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Fermilab Reports Dark Energy Not Needed
An anonymous reader writes "A Fermilab press release
reports that the expansion of the universe may be
explainable without the need for dark energy or a
cosmological constant. Apparently, ripples from
inflation in the early universe may account for
the observed expansion rate of the universe."
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but Sean Carroll is. And he's not convinced.
The other responses to this thus far are completely off. Dark matter and dark energy are not (by any current theory at least) related anything like how normal matter and energy relate via e=mc^2.
Dark Matter is a hypothetical unknown "stuff" with normal mass just like regular matter but which we cannot observe with light; it doesn't appear to be emitting or noticably obscuring any kind of radiation. But we see the movement of galaxies in such a way that they appear to be responding to the mass of something that we can't see. Hence "dark matter" - we can't see it, but it seems to be some sort of matter. Think of it like leaves blowing in the wind - we can't see the wind, but we see the motion is causes.
Dark Energy is another hypothetical unknown "stuff" that seems to be adding, somehow, to the velocity of all objects in the universe. It is postulated because the universe appears to be accelerating in its expansion, which does not make sense given an empty, neutral vaccum and a bunch of matter in it. It should be slowing down or at best, expanding at a steady rate. Hence "dark energy" - we can't detect it, but some source of energy which is causing the expansion of the universe to accelerate.
Hope this helps.
-Forrest Cameranesi, Geek of all Trades
"I am Sam. Sam I am. I do not like trolls, flames, or spam."
Not really related to the ripple they are talking about in the article, but "repulsive force" doesn't require something to push against. When students first learn of Newton's third law, "for every action, there's an opposite and equal reaction", teachers often give as an example that when you push against a wall, the wall pushes against you. That gives the idea to students that there must be something to push back against you (don't feel bad, some early rocket scientists thought the same thing). That is, however, not true. You don't need something to push against, you just need to exert a force in one direction, and there will be a a force in the opposite direction.
Because of this misconception, it was originally thought that rockets wouldn't work in space, because the exhaust they put out wouldn't be able to push against the atmosphere. But hey, they do!
Let's say we've reached the edge of the universe, what happens when we step beyond that boundary? What is out there that would possibly hold back further expansion of our universe?Gravity. There's an attractive force between every object with mass. When you jump, you move away from the center of the earth, but only for a short time. You accelerate up, but then you start decelerating. You reach a maximum height, then you start accelerating back down. During the big bang, the universe started expending. It was originally thought that there would be a "big crunch", and the universe would stop expanding, than start collapsing towards the center. Then we discovered the universe was not only not slowing down in it's expansion, but actually accelerating. That made no sense, so Dark Energy was used to explain it (a force like gravity, but pushing outwards). Under that scenario, the universe would end not through a big crunch, but would simply become dark as suns die and black holes evaporate. If we don't need Dark Energy, maybe the big crunch theory will come back.
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although I am not knowledgeable enough to tell you if the cosmic horizon is the limit of what we can see because of the distance, because of a higher-dimensional curvature of the universe, or because of something else.
Because of light speed. Since nothing can travel faster than light, you can't receive a signal from anywhere in the Universe that is farther from you than the distance light could have travelled since the formation of the Universe (14 billion years approx.)
Their budget has been slashed almost in half. After all, low quality bombs are far more important than high quality science. In fact, spending on basic research is dropping at an alarming rate through all the national laboratories. This does not bode well for our future.
"To those who are overly cautious, everything is impossible. "
The hypothesis in question was explained here
Also here for the braver souls.
The best education consists in immunizing people against systematic attempts at education. - Paul Feyerabend
And you know why conservation of momentum holds true? It's because of Newton's Third Law!
After one has studied physics for a while, the reasoning that Newton's Third Law ==> Conservation of Linear Momentum is generally replaced by more of a feeling that momentum conservation is more basic (due to, in part, Noether's Theorem http://en.wikipedia.org/wiki/Noether's_theorem) and that Newton's Laws are simply a consequence of momentum conservation. The advantages of this abstraction are manifold (*cough*), but the "obvious" ones are cases in which forces are very hard to identify (such as in radiation reaction), and systems which may be much more immediately approached through Hamiltonian's or Lagrage's formulations rather than Newton's laws.
The preprint is here.
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Uhmm ... rockets work due to conservation of momentum, not because of some mysterious reaction force.
.05G's to the halfway point, then turn around and decelerate at that same rate. THat of course means it would have to be launched into leo by some other means before lighting the torch. If not turned around and slowed down, it would go by Alpha Centari at truely relativistic speeds, near .99 C. I haven't personally ran the math, but the article I read 2 decades back sure did. ISTR the article said it would only take about 20 tons of gas. That was with estimates of about 5 tons for the whole nuclear light bulb reactor so the total vehicle weight wouldn't be near as heavy as a Saturn5 at launch. We played with such a reactor at Rocky Flats for a while, but the natives (thats us people folks) got restless. I don't know if there ever was a true 'accident' involving one of them because basicly if the 'light bulb' envelope fails, the reaction is self quenching. One of the safest reactors ever developed, but the word nuclear was its death knell.
Not a very good explanation. Rockets work in space because the exhaust gases are, in the process of being accelerated out the back of the engine, are pushing equally hard on the walls of the engine thats burning the fuel. Since thats cone shaped, wider at the rear, its the net circular square area of the back flange of the motor that the gasses push against, and its anchor to the rest of the rocket transfers that push to the rocket proper. And don't forget that a little like the e=mc2 of Einsteins famous equation, the net power, minus some losses here and there, is still e=mv2, so the holy chalice/grail of the rocket is the one that moves the gas at the highest velocity at the face of the nozzle, with some of the flame cone actually being a velocity to pressure translation so in the end, the gas velocity, being highest out near the tip of the flame, serves to increase the felt pressure pushing forward on the engine proper.
The ion and plasma drives that use zenon gas, electrostaticly or thermally accelerated to a fraction of C speed, are many times more efficient in terms of the amount of push per pound of expendable than any chemically fired rocket can ever hope to be simply becasue of the 'fraction of C speed' is many times what a chemical fired gas generator can do.
I've heard/read of estimates that a xenon gas rocket, fired by a nuclear light bulb heat source (circa 30,000 degrees C) making a plasma out of the gas, could go to Alpha Centari in just a few years, as it would accelerate at a steady
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