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B Questions regarding to the Accelerating universe from Novice

  1. Aug 11, 2015 #1
    The Universe is constantly expanding, and from the observation of Hubble, super novas, the figure turns out to be a acceleration. The theory that's most accepted seems to be dark energy at work.

    As the story goes, before big bang, there was nothing, maybe not even time or space, so how does something expand into nothing? I mean, if gravity slows us down and if dark matter is repelled by gravity (according to some theories), and then dark matter is what fills up the space for universe to expand to, shouldn't we slow down?
    I understand this is a impossible question with no definitive answer, since we can't even answer what's outside the universe (some say extreme heat, then there's the multiverse idea and also plain nothing). But this is all just mind spinning. If gravity is caused by a warp in space time, does space time also be stretched as the universe is expanding? I mean, the thing should be created by big bang right? else anything that was here before it should've been pushed back as the universe expands.

    What if there IS something outside the universe? substance that attract matter? I mean, space time seem pretty consistent whether here on earth or some place a few million light years away, so what if space time isn't expanding and accelerating, just us and matter/energy? farther we travel faster we go? all our observation comes from observing stars and explosions. Would this be a possibility? (probably just random B.S.) But really, the lack of possible explanation and info on this just send my head into oblivion, I need help :cry:.

    Please offer some sort of wild explanation or good reasoned ones, maybe even ideas on how I can stop thinking about this question from time to time.

    Thank you all for taking your time to read this nonsense.
  2. jcsd
  3. Aug 12, 2015 #2

    D H

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    FIrst, a comment. Asking about what happened before the big bang makes no sense currently.

    You are confusing dark matter and dark energy here. Dark matter gravitates just like ordinary matter. The "dark" in dark matter has two meanings. One is that it doesn't interact electromagnetically. That makes it "dark" in the sense that we can't see it. The other is that scientists are in the dark regarding what dark matter is. As deep a mystery as dark matter is, dark energy is even darker. Dark energy is essentially a placeholder name for whatever it is that is causing the expansion of the universe to accelerate.

    You want answers. There are no answers, yet. There are guesses about what dark matter might be, but those are just guesses with no evidence to back them up so far. Regarding dark energy, there aren't even any good guesses. There is however plenty of evidence that they do exist, whatever they are.

    Regarding the question about what the universe is expanding into, you are thinking too Euclidean. One of the downsides of the various non-technical models of the expanding universe (e.g., the balloon model, the loaf of raisin bread model) is that they all give the idea that the universe is expanding into something. The universe is not Euclidean. It doesn't need to have something to expand into to be able to expand.

    Look at the bottom of this window. You'll see a list of similar discussions. I suggest you read some of them, then come back with more questions.
  4. Aug 12, 2015 #3


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    The big bang theory does not say that nothing existed prior to the big bang. In fact, it doesn't even have a 'big bang' anywhere in it. The big bang theory describes the universe as expanding from a very hot, very dense state to its current cooler, less dense state. At no point in the theory is there ever a time where everything ceases to exist. The model simply stops making accurate predictions past a certain point in time which we tend to call t=0.

    Dark matter is not repelled by gravity nor does it fill up the space that we are 'expanding into'. As DH said, we aren't expanding into anything. The distance between all currently existing objects simply increases over time.

    There are no accepted theories explaining what happened prior to t = 0 at this time, and even the theoretical ones I've seen do not say that we are expanding into anything.

    The idea of something 'outside' the universe makes little sense. In cosmological terms, the universe is effectively everything that exists. Something outside the universe couldn't interact with us. If it could, then it would be, by definition, within the universe. In any case, for this to be true the universe and whatever was outside of it would have to have some pretty strange physics, stranger even than the current theory.
  5. Aug 13, 2015 #4
    thank you very much for the patience to read and answer me.

    I love cosmos but the lack of information compared to universe is simply mind boggling. it just amazing how big the whole damn thing is and that its created by a really hot, infinite dense, singularity in a flash of time. I mean, everything we know is composed of matter, which break down into energy, If I look at the universe as a closed system, all that energy came from that one single event, I simply can't comprehend how such singularity can exist, then rapidly expand and speeding up. its like the wall between our universe and the other side is the boundary between natural laws and chaos.

    I also understand what D.H. is saying, I'm sorry for the idiot mistake of mixing up dark matter and energy. But the only explanation about the acceleration we have is dark energy, 68% or so of it, but how can we observe something that give no property to observe, other than their effects on stars?

    If dark energy still behaves similar to energy, shouldn't it spread out? and if the universe is accelerating, shouldn't the density of dark energy lower and instead become deceleration?

    sorry for all these random questions, to me its just amazing. thank you.
  6. Aug 13, 2015 #5


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    Well, the problem is that cosmologists don't really believe there was actually a singularity (or so I've been told). It is believed that our math breaks down and we get bad results (aka a singularity) because we don't understand the laws of physics at the energy and density scales of the very early universe.

    The same way we observe gravity and the other fundamental forces of nature. We observe things which cannot be explained by current laws and find new theories and laws to describe them. Even if we have to observe things at the scale of galaxies and above. Just because dark energy doesn't affect us here at our scale doesn't mean we can't study it.

    Dark energy doesn't behave similar to normal energy. It's got very different rules, one of which is that it doesn't appear to dilute and spread out as the universe expands. It might be better to think of dark energy not as a substance that is pervasive throughout the universe, but as a kind of geometrical law within general relativity. In fact, the idea that dark energy is simply an effect of a cosmological constant (a mathematical term first introduced by Einstein) turns out to be the simplest explanation that fits the data so far.
  7. Aug 13, 2015 #6
    Ah, thank you again

    I was also told about the math problem regarding to the singularity, is it true the main cause is simply because all of our universal laws are created on the basis of the current universe, while before the creation, the laws might not even exist? (maybe even the concept of time?)

    I severely lack of knowledge regarding to the properties of dark energy, I had always assume that dark energy would be a new type of energy, but if dark energy is pervasive through out the universe yet a simply effect of a constant, how does it increase? Although recent observation suggested universe might not be accelerating that quick, its a→ is still increasing, there has to be a increase in amount of dark energy, else there can't be a change in acceleration. (assuming, the force of repulsion for dark energy does not change over time).

    One of the explanation for dark energy is it's a property of space, which is why the first post ended up with the term "filling up into something". My current understanding of space is that space by it self is still and frozen, in our 4 dimension world, the 3 dimension much reside on space, else it cannot exist. But time shouldn't matter, with or without space, time should exist. according to Einstein's relative theory, travelling at speed of light will slow down time but that's only when comparing the object to the rest of the universe, without a comparable object, time still goes the same speed for the object travelling at such speed. (I probably screwed up somewhere).

    To me there is no situation where t=0, there should always be something before t=0, therefore t≠0, I mean, no theory can justify the existence of nothing. Even if one of the new bizarre theory states "we are simply a program created by another entity" that entity still has to be there in the first place. Same thing with my problem regarding to cosmos and the nonsense "what are we expanding/replacing (outside the universe)", there can't be nothing, even the emptiness between quark there is something, there is space.

    That's my issue, the whole accelerating universe is observed originally from supernova and their distances. But what about space it self? is it expanding or does it not? even in the event of big rip, where w<-1 in equation of state, the obliteration of atoms would not effect space it self. would it just keep expanding indefinitely? And how can something expand indefinitely if it came from something? if the previous is false then is "space" not a product of big bang? I know this sounds trippy because you can't measure "space" but I just can't stop thinking about it.

    I seriously waste away too much time thinking about this nonsense and it won't go away. to a point where I am loosing sleep, so right now im not searching for a answer, but rather someone who can deny me wholly of this idea or shine some logic onto it. (and I swear I am psychologically fine right now and sober).
  8. Aug 13, 2015 #7
    The stress energy tensor for example tells space how to curve, note the energy density to pressure terms in the equation.


    Here is a basic intro onto Lorentz transformation. Which will help see that GR /SR is a coordinate metric

    Lorentz transformation.

    First two postulates.

    1) the results of movement in different frames must be identical
    2) light travels by a constant speed c in a vacuum in all frames.

    Consider 2 linear axes x (moving with constant velocity and [tex]\acute{x}[/tex] (at rest) with x moving in constant velocity v in the positive [tex]\acute{x}[/tex] direction.

    Time increments measured as a coordinate as dt and [tex]d\acute{t}[/tex] using two identical clocks. Neither [tex]dt,d\acute{t}[/tex] or [tex]dx,d\acute{x}[/tex] are invariant. They do not obey postulate 1.
    A linear transformation between primed and unprimed coordinates above
    in space time ds between two events is

    Invoking speed of light postulate 2.

    [tex]d\acute{x}=\gamma(dx-vdt), cd\acute{t}=\gamma cdt-\frac{dx}{c}[/tex]

    Where [tex]\gamma=\frac{1}{\sqrt{1-(\frac{v}{c})^2}}[/tex]

    Time dilation
    dt=proper time ds=line element

    since [tex]d\acute{t}^2=dt^2[/tex] is invariant.

    an observer at rest records consecutive clock ticks seperated by space time interval [tex]dt=d\acute{t}[/tex] she receives clock ticks from the x direction separated by the time interval dt and the space interval dx=vdt.


    so the two inertial coordinate systems are related by the lorentz transformation

    [tex]dt=\frac{d\acute{t}}{\sqrt{1-(\frac{v}{c})^2}}=\gamma d\acute{t}[/tex]

    So the time interval dt is longer than interval [tex]d\acute{t}[/tex]

    (Lol I'll have to find tune my latex on this site. Gotten used to a different site)

    Anyways the above is from Matt Roose " Introduction to Cosmology" I don't have the page number handy, as I'm currently working up North
    Last edited: Aug 13, 2015
  9. Aug 14, 2015 #8


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    Mordred, Modern cosmologists are reluctant to ascribe any parttcular volume or geometry to the universe. That leads to a Machian interpretation that has been losing popularity for nearly 100 years. And, I would argue our current model of gravity is insufficient to account for all observational evidence. I remain on the fence in a wait and see mode.
  10. Aug 14, 2015 #9
    This has nothing to do with Mach's principle.
  11. Aug 14, 2015 #10


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