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Expanding universe

  1. Jun 7, 2009 #1
    Why the early universe started expanding
  2. jcsd
  3. Jun 7, 2009 #2
    a big reason would be because most scientists accept the Big Bang theory, which follows that a large star essentially died and exploded, and all of the different gases and chemicals began propelling themselves outward in all directions. as they mingled, they formed all of the major elements we recognize today and according to Einstein's Theory of Relativity essentially, the major gravitational rip in the 'center' of the universe has since depleted when the star exploded, and therefore everything continues to move outward because of its inertia.
  4. Jun 7, 2009 #3


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    Hi dsmith23! :smile:
    Nooo … there were no gases or chemicals …

    there weren't even any atoms …

    I'll let someone else describe exactly what there was at the Big Bang (and why the early universe started expanding), but it certainly wasn't the remains of anything remotely starlike, it was only very elementary particles :wink:
  5. Jun 7, 2009 #4
    Hey my term paper covers this stuff! :p

    So yeah. Big bang.

    It's basically the beginning of the universe, as it was a point mass (singularity) of infinitesimally small dimensions. Inside this contained subatomic particles we know and love and stuff we don't even know and understand. It was a dense ball that soon expanded in an event called the big bang. If you'd like you can use the term exploded though that's probably not EXACTLY the right term. From then on we entered the planck era . We don't know much about this time because it was 10^-49 seconds after the big bang. Physics is just a newborn baby crying about milk and pooing it's diaper at this time, so quite a few things aren't as they are at present.

    The universe is continuing to expand from its initial big bang (you can think of it like conservation of momentum or inertia to make things easier... something that started expanding at the big bang is going to continue expanding).

    I'm going to speed things up because I have to get back to the paper.

    As it continues expanding more and more interactions take place between these subatomic particles and they make even more subatomic particles and anti-particles. The anti-particles clash with regular particles and they annihilate each other in a process called baryogenesis. This results in the domination of matter over anti-matter. As the universe continues to expand and cool, particles we all know and love like protons and electrons are created. These particles can't however make a hydrogen atom just yet because of the residual radiation resulting from all those anti-matter matter annihilations prevents and generally ionizes any hydrogen atom that is trying to form. However the universe expands more and more with time, and the radiation temperature decreases, recombination occurs and protons and electrons are combining to form hydrogen atoms and helium atoms and some even lithium atoms. Then the residual radiation decouples from this hydrogen and becomes what's called the Cosmic radiation background or now, the Cosmic Microwave Background (CMB). Since there's no more ionized particles we enter whats called the dark ages. Swirling gases of hydrogen and helium form dense clumps as can be seen as anisotropies in the CMB. The denser regions get more dense and the sparse regions get more sparse until of course gravitation centers start forming and the first protostar is formed 1 billion years after the big bang. Then stars formed other stars and galaxies and planets and finally your forum post.
    Last edited: Jun 7, 2009
  6. Jun 7, 2009 #5


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    Why did the universe start expanding?

    Because an expanding universe is a stable solution to Einstein's equations?
  7. Jun 8, 2009 #6


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    Hi protonchain, that's a nice explanation of the Big Bang. I have just a small quibble about it however, if you are doing an assignment on this hopefully this will help!

    There are two subtley different concepts here. One is the prediction of the model, and the other is something physically sensible. So in our general relativistic description of the Universe, the model has a singularity about 13-14 Billion years ago when the a(t) (scale factor) goes to zero. However, this is almost certainly not what 'really' happened. Our physical theories are incomplete, and we don't know what happens when both relativity and qauntum mechanics become important, i.e. when things are so dense gravity is of comparable strength to the other forces (if you think of an atom the electrostatic force between protons and electrons is obviously many many times stronger than their mutual gravitational attraction). This means that in fact we really have no idea what happened once the model tells us the Universe becomes denser than a certain amount.

    This is often mistakenly described as 'it isn't sensible to talk about when happened before such and such a time'. It is sensible to talk and think about it, we just haven't yet come up with a sensible solution! In any case, if you want to describe present knowledge a much better description is not that the Universe started from 'a singularity', since a singularity is not a physical thing, just a sign that the maths stops working. Instead you should point out that we just don't know (yet) what went on when the Universe was extremely dense.

    Our present theories in a sense start from a point where the Universe was very hot and some very high but finite density, and for some reason went through a very rapid expansion (inflation). So the present description doesn't start, in a physical sense, from a time when the Universe had zero volume, or infinite density. It is 'sensible' to ask what happened before this time, we just don't have a sensible answer (yet).

    I do however really like;

    Very good! Many people (including some long time posters in this forum) get quite confused about this issue, but that is a nice succinct explanation.
  8. Jun 8, 2009 #7


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    Good question, the short answer is that we don't really know. It is a (very) active research area, but we have no definate answer as yet. There are theories that describe the start of the expansion, such as inflation, but we don't know what caused inflation, and not knowing that, we can't say why it happened.
  9. Jun 8, 2009 #8
    Hey Wallace, thanks for clarifying up some issues!

    I was actually writing it all in a hurry and yesterday my brain was partially fried so some part of me didn't want to say "We don't fully understand/have a physical solution for why X happens". I don't know why, I'm throwing a literal pie in my face for that one.

    Actually my term paper involves the dark ages of the universe, 21 cm emission, and WMAP results for the constraints on cosmological parameters, so I'm not too focused on the timeline of the big bang.

    That said however I will try to summarize the epochs in 1 slide and I will definitely consider your statements and make any changes/corrections to my powerpoint as needed :).
  10. Jun 9, 2009 #9


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    I dont buy into the 'infinite' universe proposition, but, allow it as a possibility. The markers all suggest we live in a finite universe, IMO.
  11. Jun 9, 2009 #10


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    This comment doesn't appear relevant to this thread, but in any case it is unstubstantiated. There are no 'markers' in any observational data that speak greatly to this. The observations are consistant with either an infinite Universe, or at least one many many times greater than the observable patch. In the case that we are using the wrong model to fit the data, then all bets are off and we can't really say anything either way.
  12. Jun 9, 2009 #11
    hey- just wanted to say i appreciate the correction, admittedly i'm just a student with an interest for physics and its effects so much of my assumptions comes from the 'watered down' discovery channel versions =p
  13. Jun 13, 2009 #12
    No one knows that.. we only know that it is expanding first motivated from Hubble's observations, this led Einstein to throw away his cosmological constant that made the universe static. thus general relativity also wanted an expanding universe. but then came inflation, which needed more density than that is generally observed from summing matter and radiation, and then the vacuum energy and cosmological constants were born. remember that vacuum energy although a result predicted much ago as a solution of field eqns was never thought to be physically realizable.
    not only that now high redshift supernovae predict that it is also accelarating.. many models like scaler field, tachyon field, phantom field, ghost potentials have been proposed to explain them.
  14. Jun 13, 2009 #13
    this is not correct .. what that singularity held is still beyond present theories of physics
  15. Jun 13, 2009 #14
    Post #7 is right on..nobody really knows...a simplistic answer would be that the energy of expansion exceeded the energy of contraction...that is, repulsive energy density exceeded gravitational attractive energy density....but of course nobody knows why....

    Big bang theory suggests maybe a random vacuum energy fluctuation caused the start.....but that current accelerated expansion results from dark energy...in QM all random fluctuations will occur sooner or later so an outrageous one like a "big bang" is deemed possible... just "improbable"

    is a good way to think about....it's called a phase transition in this case from an unstable high temperature high energy state to the lower more stable environment we experience today...Before this general relativity and quantum mechanics break down.

    This is unproven... and I suspect outright false. If initial energy conditions change, expansion changes. Proof that the above quoted statement is false is that the universe has been experimentally shown (Hubble) to be in a period of accelerated expansion...expansion is speeding up......NOT what the above logic would dictate. As various energy densities vary with cosmic expansion, inflation also changes. For example, gravitational potential energy density decreases and apparently dark energy density remains virtually constant with cosmic expansion....hence a current period of accelerated expansion...

    A somewhat different view of expansion is described in my separate thread here in cosmology A CYCLIC MODEL OF THE UNIVERSE.
    Last edited: Jun 14, 2009
  16. Jun 14, 2009 #15


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    I was reading too much into the OP, Wallace. Apologies to all.
  17. Jun 17, 2009 #16
    I just meant it as a metaphor. I didn't mean that that is the exact mechanic for the expansion for the universe. My mistake.
  18. Jun 18, 2009 #17


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    The universe appears to have expanded at superluminal velocity in its infancy. This is the premise of 'inflation' as proposed by Alan Guth. Inflation solves a number of problems with the big bang model. It is an effective theory - one that does a good job explaining what we observe, but, has no firm theoretical basis.
  19. Jun 18, 2009 #18


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    One could get the idea from Chronos post that mere superluminal distance expansion rates is the defining characteristic of inflation.
    (Pop sci accounts often suggest this, but it is misleading. So the misconception needs to be corrected.)

    I believe the essential feature of inflation is exponential growth of the scale factor, or at least rapidly accelerating expansion.

    Merely having superluminal expansion of some distances (in standard model terms), particularly if the rates of increase were decreasing, would not by itself have the effects attributed to inflation. It wouldn't for example explain near-flatness, nor the near-uniformity of background temperature.
  20. Jun 18, 2009 #19


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    Describing inflation as a period of 'super-luminal' expansion I think relates to the particle horizon, in that during this phase the horizon shrinks, i.e. the co-moving part of the Universe accessible in any possible future for a given particle gets smaller. I can't remember if the condition for this is any acceleration (equation of state w < -1/3) or if it requires the exponential expansion you refer to (w=-1).

    I agree that it is an often misused term though, but as I say the closest thing to a technical defintion of a 'superluminal expansion' phase is defined in terms of the horizon. On the whole it is probably safer to just say exponential expansion, for the reason noted by Marcus, but none the less it is a very common way of describing the inflationary era.

    Note that this is precisely how inflation solves the horizon problem, in that regions that are connected, and hence in thermal equilibrium, prior to inflation get causally disconnected by the inflation phase. Without inflation you wonder why these casually disconnected regions can be so close in temperature, with inflation it is relatively simple thermodynamics that does the trick.
  21. Jun 19, 2009 #20
    I am wondering why the universe seems to be expanding equally in all directions from our point of view. If the galaxies are moving away from a central point of expansion, the ones nearer the point should seem to be moving away at a slower speed.
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