When you think of the Big Bang, you just mean inflation, right? You're not adding a singularity to it, are you?

The problem with putting initial conditions off limits is that virtually everything we observe in the universe is dependant on initial conditions. That is, of the set of all physically possible things we could see, we shouldn't expect to see one universe more than the other. Thus, if we come to see "Christ is King," "Zeus wuz here," "Led Zeppelin rules!," scrawled out in quasars and galaxies at the far end of the cosmos, this shouldn't raise an eyebrow? Because, provided the universe is deterministic, such an ordering would be fully determined by those inscrutable initial conditions.

If there is a mixup here, it is a mixup of terminology. When people say Big Bang, they can mean t = 0 in the Big Bang chronology (the theoretical singularity in the classical relativistic model on which Big Bang theory is based), or they can mean the earliest period where the Big Bang theory is applicable, which comes a little bit after t = 0 (and which would be preceded by Inflation), or they can even mean the entire period from there till now and beyond (the Big Bang universe). The worry about time ending or becoming physically meaningless as it approaches t = 0 is not unfounded, for although we know little about that earliest period, there is reason to think that physical clocks that give time its meaning beyond a mathematical formalism may no longer work there.

Just returning to this: the problem I see here is that assuming "The Big Bang" = T0 is begging the question (not saying you are doing this, just pointing out that this is often done). The Big Bang is a specific scientific theory about the origins of the universe, one which did not initially include, and which does not require Cosmic Inflation. Time does seem to break down at T0, provided T0 exists. However, this does not entail that time breaks down at the Big Bang because the two are not necessarily identical.

I agree that common usage is to call Cosmic Inflation and the Big Bang by the same name, but quite a lot of in-depth treatments of the topic refer to Cosmic Inflation as occuring before/being the cause of the Big Bang, and this is certainly how it was described when the theory was just a hypothesis.

My only point is that we have already taken one giant causal step back from the Big Bang. There are several theories that propose that we might take yet another step back (e.g. Black Hole Cosmology, or the idea that another Big Bang could spring to life in our universe at some point in the far distant future, aeons after it reaches thermodynamic equilibrium).

The problem with putting initial conditions off limits is that virtually everything we observe in the universe is dependant on initial conditions. — Count Timothy von Icarus

And if we expand our limits past the "initial conditions", we're forced to once again address the age old problem of the infinite regress vs the uncaused cause.

Thus, if we come to see "Christ is King," "Zeus wuz here," "Led Zeppelin rules!," scrawled out in quasars and galaxies at the far end of the cosmos, this shouldn't raise an eyebrow? Because, provided the universe is deterministic, such an ordering would be fully determined by those inscrutable initial conditions. — Count Timothy von Icarus

As your examples imply, if we were to treat such patterns as evidence - rather than just shrug - the only answer that suggests itself is to posit a deliberate creation. But then we open ourselves to the immediate counterargument that this just shifts back the "initial conditions" back to the conditions of the creator.

It seems to me we're caught between a rock and a hard place here.

When you think of the Big Bang, you just mean inflation, right? You're not adding a singularity to it, are you? — frank

Historically, the so-called Big Bang theory came first, and theor(ies) of inflation were developed later, around 1980s. Inflation pushes the Big Bang chronology a little further back, introduces some new theoretical posits, but in return it rather neatly explains some of the later features of the early universe.

As I was saying earlier, the informal name Big Bang is variously attached to different theories, periods and events. Sometimes it is even used to refer to the entire cosmological timeline, going back to time zero (which historically has been called "Big Bang singularity," although few believe in an actual singularity.)

In the context of the Past Hypothesis, again for historical reasons, we can take the initial state to be after the hypothetical inflation, perhaps somewhere around the beginning of nucleothynthesis, when hydrogen and helium ions formed. The precise cutoff is not important, because the same considerations can be extended to earlier periods.



The difficulty of applying 19th century equilibrium thermodynamics to the early universe becomes even more severe in earlier periods, however, because they are extremely brief and thermodynamically unstable.

The problem with putting initial conditions off limits is that virtually everything we observe in the universe is dependant on initial conditions. — Count Timothy von Icarus

If that is how the theory is structured, yes. But that's a feature, not a bug. You could alternatively explain initial conditions in terms of later features - that is essentially what anthropic explanations do.

That is, of the set of all physically possible things we could see, we shouldn't expect to see one universe more than the other. Thus, if we come to see "Christ is King," "Zeus wuz here," "Led Zeppelin rules!," scrawled out in quasars and galaxies at the far end of the cosmos, this shouldn't raise an eyebrow? Because, provided the universe is deterministic, such an ordering would be fully determined by those inscrutable initial conditions. — Count Timothy von Icarus

I am not sure what this fantastical hypothetical is supposed to argue.

Cool, thanks

Seeing text written in English using galaxies wouldn't undercut the Copernican Principal? I mean, the universe would be writing in human language on the largest scales we can observe... at that point, if you keep the principal it has become dogma, something religious that can't be overturned by new observations.

Likewise, if we uncovered some sort of ancient Egyptian code in our DNA that said something like "we came from other stars to give you intelligence, send some light beams at these points when you read this," I would certainly start to take the History Channel loons more seriously, rather than shrug.

The likelihood of such a code is such that it would be solid evidence for ET conspiracies IMO. But if both potential causes of the message, random fluke and alien intervention, are both entirely dependant on initial conditions and their deterministic evolution, then why would we assign more likelihood to one versus the other?

But this equal likelihood assignment is prima facie unreasonable and isn't how science operates in the first place.

Now if the universe isn't deterministic, then I don't know why what we know about the stochastic nature of the universe doesn't start applying to likelihoods off the bat, at T0 +1, in which case some aspects of the earliest observable positions of the universe can be more or less likely.




It's supposed to argue that, were we to see something like a message in English written in block letters using galaxies as pixels, I doubt anyone would seriously say 'welp, that's consistent with the laws of physics and could have been caused by initial conditions. We can't say anything about this observation being surprising.'

That seems like a bug not a feature. It reduces explanations of all phenomena consistent with fundemental physics to the inscrutable origins of being.

Not that people haven't made this argument. There are arguments for "law-like," initial conditions, as in "like physical laws, they are brite facts." But these normally posit some sort of loose boundary condition on initial conditions, along the lines of "it is a brute fact that the universe must start with low entropy."

But to claim that all initial conditions are completely immune to statistical analyses is to say that, if the universe is rigidly deterministic (a popular view), then all observations are necessary due to initial conditions; they occur with P = 1.

I don't see how this can't reduce debates about topics like the likelihood of life coming to exist in the universe to triviality. That is, the presence of "organic" compounds in asteroids, the number of potential DNA codons, the possible combinations of amino acids, the likelihood of RNA overcoming error rate problems in x hundred million years, etc. seems to become nonsense. If life occured, it is because of the initial conditions of the universe specify it with P = 1. So, if life only occurs on Earth, if life spreads out from Earth for another 10 billion years, moving on to other galaxies, never finding another origin point, our descendants should still say that there is nothing surprising about life's starting on just one planet out of trillions? But then this is not consistent with the frequentist perspective since the frequency of planets generating life is indeed vanishingly low.

The problems I see here are:

1. How does one ground probabilities when everything in the universe happens with probability = 1? A subjective approach maybe?

2. If the universe isn't rigidly deterministic, if it is stochastic, like it appears to be observationally, then the entropy of the early universe is going to be dependent on stochastic quantum fluctuations existing at the beginning of the universe. We don't have a great idea how physics works at these energy levels, but we can certainly extrapolate from high energy experiments and the underlying mathematics, in which case it seems we can say something about the likelihood of the earliest observable conditions.

3. I still don't see how you're able to use statistical analysis in early universe cosmology to support or reject any theories in this case. If someone says "look, this background pattern is extremely unlikely unless the value of the weak force changed in such and such a way, a change that seems possible due to experimental data from CERN," why isn't the response "but other things could cause that same pattern and we can say nothing about the likelihood of such a pattern emerging since it is dependent on initial conditions."

Also, I don't see why observing seemingly unlikely phenomena requires positing any sort of creator or designer. Why can't we just assume some sort of hitherto unforseen mechanism that makes the seemingly unlikely, likely? E.g., people used to think the complexity of life required a creator, but then the mechanisms underpinning evolution were discovered.

Seeing text written in English using galaxies wouldn't undercut the Copernican Principal? I mean, the universe would be writing in human language on the largest scales we can observe... at that point, if you keep the principal it has become dogma, something religious that can't be overturned by new observations. — Count Timothy von Icarus

To me, the Copernican Principle is just that - a principle. It's not a physical law that's based on observation. It's more a method of inquiry.

The purpose of the principle is to prevent a kind of empirical special pleading, by insisting that we should avoid invoking some unexplained special local condition when explaining phenomena.

Likewise, if we uncovered some sort of ancient Egyptian code in our DNA that said something like "we came from other stars to give you intelligence, send some light beams at these points when you read this," I would certainly start to take the History Channel loons more seriously, rather than shrug.

The likelihood of such a code is such that it would be solid evidence for ET conspiracies IMO. But if both potential causes of the message, random fluke and alien intervention, are both entirely dependant on initial conditions and their deterministic evolution, then why would we assign more likelihood to one versus the other? — Count Timothy von Icarus

I'm not following your example here. What you describe is a standard case of evidence for a particular theory - in this case that human life was seeded by aliens. There's nothing here that poses any problem for the scientific method.

The problems only start when we contemplate evidence for the source of all possible observations (the "initial conditions").

The scientific method is based on the premise that observation is the arbiter of truth. But that requires that you can differentiate between inputs based on the output. In other words it must be possible to construct a theory that predicts only some observations but not others.

That does not seem to be possible when we contemplate the "initial conditions". Whatever they are, they must always explain all possible observations. It is therefore in principle impossible to assess these using the scientific method.

Also, I don't see why observing seemingly unlikely phenomena requires positing any sort of creator or designer. Why can't we just assume some sort of hitherto unforseen mechanism that makes the seemingly unlikely, likely? E.g., people used to think the complexity of life required a creator, but then the mechanisms underpinning evolution were discovered. — Count Timothy von Icarus

Talking about "intial conditions" seems to rule out any mechanism since, by definition, the conditions must be present before any mechanism has operated.

The initial conditions cannot be contingent, else whatever they're contingent on is actually the initial conditions.

Sure, but how do you ever know what are actually initial conditions and what just appear to be the earliest initial conditions you can make out? This is why I think it is confusing to have turned "the Big Bang" into "whatever moment is T0," i.e., making it tautologically equivalent to T0, such that inflation is now just "part of the Big Bang." If internal inflation is the case, which is not an unpopular opinion, there is no T0 in the first place. Such a move requires you to assume a T0.

The initial Big Bang theory left several things unexplained. I don't get how you can accept inflation and claim initial conditions are unanalyzable consistently. Alan Guth proposed inflation because CMB uniformity and black body radiation seemed incredibly unlikely without some sort of mechanism at work.

From the 1920s up through the 1970s, scientists thought they had a satisfactory story for our cosmic origins [the Big Bang], and only a few questions remained unresolved. All of them, however, had something in common: they all asked some variety of the question, "why did the Universe begin with a specific set of properties, and not others?" — Ethan R. Siegel

• Why was the Universe born perfectly spatially flat, with its total matter-and-energy density perfectly balancing the initial expansion rate?
  
• Why is the Universe the exact same temperature, to 99.997% accuracy, in all directions, even though the Universe hasn't existed for enough time for different regions to thermalize and reach an equilibrium state?
  
• Why, if the Universe reached these ultra-high energies early on, are there no high-energy relics (like magnetic monopoles) predicted by generic extensions to the Standard Model of particle physics?
  
• And why, since the entropy of a system always increases, was the Universe born in such a low-entropy configuration relative to its configuration today?

In physics, we have two ways of dealing with questions like these. Because all of these questions are about initial conditions ⁠— i.e., why did our system (the Universe) begin with these specific conditions and not any others ⁠— we can take our pick of the following: — Ethan R. Siegel

• We can attempt to concoct a theoretical mechanism that transforms arbitrary initial conditions into the ones we observe, including that reproduces all the successes of the hot Big Bang, and then tease out new predictions that will allow us to test the new theory against the old theory of the plain old Big Bang without any alterations.
• Or, we can simply assert that the initial conditions are what they are and not only is there no explanation for those values/parameters, but we don't need one.

Although it's not clear to everyone, the first option is the only one that's scientific (emphasis mine); the second option, often touted by those who philosophize about the landscape or the multiverse, is tantamount to giving up on science entirely.

The big idea that actually succeeded is known, today, as cosmic inflation. In 1979/80, Alan Guth proposed that an early phase of the Universe, where all the energy wasn't in particles or radiation but in the fabric of space itself instead, would lead to a special type of exponential expansion known as a de Sitter phase....

In fact, our entire observable Universe contains no signatures at all from almost all of its pre-hot-Big-Bang history; only the final 10-32 seconds (or so) of inflation even leave observably imprinted signatures on our Universe. We do not know where the inflationary state came from, however. It might arise from a pre-existing state that does have a singularity, it might have existed in its inflationary form forever, or the Universe itself might even be cyclical in nature.

There are a lot of people who mean "the initial singularity" when they say "the Big Bang," and to those people, I say it's long past due for you to get with the times. The hot Big Bang cannot be extrapolated back to a singularity, but only to the end of an inflationary state that preceded it. We cannot state with any confidence, because there are no signatures of it even in principle, what preceded the very end-stages of inflation. Was there a singularity? Maybe, but even if so, it doesn't have anything to do with the Big Bang.
— Ethan R. Siegel

https://www.forbes.com/sites/startswithabang/2019/10/22/what-came-first-inflation-or-the-big-bang/?sh=508320894153

The point about the aliens is just this: if everything observable evolves deterministically from initial conditions, and initial conditions are unanalyzable, then the likelihood of any observation has no objective value. You could still ground probability in subjective terms, which is perhaps justified anyhow for other reasons, but it seems like a bad way to arrive at such a conclusion. The thing that has always kept me from embracing a fully subjective approach to probability is: (1) the existence of abstract propensities that seem isomorphic to physical systems and; (2) that fully subjective probability makes information theory arguably incoherent, which is not great since it is a single theory that is able to unify physics, biology, economics, etc. and provides a reasonable quantitative explanation for how we comes to know about the external world via sensory organs (leaving aside the Hard Problem there).

If everything doesn't evolve deterministically from initial conditions, then we can say some observations of the early universe are more likely than others by generalizing our knowledge of QM back to the very earliest moments of the universe (which are only observable indirectly in how later epochs look). That is, we can use our current knowledge to say how clumpy the universe should be based on our understanding of the fluctuations that ostensibly caused those clumps. And if the earliest observable parts of the universe are subject to those probabilities, then the Past Hypothesis is still fair game.

I can't really comment on the merits of theories on the early universe from a physics perspective. I don't have the requisite background.

But in terms of general principle, I don't think there's any method to determine the "real" initial conditions empirically. Nor do I see any other source of this information. In general, it's not possible to assess whether your information about speculative questions is complete.

To avoid a misunderstanding: I'm not saying nothing can be known about initial conditions. There's a difference though between asking what the conditions might have been, and why they are.

In physics, we have two ways of dealing with questions like these. Because all of these questions are about initial conditions ⁠— i.e., why did our system (the Universe) begin with these specific conditions and not any others ⁠— we can take our pick of the following:

We can attempt to concoct a theoretical mechanism that transforms arbitrary initial conditions into the ones we observe, including that reproduces all the successes of the hot Big Bang, and then tease out new predictions that will allow us to test the new theory against the old theory of the plain old Big Bang without any alterations.
Or, we can simply assert that the initial conditions are what they are and not only is there no explanation for those values/parameters, but we don't need one.

Although it's not clear to everyone, the first option is the only one that's scientific (emphasis mine); the second option, often touted by those who philosophize about the landscape or the multiverse, is tantamount to giving up on science entirely. — Ethan R. Siegel

Siegel's conclusion here strikes me as rather ridiculous. Science, in the strict sense, is a specific method for specific kind of question. There is no reason to assume every question must have an answer.

It is also somewhat ironic, because in practice, option one is really just a way to sidestep dealing with option two. If there is an event horizon that causality can pass through in one direction, but cannot be followed through backwards, then that neatly rescues determinism from the paradox of the first cause. The problem is that while inflation may scramble information about the initial conditions so as to make the unreadable, under a deterministic framework it cannot destroy that information. So all we do is draw a veil over the initial conditions and declare them inaccessible, but that does not make them disappear.

The point about the aliens is just this: if everything observable evolves deterministically from initial conditions, and initial conditions are unanalyzable, then the likelihood of any observation has no objective value. You could still ground probability in subjective terms, which is perhaps justified anyhow for other reasons, but it seems like a bad way to arrive at such a conclusion. The thing that has always kept me from embracing a fully subjective approach to probability is: (1) the existence of abstract propensities that seem isomorphic to physical systems and; (2) that fully subjective probability makes information theory arguably incoherent, which is not great since it is a single theory that is able to unify physics, biology, economics, etc. and provides a reasonable quantitative explanation for how we comes to know about the external world via sensory organs (leaving aside the Hard Problem there). — Count Timothy von Icarus

I don't think the conclusion is necessary. I think you're getting trapped here in insisting on the deterministic connection, as this will inevitably reduce all events to an inscrutable ballet of energies and forces. When we make sense of the world, we construct models, and within these models probabilities are still objective in the sense you're using it.
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