Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Big Bang for Dummies

  1. Oct 8, 2013 #1
    Hi Guys,

    Just finished watching an episode of Through the Wormhole, where they discuss the Big Bang Theory. I must say I am left with a gazillion questions. Can someone please explain this to me as simply as possible? Remember, Einstein supposedly said: "If you can't explain it simply, you don't understand it well enough." I need someone who really understands it to explain these discrepancy's to me.

    1. "The Big Bang started from nowhere and no when. "
    What? If there was no space, or time for that matter, where and how the heck did an "infinitely dense singularity" appear? Never mind braneworlds and who knows what, there was no space for it to form. You need at least something for it to start forming in, don't you? I mean what happened to the first law of thermodynamics? And if it was really infinitely dense, how come we have a finite universe today? Doesn't the very term, "infinity", mean endless time, space, or quantity? The fact that we have, at least according to the BB theory, a finite universe, belies the fact that it was infinitely dense. Very dense, yes, we don't know how to calculate it, yes, but infinitely dense?

    2. Inflation
    The universe expanded from nothing, to billions of lightyears across, then suddenly slammed on the brakes, and then started picking up speed again to the accelerating universe we live in today? Really? Science can't come up with a better explanation for explaining the universe we live in today? What happened to Occam's Razor?

    3. Raisin Bread
    The analogy of a raisin bread baking in the oven is used to explain the uniformaty of the universe we see today. The raisins themselves represent the galaxy's. Now if the universe expanded really fast with inflation, and then slowed down, and the started expanding again, how the heck do galaxy's collide? The raisins inside the bread can never collide? The space between the galaxy's should always increase. So why are we heading towards Andromeda?

    4. Observational data.
    There are apparently many objects in the visible universe, which for example either proves that there are objects out there that are older than the universe, (For example HD 140283), or other data (Like NGC7603 and NGC4319) which proves we don't know enough about red shifts, to be able to make conclusions based on it.

    I guess my question boils down to this. As I understand it, real scientists have to work according to the scientific method. This means a method of research in which a problem is identified, relevant data is gathered, a hypothesis is formulated from this data, and the hypothesis is empirically tested.

    Feynman said the following on the scientific method.

    I guess what he forgot to say was that you have to make sure the results can not be duplicated by another method. This is really important. Because no matter how smart or beautiful your results, if they can be proven to be attained from a different source, you can never say that yours is the only right answer.

    Now I am a simple man, and if you find my questions stupid, and you feel the need to tell me how stupid I am, please don't bother. I already know that. If you on the other hand can explain to me in very simple terms why the big bang theory is the only possible right answer, and can not be explained by other, more simple means, I would be very grateful.

    I have no need to demean the theory, I just don't understand why scientists insist on coming up with things like inflation and dark matter to explain a universe, which could just as easily be explained by other means.

    So would someone who REALLY understands the subject, please explain it to me?
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Oct 8, 2013 #2


    User Avatar

    Staff: Mentor

    Hoo boy, you have a lot of misconceptions. Let's begin, shall we?

    This is wrong. The Big Bang theory does NOT state that the universe started from nothing. What happens is that as we look backwards in time we see the universe getting denser and denser. At a certain point that we call t=0, our math suddenly breaks and we get infinities and nonsense. It is believed that a better understanding of physics at the extremely high energy and density levels of the early universe would allows us to keep looking a bit further back. The honest answer is that we don't know how the universe was created, if it was created at all. It is possible it has been here forever and will remain so.

    Occam's Razor has been sharpened and applied over and over again. There is simply no better explanation. That's why we're using it. It may sound crazy, but it's the best fit to the data we have.

    There were no raisins when inflation was going on. It was more of a big raisin flavored goopy fog that coalesced into raisins well after inflation stopped. Also, galaxies aren't raisins in bread. They are able to move around in space, unlike raisins, which are set where they are. Galaxies that form near enough to each other are bound together through gravity and can interact repeatedly.

    HD 140283 is 14.46 ± 0.8 billion years.
    From wiki: Due to the uncertainty in the value, this age for the star does not conflict with the age of the Universe determined by the Planck satellite, 13.798 ± 0.037.[2] The star "must have formed soon after the Big Bang"

    The error range is well within the time frame of the big bang and there is no issue with its age.

    It isn't the only possible answer. It's merely the one that makes the most sense given the available data.

    Because they can't be explained by anything else with as much accuracy and with fewer assumptions. All other explanations require much more drastic modifications of current laws, don't fit the data as well, or just don't make as much sense. Usually a combination of all 3. (And this list is by no means all inclusive)
  4. Oct 8, 2013 #3


    User Avatar
    Gold Member

    "Just Wondering", NEVER EVER EVER EVER believe "science" that you hear on TV from any program since Carl Sagan went off the air. They get a lot of stuff right, but they get so much of it WAY wrong that you really can't trust any of it. That is particularly true of "through the wormhole" which I have seen to be rife with egregious errors.
  5. Oct 8, 2013 #4


    User Avatar
    Science Advisor

    And I should add to this: it really isn't all that crazy when you understand the dynamics.

    There are, for most intents and purposes, three kinds of stuff in the universe: cold matter, hot radiation, and weird stuff that has the properties of the quantum vacuum (some call this dark energy, but that name for some reason whips people into a frenzy, and is frequently the cause of confusion and much accusation that cosmologists are just throwing darts at the wall, making stuff up until something fits...and that's really not how it is). Each of these three types of stuff causes the universe to behave in a different way: the former two lead to decelerated expansion (at different rates), and the last leads to accelerated expansion. If you've got all three existing in the universe at one time, then the universe behaves approximately according to the most dominant ingredient. Now, each of these ingredients dilutes at different rates as the universe expands. So, if you've got one dominating at some early time, as it dilutes away, others will rise to dominance. In cosmology, you therefore have a sequence of different kinds of expansion, simply owing to the fact that there are different kinds of stuff in the universe, and each kind of stuff dilutes differently.

    Now, it is not far-fetched when you incorporate modern ideas of particle physics and quantum field theory into cosmology, to suppose that there were parts of the early universe possibly dominated by the quantum vacuum. These parts undergo accelerated expansion -- they inflate. As the universe inflates, the vacuum energy begins to die away, and eventually inflation ends (this doesn't have to be the case...the universe could keep on inflating forever, and likely some parts of it are. The part of the universe that we inhabit, however, clearly stopped inflating). The end of inflation is very much like a phase transition (think steam cooling (inflation), with droplets of water forming in it (universe after inflation)). This post-inflationary phase is one filled with hot radiation -- this is effectively the hot big bang. From here, the universe expands and cools; eventually radiation dilutes away, and cold matter takes over; the expansion rate changes accordingly.

    This is just how you'd expect a universe with the 3 kinds of stuff in it to evolve. Now, the present-day universe has thrown us a curve ball of sorts. As the cold matter continued to dilute, we appear to have found another epoch of vacuum energy-dominated expansion! Where did that come from? This we are not sure about. This could be the quantum vacuum associated with empty space, but these calculation don't work out right. So, yes -- a mystery.

    We don't know how all the pieces fit together, but the story isn't as ad hoc as it might sound from the TV show.
  6. Oct 8, 2013 #5
    Hi Guys, thanks for your responses.

    Don't blame me, blame tv. :biggrin: What I don't get is that they say that the people they interview are all well known and respected people in Cosmology/Physics etc etc. Is this not the case? And if so, who are these people then?

    Well, not very helpfull, but at least very honest, thank you. :shy:

    The astronomical predictions of Ptolemy's geocentric model was "absolute fact", and was gospel for over 1500 years. You could even be burned at the stake for questioning this. They believed this, not because they were stupid, but because they could, with mathematical precision, predict where and how planets would behave, for many years in advance. If nobody ever questioned it, we would probably still have believed in it. :shy: Point is, just because the math makes sense, does not mean it's the only correct assumption.

    This is probably true, but with the Hubble Ultra Deep Field for example, they can see all the way back to between 400 and 800 million years after the Big Bang. And in it they can see clearly defined galaxy's. How could galaxy's have coalesced from a very fine "soup" to complete galaxy's in such a short space of time? On wiki it says that "The most accepted theory of how these structures came to be is that all the large-scale structure of the cosmos we observe today was formed as a consequence of the growth of the primordial fluctuations, which are small changes in the density of the universe in a confined region." But even they have to admit that "Despite its many successes, this picture is not sufficient to explain the variety of structure we see in galaxies." Or in other words, they just don't know.

    What I don't get is that when they first found it, they calculated it to be over 16 billion years old. As this made no sense, they went back and had to redo the math, until they reached the current conclusion of 14.46 ± 0.8 billion years, in a universe that is 13.7 billion years old, which just barely but conveniently puts it in the range of the age of the universe. If you keep in mind that this sun also needed time to become a sun from the "primeordial soup", which takes a couple of years in itself, you must understand my scepticism.

    I understand that this list is by no means comprehensive, but logic dictates that if a lot of the answers are, that we just don't know, it might be prudent to investigate other possibilities as well. Do any of you know of any other current research going on to prove other theory's? From what I have read so far, it seems like as soon as someone says anything to contradict the big bang theory, they are dismissed, almost like heretics. Is this the case in the scientific community? Or are there serious research into other possibilities. And if so, where can read some of it?

    Thanks for all the help so far. :thumbs:
  7. Oct 8, 2013 #6
    I heard this analogy once, where someone said that you should imagine you are sitting in a living room in someone's house. And that you can not move anywhere else. You have no idea that there is a bathroom, kitchen, bedrooms, and almost no idea of what goes on outside. You now have to explain how the world works around you. You somehow have to figure out that the wind blows outside, that sometimes it rains, you can get thunder and lightning. You have day and night cycles etc etc. You can develop a lot of theory's, but if you can't get out of the living room, you can never even know of the existence of the bathroom or the kitchen. Let alone the world outside of that. Or the immensity of space outside of that.

    I am perfectly happy to accept that there are a lot of theories about the universe we live in. But what I don't get is this. Even though it may be the best theory we have, if there are so many "We don't really know" in it, Isn't it at least possible that it is completely wrong, and that in a couple of hundred years, people will look back at this time in human history, and snigger at how silly people used to be?

    Like Feynman said, no matter how elegant the theory, if observation or experiments proves it wrong, it's wrong. Find another theory.

    And if I look at some of the evidence, for example the anomaly's in the red shifts of NGC 7603 and NGC 4319, shouldn't scientists just say, ok, there is clearly something we don't understand about red shift, so until we do, we can not depend on hubbles constant anymore?

    You see, I have autism, and one of the problems I have with that is that I am not really good at seeing the big picture. I am however really good at focusing on small details. And when I break the theory down into its constituent parts, to try and get my head around it, I hear a lot of reasons why it is so, but they almost all end with, "but we don't really know".

    This just really boggles my mind. Maybe I should ask what do we know for fact. Can you maybe tell me a couple of facts about the big bang theory that we know for certain? That can not be explained by any other means?

    That would really be helpfull.
  8. Oct 8, 2013 #7
    You are correct, just because the maths makes sense, does not mean it's the only correct assumption. However no one is assuming a picture based on maths only, but also observations. The current model fits the observed data and that’s what’s important.
    There is no contradiction between what the Hubble deep field observed and the current model of cosmology. There are serious cosmologists who do challenge the accepted cosmology. Neil Turok is one example, but he would not be so misguided as to use the Hubble ultra deep field to do so. There is no problem with the existence of galaxies going back to what the Hubble can detect. Where real disagreement lie are in much earlier epochs.
    The age of the universe has changed over time, but that’s because we have better instruments now than we had before. I see no evidence the numbers were fudged to get an answer that was convenient.
    There is no problem with the sun forming 9 billion years after the big bang.
    People do investigate alternate cosmologies but they have to do so within the framework of what is already observed. As you go further back in time there is more room for disagreement. There is a lot of debate about what happened at the big bang with many models being discussed and no consensus yet. Inflation has abroad consensus but it’s not water tight and there are still respected scientists that dispute it. After inflation there is more agreement. There is not much room to question the basic narrative of the standard (LCDM) model after inflation. We have too many independent lines of evidence that tells us the universe expanded form a hot dense state. So if you doubt the universe came from nowhere and no when you have good company with many cosmologists. But if you doubt that the universe expanded forma hot dense state then you are going against a wealth of observational evidence.
  9. Oct 8, 2013 #8
  10. Oct 8, 2013 #9
  11. Oct 8, 2013 #10


    User Avatar
    Science Advisor

    This is an excellent point, and it's what makes science so hard. We need to be able to distinguish true anomalous behavior from statistical flukes. Hubble's law is an approximation that is based on the assumption of a homogeneous and isotropic universe. This law is useful only so far as it is applied to the universe on sufficiently large scales, where homogeneity and isotropy are a good characterization. We expect and observe deviations (sometimes significant ones) on smaller scales: it's not that redshift isn't understood; it's that a simplistic characterization does not apply to all cases.

    Regarding your concern over the correctness of current theory, I think it's honest to say that we understand the broad concepts of the physics governing the evolution of the universe. Of course, there are places where things are murky, especially when one gets into the details of galaxy formation and other highly complex systems. And, yes, sometimes there are BIG surprises, like the present-day accelerated expansion of the universe. These things are not well understood, and indeed they have forced cosmologists to look again at the big picture -- even questioning whether Einsteins theory of general relativity correctly describes gravitation on the largest length scales. So, many of these avenues are pursued, and it's important to keep an open mind and to be honest about the status of current theories. But the consensus view of the evolution of the universe, despite an imperfect understanding of the cause of the accelerated expansion, is remarkably successful.
  12. Oct 8, 2013 #11


    User Avatar
    Science Advisor
    Gold Member

    The claims of anomalous redshift in NGC 7603 and NGC 4319 have been a source of controversy since Arp pushed them front and center in his quest to discredit cosmological redshift. The arguments against these as examples of anomalous redshift have been sufficient to appease any doubts about cosmological redshift with modern science. Remember, in science nothing is an indisputable fact. The preponderance of evidence is the standard. The evidence redshift is predominantly of cosmological origin is vast and compelling.
  13. Oct 8, 2013 #12


    User Avatar
    Gold Member

    Many of these people ARE respected physicists but there is apparently an absolute rule on these TV programs that they all MUST say at least one thing that is so stupid it would be treated as a joke in a room full of physicists.
  14. Oct 8, 2013 #13


    User Avatar

    Staff: Mentor

    First, practically every show is going to come out and blatantly say, "so and so is a well respected member of the scientific community", no matter who it actually happens to be. Also, remember that this is a TV show and the people being interviewed are normal people with bias, possible misconceptions, and a plethora of other human issues. The show is trying to get the most people to watch it while remaining as accurate as they feel is reasonable. This means it is MUCH easier and MUCH more sensational to say "The universe came from nothing" instead of saying "We have no idea how the universe began" and explain why.

    But the Big Bang Theory isn't "absolute fact". We are always measuring, observing, doing math, coming up with new ideas and new models, etc. The theory is constantly being updated and improved. The reason Ptolemy's geocentric model failed, was because it DIDN'T match up with observations well enough. The heliocentric model was developed for this very reason and it turned out, once people starting calculating events using it, to be much more accurate AND easier to use.

    You could say that at the time there was a lot of data that didn't quite match up with the model, but there simply wasn't anything better to use until the Heliocentric model came along. That is not the case today. Today we have a HUGE number of models being proposed for all sorts of things. I know of at least 3-4 models on what dark matter might be. What we have is a lack of necessary data to let us know which models work the best. That's why we keep observing, to get that critical data.

    Our knowledge isn't sufficient to explain the details, but there is no question that the rules of the universe allow for galaxy formation within that time frame. And c'mon, we're talking about HUNDREDS of MILLIONS of years here. That's a very long time.

    I think your skepticism comes from a lack of understanding of how star formation works and just how long of a time scale we're talking about here. The Sun is only 4 1/2 billion years old and that's only 1/3 of the age of the universe. And it's already halfway to death. There have been countless other stars that have already lived out their lives, going from protostars, to main sequence, and then either going supernova or fading away into small white dwarfs that are cooling down, eventually becoming inert balls of cold, compressed matter.

    As for redoing the math, that's a pretty standard procedure when your answer comes out to be something that shouldn't be possible. I'd be willing to bet this happens a lot more than one would think. That's why you go back and check and make sure you did everything correctly. People make mistakes, misinterpret data, etc. That's why peer-review is so very important.

    First, ask yourself what "other possibilities" even means. Does this mean that ALL of the predictions of the Big Bang Theory are wrong? (Because that's what theories and models do, they allow us to make predictions and test them) Are only some of them wrong? The issue is that everything is connected. Did you know that the Big Bang Theory is used to predict the actual ratio of elements in the universe? And when we compare this to observations, the numbers are very very close, meaning that the theory made an accurate prediction. So if the BBT is wrong, why does it predict this so well? And that's only one example, there are many many more.

    So the reason people are called idiots and crackpots when they try to contradict the BBT is that by contradicting the BBT they are actually contradicting a huge portion of the body of scientific knowledge. In order for their contradicting theory to be correct, so much of science would have to be incorrect that it's utterly absurd!
    Last edited: Oct 8, 2013
  15. Oct 9, 2013 #14
    Hi guys,

    First off let me thank all of you for your contributions. I really appreciate it. I am slowly reading through all of it, but as it is quite a lot, it will probably take some time.

    Time. Funny, it is a strange thing, isn't it. I mean the whole space-time continuum thing. I absolutely believe it is true, as it can very simply be proven, by even simple things we use every day. Like GPS for example. So no, I do not have a problem with it, I was just wondering how it fits into the whole Big Bang theory.

    I was wondering if I might run a simple scenario past you guys.

    First of I would like to say, that for now, whenever I refer to time, I do not mean the abstract construct we created to fit our lives here on earth. I mean universal time. In other words, the universe is expanding at a certain velocity, and that is connected to a certain time. Let's call it U-time, and our time on earth E-time.

    Now space is apparently expanding, and not just expanding, but the expansion is accelerating. So it logically follows that U-time must be slowing down. Now if you run this space-time model backwards, it would seem obvious that the rate of expansion slows down, and U-time speeds up.

    At some point in the past, as the expansion comes to a stop, U-time would be going really fast. Now imagine for some reason U-time starts to slow down, and space very slowly begins to expand. Could this not be the Big Bang we are talking about? But instead of inflation, where things has to expand really fast, space gradually and very slowly expands, and has a heck of a long U-time to do it in. Which would explain the homogeneity of space.

    Then, when we get to where we are now, in U-time that is, we observe the universe still expanding, and not just expanding, but apparently accelerating. We can not see U-time, all we are aware of is our arbitrarily define E-time. But we can see space, and that is expanding.

    Don't know if I am explaining my runaway train of thought clearly enough here, but isn't this a possibility? It would explain a lot of the phenomena that we see today.

    This is probably just a stupid idea, and I am probably ignoring a lot of other variables. But can you maybe tell me what it is I am missing? Thing is the fact that space and time is connected is very well established as well. And it is simple enough for me to understand.
  16. Oct 9, 2013 #15


    User Avatar
    Science Advisor

    Why the connection between expansion and time? Can't time exist in the static spacetime of special relativity?
  17. Oct 9, 2013 #16


    User Avatar

    Staff: Mentor

    Time is not an abstract concept. It is real and measurable, just like space is. (Or to be more accurate, we are measuring the difference between points in space and time)

    Also, the expansion of the universe is not a velocity. It is a rate. For example, if we ignore the acceleration of the expansion we can say that it takes X time for the universe to double in size. After this doubling, everything is twice as far apart. Now, if the distance doubled, that obviously means that objects which started close together didn't move as far or as quickly as objects which were much further apart. So when the distance between objects is large, the recession velocity is greater than when the distance between the objects was small. So it is the position of objects relative to each other that determines how fast they are moving away from each other.

    So how can universal time fit into this? I don't honestly know. I'd have to first find out if recession velocity causes time dilation, along with a lot more.
  18. Oct 9, 2013 #17
    Well, my reasoning is that spacetime is not static. Space is moving/changing (expansion is per definition movement with an inherent velocity), and therefore so must "time" be moving/changing.

    The two are fundamentaly connected, as per relativity. If one goes up, the other must come down. At least that is how I understand it. :confused:
  19. Oct 9, 2013 #18


    User Avatar
    Science Advisor

    The office in which I now sit is not expanding, and yet the clock is ticking away on the wall...
  20. Oct 9, 2013 #19
    Well this is a "prediction" that my "theory" would make. So it could be subjected to experiment.

    I am sure someone else would have thought of this by now, so there must have been experiments which would either confirm or disprove it.

    Just can't find any, but as english is not my first, or even second language, I sometimes have trouble guessing what people call things, and that makes it hard for me to find info.

    You guys are much more at home with the current scientific vernacular, and should therefore be able to find the results of such experiments much easier than I ever could.

    So if you could point me in the right direction, it would be greatly appreciated. :thumbs: :blushing:
  21. Oct 9, 2013 #20
    I am not sure what your point is here. :confused:
  22. Oct 9, 2013 #21


    User Avatar
    Science Advisor

    Just that time ticks by independent of any expansion. My reading of your previous post was that expansion was somehow needed in order for time, as a concept, to be well conceived. The theory of special relativity, which applies to static spacetime, defines time independent of any expansion; so your point about expansion and time being fundamentally connected by relativity evades me.
  23. Oct 9, 2013 #22
    Well as I understand it, special relativity states that time is a variable, dependent on your relative velocity through space.

    I propose that as space itself is expanding, this changes the value of the space part of the space time continuum, and as a consequence changes the time variable of the space time continuum itself.

    It is just a question, I am not saying this is the way it is, I'm just saying that if one changes, so must the other. At least that is how I understand it. All things are connected to all things. That's all.
  24. Oct 10, 2013 #23


    User Avatar

    Staff: Mentor

    I don't think so. Time dilation is due to your velocity relative to another observer, not to space. You actually can't assign a velocity relative to space since it isn't an object.

    I'm not sure GR works that way. (Which is what we're really dealing with here)
    Relative velocity and the geometry of spacetime are not the same things.
  25. Oct 11, 2013 #24
    This thread was going great. Comments were coming out like - we are doing the best we can with what we have. Others outside the field expect us to know more than we do and sometimes we pretend a little bit but when we get to p(t) = 0 we really are clueless.

    But then the term time and space as tangible concept come up and people start talking spacetime and once again we are wandering around in the dark with a paper bag over our head.

    Spacetime will become the gobble gook concept of physics. It will come to be known as the biggest conceptual mistake ever.

    Relative time is an acceptable concept. Absolute time is not.

    Where it really gets impossible is when you try to develop a rigorous definition of space. Not an ethereal mathematical definition but a real fundamental definition of space.

    The cleverest of tricks - space has always been assumed.

    So let's mix space and time together before anyone realized The Emperor has No Clothes.

    What has and is lacking is a rigorous definition of space that does not rely purely on mathematical convention.

    If we are lucky - spacetime will have something to do with the Higgs Boson.
  26. Oct 11, 2013 #25


    User Avatar
    Gold Member

    I am not able to make any sense at all of this blather.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook