0502: "Dark Flow"

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phlip
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Re: "Dark Flow" Discussion

faunablues wrote:If [...] the smaller objects are moving a little faster

They're not.
faunablues wrote:(if we see the big bang as an explosion, [...]

It wasn't.

Hubble's law shows that the velocity at which distant objects are moving (almost) directly away from us is (roughly) proportional to the distance between us and them. Regardless of their mass. This fact, and some geometry, shows that every distant object is moving directly away from any reference point you'd care to name.

I'm not familiar with the details of the paper mentioned in the comic, but I guess it means that the "almost" and "roughly" in the previous paragraph are out by an amount such that they indicate a gravitational attraction to an object even more distant, such that we can't see it. Or something along those lines. If someone has a more specific link (or even the name of the paper... "this A.S.T. paper" isn't enough to get useful results from Google) I'd be a happy man.

Acutally, now that I think about it, does the idea of an object outside the visible universe affecting an object within the visible universe even make sense? Or does the term "visible universe" in this context mean something different to the "theoretically-observable universe"?

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Smok3y
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Re: "Dark Flow" Discussion

Trying to comprehend the size of "supermassive" objects gives me a headache.

I gotta go lie down.

sje46
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Re: "Dark Flow" Discussion

Aspergia wrote:OK, today's xkcd wins on so many levels it finally got me out of lurk mode. I love it, very very much!

(Um, hello and all that, etc etc)

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mache-b123
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Re: "Dark Flow" Discussion

Don't know... it's the worst "ya mom" joke I've ever seen (actually, I aint seen so many "ya mom" jokes, we in Latam prefer "you're gay!" joking as everything could have a homosexual meaning); and that cute side for Beret Guy is so complex... I didn't like it, gotta say.

Maybe I don't have to read xkcd while tryin' to pull up a damn virus let there by a little sister whose Internet got bounds at Windows Live Messenger, Facebook, Hi5 and some Wikipedia (just school stuff), I think.

Catdrake
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Re: "Dark Flow" Discussion

Perhaps that all does explain the mass of those objects...
Nothing to see here, move along.

JohnDoe
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Re: "Dark Flow" Discussion

This is one of the few rare xkcd comics just made me laugh out loud.

gpvos
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Re: "Dark Flow" Discussion

What does A.S.T. stand for?

troyp
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Re: "Dark Flow" Discussion

gpvos wrote:What does A.S.T. stand for?

I don't know, but I think he's referring to this.

armandtanzarian
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Re: "Dark Flow" Discussion

Really sweet comic. Makes me wanna call up my mom too.

Aspergia
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Re: "Dark Flow" Discussion

sje46 wrote:
Aspergia wrote:OK, today's xkcd wins on so many levels it finally got me out of lurk mode. I love it, very very much!

(Um, hello and all that, etc etc)

Thank you.

I called my Mum tonight, too. I'm still in a state of goodwill to my fellow creatures after this comic - even now that I've had an hour-long update on Mum's sweet potato vine and her arthritis. Gotta love mothers.
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Purky
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Re: "Dark Flow" Discussion

Must stop reading XKCD at work, people look at me funny when I'm sat alone in my office laughing to myself...
Todays comic is made of win.

ikefalcon
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Re: "Dark Flow" Discussion

Protagonist wrote:I'm not sure what you're saying. We aren't moving away from all the galaxies, we're watching galaxy clusters moving toward a region of space for no apparent reason.

Edit: Are you meaning that everything is staying in the relatively same position except for the Milky Way? If that's the case, you're wrong. Everything's moving away from the center of the universe.

There is no "Center of the Universe." From our frame of reference, the Earth may as well be the center of the Universe since we can only see about 15 billion light years in every direction. Light from beyond that approximate distance hasn't gotten here yet. Perhaps you were referring to the point where the Big Bang occurred, but for all intents and purposes the Big Bang happened at every point in the Universe because the Universe was a singularity when the Big Bang occurred.

Hubble's law shows that all distant objects are moving away from us and that objects that are further away are travelling faster away from us. Assuming that this continues forever, the distance between the Earth (assuming the Earth never stops existing) and all distant objects will approach infinity. My question is: If you are an infinite distance from all objects, will they not all appear to be at the same point? To give an example, we make this sort of assumption if you are sufficiently far from an electric or magnetic dipole in order to simplify calculations.

This paper apparently shows that all objects are drifting toward a particular point in space, but it would seem to me that this would happen anyway if we are moving away from all objects.

phlip wrote:Acutally, now that I think about it, does the idea of an object outside the visible universe affecting an object within the visible universe even make sense? Or does the term "visible universe" in this context mean something different to the "theoretically-observable universe"?

This is a great point that I hadn't initially considered. That would depend on whether or not gravity acts instantaneously over a distance. I want to say that it does, but I honestly don't know the answer.

phlip
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Re: "Dark Flow" Discussion

ikefalcon wrote:My question is: If you are an infinite distance from all objects, will they not all appear to be at the same point? To give an example, we make this sort of assumption if you are sufficiently far from an electric or magnetic dipole in order to simplify calculations.

No, because you can have one object an infinite distance in one direction, and another an infinite distance in another direction.

Like, in 2D for simplicity, if we're at (0,0) and stationary (in our reference frame), an object is at (1,0) and moving right at constant velocity, and another object is at (0,1) and moving up at constant velocity (and the same speed as the first object), then over time the distance of those two objects from will grow without bound, but they won't seem to "approach" each other... one will always be to the right, and one will always be up.

And from the point of view of, say, the object on the right, we're at (-1,0) moving left, and the other object is at (-1,1), moving up and left at [imath]\sqrt 2[/imath] times the speed. That is, everything's moving directly away from it, at speeds proportional to their distance. Thus still satisfying Hubble's law from that perspective too.

ikefalcon wrote:This is a great point that I hadn't initially considered. That would depend on whether or not gravity acts instantaneously over a distance. I want to say that it does, but I honestly don't know the answer.

Gravity propagates at the speed of light, or close to it (if I recall correctly, it's exactly c in theory, and experiments put it somewhere between 0.8c and c, or something like that).

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keithsdragons
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Re: "Dark Flow" Discussion

Where did his legs go in the second panel?!

stelei
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Re: "Dark Flow" Discussion

Aw. This is the kind of comic that made me fall in love with xkcd in the first place, combining cutting-edge (tee hee, pun) science with naive human feelings.

Kind of related to the comic, I vaguely remember my dream last night involving something like the entire known universe approaching this unknown object faster and faster. The effect of its gravity became so strong it disintegrated people - painlessly!
Oooh, what does this button do?

umbrastone
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Re: "Dark Flow" Discussion

ikefalcon wrote:Maybe this is wrong, but wouldn't that effect just be due to parallax? I.e. if you're an infinite distance away from everything, everything would appear to be at one point. So, as we move away from all galaxies, they would all appear to converge. I don't see why a large mass would be needed to explain this.

Someone please correct me if I'm wrong.

vodka.cobra
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Re: "Dark Flow" Discussion

Hahaha. Beret man is stupid.
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SJ Zero
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Re: "Dark Flow" Discussion

This one hits close to home. "Your mom" jokes are like a diamond cut to an incredibly sharp point. Beautiful, but dangerous.

Just be glad our friend the existentialist didn't use the opportunity for a rant about the nature of life and death.

jonassimple
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Re: "Dark Flow" Discussion

Nice... Genius... Like some people already said. I was moved from laughter to nostalgia and finally to sadness in just three panels.

fedexrico
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Re: "Dark Flow" Discussion

I baawwwwww'd

damn

thebeaky
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Re: "Dark Flow" Discussion

Protagonist wrote:Edit: Are you meaning that everything is staying in the relatively same position except for the Milky Way? If that's the case, you're wrong. Everything's moving away from the center of the universe.

The center of the universe, you mean earth right?

vodka.cobra wrote:Hahaha. Beret man is stupid.

your appraisal of this comic is incredible in both its depth, and knowledge of the characters.

I dont know for sure if I laughed outloud, but I definitely found it funny.

Hahaha. Beret man is stupid.

TheAzreal
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Re: "Dark Flow" Discussion

It reminds me a lot of a softer world-- I've loved that comic forever, and one of my friends directed me to xkcd by saying, "Hey, he says he likes a softer world, too." The strange switch in the last panel is somewhat characteristic, there, as well as the sudden jolt between apparent normalcy and the introduction of a bizarre but heart-wrenching situation. [http://www.asofterworld.com/index.php?id=283 , for example.]

I would really be hard pressed to say which comic I love more, although xkcd is easier to plug. "Math, technology, and general geekiness, with occasional heart, a lot of wit, and some really excellently drawn stick figures-- I think you'll like the guy in the hat/it's sincerity/that it occasionally references the Ronpaul."

a softer world is just strange in a lot of ways. Controversial in subject matter (Why is cannibalism such an overarching theme, I ask you!), frequently and unexplainably violent, dark, cynical, and yet above and beyond all that, often strangely sentimental and even outright inspiring. [This has to be one of my favorites: http://www.asofterworld.com/index.php?id=119 , and maybe this: http://www.asofterworld.com/index.php?id=102 , although honestly, I love every one.]

On the other hand, xkcd makes me laugh almost every time, whereas a softer world mostly makes me feel sad. Maybe they aren't comparable.

P.S I got shook out of lurk mode, too.

Pxtl
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Re: "Dark Flow" Discussion

Today's Xkcd homework assignment is to hug your mother.

Those of you whose mothers have passed on get an automatic failing grade. Sorry.

SJ Zero
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Re: "Dark Flow" Discussion

Today's Xkcd homework assignment is to hug your mother.

Those of you whose mothers have passed on get an automatic failing grade. Sorry.

I envy anyone who can visit their mother without spending 30 hours straight driving(60 hours straight driving if I plan on making it home after the hug). I fail it.

Rowsdower
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Re: "Dark Flow" Discussion

Crap. Now I've got the warm fuzzies.

Save the dragon! Clickie!

gypkap
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Re: "Dark Flow" Discussion

Pxtl wrote:Today's Xkcd homework assignment is to hug your mother.

Those of you whose mothers have passed on get an automatic failing grade. Sorry.

Nope. My arms are long enough to hug my late mom anyway...along with my kids' grandma (from wife's previous marriage), who's very much alive and is a sweetheart.

libra
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Re: "Dark Flow" Discussion

RSMaster wrote:My favorite comic in a month!

My friends tease me about it, but I always fall for these kind of characters... childlike, oblivious to mockery, kind of a personification of kindness and innocence... I want to give them a huge hug and tell them it will always be all right.

Perhaps it is because these people do not exist. Real life ruins everything.

They're known as "Woobies."

Paranoid__Android
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Re: "Dark Flow" Discussion

Aren't all the galaxies in the universe meant to be moving away from eachother- not coming together?

*hugs mum*
Last edited by Paranoid__Android on Mon Nov 10, 2008 3:25 pm UTC, edited 1 time in total.
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meat.paste
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Re: "Dark Flow" Discussion

I really liked this one. It reminded me, in spirit at least, of a haiku.

Spirits hide in singularity
Crushed to God’s bosom

(It is indeed a haiku, just not in the form taught by elementary school teachers)
Huh? What?

Darkscull
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Re: "Dark Flow" Discussion

phlip wrote:Acutally, now that I think about it, does the idea of an object outside the visible universe affecting an object within the visible universe even make sense? Or does the term "visible universe" in this context mean something different to the "theoretically-observable universe"?

The visible universe (or rather, the "theoretically-observable universe", as you put it) is the volume defined by the distance from which light can have reached us since the big bang.
The particular volume we call the visible universe only fits that definition for us. We can only be affected (and have affected) things in that volume.
However next galaxy over has a different visible universe it can have been affected by, no?
Half (or around about that) of what can affect it can't have affected us yet, and vice versa. Think venn diagrams.

So yes, the idea of an object outside of our visible universe affecting an object within our visible universe does make sense.
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exploto
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Re: "Dark Flow" Discussion

Darkscull wrote:
phlip wrote:Acutally, now that I think about it, does the idea of an object outside the visible universe affecting an object within the visible universe even make sense? Or does the term "visible universe" in this context mean something different to the "theoretically-observable universe"?

The visible universe (or rather, the "theoretically-observable universe", as you put it) is the volume defined by the distance from which light can have reached us since the big bang.
The particular volume we call the visible universe only fits that definition for us. We can only be affected (and have affected) things in that volume.
However next galaxy over has a different visible universe it can have been affected by, no?
Half (or around about that) of what can affect it can't have affected us yet, and vice versa. Think venn diagrams.

So yes, the idea of an object outside of our visible universe affecting an object within our visible universe does make sense.

No, I don't think this makes sense. If gravity can only travel at the speed of light and no faster http://en.wikipedia.org/wiki/Speed_of_gravity then any object outside the visible universe should both have no effect on us, even indirectly, and we should have no way of detecting its existence, even indirectly. Yes, it will have effected other galaxies that are closer to it earlier than when it effects us, but we wouldn't be able to detect these changes either since we would still have to wait for the light from these galaxies post change to reach us. (right?)

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Re: "Dark Flow" Discussion

# Hoooooow cute! (Yeah, I'm a crybaby. *sniiiiif!*)

# It remembers me my mother's birthday is on Thursday. I want her here. Now.

vodka.cobra
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Re: "Dark Flow" Discussion

thebeaky wrote:
Protagonist wrote:Edit: Are you meaning that everything is staying in the relatively same position except for the Milky Way? If that's the case, you're wrong. Everything's moving away from the center of the universe.

The center of the universe, you mean earth right?

vodka.cobra wrote:Hahaha. Beret man is stupid.

your appraisal of this comic is incredible in both its depth, and knowledge of the characters.

I dont know for sure if I laughed outloud, but I definitely found it funny.

Hahaha. Beret man is stupid.

I can't tell if you're being sarcastic or not. Dang text-based communication medium.

Just in case you are: You, sir, are and idiot. :\
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war3_master
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Re: "Dark Flow" Discussion

My mom bring all the planets to the edge of the sky
Yeah, my mom is bigger than yours
Yeah, my mom is bigger than yours

Turiya
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Re: "Dark Flow" Discussion

This actually made me cry...damn you Randall, get out of my amygdala.
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keithsdragons
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Re: "Dark Flow" Discussion

Srsly guys, srsly...

guys?

Where are his legs?!

Random832
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Re: "Dark Flow" Discussion

ikefalcon wrote:
Protagonist wrote:I'm not sure what you're saying. We aren't moving away from all the galaxies, we're watching galaxy clusters moving toward a region of space for no apparent reason.

Edit: Are you meaning that everything is staying in the relatively same position except for the Milky Way? If that's the case, you're wrong. Everything's moving away from the center of the universe.

There is no "Center of the Universe." From our frame of reference, the Earth may as well be the center of the Universe since we can only see about 15 billion light years in every direction. Light from beyond that approximate distance hasn't gotten here yet. Perhaps you were referring to the point where the Big Bang occurred, but for all intents and purposes the Big Bang happened at every point in the Universe because the Universe was a singularity when the Big Bang occurred.

Hubble's law shows that all distant objects are moving away from us and that objects that are further away are travelling faster away from us. Assuming that this continues forever, the distance between the Earth (assuming the Earth never stops existing) and all distant objects will approach infinity. My question is: If you are an infinite distance from all objects, will they not all appear to be at the same point? To give an example, we make this sort of assumption if you are sufficiently far from an electric or magnetic dipole in order to simplify calculations.

Right, but the dipole's poles are not an infinite distance from each other, so they are infinitely far away in the same direction (or a *very* large distance away in *nearly* the same direction). It does not work for objects that are infinitely distant in different directions.

Sprocket
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Re: "Dark Flow" Discussion

::sniffles:: awe...
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pruwyben
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Re: "Dark Flow" Discussion

Borrowed Irony wrote:How did his mom get beyond the edge of the visible universe? That kind of sounds like a set-up for another your mom joke.

Yo mama's so dumb, she got lost on the way to the store and ended up beyond the edge of the known universe.
"Plus it is fun to drunk dial congress." -M.B.

war3_master
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Re: "Dark Flow" Discussion

pruwyben wrote:
Borrowed Irony wrote:How did his mom get beyond the edge of the visible universe? That kind of sounds like a set-up for another your mom joke.

Yo mama's so dumb, she got lost on the way to the store and ended up beyond the edge of the known universe.

I wish I was that dumb. Being beyond the edge of the visible universe would be awesome. I'd boldly see things no man has seen before. Until I died from lack of oxygen, that is.