Posted on 02/28/2005 6:29:00 PM PST by AntiGuv
WASHINGTON, D.C. - Scientists may not have to go over to the dark side to explain the fate of the universe. The theory that the accelerated expansion of the universe is caused by mysterious "dark energy" is being challenged by New York University physicist Georgi Dvali. He thinks there's just a gravity leak.
Scientists have known since the 1920s that the universe is expanding. In the late 1990s, they realized that it is expanding at an ever-increasing pace. At a loss to explain the stunning discovery, cosmologists blamed it on dark energy, a newly coined term to describe the mysterious antigravity force apparently pushing galaxies outward. This repulsive, unknown force is believed to make up more than 70 percent of the mass-energy budget of the universe. But the existence of dark energy is far from proven, and some researchers believe they and their colleagues simply don't understand gravity at larger scales. The gravitational pull between any two objects becomes less with distance. But in Dvali's view, it weakens more than standard theory predicts. Dvali would modify the theory of gravity so that the universe becomes self-accelerating, eliminating the need for dark energy. He presented his work here earlier this month at the annual meeting of the American Association for the Advancement of Science.
Dvali borrows from string theory, which states that there are extra, hidden dimensions beyond the four we are familiar with: three directions and time. String theory suggests that gravitons -- hypothetical elementary particles transmitting gravitational forces -- can escape to other dimensions. Dvali says this would cause "leaks" in gravity over cosmic proportions, reducing gravitational pull at larger distances more than expected. "The gravitons behave like sound in a metal sheet," says Dvali. "Hitting the sheet with a hammer creates a sound wave that travels along its surface. But the sound propagation is not exactly two-dimensional as part of the energy is lost into the surrounding air. Near the hammer, the loss of energy is small, but further away, it's more significant." The effect is to alter the space-time continuum, speeding up universal expansion. "Virtual gravitons exploit every possible route between the objects," Dvali said, "and the leakage opens up a huge number of multi-dimensional detours, which brings about a change in the law of gravity." The speeding up of the universe suggest that Einstein’s laws of General Relativity, describing the interaction of space and matter, must be modified at large cosmic distances. "It is this modification, and not dark energy, that is responsible for the accelerated expansion of the universe," Dvali concludes. The idea might be testable.
Gravity leakage should create minor deviations in the motion of planets and moons. Astronauts on the Apollo 11 mission installed mirrors on the lunar surface. By shooting lasers at the mirrors, a reflected beam can be monitored from Earth to measure tiny orbital fluctuations. Dvali said deviations in the Moon's path around Earth might reveal whether gravity is really leaking away. This article is part of SPACE.com's weekly Mystery Monday series.
Not true! Depends leak.
One could think of space as a miniature golf course. Any large object creates gravity, which then makes a "hole" which things could fall into. If you rolled out an object, it might keep going forever, unless it encountered a hole. We're sitting at the bottom of the Earth's hole, which is why it's so hard to launch things into space (we have to fling rockets out first before they can go anywhere).
When we sent out the Voyager probes, they never encountered any holes (they never got trapped by a planet's gravity and fell to the surface), so they're still going, somewhere out there. When we sent out the Apollo capsules, we flung them at the big gravity hole of the Moon, and they successfully went down it. (And came back up, and then returned and went down the Earth's hole.)
First, it was black holes were keeping the universe from flying apart. Lately, it is an invisible dark matter. Now gravity must be leaking. I can’t believe this stupidity to justify a flawed theory of an expanding universe. As our deep space vehicles have shown, there is some unknown phenomenon that is slowing them down. Isn’t obvious that the same degradation happens to light. Older the light, farther it will have slowed and shifted toward the red. The universe is not expanding, light (and other radiate energy) shift to lower frequencies with time until they blend with the background radiation (do photons have a half-life of 12 billion years?). The universe is supposed to be 15 billion years old yet we can see well-defined galaxies 12 billions years old in any direction we look. Then there is nothing for 14 billion years from sources that we cannot even define as proto galaxies. How long does it take for a galaxy to form? Probably longer than 3 billion years for that much matter to gather (unless the gravity constant had changed in those early years). Is Hubble such an icon in the astronomy community that the red shift can never be questioned even when new empirical evidence gives another explanation?
leaking gravity?? don't they have...um... pads for that?
Leaking gravity? No wonder. I've been trying to squeegie whatever this stuff is, all day.
Good one.
"It's a fudge factor," says Nieto. "And there's a fudge factor in every galaxy." ...[Stacy] Milgrom, a 57-year-old physicist at the Weizmann Institute in Rehovot, Israel, argues that a fudge factor like dark matter isn't necessary if physicists make just one small tweak to Newton's laws of gravity... Milgrom proposed that Newton's laws might change at these paltry accelerations. Below a transition acceleration equal to one 10-billionth of a meter per second per second... the force of gravity might no longer be directly proportional to acceleration, as decreed by Newton. -- Tim Folger, "Nailing Down Gravity", pp 36-38
All I know is that gravity is my enemy these days.
My research advisor here at the U. of Tennessee is actually working on a satellite that will test this. It will be able to see if the inverse square law is really an inverse square over a range of a few meters.
Nifty stuff!
Something's wrong with the space-time continuum too. The more time goes by, the more space I take up.
Hmmm... but then shouldn't some "virtual gravitons" also be leaking back into our dimensions? Perhaps there's a constant equilibrium ratio between the amounts of gravitons going in either direction. But OTOH, what if some local phenomenon can change that ratio? We should then be able to detect small variations at those locations.
No liability insurance.
The idea, I think, is that gravity actually works over more than the three spatial dimensions. It is different from the other three forces somehow in that it is roughly inverse square. During the Big Bang at high temperature and pressure and very small dimensions, all four forces were one, but as things cooled there was a phase change and these different behaviors began.
These threads always amaze me. I think I understand things and then WHAM!! upside the head.
Just how does gravity leak - could it be evidence for my theory of particulate gravity? (Actually I'm looking for evidence of particulate time, too. It would help with my time machine).
And where is it leaking to? And why aren't they leaking back too.
Hmmmmm. I've seen a lot of leaks on babies and gravity certainly plays a role.
Yes, there are some anomalies in the data. Something is going on with the Voyagers. That needs explaining.
Can't find it--it's too far away.
...
I suppose you're entitled to a serious answer after this much kidding around; here's what a first year physics student might tell you: The force pulling toward the earth, due to gravitational acceleration, is more or less exactly matched by the force pulling away from the earth due to angular acceleration (the force that makes a stone on a string fly away from you when twirl it over your head and let go). So, since the forces are matched, the moon isn't inclined to go either up or down.
You're pretty close to what led Einstein toward the unified field theory--the notion that force due to acceleration ultimately comes from the same place as force due to gravity. If Einstein were not right, it would be a pretty remarkable coincidence that when gravity and acceleration were in balance, that happens to be the exact recipe for producing stable orbits.
The more familiar statement of this coincidence is the famous elevator question: why, when you drop an elevator from high in an airless sky, while it is falling, everyone inside experiences exactly 0 gravity?
Personally, I think it has something to do with angel dust.
Which begs the question of the effects of that leaked gravity in the other dimensions. It also makes for interesting thought experiments on the possibilities of increasing or reducing said leakage.
In the Star Trek universe, yes. In the real world, no. Newton's Law (one of them): An object in motion remains in motion; an object at rest remains at rest -- unless acted upon by an outside force. If the bird was in space, travelling without outside interference (like gravity), it would remain moving at its same speed and in the same direction forever.
If, however, you launched your bird from Earth into space, gravity would eventually overcome the forward momentum you applied to the critter, and it would fall back down -- unless you launched it with enough force to escape the effects of gravity. This is called "escape velocity" and on Earth it is on the order of about 11,000 meters (~7 miles) per second.
Technically, because it's falling toward the Earth and missing.
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