Size of the Big Bang singularity?

[WayneFrancis]

Well, I am way out of my comfort zone here, but as far as I can tell, that all depends on why it started expanding.

But it seems logical to assume that if the whole universe was similar to the parts we can see, then it might undergo similar processes, for similar reasons.

Do the contents of the universe need to know that conditions elsewhere are the same before they react to what is going on locally? If everywhere is similar (subject to quantum fluctuations, of course!), then surely if our part expanded, it might have all have acted in pretty much the same way, even if it were infinite.

Those quantum fluctuations are thought to have introduced the seeds of large scale structure in our universe, but only after they were stretched out by the inflationary epoch to introduce that slight inhomogeneity. So perhaps in an infinite universe the quantum fluctuations elsewhere would cause very different end results outside of our observable universe, but surely that is the case however much larger the fundamental domain is, rather than being relevant to the issue of an infinite fundamental domain. But unless there were large differences in density across that domain before inflation, I do not see how the contents would not start off by expanding, like parts we see did.

Can anyone provide a source that says the Big-Bang universe cannot be infinite?

I agree. Analogy would be space/time is like water. It has certain properties and reacts in certain ways. DE might be something like the freezing point of water. 2 points in space don't need to be causally connected for water to freeze at a certain temperature under a certain pressure. The 2 don't need to know about each other to react in a similar manner when in similar environments.

So complaining that 2 points in space could not expand in a similar manor at a given Hubble time because they are causally disconnected is a non sequitur.

So 2 points in space that are separated by a distance that causes them to be causally disconnected can still expand in similar manner if they have similar conditions. Not this also means that vastly different points could evolve differently if the conditions in the 2 points where different.

[WayneFrancis]

I doubt it.



One of the pieces of evidence for the inflationary period is the homogeneity of the observable universe. It is therefore deduced to have come from a sufficiently small volume that it was all causally connected.

It is assumed that that homogoneity extends beyond what we can see (if only because it would be rather implausible for the bit we can see to be different from the rest). However, IF the universe is infinite, there is no reason to assume that all of it is equally homogenous. In which case there is (as far as a I know - and like speedfreak I am outside my area of expertise) no problem with different parts of the (infinite) dense primordial universe starting to expand (bang) at different times.

Again agreed. I think, but not sure, if it was Leonard Susskind that I heard say something like this.

The universe may be infinite or finite but if it is finite then it is most certainly many orders of magnitude larger then what we observe. It also appears to be flat but this could just be expected by how expansion works and where we are located. On scales much larger then the observable universe it might be as wrinkled as a raison with some parts positively curved and others negatively curved.

[Jeff Root]

Wayne,

Why does a water molecule in one location have properties
similar to those of a water molecule in another location?

Why does a house in Minneapolis look very much like a
house in Adelaide?

-- Jeff, in Minneapolis

[tnjrp]

Maybe i am misunderstanding your defintion of "Our Big Bang".

If OBB caused the observable portion of the universe, and the entire universe is larger than the observable portion of the universe, then i don't see how more than the observable portion of the universe could be caused by the BB event that caused the observable portion of the universe (OBB by your definition as i understand it).

OBB by my definition is the event that caused our observable universe to move into a state the latter evolution of which we are now observing. I did not explicitly reassert that it also very strongly seems to have produced a vastly larger part of the universe as well, since I said it already in my earlier post, but I don't think I explicitly ruled this out either. If I did I hereby recant my previous stupidity

To restate, by OBB I mean the event which created the observable universe (OU). It's widely believed to have created a lot more as well ("the uniform patch" or "the local inflation bubble", let's say LIB for short) of which OU is a subset. And from there you can't trivially extend the notion into there having been "more stuff" besides that which went into "creating" LIB. In other words, while it's possible that there is more something than LIB in the universe, this does not trivially follow from LIB's existence.

[noncryptic]

My goal is the understanding that two places in the Universe are
extremely unlikely to have nearly identical conditions unless
something caused them both to have those conditions. That is,
they have been in causal contact.

If the Universe is infinite in extent, and all of it has similar
conditions, as you suggested in post #20, then we have a big
problem of how the conditions got that way.

Is it scientifically possible that the BB event started with a quantum fluctuation that caused a cascading of quantum fluctuations? I think that would fit with speedfreak's suggestion about all parts of the universe are indirectly causally connected.

with nothing at all preceeding it to make it that way.

If it the universe started with one or more quantum fluctuations, then there must have been something where those could take place. For instance "empty space" filled with virtual particles (probably at a density different than empty space as we know it).
Given that nothing else was there to begin with, could conditions throughout that primordial empty space not have been very similar in spite of not being causally connected?
(In my view that primordial empty space -is- not the BB singularity, it produced one or more BB singularities. Also there was time in that primordial empty space (no space and energy without time), and assuming that the BB singularity was not physically zero size, there was time there to.)

To restate, by OBB I mean the event which created the observable universe (OU). It's widely believed to have created a lot more as well ("the uniform patch" or "the local inflation bubble", let's say LIB for short) of which OU is a subset. And from there you can't trivially extend the notion into there having been "more stuff" besides that which went into "creating" LIB. In other words, while it's possible that there is more something than LIB in the universe, this does not trivially follow from LIB's existence.

I think i get it. I somehow interpreted "OBB event" as referring to the physical BB singularity, regarding its size. I also misunderstood what "the uniform patch" is.
My point is that because the LIB is larger than what we see, the singularity that caused it is larger than the part of that singularity that corresponds to the pre-BB volume of the observable universe.

Problems with that line of thinking arise when it is assumed that the entire universe is infinite in expanse and emerged from one single BB event.

Am i understanding correctly that given what is scientifically understood about inflation etc, the LIB can be many orders of magnitude larger than the observable universe?
Is the fact that the size of the LIB is quantified as "many" (times larger than the OU) an indication as to how little is scientifically understood about the underlying mechanism?


(when i say "singularity" i do not mean the mathematical concept (zero size, infinite density, no time), i mean the physical thing that this mathematical concept refers to regarding the big bang.)

[cosmocrazy]

problem with this thinking is thinking there was any time in the singularity. So it is more that the universe popped into infinity. I guess you can think of it as always being infinite in nature but at T0 by definition there was no time so size didn't have meaning at that point.

This would be my way of thinking also.

[cosmocrazy]

A,B,C,D & E

Expansion is a property of space and not isolated to a particular point in space. That is what the data shows in my view.

Again I'd have to agree, I don't see how these locations can even be defined at the point of the BB. If indeed the BB originated from a singularity then time would be 0 along with space until the inflation began.

Jeff I don't understand what point you are trying to make?

[kevin1981]

OBB

("the uniform patch" or "the local inflation bubble", let's say LIB for short)

I mean the event which created the observable universe (OU)

I'm loving the way you keep adding acronyms to long worded concepts so you don't have to keep typing them out

[WayneFrancis]

Wayne,

Why does a water molecule in one location have properties
similar to those of a water molecule in another location?

Because the physics are the same. My point is that 2 points in space don't have to be causally connected to have the same properties.

Why does a house in Minneapolis look very much like a
house in Adelaide?

-- Jeff, in Minneapolis

That's a bit subjective. I've never been to Minneapolis but I'm originally from just south of Boston and have lived in VA, GA and HI and I can tell you houses in VA, MA look pretty similar, houses in HI look a fair bit different and over here in Adelaide South Australia they also look different, a bit like houses in GA. They seem to have different properties based on the types of environments that they are in. Not sure what you are trying to get at.

[Cougar]

My point is that 2 points in space don't have to be causally connected to have the same properties.

But this is the Horizon Problem. It's awfully coincidental that photons are almost exactly the same temperature, yet those photons are coming from regions of the universe that are so far apart, they could never have been causally connected. How can they have the same temperature, then? Coincident is not a satisfactory answer. Of course, this is one of the big problems that cosmic inflation appears to solve.¹

_________________________
¹ Sorry if I'm coming in late here and missing the context of your point...

[speedfreek]

I think there is a subtle distinction to be made between the causal connection required for an even temperature across the observable universe (the horizon problem), and the question of whether it is required for the whole universe to have been at an even temperature in order to partake in the expansion.

In other words, the horizon problem was brought about by the observation that our observable universe seems to have an even temperature at distances too far apart to have been causally connected. Does this mean the whole universe has to be the same temperature as our observable part of it?

Is universal smoothness required for expansion?

To explain a little further, cosmic inflation solves the horizon problem by allowing time for the contents of the observable universe to reach equilibrium, before inflation. Does this mean the whole pre-inflationary volume had to have reached equilibrium, or may there be ever so slight differences on scales much larger than we can ever see? What if the background temperature is 0.01 of a degree different, 100 billion light-years away? Or 0.1 of a degree different a 1000 billion light-years away. Perhaps it fluctuates (sometimes wildly!) all the way out to infinity?

[WayneFrancis]

But this is the Horizon Problem. It's awfully coincidental that photons are almost exactly the same temperature, yet those photons are coming from regions of the universe that are so far apart, they could never have been causally connected. How can they have the same temperature, then? Coincident is not a satisfactory answer. Of course, this is one of the big problems that cosmic inflation appears to solve.¹

_________________________
¹ Sorry if I'm coming in late here and missing the context of your point...

I agree there cougar. I'm not talking about the uniformity of temperature. What I'm talking about is more a derived property of something. I know "temperature" is a property but ... well like I said the freezing point of water is a good one. Given a certain pressure water has a certain boiling point. There is no reason to think that water beyond our event horizon is acting differently then water here.

So expansion of space might just be a derived property that is based on something else but inherently the same for any patch of space if they are in similar conditions.

[WayneFrancis]

I think there is a subtle distinction to be made between the causal connection required for an even temperature across the observable universe (the horizon problem), and the question of whether it is required for the whole universe to have been at an even temperature in order to partake in the expansion.

In other words, the horizon problem was brought about by the observation that our observable universe seems to have an even temperature at distances too far apart to have been causally connected. Does this mean the whole universe has to be the same temperature as our observable part of it?

Is universal smoothness required for expansion?

To explain a little further, cosmic inflation solves the horizon problem by allowing time for the contents of the observable universe to reach equilibrium, before inflation. Does this mean the whole pre-inflationary volume had to have reached equilibrium, or may there be ever so slight differences on scales much larger than we can ever see? What if the background temperature is 0.01 of a degree different, 100 billion light-years away? Or 0.1 of a degree different a 1000 billion light-years away. Perhaps it fluctuates (sometimes wildly!) all the way out to infinity?

Yup. And this is where my post #32 comes in.

I don't care if the universe isn't uniform at the largest scale. I don't think it is a requirement. I think that it is very understandable that the observable universe looks very homogeneous even if the entire universe is as wrinkled as a raisin

[tnjrp]

While it's not to be ruled out that we have a Tegmark Level I multiverse on our hands, I also can't recall any propositions that absolutely posit the entire universe must be uniform. So the entire universe, if larger than LIB, might also consist of multiple "uniform patches" that aren't uniform with their neighbouring ones. I think the Finnish cosmologist Kari Enqvist came up with the analogy of cosmos as a river where we might exist in "a peaceful eddy in a raging torrent".

[Jeff Root]

Is it scientifically possible that the BB event started with a
quantum fluctuation that caused a cascading of quantum
fluctuations?

Something vaguely like that is widely considered to be the
most "mainstream" concept of how things got started. It is
how I imagine it, at least.

I think that would fit with speedfreak's suggestion about all
parts of the universe are indirectly causally connected.

You describe something that starts small and grows larger.
That cannot produce an infinite Universe in finite time.

If it the universe started with one or more quantum fluctuations,
then there must have been something where those could take place.
For instance "empty space" filled with virtual particles (probably at
a density different than empty space as we know it).

That is a possibility. However, general relativity predicts that
there was no empty space before the Big Bang. It predicts that
there was nothing before the Big Bang, not even time, certainly
no virtual particles. The quantum fluctuation which began the
Big Bang would have been the first thing that ever happened.

Given that nothing else was there to begin with, could conditions
throughout that primordial empty space not have been very similar
in spite of not being causally connected?

Why would they be similar? I require some good reason for the
conditions to be similar before I will accept that they likely *were*
similar.

-- Jeff, in Minneapolis

[Jeff Root]

A,B,C,D & E

Expansion is a property of space and not isolated to a particular
point in space. That is what the data shows in my view.

Again I'd have to agree, I don't see how these locations can even
be defined at the point of the BB. If indeed the BB originated
from a singularity then time would be 0 along with space until
the inflation began.

Jeff I don't understand what point you are trying to make?

The assertion is made that the Big Bang was the beginning of the
Universe, the resulting cosmic expansion involves everything that
exists, and the Universe looks pretty much the same everywhere.
Another assertion is made that the Universe may be infinite.

I agree that each of those assertions may be correct. But not both
together. I say that they are mutually exclusive. One or the other
may be the case, but not both.

If everything is participating in the cosmic expansion and the
Universe looks pretty much the same everywhere, and is also
infinite in extent, then the Big Bang was a coincidence of the
most absurd proportions. Every part of an infinite space began
to expand at the same instant, just by coincidence.

If you say that the expansion did not begin everywhere at the
same instant, then you are saying that the Universe does not
look pretty much the same everywhere. You are saying that
the Universe does not appear to be about 13.7 billion years old
everywhere. You are saying that some parts of the Universe are
very different from other parts. You are essentially saying that
the Big Bang did not involve the entire Universe.

-- Jeff, in Minneapolis

[Strange]

The assertion is made that the Big Bang was the beginning of the
Universe, the resulting cosmic expansion involves everything that
exists, and the Universe looks pretty much the same everywhere.
Another assertion is made that the Universe may be infinite.

I agree that each of those assertions may be correct. But not both
together. I say that they are mutually exclusive. One or the other
may be the case, but not both.

Anything at t=0 is, of course, speculation (even if tightly constrained by what we know of the universe shortly afterwards).

But I think you are exaggerating the inconsistency here. They key point is "looks pretty much the same everywhere"; i.e. this is a statement about the visible universe. The only bit we can make specific statements about.

If you say that the expansion did not begin everywhere at the
same instant, then you are saying that the Universe does not
look pretty much the same everywhere.

It is unlikely that at the edge of the visible universe, the laws of physics suddenly change, or that the density or expansion dramatically change. Over the entire universe, outside the bit we can see, it is possible that conditions change gradually. Perhaps if we could see further we would know it is not as homogeneous as it appears locally. If the universe is very large or infinite then there may eventually be parts which are completely different from here. Or maybe not. Perhaps the laws/constants of physics are the same everywhere and so the variation over the entire (infinite) universe is quite small.

You are saying that the Universe does not appear to be about 13.7 billion years old everywhere.

Maybe some parts are 13.7 billion years + 1 attosecond. Or +/- 1 million years. I'm not sure it makes much difference.

Without knowing what was there at t=0, what triggered it to become the big bang, etc we can't say anything for certain.

You are saying that some parts of the Universe are very different from other parts.

Different, probably. Very different, maybe. But if, for example, the laws of physics at that point were the same everywhere (for whatever reason) then there would be a limit to the varaiation.

You are essentially saying that the Big Bang did not involve the entire Universe.

That seems like a leap of logic to me.

BTW. I am not arguing that the universe is infinite. Just that I see no logical reason why it shouldn't be.

[Jeff Root]

The assertion is made that the Big Bang was the beginning of the
Universe, the resulting cosmic expansion involves everything that
exists, and the Universe looks pretty much the same everywhere.
Another assertion is made that the Universe may be infinite.

I agree that each of those assertions may be correct. But not both
together. I say that they are mutually exclusive. One or the other
may be the case, but not both.

Anything at t=0 is, of course, speculation (even if tightly constrained
by what we know of the universe shortly afterwards).

But I think you are exaggerating the inconsistency here. They key
point is "looks pretty much the same everywhere"; i.e. this
is a statement about the visible universe. The only bit we can
make specific statements about.

The two assertions are about 1) the entirety of the Universe, not
just the part we can ever observe, and 2) the possibility that the
Universe is infinite in extent, whch obviously is infinitely more
than we can ever observe.

I don't think I'm exaggerating the inconsistency at all. The two
possibilities are clearly (to me) mutually exclusive.

(The fact that the Universe looks pretty much the same in all
directions from where we are is generalized to the cosmological
principle, that the Universe looks pretty much the same nomatter
where you are.)

If you say that the expansion did not begin everywhere at the
same instant, then you are saying that the Universe does not
look pretty much the same everywhere.

It is unlikely that at the edge of the visible universe, the laws of
physics suddenly change, or that the density or expansion
dramatically change.

I'm not suggesting a change at the edge of the visible Universe.

I'm saying that if the Big Bang occurred at different times in
different places, then there would be dramatic differences in
the appearance of the Universe from one part of the Universe
to another, though possibly only on scales larger than the
observable Universe.

Over the entire universe, outside the bit we can see, it is possible
that conditions change gradually. Perhaps if we could see further we
would know it is not as homogeneous as it appears locally. If the
universe is very large or infinite then there may eventually be parts
which are completely different from here.

In that case, you are dropping the assertion that the Big Bang was
the origin of everything. If parts of the Universe are completely
different from how they are here, then those parts of the Universe
were not involved in the Big Bang and are not participating in the
cosmic expansion.

Or maybe not. Perhaps the laws/constants of physics are the
same everywhere and so the variation over the entire (infinite)
universe is quite small.

In that case you are proposing an astounding coincidence that
everything in an infinite volume just happens by chance to be
almost identical, such that it all started expanding at once.

You are saying that the Universe does not appear to be about
13.7 billion years old everywhere.

Maybe some parts are 13.7 billion years + 1 attosecond.
Or +/- 1 million years. I'm not sure it makes much difference.

If the Big Bang occurred later at some places than at others,
the Universe overall would be very different from what we see
in the visible part of the Universe. If the time difference were
one second between two locations one light-second apart, the
difference in appearance would be dramatic.

You are saying that some parts of the Universe are very different
from other parts.

Different, probably. Very different, maybe. But if, for example, the
laws of physics at that point were the same everywhere (for
whatever reason) then there would be a limit to the variation.

What would make these "laws" the same in different places?
Coincidence? If not coincidence, what?

You are essentially saying that the Big Bang did not involve the
entire Universe.

That seems like a leap of logic to me.

If the expansion did not begin everywhere in an infinite Universe
at the same instant, then the Big Bang did not involve the entire
Universe. If the Big Bang started in one place and spread from
that place at a trillion times the speed of light, it still would not
involve the entire Universe a trillion trillion trillion years later.

-- Jeff, in Minneapolis

[noncryptic]

You describe something that starts small and grows larger.
That cannot produce an infinite Universe in finite time.

While i did propose that the universe can be infinite, it is not crucial to the point i was trying to make.
The universe can be finite and still large enough to contain parts that are not directly causally connected (but might be indirectly connected).

general relativity predicts that
there was no empty space before the Big Bang

But is it not true that GR breaks down at the big bang singularity? That is to say, GR predicts zero size and infinite density, which isn't likely to be physically possible. At least people such as Lawrence Krauss seem to think so.

Why would they be similar?

Because in the primordial empty space conditions were the same everywhere: just empty space with virtual particles, nothing else. I see no reason why conditions would be different in different locations of 'nothingness'.

[Strange]

The two assertions are about 1) the entirety of the Universe, not
just the part we can ever observe, and 2) the possibility that the
Universe is infinite in extent, whch obviously is infinitely more
than we can ever observe.

Then that is a strawman. How can anyone assert that the entire universe (finite or otherwise) is equally homogeneous.

I'm saying that if the Big Bang occurred at different times in
different places, then there would be dramatic differences in
the appearance of the Universe from one part of the Universe
to another

Why "dramatic". If there are fixed and immutable underlying laws and constants of nature (which, of course, may not be true) then why wouldn't it be roughly the same everywhere (for suitable values of roughly).

On the other hand, perhaps there are.

In that case, you are dropping the assertion that the Big Bang was
the origin of everything. If parts of the Universe are completely
different from how they are here, then those parts of the Universe
were not involved in the Big Bang and are not participating in the
cosmic expansion.

Why?

In that case you are proposing an astounding coincidence that
everything in an infinite volume just happens by chance to be
almost identical, such that it all started expanding at once.

Not a coincidence if that is the way the laws of physics are.

What would make these "laws" the same in different places?
Coincidence? If not coincidence, what?

Who knows. Maybe forrest noble's "simple theory of everything" will tell us. Maybe quantum gravity will tell us. Maybe that's just the way it is. Maybe it was created that way. Maybe its not true.

[Grey]

Jeff, I'd be wary of making any a priori conclusions about how the universe has to be, based on pure reasoning alone. The historical evidence suggests that a lot of pretty bright people have tried that, and turned out to be wrong. Sometimes the universe is weirder than we imagine. There are a number of solutions to your apparent conundrum. Here's the simplest that comes off the top of my head. We know that our descriptions of the laws of physics break down at a certain energy density. Suppose that when you get to a domain of such high energy density, causal effects are not limited to light speed. They might have no speed limit at all, and one object can have a causal influence on another at any distance, with no time elapsed. Actually, we already have an inkling that objects are linked in some nonlocal fashion, a la quantum entanglement. So you've got a third unstated assumption in with the two you've stated, that causal influences can only travel at some finite velocity. I can keep your first two conditions if I drop the third one. And it's no great shakes to drop the locality requirement, because we've already had to give it up. Bell showed more than 40 years ago that no purely local theory can properly describe the quantum facts. There are probably other assumptions you've made without realizing it (or maybe you're assuming that any alternative is absurd, and clearly false; again, I'd be careful about doing that), and you might be able to keep an infinite universe all involved in a single big bang by discarding one of those other unstated assumptions instead.

For the record, I actually favor a finite universe, but it's mostly because that's a lot easier for me to envision. I'm always careful to segregate what I'd prefer from what is actually constrained by the evidence. And I try to be careful what ideas I reject as "absurd", because our track record shows that to be an easy way to get into trouble.

[cosmocrazy]

Grey, nicely put!

To Jeff, I can see where you are trying to come from with this but I still don't see how it would apply at the initial conditions of the BB. If Space and time began at the BB then the terms Infinite & Infinity at this point have no definable meaning. Why could space not become instantly infinite as soon as time began at this point?

[noncryptic]

As i understand it, to little is understood to say anything definitive about the BB event itself and the conditions - if any - from which it emerged. That leaves room for hypothesis such as the multiverse/metaverse (Sean Carroll and others), which follows from established theories and which does involve time, space and (vacuum) energy from which a universe can emerge.

[WayneFrancis]

Anything at t=0 is, of course, speculation (even if tightly constrained by what we know of the universe shortly afterwards).

But I think you are exaggerating the inconsistency here. They key point is "looks pretty much the same everywhere"; i.e. this is a statement about the visible universe. The only bit we can make specific statements about.



It is unlikely that at the edge of the visible universe, the laws of physics suddenly change, or that the density or expansion dramatically change. Over the entire universe, outside the bit we can see, it is possible that conditions change gradually. Perhaps if we could see further we would know it is not as homogeneous as it appears locally. If the universe is very large or infinite then there may eventually be parts which are completely different from here. Or maybe not. Perhaps the laws/constants of physics are the same everywhere and so the variation over the entire (infinite) universe is quite small.



Maybe some parts are 13.7 billion years + 1 attosecond. Or +/- 1 million years. I'm not sure it makes much difference.

Without knowing what was there at t=0, what triggered it to become the big bang, etc we can't say anything for certain.



Different, probably. Very different, maybe. But if, for example, the laws of physics at that point were the same everywhere (for whatever reason) then there would be a limit to the varaiation.



That seems like a leap of logic to me.

BTW. I am not arguing that the universe is infinite. Just that I see no logical reason why it shouldn't be.

Well put.

[Cougar]

As i understand it, to little is understood to say anything definitive about the BB event itself and the conditions - if any - from which it emerged. That leaves room for hypothesis such as the multiverse/metaverse (Sean Carroll and others), which follows from established theories and which does involve time, space and (vacuum) energy from which a universe can emerge.

That sounds about right. Of course there is zero evidence for any multiverse scenario. Since we know next to nothing about the BB 'event', and certainly nothing about anything before that, even if there was a 'before', then a lot is left open to speculation, as you say. A lot can be ruled out, though. A turtle or pink unicorn would definitely be completely decomposed into its separate fundamental particles due to the known extreme temperature, pressure, and density, for example.

The purpose of these recent speculations into such unprovable possibilities appears to involve explaining the so-called fine-tuning problem. They're creative and sensical, but not that satisfying, obviously, since any 'observational support' of such ideas appears to be at least 500 years beyond our current technology....

[WayneFrancis]

...

Very well put. I'm agnostic on if the universe is infinite or finite though I agree it is easier to grasp if it is finite but such is the conundrum of infinity. It should always be easier to understand the finite over the infinite.