) than myself, I wont know if I'm anywhere near correct.

Now, about the light elements being created in the first few minutes post BB, by that time, the universe was supposedly already at least as big as the visible universe today. Was all that matter more or less concentrated in close proximity to 'space zero', or had it been dragged along by the rapid expansion? This gets back to the question I had about whether or not there was sufficient gravity to appreciably slow down the expansion, until the recently discovered acceleration kicked in.

One still finds substantial agreement between datasets even when any one of the datasets is removed from the analysis. All of these huge experimental undertakings have observations that overlap and they all point to the same conclusions.

At least until some hitherto unknown aspect of physics is discovered. And don't say it can't happen, because look at what happened to Newtonian physics! It worked great for a while, and still does for most applications, but there were things it couldn't explain. Enter Mr Einstein and company. It seems to me that we are facing a similar situation at present. The 'Standard Model' predicts the existence of the 'Higg's Boson', yet it is still to be found. From what I have read, the 'Standard Model' could be in serious jeopardy if it turns out that the 'HB' does not, in fact, exist. Does the 'SM' predict, or even have room for whatever particles 'DM' is made of?

We still do not know what the dark matter and dark energy are, but the evidence for their existence is extremely compelling.

Compelling, but not conclusive.

Thanks for the response. I hope no-one thinks I'm being deliberately obtuse. It's just the way Ed made me!

 

[wollery]

I am aware of some, but not all the evidence. That's why I asked about that in my OP. Still, most of what I have read has a tendency to treat DM/DE as fact, rather than theory.

The evidence is so overwhelming that most astronomers do treat it as fact. They shouldn't, but there you go, human nature and all that.

What about super-massive black holes that supposedly dwell at the heart of most, if not all, galaxies? As for the rotation curves, I wonder if the short length of time (cosmologically speaking) we have been able to observe galaxies in sufficient detail is anywhere near long enough to accurately calculate their angular momentum, velocity, etc.

The supermassive black holes account for the rotation curves in the inner regions, but the curves for the outer regions flatten out rather than falling off, which means that there must be huge amounts of gravitating matter in the outer regions. The velocities are measured from the variation in redshift (and thus recession velocity) across a galaxy. This does not require long term monitoring.

Several times the visible matter? As in any 3 (we'll ignore time, for the moment) dimensional object, the visible makes up only a small percentage of the whole. In fact, even from the outskirts, much of our own galaxy is obscured by dust clouds. As far as gravity is concerned, until a quantum theory of gravity is established (if, indeed, one even exists), it cannot be easily dismissed as the main, if not sole, architect of any anomolous celestial motions we might observe.

The dust is counted as visible matter. We can see all of the gas and dust. It doesn't radiate in the visible part of the spectrum, but all baryonic matter that has a thermodynamic temperature above absolute zero radiates EM waves that we can detect. There simply isn't enough of it there to account for the rotation curves.

One thing that has always bothered me about the CMB is, where is it coming from? AFIK, it reaches Earth from all directions at once, rather than from a specific direction that might point to 'ground zero', as it were (should I say 'space zero'?). And even if the CMB is radiation left over from the BB, why has it taken so long to reach us? Should it not have gone screaming past us at 'C' billions of years ago? I realise my questions are probably incredibly naiive, but everything I read seems to assume a certain level of knowledge which I am not in possession of!

The CMB was radiated from all points in the Universe at the same instant when matter and photons decoupled. This means that wherever we look in the Universe we will be seeing a photon that was emitted at that point in time.

I'm not in a position to question the data, but do tend to wonder about the conclusions that have been drawn from it. We can see so little of the universe that it seems presumptious to me for anyone to extrapolate what little is known and apply it to the entire universe.

There's nothing wrong with questioning the conclusions. A lot of what you are asking about is theoretical in nature, so it must by necessity be extrapolated from what we know. We must also assume (and observation supports this assumption) that the laws of physics are the same everywhere, and for all time. If they aren't then the theories get really screwy!

Red shifts are another pet peeve of mine! lol How can there be a doppler shift if electromagnetic radiation travels at 'C' regardless of the relative motion of the observer?? Never mind, I think I just visualised it. It's because light can only travel at 'C' that receding objects appear red-shifted, correct? At least all these questions are forcing me to look for possible answers, rather than just scratch my head and say "I dunno...sounds kinda fishy to me..." Trouble is, without someone more knowledgable (as if that would be difficult!) than myself, I wont know if I'm anywhere near correct.

The way I picture it is to assume (naively) that light is a simple transverse wave and concentrate on each successive crest of the light wave. The 1st crest is emitted at time t1, the 2nd at t2, the 3rd at t3 and so on. If an object is moving away from you at V m/s then the crest emitted at t2 will be emitted V(t2-t1) metres further than the crest emitted at t1. Since the speed of light is constant the wavelength of the light you recieve from the object is V(t2-t1) metres longer. It's redshifted.

This is a highly simplified version of what happens, but it helps to visualise the basic principle.

Now, about the light elements being created in the first few minutes post BB, by that time, the universe was supposedly already at least as big as the visible universe today. Was all that matter more or less concentrated in close proximity to 'space zero', or had it been dragged along by the rapid expansion? This gets back to the question I had about whether or not there was sufficient gravity to appreciably slow down the expansion, until the recently discovered acceleration kicked in.

The matter and energy expanded with the fabric of spacetime, because the matter and energy are part of the fabric of spacetime.

Thanks for the response. I hope no-one thinks I'm being deliberately obtuse. It's just the way Ed made me!

Question away, apart from anything thinking about how to explain this sort of stuff to people who are interested helps me to get it straight in my own head.

 

[Oolon Colluphid]

The evidence is so overwhelming that most astronomers do treat it as fact. They shouldn't, but there you go, human nature and all that.

Precisely. Just think how many people treat god as fact. Even in the face of, to be charitable, a dearth of evidence.

The supermassive black holes account for the rotation curves in the inner regions, but the curves for the outer regions flatten out rather than falling off, which means that there must be huge amounts of gravitating matter in the outer regions. The velocities are measured from the variation in redshift (and thus recession velocity) across a galaxy. This does not require long term monitoring.

I can actually see that, now. When I read your post, it began ringing a bell, that, unlike Pillory, did in fact chime. I read about the red shift quite some time ago, but somehow it got buried under endless mounds of minutiae. I'm still not quite getting how the rotation curves of the outer regions could not have caught up with the inner, given enough time, though. Clearly, there was some rotational disparity at some point in galactic evolution, at least where spirals are concerned. Otherwise they wouldn't be...spirally! Tell me, do you happen to know if there is any disparity between the inner and outer regions of Saturn's rings? There probably is, but it would be interesting if there isn't!

The dust is counted as visible matter. We can see all of the gas and dust. It doesn't radiate in the visible part of the spectrum, but all baryonic matter that has a thermodynamic temperature above absolute zero radiates EM waves that we can detect. There simply isn't enough of it there to account for the rotation curves.

Well, I'll have to take your word for that. I can't honestly say that I'm not sceptical, though. Galaxies are heterogeneous by nature. Who knows what secrets might lie at their heart. lol Occam's razor, right. Don't go inventing more stuff than is necessary to explain other stuff. lol

The CMB was radiated from all points in the Universe at the same instant when matter and photons decoupled. This means that wherever we look in the Universe we will be seeing a photon that was emitted at that point in time.

Brilliant! I can see that, too, now. I was envisioning something akin to an atomic blast which propelled us away from the centre, with the result that any residual radiation would necessarily be unidirectional from our perspective. Not so!

There's nothing wrong with questioning the conclusions. A lot of what you are asking about is theoretical in nature, so it must by necessity be extrapolated from what we know. We must also assume (and observation supports this assumption) that the laws of physics are the same everywhere, and for all time. If they aren't then the theories get really screwy!

Well, from a lay-persons point of view, the theories already seem really screwy! lol But that's clearly a function of ignorance on their (okay, my) part! My best bet would be to ditch my life of lorry driving drudgery and immediately matriculate in a house of higher learning, with the aim of learning as much physics/astrophysics as my tiny mind can absorb. But I think it's a little too late in the day for that!

The way I picture it is to assume (naively) that light is a simple transverse wave and concentrate on each successive crest of the light wave. The 1st crest is emitted at time t1, the 2nd at t2, the 3rd at t3 and so on. If an object is moving away from you at V m/s then the crest emitted at t2 will be emitted V(t2-t1) metres further than the crest emitted at t1. Since the speed of light is constant the wavelength of the light you recieve from the object is V(t2-t1) metres longer. It's redshifted.

This is a highly simplified version of what happens, but it helps to visualise the basic principle.

Visualising it is half the battle.

The matter and energy expanded with the fabric of spacetime, because the matter and energy are part of the fabric of spacetime.

I hope this isn't heresy, but whenever I hear talk of 'spacetime', a little voice in my head says 'aether'! If 'spacetime' is considered analogous to a 'fabric', why can't it be thought of as the 'aether'? Or perhaps 'dark matter' is simply a manifestation of some cosmic medium? As usual, my imagination is light years away from my understanding!

Question away, apart from anything thinking about how to explain this sort of stuff to people who are interested helps me to get it straight in my own head.

I really do appreciate the time youself and others have taken to address my concerns, for lack of a better term (maybe, ignorance!). My brain doesn't get used too much in my line of work (people in Canada will now be terrified to think that there's a truck driver out there on the highway in a state of complete zombiefication!), so I often let my mind wander and wonder about the wonders of the wandering universe. So long as I keep between the lines and don't hit anything (or equally, not let anything hit me!), the driving part is easy!

 

[wollery]

I'm still not quite getting how the rotation curves of the outer regions could not have caught up with the inner, given enough time, though. Clearly, there was some rotational disparity at some point in galactic evolution, at least where spirals are concerned. Otherwise they wouldn't be...spirally! Tell me, do you happen to know if there is any disparity between the inner and outer regions of Saturn's rings? There probably is, but it would be interesting if there isn't!

The spiral arm structures of galaxies, including our own, are due to differences in stellar density and star formation rates. They aren't simply long strings of stars. Galactic structure and dynamics is a pretty complex issue, with stars coming in at least five different kinematic classes! Saturn's rings are formed in a totally different way, but they're pretty complex. This Wikipedia page is a good place to start if you want more info.

Well, I'll have to take your word for that. I can't honestly say that I'm not sceptical, though. Galaxies are heterogeneous by nature. Who knows what secrets might lie at their heart. lol Occam's razor, right. Don't go inventing more stuff than is necessary to explain other stuff. lol

Supermassive black holes and high stellar densities are what lie at their heart!!

 

[Piggy]

Supermassive black holes and high stellar densities are what lie at their heart!!

I once dated a girl like that.

 

[Iamme]

They may have proof of the expansion theory now. (At least they think they do) See my thread, as I posted on it this morning. If you lkike...if you are REALLY interested in all this...ask Google or Yahoo then names of key terminologies or people/universities/organizations that I listed.

 

[Unnamed]

Hi, SpaceFluffer! Thanks for posting, I'm a fan.

Well, clearly neither of you are aware of the masses of evidence that point to the existence of both. I certainly wouldn't claim that both dark energy and dark matter definitely exist, but no other theory has come close to accounting for the observations.

Just to clarify, I am not denying the evidence. I unconditionally believe the evidence and I also believe that dark matter and energy are the best explanation so far. My problem is that these are labels for two things that we don't understand, and that "should not" exist, given our current understanding of physics.

tons of evidence...

One still finds substantial agreement between datasets even when any one of the datasets is removed from the analysis. All of these huge experimental undertakings have observations that overlap and they all point to the same conclusions.

We still do not know what the dark matter and dark energy are, but the evidence for their existence is extremely compelling.

I am just hoping for a more satisfactory explanation for the evidence. Maybe we will even use the terms "dark matter" and "dark energy" for historical reasons. But I will be really glad if a whole new field of physics is opened while searching for this explanation.

 

[Oolon Colluphid]

http://news.bbc.co.uk/2/hi/science/nature/4679220.stm

"Astronomers have for the first time put some real numbers on the physical characteristics of dark matter.

This strange material that dominates the Universe but which is invisible to current telescope technology is one of the great enigmas of modern science.

That it exists is one of the few things on which researchers have been certain."

Then:

"Astronomers cannot detect dark matter directly because it emits no light or radiation.

Its presence, though, can be inferred from the way galaxies rotate: their stars move so fast they would fly apart if they were not being held together by the gravitational attraction of some unseen material."

Just how 'certain' can anyone be of the presence of something whose presence can only be 'inferred'?

Then they go on to say they've measured the density and velocity of this 'enigma' whose presence can only be inferred? Hmmmm....

I'd still like to know how this DM fits into the standard model. If all this DM stuff is true, physics is in for a major overhaul, imo.

 

[Piggy]

if you are REALLY interested in all this...ask Google or Yahoo

Oh, my head hurts!

 

[wollery]

Oh, my head hurts!

Somehow the word "irony" just doesn't do it justice!!!

 

[MattusMaximus]

Just how 'certain' can anyone be of the presence of something whose presence can only be 'inferred'?

Whoa, be careful here... a similar inference of the existence of both neutrons & neutrinos was made long before any such thing was detected directly.

Neutrons were inferred to exist because of inconsistencies in the calculations of momentum & energy conservation for subatomic particle interactions early last century. They were hard as hell to detect directly because they had no electric charge, but they eventually were found.

And neutrinos, they were inferred to exist for the same reason, but they were even harder to detect since they not only have no charge but no appreciable mass either.

Here's more on this:
http://en.wikipedia.org/wiki/Neutron#Discovery
http://en.wikipedia.org/wiki/Neutrino#Neutrino_detection

My point is this, don't get too high-and-mighty with the "we-can't-trust-inference" talk. Many things in science (especially astronomy) are inferred precisely because we cannot make direct measurements.

And I seriously question whether you doubt the existence of neutrons

Cheers - Mattus

 

[0oTITANo0]

Im not a deep student of Physics, but when the question of Dark Matter comes up I have trouble with it. I think there are some errors in our descriptions of the mechanics and behavior of matter instead. When the matter that we do see has a tendency to congeal and emit radiation through the friction of interaction how can "dark matter" exert a gravitational effect on the visible matter on the one hand and not coalesce through gravitational attraction like everything else? The matter we can see tends to collect and emit radiation up until the point where the gravity field is so strong that a black hole forms (if it forms). Stars, solar systems and similar structures are the ubiquitous evidence of this. To claim that a significant amount of matter is following some but not all of the rules is hard for me to accept, especially when we cannot detect this matter directly. But the equations start working out nicely once you start solving for the missing mass, so we are definitely on to something here. But I think that "dark matter" is a description of our misunderstanding of the mechanics of matter. Unfortunately the equations that are coming out in an attempt to solve the missing mass problem without involving dark matter have major holes in them. So, the dark matter hypothesis fits best with what we know about the way matter behaves. Mixing in a bit of dark matter makes the calculations come out squeaky clean but raises a lot of questions at the other end. This leads me to conclude that what we know about how matter behaves is incomplete before it leads me to believe that matter exists that we cannot experience in any way other than the fact that existing matter is not behaving as expected.

 

[SpaceFluffer]

I'd still like to know how this DM fits into the standard model. If all this DM stuff is true, physics is in for a major overhaul, imo.

You're quite right. Non-baryonic dark matter is non-standard model stuff. Usually the term dark matter is used to mean what is technically non-baryonic dark matter. THis is the stuff that comprises about 22% of the universe's energy density and about 85% of the total matter in the universe.

Anyway, the non-baryonic dark matter must be non-standard model stuff (that's what the non-baryonic part means). We're not too worried about this for a couple reasons:

1) We have strong evidence that the standard model of particle physics is incomplete. eg. measurement of the magnetic moment of the muon, CP violation measurements, etc. show that it's very accurate but not quite right.

2) The next extension to the standard model can come from a variety of places, theoretically speaking. The current most popular theory is Supersymmetry (SUSY) which is also nice becuase it would quite naturally give a candidate for the dark matter if true.

I think using the word 'overhaul' is unfair, because theories in physics are always built with past, well-established, theories in mind. Old theories that have been used are never thrown out to make way for the new.

FOr example, if the standard model is superceded by supersymmetry, it's won't suddenly become 'wrong'. It's more that it's an approximation to a deeper theory.