Big Bang Confusion: Exploring the Singularity

[Michaela SJ]

(I am not a physicist - so, please be gentle )

I was watching a program last night on the SCI channel, Did the Big Bang Really Happen? and they were describing the 'Singularity' as being this super hot very dense and infinitely small thing. And then when it went through the 'inflation' and became trillions of degrees hot.

If I remember what happens to gases (which I know is not like the 'singularity') when a gas is compressed it heats up and when it is decompressed it cools.

Why would the singularity be of any temperature if it was essentially a nothing and why would it become super heated during the 'inflation' period?

 

[phinds]

(I am not a physicist - so, please be gentle )

I was watching a program last night on the SCI channel, Did the Big Bang Really Happen? and they were describing the 'Singularity' as being this super hot very dense and infinitely small thing. And then when it went through the 'inflation' and became trillions of degrees hot.

If I remember what happens to gases (which I know is not like the 'singularity') when a gas is compressed it heats up and when it is decompressed it cools.

Why would the singularity be of any temperature if it was essentially a nothing and why would it become super heated during the 'inflation' period?

You have likely somewhat misunderstood what they said (although not necessarily) and MUCH more importantly it's very like that what they said was nonsense.

We have to debunk this kind of pop-sci crud regularly here on PF. First, the Big Bang Theory does not posit any creation event. It describes the universe starting with an extremely hot (and I mean EXTREMELY) that may or may not be infinite in extent (we don't know, but it definitely was not an infinitely small point in space, it was a point in time) and yes, things do cool with the expansion.

The "inflationary period" just after the start of the Big Bang is still not a proven fact, although it does the best job so far of solving several significant issues with the evolution of the universe. But whether that inflation happened or not, the universe has definitely been expanding since the start and it is now expanding at an accelerating rate and is now about 1,000 times cooler than it started out.

Google "Surface of Last Scattering" and "Cosmic Microwave Background" for more on that.

 

[Arman777]

Google "Surface of Last Scattering" and "Cosmic Microwave Background" for more on that.

I think the OP is talking about reheating after inflation period

although it does the best job so far of solving several significant issues with the evolution of the universe

Inflation actually does not solve anything but changes the initial conditions for universe to different kind of inflation related conditions. Qhich that's not much of a success

 

[phinds]

Inflation actually does not solve anything but changes the initial conditions for universe to different kind of inflation related conditions. Qhich thats not much of a success

I disagree; see this:

https://en.wikipedia.org/wiki/Inflation_(cosmology)

in particular "motivation"

 

[Arman777]

I disagree; see this:

https://en.wikipedia.org/wiki/Inflation_(cosmology)

in particular "motivation"

The problem is initial conditions as I said.

We don't need inflation theory if we claim these initial conditions;

• the Universe started out with small seeds of structure
• the Universe started out with the same temperature everywhere
• the Universe started out with a perfectly flat geometry
• the Universe started out expanding

With the help of the inflation theory, we can get rid of these initial conditions. But the questions do not disappear by inflation, it just changes form. For example, the question of "Why the universe is so flat ? " turns to "why the inflation potential is so flat? " So the flat inflation potential is also another initial condition.

The main problem is why should we assume that the above initial conditions are wrong and should replace it by inflation, which changes to initial conditions to some other initial conditions?

Simply I can ask why the universe should not start with the same temperature everywhere, or flat? And there's no answer to that question.

You can read more on this article. https://www.mso.anu.edu.au/~charley/papers/canberra.pdf

 

[PeterDonis]

I was watching a program last night on the SCI channel

Be very, very, very careful about inferring anything to do with actual science from what is said on a pop science TV program. The best way to learn the actual science is to look at actual science sources: textbooks and peer-reviewed papers.

Why would the singularity be of any temperature

There is no singularity in our current best model of the universe; the "initial singularity" is an artifact of a particular kind of idealized model that is sometimes useful for pedagogy, but is not the model cosmologists actually use to study the actual universe.

why would it become super heated during the 'inflation' period?

During inflation, all of the energy in the universe is in a field called the "inflaton" field (which basically means "the field that causes inflation"). That energy is not usefully viewed as due to high temperature.

However, at the end of inflation, all of the energy in the inflaton field gets transferred to the fields we are familiar with from the Standard Model of particle physics: photons, electrons, quarks, etc. This process is called "reheating". The effect of this is to produce a very hot, very dense, rapidly expanding state that is what the term "Big Bang" properly refers to.

 

[PeterDonis]

whether that inflation happened or not, the universe has definitely been expanding since the start and it is now expanding at an accelerating rate and is now about 1,000 times cooler than it started out.

No, it's much, much, much cooler than that. It's about 1,000 times cooler (more precisely, the CMBR is about 1,000 times cooler) than it was at the surface of last scattering: but the surface of last scattering is several hundred thousand years after the Big Bang, which is when inflation ended and "reheating" took place (see my previous post). I don't know how firm our best current estimates of the temperature of the universe at reheating are, but I know it's a lot, lot, lot more than 1,000 times the current CMBR temperature.

 

[PeterDonis]

We don't need inflation theory if we claim these initial conditions;

• the Universe started out with small seeds of structure
• the Universe started out with the same temperature everywhere
• the Universe started out with a perfectly flat geometry
• the Universe started out expanding

You have these wrong. The first, which is correct, is inconsistent with the second and third, since perfect flatness and perfectly uniform temperature make it impossible to have any seeds of structure.

The correct initial conditions for the Big Bang (i.e., the hot, dense state that is at the end of inflation in inflationary models) are that the temperature was very uniform, but not completely so--there were very small fluctuations--and the spatial geometry was very flat, but not completely so--there were very small fluctuations. Those small fluctuations were the "small seeds of structure". Inflation explains them as arising from small quantum fluctuations in the inflaton field at the time of reheating; that removes the need to assume them as initial conditions, since small quantum fluctuations in the inflaton field are expected, as they are for any field.

Inflation also explains the "expanding" part, since inflation was an extremely rapid expansion, caused by the known properties of a scalar field (which are similar to the properties of dark energy that are currently causing the expansion of the universe to accelerate--but the energy density in the inflaton field was many, many orders of magnitude larger than the current dark energy density, so the accelerating expansion was correspondingly many, many orders of magnitude faster).

the question of "Why the universe is so flat ? " turns to "why the inflation potential is so flat? "

No, it doesn't, because the inflaton field did not have to start out flat; the process of inflation makes it flat (more precisely, very, very flat, with small fluctuations due to the quantum fluctuations of the inflaton field, as noted above) no matter how it started out. So inflation removes the need for a flat initial condition (which is how it solves the flatness problem).

You can read more on this article

This is the same article that, in another recent thread, we found to be incorrectly describing the effect of inflation on conformal time. So I would be careful about using it as a reference.

 

[phinds]

No, it's much, much, much cooler than that. It's about 1,000 times cooler (more precisely, the CMBR is about 1,000 times cooler) than it was at the surface of last scattering: but the surface of last scattering is several hundred thousand years after the Big Bang, which is when inflation ended and "reheating" took place (see my previous post). I don't know how firm our best current estimates of the temperature of the universe at reheating are, but I know it's a lot, lot, lot more than 1,000 times the current CMBR temperature.

OOPS

 

[Michaela SJ]

I think the OP is talking about reheating after inflation period

No, I was trying to understand how the singularity has any temperature whatsoever and why DURING the inflation the temperature rose to trillions of degrees of heat. I understand (at least I think I do) how the growth of the universe both spatially and time wise cooled down to the current ± 2,725° Kelvin.

 

[PeterDonis]

I was trying to understand how the singularity has any temperature whatsoever and why DURING the inflation the temperature rose to trillions of degrees of heat.

And the answer is that our current best model of the universe doesn't have a singularity at all (and certainly doesn't assign a temperature to one), and that there was no high temperature DURING inflation; reheating occurred at the END of inflation. See post #6.

 

[Michaela SJ]

I apologize for my strong headedness - but there must have been something that heated before something was reheated. What was 'inflated' if not a something some call a singularity.

 

[PeterDonis]

there must have been something that heated before something was reheated

Not if the energy was stored in something other than kinetic energy of particles, which was the case during inflation (the energy was stored in the "false vacuum" state of the inflaton field, which is not a state with kinetic energy--the best analogy would be that it was potential energy stored in the field that didn't manifest as "motion" of anything or temperature of anything).

What was 'inflated' if not a something some call a singularity.

We don't know what came before inflation; different models make different hypotheses about this, and we have no way to test any of those hypotheses against any evidence at this point. But none of the hypotheses are an "initial singularity" as that term is used in the idealized model I referred to before.

 

[Ibix]

I apologize for my strong headedness - but there must have been something that heated before something was reheated. What was 'inflated' if not a something some call a singularity.

If you look at the universe today, on large scales it's pretty much the same everywhere, and it's getting bigger. So if you imagine running time backwards, everything gets closer together. If you formalise that description mathematically, you get something called the Friedmann-Lemaitre-Robertson-Walker (FLRW) spacetime, and at the beginning of time the universe is infinite in extent but there's zero distance between any two points. That's the singularity. If it sounds nonsensical, don't worry, we don't believe it either.

Inflation takes this model and adds a thing called the inflaton field. The behaviour of this field modifies the model so that it no longer predicts a singularity. It also explains things like the large-scale homogeneity of the universe and makes testable predictions about the scale of the fluctuations in that homogeneity, which we don't know how to explain otherwise.

Not if the energy was stored in something other than kinetic energy of particles, which was the case during inflation (the energy was stored in the "false vacuum" state of the inflaton field, which is not a state with kinetic energy--the best analogy would be that it was potential energy stored in the field that didn't manifest as "motion" of anything or temperature of anything).

Am I right in thinking that there was normal matter during inflation - just not much of it, relatively speaking? And that its temperature dropped during inflation before skyrocketing at the end when all the energy leaves the inflaton field?

 

[PeterDonis]

Am I right in thinking that there was normal matter during inflation - just not much of it, relatively speaking?

AFAIK the inflation models assume no energy in the Standard Model fields. That's the reason for the "vacuum" part of the term "false vacuum" that describes the state of the inflaton field during inflation.

 

[Bandersnatch]

This is the same article that, in another recent thread, we found to be incorrectly describing the effect of inflation on conformal time. So I would be careful about using it as a reference.

I think that's a bit too harsh. AFAIK the only objectionable thing the recent discussion brought about was that Lineweaver drew a couple lines on one of his space-time graphs to quickly illustrate something, without redrawing the entire graph. I thought that was a bit lazy of him, but perhaps excusable on the grounds that one can assume a reader sufficiently advanced to understand the article would also understand that inflation doesn't actually change the existing history of the scale factor, or redshifts of currently observable galaxies.

 

[PeterDonis]

ps excusable on the grounds that one can assume a reader sufficiently advanced to understand the article would also understand that inflation doesn't actually change the existing history of the scale factor, or redshifts of currently observable galaxies.

I might buy this, except that the article specifically says, in the abstract, that it's a "pedagogical review". So I don't think the author should just blithely assume that all readers will understand that Fig. 4 is mislabeled on both the left and right sides.

 

[Michaela SJ]

Head is hurting -
So, we don't have a singularity, or maybe we do, that contains or doesn't contain everything in an infinitely tiny kernel that magically goes through an inflation turning into all of the Standard Model particle that was not hot a very small period of time before but is then trillions of degrees.

Or, it was something else. I realize there is a period of time that we will never understand.

 

[Arman777]

You have these wrong. The first, which is correct, is inconsistent with the second and third since perfect flatness and perfectly uniform temperature make it impossible to have any seeds of the structure.

Well apparently "perfectly" is a wrong word that is used by the author. I agree with you that it might not be the best source for quoting.

The correct initial conditions for the Big Bang (i.e., the hot, dense state that is at the end of inflation in inflationary models) are that the temperature was very uniform, but not completely so--there were very small fluctuations--and the spatial geometry was very flat, but not completely so--there were very small fluctuations. Those small fluctuations were the "small seeds of structure".

Well, that was my point. With the help of initial conditions, we might not need inflation.

No, it doesn't, because the inflaton field did not have to start out flat; the process of inflation makes it flat (more precisely, very, very flat, with small fluctuations due to the quantum fluctuations of the inflaton field, as noted above) no matter how it started out. So inflation removes the need for a flat initial condition (which is how it solves the flatness problem).

But why that particular inflation potential that creates a flat universe. This is another initial condition, isn't it? I know that there are many inflation models, which in my research I find that even some of them can create an open universe.

My point was, there are also initial conditions for inflation models. Hence why the inflation models with initial conditions would be superior or more "right" then the universe model without inflation, which claims that the universe started with some initial conditions.

 

[Ibix]

So, we don't have a singularity, or maybe we do

It's important to distinguish between our physical models and reality. Physicists can be very sloppy about their language and talk about theories (which are detailed working models of the world) as interchangeable with reality. That's fine most of the time, but at the bleeding edge of research there are usually many models with different predictions and descriptions of the world which may or may not match what actually happened.

Our current best models do not have a singularity. More primitive models do, but this is evidence that they aren't quite right. In physical models, singularities are the maths' way of saying "here be dragons". We strongly suspect that a theory throwing up a singularity stops being an accurate description of the real world simewhere "near" its singularities. What the most accurate theory is, time (and experiment) will tell.

that contains or doesn't contain everything in an infinitely tiny kernel that magically goes through an inflation

I don't think everything was ever infinitely tiny, even in the primitive model with the singularity. Although I can see why people do say that. It's one of those things that really needs maths to describe. Natural language was developed so that one ape could ask another where the tasty fruit was - it doesn't do such a good job with non-Euclidean geometry.

And inflation isn't magical. The idea that quantum fields can exchange energy is pretty routine - whatever device you are reading this on transfers energy from its matter fields into the electromagnetic field all the time. It doesn't have a bag of photons in its screen, it creates them as it needs them. The dynamics of the inflaton field are strange, yes, but dumping its energy into matter fields isn't as bizarre as it might sound. The implications of proposing it are testable and, I gather, they do match observation as far as we can tell.

all of the Standard Model particle that was not hot a very small period of time before but is then trillions of degrees.

@PeterDonis seems to be saying that its more that the physical processes we use to define "temperature" were not running during inflation. They kick in during reheating, and it was really hot. I don't know enough to comment (and Peter may object to this paraphrase).

Or, it was something else.

I don't think anybody thinks we have a complete grasp on the history of the early universe yet, no. All we can tell you is our best models, and encourage you and help you if you want to learn about them. Unfortunately, if you really want to understand them in a non-head-hurty way that means advanced maths. Any natural language description is like trying to understand why Shakespeare is considered a great playwright working only from Wikipedia's plot summary of his plays.

 

[PeterDonis]

we don't have a singularity, or maybe we do

AFAIK there are no proposed models that have one, so we don't.

 

[PeterDonis]

With the help of initial conditions, we might not need inflation.

It's not "with the help of initial conditions" only if you don't have inflation. You have initial conditions with inflation models too; but you have fewer of them. That's why inflation models are preferred by most cosmologists.

why that particular inflation potential that creates a flat universe. This is another initial condition, isn't it?

No; every inflation model creates a flat universe.

I know that there are many inflation models, which in my research I find that even some of them can create an open universe.

Do you have any valid references? Or by "my research" do you just mean your personal attempts to construct models? If it's the latter, first, you need to take note of PF's policy on personal theories, and second, you are most likely making a mistake somewhere, since it's well known in cosmology that every inflation model creates a flat universe, as I said above.

My point was, there are also initial conditions for inflation models.

Yes. But fewer than models without inflation. See above.

why the inflation models with initial conditions would be superior or more "right" then the universe model without inflation, which claims that the universe started with some initial conditions.

See above.

 

[Arman777]

Do you have any valid references? Or by "my research" do you just mean your personal attempts to construct models? If it's the latter, first, you need to take note of PF's policy on personal theories, and second, you are most likely making a mistake somewhere, since it's well known in cosmology that every inflation model creates a flat universe, as I said above.

How can I create my own theory..?? I have seen it somewhere but I don't remember it now.

Yes. But fewer than models without inflation. See above.

I am not saying that inflation is a wrong model. Its nice actually and it is true maybe but we don't know. And fewer initial conditions don't make things right.http://www.ctc.cam.ac.uk/outreach/origins/inflation_three.php
https://arxiv.org/abs/hep-ph/9411206

 

[rootone]

It's best not to consider 'the singularity' as an actual thing or object.
The term really means that extrapolating our best current theories far enough back in time leads to unphysical nonsense,
such as infinite density.
This does not mean that the theories are nonsense,
just that they can't be applied to the state of the Universe a few zepto-seconds after time begins.
It probably means there is unknown physics going on at this point, maybe physics which can never be known.

 

[kimbyd]

(I am not a physicist - so, please be gentle )

I see that you marked this question as intermediate. Note that the challenge rating is not for us, but for you. If you would like a more basic, down-to-earth response, best to mark it "B".

I was watching a program last night on the SCI channel, Did the Big Bang Really Happen? and they were describing the 'Singularity' as being this super hot very dense and infinitely small thing. And then when it went through the 'inflation' and became trillions of degrees hot.

This is sort of kinda correct. Inflation started when the universe was at an unknown temperature. In simple models, It's expected to be incredibly hot, though it didn't necessarily have to be. Ultimately it doesn't matter: inflation itself cooled the universe to essentially absolute zero, so the original temperature is irrelevant. But the field which drove inflation had a lot of energy. When inflation ended, that field decayed, and that huge energy turned into a really, really, really high temperature. This is known as reheating.

If I remember what happens to gases (which I know is not like the 'singularity') when a gas is compressed it heats up and when it is decompressed it cools.

Why would the singularity be of any temperature if it was essentially a nothing and why would it become super heated during the 'inflation' period?

The singularity itself is a fiction. It's a place where our math breaks down, and cannot exist at all as a real object. The existence of the singularity in the math means that these models are incomplete: they can't describe the hottest, densest times in our universe at all.

 

[Arman777]

No; every inflation model creates a flat universe.

I also find a chapter in Andrew R. Liddle Cosmological Inflation and Large-Scale Structure -Page 236

 

[PeterDonis]

http://www.ctc.cam.ac.uk/outreach/origins/inflation_three.php
https://arxiv.org/abs/hep-ph/9411206

I also find a chapter in Andrew R. Liddle Cosmological Inflation and Large-Scale Structure -Page 236

Yes, I'm reading through the paper you linked to. It looks like my statement was a little strong: there are inflation models that allow an open universe at the end of inflation. However, these models still predict that the universe at the end of inflation will be closer and closer to flat the longer inflation goes on, so they are no different from other inflation models in this respect; no additional initial condition is required to explain a flat universe.