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

What is Causing The Universal Expansion?

  1. Apr 7, 2009 #1
    I read Somewhere that the universe is expanding due to dark energy but how can dark energy create space and time?
  2. jcsd
  3. Apr 7, 2009 #2


    User Avatar
    Science Advisor

    There are two distinct (but related) concepts you are confusing. Expansion of the universe originated with the big bang. It was presumed by everyone that it would be slowing down. However about ten years ago it was discovered that the expansion is speeding up. To explain this the idea of dark energy was introduced. There are a lot of unanswered questions about this whole subject.
  4. Apr 7, 2009 #3


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Space and time are not substances that need to be created.

    You may be confusing the increase of distances with the "creation of space".
    there is a pattern of increasing distance called Hubble law, currently certain percentage increase per unit time. That does not create space.
    It is just a change in geometry.

    Geometry is dynamic and interacts with matter. It is determined by a differential equation---Einstein equation---a "rate of change of rate of change" equation. In the simplest case where there is no matter or almost none, the equation says spatial geometry should be Euclidean (parallel lines, angles of triangle add up to 180, pythagoras hypoteneuse etc.)

    Another reason we experience nearly normal Euclidean geometry is that, in the universe at large, distances are expanding only very slowly percentagewise---about 1/140 percent in a million years. Small local distances don't even take part in this, in any case.

    Thanks to the Einstein equation, since the density of matter around us is comparatively low, and expansion is very slow, the spatial geometry we experience is almost exactly Euclidean. Angles really do add up to nearly 180 degrees. Distances between stationary objects stay the same etc etc. But you shouldn't expect this to hold universally.

    Einstein equation, that governs geometry, does not allow for abrupt changes in the rate of expansion. Once an overall largescale pattern gets started (eg. because of a bounce) it is going to continue. It can only slow down gradually---or in the presence of dark energy speed up gradually. If nothing is done it will coast along, gradually slowing down. The main thing is the equation tells to expect no drastic change, what has been happening will continue, more or less---it doesn't need a "cause" to drive it. :biggrin:

    There does not have to be any cause of universal expansion. Einstein equation governing geometry is a differential equation (rates of rates change of rates of change, affected by the concentrations of matter). It allows gradual changes in geometry to persist once they have gotten started.
    Last edited: Apr 7, 2009
  5. Apr 8, 2009 #4


    User Avatar
    Science Advisor

    Hi Stratospher. Your question derives from an unfortunate misunderstanding that seems to have been perpetuated in the popularisation of cosmology. Unfortunately, somewhere along the line people explaining the expansion of the Universe have gotten the causality completely bass ackwards.

    There is nothing strange that 'causes' universal expansion, the things in the universe are generally moving away from everything else in the universe simply because they were doing so in the past. It is simply the familiar concept of momentum.

    If you pick any two galaxies in the universe then you will observe that they are moving away from each other (as long as they are not in the same cluster, buts that's a more technical point). Using knowledge of basic physics we can understand that unless some force acts upon these two galaxies then they will continue moving away from each other, hence the expansion of the universe does not need anything to 'cause' it to keep expanding. It did require something to start it in the first place (the Big Bang) but after that intial 'kick' everything happily keeps on drifting.

    Now, there are forces operating between the galaxies. The graviational attaraction of all matter in the universe to all other bits of matter cuases the rate of expansion to slow down. The velocity between any two galaxies is decreased over time because of this. However, what we can infer from our measurements is that while this happened in the early history of the Universe, in the last few billions years the expansion has been accelerating. This means that there is some force pushing galaxies apart. We attribute this to dark energy (although we are not sure that this is right, as mathman says there are many unanswered questions). Note that this force is not needed for expansion to continue, this would happen anyway, but it does cause the rate of expansion to increase.

    So, dark energy does change the rate of expansion, but it does not cause it. The only 'cause' of expansion is the first cause, the Big Bang (technically we think it was a process called inflation that originally got the expansion started, but we aren't completely sure about the details of this) and since then the expansion has continued simply due to inertia, with the expansion rate altered by the force of gravity and the effects of dark energy.
  6. Apr 8, 2009 #5
    So what caused the Big Bang then?
  7. Apr 8, 2009 #6


    User Avatar
    Science Advisor

    Answer that and http://en.wikipedia.org/wiki/Nobel_Prize" [Broken] will be yours :biggrin:
    Last edited by a moderator: May 4, 2017
  8. Apr 18, 2009 #7
    why is the andromeda galaxy moving towards us then?
  9. Apr 18, 2009 #8
    because of gravity. Andromeda is actually fairly close (and by that i mean thousands of light years) meaning gravity dominates over the rate of expansion.

    And just to add to the thread...

    Two months or so ago an article about dark energy came in Scientific America. It showed how dark matter may not be uniform, or in other words certain areas of the universe may have more dark matter than others. If this is the case than our galaxy may live in a "void". Making it seem that the universe is accelerating faster. Think about a 2D surface. IN that surface there are two objects, one is the milkyway the other is a supernova. These two objects is in one big circle (e.g. the universe) that expands uniformally. In this model, the older the universe gets the greater the time it takes for light to reach us (they calculate the time via the red shift affect). As the universe gets older and older gravity should dominate and start to slow the expansion down, meaning for every calculated distance the difference should get less. THis is not the case though.

    With evidence of an accelerating universe due to dark matter people think the universe is accelerating uniformaly, but what if it wasn't? What if ceratain areas of the universe had more dark matter? Try thinking of that 2d model again. however this time place our galaxy in a bulge out of the circle. The bulge represents an accelerating space-time, more dark matter. While the rest of the universe accelerates normally our "bulge" accelerates faster. Making it seem that the supernova is further away.
  10. Apr 21, 2009 #9
    Do u have a link to this article
  11. Apr 21, 2009 #10
    No sorry.:frown:
  12. Apr 30, 2009 #11
    Hello fellows, It's been a while. I have something related to this topic as well. I know tjey have superluminal velocities (superC velocities) but I know as well that thing don't travel faster than the speed of light. I was thinking, there's a theory that the universe will continue to expand and atv a certain point in time will come to a halt and start to contract. If this is the case, we know that there should be a certain distance from the center of the universe that shold be covered. Using Hubbles law and equating the recession velocity to C, we get something around 13.05 billion C.Y.. What do you make of this. If there sould be any other thing other than this do you think we should try and accept and also read more about Roger Penrose's new theory on before the big bang and what will happen at the end just before the next big bang?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook