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I The Big Bang Confusion

  1. Feb 13, 2019 at 12:33 PM #1
    (I am not a physicist - so, please be gentle :redface:)

    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?
     
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  3. Feb 13, 2019 at 12:50 PM #2

    phinds

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    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.
     
  4. Feb 13, 2019 at 1:49 PM #3
    I think the OP is talking about reheating after inflation period

    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
     
  5. Feb 13, 2019 at 2:25 PM #4

    phinds

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    I disagree; see this:

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

    in particular "motivation"
     
  6. Feb 13, 2019 at 3:03 PM #5
    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
     
  7. Feb 13, 2019 at 4:00 PM #6

    PeterDonis

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    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.

    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.

    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.
     
  8. Feb 13, 2019 at 4:02 PM #7

    PeterDonis

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    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.
     
  9. Feb 13, 2019 at 4:10 PM #8

    PeterDonis

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    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).

    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).

    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.
     
  10. Feb 13, 2019 at 5:55 PM #9

    phinds

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    OOPS egg_small.jpg
     
  11. Feb 13, 2019 at 7:37 PM #10
    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.
     
  12. Feb 13, 2019 at 7:42 PM #11

    PeterDonis

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    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.
     
  13. Feb 13, 2019 at 7:52 PM #12
    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.
     
  14. Feb 13, 2019 at 8:41 PM #13

    PeterDonis

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    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).

    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.
     
  15. Feb 13, 2019 at 8:57 PM #14

    Ibix

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    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.
    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?
     
  16. Feb 13, 2019 at 9:01 PM #15

    PeterDonis

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    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.
     
  17. Feb 13, 2019 at 11:07 PM #16

    Bandersnatch

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    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.
     
  18. Feb 13, 2019 at 11:16 PM #17

    PeterDonis

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    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.
     
  19. Feb 14, 2019 at 1:00 AM #18
    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.
     
  20. Feb 14, 2019 at 1:13 AM #19
    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.

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

    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.
     
  21. Feb 14, 2019 at 3:17 AM #20

    Ibix

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    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.
    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.
    @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).
    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.
     
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