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

Universal Expansion

  1. May 9, 2007 #1
    I was thinking yesterday about the fact that the universe is supposedly expanding at an ever increasing rate and i thought of i reason why.
    I'm pretty sure its wrong but so far nobody could explain to me why.
    If you assume that the there is an infinetly big nothingness for our current universe to expand into, i.e that there is no point where the universe will hit a wall and stop growing; then i thought you could explain it using pressure.
    So if this infinetly big space has nothing in it then the pressure must be infinetly low because of P=m/V, where as our current universe has a much smaller volum and much more mass meaning it must therefore have a greater pressure which would mean that our universe would be being pushed outwards due to the pressure differenece, and thus would accelerate because a=F/m there fore as long as there is a force it must accelerate.

    Can someone try to find where i went wong because it is really bothering me?
    Last edited: May 9, 2007
  2. jcsd
  3. May 9, 2007 #2


    User Avatar
    Science Advisor

    The current model of the universe says that the Universe first decelerated then started accelerating (roughly speaking the acceleration started ~5 Billion years ago while the universe is around 13.7 Billion years old). Regardless of anything else your model wouldn't explain the change in sign of the acceleration.

    In any case you are confusing expansion with acceleration. Your model in fact contains a universe that is expanding, but the expansion would be decelerating, since as the pressure difference dropped the expansion would slow.

    In addition the matter in the universe is in fact pressureless on large scales since the mean density of the universe is low. Gravity is the only force acting on large scales and pressure (which is due to electromagnetic forces between particles) is only significant on small scale high density objects such as stars and galaxies.
  4. May 9, 2007 #3
    i was wondering
    However some of your arguement is worng because if thhere was a large enough pressure difference then a force would be exerted which would cause acceleration due to the lack of frictional forces and a=F/m so if there were a force there would have to be some acceleration.
  5. Dec 22, 2007 #4
    In response to the last post; you are correct in saying there would be acceleration, but what Wallace is saying is that the pressure would only provide an acceleration value which becomes lower over time. As you said yourself, a = f / m, so if the force becomes lower because of the decreased pressure resulting from the expansion, so does "a." The acceleration is still happening, but it is becoming less pronounced. There must, therefore, be some other force acting on these bodies unrelated to anything mentioned herefore.
  6. Dec 23, 2007 #5


    User Avatar
    Science Advisor
    Gold Member

    Wallace is not saying that!

    Let us clear up a misconception.

    In GR cosmology pressure in the universe does not 'push the universe' so it is "expanding at an ever increasing rate".

    If there is pressure in the universe it acts to slow the expansion down and may eventually reverse it, it decelerates the expansion! This is the opposite of what might be expected.

    This happens because pressure represents a kind of energy and that is an extra source of attractive gravitation.

    To make the universe accelerate in its expansion, as is thought to be happening in the recent universe (since z~1), you need to have negative pressure, to provide a repulsive gravitation, this is thought to be delivered by Dark Energy.

    There have been many posts recently in different Forums all making the same error, I hope this makes it clear now!

    Last edited: Dec 23, 2007
  7. Dec 23, 2007 #6
    How large is the material which is supposed to support the increasing expansion really? And where can I find the calculations?
  8. Dec 23, 2007 #7


    User Avatar
    Science Advisor
    Gold Member

    Hi henxan!

    You can find a very good introduction about Dark Energy here on Scholarpedia. (Note Scholarpedia, unlike Wikipedia, is only written by experts and articles are reviewed by experts. I recommend it, although at present it is short of articles that have passed the review stage, but time will only improve that.)

    If you are asking about the Hubble Expansion itself there is much evidence which is reviewed here on Wikipedia.

    If you want further information keeping asking questions on this Forum, we will try to help you.

    Last edited: Dec 23, 2007
  9. Dec 23, 2007 #8
    well.. what particularly I was looking for was:

    - How many observation are included in the basis for the accelerating expansion theory?
    - How does the calculus go? I would really like to see a complete set of equations, so I could get a bigger grasp of the subject myself.

    I have not had that much cosmology/astronomy, only one subject called "astronomy and astrophysics," which was a bit too basic, though wide ranging. I am soon-to-be a civil-engineer, in physics, so I have an OK background in physics ;).

    I have to say that dark energy, dark matter; it all seems a bit too extravagant.. You know, epicircle-upon-epicircle, I think this is probably not the case. Why so complicated? All other progress in physics have been done resulting in EASY calculation. Newton, Einstein. In theory its wery easy in the basics. So should a theory for the expansion of the universe be.

    So, we haven't seen dark matter or energy. ->Occam's razor->doesn't exist.

    Well, thats my "feeling" though, but I would like to get to review the material on the subject before I make any conclusive choice of my own. :)
  10. Dec 23, 2007 #9


    User Avatar
    Science Advisor
    Gold Member

    Did you read the link to the Scholarpedia article? (Just left click on the word "here").

    Actually if you read my other posts I too have questions about the standard model.

    However, here suffice it to say that it fits the data very well from several different independent sources.

    The problem with it though is that it relies on the physics undiscovered in the 'laboratory' of Inflation, Dark Matter and Dark Energy.

    If an alternative modification of GR is found that does not require these entities then they would then indeed be seen as 'epicycles' introduced to 'save the appearances'.

    But that day has not arrived yet and tomorrow, who knows, perhaps they will be discovered in the laboratory with all the correct properties, thus confirming the standard [itex]\Lambda[/itex]CDM model.

    Of course there is nothing to say that "a theory for the expansion of the universe" should be "EASY", and certainly in any practical application I wouldn't say that about GR either!

    Last edited: Dec 23, 2007
  11. Dec 23, 2007 #10
    Thanks for clearing that up for me, Garth. I was unaware of the attraction provided by the pressure; and yes, it is counterintuitive. Recent work done in Spain provides yet another view of expansion, however. The head researcher suggested that the observed acceleration simply results from time slowing down throughout the universe, which would provide the necessary illusion. I have my doubts about this one, however; if time were slowing down at a uniform rate, then no acceleration would be observed, since time on Earth would be passing at the same rate relative to objects in distant parts of the universe. But for time dilation to account for the observed acceleration, Earth and the solar system would have to be accelerating, bringing one right back to accelerated expansion. If anyone has information about this study which I am lacking, I would appreciate hearing it, as it would clear up a number of problems I have with this view.
  12. Dec 23, 2007 #11

    Yes, I did read the article on dark- energy & matter. But I didn't see any relevant links to the "the universe is expanding at an accelerating rate" proof. This is very important. Because it is the physical reality which govern the theories deduced. If you've got the wrong data, you just wont get the right formulae.
  13. Dec 23, 2007 #12


    User Avatar
    Science Advisor
    Gold Member

    The evidence is in the diagram Figure 1.

    Of course it has been rigorously analysed statistically (with the assumption that such supernovae are standard candles over cosmological epochs) and there are lots of papers about Super Novae Type Ia and fitting the WMAP data.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook