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### Author Topic: Could the accelerated expansion of the universe be due to an approaching brane?  (Read 2228 times)

#### DoctorBeaver

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##### Could the accelerated expansion of the universe be due to an approaching brane?
« on: 29/07/2008 19:52:02 »
In the book I've just been reading, the author writes about branes in a higher dimensional bulk. Our universe is located on 1 of these branes, but there are others. Apparently there is no reason to think that particles and forces on other branes are the same as on our brane.

One theory I've seen for the weakness of gravity compared to the other known forces is that it originates on another brane and decreases in strength as the inverse of the exponential of the distance between the "gravity brane" and our brane; this being due to the decreasing probability of finding a graviton as distance from the gravity brane increases.

My little beaver brain mulled those 2 things over and spat this out...

If there is another brane approaching ours through the bulk that houses a force that is not natural to our brane, and its strength works similar to gravity (i.e. its strength decreases as the inverse of the exponential of the distance between the branes), then as our branes approach one another, the strength of that force would increasingly affect us. Let's say that this force is a type of anti-gravity.

At large separation, there would be very little increase in strength as the branes approach; but the closer they get, the more quickly the strength of the force increases (remember, it's exponential). That means that we would increasingly notice the effect of this anti-gravity.

Isn't that what we are seeing with the accelerating expansion of the universe?
« Last Edit: 29/07/2008 19:55:21 by DoctorBeaver »

#### LeeE

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##### Could the accelerated expansion of the universe be due to an approaching brane?
« Reply #1 on: 30/07/2008 00:09:04 »
The idea of another approaching brane affecting our brane seems plausible enough but I wouldn't be so happy to accept that the other brane is likely to be very different to our one.

If both branes exist within the bulk it seems to me that the branes must share some dimensions of the bulk, like a plane intersecting a solid.  If this is so, and even if we assume that the bulk is non-homogeneous i.e. not smooth, then as the two branes approach each other they should become more and more alike.  Also, at some point the approaching brane must pass through the gravity brane (unless it's always been between us), at which point it should have been identical to the gravity brane.  That is unless the branes have additional properties that are outside the bulk, and effectively outside the universe.

Perhaps we could imagine a similar effect by having the two branes stationary with respect to each other, but both moving through the bulk together - imagine two parallel planes moving along the axis of a cone (away from the apex).  The expansion could then be attributed to the 'widening' of the bulk (cone) whilst not affecting gravity (which is dependent on the distance along the axis to the other brane).  This would suggest that the bulk is homogeneous though (unless the laws of physics have actually changed over time), and then begs the question: how and why is the other brane different to ours, if it's only just further back down the bulk in a region that we have already passed through?

#### DoctorBeaver

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##### Could the accelerated expansion of the universe be due to an approaching brane?
« Reply #2 on: 30/07/2008 08:05:27 »

If both branes exist within the bulk it seems to me that the branes must share some dimensions of the bulk, like a plane intersecting a solid.  If this is so, and even if we assume that the bulk is non-homogeneous i.e. not smooth, then as the two branes approach each other they should become more and more alike.

Not necessarily. Particles & forces could be localised or sequestered. They could either be confined to the brane on which they originate, or could not travel far from it, and, in the case of localisation, the strength of the forces would decrease exponentially as you moved away.

They wouldn't be "sharing" the same bulk dimensions, they both exist within it. Particles & forces that are confined to the branes would be unaware of the bulk (unless they are affected by any particles or forces that travel through the bulk).

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Also, at some point the approaching brane must pass through the gravity brane (unless it's always been between us), at which point it should have been identical to the gravity brane.

It could be approaching us from the other side.

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and then begs the question: how and why is the other brane different to ours, if it's only just further back down the bulk in a region that we have already passed through?

There has been a lot of speculation that different realms (or branes) could have different properties. The anthropic principle suggests just that.
« Last Edit: 30/07/2008 08:11:33 by DoctorBeaver »

#### LeeE

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##### Could the accelerated expansion of the universe be due to an approaching brane?
« Reply #3 on: 30/07/2008 23:29:08 »
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Particles & forces could be localised or sequestered. They could either be confined to the brane on which they originate, or could not travel far from it, and, in the case of localisation, the strength of the forces would decrease exponentially as you moved away.

In the RS scheme, I believe that the branes are four-dimensional spacetimes and the bulk is a five dimensional spacetime.  Now while it sounds reasonable for different forces and particles to exist in two different branes, it seems to me that because the two branes both exist within the same bulk they should be similar unless the nature of the bulk is different at the two locations occupied by the two branes.

For example (using arbitrary qualities), if the region of the bulk where our brane is located is 'apple' but the region of the bulk where the gravity brane is located is 'orange' then there would be differences between the two branes and the forces and particles that can exist within them.  However, if another brane is moving through the bulk towards us from the 'banana' region, it would start with 'banana' properties, and 'banana' forces and particles but they would become 'appley' as it got closer to us and moved towards the 'apple' region.

Ok - much smarter people than me have come up with the stuff that you're pointing out but I just can't see how the branes are not sub-sets of the bulk and so therefore must inherit their properties from it.  If the bulk is non-homogeneous and the branes are stationary, then it all seems fine, but if branes are moving through a non-homogeneous bulk then it seems to me that their properties must change as they move.

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##### Could the accelerated expansion of the universe be due to an approaching brane?
« Reply #3 on: 30/07/2008 23:29:08 »