B If the Multiverse is correct why don't they appear inside our own?

[Chris Gascoigne]

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If the initial conditions for our Universe was empty, timeless space and a quantum fluctuation created the entire Cosmos why have we not seen this repeated endlessly in the empty space within our own Universe?

 

[PeterDonis]

The "empty, timeless space" you refer to, that occurs in hypotheses about the universe being created by a quantum fluctuation, is not at all the same as the "empty space" within our own universe.

 

[Chris Gascoigne]

What are the differences?

 

[PeterDonis]

What are the differences?

It depends on the specific hypothesis. Can you give any references to particular examples that you are interested in?

 

[Chris Gascoigne]

What is the nature of space pre big bang and what is the nature of empty space inside our universe?

 

[PeterDonis]

What is the nature of space pre big bang

As I said, it depends on the particular hypothesis about what was "pre big bang". There are multiple hypotheses, which is why I asked you for particular references since I don't know from where are you getting your impression of what the "pre big bang" state was like.

The front runner seems to be eternal inflation, in which the state pre big bang is a "false vacuum" state of the inflaton field--i.e., a state which is not really a vacuum because there is a lot of energy stored in the inflaton field, but in which the usual Standard Model fields that make up the matter and radiation we are familiar with (quarks, leptons, photons, etc.) are all in their vacuum states.

what is the nature of empty space inside our universe?

A state in which all fields are in their vacuum states, i.e., a true vacuum state.

 

[Chris Gascoigne]

So we do not know the conditions of anything pre big bang we only assume that it must have been a state capable of producing a quantum fluctuation capable of inflating into the known universe. If we can only hypothese about this state how can we be certain that it is different from so called empty space? I say so called empty space because I was under the impression that what we call a vacuum state is actually a seething with quantum fluctuations and matter and anti-matter particles that come into existence from 'nothing' and instantly annihilate each other. See this attached computer simulation of a true vacuum.

 

[PeterDonis]

we do not know the conditions of anything pre big bang

It's true that we don't know exactly what those conditions were. Proponents of inflation make arguments that we do actually have evidence to show that there was an inflationary epoch preceding the big bang; but it's not clear that those arguments have become generally accepted by cosmologists.

But that's not the same as saying we know nothing about what those conditions were. We know that they must have been quite different from the state of "empty space" in our current universe, because we know that that state is not capable of causing another big bang. See further comments below.

we only assume that it must have been a state capable of producing a quantum fluctuation capable of inflating into the known universe

It's not entirely clear to me that this is an accurate description of any of the hypotheses being considered about what the pre big bang state was like. That's why I keep asking you for a reference.

If we can only hypothese about this state how can we be certain that it is different from so called empty space?

Because the "empty space" of our current universe--meaning a state in which all quantum fields are in their vacuum state--cannot cause another big bang. All of the hypotheses about the pre-big bang state involve the presence of something that stores a large energy density (in inflation models it's the inflaton field); that large energy density then gets transferred to the Standard Model fields in the big bang event. Our current "empty space" state does not have anywhere to store such a large energy density, since that requires at least one field (again, in inflation models it's the inflaton field) to not be in its vacuum state.

I say so called empty space because I was under the impression that what we call a vacuum state is actually a seething with quantum fluctuations

This is a wrong impression, although unfortunately many pop science sources promote it. See this Insights article:

https://www.physicsforums.com/insights/vacuum-fluctuation-myth/

You will also find plenty of previous PF threads discussing this.

See this attached computer simulation of a true vacuum.

Where are these from? Again, you need to give a reference.

 

[mathman]

This discussion will get nowhere. What was before the big bang (even if definable) and how it happened is pure guesswork!

 

[DaveC426913]

Not quite guesswork - a better term is conjecture.

 

[PeterDonis]

What was before the big bang (even if definable) and how it happened is pure guesswork!

Tell that to all the physicists who are working on inflationary models and who have proposed possible tests of those models based on cosmological data.

 

[Drakkith]

This discussion will get nowhere. What was before the big bang (even if definable) and how it happened is pure guesswork!

Not necessarily. This assumes that the big bang was 'the' creation event of the universe, something which may or may not be true. It is entirely possible that the big bang was simply a transition phase from a previous state for a universe that already existed. Many other possibilities exist.

 

[PeterDonis]

This assumes that the big bang was 'the' creation event of the universe, something which may or may not be true.

In fact, if the term "big bang" is correctly interpreted according to how cosmologists actually use it in practice (as opposed to how it is used in many pop science sources), it is almost certainly not the "creation event" of the universe. The big bang as cosmologists use the term in practice means the hot, dense, rapidly expanding state, with all of the Standard Model fields having a very high energy density, that is the earliest state for which we have reasonably good evidence. But that state almost certainly arose from something else (the best current front-runner being a previous epoch of inflation) and didn't just pop into being from nothing.

 

[Chris Gascoigne]

The initial condition of our universe must have been absolutely nothing. Pre inflaton field, pre big bang, pre any hypotheses. Any initial condition would be immutable and would therefore still remain - anything subject to change can only do so on a background not subject to change. Inside our universe, between the various kinds of matter, exists absolutely nothing. If universe producing big bangs are a natural consequence of nothing then they would be popping into existence anywhere that nothing exists. This would be observable within our own universe. As we do not see this it must follow that the multiverse theory is fundamentally flawed and not worth hypothesising about.

 

[Drakkith]

The initial condition of our universe must have been absolutely nothing.

That assumes that there was an initial condition to the universe, which is not necessarily true, especially in the case of an infinitely old universe. Besides, if absolutely nothing exists, no matter, no fields, no nothing, then it's hard to say that the universe exists at all. And then you're back to the whole 'something from nothing' dilemma.

Inside our universe, between the various kinds of matter, exists absolutely nothing.

That's not strictly true. The underlying fields that give rise to matter and radiation still exist.

If universe producing big bangs are a natural consequence of nothing then they would be popping into existence anywhere that nothing exists.

Even if this is true, the universe has no truly empty space if we count the underlying fields.

As we do not see this it must follow that the multiverse theory is fundamentally flawed and not worth hypothesising about.

Nonsense. There are several different theories regarding multiverses, not all of them requiring some kind of big bang event separate from the one that possibly gave rise to our universe. If they were all fundamentally flawed, at least fatally, they would not be given any real treatment in science. Besides, our understanding of the universe is still very limited, and these multiverse theories are not thought of as anywhere near complete and certainly aren't accepted as fact.

 

[Klystron]

{snip ... contradictory statements.}
Inside our universe, between the various kinds of matter, exists absolutely nothing. {...snip}

Ignoring other fields for the sake of discussion, cosmic microwave background radiation (CMBR) permeates 'our universe between the various kinds of matter', readily detected by sensitive radio frequency (RF) receivers.

 

[PeterDonis]

The initial condition of our universe must have been absolutely nothing.

Inside our universe, between the various kinds of matter, exists absolutely nothing.

Wrong in both cases. Posts #15 and #16 have given good explanations of why.

If universe producing big bangs are a natural consequence of nothing

They aren't. I've already explained what they are a natural consequence of, and it's not "nothing", nor is it "the empty space that exists in our current universe".

At this point you do not seem to be responding to what others are saying, but simply to continue repeating your initial points. If you cannot address the various responses you have been given, there is no point in continuing this thread. Can you?

 

[Chris Gascoigne]

I can indeed although by now I am confused. Firstly PeterDonis tells me that space within our own universe is a complete vacuum (which I did not accept) then I am told that everywhere in our universe if full of the fundamental fields. Then Drakkith suggested that I am talking nonsense shortly followed by the assertion that we know very little about our universe.
What I am seeking to do is eliminate the Multiverse theories from my cosmological enquires by way of reason. We do not know the conditions that gave rise to the big bang we only understand a portion of what was created 'chronologically' by this event. It is unlikely that the initial conditions would be self destructive and therefore universes would be compelled to be produced in every instant for infinity. Even if we follow Roger Penrose's eternal inflation redefining of universal scale hypothesis we still require a primary state and we may as well start with our own big bang, or pre big bang.
If our universe is infinite then there must be pockets within it consisting of the exact same conditions from which our own big bang was created and therefore these would be observable. If our universe is not infinite then, from the very moment of it's inception, there would be universes constantly created from every single point that exists beyond the rim of our universe and these would be, relative to us, inward facing. The visual image here is of an expanding bubble, on the outer edge of this sphere whatever the circumference was would exist the same initial conditions that created our own bubble. These bubbles would have equal force and therefore our universe would look like heavy rain on a pond as each universe interfered with each other, ad infinitum. Life such as ours would not be possible in such a Cosmos.
For me the two questions are: how can there be anything at all and how can there be only one thing, our Universe. The first question is easily answered because in a state where time and space do not exist all possibilities are inevitable, we live in one of those possibilities. The second question is more vexing.

This from Sabine Hossenfelder goes part way to dismissing the Multiverse theories, for me anyhow.

 

[Drakkith]

What I am seeking to do is eliminate the Multiverse theories from my cosmological enquires by way of reason.

You cannot unless you throw away reason itself. We just don't know enough to confidently say that multiverses, in every conceivable form, are impossible.

For me the two questions are: how can there be anything at all and how can there be only one thing, our Universe. The first question is easily answered because in a state where time and space do not exist all possibilities are inevitable, we live in one of those possibilities. The second question is more vexing.

You haven't answered the first question, you've merely made an assertion that does not hold up under even cursory examination. Why would any possibility, let alone all possibilities, exist in a state in which the fundamental aspects that make up our universe don't exist themselves? The existence of possibilities requires that something exist, some thing that can vary in either time or space. It's like saying it's possible to play roulette in a universe without a roulette table!

If our universe is infinite then there must be pockets within it consisting of the exact same conditions from which our own big bang was created and therefore these would be observable.

You are correct here except for the part about them being observable. The finite age combined with the expansion of the universe places a limit to how far away we can see, and our viewing boundary is very, very small compared to how far you would have to travel to have a reasonable chance of stumbling upon one of these identical 'sub-universes'.

We do not know the conditions that gave rise to the big bang we only understand a portion of what was created 'chronologically' by this event. It is unlikely that the initial conditions would be self destructive and therefore universes would be compelled to be produced in every instant for infinity.

I agree with the first sentence. However the second sentence is just another unsupported assertion. You have given no explanation as to why universes would be compelled to be continually produced for all of time. It can't be based on known science, as cosmologists readily agree that we don't know the initial conditions.

If our universe is not infinite then, from the very moment of it's inception, there would be universes constantly created from every single point that exists beyond the rim of our universe and these would be, relative to us, inward facing. The visual image here is of an expanding bubble, on the outer edge of this sphere whatever the circumference was would exist the same initial conditions that created our own bubble.

Beyond what I've already said about universes constantly spawning, I think you're missing the possibility that the universe is finite, but unbounded in some manner. That is, it is finite in size and volume but lacks any kind of hard boundary or border (i.e. a closed manifold). Moving in one direction would, eventually, lead you all the way back around to your starting point without you every having to change direction. Imagine the 3d equivalent to the 2d surface of a sphere. You can move around the surface (and only the surface, not up or down) forever, in any direction, without ever encountering a boundary. Go up a dimension and you can move in any direction in 3d space and both never encounter a boundary and never leave this finite region of space.
See more here: https://en.wikipedia.org/wiki/Shape_of_the_universe#Global_universe_structure

This from Sabine Hossenfelder goes part way to dismissing the Multiverse theories, for me anyhow.

Just looking at the title makes me want to smash my head on my keyboard. Anyone saying science, or any part of science, is like religion almost certainly has a profound misunderstanding of both science and religion.

I don't really understand the problem you have with multiverse theories. They are, at best, infant theories that will probably be waiting a very, very long time for any kind of experimental or observational results to support them sufficiently to be accepted as 'correct'. Like all theories that are lacking supportive evidence to validate and distinguish them from their competitor theories, these should not be treated as being accurate or correct. They should merely be given the status of 'interesting possibilities in need of further investigation'.

 

[Chris Gascoigne]

You have given no explanation as to why universes would be compelled to be continually produced for all of time.

For exactly the same reasons as our universe came into existence.

I don't really understand the problem you have with multiverse theories.

I see them as a waste of time and energy and the responses to my initial query have served to confirm this. Every response has been pure conjecture; often contradictory.

Just looking at the title makes me want to smash my head on my keyboard. Anyone saying science, or any part of science, is like religion almost certainly has a profound misunderstanding of both science and religion.

I believe Sabine Hossenfelder is a highly respected physicist - don't shoot the messenger!

The question ought not to be 'where are the multiverses?' but 'what initial conditions would be capable of producing just one universe?'. The Cosmos is the answer we just don't know what the question was. Multiverse is a rabbit hole.

 

[weirdoguy]

universe is a complete vacuum (which I did not accept) then I am told that everywhere in our universe if full of the fundamental fields.

Do you understand what "vacuum" means in the context of QFT? Do you know what a state of a field is? I think that your problem is exactly this - lack of understanding of some aspects of the topic. People here are using words in their technical meaning. Vacuum is not a synonim for simply "empty space" because that phrase is ambiguous as is stated. Ignoring what other people say and repeating your own misunderstandings won't help. Ask questions instead.
Besides, you've been asked for references e.g. for the pictures in post #7. Another thing you ignored.

I believe Sabine Hossenfelder is a highly respected physicist

And yet she's making some stupid comparisons. Being a highly respected physicist doesn't mean that every word you say is a revelation. That's why scientific papers are peer reviewed. This video and her other personal opinions are not.

 

[Ibix]

Do you understand what "vacuum" means in the context of QFT?

@Chris Gascoigne - note, for example, the distinction between false and true vacuum made by PeterDonis in #6, and Peter's use of scare quotes around "empty space" when he uses it in two different senses in #2, both of which you seem to have missed.

 

[Chris Gascoigne]

There is nothing that has been mentioned or proposed here that I haven't heard or considered many times before.

So in answer to the request for asking a question here we go.
Q1 Is there any evidence that there is more than one universe?
Q2 Is there any way of testing any hypotheses which postulate the existence of multiple universes?

 

[weirdoguy]

There is nothing that has been mentioned or proposed here that I haven't heard or considered many times before.

We're not asking if you've heard it, we're asking if you understand it. And for me it is clear that you don't, and it's also clear that you don't want to. You ignore everything and everyone.

 

[Chris Gascoigne]

I understand the concepts that have been proffered but not the mathematics that supports them. I am a philosopher not a physicist or mathematician.

So with that cleared up can you please answer the the two questions I posed in response to your request?

Q1 Is there any evidence that there is more than one universe?
Q2 Is there any way of testing any hypotheses which postulate the existence of multiple universes?

 

[Chris Gascoigne]

Please see attached.

 

[Vanadium 50]

I understand the concepts that have been proffered

No, you really don't. You say "multiverse" when (and it took some time to sort this out) you really mean "eternal inflation".

I believe Sabine Hossenfelder is a highly respected physicist - don't shoot the messenger!

Not so much as you think. She's more famous as a blogger. In the last five years her papers have received 75 citations, of which 10 were self citations. To compare, I am not even a theorist, but have written a couple of theory papers. The least-cited single paper of mine has 115 cites, of which 3 are self-cites.

This thread's goalposts keep moving. The original question was "If the Multiverse is correct why don't they appear inside our own?" and now it's "Is there any evidence that there is more than one universe?" and "Is there any way of testing any hypotheses which postulate the existence of multiple universes?"

To answer your second set of questions, you have to define "universe" and discuss if and how it differs from "region on our universe that we have not observed and will never observe". To answer your original question, "for the same reason rocks roll spontaneously downhill but not uphill."

It is starting to sound, though, that you are not asking questions so much as pushing a position. I do not recommend it, seeing as how by your own admission you are not an expert in the physics."

 

[weirdoguy]

I understand the concepts that have been proffered but not the mathematics that supports them.

So you don't understand the concepts. Understanding concepts (especially when it comes to such and advanced topics like the ones you try to discuss) means mainly understanding the maths behind them.

Q1 Is there any evidence that there is more than one universe?

Of course not, but that is not what you were initially asking. You asked some questions, got some lengthy replies pointing out your misunderstanings, and then you ignored it, restated again your wrong understanding as if nothing happened and then changed the subject. Is that a way to learn? Is that a way to discuss?

 

[Chris Gascoigne]

The root of the word 'discuss' is the same as the roots of the words 'percuss' and 'concuss' and means literally hitting each other with words. I prefer 'converse' which means to turn together in dialogue. In a short space of time I have noticed a tendency to attack questions and questioners when what they (certainly in this case) are asking is quite obvious although lacking in technicality.
Where are the Aliens? is a valid question and so is where are the other universes?
The term universe here applies to the cosmos we inhabit and observe and know very little about. The term multiverse here means more than one of these. Is it probable that there is more than one of these grand objects of perception we generally refer to as a universe? The answer to this ought to be yes on the assumption that the initial conditions from which our universe spawned was the simplest of states devoid of entropy. The initial conditions here are those that we would encounter were we to reverse time to the moment just before the big bang. Whenever I see models of multiverse theories they postulate an infinite number of bubble like universes that never intersect. The mathematics behind why they never actually intersect seems compelling to a non mathematician - with one fundamental flaw which is that these bubble universes have the same point of origin as though there would be one giant bubble making machine that was fixed. In answer to the question 'what is the centre of the cosmos' the answer is here and here and there; in fact everywhere because prior to spacetime there were no co-ordinates therefore any point of origin is pervasive. So any bubble making mechanism would be creating universes in all directions at the same time and therefore they would have to intersect. Maybe this is true and the interference pattern created by an infinite number of universes from an infinite number of points collapses into this one single universe.

 

[Vanadium 50]

Being told one is wrong when one says a wrong thing is not an attack.
Argument by repetition is not much of an argument.
Further discussion is pointless.

 

[Chris Gascoigne]

Ok, I agree that further discussion is pointless! But I don't agree with your use of the word 'wrong'. My profession is a teacher and physics is a lifelong interest (but obviously not what I teach!). If one of my learners asks me a question I try to check that I understand what it is they actually want to know then I try and answer to the best of my ability. I would never, ever, tell them that they are ignorant or that their question is wrong. On this forum and in this relationship I am the learner and you are the teacher(s). I would like to know why we do not see any evidence of other universes. What I am being told is that I don't know what I am talking about - which is quite true and precisely why I am asking questions. Can anyone point me to an explanation given here that might help me understand? I am afraid I haven't found this foray the least bit helpful so I will have to take my obtuseness elsewhere.

 

[Ibix]

If one of my learners asks me a question I try to check that I understand what it is they actually want to know then I try and answer to the best of my ability.

You'll note that you were asked several times for a reference to which model you were talking about, precisely because we didn't know which one you wanted to know about. That you don't seem to know which one it is and are unwilling to provide a reference to whatever you've been reading so that we can find out for ourselves is a huge part of the reason you are running into resistance - you aren't helping us to help you.

I would like to know why we do not see any evidence of other universes.

I think you have this backwards, basically. Anything we can see is part of the universe, by definition. If you want to posit other universes, then, they must be regions that are somehow disjoint from our universe. My understanding is that different models achieve that in different ways. Eternal inflation, which V50 thinks is the model you are talking about, achieves it (if I understand correctly) by having the spaces between bubble universes growing faster than the bubble universes. Bubble universes cannot form inside others because the interiors of the bubbles are true vacuum not false vacuum and can never overlap if they don't overlap initially, so we can never see another universe.

One point to take into account is that, observationally, we do not see new universes forming. Any theory that posits other universes must therefore provide a mechanism by which we don't see them, otherwise it would be immediately dismissed unless all scientists discussing them were total idiots. Thus you dismissing multiverse theories in general on the grounds that we don't see other universes is somewhat disrespectful.

Again - different models have different mechanisms for explaining why we don't see other universes. Unless you are willing to specify a model, or tell us what you've been reading that prompted the question, you are unlikely to receive a specific answer.

 

[PeterDonis]

I understand the concepts that have been proffered but not the mathematics that supports them.

Then you need to go learn the math. Consulting a cosmology textbook (Liddle seems to be a good one) would be a good start.

PeterDonis tells me that space within our own universe is a complete vacuum (which I did not accept)

Since you admit you don't understand the math, you have no valid basis on which to either accept or to not accept what I said. What you should be doing is learning the math, so you can make a valid judgment.

then I am told that everywhere in our universe if full of the fundamental fields

Which is perfectly consistent with what I said about empty space in our present universe. Go read what I said in post #8 again about the empty space in our current universe. Notice the word "fields" in it, and what I said about them.

 

[PeterDonis]

What I am seeking to do is eliminate the Multiverse theories from my cosmological enquires by way of reason.

You can't eliminate hypotheses by way of reason alone unless they are logically inconsistent. Multiverse theories are not. You might not like them (and you would find plenty of physicists to agree with you), but you can't rule them out on purely logical grounds. That's why criticisms of such hypotheses by physicists don't attempt to rule them out on logical grounds; they criticize them based on lack of evidence.

Can anyone point me to an explanation given here that might help me understand?

You've already said you don't understand the math involved, and without that you won't be able to understand anything we have said in this thread in response to your questions. So, as I said in my previous post, you need to go learn the math.

Trying to understand physics without understanding the math is generally not a good idea. As Richard Feynman said, "If you want to understand Nature, you must learn the language She speaks in".

 

[PeterDonis]

I agree that further discussion is pointless!

Then you agree that this thread can be closed. So it's closed.