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.
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.)
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.
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.
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.
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.
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.
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.
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 :-)
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.
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?
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.
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.
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.
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.
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.
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:
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?
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.
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?
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.
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?
> "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.
Not exactly. A universe can expand, slow down, then collapse. In this case, bouncing back out.
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
Yes and no. You're not thinking about contraction. With relativity we can fit a 100ft ladder inside a 10ft barn.
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!
>> 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)
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.
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.
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.
Current observations make it likely that our observable universe expands (think "stretches"), and the expansion will continue forever.
If it's expanding, then it was smaller earlier. Asking about the far past is a natural reaction, and the Big Bang theory is a pretty good attempt at explaining that.
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.
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.
Your first statement right off the bat is a bit of an assumption, why can’t the parent universe also have been formed inside a black hole? Why did you assume that?
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?
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.
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.
> 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.
> 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?
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.
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
I'm guessing it'd look something like this on a 1-dimensional number line:
--- > | > >> . << < | < ---
The dot in the middle would be the singularity, the pipes the event horizon, and the contents would be increasingly warped spacetime that may or may not exist, depending on your interpretation of things.
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.
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.
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?
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.
It may just be that the physical conditions of our universe just prior to the big bang are indistinguishable from that of the interior of black holes.
In that sense black holes are areas where our universe has reverted from it's low entropy state all the way back to the initial nearly infinite entropy state.
I feel like quantum physics is gently pointing us towards the idea "everything you can imagine is real at once". As in, all possible universes and physics systems and whatnot do exist in some sense of this word, and we happen to inhabit one. Just like Earth is a totally unremarkable planet in a totally unremarkable solar system in a totally unremarkable galaxy, except we popped up here so for a long time we thought there's something deeply special about Earth.
I find it more useful to anchor the concept of "real" in what one has direct access to. Beyond that there are many ways to describe our shared reality and the space of possible realities, including the past and future, some of which are more real than others, and go far beyond what we can imagine. Quantum physics gives us a language to expand what we can describe and imagine.
Not only does the sun not rotate around us, the rest of the galaxy doesn't even care to think that we exist. An interesting evolution in thought nonetheless.
If we wait until we understand everything perfectly before publishing, we’d never publish anything. That question may remain, but so do many others, this paper can’t address them all.
Block Universe.
The more you think about it, the more probable it seems.
Why should a universe pass time like a movie, if all moments could exist simultaneously? If there is no time, and it’s just a simulation formed in our brain, there doesn’t have to be a beginning nor end.
I'd put it a bit differently, that it remains a scientific endeavor, but leaves us in the same predicament as we're in now, which is the difficult work of forming a scientific theory that can only be tested indirectly.
There is no other entity. We're nothing. An algebra of nothing. Combine nothing with nothing in various ways (like S-terms) and it gives you physics, among many other things. From the inside we see a universe, from the outside you would see nothing.
This is why the "but the universe couldn't spawn out of nothing!" style arguments are so annoying. They completely accept that an all powerful all knowing entity could exist for all of time and not need a creator without any supporting evidence. But the origin of the universe specifically needs to be explained in detail or science is a sham.
Maybe there was no cruelty, and we were just plain matter that fell into our encapsulating black hole. Like what happenswith our own universes black holes.
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.
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.
> 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.
>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?
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.
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.
> 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.
Yeah I was referencing the event horizon as the most meaningful measure of size.
And whether the density is fixed over time or not doesn't affect the question. Let's take the universe at its current average mass/energy density - whatever the "true" measure of that is.
To the best of our understanding, at large scales the density is uniform. So if we consider a suitably large spherical volume of space within our (presumably infinite) universe.. that volume will have an average mass/energy content greater than the threshold amount for a black hole of that apparent volume (again, using the external event horizon frame).
So that suggested to me that either we live in a finite universe, or we must be on the inside of an event horizon. It seems like an unavoidable conclusion.
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.
That is a very interesting idea… the equation and its assumptions doesn’t seem to have any exceptions so it does strongly suggest our universe is a black hole, inside a black hole?
> 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.
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".
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.
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.
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.
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].
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.
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"
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.
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.
Belief in anything is completely trivial unless you act based on those beliefs. No one is going to waste time worshiping, or murder someone over, the "nothing" from before "something".
Readit News
(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.)
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.
> 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.
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https://www.amazon.com/World-Physics-Library-Literature-Anti...
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?
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.
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.
That’s not mentioned in the summary. After inflation the event horizon would not exist.
(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.
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.
[0]: https://observablehq.com/@tophtucker/theres-plenty-of-room-i...
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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.
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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?
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.
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?
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.
Nitpick: We couldn't be anywhere else, except nonexistent.
Does that repeat forever? Does it lose energy in the bounce? If so, to where and how?
Yes and no. You're not thinking about contraction. With relativity we can fit a 100ft ladder inside a 10ft barn.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!
>> 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)
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.
If it's expanding, then it was smaller earlier. Asking about the far past is a natural reaction, and the Big Bang theory is a pretty good attempt at explaining that.
https://en.wikipedia.org/wiki/Expansion_of_the_universe
https://en.wikipedia.org/wiki/Chronology_of_the_universe
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.
I guessed c once. It would be a constant. Maybe all the constants are spaghettified remains of a superior universe.
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.
Or is that too simplified?
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.
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?
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I really like this analogy for "what is outside of our universe", thank you
https://en.wikipedia.org/wiki/Cosmological_natural_selection
I don’t think it has a hypothesis for the origin, though
In that sense black holes are areas where our universe has reverted from it's low entropy state all the way back to the initial nearly infinite entropy state.
I do agree that it makes sense, but not because of what quantum mechanics says.
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As for where I came from, I gotta admit I feel curious about that too, but mostly I’m just happy to be here. Real excited to see what you do next.
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.
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.
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.
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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.
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.
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?
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.
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.
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
The center of a black hole is infinitely dense. That's why it even exists. The event horizon is not the black hole.
> and the fixed density of our observed universe
Our universe is expanding. It's density is not fixed.
You really want to be thinking about this in terms of entropy and not matter.
And whether the density is fixed over time or not doesn't affect the question. Let's take the universe at its current average mass/energy density - whatever the "true" measure of that is.
To the best of our understanding, at large scales the density is uniform. So if we consider a suitably large spherical volume of space within our (presumably infinite) universe.. that volume will have an average mass/energy content greater than the threshold amount for a black hole of that apparent volume (again, using the external event horizon frame).
So that suggested to me that either we live in a finite universe, or we must be on the inside of an event horizon. It seems like an unavoidable conclusion.
Arguing semantics is rather boring when it's obvious you understood the point he was trying to make.
> Our universe is expanding. It's density is not fixed.
None of that precludes uniform density at large scales.
> The center of a black hole is infinitely dense. That's why it even exists. The event horizon is not the black hole.
>> and the fixed density of our observed universe
> Our universe is expanding. It's density is not fixed.
These are both correct and germane points. So why was this post downvoted? Physics isn't a popularity contest, it relies on evidence.
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.
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.
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".
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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.
> 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].
https://en.wikipedia.org/wiki/Gravitational_singularity
https://en.wikipedia.org/wiki/The_Last_Question
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.
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