Comment by barishnamazov
4 hours ago
I foolishly sat in 8.821 [0] while at MIT thinking I could make sense out of quantum gravity. Most of the math went over my head, but the way I understand this paper, it’s basically a cosmic engineering fix for a geometry problem. Please correct me if necessary.
String theory usually prefers universes that want to crunch inwards (Anti-de Sitter space). Our universe, however, is accelerating outwards (Dark Energy).
To fix this, the authors are essentially creating a force balance. They have magnetic flux pushing the universe's extra dimensions outward (like inflating a tire), and they use the Casimir effect (quantum vacuum pressure) to pull them back inward.
When you balance those two opposing pressures, you get a stable system with a tiny bit of leftover energy. That "leftover" is the Dark Energy we observe.
You start with 11 dimensions (M-theory) and roll up 6 of them to get this 5D model. It sounds abstract, but for my engineer brain, it's helpful to think of that extra 5th dimension not as a "place" you can visit, but as a hidden control loop. The forces fighting it out inside that 5th dimension are what generate the energy potential we perceive as Dark Energy in our 4D world. The authors stop at 5D here, but getting that control loop stable is the hardest part
The big observatiom here is that this balance isn't static -- it suggests Dark Energy gets weaker over time ("quintessence"). If the recent DESI data holds up, this specific string theory solution might actually fit the observational curve better than the standard model.
[0] https://ocw.mit.edu/courses/8-821-string-theory-and-holograp...
> we perceive as Dark Energy in our 4D world
This is a bit of a technicality, but we don't live in a 4D world, we live in a 3+1D world - the 3 spacial dimensions are interchangeable, but the 1 time-related dimension is not interchangeable with the other three (the metric is not commutative).
I'm bringing this up because a lot of people seem to think that time and space are completely unified in modern physics, and this is very much not the case.
To expand on this a little for those interested, time has properties space doesn't. For example, you can turn left to swap your forward direction for sideways in space. You cannot turn though, in a way that swaps your forward (as it were) direction in space for a backward direction in time.
Equally, cause always precedes effect. If time were exactly like space, you could bypass a cause to get to an effect, which would break the fundamental laws of physics as we know them.
There's obviously a lot more, but that's a couple of examples to hopefully help someone.
How is the difference between them characterised in physics?
It seems like it would be hard to distinguish from the point of view of a 4D unit vector XYZT if T was massively larger. Is it distinguished because it's special or is it just distinguished just because the ratio to the other values is large.
Imagine if at the big bang there was stuff that went off in Z and XY and T were tiny in comparison? What would that look like? Part of me says relativity would say there's no difference, but I only have a slightly clever layman's grasp of relativity.