Chandra Provides Support For Dark Energy 350
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)."
Re:Dakr Matter (Score:4, Informative)
> 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.
Er, I mean Nibbler. Oh never mind... (Score:2, Informative)
Re:Dakr Matter (Score:5, Informative)
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.
Re:Dakr Matter (Score:3, Informative)
This is effectively how the Alcubierre warp drive works.
Re:Dark matters (Score:2, Informative)
- New Clues About the Nature of Dark Energy [slashdot.org] dark matter is here
- Missing Matter... Still Missing [slashdot.org] dark matter is gone
Now, with this story, it's back again.Re:Dakr Matter (Score:2, Informative)
Expansion of universe (Score:5, Informative)
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 [utk.edu], 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.
Re:Dakr Matter (Score:5, Informative)
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.
Re:Dakr Matter (Score:3, Informative)
(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.
Re:Dakr Matter (Score:3, Informative)
Re:Dark Matter (Score:2, Informative)
Re:Dark matters (Score:3, Informative)
"Dark matter" != "Dark energy" (Score:5, Informative)
Re:Ptolemy's back! (Score:2, Informative)
Re:"Dark matter" != "Dark energy" (Score:5, Informative)
Dark matter is normal matter. "Cold dark matter" has a pressure of 0 (or very low in relativistic terms), just like all regular gas, stars, planets, etc.
Dark energy is freaky. It has *negative* pressure.
The two are extremely different things.
-Rob
Re:Dakr Matter (Score:3, Informative)
Re:Dakr Matter (Score:5, Informative)
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.
"The Inflationary Universe" by Guth (Score:2, Informative)
I imagine a lot of the theories have been proven/disproven by now (published in 1998, I think?), but it was a good read. A little dry in some parts, and somw parts assume that you know a few basics of astronomy/expansion theories, but overall a very good read.
Give it a shot.
Re:Ptolemy's back! (Score:5, Informative)
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.
Re:Dakr Matter (Score:3, Informative)
Re:But seriously... (Score:2, Informative)
No. There is no "center". Or, alternately, every point is the center.
It's not like an explosion of an object into space. It's the explosion of space itself.
Re:Dakr Matter (Score:2, Informative)
Photons travel at (or below, depending on the medium) c. However, there's nothing in special relativity to prevent there being particles that always travel faster than c; these purely theoretical particles have been dubbed tachyons [wikipedia.org], and they are something of a science fiction staple.
Re:Chandra == Moon (Score:3, Informative)
Re:Expansion of universe (Score:2, Informative)
> which has a value of about 50 km/s/Mpc.
Actually, the currently accepted value is around 71 +/- 4 km/s/Mpc, based on WMAP (http://map.gsfc.nasa.gov/) observations.
Re:"Dark matter" != "Dark energy" (Score:5, Informative)
Dark matter, on the other hand, is the name confusingly given to a number of unsolved phenomena. By looking at how the outer parts of galaxies rotate, we get a sense of how much matter is in a given galaxy, as well as its distribution. It seems that there is a great deal of matter in the outer regions of galaxies that does not 'glow' like stars do. In addition, by studying how galaxies move in clusters, we strengthen the case for lots of matter existing between galaxies that is invisible to us. The candidates for this dark matter are many and varied, from innumerable Jupiter-sized objects to cold white dwarfs to small black holes. Current observations are undertaken to rule in or out some of these. Even so, standard Big Bang theory predicts an upper limit to the amount of "ordinary" (baryonic) matter present, so it is possible that some of this dark matter might be weird stuff.
Re:But seriously... (Score:3, Informative)
The balloon surface analogy is a 2D example. Yes, there's a center and edge to the balloon, but that's in 3D. The surface doesn't have an edge. Similarly in 4D, the universe can have a center and edge, and does when time is the fourth dimension, but that is measured in time and not in 3D space. In that case, current time is the "edge" and the big bang is the "center". And we're definitely not expanding at the speed of light at the current time.
Re:dark energy and energy conservation (Score:5, Informative)
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!
Not understanding science (Score:3, Informative)
>everything it states about ratios and measurements involves assumptions. This isn't science.
Yes, it is science. There are observations made that are attempting to confirm or disprove predicitons made consistent with their hypotesis. As for your distaste for the choice of language, particularly the weasle words; that's the way scientists write."Recent observations of a massive shockwave, intense gamma, beta, and alpha radiation, together with so far unrepeated visual observations of what is thought to be a very large smoking crater located at what appears to be the former site of the City of Los Angeles are not inconsistent with the suggestion that a large thermonuclear device or some similarly destructive object may possibly have detonated in Southern California. Our research group is meeting to design further field tests of this hypothesis and it is anticipated that a team of sacrificial graduate students will be sent to the site in the reasonably near future for purposes of further data collection."
Re:Dakr Matter (Score:3, Informative)
Yes, and that 4th dimension would be called time. The "center" of the balloon would be the equivalent of the big bang, but as you say there is no "center" in the normal dimensions, i.e., no center of the balloon surface and no center of the 3D universe at any given time.
Re:Ptolemy's back! (Score:3, Informative)
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".
Re:Universal Catapult (Score:4, Informative)
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".
Summary of technique (Score:5, Informative)
Here's a quick summary of the technique:
[TMB]
Re:Universal Catapult (Score:2, Informative)
As for gravity.. gravity is a force. If not countered by another force, it causes acceleration. In fact, acceleration is caused by the net force on an object. Your weight is the tension caused by the countering forces of gravity, and the molecular bindings of what you're standing/sitting/laying on resisting.
Now, as for the universe expansion. Objects further away from us are moving away from us, faster, the further they are. Imagine being stuck on the middle of a piece of elastic that is being stretched, and seeing how fast various spots are travelling away from your spot. Due to the quirk of einsteinian space, from each 'edge' of the universe, the other edge seems to be moving away at very close to the speed of light. From the middle, both edges seem to be moving away at near the speed of light. Just not as near. (There is an infinite amount of measurement in 'nearness' to the speed of light. No matter how near you are, you can always get nearer.)
Now, if gravity is the predominant force, the velocity (near the speed of light) of these far distant objects would be getting farther from the speed of light, slowing down. If there was no predominant force, each spot on the elastic would keep going away from us at that constant speed. And if there was a countervailing force, the speed would keep going up as they left us.
Now, because scientists are throwing dark energy and dark matter around like so many bad ideas that make good t-shirt designs (anybody remember chaos theory?), I am unsure if they mean that the universe, while slowing down its expansion, will never reach zero and then reverse (ie, it will achieve escape velocity.. no matter how long, even if the expansion slows, will it ever stop). This, to me is not 'accelerated'. Even if there is a countering force that is preventing gravity from being as effective as it can at slowing things down.
Alternatively, if not only is the universe not going to close, the relative velocities are actually increasing over time, then the universe is going downhill fast. (and faster, and faster)
As for the people talking about space itself moving, they are misusing concepts that apply only when there is a great amount of mass in one place, like the early universe. Mass has the ability to drag spacetime with it (since spacetime really is a product of mass anyway, remember the rubber sheet idea?) Unless the vast majority of mass in the universe is at the rim, this has no significant affect. (and, in fact, the universe has some pretty decent mass dispersion.. the variation is incredibly, incredibly, minute. Almost every direction you look in has the same amount of mass.)
MNRAS article (Score:3, Informative)
[TMB]
Re:Universal Catapult (Score:4, Informative)
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 [uchicago.edu] 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.
Re:Dark Matter (Score:3, Informative)
The reason for this provide in the sibling response.
Re:"Dark matter" != "Dark energy" (Score:2, Informative)
The answer to (1), then, is just that if such an exponential dependence exists, it must have an effect smaller than we've been able to measure. Nothing of the kind has been seen.
For (2), there are good theoretical reasons for asserting that the *rest* mass of photons is dead, flat, zero. Any photon will have some relativistic mass due to its energy density, but their rest mass must be 0. Also, the current ratio is about 1 billion photons/particle, so on a universal scale, photons would have to have a rest mass on the order of 1 eV to have an effect, since protons have a mass of about 1 billion eV.
Re:Ptolemy's back! (Score:3, Informative)
This model had been rejected by other philosophers at the time, but the meme was out there even then.
Re:Dakr Matter (Score:4, Informative)
I don't, but "Brief history of Time" [amazon.com] by Hawking or "The Elegant Universe" [amazon.com] by Green have pretty good explanations of relativity.
Actually, I'd recommend trying a book written by Einstein called "Relativity" [amazon.com]. 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).
Re:Dark matters (Score:5, Informative)
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.
Not moving faster than c (Score:2, Informative)
Where u and v are the velocities of those spaceships, the formula is something like this:
u' = (u-v)/(1+uv/c^2)
Re:"Dark matter" != "Dark energy" (Score:5, Informative)
Some responses...
Check... although photons can have almost any energy. Low-frequency photons (think IR) have low energy, and high-frequency photons (think gamma rays) have high energy.
I have no idea what you're saying here. Photons have no charge and no mass. They are not "negative" in any sense of the word I'm familiar with. One of the fundamental properties of photons is that they are always moving at the speed of light - that's why they move away from their source.
Why must it? If you begin to study physics seriously, one of the first pre-conceptions you'll have to let go of is that your "common sense" can be trusted to tell you how things behave in the quantum world. Photons have no mass.
A typical fate for a photon would be for it to be absorbed by an atom. In the process, the photon's energy is put into raising one of the atom's electrons from a lower energy state to a higher energy state.
No. All photons have a non-zero energy which equals something like h * f, where h = Planck's constant and f = the photon's frequency. I may be off by a factor of 2 pi... it's been a long time since I took Modern Physics!
Hope this helps.
Sean
Re:Summary of technique (Score:3, Informative)
Re:Dark matters (Score:3, Informative)
Not offhand, but a couple of good places to look would be to check the various online bookstores for quantum mechanics textbooks to see what are recent/available and get good reviews, and course pages at various universities to see what textbooks they use and what online resources they have available. Expect to pay $100 or so for a good textbook on any given topic.
There are a number of online physics tutorials that should cover some of it, but I'm afraid I don't know where they are.
Thinks like wikipedia and everything2 have at least an overview, but detail will be quite spotty.
Good luck