# Question about gravity & the Big Bang

#### Bolo1

I'm a complete layman, and very curious, but don't have a lot of in-depth knowledge to tackle the following:
Recently, someone presented an argument to me against the Big Bang. His argument was that taking into account the matter of the universe and the nature of gravity, the universe should have collapsed back into itself shortly after the Big Bang. I read in "The Fabric of the Cosmos" that a theory exists t hat gravity can have a repulsive effect rather than an attractive force under certain conditions, and the time shortly after the Big Bang was one of those brief moments (in addition to light traveling faster than it does now.)

In addition, this person posed the question "Why don't Population II stars contain heavy elements?" I'm not sure how this ties into the Big Bang, or disproves it, but maybe someone can shed some light. He seemed pretty sure that for the Big Bang to exist, it should have produced heavy elements in ALL stars.

I'm showing my ignorance here, but this looks like the best place to get some answers.
I'd appreciate it!

#### Ich

Welcome Bolo1,

His argument was that taking into account the matter of the universe and the nature of gravity, the universe should have collapsed back into itself shortly after the Big Bang.
It's a matter of initial conditions.
It is believed that very early inflation came into play, a vastly repulsive "field" that started expansion. There are also speculations about gravity being repulsive if curvature gets sufficiently high. We don't know the nature of these things, but we see the results.
I also don't see how this could be an argument against BB. I mean, the fact that the universe evolved from a hot, dense state is just this, a fact. There are many lines of evidence for it, and we can trust the extrapolations of our theories.
OTOH, an argument based on a classical theory under conditions where the theory is bound to break down sounds not really convincing.
"Why don't Population II stars contain heavy elements?"
Because there wegre none there when they formed. That's actually a prediction of BB theory. Heave elements were generated by stars, not the BB.
There is indeed some thension between data and model, but it doesn't appear that your someone knows enough about these things as to argue about fine points.

#### Freeman Dyson

i actually have an astronomy final tomorrow where identifying and listing the properties of pop 1 and 2 and stars is one of the many things i am supposed to know. We have to wait for the first stars to die before those heavy elements are added to the ingredients that stars can use. nuclear fusion.

and they do contain some heavy elements just not as many as pop 1.

#### bapowell

It's also important to point out that one does not need inflation or any other form of repulsive gravity to keep the early universe from recollapsing on itself. As Ich says, it's a matter of initial conditions. If the early universe possesses an energy density equal to or smaller than its critical density, it will go on expanding forever. Only if the energy density is too large will the universe recollapse on itself.

#### friend

It's a matter of initial conditions.
It is believed that very early inflation came into play, a vastly repulsive "field" that started expansion. There are also speculations about gravity being repulsive if curvature gets sufficiently high. We don't know the nature of these things, but we see the results.
I fail to see how a field defined on a space has any influence on space itself expanding, adding more space with time. The field is defined on a pre-existing space, not a space defined on a pre-existing field, right? So whether gravity is attractive or repulsive should have nothing to do with space expanding, right?

#### emc2cracker

I recommend you read "reinventing gravity" by professor John Moffat. In his relativistic modification of gravity the universe has a beginning point but its very different than the standard model.

I still have doubts about dark matter, dark energy, and quantum theories that relativity seems to depend on or fall short on explaining away. It is entirely possible our current view of the universe and how it became to be is flawed, I will probably be crucified for saying this here but it should be said.

Until the day someone produces some proof of dark matter through one of the three current experiments searching for it we need to be open minded and compare other models to data.. if that day comes I'll gladly subscribe whole hearted to relativity. Frankly I enjoy the ideas of singularities and other extreme aspects of relativity, they are very exciting.. however I like to keep an open mind under such circumstances as we now face in modern cosmology.

#### Bolo1

Thank you, everyone! Keep the answers coming! I GREATLY appreciate it. I'll be reading posts from this forum quite a bit now...

#### emc2cracker

You know I am just a layman too with an inquisitive imagination, after hanging out here I decided to teach myself physics. If your passion is in cosmology then don't let your background stop you, I have letters from Hawking, Moffat, Brownstien, Hughes, and others that have helped me with my science fiction writing. If you get a good idea and it makes sense people will notice, so keep reading and remember to do a little research on your own and you will have the respect of everyone despite your academic level.

#### Bolo1

Thank you! In fact , I was thinking the same thing, considering the universe right after the Big Bang consisted of superheated gasses...

#### Ich

friend said:
I fail to see how a field defined on a space has any influence on space itself expanding, adding more space with time.
You have to have a good definition of what "expanding space" means. Have a look at http://math.ucr.edu/home/baez/einstein/" [Broken], where you get a rather operational definition. "Space" is defined by the distances of test particles. Without test particles, no definition of space.
So the field makes particles accelerate away from each other. If such a thing happens everywhere, and space is finite, that's clearly "adding more space with time". If space is infinite, that's still called "expansion of space". And yes, inflation works on "predefined space". It's not the "final cause", it's merely something that happens very early, maybe even as early as there is a notion of "space".

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#### emc2cracker

Think about space as a pizza dough, toss it up with a spin. The dough will have irregularities in it, what if the dough breaks and cracks in the middle?

What if dark energy is where space has a crack or is soooo thin its almost not there, but its not torn by mass as in a black hole but by the inflation of space itself. As the center of your Pizza dough tears and cracks the edges will flay apart even faster because the tension from the center is all of a sudden gone!

So basically you could think of dark energy as white holes.. but instead of spitting out matter from the black holes its just spitting out space itself.

Now I'm not saying thats a perfect comparison but it should help you visualize space expanding faster and faster.

I do in fact think dark energy may be a very real thing.. not so sure about dark matter yet. I'm also not certain inflation has always happened, it may have happened due to other circumstances besides a big bang. I do wonder if spacetime pulling itself apart may in fact cause dark energy. Why this is very interesting to me, eventually spacetime will develope these cracks at its outer edges... maybe possibly turning our expansion to contraction. In our pizza dough if we kept spinning it and spinning it until the edges broke it flings itself apart... however spacetime doing this would be subject to only the physics of the dark energy itself. Maybe once the edges of space are broken then the entire outer nothingness will become a source of dark energy?

Interesting things to ponder for me lol.

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#### Calimero

So basically you could think of dark energy as white holes.. but instead of spitting out matter from the black holes its just spitting out space itself.
I prefer to think that dark energy originates from many, many dwarfs standing around edge of universe, and pulling ropes, thus stretching it. So, basically, cosmological constant is just measure are dwarfs well fed, and how hard they can pull. It seems that they are constantly well fed, which implies that there is unlimited supply of pizza dough just outside of universe.

Seriously man, what are you talking about?

#### bapowell

I'm also not certain inflation has always happened, it may have happened due to other circumstances besides a big bang.
It did. Inflation has nothing directly to do with the big bang.

#### friend

You have to have a good definition of what "expanding space" means. Have a look at http://math.ucr.edu/home/baez/einstein/" [Broken], where you get a rather operational definition. "Space" is defined by the distances of test particles. Without test particles, no definition of space.
So the field makes particles accelerate away from each other. If such a thing happens everywhere, and space is finite, that's clearly "adding more space with time". If space is infinite, that's still called "expansion of space". And yes, inflation works on "predefined space". It's not the "final cause", it's merely something that happens very early, maybe even as early as there is a notion of "space".
And then there is the metric that defines the distance between prior-existing points. The metric can be considered to be a field on an underlying space of points on a manifold (IIRC). But the usual particle fields are defined on a background metric. These are the fields responsible for dark energy, dark matter, and the usual matter and light that we see. It would seem that particle fields have no influence on the background metric.

There would have to be something common to both particle fields and the metric for one to influence the other. In which case there would be a sort of equality between particles and space. Maybe this is the case with dark energy.

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#### emc2cracker

May I ask does physicists generaly enjoy talking down to people?

I can't stand a matter of fact one liner, or these little comments that simply make fun of another post. Or do you think its just so advanced you can't dumb it down? Instead of just making a statement why don't you take a little effort and enlighten me. I want to hear about this big bang without any inflation or related to inflation because I don't buy that at all.

#### Calimero

May I ask does physicists generaly enjoy talking down to people?

I can't stand a matter of fact one liner, or these little comments that simply make fun of another post. Or do you think its just so advanced you can't dumb it down? Instead of just making a statement why don't you take a little effort and enlighten me. I want to hear about this big bang without any inflation or related to inflation because I don't buy that at all.
Sorry, my intention was not to insult you, but to illustrate the following: Can you refute anything I wrote with any piece of observational evidence? I think that you can't. How do we know then that dwarfs are not responsible for dark energy? We don't. Does it make my statement worth anything? No, it does not. That is my point.

If you ask me where the dark energy comes from, I think that it is inherent and inseparable with space. It is simply space companion, the underlying principle, or if you wish, you can't have space without DE. Why it is 120 orders of magnitude less then what we predict, I don't know. That is probably Nobel prize worthy question. I really think that invoking concepts like "space being created", or "new space being added", or "something like white holes spitting space" is not helpful at all.

#### AWA

I prefer to think that dark energy originates from many, many dwarfs standing around edge of universe, and pulling ropes, thus stretching it. So, basically, cosmological constant is just measure are dwarfs well fed, and how hard they can pull. It seems that they are constantly well fed, which implies that there is unlimited supply of pizza dough just outside of universe.
Finally, a comic touch in this forum where many people post as if they had a bad case of tooth-ache.
Thanks Calimero

#### emc2cracker

Please correct me if I am wrong here. I am under the impression our best current guess on the subject is that the big bang had irregularities, these not so hot spots turned into empty space areas, those empty space areas then become the source for what they call dark energy and inflation.

I am not saying this is correct, I am just trying to help this man grasp the concept that the theory spells out. I like to have a decent understanding of ALL theories and all alternatives.

I am with you I don't like invoking dark energy as a space making factory... but its more likely I believe than say dark matter and some of the exotic particles we are searching for. And my little analogy is just meant to help grasp the theory in a layman's world, sorry if I gave the impression as if I"m trying to prove it with simple logic.

And I apologize for snapping, its just frustrating when someone like me really tries to understand cosmology and gets made fun of or shrugged off like an annoying fly lol. Sometimes I deserve it, but sometimes I've put alot of effort into my thoughts and am merely trying to expand my own knowledge or help someone else.

.... it was pretty dang funny though lol

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#### emc2cracker

Things brings me to another notion. I have tried to research this question but I can't find any information about it. Is timespace supposed to be uniform? Is it possible that dark energy isn't new space being made but the abscence of it like say a hole of nothing?

#### bapowell

May I ask does physicists generaly enjoy talking down to people?

I can't stand a matter of fact one liner, or these little comments that simply make fun of another post. Or do you think its just so advanced you can't dumb it down? Instead of just making a statement why don't you take a little effort and enlighten me. I want to hear about this big bang without any inflation or related to inflation because I don't buy that at all.
I'm not talking down to you. I'm correcting a false statement that you made for the benefit of the forum. People read these forums and when you make false statements in an ostensibly authoritative tone, it's possible that people will come away misinformed.

The big bang theory does not address the initial moment of expansion. It is a model for the expansion of the universe from an early hot, dense phase. This expansion, prior to 1980, was believed to have been non-accelerating, the way one might expect it to have gone given that gravity is attractive. And so the early universe is radiation-dominated and expanding at a non-accelerating rate, as it expands it cools, and eventually radiation redshifts away leaving a matter dominated universe. In 1980, Alan Guth suggested inflation as a means of addressing various classical problem of big bang cosmology. Inflation, by definition, is accelerated expansion. Guth hypothesized the existence of an exotic form of matter (the vacuum energy of a scalar field is one implementation) that, when placed as the source in Einsteins Eqs, led to accelerated expansion.

So, inflation can be considered part of the big bang model, but the big bang model by no means requires inflation on any fundamental level in order maintain consistency.

#### Ich

friend said:
It would seem that particle fields have no influence on the background metric.

There would have to be something common to both particle fields and the metric for one to influence the other.
Yes, gravity. The energy density of the fields equals spacetime curvature. That's (a simpified version of) Einstein's field equation. Surely you know Wheeler's "Space tells matter how to move and matter tells space how to curve."

#### emc2cracker

You said yourself inflation is considered a part of the big bang model, this man asked about it and I tried to help him understand it. I never said the model was correct. And I apologize if I am not being humble enough, I believe I told the guy I'm just a layman as well not a physicist.. yet.

#### friend

Yes, gravity. The energy density of the fields equals spacetime curvature. That's (a simpified version of) Einstein's field equation. Surely you know Wheeler's "Space tells matter how to move and matter tells space how to curve."
Right. But that does not tell us why matter curves spacetime. How particle fields influence spacetime, in the form of inflation, would tell us why matter curves space. The problem seems to be that fields defined on a background metric does not tell us how the metric depends on the fields. The metric itself can be considered to be a field, gravitational field. But that does not tell us that particle field should influence the metric.

#### bapowell

Right. But that does not tell us why matter curves spacetime. How particle fields influence spacetime, in the form of inflation, would tell us why matter curves space. The problem seems to be that fields defined on a background metric does not tell us how the metric depends on the fields. The metric itself can be considered to be a field, gravitational field. But that does not tell us that particle field should influence the metric.
What do you mean by saying that fields are defined on a background metric? This metric itself is indeed a field, and it is determined by the matter fields present:

$$G_{\mu \nu} = T_{\mu \nu}$$.

Einstein's equations tell us that matter curves spacetime. It tells us how particle fields influence spacetime. If it doesn't tell us these things as you seem to be suggesting...then what does it tell us?

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