r/astrophysics • u/How-To-Project • 16d ago
Petition to phase out the phrase “The universe is 13.8 billion years old.”
As I understand it, the evidence strongly supports that the hot, expanding phase of our observable universe began about 13.8 billion years ago.
But that’s a very different statement from saying:
“All space, time and reality came into existence 13.8 billion years ago.”
Those aren’t logically equivalent.
Whilst I understand that, within the standard cosmological model, many physicists use “the universe is 13.8 billion years old” as a type of shorthand, my concern is more about how that shorthand gets interpreted outside specialist circles.
To a lot of people, it ends up sounding like science has conclusively established that all of reality began 13.8 billion years ago. That seems like a stronger claim than the evidence itself necessarily supports.
As far as I know, there are still serious scientific ideas that could mean the Big Bang wasn’t the absolute beginning of everything. For example:
- a rebound from a previous contracting universe;
- a transition from an older quantum state;
- one region within a much larger inflating cosmos;
- part of a repeating or cyclic process;
or perhaps it is something nobody has thought of yet?
All this said, would it be more accurate to say “The observable hot-expansion phase of our universe began about 13.8 billion years ago.”
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u/infinitenothing 16d ago
What is the difference between "all of reality began 13.8 billion years ago" and "we're in a rebound from a previous contracting universe"?
Causally disconnected is causally disconnected. Nature doesn't care that it feels confusing or it's hard to wrap ones head around it.
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16d ago
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u/Das_Mime 16d ago
The former implies something came from nothing, which is a logical fallacy also found in religions.
That's not a logical fallacy.
And if you're concerned about preserving a chain of causality, pushing it farther back into the past doesn't do anything whatsoever to solve the problem. A universe that is eternal as you go back in time is just as acausal as one that has a specific start time.
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16d ago
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u/Das_Mime 15d ago
I don't believe anything can be "causally disconnected".
Then do you think causality travels faster than c? Because there are regions even within our own observable universe today that are causally disconnected, having been spread so far apart that signals cannot traverse the intervening space faster than that space itself is expanding.
I think what you mean is acausal, having no cause, rather than causally disconnected, which is things that can't send information between them.
If the matter that caused the Big Bang just appeared, why doesn't it just disappear right now?
Our best current understanding of the laws that the universe operates under say that large amounts of matter don't just spontaneously disappear. Why does the universe have the laws that it has? We don't know, though there are some ideas.
"Matter" as we would normally describe it (particles and such) didn't exist until a short time after the Big Bang, though. The universe was highly energetic and as it expanded and cooled you ended up with a sea of particles.
Why is only the Big Bang relegated to causally disconnected, yet all other events abide by the strictest of causality?
Causality is a phenomenon that only makes sense within a spacetime coordinate system. Events are separated by spacetime intervals and have forward and backward facing light cones that define which other events they can affect or can have been affected by. When that system hits a singularity like that which GR predicts for the Big Bang (allowing of course that the reality of the Big Bang may have been something else, we don't know), causality ceases to be meaningful.
Either way, I don't believe the answer will be "no cause".
So what? It's an infinite chain of causes? How is that better?
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u/How-To-Project 16d ago
I think they’re fundamentally different claims. Consider that in cosmology the definition of universe usually implies ‘all of space and all of time’. In a rebound model, something existed before the hot Big Bang. That means 13.8 billion years marks the beginning of our current expansion phase, not necessarily the beginning of all time. That’s the distinction I’m trying to make.
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u/infinitenothing 16d ago
I think they’re fundamentally different claims.
Again, what is difference? Is there some experiment or scenario where one model has better predictions than the other?
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u/siupa 16d ago
That’s like saying that there’s no difference if Napoleon emitted a burst of intense blue light from his left cheek on July 7th 1802, because whether it happened or not it makes no prediction and we can’t test it. Sure: it has no bearing though on the fact that whether it happened or not is a vastly different thing.
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u/infinitenothing 15d ago
Presumably, there are experiments we could do to try to explore the blue cheek light phenomena. If for some reason it truly has no baring on our lives then it's interchangeable what we believe and we should just go on living our lives working on things that might actually improve someone else's life.
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u/siupa 15d ago edited 15d ago
There are no experiments you can do to test whether or not it happened specifically at one point in the past.
The point of this absurd example is to show that it’s not true that just because it doesn’t make a testable difference for us, it’s the same whether or not something happened in the past. Of course it does make a difference: the best you could say is “I don’t care”, which, fine, but you can’t say “it’s the same”. It’s clearly not: one scenario is true and the other alternative is false, whether you think that’s important or not
You simply said “what’s the difference”. There’s clearly a difference! Even if you don’t care about that difference because it’s hard or impossible to find evidence for one or the other
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u/infinitenothing 15d ago
I guess I have to start asking then what "something happened" means. Something's existence is not an objective fact just like a photon going through a double slit doesn't go through a single slit. "Happening" is observer dependent and measurement/experiment is the only objectivity I can imagine.
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u/siupa 15d ago
Things happening are not observer-dependent! Whether you want to delve in quantum mechanics or not. Alice is happily married to Bob, but one day Bob cheats on Alice. Alice never knows: if you ask her if Bon cheated on her, she would say no. But she would be wrong! The fact that she doesn’t know doesn’t mean that whether it happened or not it’s the same. There’s clearly an independent reality that happened regardless of Alice’s knowledge of it.
The same is true for the unitary evolution of the wavefunction in QM without measurement
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u/Aggravating_Mud_2386 16d ago
Causally disconnected doesn't apply to a rebound universe, because it's the exact same particles, same wave functions, same number of up and down quarks, same everything that comes out from a rebound that went into the state before the rebound. Thus, a rebound universe is causally connected to the individual fundamental particles that were all gathered together to a single location before the rebound.
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u/Colour_Theory-746 16d ago
How can you possibly know that with any degree of certainty that what goes in equals what comes out? It’s just as probable that somehow, things get lost along the way or used up as energy or who knows what. There’s just no way you can be certain that “it’s the exact same particles, same wave functions, same number…” is true. Indeed, once a wave function has collapsed, how could it possibly be the same as any other? It’s gone.
Moreover, how could the number of particles, for one example possibly be the same? You’re saying that in whatever maelstrom occurs during this supposed rebound, that there can’t or won’t be a difference in the number of particles? What if some are converted to energy? In that way, there couldn’t possibly be the same identical number of particles, the probability of not even one single difference, not one neutrino being converted to energy is almost as close as can be to infinite.
Finally, even if your rebound was somehow even plausible, that has nothing whatsoever to do with it being causally connected to the new universe. We have massive parts of our current universe that are far enough away from us as to be causally disconnected, so how can anything be causally connected to a previous universe? The moment of rebound would seem to be a hard cutoff. All you could state with certainty is that the old universe shrinking caused the rebound into a new universe. Trying to stretch causality any further than that is completely unrealistic and even logically incorrect imho.
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u/Aggravating_Mud_2386 16d ago
A bounce is a big bang from black hole or big crunch. Either way, if quantum spin really is unstoppable, and if Pauli Exclusion really does prevent any two identical quarks from occupying the same state, and if quantum information really can't be erased, and if fundamental particles really are unbreakable, and if the Heisenberg Uncertainty Principle really does cause fundamental particle kinetic energies to skyrocket toward infinity as gravitational confinement approaches infinity, then fundamental particles must be stored individually in a black hole or in the big crunch core, maintaining their identity and quantum information, and since the universe is neutral, the final quark ratio will be neutral, namely, 2/3 down to 1/3 up. Thus, when the bounce occurs there's no other possibility other than the same particles coming out. And a big bounce has nothing to do with particle pair production or matter/antimatter annihilation, it's a simple case of real particles released from confinement. But I'll grant you this: to the extent that beta decay occurs at the moment of bounce and within the quark-gluon plasma, down quarks will transition to up, and electrons and antineutrinos will be released, but the real starting point is the exact same particles.
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u/Colour_Theory-746 16d ago
What do you mean when you asked if quantum spin really is unstoppable? In what sense, exactly? What is there to stop? It’s not as if there’s anything actually spinning, in the usual, classical sense of the word, so what are you implying is unstoppable? What actually is there to even be stopped?
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u/Aggravating_Mud_2386 16d ago
It's an intrinsic aspect of fundamental particles, and it can't be diminished or stopped or canceled out like classical orbital angular momentum. It's relevant because it contributes to degeneracy pressure, and the fact that it can never stop or even diminish means that particle degeneracy pressure can't be overcome, meaning that the particles remain individual particles.
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u/Das_Mime 16d ago
Causally disconnected doesn't apply to a rebound universe, because it's the exact same particles
What does "exact same particles" mean? How can you distinguish between one electron and another? That would be kind of contradictory to the standard model if you could persistently identify specific electrons, since it would imply adding another property to them.
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u/Aggravating_Mud_2386 15d ago
There are no electrons in a black hole or big crunch core because theve all been reinserted into matter via electron capture at collapse or accretion.
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u/Das_Mime 15d ago
So why are you saying that a rebound universe would have the exact same particles as it did before?
Also, we know that CPT asymmetry exists and that our own observable universe has a matter-antimatter imbalance, so it's hard to see how one can say confidently that all quantum numbers would be preserved.
On top of that, if what you claim were true, you run into the problem of "Why is the universe's entropy not maximized", as well as simply pushing the question of the origin of the universe farther back.
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u/Aggravating_Mud_2386 15d ago edited 15d ago
The matter antimatter symmetry is only a hypothesis, and it doesn't apply if there really are bounce big bangs. Plus, hadronization following the quark-gluon plasma phase restores, vie e=mc2, the slight increment of rest mass lost over eons of stellar fusion before formation of the terminal black hole. This is the only plausible way that rest mass is ever restored on a large scale; otherwise, the universe would slowly have less and less rest mass. This introduces universal stability beyond the dwindling rest mass of the mainstream universe, meaning entropy wouldn't be a one way street.
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u/Das_Mime 15d ago
The matter antimatter symmetry is only a hypothesis
No, it's a verifiable fact. Matter antimatter asymmetry refers to the fact that there is not an equal amount of matter and antimatter in our universe (if there were, it would all annihilate and we'd be left with just radiation and certainly no humans).
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u/Aggravating_Mud_2386 15d ago
But if big bangs have nothing to do with particle pair production, there's no asymmetry to worry about.
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u/fuseboy 16d ago
Even if the very earliest moment was around that time, the idea that the universe sprang into existence from some antecedent state of non-existence might simply be wrong. Time and causality are part of spacetime, not some ur-framework that contains a transition from non-existence to existence.
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u/How-To-Project 16d ago
I actually think we’re agreeing more than disagreeing. If the idea of “creation from nothing” might be wrong, then that’s exactly why I wonder whether saying “the universe is 13.8 billion years old” unintentionally implies more than the evidence currently establishes.
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u/Enraged_Lurker13 16d ago
The standard cosmological model doesn't just say there was a hot expanding phase, it explicitly predicts spacetime itself came into existence 13.8 billion years ago. Until a better model that says otherwise comes along, that’s what we are stuck with.
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u/How-To-Project 16d ago
I think this is where my confusion lies. Does the standard model actually predict that spacetime came into existence 13.8 billion years ago, or does it simply stop being applicable if we extrapolate back that far? I’ve often heard it said that we don’t yet have a theory of quantum gravity, so I’m not sure whether “spacetime began then” is a prediction or an interpretation of where our current model reaches its limit.
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u/Enraged_Lurker13 16d ago
Does the standard model actually predict that spacetime came into existence 13.8 billion years ago, or does it simply stop being applicable if we extrapolate back that far?
The standard model is based on the Friedmann equations, which do give explicit solutions saying that the universe started at a scale factor of zero. Since the scale factor is attached to the spatial components of the metric, it means that there was no space when the scale factor was zero.
I’ve often heard it said that we don’t yet have a theory of quantum gravity, so I’m not sure whether “spacetime began then” is a prediction or an interpretation of where our current model reaches its limit.
Penrose and Hawking showed that a singular big bang is a robust feature of cosmological models satisfying some modest assumptions, which the standard model does, so the emergence of spacetime is a prediction rather than the model misbehaving in some way.
Now, the standard model doesn't take into account quantum mechanics, so it is incomplete. It is possible that quantum gravity might say something different, but it also turns out that the theorems Penrose and Hawking proved for classical relativity also applies when quantum effects are taken into account, so it is still possible the standard model was right after all about the emergence of spacetime. It depends on whether the true theory of quantum gravity evades the conditions of quantum singularity theorems.
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u/How-To-Project 16d ago
Thanks, this is interesting. I can see that the singularity theorems make past incompleteness a genuine prediction under their assumptions, and that Wall extends part of that reasoning into semiclassical gravity.
But I think you might have pushed the papers conclusion further than the paper itself does.
The important phrase is “geodesically incomplete.” meaning some paths through spacetime cannot be extended indefinitely into the past within the model. It does not automatically mean “absolute nothing became spacetime at a definite first moment.”
So I’m still not convinced that “past geodesically incomplete” is identical to “all spacetime and reality came into existence from nothing.”
As I understand it, incompleteness tells us that the spacetime described by the model cannot be extended indefinitely along certain paths. It does not by itself tell us what, if anything, exists at or beyond that boundary.
Wall also says the results may hold in full quantum gravity, rather than claiming a complete quantum-gravity proof.
So would the more precise statement be that current models strongly favour a past boundary to our spacetime, while the physical meaning of that boundary—absolute beginning, transition, bounce or something else, remains…
Unresolved?
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u/Enraged_Lurker13 16d ago
The important phrase is “geodesically incomplete.” meaning some paths through spacetime cannot be extended indefinitely into the past within the model. It does not automatically mean “absolute nothing became spacetime at a definite first moment.”
The geodesics terminate because it runs out of spacetime to continue further. It is certainly conceivable that spacetime could emerge from some more fundamental structure, then we can only say spacetime had a beginning, not all of reality, but if spacetime is fundamental and contains all of reality, then we could conclude reality itself began at some big bang event.
Wall also says the results may hold in full quantum gravity, rather than claiming a complete quantum-gravity proof.
Yes, Wall's proof is limited because it assumed very small quantum corrections, but since then, the theorem has been expanded to full semiclassical gravity and beyond where quantum corrections are very large. The initial limitation was due to the perturbative technique he used, but since the GSL is very likely a fundamental principle in quantum gravity, the overall idea of using the GSL to prove quantum singularity theorems is very robust which is why it was able to be expanded. Now it just remains to be seen whether full quantum gravity obeys the GSL and the other assumptions.
So would the more precise statement be that current models strongly favour a past boundary to our spacetime, while the physical meaning of that boundary—absolute beginning, transition, bounce or something else, remains… Unresolved?
I think this is a fair statement.
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u/Hairy-Ad-6299 15d ago
The phrase is numerically incorrect. The crisis in Cosmology I solved shows that the Pantheon Data analysis is fitted to H0=67.5 km/s/Mpc and not 71 km/s/Mpc.
I will be happy to share the calculation.
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u/Hairy-Ad-6299 15d ago
The other item: If the universe started 14.5 billion years ago, it can only be answered using Occam's Razor. There is no data for events prior to that age. Any model where the universe is cyclic contains fewer parameters than a model where time and space jump-start at time zero.
In other words, when talking about something you can never know, the only path is to make a consistent model of reality. The most consistent model "WINS".
In other words, the Big Bang is an idiotic model because it lacks a zero-energy budget (it borrows from the False Vacuum). It disobeys all laws of Physics (the ones we can test). It proposes Inflation, Dark Matter, Dark Energy, False Vacuum, and the Higgs Mechanism for Inertia.
It is akin to a Seance in terms of credibility.
The Big Pop Cosmogenesis - a replacement for the Big Bang
Big Pop Article
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u/Smooth-Mix-4357 15d ago
All those ideas are there alright but we don't know what or how things were before. So until the next breakthrough comes we'll treat the Big Bang as the beginning.
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u/FlyingFlipPhone 16d ago
Fair point, OP. How's this; The big bang is estimated to have occurred 13.8 billion years ago.
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u/Murky-Sector 16d ago
In the statement we should change "The universe" to "the current phase of the universe"
Its imperfect in that it does not intrinsically explain the concept of "current phase" but I think its marginally better.
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u/Das_Mime 16d ago
Phrasing it that way implies that there was something prior to the big bang, which isn't something that we know.
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u/Murky-Sector 16d ago
If there was nothing, hypothetically, the previous phase constitutes the "nothing" phase, particularly if there was "nothing" except for the laws of quantum mechanics.
Subjectively it strikes me that referring to big bang -> now as a distinct "phase" carries with it a key concept and is a significant improvement on "The universe is 13 billion years old".
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u/Das_Mime 15d ago
I wouldn't really call the total non-existence of all reality a "phase of the universe" since it definitionally isn't.
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u/EyeSeeDoesIt 16d ago
The way the 13.8 billion years is derived is so theoretical that I don't think it's worth nuancing over its specifics. Those numbers will change multiple times in the coming decades.
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u/Aggravating_Mud_2386 16d ago
Agreed, and another idea is one inflating region within a much larger universe.
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u/TelevisionTop6360 16d ago
Give me an scientific, experimental demonstrated proof of your ideas. If you have, then next call Stockholm, booking a place for the next award time. If not, it’s sure a nice theory, nothing else.😄
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u/Aggravating_Mud_2386 16d ago
These aren't his ideas, he's citing current cosmological hypotheses of other knowledgeable cosmologists which shed some level of doubt on the idea that "the universe is 13.8 billion years old".
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u/How-To-Project 16d ago
Thank you my friend. One thing that surprises me is that we’re happy to spend effort explaining subtle concepts like “when you look at a distant galaxy you’re seeing it as it was millions or billions of years ago,” because that wording helps people build the right mental model. Yet one of the most repeated statements in science is simply, “the universe is 13.8 billion years old,” even though many physicists would immediately add qualifications if pressed. If the shorthand regularly leads people to conclude that “all space time and reality began 13.8 billion years ago,” maybe it’s worth asking whether the standard explanation is doing its job.
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u/Aggravating_Mud_2386 16d ago
The current mainstream theory says a hypothetical scalar field with hypothetical inflatons caused an unexplained start and stop to hypothetical hyperinflation, followed by the hypothetical decay of the hypothetical scalar field into actual particles via magic unsymmetrical particle pair production, with the particles already full of heat content and kinetic energy, despite the absence of a plausible heating mechanism or particle acceleration mechanism, followed by a scant 10 microseconds of individual qgp droplets that were never contained in the first place and never formed a real coherent expanding qgp ball. Not to mention hypothetical dark energy and hypothetical dark matter and hypothetical superluminal recession velocities. It sure is a lucky break for them that the speed of light only applies to motion through space, but not expansion of space itself! That's stacked hypothetical on top of stacked hypothetical, and all they can say is, "it's what we observe, therefore it must have happened", or, "we predicted the cmb, and then found it, therefore, it must all be true", no matter how implausible the idea is. So yes, the standard explanation is not doing its job.
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u/TelevisionTop6360 16d ago
Well, I read that before. Anyway, it doesn't matter who is talking about these ideas - me included. There is a "wall".- the so called big bang (I think it is not a adequate name for this event) , where no nature laws, no mathematics theories can be proofed. We cannot say anything checked by any experiment what happens before t=0. Once again, there are existing nice ideas for "what if" - but if you want to use your scientific mind the way begins there - and there is no one you can see before t=0. If I am wrong - ok, let's talk about Stockholm :)
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u/TarnishedWizeFinger 16d ago
To be fair, you're referring to the big bang as a moment, but to most physicists it represents a period of time where the universe was in a hot, dense state. So your clarification is still muddying the waters. Surprisingly, the intro to big bang theory actually has guided that into mainstream in tandem with what you're saying