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Comment by sulam | Hacker News Reader

This spawned a question for me that seems like it must be nonsensical but I don’t know the answer.

Why did the Big Bang not just produce a Big Hole? IOW, if all the current mass of the universe was in a small volume wouldn’t it have promptly created a black hole?

I’m assuming the answer has something to do with mass not actually existing in its current form until the universe expanded enough. However I don’t have any mental framework to understand why that would be true. If someone reading this knows the answer and can point me at it, I’d be grateful. TIA!

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The early universe was ridiculously homogeneous. We can infer this with reasonable certainty from the extreme homogeneity of the CMB and from the gravitational instability of density fluctuations: as you expect, they attract more mass and accrete to form stars and galaxies. So if you start from the universe as it looks now and run time backwards, you get ridiculous homogeneity at the bang.

And that's why you didn't get a giant black hole. There was a lot of mass at very high density, and gravity was pulling very hard at it - but it was pulling (almost exactly) equally hard in all directions.

So, are we assuming the universe is infinite? Otherwise there'd be a border to that.

The universe can be finite without a border. Think about living on the surface of a sphere or a torus (finite, no edge), as opposed to living on a sheet of paper

Part of me thinks that black holes are the border, and we might be inside one.

The Earth's surface is finite, but without a border.

A finite universe would be similar, just adding a dimension.

I think almost all options are currently still in play. Finite with border, finite in infinite space, infinite.

If it was finite, does it mean it was definitely not flat?

Treat the "Universe" that was created in the big bang like a firework that has just gone off. The "boundary" of the universe is just the size the explosion has expanded to for a given time. And today, 13.8B years after the big bang, our universe is 46.5 billion light-years in diameter. But just like a firework explosion, there isn't really a solid border to it that you can plant a stake or flag in.

We have no clue what is beyond the explosion itself and, for intents and purposes, that expanse is infinite by current understanding. Does that expanse contain other explosions? Don't know, there hasn't really been an observed overlap so far.

How can the diameter be larger than 2 * the speed of light * time ?

The given number of 46.5 Gly is actually the radius, not the diameter. This can be larger than the age of the universe times the speed of light because the universe expands, i.e. while the light was traveling towards us the point of emission expanded away from us. So 46.5 Gly is the distance to the point of emission today but when the light we observe today was emitted there 13.8 billion years ago the distance was much smaller. Also note that the expansion of the universe is not limited to the speed of light because it is space itself expanding and not something moving through space which in turn means that there is no limit to the speed at which the point of emission can expand away from us.

Thanks. I kind of understand enough of that to know what to search for further.

IANAP, but the big bang, name notwithstanding, is completely unlike a conventional explosion. It is not a wavefront expanding in three dimensions off a center, there is no frontier nor a center at all in our three dimensions and each point of the universe[1] is getting farther than every other point, i.e. not moving radially off a center.

You could imagine that the explosion is happening on an extra dimension and our three dimensional universe is the wave front of that explosion. Except that the math doesn't actually require the extra dimension to exist nor there is any evidence that it does.

edit: also the cited diameter is only for the visible universe. The universe is currently understood to be infinite.

[1] my understanding this applies only to points of the universe that are not otherwise gravitationally bound with each other.

The universe is infinite by definition. It may be closed, which mean it will repeat itself indefinitely, but that's still infinite.

What isn't infinite is the observable universe. But the observable universe is just the view we have, its boundaries are defined by the observer. In fact my observable universe is different from your observable universe, by an insignificant amount.

When physicists talk about the universe, it usually means the observable universe. Simply because for us, that's all we have.

How is the universe infinite by definition?

Wikipedia for one says the following. There is no mention of infinity.

"The Universe is all of space and time[a] and their contents,[10] including planets, stars, galaxies, and all other forms of matter and energy. While the spatial size of the entire Universe is still unknown,[3] it is possible to measure the observable universe."

We can measure that the Universe is homogeneous and isotropic on sufficiently large scales (it is according to the measurements). We can also measure the 'flatness' of the Universe (it is very likely to be flat, according to measurements).

I don't know the details, but I was told that with these assumptions, proving that the Universe is infinite is only a matter of mathematics.

A second issue is in Time 0.0001 seconds the observable universe is only a 0.0001 light seconds radius which limits how much mass could form a black hole.

No, it doesn't. The fact that you can't see beyond a certain point at a certain time doesn't mean the mass beyond that point does not count.

If an object would react to mass outside it’s light cone.

That would mean you can communicate faster than light by moving a large mass.

> If an object would react to mass outside it’s light cone.

That mass was not outside its light cone in the past. (This is one of the main points given in favor of inflation models: that they solve the "horizon problem" because the inflationary expansion means that mass over a region much wider than our observable universe was within our past light cone at the end of inflation.)

Ahh, ok, based on context I assumed you would understand that as an actual light second in spacetime as in actual distance traveled. Not light second as a unit of distance in 3d space at t=0.001 seconds which is a rather meaningless number at that point.

It’s a meaningful difference, but still avoids the black hole problem.

PS: The math really does not say anything about t=0, after ~t=10^-30 to t=1 seconds you don’t get black holes.

> I assumed you would understand that as an actual light second in spacetime as in actual distance traveled

If you are talking about actual light, it is not correct to say that it travels, say, one light-second in spacetime. The arc length along a light ray's path in spacetime is zero, because it's a null worldline. The same goes for the boundaries of light cones, which are what I think you are actually trying to get at: light cone boundaries are null surfaces, so arc length in spacetime along them is zero.

However, I don't think what you actually meant by "one light-second" was "distance in spacetime along the boundary of the past light cone". See below.

> Not light second as a unit of distance in 3d space at t=0.001 seconds which is a rather meaningless number at that point.

No, it isn't. It's a perfectly meaningful number: a distance in the surface of constant comoving time labeled by the coordinate t=0.001 seconds (or whatever time you want to pick). And if you are trying to describe "the size of the observable universe", this kind of distance is indeed what you need to specify.

Your error, however, is to assume that at t=0.001 seconds, the observable universe was 0.001 light-seconds in size. It wasn't. Such a conclusion would only be valid in flat spacetime, but the spacetime that describes the universe is not flat. It is actually non-trivial to come up with a correct expression for the size of the observable universe as a function of comoving time (and the actual expression is model-dependent--for example, it depends on how long your model says the inflation epoch lasted).

> The math really does not say anything about t=0

I agree. I wasn't saying that it did. The inflationary epoch is not modeled as starting at t=0.

> after ~t=10^-30 to t=1 seconds you don’t get black holes.

Where are you getting these numbers from?

Homogeneity of the early universe by itself is not enough to prevent collapse. You need rapid expansion as well. A homogeneous universe that is not expanding at all will collapse.

You are answering a different question. The collapse of a homogeneous (i.e. FRW) universe produces an increasingly dense, but still homogeneous universe. Without a central singularity and an outside, it looks nothing like a black hole (as asked).

BTW, expansion alone is not quite enough to answer why the early universe didn't collapse. You can have very rapid expansion, yet a closed universe which eventually comes to a standstill and then starts shrinking. A better answer is that the universe is very close to having critical density, i.e. just enough mass to expand forever at an asymptotically declining rate, absent new drivers of expansion (i.e. dark energy).

One thing our answers have in common is fine-tuning: the universe started out ridiculously homogeneous, and ridiculously close to the critical density. Inflation provides a way to explain both those properties, but (as critics are fond of pointing out) at the cost of fine-tuning the hypothetical microphysics needed to drive it.

Yes, these are all valid points.

This is an important question with a slightly surprising answer.

First you need to understand that the Big Bang was not an explosion of matter into space, it was an explosion of matter and space (technically space-time). This might be hard to wrap your head around, it might be easier to think about space-time as non-infinite, say curving back on itself like the surface of a balloon (though as far as we can tell it is infinite).

Second, during the early Universe just after the Big Bang when the energy density of the Universe was incredibly high the density was nevertheless incredibly uniform. So, let's say you are inside a soup of matter and energy with a density as high or higher than the core of a star, but there is matter in every direction, what way is gravity pulling you? This is the other non-intuitive bit, gravity isn't pulling you in any direction. Pick a volume of matter in space around you and consider how its gravity affects you, for every single volume there is a corresponding volume on the opposite side of you that has exactly the same mass and is exactly as far away from you, cancelling out the gravity from the first chunk. And this is true for every single bit of volume around you, there's a 1:1 mapping of bits of stuff around you with other bits of stuff on the opposite side of you which exactly counterbalances the gravitational force from the first bit. The gravitational field is uniform, so nothing collapses, no black holes form, no stars or planets form either.

It's only after the Universe continues to expand and minute variations in local density are gradually amplified (over millions of years) into denser and denser clumps that it becomes possible to form galaxies and galaxy clusters/super-clusters (and thus black holes).

Now, we don't know for sure whether or not there were isolated little "super clumps" of extra density in the early Universe which allowed for the formation of black holes prior to the formation of galaxies (so-called primordial black holes). That's one theory for explaining the existence of supermassive and especially ultramassive black holes, but it's unconfirmed. However, now it seems as though we have enough evidence to show that primordial black holes can't be the explanation for the evidence for dark matter.

"First you need to understand that the Big Bang was not an explosion of matter into space, it was an explosion of matter and space (technically space-time)."

This is a type of statement I have read many, many times, but to me, it's the same as saying "wakalixes!". I've never read anything that gave me a hint of why and how there should be a difference between two objects receding from each other and "space itself" expanding.

New space is being created between those two things.

When they move 1 meter apart from each other through space, they'd move 1 meter back again to meet up. But when space itself is expanding between them, they've moved 1 meter apart but now to get back together they have to move 2 meters.

This has some other subtler ramifications. It means that the space between the two objects can increase at faster than the speed of light (because you can't move through space faster than light, but space can expand faster than light). Also, with the Higg's field's nonzero rest value _energy can be created from nothing_ by creating space. Yes, this violates conservation of energy.

>>It means that the space between the two objects can increase at faster than the speed of light

Say they are two objects in the universe A and B. New space is created between A and B faster than the speed of light. There is a USB drive carrying a dump of wikipedia on B.

Does this imply information is now moving faster than the speed of light?

No because the USB stick is going nowhere, it just remains at B. Imagine a tiled floor where A and B are the centers of two tiles, the USB stick also resting on the B tile. Expansion of space just means that you are constantly increasing the gaps between tiles maybe inserting new tiles every time the gaps become wide enough. The USB stick however just rests on the B tile not moving through space, i.e. reaching another tile, ever.

Explain to me how this works. A and B remain stationary while only new space gets created between them?

By geometry, the shortest distance between two points is a straight line. If A and B are two stationary objects, with a straight line being shortest distance between them. Then space is basically a twisted thread like path between A and B, More thread is created every time instant 't' than light can travel through it?

And if some one figures out a way to walk from A to B in a straight line then they can walk faster than anything that can walk on the thread?

Unfortunately I don't understand what you are suggesting or asking. Maybe this short video [1] with the classical balloon analogy will help you understand it better.

[1] https://www.youtube.com/watch?v=mnENgtCdObo

Figure 1:

     _                 _ 
    |_|---------------|_|
     A                 B

Figure 2:

     _    -----\____   _
    |_|--/          \-|_|
     A                 B

Let's say there is constant speed 'c' beyond which one can't travel. Lets say we travel at 'c'.

Figure 1 is the fastest possible way of going from A to B

If more space is added, and yet A and B remain at fixed positions, the only way this model will work is the path(possible way to go from A to B) gets looped/twisted and turned. In this case A and B remain at fixed positions but new space gets created. Its also possible in this model that as more and more loops get created fast enough, even by travelling at 'c' one can never arrive at B.

If the space is not looped or twisted this how A and B will move apart at increasing time instances t1, t2, t3

t1:

     _    _
    |_|--|_|
     A    B

t2:

     _     _   
    |_|---|_|
     A     B

t3:

     _      _
    |_|----|_|
     A      B

t4:

     _       _
    |_|-----|_|
     A       B

...

tn:

     _
    |_|----------------
     A

As you can see when we keep the space expansion a straight line the objects always move. But if we have to keep the objects fixed we have to loop, twist and turn the path(space).

If the objects were stationary, this is how it will look

t1:

     _          _
    |_|--------|_|
     A          B

t2:

     _          _
    |_|---/\---|_|
     A          B

t3:

     _          _
    |_|---/|\--|_|
     A          B

tn:

     _     ---  B
    |_|---/   |---
     A  (many loops)..

Is this understanding correct?

Now I see what you mean, you are confusing two spaces. Imagine a finite two dimensional space, say one meter in each dimension. You are now almost certainly picturing something like a sheet of paper one by one meter floating in our three dimensional space in front of you or maybe the surface of a table of that size. The important thing is that this is not a picture of a finite two dimensional space, it is the picture of a finite two dimensional space embedded in a three dimensional space.

If you would crumble up the sheet of paper to a small ball than that would be a different embedding of the two dimensional space in our three dimensional space but importantly the two dimensional space would remain unchanged. That is a two dimensional creature living on the sheet of paper could not tell the difference between living in a nicely flat sheet of paper and living in the crumbled up ball of paper.

And that is the same in your picture, space is the string between A and B and you can not tell the difference between it being embedded in a straight or curled up manner. Even more importantly a space does not require any embedding at all. It is a limitation of our brains that we can only imagine a two dimensional space as a sheet embedded in our three dimensional space. A and B can get further apart without being embedded in any space and moving away from each other in the embedding space or having new space curl up in between them.

So your mistake is to assume that our space must be embedded in another space in order to be able to expand. This might be the case but it is not necessary and we have no evidence for that. It also starts an infinite regress, why wouldn't the space our space is embedded in be embedded in yet another space? And then that one and the next one and so on.

If our space was indeed embedded in another space and curled up there, than your suggested faster trip from A to B would be the equivalent of a two dimensional creature leaving the crumbled up ball of paper into the third dimension, moving a bit through our three dimensional space, and then falling back into the paper at a point nearby in our three dimensional space but potentially far away from the starting point if you moved only inside the crumbled up sheet of paper. You would literally have to leave space-time and move through a space that is not our space-time.

This is in some sense very similar to wormholes but in that case you do not have to leave space-time and move freely through the embedding space but you can follow shortcuts inside space-time due to the topology of it. But again there is no need for any embedding space even if all the pictures of wormholes show space-time with the wormhole embedded in another space.

> And this is true for every single bit of volume around you

... except on the edges... but maybe there wasn't an edge and space is sphericalish? And if it was back then, why wouldn't it be now? Which brings the question: if you go far enough in one direction, do you come back where you started? Can we figure out whether that's the case?

I don't know the technical/methematical explanation for this, or if one is really needed (though it seems like a valid question to pose), but it is noteworthy that black holes and the big bang are the original "singularity", and that there are several related theories in which they are directly related, notably Big Bounce[1] and my favorite (which I learned of from Lee Smolin's amazing Life of The Cosmos[3]) Cosmological Natural Selection[2]. There's also a variant of Cosmological Natural Selectionm CNS-I[4], but that one possibles borders on religious, though it's possible it could be falsifiable/supportable some day by modeling.

I suppose it's even somewhat plausible that the answer to your question, in combinations with the other theories, are "all true", and that for various reasons or models the first N big bangs did result in immediate/fast collapse back to another big crunch/black hole, and that each subsequent crunch/black hole created another universe, until there was a universe that survived long enough to create several black holes, and so on and so on...

[1]https://en.wikipedia.org/wiki/Big_Bounce [2]http://evodevouniverse.com/wiki/Cosmological_natural_selecti... [3]https://www.amazon.com/Life-Cosmos-Lee-Smolin-ebook/dp/B004T... [4]http://evodevouniverse.com/wiki/Cosmological_natural_selecti...

> if all the current mass of the universe was in a small volume wouldn’t it have promptly created a black hole?

No, because it was expanding too rapidly. The usual intuitions about how much mass in how small a volume it takes to make a black hole assume that the mass is stationary, or nearly so.

This matches a description I was able to find online. What that seems to imply though is that expansion was literally faster than light. Yes? No?

[edit] and it seems this is true and can be explained by the space-time matrix not being bound by light speed limits.

> What that seems to imply though is that expansion was literally faster than light. Yes? No?

No. No particle of matter outruns a light ray at the same location and moving in the same direction. Or, to put it another way, all pieces of matter are moving within the light cones at their spacetime locations. "Moving within the light cones" is the correct General Relativity version of "nothing goes faster than light".

The coordinate speed of pieces of matter can be greater than c, but in a curved spacetime that, in itself, is not a problem.

Yes it was. I think it's called the inflation period and is used as a possible explanation of why there are big voids in the universe.

The expansion of space is always faster than the speed of light if you pick any two points far enough apart from each other, at least as long as space is expanding and the universe is infinite. Or if the universe is finite, some time after the bang there will be points which separate faster than the speed of light from each other. These points aren't moving, of course.

I like u/Retric's comment on this too:

> A second issue is in Time 0.0001 seconds the observable universe is only a 0.0001 light seconds radius which limits how much mass could form a black hole.

That is, if you imagine all that mass-energy springing into existence in a vast universe (but with every point much closer to each other than today), there was not enough time for all that energy to notice the gravity of the surrounding mass-energy density and collapse into a black hole. Add to that the expansion rate (inflation), and the extreme uniformity of the young universe (which means gravity would have been pulling every thing in all directions rather than "inward" towards a soon-to-be-black-hole-center), and a collapse was impossible.

> if you imagine all that mass-energy springing into existence in a vast universe

You can't; this violates conservation laws. In other words, there is no solution of the equations of GR that describes mass-energy springing into existence in a universe that was empty up to that point.

At the end of inflation, matter and radiation in the form we know them today (the quantum fields described by the Standard Model of particle physics) got a huge amount of energy pumped into them; but that energy did not come from nowhere. It came from the inflaton field--the field that caused inflation up until that point--so that energy was already there and there had already been plenty of time for its gravitational effect to be "noticed".

My naive (and likely wrong) understanding is that cosmological inflation was caused by a "false vacuum" of the higgs field. Think of it as if the energy pressure of a system is higher than a vacuum, but has similar pressure properties of a vacuum. That had the effect of gravitational pressure pushing matter outwardly, instead of pulling it inwardly like we have it today (to answer your question). That caused enormously fast expansion of the universe. Very shortly afterwards, the false vacuum "decayed", and gravity started behaving as you know it.

Check out Alan Guth's inflationary universe - a surprisingly accessible book on a deeply complex subject.

Someone please correct me where I'm wrong!

Your description of what happened during inflation is basically correct. However, in order to explain why a black hole didn't form as soon as inflation ended and gravity started behaving "as we know it", you have to include the fact that all of the matter that was formed at the end of inflation was expanding very, very rapidly. (The universe was actually vacuum during inflation--at the end of inflation all of the energy in the false vacuum got transferred to the Standard Model fields, creating matter and radiation at extremely high temperature and expanding.)

Maybe the big bang did produce a black hole, and we're living inside it?

Back in my undergrad physics days, I postulated that our "universe" could be a big black hole inside some "extraverse". I worked through classical gravitational calculations to determine, from an estimate of the mass density of the universe, what the Schwarzschild radius (aka event horizon radius) of that black hole would be, and it came out to between 5 and 40 billion light years or so (the density of the universe is hotly debated, see other discussion in this thread about dark matter), which is at least the right order of magnitude.

How is it the right order of magnitude? Did you compute that based on the size of the observable Universe? But that's arbitrary, the Universe is way bigger than that.

No. The input into the equation is the density of matter in space, nothing having to do with the size of the observable universe. On large scales in the visible universe, matter is fairly evenly distributed at a density of about 1 atom of hydrogen per cubic meter.

For any constant nonzero density form of matter, if you pack enough of it together in a sphere in a flat spacetime, its Schwarzchild radius will eventually exceed its radius. This is because the Schwarzchild radius is proportional to the mass, but the radius of the actual matter is proportional to the cube-root of the mass.

It's a fun thought experiment, but not physically meaningful in this universe because our spacetime is expanding, and the original calculations assume there's nothing at the edge pulling 'outward', but in our universe there is.

> For any constant nonzero density form of matter, if you pack enough of it together in a sphere in a flat spacetime, its Schwarzchild radius will eventually exceed its radius.

You can't do this in flat spacetime, because the matter curves spacetime, and the denser you pack it, the more it curves spacetime.

Obviously.

The point of the thought experiment was that you start with a universe with no other matter in it that's not expanding, both of which conditions our universe violates.

Thank you, you're adding more concrete verbiage to my suspicions that my calculations were too naive: I was only doing classical, pre-General-Relativity, calculations. On such large scales, the dynamic nature of spacetime itself would play an important role. Especially if, as evidence suggests, gravitational waves travel no faster than light.

I think inflation had something to do with it, the only time in the history of the universe something faster than the speed of light was possible--but only because space and time itself was expanding so quickly.

Just stumbled upon this: https://www.reddit.com/r/askscience/comments/8p7hnn/where_an...

Ob: I know nothing. But the way I look at it is: what is the difference between a point and infinite space? Taken from the perspective of light (travelling at the speed of light), the universe is still a single point. That is, the size of the universe is Lorenz contracted to zero. It also takes zero time to go from one end to the other ;-)

From the perspective of light, what is the difference in the universe at the instant of the big bang and some time after it? Well... potentially nothing. The structure of the universe only makes sense from within the context of the universe itself. The space of the universe only makes sense inside the universe. The time of the universe only makes sense inside the universe.

It's not the case that the big bang exploded material into empty space. The amount of energy/mass in the universe has never changed from the perspective of the universe. The big bang is the distinction of structure in the universe. What is the difference between a point and an infinite space of uniformity? When an infinite space of uniformity loses absolute uniformity, what happens?

Why did the universe not collapse into a singularity? Because it was already a singularity. It would require some kind of structure in space in order for a "collapse" to make sense. As soon as the structure is created, it is no longer uniform, and so it will no longer collapse uniformly - especially if space is expanding at the same time. I suspect there are several black holes at the "centre" of the universe, but I can not think of a way for all the matter to collapse into a singularity.

There are major challenges/issues with Big Bang theory. The easiest issue to see and understand is temperature in the CMB appears to indicate “communication”... basic descriptions of the problem here https://youtu.be/dbm3M9Bz4RE

No one really knows anything about the universe before 10^-42 seconds, and there is no physics to describe what likely happened. Photons/light and matter as we known it didn’t exist and wouldn’t for another 400,000 years.

There are many very deep issues that are explained pretty well here https://youtu.be/JDmKLXVFJzk It’s pretty heady so be ready

In my mind:

There is Universe n-1 (before ours). Last matter and anti-matter collide. Then, as there is nothing and as there is everything, the cycle begins again since we can't achieve "the balance". A big bang occurs, a universe n is born, with it's own set of laws (the laws of physics, like a new buildconfig). Everything slowly spreads and gets into perfect balance. Then everything dies. Then the last bits of matter stop existing. The Boom. A new universe is born, n+1. It has it's own set of constants. Maybe they aren't balanced properly, so it lives for a short moment and then dies and n+2 is born. And so on and so on.

I'm absolutely not an expert, but I believe there wasn't any mass at first; it was all energy. It had to slow down to become mass, at which point it was spread out enough. Or something along those lines.

Another possibility is that black holes are places where lots of mass accumulates in the same place in space, while the big bang was a rapid expansion of space itself. Even if there was enough mass in one spot to be what we'd consider a black hole, the space it occupied expanded, dragged the mass along with it, and ripped the black hole apart.

Doesnt E=mc^2 form an equivalence? I would expect energy to warp space-time just as mass does.

I believe it does. If you have a bottle full of chemicals with potential energy, and then you make them react and get the energy from them (via, I dunno, heat), while all the particles remain in the bottle, then I think the bottle weighs less afterwards—though probably by an undetectably small amount. I think I read that the change in mass is detectable after, say, nuclear fission.

> I would expect energy to warp space-time just as mass does.

It does. So do pressure and stresses. The full "source" of gravity in General Relativity is the stress-energy tensor, which includes all of these things.

> it was all energy. It had to slow down to become mass

If there was no mass then what made the energy slow down?

For example a photon will travel at the speed of light in a vacuum indefinitely.

In what scenario does energy just slow down without interacting with matter?

"Mass" is just another name for "rest-energy".

A single photon cannot have mass because it has zero energy in the reference frame where it is at rest (zero net momentum). But a pair of non-parallel photons can have rest-mass, and indeed the mass of the photons inside a star is considerable, for example.

> "Mass" is just another name for "rest-energy".

Not if you're talking about what the source of gravity (spacetime curvature) is. Massless particles still have energy and still cause spacetime curvature.

The "source" of gravity is the stress-energy tensor. "Mass" (rest-energy) is a simplification that is useful for approximations and conceptualizations.

> "Mass" (rest-energy) is a simplification that is useful for approximations and conceptualizations.

Yes, but not in a context where radiation is present, since radiation can have energy but has no rest mass (rest energy).

> A single photon cannot have mass because it has zero energy in the reference frame where it is at rest (zero net momentum)

I didn't catch this before, but it is not correct. There is no such thing as a frame in which a photon is at rest.

You can make black holes with massless particles: https://en.wikipedia.org/wiki/Kugelblitz_%28astrophysics%29

> I believe there wasn't any mass at first; it was all energy. It had to slow down to become mass

This is partly true. During inflation, the universe was vacuum; all of the energy was in the "false vacuum" state of whatever field caused inflation (it's usually called the "inflaton" field, which IMO is a very confusing name). When inflation ended, all of that energy got transferred to the Standard Model fields, forming matter and radiation at very high temperature and expanding very rapidly.

So the "all energy at first" part is true; but the "had to slow down to become mass" part is not. The matter and radiation was there as soon as inflation ended; it didn't have to slow down first.

But did it end?

You mean, did inflation end? Yes, it did.

So the inflation is different from the current expansion.

Yes.

Neither space nor time existed prior to the big bang and energy levels were so high that a different set of laws of physics dominated the early evolution.

You assume it didn't. Perhaps the universe as we know it is the inside of a black-hole.

Perhaps once a black hole has nothing left to suck in, it explodes?

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