Research suggests Big Bang may have taken place inside a black hole
595 comments
·June 11, 2025carbocation
n2d4
Yea, and it was a great read too. I wish more researchers would publish blog posts alongside their technical whitepapers, although I acknowledge that not everyone involved in science has or wishes to acquire the skills needed to write blog-form content.
(I'd also be worried about a world where researchers are evaluated based on the virality of their blog posts, vs. how impactful their work was.)
rendaw
Communication skills are often missing in engineering too, but I think I'd argue they should be required - all work is fundamentally collaborative.
Being able to effectively communicate to different people on your team, outside your team, managers, business people, etc is not optional and more than once I've seen things get stalled or turn into a mess because communication didn't happen.
STEM is often a haven for neurodivergence but I think communication skills are something that is largely learned and not something that comes naturally for everyone. People who are good at communicating spend a fair amount of effort rewriting, trying different wordings, different introductions, getting feedback from people, etc.
FWIW I see things like being able to sell a proposal, managing expenses, planning, etc as optional - these are good to have, but someone else can do them if you can communicate well, but in the end the only person who can communicate what you're thinking is you.
jasonm23
"Required" is a bit of a gatekeeper, while I agree good communication skills are valuable.
Blog form content in particular, _requires_ proofing, re-editing, and so on and there's a whole skill set which contributes to makes such content sticky and engaging.
You also seem to be confounding your own point. Indeed all work is collaborative, someone who lacks communication skills, will generally team up with other collaborators who can bring those skills to bear.
graemep
I think the benefits greatly outweigh any dangers. I far prefer to read something like this than something written up by a journalist.
> I acknowledge that not everyone involved in science has or wishes to acquire the skills needed to write blog-form content.
They should. If your research is publicly funded you should make it as available to be public as possible. Academics should be able to communicate, and I very much doubt they are unable to acquire the skills
> I'd also be worried about a world where researchers are evaluated based on the virality of their blog posts, vs. how impactful their work was
Given how bad the measures of impact and the distorted incentives this produces I am not even sure this would even be a bad thing.
If nothing else it improves transparency about what they are doing, again with public money.
quantum_magpie
>They should. If your research is publicly funded you should make it as available to be public as possible. Academics should be able to communicate, and I very much doubt they are unable to acquire the skills
So in addition to being:
-professional researchers
-professional teachers
-professional project managers
-professional budget specialists
-professional scientific writers
-a failed idea away from losing it all
They should also become:
-professional PR managers
-professional popular writers
While still being paid (poorly) for a single job of all of these.
gms7777
A few years ago, at least in my field, there was definitely a trend of people at least doing twitter threads explaining the key findings of their papers. It's obviously less in-depth than a blog post would be, but it was still usually a far more accessible version of the key ideas. Unfortunately, this community has basically dissolved in the last few years due to the changes in twitter and to my knowledge hasn't really converged on a new home.
pkaodev
[flagged]
bdbenton
It's a controversial observation, but it is very true. I work with AI models and have to read recently published research to work with the latest developments in the field.
Do a quick keyword search on papers related to the subject. So much of it is completely useless. It is clearly written to keep people busy, earn credentials, boost credibility. Papers on the most superfluous and tangential subjects just to have a paper to publish.
Very little of it is actually working with the meat of the matter: The core logic and mathematics. It is trend following and busywork. Your sentiment is controversial because people are religiously loyal to the intellectual authorities of these credentialed systems, but a lot of published research does not push any boundaries or discover anything new. This paper seems to be an exception.
I would argue that a lot of the research published in the social sciences also falls under this category. It is there so that someone has a job. I'm not discrediting social sciences in general, am just pointing out that there is a lot of ways to creatively take advantage of academia to secure a paycheck and this is certainly exploited. The kneejerk reaction to reasonable criticism just proves this point even further.
tracerbulletx
This is a good thing. This is where the economy surplus went. Not to 5 days of leisure for everyone. But to jobs that keep us occupied, engaged, and motivated but aren't strictly required. The alternative is just either starving everyone to death, except for a few elite and their slaves, or everyone being bored out of their minds and wondering what the point of life is.
4ndrewl
Can you cite your sources please?
torial
There used to be a common practice of scientists writing summaries of their research for lay people. I think they viewed it as their civic duty. I had a collection called the World of Physics which included essays written by various scientists. I originally had it in the 90s and found it again after many decades. Would highly recommend.
https://www.amazon.com/World-Physics-Library-Literature-Anti...
beloch
It's far preferable to having university PR people write some hype piece. Where they'd spend the whole time gushing about it being a world first, paradigm shifting, blah blah blah, the author focuses on things that actually matter. e.g. Is it testable? Yes, here's what to look for.
fracus
Yeah, wow. That was great. His solution seems so simple and clears all the previous model's problems. I guess every black hole could contain its own universe.
NKosmatos
Too bad the author didn’t explain more the concept of the “parent” universe and how our own (contracting & expanding) universe got created. Nice things to read/consider/ponder late at night :-)
csours
Unfortunately, it appears that the universe does not care very much about human satisfaction. Fortunately, other humans do.
alkyon
I would be surprised if the size doesn't matter in this case. On the one hand, tiny black holes tend to be rather short-lived. On the other, I suppose some threshold mass/energy is needed to generate a child universe that doesn't collapse immediately.
raxxorraxor
Ironically that was basically the first thought many had when it was clear we cannot explain what happens in the edge case of a singularity. It was always "perhaps another unsiverse or a way into a parallel one".
It still leaves a lot of questions though, especially if you try to marry quantum mechanics to these makroscopic models. Where did the initial black hole come from and should a corresponsing anti matter black hole exist?
oreilles
Well that's an indsight bias if I've seen one. This is the first time I ever read that the "bottom" of a black hole could be a entirely new universe. If there ever "always was" a common hypothesis, it was the wormhole.
jarend
The article is based on a physics paper (arXiv:2505.23877), not management theory or institutional metaphors.
What the paper actually proposes is that the Big Bang may have been a gravitational bounce inside a black hole formed in a higher-dimensional parent universe. Quantum degeneracy pressure stops the collapse before a singularity forms. From the outside, it looks like a black hole. From the inside, it evolves as a 13.8 billion year expansion. That is general relativity applied across frames.
Simply put this is a relativistic collapse model with quantum corrections that avoids singularities and produces testable predictions, including small negative curvature and a natural inflation-like phase.
leiroigh
>in a higher-dimensional parent universe
That's incorrect: The parent universe is not higher-dimensional, it's the same good old 3+1 as our universe.
What they propose is: Let's take our good old GR, and start with a (large, dilute) compactly supported spherically collapsing collapsing cloud of matter. During that, you get an event horizon; afterwards, this looks like a normal black hole outside, and you never see the internal evolution again ("frozen star", it's an event horizon). Inside, you have the matter cloud, then a large shell of vacuum, then the event horizon.
Quantum mechanics suggests that degeneracy pressure gives you an equation of state that looks like "dilute = dust" first, and at some point "oh no, incompressible".
They figure out that under various assumptions (and I think approximations), they get a solution where the inside bounces due to the degeneracy pressure. Viewed from inside, they identify that there should be an apparent cosmological constant, with the cosmological horizon somehow (?) corresponding to the BH horizon as viewed from the outside.
All along the article, they plug in various rough numbers, and they claim that our observed universe (with its scale, mass, age, apparent cosmological constant, etc) is compatible with this mechanism, even hand-waving at pertubations and CMB an-isotropies.
This would be super cool if it worked!
But I'm not convinced that the model truly works (internally) yet, too much hand-waving. And the matching to our real observed universe is also not yet convincing (to me). That being said, I'm out of the cosmology game for some years, and I'm a mathematician, not a physicist, so take my view with a generous helping of salt.
(I'm commenting from "reading" the arxiv preprint, but from not following all computations and references)
PS. I think that they also don't comment on stability near the bounce. But I think that regime is known to have BKL-style anisotropic instability. Now it may be that with the right parameters, the bounce occurs before these can rear their heads, and it might even be that I missed that they or one of their references argue that this is the case if you plug in numbers matched to our observed universe.
But the model would still be amazing if it all worked out, even if it was unstable.
mr_toad
> with the cosmological horizon somehow (?) corresponding to the BH horizon as viewed from the outside.
That’s not mentioned in the summary. After inflation the event horizon would not exist.
Agentlien
> What the paper actually proposes [...]
(Emphasis mine)
I haven't read the paper yet, but this sounds like a (good) summary of exactly what the article is saying. It makes me wonder what, if anything, you feel is different from the way you put it and the way it is explained in the article? As a layman they seem the same to me.
Voloskaya
The article was written by the main author of the paper, so yes, it's a good summary :)
Agentlien
I meant that the parent comment to mine was a good summary of the article.
However, the comment was worded as if it meant to highlight some difference between how the article summarized the paper and what the paper is actually saying. Since I couldn't see a difference between the above poster's summary and that in the article, I was curious what I was missing.
empiricus
Looking at the paper, I don't see any higher dimensions of the parent universe, it is still using the same 4D General relativity framework for the parent.
ASalazarMX
So, could the same interaction create planar universes inside our own black holes? Linear universes inside those as well?
It's incredible how big a 4-D universe would have to be to contain our own, even crazier if there are more levels; but our own universe could contain easily uncountable planar universes.
dleeftink
Isn't it more a matter of how space is folded in higher dimensions rather than an increase in volume that accounts for containment? There is plenty of space in the corners:
[0]: https://observablehq.com/@tophtucker/theres-plenty-of-room-i...
potamic
They have basically disproved Penrose-Hawking's theories of singularity? Isn't that like a pretty big deal? To people working in this field, what is the reaction to this paper?
null
eggn00dles
seems like this is just giving up on quantum gravity and saying the pauli exclusion principle will hold regardless of the gravitational force.
kosh2
I have two problems / questions with this:
1. This theory requires a parent universe that can't have been formed inside a black hole. This means there must a be second "universe creation" mechanism that we can / may never know about from our child universe. For me, this doesn't really answer the true question: "How did our universe begin?" Yeah, it may the "unknown field with strange properties" but instead we get an unknown parent universe with strange properties.
2. The black hole in the parent universe must be much much bigger than anything we see in ours since it has to contain all the matter that we see. How is a black hole supposed to form that is 750 billion times bigger than the largest black hole we know about?
PaulHoule
I don't see this idea as very new.
There are many models of black holes, such as the Schwarzchild solution, that have an area of "asymptotically flat spacetime" which is, from the viewpoint of our universe, part of the black hole. That something happens around the singularity that creates this new universe doesn't sound that crazy.
If our universe is a child of another universe and that is a child of another universe and so forth it fits into the kind of "multiverse" model that addresses issues such as "why does the universe have the parameters it does?" Either there are a huge amount of universes such that we're lucky to be in one we can live in, or there is some kind of natural selection such that universes that create more black holes have more children.
As for the relative size of the parent black hole, conservation of energy doesn't have to hold for universes in the normal sense. One idea is that the gravitational binding energy of the universe is equal to the opposite of all the mass in the universe such that it all adds up to zero so we could have more or less of it without violating anything.
corry
Do you find the idea of an infinite regress -- "our universe is a child of another universe and that is a child of another universe and so forth" -- holds much explanatory power for you?
To me it's prima facie a hollow explanation. I get that some models, like eternal inflation or certain cyclic cosmologies, entertain the idea of an infinite past or blur the standard arrow of time... but how does pushing the origin question back indefinitely actually resolve anything?
blamestross
The problem is that:
- We have a parent universe we will never be able to observe.
could be a true statement.
The "infinite sequence" part is just a likely implication, it isn't necessarily true. We would need information we can't access to find out.
lugu
I doubt you understand what science is about. The proposed theory, like any theory, should be judged on its power of prediction and simplicity. It doesn't matter if it doesn't satisfy your curiosity.
D-Coder
> we're lucky to be in one we can live in
Nitpick: We couldn't be anywhere else, except nonexistent.
thomasweiser
The anthropic principle
nurettin
> "why does the universe have the parameters it does?"
To those who say "oh but if this parameter was slightly off, that thing I subjectively decided to pick wouldn't have happened!":
How would you know that this universe could exist in any other way? Wouldn't things just stabilize into certain frequencies and lengths after some time?
To me "fine tuning" isn't really a conundrum, it is just question begging and you don't need to wish it away with multiverses.
godelski
> requires a parent universe
Does that repeat forever? Does it lose energy in the bounce? If so, to where and how?
> The black hole in the parent universe must be much much bigger than anything we see in ours
Most importantly, you don't need everything all figured out at once to publish. Then no one would always publish. There'd be nothing to improve on. Only one publication that says everything. Till then, everything does have criticisms and is incomplete. It's good to have criticisms! They lead you to the next work!
mcswell
>> The black hole in the parent universe must be much much bigger than >> anything we see in ours
>> Yes and no. You're not thinking about contraction. With relativity >> we can fit a 100ft ladder inside a 10ft barn.
I believe the OP was talking about mass, not linear dimension. (And if he wasn't, I am.) Unless somehow mass inside a black hole is not constant? (ignoring accretion)
godelski
Relativity applies to mass too. Accelerate and you become heavier.
Remember, mathematically, a blackhole is mass in an infinitely small point. You are dividing by 0. I don't know the answer, but if someone is saying that from the outside the apparent mass is different than from the inside, that doesn't set off any alarm bells. We literally are talking about Dr Who style "it's bigger on the inside". Even the ladder example should make you think about mass. Without relativistic effects the mass inside the barn is only part of the ladder. With relativity, the whole ladder, and thus mass, is inside. So yeah, weird things happen.
2OEH8eoCRo0
Where does the information of the previous universe before the bounce go? Is it destroyed?
blamestross
Cosmic Background Radiation distribution could be that information. The distribution of mass hitting the event horizon then bouncing
JKCalhoun
It's been suggested it is gone and that perhaps even new laws of physics are created with each iteration (but I don't know why that would be).
mordae
Maybe we live inside an universal hash function.
mr_toad
The outer universe could have always existed, but unlike ours it eventually collapsed. By contrast ours did the reverse, and it looks like it will expand forever. There is a neat symmetry. I guess you could make the case that it’s really just one universe, and the collapse and expansion mirror each other.
meowky
1. It is possible that every universe is formed in a blackhole – an infinite universe-blackhole-universe chain. We don’t know what “infinity” means in this scenario, so we can’t simply rule it out. For comparison, Aristotle ruled out an infinite chain of causes, which we now know (with the help of hindsight, of course) is a flawed conclusion.
2. We don’t know whether our universe is big or small compared with other universes. We don’t know whether, or how, it makes sense to compare sizes between universes.
Big Bang is arguably the biggest speculation in modern science.
nbulka
We think the universe had to "begin" because we "began" and tend to anthropomorphize. Is that necessarily true? The universe is under no obligation to have a beginning. Sail around the Earth and you might just end up right where you started.
jungturk
Yes, but earth still had a beginning.
I agree with you, though - causal explanations are compelling and confer a sense of certainty and humans seem to like that, but it doesn't make them necessary.
mc32
The Sun had to begin. At one point it was just accreting gasses, then at some point gained enough mass to ignite. People also start at some point they begin as a daughter and grow eventually into a viable life. But also our galaxies had to form before our sun. So, yes there are beginnings to things. At one point they weren’t, at another point they were.
bagacrap
Wouldn't every theory/model of the universe leave room for follow up questions? Why is it problematic if it doesn't answer literally every conceivable quandary?
bhk
It's black holes all the way down!
molticrystal
If the crux of the article is the fermion bounce, and you compare that to how much matter and energy we are aware of, that is quite the black hole, which leads one to start wondering what environment it existed in to become that size. Even if it is now stuck due to a positive curvature of just bouncing back and forth.
I would like the article to acknowledge a bit more though that blackhole universe theories and speculation are quite old now, not radical and a striking alternative, as it is natural to think about it once you learn of the concept of event horizons. What differentiates this though is the analytical solution.
null
afarah1
Interesting read, but even if we assume the author is correct, and the cosmos formed as a black hole in a larger universe, the question remains, how did this larger universe formed, then? Might just be impossible to know.
randomtoast
Questions like what was before the big bang or what is outside of our universe seem to be quite natural. However, we still don't know if these questions are well defined and have a proper meaning. For instance, a few hundred years ago, one might have asked, what happens if I go to the edge of the (flat) earth? Or one might ask: What is north of the north pole?
helsinki
Thanks, GPT 4.1. It told me the same thing twelve hours ago when I asked it what was at the top. “what’s north of the North Pole”?
randomtoast
It is well possible that GPT-4.1 references Sean Carroll, either directly or by regurgitation.
> One sometimes hears the claim that the Big Bang was the beginning of both time and space; that to ask about spacetime “before the Big Bang” is like asking about land “north of the North Pole.”
Source: https://www.preposterousuniverse.com/writings/dtung/
I'm a regular listener of his Mindscape podcast, and that's where I got this phrase. I can highly recommend his podcast: https://www.preposterousuniverse.com/podcast/
timonofathens
> What is north of the north pole?
I really like this analogy for "what is outside of our universe", thank you
layer8
Maybe the larger universe is identical to the contained universe, like a fractal. That would solve the question. ;)
downboots
Might I suggest Brouwer's theorem while we figure it out
joshdavham
Could you elaborate? It's been a while since I've done any real analysis/etc.
mikrl
Then we gotta find the black hole in our universe that contains that universe, and nuke it before they come to take our fluids!
skeaker
Might not be the best idea, unless we just so happen to be all the way at the top of the sequence[1]...
revskill
Why selfish ???
teaearlgraycold
Would be fun if we find a function f(state, time) such that for f(singularity, 14 billion years) we get our current universe. i.e.: every singularity turns into our exact universe.
unsupp0rted
Implying there’s no such thing as randomness, at any level?
conradev
This theory is in the same space:
https://en.wikipedia.org/wiki/Cosmological_natural_selection
I don’t think it has a hypothesis for the origin, though
phatskat
See also the recent HN discussion about Blowtorch Theory, which has roots in (but doesn’t necessitate) CNS
conradev
building on ten years of earlier research for a book on cosmological natural selection
I loved this overview on our current approaches to measure the expansion: https://youtu.be/WNyY1ZYSzoU
postalrat
That's a very 3D way to think of a universe.
null
PUSH_AX
Just casually adding the biggest question its possible to ask
dboreham
It's just recursion in the simulator.
meindnoch
I've read somewhere an article which posited that our 3D universe might be inside a 4D black hole. When you cross a black hole's event horizon, the radial coordinate becomes timelike, so you lose one degree of spatial freedom. Movement is still possible in the tangential directions however, so what you get is basically an N-1 dimensional universe. So maybe our 3D universe is actually matter that fell into a 4D black hole, and our 3D black holes contain 2D flatland universes. And of course, the outer 4D universe might be in a 5D black hole, etc.
unyttigfjelltol
Yes, and then there's the parlor game of guessing what familiar property of our known universe is actually a spaghetified fourth dimension.
I guessed c once. It would be a constant. Maybe all the constants are spaghettified remains of a superior universe.
jfengel
I don't think c is a good candidate, because it's not really a parameter. It's just a correction factor for our mis-judgment in picking different units for time and space.
In "natural units", we define the units so that the important conversion factors (c, G, h-bar, etc) work out to exactly 1. You can say that c is one light-year per year and then forget about it.
The true parameters of the universe are the dimensionless constants: the fine structure constant, proton-electron mass ratio, 3+1 dimensions, etc.
throwawaymaths
> I don't think c is a good candidate, because it's not really a parameter.
dont be so sure! there is no way to experimentally know if c is a parameter or not. there are consistent physics formulations which have variable, even anisotropic c. physicists dont usually explore them (e.g. tangherlini relativity) though because the math is considerably harder.
jldl805
Gravity, obvs.
AtlasBarfed
I thought that's what the high dimension counts of string theories were: taking constants and turning them into dimensions.
Or is that too simplified?
codethief
> When you cross a black hole's event horizon, the radial coordinate becomes timelike, so you lose one degree of spatial freedom.
The second half is incorrect. Since the time coordinate becomes spacelike in turn you'll still have 3 spatial degrees of freedom. Dimensions can't just vanish if you believe that spacetime is a 4D Lorentzian manifold (as physicists do).
Moreover, the singularity is not a place you can poke with a stick, once you've entered the black hole. It lies in your future, in the same way as your death.
BoiledCabbage
> The second half is incorrect. Since the time coordinate becomes spacelike in turn you'll still have 3 spatial degrees of freedom. Dimensions can't just vanish if you believe that spacetime is a 4D Lorentzian manifold (as physicists do).
Can we say that one of the spatial dimensions (the radial dimension) and the time dimension combine into a single dimension? After crossing the event horizon aren't they 1:1 correlated?
raattgift
No, there's no change in dimensionality.
The swapping of timelike and radial dimensions are a "game" frequently played with families of coordinates, including Schwarzschild coordinates. One can apply any system of coordinates on a physical system without changing the behaviour of the physical system: coordinates are unphysical. Think of navigating around in a neighbourhood: you can talk about going forward a few blocks then turning left, after which you go forward two more blocks; or for the same journey, going "city north" a few blocks then going "city west" two blocks. Here assuming that (initially) "forward" is in the "city north" direction (and "city north" is not necessarily exactly magnetic north nor a section of a meridian of longitude). After the left turn, "forward" is "city west". There's an analogue to the discussion's (ab)use of Schwarzschild coordinates.
In Schwarzschild spacetime, without applying any system of coordinates, just floating in free-fall far from a black hole extremizes your travel in the timelike dimension. (You can do this at home: you stay put at some point on Earth (whether you use GPS latitude/longitude/altitude or some other system of coordinates) but your wristwatch keeps ticking). Inside the black hole horizon, just floating in free-fall extremizes your travel in the direction of the singularity. Far from the black hole, accelerating as strongly as you can in any direction takes travel from the timelike dimension and puts it into one or more spatial dimensions. In particular, you have the freedom to increase the spacetime interval between you and the singularity. Within the horizon, however strongly you accelerate the spacetime interval between you and the singularity shrinks. This behaviour seems to invite the use a different set of coordinates applied to a patch of space around an observer far from the black hole and a patch of space around an observer inside the horizon. It's some human cognition thing, and in the early 20th century it took decades to discover systems of coordinates that work for observers far from the black hole, at the horizon, and inside the horizon. And even today, most people don't seem to try to enhance physical intuitions by swapping among arbitrary systems of coordinates (including no coordinates) on a single physical system like a black hole and a pair of observers (one inside the horizon and one far outside the black hole).
The Schwarzschild black hole interior is still locally Lorentz-invariant everywhere (because the whole Schwarzschild spacetime is a Lorentzian manifold).
The various local interactions of the Standard Model will keep working inside a black hole. In a really tiny patch around every point, everything behaves as if its in Minkowski space (flat 4-d (3 spatial + 1 time) spacetime).
(That's one of the problems of quantum field theory on curved spacetime: the "focusing-pressure" [for experts: this is encoded in the Weyl curvature tensor; my "scare quotes" take a view of this in a Raychaudhuri equation way] gets so high that the unknown ultraviolet behaviour of the Standard Model (a quantum field theory) becomes relevant. The Weyl behaviour in Schwarzschild is that quasispherical objects are strongly prolated with the long axis aligned radially: a soccer ball or basketball starts looking like an American or Canadian or Rugby football ball. The radial stretching "spaghettifies" by ripping apart weaker bonds (like intermolecular ones, and molecular ones, and ionizing atoms), but the tangential squashing ("focusing") must eventually generate more nuclear interactions, probably up to quantum chromodynamics (QCD) energies possibly before the radial stretching starts generating hadronization.
How this works in the Standard Model is just unknown. However simpler "test" quantum field theories (fewer, or even no, interactions; and often no colour-confinement-like processes) raise really difficult questions.
Finally, back to the Standard Model as local theory: how does any allegedly quantum nonlocality behaviour work? Local here in the sence that states can be distinguished by local measurements alone. Related questions: can you entangle particles deep inside a black hole? If an entangled pair fall in together, how does the entanglement evolve? Or obsessing black hole information people, what if you throw in only one half of an entangled pair and locally measure the outside half? Nobody has great answers for these sorts of questions at present, and there's no near-term hope of testing any proposals in laboratories or via astrophysical observation.
XorNot
I don't think the spacetime swap idea is particularly well explained though? Like although it's sort of mathematically true, my impression was that it's not like time suddenly becomes a dimension you're moving in once inside the event horizon, just that spacetime is acting so weird because there's now a deliberate direction where one did not exist before.
codethief
> I don't think the spacetime swap idea is particularly well explained though?
What exactly do you mean by "spacetime swap idea"? If you're saying the behavior of Schwarzschild coordinates at the event horizon is not well-understood, then I disagree. There is nothing particularly weird or surprising going on, there's just a trapped surface[0].
null
paulnovacovici
I’ve always like to explore the idea of our universe being in a static 5th dimension where the 5th dimension represents randomness/entropy. The same way to think about exploring a 2d plane in a 3 dimensional space where the 3rd dimension is constant. We just happen to be in a random big bang in this 5th dimensional space
mousethatroared
You can't have the curl operator in 4D.
account-5
What's in a 1D black hole?
modderation
I'm guessing it'd look something like this on a 1-dimensional number line:
--- > | > >> . << < | < ---
SlightlyLeftPad
/dev/null I presume
twothreeone
What? Wouldn't that mean an object's speed in some direction determines how time passes once it crossed over, and conversely, it would experience its old time dimension as spatial and be able to "move through (old) time" freely after crossing the event horizon?? My head hurts.
kannanvijayan
Oh I have so many questions on this topic.
I've often wondered about this. I don't have any direct physics training, but it's something that felt really plausible after I learned that the mass of a black hole is linearly proportional to its swarzschild radius.
As the size of the black hole goes up, its overall density must decrease. Combined with the other observation that our universe has uniform density at large scales, it seemed really obvious to me that there existed some threshold at which the decreasing density of a very large black hole, and the fixed density of our observed universe.. would cross.
I used to muse about this question with some other tech colleagues that liked talking about physics stuff but never really got a clear answer to this.
On a side note - I'm absolutely fascinated by the implications relating to this. I'll post a follow-up thought I'm hoping somebody else has also thought about:
I've seen discussion of dark energy mostly presented as a surrogate for real energy. That there is some underlying energy "accelerating things away from each other".
I always felt uncomfortable with that characterization. It seems more reasonable to me to think of dark energy as _negative energy_ - i.e. a loss of overall energy.
In a classical system, two things moving away from each other stores potential energy that can be recouped at some later time. Dark energy doesn't work this way - things accelerate away from each other the further apart they are. From a global perspective, it's an energy loss.
The energy loss pervades to the quantum world as well - photons that start off high frequency arrive low-frequency.
It somehow feels more appropriate to me to think of dark energy as energy being extracted out of the universe, in some form never to return. Maybe like a black hole evaporating as observed from the inside?
When I asked this of some people in real life, I was pointed to answers that indicated that the "energy" component in dark energy is normalized into the "tension" of space somehow. As I mentioned before I'm not really studied in physics, but that explanation felt unsatisfactory to me.
dvh
Plug estimated mass of universe to your schwarzschild formula and be amazed how close it is to observable size of the universe.
kannanvijayan
I tried once, but I'm not sure what terms to throw in there. Visible matter, estimated dark matter.. anything else?
I think my estimate came out to less dense than the required threshold but it was a while back now and cobbled together with some queries to wolfram.
eapriv
This is true almost by definition, and doesn’t tell us anything interesting about black holes.
account-5
There was a thread a while ago on here where the hypothesis for why things are moving apart at faster rates is down to time moving at different speeds due to mass.
So time in the void between galaxies is moving quicker than time in the galaxies, but on the grand scale of the universe the differences as up a lot.
I quite liked this theory, think is make sense, at least from my very limited understanding of this stuff.
__turbobrew__
Would make sense if our universe is a simulation. It takes more compute power to simulate areas of high density so time naturally flows slower there.
account-5
Yeah, but also that's how time actually works too, time runs slower for us on earth than say GPS satellites so adjustments need calculated to sync the two. Again caveat is I'm more than likely either just wrong or misunderstanding it or massively oversimplifying it.
codethief
Aka Timescape Cosmology, https://en.m.wikipedia.org/wiki/Inhomogeneous_cosmology
null
nathan_compton
> It somehow feels more appropriate to me to think of dark energy as energy being extracted out of the universe, in some form never to return. Maybe like a black hole evaporating as observed from the inside?
But in this story the black hole increases in size as matter falls into the horizon and shrinks as it evaporates, so cosmic expansion would be associated with more energy falling into the black hole than leaving it.
kannanvijayan
I thought about this part. I'm not sure we can link apparent size from outside the event horizon to apparent size from inside.
Apparent distance is something that's affected by relative frames of reference and the frames of reference are as different as as can be in this case.
burnt-resistor
A black hole is really just a singularity with infinite density by definition, but finite mass.
The size and density of the Schwarzschild volume is determined only by mass (stationary, non-rotating). It's proportional to the inverse square of mass. Density = 3c⁶/32πG³M².
SMBHs have densities ~0.5 kg/m³ between thin air and water.
Stellar BHs are ~1e19 kg/m³ several orders of magnitude more than a neutron star.
afarah1
>follow-up thought I'm hoping somebody else has also thought about [...] dark energy as _negative energy_ [...] Maybe like a black hole evaporating
Another layman's thoughts: Isn't the energy theoretically lost by black holes so faint it's currently undetectable? And isn't the amount of dark energy theorized to be the major component of the observable universe? It seems like the numbers wouldn't add up?
kannanvijayan
I don't have enough of the background to speculate about the numbers. Dark energy feels "big" if we think of it in terms of the actual energy it would take to accelerate the universe away from itself at the rate that we see.. but the rate that we see is affected by our frame of reference, along with distances and everything else.
I'm gonna pull out my lay understanding again. An evaporating black hole, as it gets smaller, should get more dense and be associated with a higher local spacetime curvature, no? The effect of which would be to slow down apparent time for observers within the system. Maybe that affects observed distance and rates of speed at which things seem to be happening when we look out into the sky?
Sometimes I regret not caring enough about calculus in university.
codethief
> Combined with the other observation that our universe has uniform density at large scales
s/has/had at the time of recombination
It is largely an assumption of LCDM that we can treat the universe as practically homogeneous throughout its entire evolution but potentially not a very well-founded at that [0, 1].
> I always felt uncomfortable with that characterization. It seems more reasonable to me to think of dark energy as _negative energy_ - i.e. a loss of overall energy.
Your intuition is correct. If the Lambda term in the Einstein field equations is moved over to the side of the energy momentum tensor, it takes on the role of a negative contribution (provided Lambda > 0, as observations seem to indicate).
> In a classical system, two things moving away from each other stores potential energy that can be recouped at some later time. Dark energy doesn't work this way - things accelerate away from each other the further apart they are. From a global perspective, it's an energy loss.
Note that there is no global energy conservation in General Relativity[2], only at a local scale[3]. Heck, you'll already struggle to define what the energy is of a given piece of spacetime in a meaningful and generic manner[4, 5]. In other words, violations of energy conservation due to spacetime expanding or contracting (a strictly non-local phenomenon), like in the case of the cosmic redshift, are expected and our intuition from classical mechanics only takes you so far.
> It somehow feels more appropriate to me to think of dark energy as energy being extracted out of the universe, in some form never to return.
Dark energy aka the cosmological constant term in the Einstein field equations is a constant term, as the name suggests. Yes, there can be energy loss due to spacetime expanding (see above) but that doesn't change the gravitational constant.
[0]: https://en.wikipedia.org/wiki/Cosmic_web
[1]: https://en.m.wikipedia.org/wiki/Inhomogeneous_cosmology
[2]: https://en.m.wikipedia.org/wiki/Conservation_of_energy
[3]: https://en.m.wikipedia.org/wiki/Stress%E2%80%93energy_tensor
[4]: https://arxiv.org/abs/1510.02931
[5]: https://en.m.wikipedia.org/wiki/Mass_in_general_relativity
kannanvijayan
Interesting reading - this is the first thorough response I've gotten to some of these question. Will check out the reading material.
raattgift
I think given time at a blackboard we could walk through Newton's cannon in the context of Poisson gravity, and for extra credit with the cannonball inducing a perturbation of the Poisson vector field. Even without the cannonball's backreaction, the Poisson picture offers a nice image of the gravitational potential energy at the top of the cannonball's inertial (ballistic) curve. We would then consider a cosmology like our own but with a recollapse: at maximum extent there is some (quasi-)Newtonian notion of gravitational potential energy for all the galaxies, since they are at the point where they begin free-falling back into a denser configuration. It's then the usual story of relating kinetic and potential energy, and recognizing that the standard cosmological frame is close to Newtonian by design. (We also have to stop this approach when the galaxies are merging enough that radiation pressure and gas ram pressure become relevant, because the errors become astronomical).
Since we don't have a blackboard in front of us to interact with, I can suggest Alan Guth's lecture notes on Newtonian cosmology. (Guth is credited with discovering cosmic inflation.) https://web.mit.edu/8.286/www/lecn18/ln03-euf18.pdf See around eqn (3.3). You could also borrow a copy of Baumann's textbook <https://www.cambridge.org/highereducation/books/cosmology/53...> which studies the Poisson equation for various spacetimes, however a static spacetime gets most of the focus.
A universe which expands forever, or which expands faster in the later universe, makes a mess of this sort of approach to calculating a gravitational potential energy. So does any apparent recession velocity that's a large fraction of c (inducing significant redshift, whatever the recession (pseudo-)"force" might be).
However, the general idea is that there is a relationship between the kinetic energy a receding galaxy (in a system of coordinates -- a "frame" -- in which these kinematics appear) and a gravitational potential energy still occurs in a non-recollapsing universe. It's just that the potential energy climbs forever, and you get an equivalent to gravitational time dilation between galaxies at different gravitational potentials (i.e., between early-universe galaxies and higher-potential modern-times galaxies).
Accelerometers in galaxies will not show a cosmic acceleration for any galaxy; they're all really close to freely-falling (local galaxy-galaxy interactions are real -- collisions and mergers and close-calls happen -- but wash out over cosmological distances; look up "peculiar velocity" for details). Therefore we can conclude that there's no real force imposing acceleration on the galaxies. However that's also true of a cannonball in a ballistic trajectory, including one on an escape trajectory or one that enters into a stable orbit. Consequently one can draw some practical comparisons between a ballistic launch from Earth into deep space and galaxies spreading out from an initially denser early part of an expanding cosmos.
> Dark energy as energy being extracted out of the universe
No, it's just a way of thinking about whatever is driving the expansion, and that doesn't dilute away with the expansion as ordinary matter and radiation does. It's not even a "real" energy in the sense that it is only an energy in the cosmological frame, and is a frame-dependent scalar quantity, whereas in the fuller theory it's just a multiplier of the metric tensor. So it's the full relativistic metric doing the work but we absorb some of that into cosmological coordinates in the cosmological frame of reference, carving up the metric tensor into a set of vectors including an expansion vector identical at every point in spacetime.
The expansion vector can also be thought of in terms of pressure: in a collapsing cosmological frame, a pressure drives galaxies together into a denser configuration. The inverse of pressure is tension, so in an expanding cosmological frame, it's a tension that pulls galaxies apart into a sparser configuration. (The reason one uses pressure or its inverse is that the matter fields are idealized as a set of perfect fluids at rest in the cosmological frame; each such fluid has an associated density and internal pressure which evolve with the expansion or contraction of the cosmos, generally becoming less positive in the time-direction of expansion (i.e., in the future direction in a universe like ours). Another way of thinking about pressure is as a measure of isotropic inflow of energy-momentum into a point; increasing pressure at a point therefore increases the curvature at that point. Tension is an isotropic outflow, and so positive tension is repulsive as opposed to the attraction from positive pressure.)
> that explanation felt unsatisfactory to me
Hopefully the above helps a bit. Unfortunately there's only so much teaching one might do in a series of HN comments, and ultimately one probably is better served in developing some grounding in the full Einstein Field Equations / Friedmann-Lemaître equations before thinking in quasi-Newtonian ways. Going the other direction tends to lead to misunderstandings and developing false intuitions when running into situations where the quasi-Newtonian picture needs post-Newtonian correction terms.
It's cool that you have all sorts of questions. You could consider signing up for part time / non-business-hours courses in relativity at a nearby community college or the equivalent, depending on where you are, or maybe just bringing a hot lunch to a lecturer there in exchange for a quick informal tutorial. Anything like that is bound to get you to better answers than raising comments on HN threads about astrophysics in the broadest sense, as answers here are often somewhere between non-standard and unreliable.
bmacho
> The Big Bang is often described as the explosive birth of the universe – a singular moment when space, time and matter sprang into existence.
It is indeed "often described" in the media as such. However, that is _not_ the currently accepted theory. "What if there were no space and time before the Big Bang" is just Stephen Hawking's pet theory.
jerf
A more accurate summation would be that our theories do not permit us to go back beyond what appears to be the "Big Bang", and indeed, we can't quite get to it either, since the need for Quantum Gravity becomes too great as we get to what seems to be the "zero time". We have no principled, reasonable way to make any claims about what came before the point where our theories break down, and that includes the claim that there was no space or time at all before then.
Thus, anything and everything you've heard about what is there "before the big bang" has always been speculation. I mention this because sometimes people read the science media, which is always reporting on this speculation, and think that the reporting on the speculation constitutes "science" constantly changing its mind, but that's not the case here. Science has consistently not had a justifiable position on this topic, ever. It has always been speculation. It is the press that often fails to make this clear and writes stories in terms of what "science" has "discovered", but any claims of certainty in this area are not the claims of "science".
_alternator_
Interesting thing with this work is that it does create an observable, testable hypothesis: slightly positive curvature of the universe.
null
smadsen
What people seem to not be able to conceptualize, consciously or not, is that there really is no "before" the Big Bang in the standard model (Lambda-CDM), if time itself exists only after t=0.
nathan_compton
The Lambda CDM does not really say that. As other commenters have pointed out, Lambda-CDM is silent on the very earliest few moments of the universe where quantum gravity would be required.
koakuma-chan
What is the currently accepted theory?
bmacho
We have no theories working at those conditions. Wiki says
> General relativity also predicts that the initial state of the universe, at the beginning of the Big Bang, was a singularity of infinite density and temperature.[6][obsolete source] However, classical gravitational theories are not expected to be accurate under these conditions, and a quantum description is likely needed[7].
DrammBA
> [6][obsolete source]
I didn't know but apparently wikipedia treats old sources as obsolete, doesn't matter if there's new information or not that would make it obsolete.
I wonder if any sources supporting the notion that the earth is round must be updated every couple of years with a new source or study.
joquarky
This is the best theory we have:
arbll
At t=0 or "before" none
AtlasBarfed
Seems inevitable that we'll discover we aren't the only universe / only cycle.
We went from thinking the Earth was the center of the universe, to the sun being the center of the universe, and the next obvious step is our universe isn't at the center of universes.
twodave
Might as well believe in God if you’re going to believe in spontaneous accidental creation…
kibwen
Why not? If you can't observe it, test it, and reproduce it, then it lies outside the realm of science and in the realm of belief. Until someone figures out a way to experimentally verify the big bang hypothesis (or any other explanation for the origin of the universe or what came "before"), it's entirely fair to attribute it to whatever you feel like, be it a god or anything else. There is no law of the universe that guarantees that science is capable of answering all questions.
mjh2539
If you can't observe it, test it, and reproduce it, then it lies outside of the realm of (natural) science and may lie within the realm of mathematics, philosophy, or (gasp) theology.
> There is no law of the universe that guarantees that science is capable of answering all questions.
There's a name for a more nuanced version of this "law" and there's a good amount of work being done arguing for and against weaker and stronger versions of it: https://plato.stanford.edu/entries/sufficient-reason/
ACow_Adonis
Well, I think surely the entirely fair thing to do is to just admit we don't know rather than make any attribution or imply any possession of an answer to those questions?
duped
There's quite a big philosophical difference between "there exists a point beyond which it is possible to make observations" and "the universe was created by an omnipotent being"
Terr_
Verily, for all knowest that the gods live up in the sky, which is forever unreachable and unobservable by any man.
hshshshshsh
Am omnipotent being is a necessity for making observations. A lot of religions considers consciousness as God.
arbll
Anything outside of what we can observe will always be based on faith anyway. We'll probably never understand what's "before" the big bang, wether it make sense to ask that question or why something exists rather than nothing.
nathan_compton
I don't think so - god is substantially less parsimonious. But in the end, I think you're sort of using two different notions of belief as if they were the same.
I believe (lowercase b) in all sorts of stuff, scientific and otherwise, but believing in God typically indicates some kind of act of faith, which is to say, ultimately, to believe in something despite the absence of evidence for it and for some deeper reason than can be furnished by a warrant of some kind. I can believe in the spontaneous generation of the universe in the lowercase b sense of the word without really having anything to do at all with the latter kind of belief, which I think is kind of dumb.
mjh2539
Historically, in the West at least, the ability or inability to reason one's way to the existence of God determined whether you needed to rely on faith or not.
https://www.vatican.va/content/pius-xii/en/encyclicals/docum...
skellington
Sort of like believing that we have free will.
zthrowaway
The Big Bang theory was created by a Catholic priest. So yes.
mcswell
And the Big Bang was created by the priest's God.
kobalsky
You're confusing belief with accepting the current scientific consensus.
null
pharrington
Which God? Vishnu? Ra? Amatarasu?
ben_w
Prince Philip? https://en.wikipedia.org/wiki/Prince_Philip_movement
hshshshshsh
Assume God exists.
Various isolated cultures are going to come up with different names for God.
This like saying which Sun? Surya, Ra or Helios?
All are different names of sun. But there is only one sun.
0x0203
If the universe does have a positive curvature as this predicts, would that mean that if we look out into space, we could see the same galaxies multiple times? Or even our own galaxy in the past? Or is the predicted curvature slight enough that anything we might see multiple times is already beyond the limits of visibility due to universe expansion?
mr_toad
Only if the circumference of the universe is small enough for light to have made the round trip since the universe began. But we think that the universe is much larger than that.
HocusLocus
Suggested hard sci-fi light reading: "Cosm" by Gregory Benford, 1999. A universe the size of a bowling ball created in a laboratory. The scientist responsible for it, keeping it safe and on the run from gvt spooks. They want to protect it for as long as it lasts, and since its time is as relative as its size, they won't have long to wait.
mmh0000
Added.
Opinions:
A) I love all the scifi book recommendations that cone up on HN
B) i wish you’d all stop recommending great and amazing books. My queue is so backlogged and jammed I'm never going to catch up.
fibonachos
Same here. My interest in the Sci-Fi genre started with an HN comment recommending Blindsight, by Peter Watts.
Several comments and sci-fi series later, and I’m currently reading about spacefaring sentient spiders.
arto
So many books, so little time...
baw-bag
There needs to be some kind of hackernews library or goodread. I have enjoyed many books (and some no so much) but always on the look out for books.
mewpmewp2
If only we had an even bigger universe, we would have more time... is that how it works?
RGamma
What's a couple dozen books (and video games) in my backlog when I have a thousand websites there?
donohoe
Please share the queue!
veqq
Microcosmic God - Theodore Sturgeon (1941)
Xophmeister
I seem to remember a similar Star Trek episode; DS9, IIRC.
throwawaymaths
"playing god", s2e17
ars
I remember that, and the enormous plot hole that they could move the thing in a transporter!
sleepybrett
Rick and Morty episode (season 2 episode 6, 'The Ricks Must be Crazy') where it turns out rick has created a whole universe inside his spaceship battery who's whole purpose is to produce energy to run his spaceship. A scientist within this microverse creates a miniverse ....
edfletcher_t137
> The black hole universe also offers a new perspective on our place in the cosmos. In this framework, our entire observable universe lies inside the interior of a black hole formed in some larger “parent” universe.
Does it also follow that black holes in our universe contain universes internally, beyond their event horizons?! Seems like it should. Mind-blowing.
int_19h
It's not a new idea, although I don't think it would be accurate to describe the other universes as "contained" within the black hole.
https://en.wikipedia.org/wiki/White_hole#Big_Bang/Supermassi...
goodcanadian
Does it also follow that black holes in our universe contain universes internally, beyond their event horizons?!
Not necessarily. It's not clear that any are massive enough to cross the threshold required for the "bounce."
afroboy
I'm not a scientist or astrophysicist but i do believe in science, is it ok to believe that we as humans and all our scientific development still very very far from proving anything remotely close to how the universe got created? I feel this subject is for humanity in year 2600 to start discussing it.
Scientist still can show their theories and search papers and i can't understand a shit but i don't believe in any theory that proves how the universe got created.
didibus
Science is a process, not a source of truth. It has a very practical lens, which is very utilitarian, does the knowledge and models allow for invention or prediction that works in our reality for some current need.
The assumption is, you never really know, but if the model in which the theory says X, is able to predict something in the future or some experiment for Y, than that model appears to better approximate reality. Or is that knowledge and model allowing us to now do something we could not before, etc.
Over time, it course corrects to improve its knowledge and models in ways that show better results for prediction or invention.
CobrastanJorji
"Proving anything" is kind of fuzzy. We've got very solid evidence that some sort of big bang happened. We can see the galaxies flying away from a common point, and since we can count backwards, we can know roughly when they probably would've been in the same spot. The how and why, and the what happened before, those are very unknown, although we've got a surprising amount of knowledge of what the first few seconds were probably like.
ptmcc
It's not about belief, it's about observing, collecting data & evidence, and proposing possible explanations. As new observations and evidence are found the possible explanations are refined. No one credible is claiming hard proof of anything at this point.
didibus
Agreed, people often mix that up, but you have to adopt a probabilistic mindset, you can believe the coin with land on its head, but you also know that based on the weight and curvature of the coin it will land on its tail 68% of the time you flip it, etc. Then choosing tail is no longer a belief, it is simply going with the choice that is backed by prior observation, experiments, models, etc.
You might still lose, and so you might choose to also believe it will land on tail this time, but the rationale for choosing tail was not based on a belief system, but the going information and where it points too.
philipov
Before we can prove, we must first wonder. We proceed by small steps, and if we don't start discussing it now, 2600 will still be too early.
serf
I think the phrase 'believe in science' is weird; it's nearly as problematic as "I have faith in science".
It can be, but generally the concept of 'belief' isn't attributed to ground truths; it's just 'the truth', you rarely hear the phrase "I believe 2 and 2 is 4." , it's just '2 and 2 is 4.' -- I think that's important.
In fact, a lot of people insert the word 'believe' to insert a concept of self-doubt. "What was our last test results passing rate?" "I believe it was around 95 percent.."
But semantics aside here's the real question : Why do you have some kind of notion that you should 'believe' anything without being able to understand it? Just trust in the world and those around you?
We haven't figured origin yet, so let's get off that, but when a scientist of some sort makes a discovery, they release evidence and methods , and you decide to believe the conclusions without an understanding of the work -- well that's just a display of faith. Faith in the scientist themselves, the system they work within, and the society you're in.
Which leads me to say this : If you make an effort to begin to understand the frameworks and systems which lead to scientific conclusions you can largely remove the faith and belief elements up until you hit the very highest spectrums of each field where speculation comes back into play.
tl;dr : if you 'cant understand a shit', you don't put any leg-work in and make an effort to speak the language, you'll probably end back up in beliefs rather than an ever increasing codex of knowledge -- regardless of the field. That's okay -- but it doesn't offer the same benefits as knowledge -- it just lets one say things like "I don't believe in any theory..."
EasyMark
But that's why science is so cool, it doesn't matter what we believe, it only matters if your theory fits the facts and makes good predictions. If it doesn't, you can chuck it in the bin without guilt. Unlike beliefs, which often can cause psychic trauma if reality doesn't match the belief of the individual.
mkoubaa
You don't believe in science, you believe in a metaphysical claim about science that you haven't articulated.
kgwxd
Belief is acting as if something, for which no evidence has been given, is true. Imagination, taken too far. No one is telling you to believe anything here, they're suggesting we search for clues to support or disprove a theory. Or don't, it's up to you.
LordNerevar76
That's not the definition of belief. Belief can have various levels of different kinds of evidence behind it. Scientific, historical, philosophical, experiential, etc. A belief could have more or less scientific evidence than other beliefs, but rarely is belief predicated on no evidence whatsoever.
kgwxd
Do I really have to include the exact definition of every word I ever use? You know damn well I'm talking about blind faith and scientific evidence. And you know damn well OP is completely dismissing a theory because they're confusing it with a demand for blind faith. I don't care what people want to believe, but if they start labeling things incorrectly, I'm going to point it out.
null
I think it's neat that this summary is written by an author of the scientific manuscript. Oversimplification is a risk, but this approach eliminates the possibility that the writer did not understand the underlying science.