Singularity / Black Hole at T=0?

[Alan Grayson]

Alan Grayson

11:56 PM (1 minute ago) 

to Everything List

Running the clock backward, and assuming the physical size of the universe converges to a singularity with zero volume at T=0, will it form a Black Hole? TY, AG

[Alan Grayson]

Let me pose the problem differently; if the entire contracted to almost zero volume, is there anything we know that would prevent it from becoming a BH? AG 

[Alan Grayson]

On Tuesday, February 18, 2025 at 11:19:45 PM UTC-7 Alan Grayson wrote:

On Monday, February 17, 2025 at 11:59:45 PM UTC-7 Alan Grayson wrote:

Alan Grayson

11:56 PM (1 minute ago) 

to Everything List

Running the clock backward, and assuming the physical size of the universe converges to a singularity with zero volume at T=0, will it form a Black Hole? TY, AG

Let me pose the problem differently; if the entire universe contracted to almost zero volume, is there anything we know that would prevent it from becoming a BH? AG 

[Quentin Anciaux]

AG, the key issue is that the universe isn’t collapsing into a localized region—it’s expanding/collapsing everywhere. A black hole forms when mass collapses within a surrounding spacetime, creating an event horizon. The early universe, however, was homogeneous and isotropic on large scales, meaning there was no "outside" region for an event horizon to form around it.

In GR, a universe that contracts to extremely high density doesn’t necessarily become a black hole—it follows different equations that describe a hot, dense state rather than a localized collapse. The FLRW metric describes a global evolution of spacetime, not a local gravitational collapse like a black hole. That’s why the early universe could be dense without forming a black hole—it didn’t have a surrounding spacetime to collapse into.

Quentin 

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[Alan Grayson]

On Wednesday, February 19, 2025 at 1:40:13 AM UTC-7 Quentin Anciaux wrote:

AG, the key issue is that the universe isn’t collapsing into a localized region—it’s expanding/collapsing everywhere.

I am not being sarcastic to ask if that's a fact based on solid physics or your opinion? AG

 

A black hole forms when mass collapses within a surrounding spacetime, creating an event horizon. The early universe, however, was homogeneous and isotropic on large scales, meaning there was no "outside" region for an event horizon to form around it.

Firstly, I have a hard time distinguishing between homogeneous and isotropic. Also, if we assume the volume shrinks close to, but not equal to zero, there would be a region where an event horizon could form. AG

In GR, a universe that contracts to extremely high density doesn’t necessarily become a black hole—

"Doesn't necessarily", but is it possible or absolutely precluded?  AG

it follows different equations that describe a hot, dense state rather than a localized collapse. The FLRW metric describes a global evolution of spacetime, not a local gravitational collapse like a black hole. That’s why the early universe could be dense without forming a black hole—it didn’t have a surrounding spacetime to collapse into.

As I posted earlier on another thread, there could be other metrics that predict a BH result close to the BB. AG 

[Quentin Anciaux]

A black hole forms when mass collapses within an external spacetime, creating an event horizon. The early universe wasn’t a localized collapse within surrounding space—it was the entire spacetime itself contracting or expanding. That’s why an event horizon doesn’t form around it.

The difference between homogeneous and isotropic is simple: homogeneity means the universe has the same properties everywhere on large scales, while isotropy means it looks the same in all directions. Together, they describe a universe that doesn’t have a preferred center or edge, unlike a collapsing object forming a black hole.

Quentin 

To view this discussion visit https://groups.google.com/d/msgid/everything-list/8b1080ee-31a3-4b39-9ad5-54cfc0ce9b5fn%40googlegroups.com.

[Alan Grayson]

On Wednesday, February 19, 2025 at 2:46:42 AM UTC-7 Quentin Anciaux wrote:

A black hole forms when mass collapses within an external spacetime, creating an event horizon. The early universe wasn’t a localized collapse within surrounding space—it was the entire spacetime itself contracting or expanding. That’s why an event horizon doesn’t form around it.

But if we're referring to a collapse not quite to zero volume, spacetime exists for a BH horizon to form. Also, if you grant that spacetime can contract or expand, are you not implicitly assuming a finite universe? AG 

The difference between homogeneous and isotropic is simple: homogeneity means the universe has the same properties everywhere on large scales, while isotropy means it looks the same in all directions. Together, they describe a universe that doesn’t have a preferred center or edge, unlike a collapsing object forming a black hole.

ISTM that they're essentially equivalent, that one implies the other and vice-versa. AG 

[Quentin Anciaux]

AG, if spacetime contracts to an extremely small volume but remains homogeneous and isotropic, it doesn’t form a black hole in the traditional sense. A black hole requires an asymmetric collapse of mass within an already-existing spacetime, leading to an event horizon. The early universe, however, wasn’t collapsing into an external space—it was space itself evolving. The conditions for a black hole simply don’t apply when everything is contracting uniformly rather than collapsing toward a single point within a larger spacetime.

Regarding your second point, homogeneity and isotropy are not the same. A universe can be homogeneous but not isotropic (same properties everywhere but looks different in different directions). Likewise, it can be isotropic but not homogeneous (looking the same in all directions but with variations in density at different locations). Together, they imply a universe that has no special center or direction, but one doesn’t strictly require the other.

Your assumption that spacetime contraction implies finiteness isn’t correct. An infinite universe can still contract or expand without needing to be finite. The FLRW metric allows for infinite spatial extent while still having a changing scale factor over time.

Quentin 

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[Alan Grayson]

On Wednesday, February 19, 2025 at 3:56:10 AM UTC-7 Quentin Anciaux wrote:

AG, if spacetime contracts to an extremely small volume but remains homogeneous and isotropic, it doesn’t form a black hole in the traditional sense. A black hole requires an asymmetric collapse of mass within an already-existing spacetime, leading to an event horizon. The early universe, however, wasn’t collapsing into an external space—it was space itself evolving. The conditions for a black hole simply don’t apply when everything is contracting uniformly rather than collapsing toward a single point within a larger spacetime.

Why does BH formation requires asymmetric collapse?  How can you know that a collapsing universe would do so uniformly, particularly since its mass distribution is not uniform? AG 

Regarding your second point, homogeneity and isotropy are not the same. A universe can be homogeneous but not isotropic (same properties everywhere but looks different in different directions). Likewise, it can be isotropic but not homogeneous (looking the same in all directions but with variations in density at different locations). Together, they imply a universe that has no special center or direction, but one doesn’t strictly require the other.

Your assumption that spacetime contraction implies finiteness isn’t correct.

It was what you seemed to be implying in your last post. AG

 

An infinite universe can still contract or expand without needing to be finite.

If you want to consider average distances between galaxies as your criterion, then concepts of contract and expand are poor descriptors IMO. AG

 

The FLRW metric allows for infinite spatial extent while still having a changing scale factor over time.

Quentin 

Le mer. 19 févr. 2025, 11:37, Alan Grayson <agrays...@gmail.com> a écrit :

Y

[Quentin Anciaux]

AG, black hole formation typically requires asymmetric collapse because it happens within an existing spacetime with a surrounding region that remains unaffected. This allows an event horizon to form around the collapsing mass. In contrast, a globally contracting universe (if it were homogeneous and isotropic) wouldn’t have an external region for an event horizon to form—it would just keep collapsing as a whole. That’s why traditional BH formation and a collapsing universe are different scenarios.

However, if the universe were not perfectly homogeneous (which it isn’t at smaller scales), then localized collapses could happen, leading to black holes forming within the universe. But this doesn’t mean the entire universe itself becomes one giant black hole. Instead, it would just mean that density fluctuations might create many black holes during contraction.

Regarding expansion/contraction, if you think "average distance between galaxies" is a poor descriptor, what would you propose instead? The scale factor in the FLRW metric defines expansion/contraction precisely—it’s not just an "opinion," it’s how GR describes cosmic evolution. An infinite universe can still expand or contract in this framework without implying finiteness.

Quentin 

To view this discussion visit https://groups.google.com/d/msgid/everything-list/2c78a142-b5e3-4a91-8b7c-2f7322057500n%40googlegroups.com.

[Alan Grayson]

On Wednesday, February 19, 2025 at 6:12:49 AM UTC-7 Quentin Anciaux wrote:

AG, black hole formation typically requires asymmetric collapse because it happens within an existing spacetime with a surrounding region that remains unaffected. This allows an event horizon to form around the collapsing mass. In contrast, a globally contracting universe (if it were homogeneous and isotropic) wouldn’t have an external region for an event horizon to form—it would just keep collapsing as a whole. That’s why traditional BH formation and a collapsing universe are different scenarios.

However, if the universe were not perfectly homogeneous (which it isn’t at smaller scales), then localized collapses could happen, leading to black holes forming within the universe. But this doesn’t mean the entire universe itself becomes one giant black hole. Instead, it would just mean that density fluctuations might create many black holes during contraction.

The universe isn't homogeneous on large scales. We see huge filaments containing galaxies and huge voids between them. and if the volume is actually decreasing close to zero, it's possible these numerous BH's could merge and the entire universe could contract to one giant BH. AG 

Regarding expansion/contraction, if you think "average distance between galaxies" is a poor descriptor, what would you propose instead? The scale factor in the FLRW metric defines expansion/contraction precisely—it’s not just an "opinion," it’s how GR describes cosmic evolution. An infinite universe can still expand or contract in this framework without implying finiteness.

I agree that an infinite universe can have distances between galaxies increasing or decreasing. But since the volume of space is infinite and hence unchanging, it was a poor choice of words to call this increasing or decreasing, which implies changes in volume. Now that's my opinion and it's not the first time a poor choice of words has occurred in science and other disciplines. AG 

Quentin 

[Quentin Anciaux]

AG, while the universe has large-scale structure (filaments, voids, and clusters), it is homogeneous and isotropic on scales beyond a few hundred megaparsecs. The cosmic microwave background (CMB) confirms this—on sufficiently large scales, the density variations average out. So while local structures exist, GR still treats the universe as homogeneous and isotropic for cosmological modeling.

If the universe were contracting toward near-zero volume, local black holes might merge, but this doesn’t mean the entire universe becomes a black hole. The key difference is that black hole formation requires an event horizon surrounding a localized mass. If everything is contracting together, there’s no external region for an event horizon to form around. Instead, it would just result in a Big Crunch, a singularity different from a traditional black hole.

Regarding terminology, infinite space doesn’t imply unchanging volume in any meaningful way. Even in an infinite universe, the concept of expansion/contraction is well-defined in GR through the scale factor, which describes how distances evolve over time. The word choice is not an issue of misinterpretation—it’s how the theory mathematically describes cosmic evolution.

Quentin 

To view this discussion visit https://groups.google.com/d/msgid/everything-list/8a620a5a-53d2-4756-91e7-db99c5b68cf9n%40googlegroups.com.

[Alan Grayson]

On Wednesday, February 19, 2025 at 8:02:05 AM UTC-7 Quentin Anciaux wrote:

AG, while the universe has large-scale structure (filaments, voids, and clusters), it is homogeneous and isotropic on scales beyond a few hundred megaparsecs. The cosmic microwave background (CMB) confirms this—on sufficiently large scales, the density variations average out. So while local structures exist, GR still treats the universe as homogeneous and isotropic for cosmological modeling.

I really don't see how you can claim the universe is isotropic and homogeneous. On the largest scale we see huge filaments containing galaxies in one direction, and in another we see huge voids. I am sympathetic to those who want to simplify E's field equations in order to get solutions, but they're assuming something which is obviously not true. AG  

If the universe were contracting toward near-zero volume, local black holes might merge, but this doesn’t mean the entire universe becomes a black hole. The key difference is that black hole formation requires an event horizon surrounding a localized mass. If everything is contracting together, there’s no external region for an event horizon to form around. Instead, it would just result in a Big Crunch, a singularity different from a traditional black hole.

How would the singularity of a Big Crunch differ from a traditional BH? I would assume that as the numerous BH's form on a non-uniform contraction, there would be a small time interval just before all volume decreased to zero, to form the horizon you claim couldn't form. AG 

Regarding terminology, infinite space doesn’t imply unchanging volume in any meaningful way. Even in an infinite universe, the concept of expansion/contraction is well-defined in GR through the scale factor, which describes how distances evolve over time. The word choice is not an issue of misinterpretation—it’s how the theory mathematically describes cosmic evolution.

If the universe is infinite in spatial extent, the average distance between galaxies might decrease or increase, but the volume, remaining infinite, cannot change. I don't see how you can argue against this pov. AG 

[Alan Grayson]

On Wednesday, February 19, 2025 at 3:11:21 PM UTC-7 Alan Grayson wrote:

On Wednesday, February 19, 2025 at 8:02:05 AM UTC-7 Quentin Anciaux wrote:

AG, while the universe has large-scale structure (filaments, voids, and clusters), it is homogeneous and isotropic on scales beyond a few hundred megaparsecs. The cosmic microwave background (CMB) confirms this—on sufficiently large scales, the density variations average out. So while local structures exist, GR still treats the universe as homogeneous and isotropic for cosmological modeling.

I really don't see how you can claim the universe is isotropic and homogeneous. On the largest scale we see huge filaments containing galaxies in one direction, and in another we see huge voids. I am sympathetic to those who want to simplify E's field equations in order to get solutions, but they're assuming something which is obviously not true. AG  

If the universe were contracting toward near-zero volume, local black holes might merge, but this doesn’t mean the entire universe becomes a black hole. The key difference is that black hole formation requires an event horizon surrounding a localized mass. If everything is contracting together, there’s no external region for an event horizon to form around. Instead, it would just result in a Big Crunch, a singularity different from a traditional black hole.

How would the singularity of a Big Crunch differ from a traditional BH? I would assume that as the numerous BH's form on a non-uniform contraction, there would be a small time interval just before all volume decreased to zero, to form the horizon you claim couldn't form. AG 

Would you agree that cosmologists who contemplate a Big Crunch, are assuming a finite universe which collapses to zero volume in finite time? if so, the idea of a finite universe is not beyond the pale.  AG

Regarding terminology, infinite space doesn’t imply unchanging volume in any meaningful way. Even in an infinite universe, the concept of expansion/contraction is well-defined in GR through the scale factor, which describes how distances evolve over time. The word choice is not an issue of misinterpretation—it’s how the theory mathematically describes cosmic evolution.

"Infinite space doesn't imply unchanging volume in any meaningful way."  Really? If the universe is infinite in spatial extent, the average distance between galaxies might decrease or increase, but the volume, remaining infinite, cannot change. I don't see how you can argue against this pov. AG 

[Quentin Anciaux]

AG, a Big Crunch scenario does not necessarily assume a finite universe. An infinite universe can also undergo a global contraction—meaning that while distances between galaxies shrink, the universe itself remains infinite at all times. A finite universe collapsing to zero volume in finite time is just one possibility, but it’s not required for a Big Crunch model. The idea of a finite universe is, of course, not beyond the pale—it remains an open question in cosmology.

Regarding infinite space and "unchanging volume," the key issue is that volume in an infinite universe is not a meaningful quantity in the way you are describing it. Yes, if the universe is infinite now, it was always infinite, but that doesn’t mean nothing changes—the scale factor determines how distances evolve. The phrase "volume cannot change" is misleading because in an infinite universe, there is no finite, well-defined total volume to begin with. Instead, we talk about the expansion or contraction of distances within that infinite space, which is physically meaningful.

Quentin

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[Alan Grayson]

On Wednesday, February 19, 2025 at 11:54:52 PM UTC-7 Quentin Anciaux wrote:

AG, a Big Crunch scenario does not necessarily assume a finite universe. An infinite universe can also undergo a global contraction—meaning that while distances between galaxies shrink, the universe itself remains infinite at all times. A finite universe collapsing to zero volume in finite time is just one possibility, but it’s not required for a Big Crunch model. The idea of a finite universe is, of course, not beyond the pale—it remains an open question in cosmology.

Regarding infinite space and "unchanging volume," the key issue is that volume in an infinite universe is not a meaningful quantity in the way you are describing it. Yes, if the universe is infinite now, it was always infinite, but that doesn’t mean nothing changes—the scale factor determines how distances evolve. The phrase "volume cannot change" is misleading because in an infinite universe, there is no finite, well-defined total volume to begin with. Instead, we talk about the expansion or contraction of distances within that infinite space, which is physically meaningful.

Quentin

Do you concede that the universe isn't isotropic or homogeneous? What is the nature of the singularity in the Big Crunch for a finite and infinite universe? How does it differ from the standard BH? AG 

[Quentin Anciaux]

Le jeu. 20 févr. 2025, 08:05, Alan Grayson <agrays...@gmail.com> a écrit :

On Wednesday, February 19, 2025 at 11:54:52 PM UTC-7 Quentin Anciaux wrote:

AG, a Big Crunch scenario does not necessarily assume a finite universe. An infinite universe can also undergo a global contraction—meaning that while distances between galaxies shrink, the universe itself remains infinite at all times. A finite universe collapsing to zero volume in finite time is just one possibility, but it’s not required for a Big Crunch model. The idea of a finite universe is, of course, not beyond the pale—it remains an open question in cosmology.

Regarding infinite space and "unchanging volume," the key issue is that volume in an infinite universe is not a meaningful quantity in the way you are describing it. Yes, if the universe is infinite now, it was always infinite, but that doesn’t mean nothing changes—the scale factor determines how distances evolve. The phrase "volume cannot change" is misleading because in an infinite universe, there is no finite, well-defined total volume to begin with. Instead, we talk about the expansion or contraction of distances within that infinite space, which is physically meaningful.

Quentin

Do you concede that the universe isn't isotropic or homogeneous? What is the nature of the singularity in the Big Crunch for a finite and infinite universe? How does it differ from the standard BH? AG 

AG, on small scales, the universe is neither isotropic nor homogeneous due to the presence of galaxies, filaments, and voids. However, on large scales, it is effectively homogeneous and isotropic, as confirmed by the cosmic microwave background (CMB) and large-scale surveys. The Cosmological Principle—which assumes large-scale homogeneity and isotropy—remains valid for describing the universe at scales beyond a few hundred megaparsecs.

Regarding the Big Crunch singularity, it differs depending on whether the universe is finite or infinite:

Finite Universe: If the universe is closed and finite, a Big Crunch would resemble the time-reversed version of the Big Bang—space collapses to a singularity where density, temperature, and curvature diverge. It’s a true spacetime singularity in GR, where classical physics breaks down.

Infinite Universe: If an infinite universe undergoes a Big Crunch, distances still shrink everywhere, but it remains infinite at all times. The singularity would be a global state of infinite density everywhere rather than a localized point. Unlike a black hole, this singularity isn’t "contained" within an event horizon—it involves the entire universe.

A black hole singularity, in contrast, is localized—it forms due to asymmetric collapse, creating an event horizon around a specific region of space. In a Big Crunch, there’s no such horizon enclosing the universe because space itself is collapsing uniformly (on large scales). That’s the fundamental difference: a black hole singularity is localized within spacetime, while a Big Crunch singularity is the entire spacetime itself collapsing.

Quentin 

To view this discussion visit https://groups.google.com/d/msgid/everything-list/37eafd10-6762-4c36-8899-7a8163766220n%40googlegroups.com.

[Alan Grayson]

On Thursday, February 20, 2025 at 12:22:32 AM UTC-7 Quentin Anciaux wrote:

Le jeu. 20 févr. 2025, 08:05, Alan Grayson <agrays...@gmail.com> a écrit :

On Wednesday, February 19, 2025 at 11:54:52 PM UTC-7 Quentin Anciaux wrote:

AG, a Big Crunch scenario does not necessarily assume a finite universe. An infinite universe can also undergo a global contraction—meaning that while distances between galaxies shrink, the universe itself remains infinite at all times. A finite universe collapsing to zero volume in finite time is just one possibility, but it’s not required for a Big Crunch model. The idea of a finite universe is, of course, not beyond the pale—it remains an open question in cosmology.

Regarding infinite space and "unchanging volume," the key issue is that volume in an infinite universe is not a meaningful quantity in the way you are describing it. Yes, if the universe is infinite now, it was always infinite, but that doesn’t mean nothing changes—the scale factor determines how distances evolve. The phrase "volume cannot change" is misleading because in an infinite universe, there is no finite, well-defined total volume to begin with. Instead, we talk about the expansion or contraction of distances within that infinite space, which is physically meaningful.

Quentin

Do you concede that the universe isn't isotropic or homogeneous? What is the nature of the singularity in the Big Crunch for a finite and infinite universe? How does it differ from the standard BH? AG 

AG, on small scales, the universe is neither isotropic nor homogeneous due to the presence of galaxies, filaments, and voids. However, on large scales, it is effectively homogeneous and isotropic, as confirmed by the cosmic microwave background (CMB) and large-scale surveys. The Cosmological Principle—which assumes large-scale homogeneity and isotropy—remains valid for describing the universe at scales beyond a few hundred megaparsecs.

While the CMB is approximately uniform in temperature, but that's at 380,000 years after the BB, but when we observe it at later times, there are huge filaments with a plethora of galaxies, separated by huge voids. This cannot be isotropic since the scale is hugely large. It's also not homogeneous if we consider the property of density. So, IMO, whereas the universe seems to satisfy the CP at 380,000 years, its subsequent evolution contradicts the CP. AG

Do you agree that the age of the universe is finite, so if it's infinite, that condition could not have evolved over its finite lifetime, but must have been its property as an initial condition? AG

Regarding the Big Crunch singularity, it differs depending on whether the universe is finite or infinite:

Finite Universe: If the universe is closed and finite, a Big Crunch would resemble the time-reversed version of the Big Bang—space collapses to a singularity where density, temperature, and curvature diverge. It’s a true spacetime singularity in GR, where classical physics breaks down.

Do you believe in the BB as a specific event, at time defined as T=0, from which the universe emerged from some underlying substratum? IOW, do you believe the universe came into existence at the BB? AG 

[Quentin Anciaux]

AG, the Cosmological Principle (CP) applies at large scales, not at the scale of individual galaxies, filaments, or voids. While structure formation creates density variations, these variations average out when viewed over hundreds of megaparsecs. The CMB provides the earliest direct evidence of large-scale homogeneity, and while gravitational evolution has produced filaments and voids, the CP still holds statistically when considering the universe at a sufficiently large scale. The fact that structure forms doesn’t contradict the CP—it’s an expected consequence of small initial fluctuations growing under gravity.

Regarding the age of the universe, yes, it’s finite (around 13.8 billion years). If the universe is infinite now, then it must have been infinite from the beginning—infinity doesn’t "grow" in a finite time. This is why an infinite universe was already infinite at the Big Bang, just in an extremely dense and hot state. That’s not an opinion; it follows directly from how GR and the FLRW metric describe an infinite expanding spacetime.

As for the Big Bang (BB), it is best understood as a transition rather than a singular "event." The BB represents the point where classical GR models break down, and physics needs quantum gravity to describe what came "before" (if that question even makes sense). The universe didn’t necessarily emerge from nothing—the BB marks the beginning of our current phase of expansion, but there could have been a prior state (eternal inflation, a bouncing universe, or some other pre-BB phase). GR alone doesn’t tell us whether the universe "came into existence" at T=0—it just describes its evolution from an extremely hot, dense state forward. Look for hot big bang.

Quentin

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[Alan Grayson]

On Thursday, February 20, 2025 at 1:56:36 AM UTC-7 Quentin Anciaux wrote:

AG, the Cosmological Principle (CP) applies at large scales, not at the scale of individual galaxies, filaments, or voids. While structure formation creates density variations, these variations average out when viewed over hundreds of megaparsecs. The CMB provides the earliest direct evidence of large-scale homogeneity, and while gravitational evolution has produced filaments and voids, the CP still holds statistically when considering the universe at a sufficiently large scale. The fact that structure forms doesn’t contradict the CP—it’s an expected consequence of small initial fluctuations growing under gravity.

When we can observe filaments and voids, aren't we observing way beyond hundreds of megaparsecs, and isotropy clearly breaks down? AG 

Regarding the age of the universe, yes, it’s finite (around 13.8 billion years). If the universe is infinite now, then it must have been infinite from the beginning—infinity doesn’t "grow" in a finite time. This is why an infinite universe was already infinite at the Big Bang, just in an extremely dense and hot state. That’s not an opinion; it follows directly from how GR and the FLRW metric describe an infinite expanding spacetime.

As for the Big Bang (BB), it is best understood as a transition rather than a singular "event." The BB represents the point where classical GR models break down, and physics needs quantum gravity to describe what came "before" (if that question even makes sense). The universe didn’t necessarily emerge from nothing—

Right, not necessarily, but if it did emerge from Nothing, could it have become infinite instantaneously? AG

[Quentin Anciaux]

AG, while filaments and voids extend across hundreds of megaparsecs, isotropy only breaks down locally, not globally. If you look at one specific region, it may appear anisotropic, but if you average over sufficiently large volumes, the universe still appears statistically homogeneous and isotropic. Observations of the cosmic microwave background (CMB) and large-scale galaxy surveys confirm this—on scales larger than 1 gigaparsec, the universe still obeys the Cosmological Principle.

Regarding the universe emerging from nothing, if it were spatially infinite now, then yes—it would have had to be spatially infinite from the very beginning. An infinite universe doesn’t "grow" from a finite state in a finite time. If you assume the universe truly began from absolute nothing, then it must have instantaneously been infinite—which itself raises deep questions about the nature of such a transition. This is one of the reasons why many models (such as eternal inflation or cyclic universes) propose pre-existing states rather than a true emergence from nothing, which in itself is something miraculous. 

Quentin 

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[Alan Grayson]

On Thursday, February 20, 2025 at 2:45:02 AM UTC-7 Quentin Anciaux wrote:

AG, while filaments and voids extend across hundreds of megaparsecs, isotropy only breaks down locally, not globally. If you look at one specific region, it may appear anisotropic, but if you average over sufficiently large volumes, the universe still appears statistically homogeneous and isotropic. Observations of the cosmic microwave background (CMB) and large-scale galaxy surveys confirm this—on scales larger than 1 gigaparsec, the universe still obeys the Cosmological Principle.

Regarding the universe emerging from nothing, if it were spatially infinite now, then yes—it would have had to be spatially infinite from the very beginning. An infinite universe doesn’t "grow" from a finite state in a finite time. If you assume the universe truly began from absolute nothing, then it must have instantaneously been infinite—

That's why I conclude the universe is finite, to avoid the singularity of intantaneously becoming infinite. Of course, I can't prove a transition from Nothing to Something. However, that Nothing from which it might emerge, isn't really Nothing, but an underlying substratum, which erupts, roughly analogous to a volcano. Yes, my model is speculative, but so are the other models. AG

What's your take on quantum gravity models? Since measurements show the universe isn't quantize at Planck length, isn't LQG dead on arrival? In general, what are quantum gravity models trying to quantize? AG

[Quentin Anciaux]

AG, whether the universe is finite or infinite, emerging from absolute nothing is equally miraculous. If a substratum exists, its infinity remains an open question. If it’s eternal, that’s already an infinity in itself.

For quantum gravity, if spacetime isn’t quantized at Planck scales, LQG struggles. The real issue: does gravity itself need quantization, or just its interactions? Still unresolved.

Quentin 

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[Alan Grayson]

On Thursday, February 20, 2025 at 2:45:02 AM UTC-7 Quentin Anciaux wrote:

AG, while filaments and voids extend across hundreds of megaparsecs, isotropy only breaks down locally, not globally. If you look at one specific region, it may appear anisotropic, but if you average over sufficiently large volumes, the universe still appears statistically homogeneous and isotropic. Observations of the cosmic microwave background (CMB) and large-scale galaxy surveys confirm this—on scales larger than 1 gigaparsec, the universe still obeys the Cosmological Principle.

Can you cite a paper which supports your claim? Deep space surveys which show the filaments and voids at distances greater than hundreds of megaparsecs. So, since seeing is believing, at huge distances the CP seems to fail. Despite my skepticism, I am willing to read any paper that substantiates your claim. AG 

Regarding the universe emerging from nothing, if it were spatially infinite now, then yes—it would have had to be spatially infinite from the very beginning. An infinite universe doesn’t "grow" from a finite state in a finite time. If you assume the universe truly began from absolute nothing, then it must have instantaneously been infinite—which itself raises deep questions about the nature of such a transition. This is one of the reasons why many models (such as eternal inflation or cyclic universes) propose pre-existing states rather than a true emergence from nothing, which in itself is something miraculous. 

Every thing about the universe is miraculus if you think about it more deeply. I tried to form a model of the early universe which is simplest, and a finite bubble, arising from an infinite substratum, approximately spherical in shape, at an ultra-high temperature, seems to fill the bill. AG 

[Alan Grayson]

On Thursday, February 20, 2025 at 3:43:57 AM UTC-7 Quentin Anciaux wrote:

AG, whether the universe is finite or infinite, emerging from absolute nothing is equally miraculous. If a substratum exists, its infinity remains an open question. If it’s eternal, that’s already an infinity in itself.

For quantum gravity, if spacetime isn’t quantized at Planck scales, LQG struggles. The real issue: does gravity itself need quantization, or just its interactions? Still unresolved.

If spacetime can't be quantized, what can? Interactions? AG 

[Quentin Anciaux]

AG, you’re just shifting the problem—calling the universe a "finite bubble" but embedding it in an infinite substratum doesn’t eliminate infinity, it just moves it up a level. If that substratum is eternal, then you’ve already accepted one form of infinity, making your argument against an infinite universe self-contradictory.

Quentin 

To view this discussion visit https://groups.google.com/d/msgid/everything-list/3a6bc8c6-5a6e-4478-99b2-d4bd20f1fbf6n%40googlegroups.com.

[Alan Grayson]

On Thursday, February 20, 2025 at 10:55:45 PM UTC-7 Quentin Anciaux wrote:

AG, you’re just shifting the problem—calling the universe a "finite bubble" but embedding it in an infinite substratum doesn’t eliminate infinity, it just moves it up a level. If that substratum is eternal, then you’ve already accepted one form of infinity, making your argument against an infinite universe self-contradictory.

Quentin 

I'm not trying to absolutely eliminate all infinities. I'm just trying to construct a model that makes sense to me. AG 

[Alan Grayson]

On Thursday, February 20, 2025 at 8:51:53 PM UTC-7 Alan Grayson wrote:

On Thursday, February 20, 2025 at 2:45:02 AM UTC-7 Quentin Anciaux wrote:

AG, while filaments and voids extend across hundreds of megaparsecs, isotropy only breaks down locally, not globally. If you look at one specific region, it may appear anisotropic, but if you average over sufficiently large volumes, the universe still appears statistically homogeneous and isotropic. Observations of the cosmic microwave background (CMB) and large-scale galaxy surveys confirm this—on scales larger than 1 gigaparsec, the universe still obeys the Cosmological Principle.

Can you cite a paper which supports your claim? Deep space surveys which show the filaments and voids at distances greater than hundreds of megaparsecs. So, since seeing is believing, at huge distances the CP seems to fail. Despite my skepticism, I am willing to read any paper that substantiates your claim. AG 

To observe filaments and voids, how far out are they to be viewed? If you go out 500 megaparsecs, that's slightly less than 1.6 billion light years, a small fraction of the radius of the visable universe, which 46 billion light years. At that distance, if they can be viewed, the universe doesn't seem to be isotropic. AG 

[Quentin Anciaux]

Try to use the internet sometimes.... 

https://www.astro.rug.nl/~weygaert/tim1publication/cosmo2019/cosmology2019.lect3a.cosmological_principle.pdf

https://pages.uoregon.edu/jschombe/cosmo/lectures/lec05.html

To view this discussion visit https://groups.google.com/d/msgid/everything-list/ebc1acc3-eab1-4a83-b489-e329cb80e0acn%40googlegroups.com.

[Alan Grayson]

On Friday, February 21, 2025 at 1:24:18 AM UTC-7 Quentin Anciaux wrote:

Try to use the internet sometimes.... 

https://www.astro.rug.nl/~weygaert/tim1publication/cosmo2019/cosmology2019.lect3a.cosmological_principle.pdf

https://pages.uoregon.edu/jschombe/cosmo/lectures/lec05.html

Interesting. TY.The Gamma Ray Bursts are pretty convincing to establish isotropy and homogeneity. One other thing. In Penrose's oscillating universe model, does he assume the volume expands and contracts periodically, or does he assume it remains infinite in volume throughout, and that the average distances between galaxies increases and decreases periodically? AG 

[Quentin Anciaux]

Le ven. 21 févr. 2025, 09:58, Alan Grayson <agrays...@gmail.com> a écrit :

On Friday, February 21, 2025 at 1:24:18 AM UTC-7 Quentin Anciaux wrote:

Try to use the internet sometimes.... 

https://www.astro.rug.nl/~weygaert/tim1publication/cosmo2019/cosmology2019.lect3a.cosmological_principle.pdf

https://pages.uoregon.edu/jschombe/cosmo/lectures/lec05.html

Interesting. TY.The Gamma Ray Bursts are pretty convincing to establish isotropy and homogeneity. One other thing. In Penrose's oscillating universe model, does he assume the volume expands and contracts periodically, or does he assume it remains infinite in volume throughout, and that the average distances between galaxies increases and decreases periodically? AG 

It assumes it is infinite and remains infinite, only density varies.

Quentin 

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[Alan Grayson]

On Friday, February 21, 2025 at 2:31:28 AM UTC-7 Quentin Anciaux wrote:

Le ven. 21 févr. 2025, 09:58, Alan Grayson <agrays...@gmail.com> a écrit :

On Friday, February 21, 2025 at 1:24:18 AM UTC-7 Quentin Anciaux wrote:

Try to use the internet sometimes.... 

https://www.astro.rug.nl/~weygaert/tim1publication/cosmo2019/cosmology2019.lect3a.cosmological_principle.pdf

https://pages.uoregon.edu/jschombe/cosmo/lectures/lec05.html

Interesting. TY.The Gamma Ray Bursts are pretty convincing to establish isotropy and homogeneity. One other thing. In Penrose's oscillating universe model, does he assume the volume expands and contracts periodically, or does he assume it remains infinite in volume throughout, and that the average distances between galaxies increases and decreases periodically? AG 

It assumes it is infinite and remains infinite, only density varies.

Quentin 

Why do you think some cosmologists posit a universe which is changing in spatial volume, when it could be simplier to assume it's infinite and has always been infinite? Mostly, I've heard the former, which is hard to explain, that it's not like an explosion expanding into a pre-existing space. AG 

[Quentin Anciaux]

AG, some cosmologists consider a changing spatial volume because it's the natural outcome of General Relativity applied to a finite universe with curvature. In a closed (positively curved) universe, the volume changes as the universe expands or contracts.

However, assuming an always-infinite universe is simpler mathematically but conceptually non-trivial, it requires explaining how an infinite universe emerges or evolves without contradictions. The Big Bang isn’t an explosion into pre-existing space, but rather an expansion of space itself, which makes defining an initial condition for an infinite universe more subtle.

Many models, including eternal inflation and some interpretations of the ΛCDM model, favor an always-infinite universe, while others consider a finite but vast one. The debate persists because both scenarios have theoretical and observational challenges.

Quentin 

To view this discussion visit https://groups.google.com/d/msgid/everything-list/8b225aa1-ae4d-44cb-b095-365e303b3e9fn%40googlegroups.com.

[Alan Grayson]

On Friday, February 21, 2025 at 3:20:16 AM UTC-7 Quentin Anciaux wrote:

AG, some cosmologists consider a changing spatial volume because it's the natural outcome of General Relativity applied to a finite universe with curvature. In a closed (positively curved) universe, the volume changes as the universe expands or contracts.

However, assuming an always-infinite universe is simpler mathematically but conceptually non-trivial, it requires explaining how an infinite universe emerges or evolves without contradictions. The Big Bang isn’t an explosion into pre-existing space, but rather an expansion of space itself, which makes defining an initial condition for an infinite universe more subtle.

In an always-infinite universe, there is no need to assume a BB. It's superfluous. It applies to a spatially finite universe which has a creation event (called the BB) and implied by the existence of CMR. This universe has always been expanding, and the contraction phase is hypothetical and will occur under certain specific conditions. AG 

[Quentin Anciaux]

AG, an always-infinite universe doesn’t necessarily remove the need for a Big Bang (BB)—it just reframes it. The BB isn’t just about spatial finiteness; it’s about the hot, dense early state that led to the Cosmic Microwave Background (CMB) and the observed expansion. Even in an infinite universe, the BB still marks a transition from a uniform, high-energy state to the structure we see today.

If the universe was always expanding, that raises the question: expanding from what? Even in models like eternal inflation, there’s still a "beginning" to our observable region, even if the entire multiverse has no start.

A contraction phase remains hypothetical, but the BB framework remains necessary because it explains key observations—CMB, nucleosynthesis, and large-scale structure—not just the origin of a finite space.

Quentin 

To view this discussion visit https://groups.google.com/d/msgid/everything-list/e36ceff6-c08a-4d34-ae0b-137e855dd1dcn%40googlegroups.com.

[Alan Grayson]

On Friday, February 21, 2025 at 4:07:49 AM UTC-7 Quentin Anciaux wrote:

AG, an always-infinite universe doesn’t necessarily remove the need for a Big Bang (BB)—it just reframes it. The BB isn’t just about spatial finiteness; it’s about the hot, dense early state that led to the Cosmic Microwave Background (CMB) and the observed expansion. Even in an infinite universe, the BB still marks a transition from a uniform, high-energy state to the structure we see today.

In the always-infinite universe, the BB is not a singularity, but a state of maximum density and temperature. In this scenario, it would seem possible that we could use GR to determine that state precisely, but it's not the case. Do you have an explanation for this situation? AG

In my view, the universe had a beginning. It emerged from something else, and has been expanding ever since. AG 

[Quentin Anciaux]

AG, an infinite universe can, in principle, reach infinite density at the Big Bang, forming a singularity, but this is where General Relativity breaks down. The singularity predicted by GR is likely a sign that the theory is incomplete at such extreme conditions.

However, if the universe was always infinite, the BB singularity would not be a point-like collapse, but a state of infinite density everywhere. This challenges our usual understanding of singularities, which are typically localized (black holes). In this case, it would be a global singularity, an entire infinite space compressed to infinite density simultaneously.

Most modern models, including Loop Quantum Cosmology and string theory, suggest that quantum effects would prevent this scenario, replacing the singularity with a high-density bounce or transition. This is why many cosmologists favor a finite maximum density rather than true infinity at the BB.

Your view that the universe had a beginning and emerged from something else is plausible, but what that "something else" was remains an open question. Models like eternal inflation or cyclic universes attempt to describe this transition without a singularity, but there’s still no definitive answer.

Quentin

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[Alan Grayson]

On Friday, February 21, 2025 at 11:18:46 AM UTC-7 Quentin Anciaux wrote:

AG, an infinite universe can, in principle, reach infinite density at the Big Bang, forming a singularity, but this is where General Relativity breaks down. The singularity predicted by GR is likely a sign that the theory is incomplete at such extreme conditions.

However, if the universe was always infinite, the BB singularity would not be a point-like collapse, but a state of infinite density everywhere. This challenges our usual understanding of singularities, which are typically localized (black holes). In this case, it would be a global singularity, an entire infinite space compressed to infinite density simultaneously.

Most modern models, including Loop Quantum Cosmology and string theory, suggest that quantum effects would prevent this scenario, replacing the singularity with a high-density bounce or transition. This is why many cosmologists favor a finite maximum density rather than true infinity at the BB.

Your view that the universe had a beginning and emerged from something else is plausible, but what that "something else" was remains an open question. Models like eternal inflation or cyclic universes attempt to describe this transition without a singularity, but there’s still no definitive answer.

Quentin

What is the nature of the singularity when a high mass star collapses to a BH? Is it infinite density at its center, and if so, why doesn't GR breakdown at this point? AG 

[Liz R]

On Saturday, 22 February 2025 at 13:01:41 UTC+13 Alan Grayson wrote:

What is the nature of the singularity when a high mass star collapses to a BH? Is it infinite density at its center, and if so, why doesn't GR breakdown at this point? AG 

It almost certainly does. Unless spacetime is truly a continuum, hence infinitely divisible, there must come a point where it turns into something else - or always is something else, if you prefer - a quantum foam, or whatever.

[Alan Grayson]

I was trying to make a specific point; namely, if a singularity is physically impossible, the question of the existence of a singularity in the interior of a BH is solved. It doesn't exist! AG  

[Brent Meeker]

Of course most physicists wouldn't think of that as a solution unless you had a theory of what replaced the singularity.  They don't take "a singularity" literally as something that exists in the black hole; they know it's just a placeholder for "Here's where general relativity breaksdown".

Brent