Gravity acting as a repulsive forces at long distances

[ashwin]

Hi, I am currently an A Level student and we today begun to discuss fields. We were discussing the four different types of forces and my teacher asked me to name one difference between electromagnetic and gravitational forces. I said that gravitational forces are always attractive whereas EM can be either- this led onto the interesting discussion of a new theory that at long distances, gravity can be a repulsive force. He said that there has been evidence from a certain type of supernova which always explodes with the same intrinsic brightness, which can be used to show that the universe is actually expanding at an increasing rate!
Now from my previous readings on this subject, (which haven't been all that extensive) i had understood that gravity was always an attractive force caused by the exchange of gravitons between particles.
Can anyone help to explain possible explanations for this phenomena. There seems to be extensive material on the topic available, but none which is very succinct or easy to follow. Any light that you could shed on the matter would be greatly appreciated.
Thank you very much
-ashwin

 

[meteor]

He said that there has been evidence from a certain type of supernova which always explodes with the same intrinsic brightness

Supernovae type Ia can serve like standard candles (distance indicators), because the peak light output from such supernovae is always the same (is always equivalent to an absolute blue magnitude of -19.6). So you can use a formula to calculate the distance to them. The acceleration of the universe is not due to repulsive gravity, the culprit is some misterious energy, with the appropriate name of dark energy

However, some inflationary theories predict the existence of topological defects called domain walls, they separate the universe in a great number of cells, and is theorized that gravity is repulsive in its vicinity

 

[Phobos]

Welcome to Physics Forums, ashwin.

It's only been recently discovered (like what...in the past decade?) that the expansion of the universe is accelerating. This has been attributed to "dark energy" (has a few other aliases) but it is still unknown what dark energy is.

Oh, and don't hang your hat on gravitons...they are not known to exist. Gravitons are one attempt (albeit an interesting quantum mechanical attempt) to explain what gravity is. But so far, gravity is still best described (I think) by General Relativity.

Regarding those "standard candles", it was determined that the expansion of the universe is accelerating based on findings that the distant candles were dimmer than expected (more distant than expected per what the predicted expansion rate was at the time).

Another answer for your teacher...gravity is a LOT weaker than EM.

 

[Mr. Robin Parsons]

Originally posted by Phobos
(SNIP) Another answer for your teacher...gravity is a LOT weaker than EM. (SNoP)

Usually, yes, but not always, Black holes show gravity can out-do EM(r) no...well..."evidence"...(Means the dark area that surrounds a BB, No EMR there)

As for the "dark energy" perhaps it is simply a hole in 'spacetime', no one knows yet that it is, or isn't, that...so,

 

[avemt1]

https://www.physicsforums.com/showthread.php?s=&threadid=12226

This is one of my theory sites, and it has a link to the scientific american artice 'OUT OF THE DARKNESS' I am sure that this would be very healpful to you. If you have any more questions my e-mail is avemt1@yahoo.com

I am 18 years old now and am in my senior year of high school. If you need an extensive explanation of the theory i can email or mail you articles in chronological sequence to your address.
https://www.physicsforums.com/showthread.php?s=&threadid=12226