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IANAP, but as a mathematician it seems extremely inelegant that there would be a start to the time dimension of the space-time object we live in, when we don't

IANAP, but as a mathematician it seems extremely inelegant that there would be a start to the time dimension of the space-time object we live in, when we don't even know if the spatial dimensions are finite themselves. It is my understanding that the density of the universe billions of years ago was radically different from the one we now observe, and since density is intrinsically tied to our perception of space and time, wouldn't it make more sense that time actually stretches infinitely the further back we go, thus nullifying the concept of a beginning? I guess I'm having a hard time with the idea that space-time could be discontinuous.

justrealist 3y ago
s/"universe"/"Observable universe"/ that's the context in which physicists are talking
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kevinventullo 3y ago
As a fellow mathematician, I think maybe the right way to think about physics is that the claims are never even supposed to be “true” in some absolute mathematical sense. It’s more like true to some first approximation. So maybe we can interpret this claim as saying that 25 billion years ago, the universe was kinda similar to what we have now, but 27 billion years ago everything was ultra compressed and gravity didn’t have a significant effect on anything. Or maybe time is like a left-open interval. Finite, but didn’t have a start. I dunno, I’m making stuff up.
- Certhas 3y ago
  Left open interval is the right way to view it. There is no contradiction or paradox here. Merely unfamiliarity. But maybe you should not expect the rules of the early universe to be very familiar...
- User23 3y ago
  Physics is only true modulo theory. It’s intense intellectual arrogance to say “because this model predicts some phenomena the world must conform to this model.” Newton, who I think it’s fair to say was not known for his humility, refused to feign hypotheses and satisfied himself with describing his observations. However in today’s day and age that way lie no fat grants, adulating television specials, or NYT bestsellers.
gmmeyer 3y ago
The question is "how long has it been since the big bang." It's an important and relevant question for cosmology and physics. It isn't really a stance on the "beginning of time," which may have started long before this moment, but it is the start of the universe as far as physics is concerned.
- davorak 3y ago
  > but it is the start of the universe as far as physics is concerned At least what we currently spend most of time studying/researching in physics right now. We can hope to expand beyond that given enough time.
  - philipov 3y ago
    > ...given enough time. There might literally not be enough time to expand beyond that, given how cosmological horizons work. Being part of the system we're trying to observe puts some nasty limits on what we can know, even in principle.
    - outworlder 3y ago
      There are even (ever shrinking) limits on how much of the universe we can observe.
    - kulahan 3y ago
      Pull the levers and see what happens, and then write it down. That's really all we can do.
    - davorak 3y ago
      I agree we practically may never know and fundamentally the rules of the universe might end up making it impossible to know somethings, but as far as I know the ultimate limits are currently unknown so 'We can hope ...'.
    - mensetmanusman 3y ago
      It's kind of cool to think that some idea that someone has at some point in the timeline of consciousness might be true; but impossible to prove.
tomp 3y ago
As a mathematician, I don't think about Big Bang as the "start of time" (or any other dimension), but simply as "fixed point" (of the "physical evolution of all particles in the universe" function).
- Certhas 3y ago
  It's not a fixed point of the equations, though. That is technically incorrect and also not the right intuition. It's the exact opposite. It's a point at which the acceleration of (the density of) all particles diverges, and thus the equations can not be continued past that point.
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marcyb5st 3y ago
While I agree with you on this "hack", I also believe that if we accept that geodesic incompleteness idea is right then time for all intense and purposes has a start. At least as observers within the bubble of space-time that is causally connected. If time/space-time existed before the BigBang is probably an unanswerable question (unless we are within a 4d black hole and we can listen to waves that perturbed the matter before the formation of our universe).
- rightbyte 3y ago
  > If time/space-time existed before the BigBang is probably an unanswerable question Why? Any sign of another Big Bang going from some other point somewhere else would indicate that "our" Big Bang might not have been the first one. It is harder to prove absence though.
  - marcyb5st 3y ago
    Several (possible) reasons: Because if inflation is also correct we lost every causal connection with whatever was there before, or is so diluted that we might not be able to detect. Another possibility could be that at the time of the BigBang the energy density was so high that everything was unified, and so when forces actually separated they "tabula rasa" anything that occupied the bubble of space-time we expanded into. However, it could also be that the fluctuations we see in the CMB are due to perturbations of what happened before. But that possible clue is better explained by comic inflation expanding quantum fluctuations at an incredible speed that disconnected them. That's why I said it's (probably) unanswerable. I also hope I am wrong though.
Certhas 3y ago
It's unclear what exactly you have in mind, but the equations of GR simply predict that the topology of space-time is not that of R^4, but includes boundaries at finite (temporal) distance, so-called space-like singularities [1]. This is not unusual. For example, the solutions to the ODE dx/dt = -1/x topologically live on the half line. [1] https://en.wikipedia.org/wiki/Penrose%E2%80%93Hawking_singularity_theorems https://en.wikipedia.org/wiki/Penrose%E2%80%93Hawking_singul...
nonameiguess 3y ago
This isn't really what "start" is usually intended to mean. They're just identifying the time of the Big Bang, beyond which any possible causal connection is lost and we can't possibly look back further. More time, more space, more something else may have existed and been causally prior, but we can't meaningfully talk about it except speculatively. It isn't part of our spacetime. Of course, people do speculate. I seem to recall some level of anisotropy in the cosmic microwave background that was a bit more than expected purely from quantum vacuum fluctuations in the pre-inflationary early universe and at least one physicist musing that it might be a perturbation from some other universe that has since lost causal connection. This, of course, makes no testable predictions, can't be falsified, isn't really science, but human intellectual curiosity goes beyond science.
lioeters 3y ago
Similarly, I find the idea of the end of time to be weirdly unreal and impossible. It feels like time is not a thing that has a beginning or an end, and that it would just stretch and dissipate infinitely into the future.
- 8note 3y ago
  You can still imagine that as an end, and define an end based on that infinite stretch. At some point, you can't tell the difference between before and after more stretching, and you'll never be able to stretch in a way that can be noticed. There's no more events to happen, and more so, no ability for more events to happen. Thats still likely an artifact of our models though, and that when you do to something like that, that new events start happening again
- empyrrhicist 3y ago
  If electrons decay, there will be a time when there's nothing in the universe which can function as a clock.
  - eis 3y ago
    Which does not mean that time stops. If no one hears a tree fall that does not mean the tree isn't falling.
    - empyrrhicist 3y ago
      Not a physicist, but I know that there are physicists who disagree with that. See: Roger Penrose and CCC.
      - eis 3y ago
        I'm not sure if Penrose suggests that time stops once there are only photons left. He says it is equivalent to the situation at the Big Bang and time certainly didn't stop there. In fact, in his conformal cyclical cosmology spacetime goes on indefinitely but just reboots regularily.
fnordpiglet 3y ago
My understanding is that in models where time began at the Big Bang space typically also began concurrently. Before this time (harhar) there was nothing, but that’s meaningless because time also didn’t exist. It’s immeasurable before a certain point. Likewise space began at that time and expanded to fill all space rapidly, which I’m often a little unclear here, but my understanding is in a similar way to how space expands today outside of galaxies by space simply expanding from within itself (I.e., without force or movement). This conceptualization might help quiet that discontinuity discomfort. I’ll search for it but PBS Space time has some wonderful intuition building visualizations and explanations in some of their parts on models for the beginning of the universe. If I can find the ones I’m thinking of I’ll edit later, but regardless I find their background material on “wtf” re: physics can be helpful.
WallyFunk 3y ago
> wouldn't it make more sense that time actually stretches infinitely the further back we go, thus nullifying the concept of a beginning? I believe it could be 'turtles all the way down' as the phrase goes. Maybe our Universe started by a collision of two or more other Universes and we got this mess called the Big Bang. This doesn't explain how those other Universes started though. But then as mere humans, we can't conceptually grasp Infinity itself. This is not some failing of ours, it's actually convenient to not imagine infinity, as it would drive us mad. The minute you include the Infinity Symbol (∞) in a math equation, all logic starts to cease and get very wobbly.
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- belfalas 3y ago
  > Maybe our Universe started by a collision of two or more other Universes and we got this mess called the Big Bang. Once upon a time I watched PBS Nova program about string theory. I remember they talked about something along these lines. It's been almost two decades since I saw it so memory is rusty. But it was something along the lines of universes existing in these big "planes" (they visualized them like these big floating membranes) that would vibrate and occasionally smack into each other. When one of these collisions would occur, there was the potential for a new plane to be formed from that collision. Like you said, that could potentially explain where new universes come from, but not how the other "turtles" got there in the first place.
  - award_ 3y ago
    That's brane or membrane theory. I think Ed Witten was a big proponent of it? I saw the same documentary btw ;-)
  - montjoy 3y ago
    There’s a theory that the birth of our universe looks eerily similar to what might happen inside a black hole when it’s first formed. Fun to think about. https://en.m.wikipedia.org/wiki/Black_hole_cosmology https://en.m.wikipedia.org/wiki/Black_hole_cosmology
ouid 3y ago
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goodbyesf 3y ago
As I understand it, 'the beginning' is when 'the universe' had a 'once-in-a-gazillion' moment where the probabilistically unlikely event of entropy shrinking to a very small value happened ( aka 'the beginning' ) and from whence entropy started to increase again as it is wont to do. It all goes back to thermodynamics and the probabilistic understanding that entropy 'always' increases. But if entropy always increases then by reason, it must have started off at small minuscule point sometime in the past. But if energy/mass are constant, how could it have gotten to the low entropy in the first place? Given enough time, a 'once-in-a-gazillion' event actually happened. At a fundamental level, it's all mathematical guesswork.
akira2501 3y ago
> that space-time could be discontinuous. Why not? Tons of things are, for example, entropy. I have a crackpot idea that the Big Bang itself was just an "entropy population inversion." The big bang is literally just the moment where the discontinuity occurs.
- misnome 3y ago
  Are you sure that you never read Asimov’s “The last Question”?
  - akira2501 3y ago
    I have not. Judging from the plot summary, it is a very similar idea... albeit from a highly metaphysical perspective.
    - mensetmanusman 3y ago
      It is very short, you should read it!
- akomtu 3y ago
  Another idea: at the end of times, when black holes finish merging into one giant black hole, will it force a big bang like event? If we assume the universe to be a large 3d sphere, what's the topology of space in it if it's just one giant black hole?
  - Thiez 3y ago
    Why would all black holes eventually merge? The mass in our observable universe is constantly decreasing, with more and more of it going forever beyond our reach. For any two distant black holes it seems more likely that they will eventually find themselves forever alone in 'their' universe than that they will merge.
soligern 3y ago
It’s all irrelevant until we can answer what does the universe exist in, which we never will.
- uwagar 3y ago
  could be 'on' too. like on a gigantic tortoise.
  - n_sweep 3y ago
    it's tortoises all the way down
    - uwagar 3y ago
      hey tortoise. u seem above me ;)
- VincentEvans 3y ago
  I propose the universe exists in itself. That the interstellar space - is the same as subatomic space. Its both at the same time, like a möbius strip. Space between stars is space between atoms, etc.
  - rewgs 3y ago
    This is my favorite hypothesis, even though/especially because it makes your mind spin. I could be wrong but I think Roger Penrose has some thoughts along these lines. I may be butchering this and/or injecting my own (admittedly ignorant) pontification into it, but: The universe will keep expanding until entropy reaches a maximum -- the "Big Freeze." Once maximum entropy is reached -- wherein all atoms in the universe are totally dispersed, everything is the same distance from everything else, and the universe is a totally uniform field -- one could say that is very orderly indeed, i.e. the theoretical minimum entropy. I imagine that all of the matter in the universe being perfectly dispersed would quite neatly "fill" the universe. So who's to say such a situation isn't infinitely dense? Much like the moment just before the Big Bang.
- 3y ago
speak_plainly 3y ago
I think that by definition if the universe was at one point a singularity then there is a start to time and space. This idea goes all the way back to Plato and his Parmenidean-inspired rejection of the Pythagorean notion of the Monad as being at the centre of the universe as the Number One implies certain properties like perfection, unity, etc., and we see none of those properties in our existence. This led Plato to argue that the One existed separately from our reality, which was just an imperfect copy associated with the idea of an indefinite Dyad. So it wasn't just that the universe was a densely packed packed ball of all the stuff we see today and it somehow spilled out or burst forward, what existed before was a monad, and all the stuff we see including space-time, the elements, and more were created at the time of the Big Bang.
downWidOutaFite 3y ago
We already have space-time discontinuities in black holes
stephc_int13 3y ago
It could be that our ideas about infinite and continuous values, so-called real numbers, are nothing more than a good model/abstraction up to some point.
upsidesinclude 3y ago
One way to consider it would be taking the limit of 'space' as time approaches negative infinity from a zero point of today. That's kind of silly considering they are coupled, but for practical purposes it is the effective point of 'the big bang'. Also, everything is waves
patrickthebold 3y ago
I don't think that makes space-time discontinuous. There's a 'most northern' point on the earth, and the surface is arguably continuous. So it seems fine to have an 'earliest' point in time and still have continuous space-time.
- nine_k 3y ago
  The surface does not even need to be smooth to be continuous. There is a tip on a needle, but the needle is continuous.
- chriswarbo 3y ago
  > There's a 'most northern' point on the earth, and the surface is arguably continuous. This was Hawking's point of view. One way of thinking about special relativity is that time is at right-angles to space, and everything travels at the same 'proper' speed (c) through this 4d spacetime, just in different directions (hence why "fast" objects experience "time dilation": their 4d heading is pointed largely in a space direction, so they don't progress much in the time direction). This extends to general relativity, which describes how these directions "bend" into each other. For example, the reason objects fall due to gravity can be described as spacetime curving such that "future" points slightly "downwards" (and hence slightly less in the usual time direction; which, again, causes time dilation). AFAIK Hawking's idea was that the curvature at the big bang was so high, that "past" is pointing completely in a spatial direction; and hence there's no such thing as "earlier" anymore. Similar to how "north" near the north pole is at right-angles to "north" at the equator.
chongli 3y ago
It is my understanding that the density of the universe billions of years ago was radically different from the one we now observe It’s even more puzzling than that. Our current model features a period of time called the inflationary epoch which lasted between 10^-36 and 10^-33 or so seconds after the Big Bang, in which the universe expanded by a factor of at least 10^26 along each linear direction [1]. This is unfathomably rapid expansion. It makes sense mathematically but not at all intuitively. It’s totally bizarre and weird and I don’t have even close to the level of background needed to challenge it! [1] https://en.wikipedia.org/wiki/Inflation_(cosmology) https://en.wikipedia.org/wiki/Inflation_(cosmology)
bb88 3y ago
> wouldn't it make more sense that time actually stretches infinitely the further back we go, thus nullifying the concept of a beginning? Matt from PBS space time covered this here. https://www.youtube.com/watch?v=K8gV05nS7mc https://www.youtube.com/watch?v=K8gV05nS7mc
mensetmanusman 3y ago
That would still lend itself to a 'beginning' of when the energy density of space time allowed for what we see today.
Ygg2 3y ago
> spatial dimensions are finite themselves They probably are. See Planck length.
shalmanese 3y ago
There’s a start to the space dimension as well. Imagine a ball 10cm in diameter, it’s possible to imagine a ball that is 9cm smaller than it but not a ball 11cm smaller.
anon291 3y ago
Other than theoretically, the spatial dimensions can only ever be observed to be c * T where T is the total age of the universe. Thus, space is bounded as well. It all comes down to what it means to be bounded. If something preceded the big bang, it cannot be observed (by our current reckoning). Similar to things farther than cT away... Unobservable; unknowable. There's nothing inelegant about it. C is the conversion factor between space and time.
rlupi 3y ago
AFAIK (pardon my limited knowledge of physical and math terms) the spacetime interval is the conserved unit and it has a lower bound in the Plank-scale limit. In the paragraph above, you assume time is a primitive concept, but it is really an emergent phenomenon. The passing of time, moving by some amount of (the plank-scale limit of) spacetime interval, can happen only across connected configurations. Think about evolution in spacetime as a combinatorial game, there can be positions where causal links are not possible, or even empty games. The best analogous I can think of arises in the field of surreal numbers https://en.wikipedia.org/wiki/Surreal_number https://en.wikipedia.org/wiki/Surreal_number, with their built-in concept of generations.
- montjoy 3y ago
  FWIW I don’t think it’s certain yet that time cannot be divided smaller than the plank limit. I remember reading an article some years ago about an experiment indicating that there seems to be no smallest “slice” of time. Unfortunately I can’t find it and my patience for searching google these days is pretty low.
jononomo 3y ago
Sounds like you're just scared of facing the fact that God exists.

justrealist 3y ago

s/"universe"/"Observable universe"/ that's the context in which physicists are talking

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kevinventullo 3y ago

As a fellow mathematician, I think maybe the right way to think about physics is that the claims are never even supposed to be “true” in some absolute mathematical sense. It’s more like true to some first approximation. So maybe we can interpret this claim as saying that 25 billion years ago, the universe was kinda similar to what we have now, but 27 billion years ago everything was ultra compressed and gravity didn’t have a significant effect on anything. Or maybe time is like a left-open interval. Finite, but didn’t have a start. I dunno, I’m making stuff up.

Certhas 3y ago
Left open interval is the right way to view it. There is no contradiction or paradox here. Merely unfamiliarity. But maybe you should not expect the rules of the early universe to be very familiar...
User23 3y ago
Physics is only true modulo theory. It’s intense intellectual arrogance to say “because this model predicts some phenomena the world must conform to this model.” Newton, who I think it’s fair to say was not known for his humility, refused to feign hypotheses and satisfied himself with describing his observations. However in today’s day and age that way lie no fat grants, adulating television specials, or NYT bestsellers.

gmmeyer 3y ago

The question is "how long has it been since the big bang." It's an important and relevant question for cosmology and physics. It isn't really a stance on the "beginning of time," which may have started long before this moment, but it is the start of the universe as far as physics is concerned.

davorak 3y ago
> but it is the start of the universe as far as physics is concerned At least what we currently spend most of time studying/researching in physics right now. We can hope to expand beyond that given enough time.
- philipov 3y ago
  > ...given enough time. There might literally not be enough time to expand beyond that, given how cosmological horizons work. Being part of the system we're trying to observe puts some nasty limits on what we can know, even in principle.
  - outworlder 3y ago
    There are even (ever shrinking) limits on how much of the universe we can observe.
  - kulahan 3y ago
    Pull the levers and see what happens, and then write it down. That's really all we can do.
  - davorak 3y ago
    I agree we practically may never know and fundamentally the rules of the universe might end up making it impossible to know somethings, but as far as I know the ultimate limits are currently unknown so 'We can hope ...'.
  - mensetmanusman 3y ago
    It's kind of cool to think that some idea that someone has at some point in the timeline of consciousness might be true; but impossible to prove.

tomp 3y ago

As a mathematician, I don't think about Big Bang as the "start of time" (or any other dimension), but simply as "fixed point" (of the "physical evolution of all particles in the universe" function).

Certhas 3y ago
It's not a fixed point of the equations, though. That is technically incorrect and also not the right intuition. It's the exact opposite. It's a point at which the acceleration of (the density of) all particles diverges, and thus the equations can not be continued past that point.
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marcyb5st 3y ago

While I agree with you on this "hack", I also believe that if we accept that geodesic incompleteness idea is right then time for all intense and purposes has a start. At least as observers within the bubble of space-time that is causally connected. If time/space-time existed before the BigBang is probably an unanswerable question (unless we are within a 4d black hole and we can listen to waves that perturbed the matter before the formation of our universe).

rightbyte 3y ago
> If time/space-time existed before the BigBang is probably an unanswerable question Why? Any sign of another Big Bang going from some other point somewhere else would indicate that "our" Big Bang might not have been the first one. It is harder to prove absence though.
- marcyb5st 3y ago
  Several (possible) reasons: Because if inflation is also correct we lost every causal connection with whatever was there before, or is so diluted that we might not be able to detect. Another possibility could be that at the time of the BigBang the energy density was so high that everything was unified, and so when forces actually separated they "tabula rasa" anything that occupied the bubble of space-time we expanded into. However, it could also be that the fluctuations we see in the CMB are due to perturbations of what happened before. But that possible clue is better explained by comic inflation expanding quantum fluctuations at an incredible speed that disconnected them. That's why I said it's (probably) unanswerable. I also hope I am wrong though.

Certhas 3y ago

It's unclear what exactly you have in mind, but the equations of GR simply predict that the topology of space-time is not that of R^4, but includes boundaries at finite (temporal) distance, so-called space-like singularities [1]. This is not unusual. For example, the solutions to the ODE dx/dt = -1/x topologically live on the half line. [1] https://en.wikipedia.org/wiki/Penrose%E2%80%93Hawking_singularity_theorems https://en.wikipedia.org/wiki/Penrose%E2%80%93Hawking_singul...

nonameiguess 3y ago

This isn't really what "start" is usually intended to mean. They're just identifying the time of the Big Bang, beyond which any possible causal connection is lost and we can't possibly look back further. More time, more space, more something else may have existed and been causally prior, but we can't meaningfully talk about it except speculatively. It isn't part of our spacetime. Of course, people do speculate. I seem to recall some level of anisotropy in the cosmic microwave background that was a bit more than expected purely from quantum vacuum fluctuations in the pre-inflationary early universe and at least one physicist musing that it might be a perturbation from some other universe that has since lost causal connection. This, of course, makes no testable predictions, can't be falsified, isn't really science, but human intellectual curiosity goes beyond science.

lioeters 3y ago

Similarly, I find the idea of the end of time to be weirdly unreal and impossible. It feels like time is not a thing that has a beginning or an end, and that it would just stretch and dissipate infinitely into the future.

8note 3y ago
You can still imagine that as an end, and define an end based on that infinite stretch. At some point, you can't tell the difference between before and after more stretching, and you'll never be able to stretch in a way that can be noticed. There's no more events to happen, and more so, no ability for more events to happen. Thats still likely an artifact of our models though, and that when you do to something like that, that new events start happening again
empyrrhicist 3y ago
If electrons decay, there will be a time when there's nothing in the universe which can function as a clock.
- eis 3y ago
  Which does not mean that time stops. If no one hears a tree fall that does not mean the tree isn't falling.
  - empyrrhicist 3y ago
    Not a physicist, but I know that there are physicists who disagree with that. See: Roger Penrose and CCC.
    - eis 3y ago
      I'm not sure if Penrose suggests that time stops once there are only photons left. He says it is equivalent to the situation at the Big Bang and time certainly didn't stop there. In fact, in his conformal cyclical cosmology spacetime goes on indefinitely but just reboots regularily.

fnordpiglet 3y ago

My understanding is that in models where time began at the Big Bang space typically also began concurrently. Before this time (harhar) there was nothing, but that’s meaningless because time also didn’t exist. It’s immeasurable before a certain point. Likewise space began at that time and expanded to fill all space rapidly, which I’m often a little unclear here, but my understanding is in a similar way to how space expands today outside of galaxies by space simply expanding from within itself (I.e., without force or movement). This conceptualization might help quiet that discontinuity discomfort. I’ll search for it but PBS Space time has some wonderful intuition building visualizations and explanations in some of their parts on models for the beginning of the universe. If I can find the ones I’m thinking of I’ll edit later, but regardless I find their background material on “wtf” re: physics can be helpful.

WallyFunk 3y ago

> wouldn't it make more sense that time actually stretches infinitely the further back we go, thus nullifying the concept of a beginning? I believe it could be 'turtles all the way down' as the phrase goes. Maybe our Universe started by a collision of two or more other Universes and we got this mess called the Big Bang. This doesn't explain how those other Universes started though. But then as mere humans, we can't conceptually grasp Infinity itself. This is not some failing of ours, it's actually convenient to not imagine infinity, as it would drive us mad. The minute you include the Infinity Symbol (∞) in a math equation, all logic starts to cease and get very wobbly.

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belfalas 3y ago
> Maybe our Universe started by a collision of two or more other Universes and we got this mess called the Big Bang. Once upon a time I watched PBS Nova program about string theory. I remember they talked about something along these lines. It's been almost two decades since I saw it so memory is rusty. But it was something along the lines of universes existing in these big "planes" (they visualized them like these big floating membranes) that would vibrate and occasionally smack into each other. When one of these collisions would occur, there was the potential for a new plane to be formed from that collision. Like you said, that could potentially explain where new universes come from, but not how the other "turtles" got there in the first place.
- award_ 3y ago
  That's brane or membrane theory. I think Ed Witten was a big proponent of it? I saw the same documentary btw ;-)
- montjoy 3y ago
  There’s a theory that the birth of our universe looks eerily similar to what might happen inside a black hole when it’s first formed. Fun to think about. https://en.m.wikipedia.org/wiki/Black_hole_cosmology https://en.m.wikipedia.org/wiki/Black_hole_cosmology

ouid 3y ago

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goodbyesf 3y ago

As I understand it, 'the beginning' is when 'the universe' had a 'once-in-a-gazillion' moment where the probabilistically unlikely event of entropy shrinking to a very small value happened ( aka 'the beginning' ) and from whence entropy started to increase again as it is wont to do. It all goes back to thermodynamics and the probabilistic understanding that entropy 'always' increases. But if entropy always increases then by reason, it must have started off at small minuscule point sometime in the past. But if energy/mass are constant, how could it have gotten to the low entropy in the first place? Given enough time, a 'once-in-a-gazillion' event actually happened. At a fundamental level, it's all mathematical guesswork.

akira2501 3y ago

> that space-time could be discontinuous. Why not? Tons of things are, for example, entropy. I have a crackpot idea that the Big Bang itself was just an "entropy population inversion." The big bang is literally just the moment where the discontinuity occurs.

misnome 3y ago
Are you sure that you never read Asimov’s “The last Question”?
- akira2501 3y ago
  I have not. Judging from the plot summary, it is a very similar idea... albeit from a highly metaphysical perspective.
  - mensetmanusman 3y ago
    It is very short, you should read it!
akomtu 3y ago
Another idea: at the end of times, when black holes finish merging into one giant black hole, will it force a big bang like event? If we assume the universe to be a large 3d sphere, what's the topology of space in it if it's just one giant black hole?
- Thiez 3y ago
  Why would all black holes eventually merge? The mass in our observable universe is constantly decreasing, with more and more of it going forever beyond our reach. For any two distant black holes it seems more likely that they will eventually find themselves forever alone in 'their' universe than that they will merge.

soligern 3y ago

It’s all irrelevant until we can answer what does the universe exist in, which we never will.

uwagar 3y ago
could be 'on' too. like on a gigantic tortoise.
- n_sweep 3y ago
  it's tortoises all the way down
  - uwagar 3y ago
    hey tortoise. u seem above me ;)
VincentEvans 3y ago
I propose the universe exists in itself. That the interstellar space - is the same as subatomic space. Its both at the same time, like a möbius strip. Space between stars is space between atoms, etc.
- rewgs 3y ago
  This is my favorite hypothesis, even though/especially because it makes your mind spin. I could be wrong but I think Roger Penrose has some thoughts along these lines. I may be butchering this and/or injecting my own (admittedly ignorant) pontification into it, but: The universe will keep expanding until entropy reaches a maximum -- the "Big Freeze." Once maximum entropy is reached -- wherein all atoms in the universe are totally dispersed, everything is the same distance from everything else, and the universe is a totally uniform field -- one could say that is very orderly indeed, i.e. the theoretical minimum entropy. I imagine that all of the matter in the universe being perfectly dispersed would quite neatly "fill" the universe. So who's to say such a situation isn't infinitely dense? Much like the moment just before the Big Bang.
3y ago

speak_plainly 3y ago

I think that by definition if the universe was at one point a singularity then there is a start to time and space. This idea goes all the way back to Plato and his Parmenidean-inspired rejection of the Pythagorean notion of the Monad as being at the centre of the universe as the Number One implies certain properties like perfection, unity, etc., and we see none of those properties in our existence. This led Plato to argue that the One existed separately from our reality, which was just an imperfect copy associated with the idea of an indefinite Dyad. So it wasn't just that the universe was a densely packed packed ball of all the stuff we see today and it somehow spilled out or burst forward, what existed before was a monad, and all the stuff we see including space-time, the elements, and more were created at the time of the Big Bang.

downWidOutaFite 3y ago

We already have space-time discontinuities in black holes

stephc_int13 3y ago

It could be that our ideas about infinite and continuous values, so-called real numbers, are nothing more than a good model/abstraction up to some point.

upsidesinclude 3y ago

One way to consider it would be taking the limit of 'space' as time approaches negative infinity from a zero point of today. That's kind of silly considering they are coupled, but for practical purposes it is the effective point of 'the big bang'. Also, everything is waves

patrickthebold 3y ago

I don't think that makes space-time discontinuous. There's a 'most northern' point on the earth, and the surface is arguably continuous. So it seems fine to have an 'earliest' point in time and still have continuous space-time.

nine_k 3y ago
The surface does not even need to be smooth to be continuous. There is a tip on a needle, but the needle is continuous.
chriswarbo 3y ago
> There's a 'most northern' point on the earth, and the surface is arguably continuous. This was Hawking's point of view. One way of thinking about special relativity is that time is at right-angles to space, and everything travels at the same 'proper' speed (c) through this 4d spacetime, just in different directions (hence why "fast" objects experience "time dilation": their 4d heading is pointed largely in a space direction, so they don't progress much in the time direction). This extends to general relativity, which describes how these directions "bend" into each other. For example, the reason objects fall due to gravity can be described as spacetime curving such that "future" points slightly "downwards" (and hence slightly less in the usual time direction; which, again, causes time dilation). AFAIK Hawking's idea was that the curvature at the big bang was so high, that "past" is pointing completely in a spatial direction; and hence there's no such thing as "earlier" anymore. Similar to how "north" near the north pole is at right-angles to "north" at the equator.

chongli 3y ago

It is my understanding that the density of the universe billions of years ago was radically different from the one we now observe It’s even more puzzling than that. Our current model features a period of time called the inflationary epoch which lasted between 10^-36 and 10^-33 or so seconds after the Big Bang, in which the universe expanded by a factor of at least 10^26 along each linear direction [1]. This is unfathomably rapid expansion. It makes sense mathematically but not at all intuitively. It’s totally bizarre and weird and I don’t have even close to the level of background needed to challenge it! [1] https://en.wikipedia.org/wiki/Inflation_(cosmology) https://en.wikipedia.org/wiki/Inflation_(cosmology)

bb88 3y ago

> wouldn't it make more sense that time actually stretches infinitely the further back we go, thus nullifying the concept of a beginning? Matt from PBS space time covered this here. https://www.youtube.com/watch?v=K8gV05nS7mc https://www.youtube.com/watch?v=K8gV05nS7mc

mensetmanusman 3y ago

That would still lend itself to a 'beginning' of when the energy density of space time allowed for what we see today.

Ygg2 3y ago

> spatial dimensions are finite themselves They probably are. See Planck length.

shalmanese 3y ago

There’s a start to the space dimension as well. Imagine a ball 10cm in diameter, it’s possible to imagine a ball that is 9cm smaller than it but not a ball 11cm smaller.

anon291 3y ago

Other than theoretically, the spatial dimensions can only ever be observed to be c * T where T is the total age of the universe. Thus, space is bounded as well. It all comes down to what it means to be bounded. If something preceded the big bang, it cannot be observed (by our current reckoning). Similar to things farther than cT away... Unobservable; unknowable. There's nothing inelegant about it. C is the conversion factor between space and time.

rlupi 3y ago

AFAIK (pardon my limited knowledge of physical and math terms) the spacetime interval is the conserved unit and it has a lower bound in the Plank-scale limit. In the paragraph above, you assume time is a primitive concept, but it is really an emergent phenomenon. The passing of time, moving by some amount of (the plank-scale limit of) spacetime interval, can happen only across connected configurations. Think about evolution in spacetime as a combinatorial game, there can be positions where causal links are not possible, or even empty games. The best analogous I can think of arises in the field of surreal numbers https://en.wikipedia.org/wiki/Surreal_number https://en.wikipedia.org/wiki/Surreal_number, with their built-in concept of generations.

montjoy 3y ago
FWIW I don’t think it’s certain yet that time cannot be divided smaller than the plank limit. I remember reading an article some years ago about an experiment indicating that there seems to be no smallest “slice” of time. Unfortunately I can’t find it and my patience for searching google these days is pretty low.

jononomo 3y ago

Sounds like you're just scared of facing the fact that God exists.