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Chandra Provides Support For Dark Energy
starannihilator writes "The Chandra X-Ray Observatory has provided new evidence supporting the existence of dark energy, the force causing the acceleration of universal expansion. The new findings support the theory that the universe will expand forever, provided there is enough dark matter. CNN and Newsday are running the story, originally reported by NASA. Chandra's site has some good images and information on the three galaxies clusters studied (Abell 2029, MS2137.3-2353, and MS1137.5+6625)."
> I thought it was decided that the universe's expansion was expanding at the
> speed of light
I don't think the acceleration in on the order of c^2, if that's what you mean. What they mean is that due to acceleration, some space (the stuff that's furthest away) is expanding at close to c.
Releativity states that the speed of light is as fast as it gets. ... for matter. Relativity makes no such claims as to the speed limit of space itself.
There are quite a number of valid GR metrics which describe space which expands faster than the speed of light, and in fact, it's thought that it did expand faster than the speed of light during the inflationary period.
Those same metrics are the basis of the Alcubierre metric, one of many ways to generate faster-than-light travel without multiply-connected spacetimes (wormholes). Like most "violate the speed of light" metrics, it requires negative energy density matter, though variations on the metric allow for very tiny amounts of negative energy matter to generate it.
No. The expansion of the universe refers to the fact that distant galaxies are moving away from us, and that the farther they are, the faster they are moving. This is expressed by the Hubble constant, which has a value of about 50 km/s/Mpc.
The acceleration of the expansion is reflected as this "constant" increasing with increasing distance.
The acceleration is caused by Dark Energy, not Dark Matter.
Dark Matter is either normal matter or subnuclear matter that makes its presence felt as increased gravity, but is not directly observable.
Dark Energy is not well understood at all.
HCG 50a = 2MASX J11170638+5455016
11h17m06.4s +54d55m02s
Yes. The universe is accelerating in its expansion. To say that the universe is expanding "at the speed of light" isn't quite right in a couple of ways. First off, if we look at objects nearby, they are moving away from us at some (quite reasonable) finite speed (i.e. the nearby Virgo Cluster is moving only at ~1000 km/s). Secondly, the somewhat more subtle point is that we generally talk about velocites not exceeding the speed of light; however, this is motion THROUGH space. The expansion of the universe (expansion OF space) doesn't necessarily need to follow this rule. . .
I should also point out that "Dark Matter" and "Dark Energy" are COMPLETELY DIFFERENT THINGS (as far as we know). Astronomers have just named them both "Dark" because they don't know what they are. They both also affect the expansion of the universe, but dark matter is slowing down the expansion of the universe (presumably via gravity) and dark energy is accelerating the expansion of the universe (by some yet-unknown force). Dark Matter is weird, but at least it seems to sortta obey the rules of the universe (i.e. gravity); dark energy is completely unlike anything we've seen before.
I couldn't tell if you were experimenting with poor-man's cryogenics or looking for the orange sherbet.
(1) the universe is not expanding at the speed of light (I think that it is less)
(2) the space-time fabric of the universe is not an object anyway, so FTL rules do not apply
It is actually possible for 2 objects to move apart faster than the speed of light even though neither is moving FTL compared to the other. This statement seems to be nonsensical, until you realize that the expansion is a 4D effect. Think of the galaxies (in 2D) as though they were on the surface of a balloon (2D). Now imagine the balloon getting larger (3D effect) at the same time that galaxies are moving farther apart. Now use that analogy with 1 higher dimension. The 2 effects are independent, but both contribute to increasing distance between galazies. As my physics professor once said "Everywhere is getting farther apart."
BTW, this is why the wavelength of the cosmic radiation is getting longer. The cosmic radiation is actually getting stretched (along with everything else) along with the universe. So while the frequency & energy stay the same, the cosmic radiation gets "red-shifted." And since they can surmise the starting wavelength (from Hydrogen energy levels), they can make predictions based on that too.
Let's go Hurricanes!!! 2006 Stanley Cup Champions!!!
As others have said, accelerating expansion means that objects very distant are moving away from us faster than closer objects are moving away from us. If you have time for some interesting reading, I'd recommend a title called Atom which is very readable and is a good primer on theory from the big bang to present time. It won't answer many questions about dark energy, but if anything, it'll give you a good idea of what we know in very readable terms and most likely get you to want to read more :) It was my first book on the subject, and it certainly had that effect. Note that I'm no physicist, just a curious reader.
Uh, dark matter and dark energy aren't the same thing.
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I think a more accurate answer is in order (not to malign the other posts, as they all contain bits of the complete answer). Accelerating means that the outer boundary of Universe ("horizon") is moving away from us at an ever increasing rate. The components of the Universe also accelerate away at a proportional rate. It does not simply mean that the Universe is growing bigger (although that is true). It means that it's growing bigger at an increasing rate. The Universe is *not* expanding at the speed of light. The speed of light does not pose a real barrier to the expansion of the horizon (in fact, it is believed that the Universe underwent a period of faster than light inflation early on), but it DOES place an upper limit on the speed at which components of the universe can move away. In other words, the speed of light applies only to objects, not imaginary boundaries. As an example, one can think of shining a laser pointer at a distant wall and moving your wrist quickly; the laser point can easily exceed the speed of light, since the point is not a physical object. But no matter how hard you throw the laser pointer itself, you'll never exceed the speed of light. When you ask whether dark matter (actually, dark matter isn't special at all, usually just normal matter that's hard to detect; you mean "dark energy") can bend the laws of physics, you've fallen into the same trap that countless others have. Popular accounts seem to really harp on the exoticness of dark matter, but never mention how "normal" it can be. Dark energy is just an energy field that exerts a pressure on the constituents of the Universe, just as a gas would exert a pressure on the walls of its container. This is one of the most trusted explanations for why the Universe is accelerating.
No offense, but that's not how relativity works. The thoery of Relativity posits that all measurements are taken from some frame of reference, and it is impossible for an object to go faster than light for any frame of reference.
So, if I'm on a spaceship going 99.9999999% the speed of light from the frame of reference of the earth. However, from the frame of reference of my spaceship, I'm stationary. Now, I can run as fast as I want in any direction, I can even sit in the back of my space ship with a super-powerful gun that shoots bullets at 99.9999999% the speed of light, and fire a couple rounds towards the front of the ship. From the frame of reference in the ship, the bullets will travel at 99.9999999% the speed of light, even when the ship is travelling at 99.9999999% the speed of light in reference to the earth.
But here is where it gets wierd: an observer on earth will not measure the speed of the bullets to be travelling 199.9999996% (99.9999999%x2) the speed of light, they will be measuring the bullet to be travelling just over 99.9999999% the speed of light.
This is because, from the viewpoint of someone on earth, the space ship will be very short, which means even if it still traverses the length of the ship in the same amount of time as it does from the viewpoint of me on the spaceship, it will not have travelled the same distance, which (since v=d/t) means the bullet didn't travel as fast relative to the spaceship (from the viewpoint of earth) as it did from the viewpoint of someone on the spaceship.
Additionally, from the viewpoint of Earth, time is travelling more slowly on the spaceship, which enhances the effect even more.
It's confusing if you don't have a handle on it, but none the less, this is how the theory of relativity works.
You are quite right.
Here is a synopsis of signs that things are due for a big shakeup:
The last two are probably not mandatory, but most people feel like any general theory should account for those two things.
HCG 50a = 2MASX J11170638+5455016
11h17m06.4s +54d55m02s
A photon cannot travel faster than light, because it is light. All massless particles that traval at c are called luxons; these include photons, gluons, W and Z particles, and hypothetical gravitons. Anything that travels slower than c (basically all ordinary matter) is a tardyon, and has positive mass. Hypothetical particles that travel faster than light and have negative mass are tachyons. As tardyons accellerate, they gain mass and time slows down for them. As you approach c, mass increases and time slows exponentially, until, at c, mass becomes infinite and time stops. This is why nothing that travels slower than light can reach it.
This is a special excite
This
The Chandra instrument is named after Subrahmanyan Chandrasekhar, an astrophysicist from early in the previous century. Not the moon.
No gods, no demons, and no masters. Secular Humanism!
Just a quick reply to this. I'm a graduate student doing computational astrophysics - in particular, cosmological structure formation (galaxies and such). The law of conservation of energy is only valid in closed systems. If the universe isn't a closed system - if there's something 'outside the universe' which is adding/subtracting energy - then energy doesn't necessarily have to be conserved. Also, there are some cosmologists that believe that energy is not conserved on cosmological scales, so the law of conservation of energy is not valid on all scales. I suppose it's fair to say that as of right now, dark energy appears to result in the non-conservation of energy on very large scales, given our current understanding of particle physics. However, there is almost certainly a lot going on that we don't really understand, so it's an open problem.
I hope that helps!
Actually, Ptolemy may be one of the great under-appreciated physicists in history. Everyone's always making him out to be the poor fool, but in another view, his work is really astounding.
Ptolemy had successfully been able to plot the motions of the stars, planets, sun, moon, etc. If I remember correctly he even guages the distances pretty well.
He had also correctly calculated variables for all the motions, only thought they were different things. What I mean by that is that he had calculated the value that accounts for the rotation of the Earth, he just thought the entire universe was rotating. He had come up with values for the revolution of the sun around the earth and the orbits of the other planets, but called them epicycles. Really, if you track all the planets, you find the center of the epicycles always correspond to the positions of the sun.
Ptolemy even seems, in places, to recognize the possible theory that the earth is moving, but doesn't like it because he's worried about what effects that will have on other mathematical investigations (e.g. he's mentions throwing a ball into the air, and it comes straight down- not to the side). So, really, all Capernicus had to do was read Ptolemy, say "We'll, Ptolemy recognizes both possibilities and thinks the first possibility is right, but I'm going with option 2." It's not as innovative as people think. Capernicus didn't even discover then means by which planets were held in their orbits- we had to wait for Newton to get any of that.
Plus, this whole issue gets murkier with the advent of the theory of relativity. People spent years arguing whether the earth goes around the sun or the sun around the earth, only to have Einstein say "Both... neither...it doesn't really matter. It's all an issue of perspective".
Doesn't gravity effecct objects regardless of the distance between them? Meaning to say, that gravity, however weak, will always have this attractive force.
so, won't this energy causing this accelerating expansion eventually burn up/out?
The amount of gravitational energy between two objects is a static amount that can be determined. The amount of energy in kenetic motion can also be determined. As two objects move apart the gravetational potential grows while the speed they are traveling away from eachother decreases. If the kenetic energy is greater than the gravatational energy, then the two objects will continue to move apart. If the gravetational energy is greater than the kenetic energy causing them to separate, then their realative motion away from eachother will slow, stop and then they will begin to come back together. This is basically an explantion of escape velocity that you always hear about in rocket launches.
A condensed explantion of what would take too long to describe here in full basically says that the average kentic energy that the obejcts in the universe has been determined as well as the average gravatational energy. If the gravetatinal energy was greater then everything would eventually come back together in what is known as a "big crunch", but the kenetic energy is greater and thus the universe will continue to expand.
Now part of this probelms comes not from actual kenetic energy, but due to that space itself is increasing. So the distance between two objects is increasing proportional to the distance between them. the rate at which this is occuring also seems to be growing. This is the acceleration of the universe you are reading about here. Reasons for this are nt well known, but one theory is that there are something like 11 total dimentions and the other 7 after three spacial and time are shrinking, causing the others to expand. These other seven are already so small that they haven't been detected yet (we're way off in string theory territory here).
If this acceleration continues to increase, then eventually the rate at which space is expanding might grow so large that it will overcome not only gravity but even the other forces that hold atoms or particles together. This senario where everything is torn apart into component parts is called the "big rip".
Here's a quick summary of the technique:
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in other words, what evidence supports that this thing is going to expand at an accelerating rate forever? seems like gravity is going to get a little upset about that eventually.
Which is exacly why scientists have postulated the existance of dark energy. You see, you're correct, the effect of gravity does suggest that the universe's expansion should be decelerating. But it's not. All of our observations say that it's accelerating. Most cosmologists would say that's it's pretty much a confirmed fact at this point. The cause of this acceleration is unknown. They're postulaing the existance of this 'dark enery' which exerts some sort of repulsive force.
Enistien actually came up with the idea first, but for a completely different reason. He didn't call it dark enegry though (I don't think), it was just a variable that he added to his equations to force the overall 'shape' of the universe's space-time to be flat. He later took it out because he thought it was stupid; it was much more logical to assume the universe wasn't flat, in which case it wasn't needed. However modern day measurements of the Cosmic Background Radiation have given very strong evidence that the universe is actually flat. So now they've put the variable back into his equations, and they're working on trying to prove it's existence.
Aw crap, ninjas!
I don't, but "Brief history of Time" by Hawking or "The Elegant Universe" by Green have pretty good explanations of relativity.
Actually, I'd recommend trying a book written by Einstein called "Relativity". I've also heard it referred to as "the short book" because apparently he wrote two, one in laymen's terms, and one filled with math/equations. If you really want to go hard-core, you can read his original papers, but takes a bit of work to get through, and it helps if you have a big physics background and are familiar with Maxwell. "Relativity" isn't too hard to understand, though. Plus, it's generally true that you'll never get such a dead-on explanation of a theory as when you get an explanation from the guy who came up with it. I've met a lot of modern physicists whose grasp on Relativity has been corrupted by hearing poor explanations. No risk of that if you go to the source (Einstein).
I am eagerly awaiting the next annoncement where someone again finds evidence to refute the dark matter claims. It seems like the science; "Dark Matter is like this" - "No, it can't be, actually it's like that".
The only articles I've seen that make statements like that are the commentaries on the commentaries on the dumbed-down press releases on the actual publications.
What's actually been happening is more along the lines of:
"There's a discrepency between galactic models and observations. What did we get wrong in the model/what needs to be added?"
"Maybe A? B? C? D?" "Let's try to test them and see."
"Not A, not D, but maybe B, maybe C." "What kind of B or C? B1? B2? B3?" "Let's try to test them and see."
"Our models only work if we have B _and_ C, and we've ruled out C1 and C2, but C3 still works."
"What kind of C3?"
"New observations show a new effect in addition to the old one. How do we explain it?"
"Maybe E? Maybe F?"
[Etc.]
This is a process of examining many possible explanations, and weeding out the ones that don't work until we have reasonable confidence that the ones left _do_ work.
We've gone from "galactic rotation doesn't match models based on stars alone, what could be causing this?" to "we know that there's about X amount of normal matter we aren't seeing, Y amount of abnormal matter that we aren't seeing, and that the properties of the abnormal matter fall somewhere in this range (that's wide but being narrowed)". There's surprisingly little backtracking. Tests that detect or fail to produce evidence for dark matter of various types all help to increase our understanding of what dark matter's actual properties are.
As for dark energy, if anything, it would be surprising if something like it _didn't_ exist. We already knew that a scalar field with similar properties was likely present in the early universe, and several models proposed universes where the _absence_ of the field was only a local effect. Even relativity contained a similar type of effect that was set to zero a priori as opposed to forced to zero through a mechanism inherent in the model.
We're still sifting through the myriad of possibilities, but we certainly are learning something each step of the way.