So aether it is, then.

So, it was with Jesus?

Where it is was never the issue. What is it , is the problem.

So, does the Universe expand forever, or does it fall back on itself every now and then?

So, either the entire universe is filled with an invisible, basically non-interacting material that passes right through normal matter, which we can't detect except through gravity measurements which are inconsistent with how we think gravity works, but it isn't clustered tightly around the centers of mass of galaxies despite interacting gravitationally....

...or there's something wrong with our current knowledge of gravity, and that's causing the errors.

Wonder which Occam will select.

Cthulhu_is_my_homeboy:

So, either the entire universe is filled with an invisible, basically non-interacting material that passes right through normal matter, which we can't detect except through gravity measurements which are inconsistent with how we think gravity works, but it isn't clustered tightly around the centers of mass of galaxies despite interacting gravitationally....

...or there's something wrong with our current knowledge of gravity, and that's causing the errors.

Wonder which Occam will select.

Yeah, about that... When was it that it became so difficult for a scientist in an interview to say "we don't know that yet?" Every current theory is touted as "the truth, the whole truth, and nothing but the truth." It looks like scientists are trying to become some sort of priest class...

Cthulhu_is_my_homeboy: ...or there's something wrong with our current knowledge of gravity, and that's causing the errors.

Wonder which Occam will select.

Occam's razor does not apply to quantum physics.

/If it wont work on the small, it probably doesn't work on the large either.

Scientists: dark side of the Moon this, dark side of the Moon that
Reality:

Scientist: Umm.. well. I mean.. Oh look, dark matter!

Cthulhu_is_my_homeboy: So, either the entire universe is filled with an invisible, basically non-interacting material that passes right through normal matter, which we can't detect except through gravity measurements which are inconsistent with how we think gravity works, but it isn't clustered tightly around the centers of mass of galaxies despite interacting gravitationally....

...or there's something wrong with our current knowledge of gravity, and that's causing the errors.

Wonder which Occam will select.

Um, no. Dark matter *is* clumped around galaxies and is responsible for outer regions of galaxies rotating at similar speeds to the interior instead of rotation speed dropping off as one would expect if only the visible matter were there. Link (new window)

The finding in the article suggests that in addition to being clumped in galaxies it also fills the regions (at a lower density) between galaxies too.

The bullet cluster blows a pretty big hole in the idea that dark matter is merely an artifact of an incomplete understanding of gravity. Suffice it to say, two clusters of galaxies are colliding but the masses of the galaxies aren't slowed down uniformly by the collision. Stars, which interact primarily through gravitation, pass through (there's a lot of empty space inside of galaxies!) without slowing too much, but the hot interstellar gas (which makes up most of the non dark matter mass of the cluster) also interacts electromagnetically and is slowed more. Dark matter, which only interacts with gravity, would be expected to pass through relatively unaffected, the same as the stars. Gravitational lensing is used to determine the location of the mass in the cluster and most of it is *not* concentrated with the gas (which constitutes the bulk of the visible matter). Instead, the strongest lensing is seen surrounding the stars (which did not get slowed) even though very little of the total visible mass is in that volume. There must be (additional) mass of some kind for the light to be distorted (the lensing) around that volume.

/sorry if that was a confusing explanation
//I'm sure there are more eloquent descriptions
///(parenthetical)

So, our buddy Voyager should be able to possible find some of this stuff "soon".

/ Soon is kind of a relative term.

Baryogenesis: Um, no. Dark matter *is* clumped around galaxies and is responsible for outer regions of galaxies rotating at similar speeds to the interior instead of rotation speed dropping off as one would expect if only the visible matter were there. Link (new window)

You didn't understand what I was saying. To explain the galactic rotation issue, dark matter has to be distributed evenly or mainly around the edge of galaxies. But if it interacts gravitationally it should sink toward the center of the galaxy where the mass is concentrated (unless the galaxy is rotating fast enough for centrifugal force to throw it out toward the edges, which seems doubtful).

The finding in the article suggests that in addition to being clumped in galaxies it also fills the regions (at a lower density) between galaxies too.

The bullet cluster blows a pretty big hole in the idea that dark matter is merely an artifact of an incomplete understanding of gravity. Suffice it to say, two clusters of galaxies are colliding but the masses of the galaxies aren't slowed down uniformly by the collision. Stars, which interact primarily through gravitation, pass through (there's a lot of empty space inside of galaxies!) without slowing too much, but the hot interstellar gas (which makes up most of the non dark matter mass of the cluster) also interacts electromagnetically and is slowed more. Dark matter, which only interacts with gravity, would be expected to pass through relatively unaffected, the same as the stars. Gravitational lensing is used to determine the location of the mass in the cluster and most of it is *not* concentrated with the gas (which constitutes the bulk of the visible matter). Instead, the strongest lensing is seen surrounding the stars (which did not get slowed) even though very little of the total visible mass is in that volume. There must be (additional) mas ...

Or the stars are heavier than we think they are.

I still think dark matter existing and having the properties it's supposed to have is less likely than its effects being explainable by an as-yet-unknown property of spacetime itself.

Correct me if I'm wrong but as far as I can tell, there's no need to invoke nonbaryonic dark matter, but cosmologists like to do so because the astronomers are generally allergic to the idea of nonstellar, nongaseous mass - they can't see it. I haven't been able to find anything saying conclusively that the dark matter in cosmology models is necessarily absolutely nonluminous; from what I grok so far, just being cold (and therefore not emitting significant amounts of light) is good enough. Uniformly dispersed gas is also ruled out because it would have noticeable effects (Snell's Law etc) on light passing through it.

Much as his embiggenatory wordsmithing irritates me, I'm nevertheless interested in seeing Phil's official take on this. Would ice balls and chondrules be a good enough explanation? Or would we be able to see those too via a blackbody radiation signature and/or light blockage similar to what gas would do?

wiki/Dark_matter#Cold_dark_matter

Today, the "cold" argument wins scientific acceptance for explaining observable phenomena. "Cold" dark matter is dark matter composed of constituents with typical speeds much below the speed of light (generally less than 0.1c). This is currently the area of greatest interest for dark matter research, as hot and warm dark matter do not seem to be viable for galaxy and galaxy cluster formation.

The composition of the constituents of cold dark matter is currently unknown. Possibilities range from large objects like MACHOs (such as black holes[65]) or RAMBOs, to new particles like WIMPs and axions. Possibilities involving normal baryonic matter include brown dwarfs or perhaps small, dense chunks of heavy elements.

wiki/MACHOs
Massive astrophysical compact halo object, or MACHO, is a general name for any kind of astronomical body that might explain the apparent presence of dark matter in galaxy halos. A MACHO is a body composed of normal baryonic matter, which emits little or no radiation and drifts through interstellar space unassociated with any planetary system. Since MACHOs would not emit any light of their own, they would be very hard to detect.

I wish more astrophysics articles would get the NEWSFLASH tab.

all this for a WIMP....

Cthulhu_is_my_homeboy: Or the stars are heavier than we think they are.

They'd have to be a fark ton heavier. And if they were, there'd be a lot more supernovas and nebulae around. Also we'd have a lot more red stars in the sky. The mass of a star is directly related to how long it lasts, and how it ends. Though it sounds paradoxical, a star with 20 times the mass of our Sun will last a much much shorter span of time. The extra mass means the gravity pulling the star in is greater, this means fusion happens faster, which means it turns its hydrogen into helium faster, and gets onto producing heavy elements faster. And eventually it runs out of that stuff. And if the star is sufficiently massive, when it runs out of stuff to fuse, it goes boom. The reason the Sun will last so long is because as far as stars go, it's not particularly big.

Treefingers: I wish more astrophysics articles would get the NEWSFLASH tab.

If Betelgeuse goes supernova, it'll get a newsflash tag. Well, I mean if we see the supernova while Fark is around it'll get it. That could be tomorrow or 500 years from now. I would love it if it's tomorrow.

pup.socket: So, does the Universe expand forever, or does it fall back on itself every now and then?

If you got an hour to kill, here's a presentation by Lawrence Krauss^ where he explains the whole thing in detail, along with a lot of other stuff.

If you don't have an hour to kill: We apparently live in an open, flat universe. This means the universe will continue to expand - slower and slower, but never quite stopping.
=Smidge=

Smidge204: - slower and slower,

Actually, it's happening faster and faster (new window)

Dear armchair astrophysicists:

Dark matter most likely exists. While there are some competing theories, they are generally discredited by the community at large. Also, if you've though about the obvious solution, chances are so has a researcher. Unlike you, that researcher actually compared the theory to data and realized how horribly wrong it is, however.

My girlfriend took it when she moved out.

Cthulhu_is_my_homeboy: You didn't understand what I was saying. To explain the galactic rotation issue, dark matter has to be distributed evenly or mainly around the edge of galaxies. But if it interacts gravitationally it should sink toward the center of the galaxy where the mass is concentrated (unless the galaxy is rotating fast enough for centrifugal force to throw it out toward the edges, which seems doubtful).

Uh, it's *orbiting* the galactic center along with the stars.

Cthulhu_is_my_homeboy: Or the stars are heavier than we think they are.

I still think dark matter existing and having the properties it's supposed to have is less likely than its effects being explainable by an as-yet-unknown property of spacetime itself.

Now you're proposing amendments to gravity and our understanding of stellar bodies while providing nothing in the way of better explanatory power. That's not the direction to go.

WhyteRaven74: Smidge204: - slower and slower,

Actually, it's happening faster and faster (new window)

You are right - I have misremembered the content of that video and I apologize.

=Smidge=

Dark matter is the lube between things.

Dark Matter is the invisible sky blizzard astrophysicists use to keep Xeno out of their brainwaves.

Much more likely is that the gravitational lensing oddities and expansion of the universe are driven by virtual particles. (new window).

PFC Obvious: Dear armchair astrophysicists:

Dark matter most likely exists. While there are some competing theories, they are generally discredited by the community at large. Also, if you've though about the obvious solution, chances are so has a researcher. Unlike you, that researcher actually compared the theory to data and realized how horribly wrong it is, however.


This is FARK, where everyone is the world's greatest expert in everything.

GeneralJim: Cthulhu_is_my_homeboy: So, either the entire universe is filled with an invisible, basically non-interacting material that passes right through normal matter, which we can't detect except through gravity measurements which are inconsistent with how we think gravity works, but it isn't clustered tightly around the centers of mass of galaxies despite interacting gravitationally....

...or there's something wrong with our current knowledge of gravity, and that's causing the errors.

Wonder which Occam will select.
Yeah, about that... When was it that it became so difficult for a scientist in an interview to say "we don't know that yet?" Every current theory is touted as "the truth, the whole truth, and nothing but the truth." It looks like scientists are trying to become some sort of priest class...

The problem is usually the interviewer (or even the editor) rather than the scientist. "We really don't know yet" doesn't make nearly as good of a story as "OMG THIS CHANGES EVERYTHING!"

Feepit: Much more likely is that the gravitational lensing oddities and expansion of the universe are driven by virtual particles.

The problem with virtual particles is, well, they're virtual. I mean sure you can describe them, do all sorts of fancy calculations involving them, but they don't generally hang around long enough to do anything except interact with other virtual particles . Plus there's the whole wave particle duality thing to deal with. And such weirdness as some virtual particles having negative kinetic energy. One of the things with virtual particles, at least those with some decent amount of mass is that when they go poof, there's often a nice little release of gamma radiation. This is particularly the case when you get interactions between virtual particles and anti-particles. In order to have enough mass to cause gravitational lensing, you'd have so many virtual particles that space would be awash in radiation coming from space itself. As it, the radiation in space, save for the cosmic background radiation, comes from specific sources, like black holes, pulsars and other stuff.

FTA: It is well known that there is a large amount of unseen matter called "dark matter" in the universe. It constitutes about 22 percent of the present-day universe while ordinary matter constitutes only 4.5 percent.

What constitutes the other 74.5% then??

WhyteRaven74:

The problem with virtual particles is, well, they're virtual. I mean sure you can describe them, do all sorts of fancy calculations involving them, but they don't generally hang around long enough to do anything except interact with other virtual particles . Plus there's the whole wave particle duality thing to deal with. And such weirdness as some virtual particles having negative kinetic energy. One of the things with virtual particles, at least those with some decent amount of mass is that when they go poof, there's often a nice little release of gamma radiation. This is particularly the case when you get interactions between virtual particles and anti-particles. In order to have enough mass to cause gravitational lensing, you'd have so many virtual particles that space would be awash in radiation coming from space itself. As it, the radiation in space, save for the cosmic background radiation, comes from specific sources, like black holes, pulsars and other stuff.

Thank you for your reply, I just wish it didn't make my brain hurt trying to wrap my head around the concepts. Honestly, I'd like to be a cosmologist if I wasn't so inept at math.

hmm...you mean to say that there is a substantial amount of matter
that we can't detect due to our limited range of observation
and limits of our technology???

SAY IT AIN'T SO!!

Get off your high horse, stop your presumptions...and get real.

You would think that your scientist is a priest who's saying the world ends at end of line of sight...
Or that you since you can't see it...it doesn't exist...as with germs
Or since you can't find it with all your tools and fellows, it simply cannot be factored in...

Like we can't find a criminal in a man-hunt of a city...so they must be dead.
We're talking about a world where we JUST got to the edge of our solar system with a tiny object years in the taking.

Suurree...we can note EVERYTHING from just the equivalent of standing on the edge of a ledge looking through a toilet roll.

It's like when all the top physics students were going through EVERY problem in the book,
and couldn't find the same solution for one in the book...so the book MUST be wrong.
(No, it took me to look away from the girl I was flirting with...glance down at it, and tell them to try the Y coordinate instead of X, to get the same answer)

Just because you've got a nice pretty piece of paper saying PhD and your buddies saying you're a good guy,
doesn't mean you know of EVERYTHING...
or can even consider it

Get real.
Do science.
Consider it, gather data, test it, analyze it, ponder it, make a guess...repeat, repeat, repeat.

And face it, you might die without THE answer.
But if you're lucky, you may be able to significantly add to the puzzle's solution.

/such is the burden we scientists and techs bear.
//and go read the Hitchhiker's guide again...until you "get it"

ipsofacto: FTA: It is well known that there is a large amount of unseen matter called "dark matter" in the universe. It constitutes about 22 percent of the present-day universe while ordinary matter constitutes only 4.5 percent.

What constitutes the other 74.5% then??

Energy and Dark Energy (new window).

GeneralJim: Yeah, about that... When was it that it became so difficult for a scientist in an interview to say "we don't know that yet?" Every current theory is touted as "the truth, the whole truth, and nothing but the truth." It looks like scientists are trying to become some sort of priest class...

You don't actually know the first thing about science, do you? I mean, aside from what your parish priest tells you after zipping up.

SnarfVader: ipsofacto: FTA: It is well known that there is a large amount of unseen matter called "dark matter" in the universe. It constitutes about 22 percent of the present-day universe while ordinary matter constitutes only 4.5 percent.

What constitutes the other 74.5% then??

Energy and Dark Energy (new window).

Okay, but what does '% of the Universe' relate to, total energy?

ipsofacto: SnarfVader: ipsofacto: FTA: It is well known that there is a large amount of unseen matter called "dark matter" in the universe. It constitutes about 22 percent of the present-day universe while ordinary matter constitutes only 4.5 percent.

What constitutes the other 74.5% then??

Energy and Dark Energy (new window).

Okay, but what does '% of the Universe' relate to, total energy?

My understanding of it is the % represents everything that exists in the universe. Since matter is a form of energy, yes, you could say it is total energy.

PFC Obvious: Dear armchair astrophysicists:

Dark matter most likely exists. While there are some competing theories, they are generally discredited by the community at large. Also, if you've though about the obvious solution, chances are so has a researcher. Unlike you, that researcher actually compared the theory to data and realized how horribly wrong it is, however.

Of course dark matter exists, but AFAIK it's an open question as to whether it's nonbaryonic. What are the observations, and assumptions behind the calculations, that are supposed to say dark energy and/or nonbaryonic matter must exist? And considering that TFA is presented as "new" knowledge that more nonstellar potentially baryonic matter exists in galactic halos than was previously thought, how does that knowledge change things?

FTFA:
More interestingly, the estimated total amount of dark matter in the outskirts of the galaxies explains the gap between the global cosmic mass density and that derived from galaxy number counting weighted by their masses.

Cthulhu_is_my_homeboy: So, either the entire universe is filled with an invisible, basically non-interacting material that passes right through normal matter, which we can't detect except through gravity measurements which are inconsistent with how we think gravity works, but it isn't clustered tightly around the centers of mass of galaxies despite interacting gravitationally....

...or there's something wrong with our current knowledge of gravity, and that's causing the errors.

Wonder which Occam will select.

Or "dark matter" is just non-radiant matter, which is to say, dust, rock, ice, and general space junk, which astronomy calculations tend to "round out" for the sake of simplicity. Or "dark matter" is just the accumulated gravitational impact of all the trillions of light years of sub-atomic particles roiling about in and out of existence between star systems. Or, if Higgs was right and mass is the drag resulting from matter passing through reality, maybe "dark matter" is the accumulated gravitational effect of the fabric of space itself.

GeneralJim: Cthulhu_is_my_homeboy: So, either the entire universe is filled with an invisible, basically non-interacting material that passes right through normal matter, which we can't detect except through gravity measurements which are inconsistent with how we think gravity works, but it isn't clustered tightly around the centers of mass of galaxies despite interacting gravitationally....

...or there's something wrong with our current knowledge of gravity, and that's causing the errors.

Wonder which Occam will select.
Yeah, about that... When was it that it became so difficult for a scientist in an interview to say "we don't know that yet?" Every current theory is touted as "the truth, the whole truth, and nothing but the truth." It looks like scientists are trying to become some sort of priest class...

About the same time universities and publishers started trying to sell scientists as celebrities. PR folks tend to think us mere mortals are too moronic to be comfortable with uncertainty.

The stars are matter, we're matter, but it doesn't matter.

FTA:

Einstein's general theory of relativity predicts that a light ray passing near a massive object such as a galaxy is bent by the effect called "gravitational lensing". For example, the effect causes the image of a distant galaxy to be deformed and brightened by an intervening galaxy. However the effect itself is very small and so cannot be easily detected for a single galaxy.

I must be missing something. Galaxy's are big. They contain billions of stars. That's a lot of mass. Add to this the fact that it can take light thousands of years to pass a galaxy. Our galaxy is about 100,000 light years in diameter. Not sure why it is difficult to detect a lensing effect given the scale.

Baryogenesis: Now you're proposing amendments to gravity and our understanding of stellar bodies while providing nothing in the way of better explanatory power. That's not the direction to go.

Historically, pointing out that we've beaten a theory into the ground and it just doesn't work has been the direction to go.

Turning Lead into Gold was not just a mad whim of alchemists, it made perfect sense within the four element theory. That is dismissed these days as nonsense, but it was a very sophisticated theory that could explain most of the known properties of matter.

It was also wrong.

Pointing out that it was wrong would be the right thing to do, even if you didn't have a full periodic table to replace it with.

Also, you say that Cthulhu_.... is proposing amendments to gravity, but I can't see how current gravitational theory would predict that dark matter would form halos around galaxies, while being undetectable anywhere else, yet that seems to be what it does. Doesn't that already imply amendments to gravity?

BarryJV: Baryogenesis: Now you're proposing amendments to gravity and our understanding of stellar bodies while providing nothing in the way of better explanatory power. That's not the direction to go.

Historically, pointing out that we've beaten a theory into the ground and it just doesn't work has been the direction to go.

Turning Lead into Gold was not just a mad whim of alchemists, it made perfect sense within the four element theory. That is dismissed these days as nonsense, but it was a very sophisticated theory that could explain most of the known properties of matter.

It was also wrong.

Pointing out that it was wrong would be the right thing to do, even if you didn't have a full periodic table to replace it with.

Also, you say that Cthulhu_.... is proposing amendments to gravity, but I can't see how current gravitational theory would predict that dark matter would form halos around galaxies, while being undetectable anywhere else, yet that seems to be what it does. Doesn't that already imply amendments to gravity?

To be fair, alchemy was based on jack shiat.

insertsnarkyusername: To be fair, alchemy was based on jack shiat.

Funny thing about alchemy? That if you stick with the definition of it, the transmutation of common elements to less common elements, we do it all the time now. Various radioisotopes are produced by neutron bombardment of comment elements.

I lurked for two years before I signed up!:

Thank you for your reply, I just wish it didn't make my brain hurt trying to wrap my head around the concepts. Honestly, I'd like to be a cosmologist if I wasn't so inept at math.

No math required! That's what cash registers are for.


Become a cosmetologist!

Broom:

GeneralJim: Yeah, about that... When was it that it became so difficult for a scientist in an interview to say "we don't know that yet?" Every current theory is touted as "the truth, the whole truth, and nothing but the truth." It looks like scientists are trying to become some sort of priest class...

You don't actually know the first thing about science, do you? I mean, aside from what your parish priest tells you after zipping up.

Behold this mighty tree -- the North American Peckerwood.

So, what's the butthurt, Buttercup? Lessee... Oh, educated in Catholic School. So, we're talking LITERAL butthurt here. Nudge, nudge, wink, wink. Say no more, say no more, Say. No. More.

Heron:

GeneralJim: Yeah, about that... When was it that it became so difficult for a scientist in an interview to say "we don't know that yet?" Every current theory is touted as "the truth, the whole truth, and nothing but the truth." It looks like scientists are trying to become some sort of priest class...

About the same time universities and publishers started trying to sell scientists as celebrities. PR folks tend to think us mere mortals are too moronic to be comfortable with uncertainty.

Well, on average, they're probably right. Your idea is more than a little depressing, but it does have that "ring of truth" thing going for it. *SIGH*

BarryJV: Also, you say that Cthulhu_.... is proposing amendments to gravity, but I can't see how current gravitational theory would predict that dark matter would form halos around galaxies, while being undetectable anywhere else, yet that seems to be what it does. Doesn't that already imply amendments to gravity?

It clumps around (visible) matter because it's attracted by gravity, no modifications necessary. It's only (currently) detectable on large scales because gravity is a weak force and dark matter doesn't interact electromagnetically.

Daraymann: FTA:

Einstein's general theory of relativity predicts that a light ray passing near a massive object such as a galaxy is bent by the effect called "gravitational lensing". For example, the effect causes the image of a distant galaxy to be deformed and brightened by an intervening galaxy. However the effect itself is very small and so cannot be easily detected for a single galaxy.

I must be missing something. Galaxy's are big. They contain billions of stars. That's a lot of mass. Add to this the fact that it can take light thousands of years to pass a galaxy. Our galaxy is about 100,000 light years in diameter. Not sure why it is difficult to detect a lensing effect given the scale.

The strength of gravity drops with the square of the distance of the light ray to the center of mass of the galaxy. 50,000 light years squared is a large number.

Smidge204:
If you don't have an hour to kill: We apparently live in an open, flat universe. This means the universe will continue to expand - slower and slower, but never quite stopping.
=Smidge=

Thanks, although my remark was just a stupid snark, I'll bookmark this for when I have an hour.