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Big bang question

  1. Mar 24, 2010 #1
    As I understand it, the big bang is based on the fact that all the galaxies are drifting apart in the universe. If we reverse this we end up with the big bang. But is this not based on the assumption that the universe has always been expanding? How do we know that 4 billion years ago the universe was not expanding? Has it been proving that this expansion has always been constant. I am aware that the expansion is speeding up but is there a chance that this is only the case in the present? Please keep it simple as I am no brainiac.
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  3. Mar 24, 2010 #2


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    We can measure the speed of galaxies further away - all the way back to the very early universe.
    Ultimately we can only see back to the point where the universe was just energy (which now makes the microwave background).
    It's possible that the universe didn't go back to a singluarity before that - but if it didn't it makes it difficult to know where the energy came from or why it suddenly decided to expand.
  4. Mar 24, 2010 #3
    So the big bang theory makes things easier to understand. But that does not make it true. I am not saying it didn't happen, so I hope no one gets offended. I am saying that it is not the final say on the matter. Do we tend to make things fit into the big bang theory?
  5. Mar 24, 2010 #4
    Hmmm, that's really more of a general commentary on the scientific method than on the Big Bang. Truth is a concept that belongs to disciplines like philosophy and mathematics rather than science. The only way to produce truths is to start from axioms taken to be self-evident and to build upon those according to rules taken to be equally self-evident, the latter usually being what we call logic.
    Science, on the other hand, has learned the hard way that it can't afford to take anything to be self-evident. Instead, the modern interpretation of theories is that they are models which explain vast and complicated amounts of observational data in simple and elegant ways. In my experience, most scientists assume that there is some connection between this process and some form of underlying truths about reality, but have given up worrying about the precise nature of that connection.

    That being said, these thoughts are particularly relevant to cosmology, because the field is by its nature more speculative than others - repeatable experiments tend to be hard to perform. ;)
  6. Mar 24, 2010 #5
    Except for the evidence:
    "The announcement today represents a rite of passage for cosmology from speculation to precision science."
    - John Bahcall of the Institute for Advanced Study in Princeton, N.J.


    "The Big Bang is the cosmological model of the universe that is best supported by all lines of scientific evidence and observation. As used by scientists, the term Big Bang generally refers to the idea that the universe has expanded from a primordial hot and dense initial condition at some finite time in the past, and continues to expand to this day."

  7. Mar 24, 2010 #6


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    Not quite. The Big Bang theory is a model for the early universe that is supported by 3 main observational sources:

    1) The Hubble expansion. As mgb_phys says, we can determine the expansion rate of the universe by measuring the redshifts of receding galaxies. These measurements reveal a uniformly expanding spacetime back to very early times.

    2) The cosmic microwave background (CMB). This is the 'left over' radiation from the hot, dense early universe. It is consistent with the picture of a hot dense universe that has since been cooling and expanding since the CMB's creation around 300K years after the big bang.

    3) Nucleosynthesis. The calculations of the chemical reactions that should have taken place in a hot dense early universe, and taking into account the change in their initial abundances as the universe evolved through its expansion, match modern observations extremely well.

    The Big Bang --defined as the expansion of the universe from a hot, dense, early state -- is well supported by modern day observations. It is not merely a device for making things easier to understand.
  8. Mar 24, 2010 #7


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    Virtually all our observational evidence and our models point to this explanation. With the exception of some minor tweaks, there are no competing theories that fit the data or the models.

    Whether it is "actually" true is not something we can act upon. We act upon what we see.

    (Like everything else in life. Did that red-hot stove burner actually burn you, or can you not count on your observations, level of pain and existing knowledge of stove burners? What difference does it make if it "really" happened? Get it treated.)
  9. Mar 24, 2010 #8
    Frankly, I'm not sure I see the difference. "Making things easier to understand" (which sometimes allows us to predict how unfamiliar things will behave and thus leads to technological progress) is a fair summary of the ultimate objective of all scientific endeavour.

    It's not hard to come up with alternative models (aka junk science) to the Big Bang that fit the data equally well, as long as we don't also impose aesthetic constraints like simplicity and elegance.
  10. Mar 24, 2010 #9
    I guess I am wondering the age we gave the universe. Has it been proven that this expansion in increasing at a fixed rate? That it has never been different?
  11. Mar 24, 2010 #10


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    OK, but surely you would need to base these alternative models on actual physical theories like GR. If not, it's not science. Once you've done that, I assume you can choose some totally funky parameterization that lets your universe evolve in crazy ways. But these models are technically more complex than the concordance big bang model, and can be statistically disfavored in a rigorous manner.

    I disagree that the big bang need not reflect physical reality. If you're in the business of statistics, then yes, your models and their parameterizations are simply tools to facilitate an understanding of the phenomena. They don't (and arguably shouldn't) need to capture anything fundamental about the phenomena. But physical science is different. Our models need to be based in reality, and should describe the physics behind the phenomena.
  12. Mar 24, 2010 #11
    No, neither observational data nor theory suggests a fixed rate of expansion. The most obvious correction is gravitational deceleration, several more exotic corrections are part of the modern Big Bang models. However, if what one is interested in is an order-of-magnitude estimate for the time elapsed since the Big Bang, simply using today's expansion rate as if it were a constant works well enough.


    That certainly was the universal opinion of the classical (pre-QM) era. However, it does rely on several common-sense notions about reality that aren't fully compatible with the philosophical underpinnings of quantum mechanics. AFAIK, mainstream philosophy of science has pretty much given up on the claim of there being a qualitative difference between a purely descriptive model on the one and a true law of nature on the other hand.
  13. Mar 24, 2010 #12


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    OK. I guess I agree with this statement. I'm just trying to emphasize the difference between a physical model and a statistical model in terms of their differing objectives regarding the understanding physical phenomena.
  14. Mar 25, 2010 #13
    Hi. This is my first post here. I'm not a physics guy, I just like to read about it.

    I was wondering about the idea that binbots has raised. The observations of the galaxies appear to show that we're all racing away from each other, and accelerating. I imagine the big bang expanding the way a balloon expands when you blow it up. The discreet points are actually spreading farther apart as you blow the balloon up.

    Is it possible that the expansion of space/time was not linear in those early days, and that time expanded with space? The idea being that as space/time expanded, the moments of time expanded...or slowed down?

    Since we're all expanding along with everything else in the universe, we don't perceive any change in the passage of time.

    At least not until we peer far enough into the past, to finally see light that is billions of years old, when space/time's moments of time were closer together. While we are observing this 'old' light from the past, the matter radiating that light will appear to us as accelerating away from us, where it is actually us slowing down.

    Keep in mind, this is not actually light slowing down, but space/time expanding. It would also mean it's not dark matter accounting for the apparent acceleration we see, but simply expansion of the universe taking longer and longer to go from moment to moment.

    It's an intriguing idea.
  15. Mar 25, 2010 #14


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    I'm not sure you quite grasped what mgb was telling you. The big bang happened around 14.7 billion years ago. Astronomers have identified galaxies as old as 14 billion years, so we know the universe has been expanding at least that long.

    But then looking at the CMB, they can determine that it has even been expanding for a lot longer, as the universe became transparent only some 380,000 years after the big bang. So we can confirm expansion all the way back to 14.6+ billion years. That's pretty solid confirmation of the theory.

    So please don't interpret your own misunderstanding of the theory as a flaw in the scientific method.
  16. Mar 25, 2010 #15
    “WMAP definitively determined the age of the universe to be 13.73 billion years old to within 1% (0.12 billion years)”
  17. Mar 25, 2010 #16
    For someone who is not a physics guy you see some things that I wouldn't expect. You actually wrote my question down for me. When it is said that the universe began 14.8 billion years ago, what does that even mean. What is time? To me, time seems like a man made concept, just like everything else. A way of explaining the physics world from our eyes. But without our eyes is there even time? Every time I think about this I keep going until I spiral myself into a headache
  18. Mar 25, 2010 #17


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    So you're suggesting that without man, there would be no time? A universe devoid of mankind certainly evolves, trudging along to a state of lower energy and higher entropy. Many consider this to define the direction of time. If you were to attach a wrist watch to the rest frame of the CMB and start it when the CMB was generated, it would record 13.7 billion years by today.
  19. Mar 25, 2010 #18
    I know you're right, i know time is based on reactions being taken out, like a second is some ridiculous amount of times that Cesium goes between hyperfine levels. But time is not linear and time could be expanding as well. If we are expanding, that means our velocities are getting larger and larger. as our velocities get larger, so does time with respect to a still frame of reference. so how do you even mesaure that kind of thing. Obviously there are people much smarter than myself who have thought about this question and come to a conclusion, i just don't know what that conclusion is.
  20. Mar 25, 2010 #19


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    This is a common misconception. While the universe is expanding, this does not mean that the objects in it are getting faster and faster. Galaxies are (mostly) at rest with respect to the expanding space -- they separate from each other because the space itself is expanding. So, with respect to their local reference frames, their velocity is zero. Of course, some galaxies do move relative to the background, and different observers in different parts of the universe should certainly be keeping different times, due to the effects of gravity. That's why it's important in cosmology to define reference frames. When we talk about the age of the universe, we are implicitly giving the time as seen by a comoving observers -- one that is locally at rest and just flowing along with the expansion of space. Of course, an observer that has been moving close to the speed of light relative to the CMB for most of the history of the universe would record a different time!

    Lastly, as for the 'expanding time' you mentioned, this also sort of happens. When you examine the Schwarzschild solution pertinent to a black hole, you find that the space surrounding the black hole is in fact static. Objects are drawn towards the black hole as a result of a 'curved' time coordinate. Same with objects that fall towards earth.

    You are right that time is a concept that is worth pondering deeply. However, as long as you follow the rules of relativity, it can be a useful concept in cosmology.
  21. Mar 25, 2010 #20
    So it is space that is expanding not matter? empty space is expanding? which brings me to another question about immense planets and suns bending light. From what I understand, it is the gravity that condenses spacetime, and this "spacetime" is what you are referencing by curved time coordinates? So it is not necessarily the space that is being altered in that certain area, it is the time that is being altered?

    I apologize if my thoughts are not very well formulated but there is still a solid amount of confusion for me revolving around this topic.
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