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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."
Comment removed based on user account deletion
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."
It has been observed that galaxies on the whole do not obey Kepler's Laws of Planetary Motion. In particular, stars along different positions moving outwards from the core have about the same measured period (to do a complete orbit of the core). Kepler's Third Law says that the square of the period of a star is directly proportional to the cube of the stars' semimajor axis. Stars further out should have a much longer period (like Pluto) than those close to the core. Dark matter is an invention that compensates for discrepancy by saying that there is perhaps a large halo of non-detectable mass around the galaxy (about 10 or more times the observed mass). There are reasons to believe (too complex to go into here) that dark matter is not like ordinary matter but still has mass.
The Hubble constant, which was discovered by Edwin Hubble, showed that the universe was expanding. If the universe began at the Big Bang (and there is a significant amount of data to support this assertion) then the speed of expansion should be fairly straightforward to calculate. What is observed at great distances is greater than that value. For this reason scientists have named (very poorly) another potential fundamental force dark energy which accounts for the expansion. Dark energy would be like gravity, but repulsive, and is only significant at extreme distances (so that it wouldn't hurt the formation of galaxies).
Suddenly, the hairy finger of a familiar monkey tapped me on the shoulder. It was time.--G. T.
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.
Warning: Opinions known to be heavily biased.
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.)
M-Theory. Gravitons can escape to other branes (other 10 dimensional surfaces that hold universes because they have a closed structure with respect to our brane (complete circle) where as all other elemental particles are open so they are physically connected to our universe and cannot escape.
I like the idea of M-Theory.. the only thing is that we do not have the mathematics to describe it yet..... Personally I think the problem lies with the undefined values of 1/0. if we can define that as u and define u*u^{-1} = 1 then we can show that 0|n because n = 0*un.
I am the Alpha and the Omega-3
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. "
Note that the topology of the universe likely makes it such that you couldn't leave the universe if decided to fly as fast as you can "away"- it's likely that you would curve back on the universe and be pointed back towards home. There are suggestions that our spatial dimensions as we know them arise from the space of the universe being arranged as a gigantic hypersphere. Imagine a two-dimensional universe on the surface of a sphere. Inhabitants could go and go and go, but unless they are somehow able to "pop" into the third dimension and leave the surface, no amount of motion on the sphere's surface will get them out of the universe. Now bump it up another dimension. Just as the two-dimensional inhabitants are stuck on a 2D surface, we are stuck on a 3D surface. Just as if you could pop out of the 2D universe, you'd see the entire universe as a sphere, if you could pop out of our universe, you'd see a hypersphere (technically, a 3-sphere, since the surface of the sphere, which we're on (I know it feels like "in") is three dimensional).
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
For anyone that can actually understand it click here (in pdf format)
Dark matter is, well, matter. Some of it is probably ordinary baryonic matter that's cold, like brown dwarves and such; some of it is probably weakly-interacting elementary particles. Dark matter, like all matter, produces an attractive gravitational force.
Dark energy is not what we think of as "matter" at all. One of its key physical properties is that, unlike matter, it has negative pressure. This is what causes the expansion of the universe to accelerate: in general relativity, not only mass-energy but pressure (and other things, like stress/strain) gravitate. Positive pressure is attractive; negative pressure is repulsive.
(Incidentally, this is why black holes form when a star undergoes gravitational collapse. You might think that it would collapse a bit and then the collapse would halt, as the star's internal pressure resists further collapse. But for a sufficiently massive collapsing star, the internal pressure builds up so strongly that its gravitational attraction outweighs the force of its repulsion, and further increases in pressure actually hasten the collapse instead of slowing or halting it.)
There have been various theories of what dark energy is; the simplest is that it's Einstein's cosmological constant, a term Einstein introduced into general relativity in order to have a static universe (a repulsion that exactly balanced the gravitational attraction of the matter in the universe, so space neither expands or contracts). Einstein retracted this idea when it was shown that space is expanding, but it was resurrected to explain how the expansion could accelerate.
It's possible to rewrite Einstein's field equation to put the cosmological term, which is ordinarily on the left-hand "curvature" side of the equation, onto the right-hand "source of gravity" side. In that case, it's interpreted as the mass-energy/pressure/etc. contributed by the vacuum itself (which is possible in quantum field theory, with the zero-point energy). Unfortunately, quantum field theory predicts the wrong value for this by over 100 orders of magnitude, leading to the "cosmological constant problem". Other approaches to dark energy involve dynamical quantum fields -- which can be sort of treated as a time-varying cosmological constant, although there can be additional differences -- with exotic properties, like negative pressure.
Wikipedia on dark matter and dark energy.
You can never rule out the existence of a phenomenon that's screwing things up in an undetectable way. Indeed, if it has no detectable effect, who cares? But you can rule out alternatives that have side effects. For instance, you could postulate that the redshifting of light from distant galaxies is not due to recession in an expanding universe, but is instead due to some other mechanism which causes light to lose energy as it travels ("tired light"). But there are many ways that various specific tired-light models fail to be consistent with other observations. If two explanations give the same predictions for all observations that we can perform, we can never tell the difference between them, but often that's not the case, and we can rule out alternatives.
Here's a bit more about the nature of Dark Matter, what theory thinks it is in a more specific manner. However, we must explain what "Normal" Matter is.
"Normal" Matter is matter that is influenced by the four forces of the universe. The first three are at the atomic and sub-atomic level, the fourth one is on the Relativistic Scale:
1. The Strong Force, keeping the atomic nucleus together, even as it is composed of neutral and positively charged particles.
2. The Weak Force, which makes the orbits of electrons decay over time. We know it also as Radiation.
3. The Electromagnetic Force, which creates attraction between atoms, creating molecules.
4. The Gravitational Force, which creates an attraction between masses.
Dark Matter of the WIMP variety (Weak Interactive Massive Particles) is UNAFFECTED by the first three forces. It is like a mist that congeals under gravity, but is never more than "ghostly", and since it cannot bond at the atomic and subatomic level, it is undetectable except by the gravity it creates.
Dark Matter of the MACHO variety (Massive Compact Halo Objects) consists of would-be stars that never made it, or stray planets and planetoids roaming around the galaxy.
Therefore, we call it Dark Matter because we haven't been able to detect it, but we can perceive the gravitational effect it has, everywhere we look. And I mean everywhere, since the figures say that between 90% and 98% of the universe consists of Dark Matter.
Lil' Thindime, lilting a lacrimose lament, krashes the kwaint konfines of Kokonino Kounty
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.
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.
This isn't quite true. DOE's funding for High Energy Physics Programs (basically, Fermilab and SLAC) is down 3.1%, or $22.5 million, from $736.4 million to $713.9 million. (I couldn't find out exactly how much Fermilab lost from those cuts, but I recall seeing a figure of about a 4% decrease w/inflation, which is pretty consistent over the past 5 years) Furthermore, run times of the accelerators would be increased over FY 2005 levels at the Fermilab Tevatron (6% more operating hours) and SLAC (54% more hours).
Along with the budget cuts, the BTeV project at Fermilab was canceled. With HEP experiments at SLAC and Brookhaven going offline in a couple of years, Fermilab will soon be the only HEP lab in the nation. Currently the CDF/D0 experiments (the two main detectors) on the Tevatron are scheduled to run until 2009 or 2010. And MINOS/NuMI will run at least that long as well.
Fermilab is going through a 5% workforce reduction, voluntary at first... The saving grace for Fermilab right now will come in the form of the International Linear Collider, the Next Big Thing(tm) in HEP. More info at http://www.interactions.org/linearcollider/ and http://ilc.fnal.gov
More insider info upon request, heh.
The preprint is here.
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You can solve the horizon problem by having the universe expand ultra-fast for a time, just after the big bang, blowing up by a factor of 1050 in 10-33 seconds. But is that just wishful thinking? "Inflation would be an explanation if it occurred," says University of Cambridge astronomer Martin Rees. The trouble is that no one knows what could have made that happen.
Unforch, either the exponent was forgotten, or got lost in an html glitch someplace, What I'm refering to is the 1050 figure used above, which should be 10^50, which is the correct value used in Martin Rees's original release on the subject.
And that is not a trivial difference folks, it is truely astronomical in its own right.
--
Cheers, Gene
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
99.34% setiathome rank, not too shabby for a WV hillbilly
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
Sometimes I swear we are our own worst enemy.
--
Cheers, Gene
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
99.34% setiathome rank, not too shabby for a WV hillbilly
Current models indicate that there is simply not enough matter to halt the expansion. It appears the Universe is headed for a heat death.
The world's burning. Moped Jesus spotted on I50. Details at 11.
The problem with the theory wasn't so much you couldn't move faster than the speed of light (apparently this was a valid way to "cheat" according to the calculations). It was that the amount of energy required to maintain such a bubble would be powerful enough to instantly destroy any event causing this phenomenon in the first place (aka a warp drive). It was akin to being inside a extremely unstable black hole.
--- Journals are boring; Go to my web page instead
There is no epicenter like there is in an earthquake, but you can define any position at rest in the frame of the background radiation itself to be as good of an "epicenter" as any other for purposes of this discussion. The Earth doesn't meet this standard, because it's moving at a speed of 365 km/sec relative to the CMBR (hence the Doppler shift), but it probably doesn't matter.
Here is a good FAQ entry regarding the difference between the observable universe and the entire universe.
Scientific American this month has a article called the Misconceptions of the Big Bang this month (March 2005). This article attempts to correct the errors that even experienced scientist make about how the Big Bang theory works. Until I read this article I was somewhat skeptical of BBT because the misinformation out there causes the theory to make little sense.
oops, forgot the link: SCIAM.
You mean, why isn't the expansion unmeasurable because our measuring sticks are expanding with space? See this FAQ.