String theory can now describe a universe that has dark energy?
quantamagazine.org48 points by nsoonhui 4 hours ago
48 points by nsoonhui 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.
Side note, there’s been a few recent publications showing that dark energy may not be needed to explain what we are seeing.
1. Inhomogeneity backreaction (Moffat 2025) Large-scale cosmic inhomogeneities such as voids and dense regions can create an effective expansion history that mimics evolving dark energy when averaged using standard homogeneous assumptions. https://arxiv.org/abs/2503.20912
2. Timescape cosmology (Wiltshire) Because cosmic voids expand faster than dense regions and dominate volume at late times, observers may infer acceleration from redshift data even if the universe is not globally accelerating. https://www.livescience.com/physics-mathematics/dark-energy/...
3. Local giant void hypothesis If the Milky Way resides inside a large underdense region, locally measured redshifts and distances can bias expansion measurements and partially explain apparent acceleration and Hubble tension. https://www.livescience.com/space/cosmology/echoes-from-the-...
4. Void universe models (LTB cosmologies) Placing the observer near the center of a large cosmic void can reproduce supernova redshift–distance relations without dark energy, though such models struggle with other cosmological constraints. https://arxiv.org/abs/0807.1443
5. Structure formation and virialisation effects The growth of cosmic structure and entropy production alters averaged expansion rates, potentially generating an apparent dark-energy-like signal without introducing a new energy component. https://www.aanda.org/articles/aa/full_html/2024/09/aa50818-...
6. Redshift drift as a discriminator Measuring how cosmological redshifts change over time can distinguish true cosmic acceleration from redshift effects caused by voids or inhomogeneous expansion. https://arxiv.org/abs/1010.0091
I don't know who wrote the title for this submission, but adding a question mark that is not in the linked article seems like a terrible editorial decision.
Agree, its editorialising and not allowed under the guidelines here (unless it was in the original and that was changed), but given the uselessness of the field you could argue that any "String Theory" claim in any title should have an automatic question mark (or perhaps several) attached afterwards.
> given the uselessness of the field
It's not useless, though. String theory can be a fad (or "difficult to prove", per Witten) but some of the mathematics used in its research or "trying to prove it" have been used in other fields.
String theory does not work with de-Sitter spaces, only with anti de-Sitter spaces. Science has proved we are living in a de-Sitter space. String theory cannot be true.
That is literally what this article is about.
> “What they have found is a 5D de Sitter solution, and we don’t live in 5D,” said > Antonio Padilla(opens a new tab) of the University of Nottingham.
> Still, the work is expected to launch a new era in matching the mathematical > elegance of string theory to the actual world we live in."
yeah, sounds real promising. string theory all over. nice maths but who cares if it doesnt map to reality, its nice maths!
Which would be fine if they were calling themselves mathematicians, we can debate if their ideas are more/less worthy of funding vs all the other mathematicians working on interesting math that might or might not be useful. However when they call themselves physicists we demand they prove they are creating useful physics. There are other areas of study in Physics that are producing results and thus seem more worthy of funding.
Remember resources are limited. We cannot fund everyone who wants it. Society needs to make choices, we are generally okay with a bit of "interesting but unlikely to produce anything important", but most of what we fund needs a return on investment.
The prediction is just 105 orders of magnitude (and an extra dimension) away, but ok.
Hm, string theory can describe a lot of things, but it's not testable with current technology. I'm pretty sure that other mathematical constructs exist that could also describe a similar set of properties, but we just happened to stumble upon string theory first, and got enamored with some of the nice properties it had initially.
Here is the latest and in my opinion the best interview with Ed Witten [1]
Things he talks about go mostly over my head. What disappointed me a little bit is that he seems to be a materialist. But that is pretty common position among physicists anyway, so not that surprising.
If materialists disappoint you, then you should check out Deepak Chopra, for all your self affirming quantum woo needs and desires. He will make your dreams come true! Just buy lots of his books, and you both will be very happy.
I'm sad to discover that wisdomofchopra is no more.
We have llms now so no need for another bullshit generator.
Robots will need to become fat and human before they can ever generate Deepak Chopra Quality Woo!
https://www.youtube.com/watch?v=aO2dPIdEaR4
"But as Deepak Chopra taught us, quantum physics means anything can happen at any time for no reason. Also, eat plenty of oatmeal, and animals never had a war! Who's the real animals?" -Professor Hubert Farnsworth
Sounds like overfitting.
Yeah I am awaiting the Copernican Revolution of quantum mechanics before I care about this stuff.
I personally have no practical application, so it does me no good to learn this stuff that will be obsolete sooner or later.
Only in universe with 5 dimensions. Shouldn't string theory be given up on at this point? This theory has existed for over 50 years and hasn't produced any results. Even the predictions made by it such as e.g. supersymmetry have not been confirmed despite searching for them at particle colliders.
As I understand it, it's still our best candidate for a unified theory of everything. Not for lack of effort in researching alternatives, either.
It has produced some good math though. That's something.
At what opportunity cost?
If you want to bash badly-spent potential look at people doing cutting edge ad research and optimization, or HFT. This is at least good base research that others can build on.
String theory has generated a lot of hype over the years, but never delivered anything. Looks to me like they are all the negatives you hate about ad research.
Fair point, but waste in one domain should not be used to excuse waste elsewhere. Unless your argument is that it's generally hard for human societies to know where to best invest their scientific talent without the benefit of hindsight.
Think of it as a playground for the exercise and training of a pool of minds that will one day either make the glove fit or kick the sand castle over replacing it with a better mousetrap.
Too many metaphors? Hmmm, maybe fold in some dimensional reduction somehow.
human societies don't decide where to invest their talent; individuals do
I've worked in academia. How grants are won and research is received and encouraged is not an individual decision.
I agree plus ST takes a person who would have researched somewhere else. The Googler or Jane Street or guy who decides to travel the world in the canoe have different reasons and probably would need way more persuading to be in academia.
100% unknown. That's always the problem with science and why political control and direction always backfires and is stupid.
Shouldn't string theory be given up on at this point?
Anyone who is anti string theory actually qualified to make statements saying string theory is wrong or not worth more investment from researchers?
Are these anti string theory posts on HN mostly just laymen hearing how string theory can’t be tested and we wasted a lot of resources on it so it needs to be repeated on every string theory post here?
Susskind himself said its dead in the water. We dont live in an anti de-Sitter space, the basis of string theory.
They had to keep the theory alive somehow.
Yes. Like Python can describe any turing machine.