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

How can space itself move/expand

  1. Feb 2, 2014 #1
    How can space itself "move/expand"

    Frequently in cosmology and sometimes in other fields you hear talk of the universe expanding of contracting. I think we finally settled on that it is expanding. I'm having a little trouble grasping that - space itself/the geometry of space is changing with time. I unfortunately lack the math background to understand any talk of manifolds and tensors(not for lack of trying), but conceptually, can someone help me get my head around that. How does can the geometry of space change? What does that even mean to have "space" expand? What are the implications of this changing nature of space?

    If the explanation involves math, I'm working on Calc III this semester, and have a fairly good grasp of "normal" Real-3 space from other sources.
  2. jcsd
  3. Feb 2, 2014 #2


    User Avatar
    Gold Member

    Basically, space DOESN'T "expand", things just get farther apart. This is a weird concept, I know. Google "metric expansion" for a discussion
  4. Feb 2, 2014 #3
    here is two articles that cover expansion and universe geometry, they are fairly lengthy but will get you started on understanding current cosmology

    the first is a descriptive of expansion and redshift, the second article cover universe geometry with the basic mathematics.



    this article has good visual aids to understand the first two articles

    this article covers the LCM model "what we have learned from observational cosmology"

    that should get you started feel free to ask any questions on any of the articles posted.
  5. Feb 3, 2014 #4
    Ah, those articles help considerably. It's not what I'd call intuitive, but what is these days? Thanks.
  6. Feb 3, 2014 #5


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    One way to get some intuition is not to start by thinking about "space" or "expands" (these carry a load of preconceptions) but rather by thinking about the soup of ancient light---the CMB---that we live in.

    Take some time to find out about it. It has a mix of wavelengths that looks like the thermal glow from something at a certain temperature, namely 2.725 Kelvin or some such. The power curve is a beautiful fit to the black body heat glow curve.

    because it is the same temperature in all directions if you are standing still relative to the soup this gives us a criterion for standing still

    Take some time to think about the fact that there is an absolute criterion of standing still. If you move in some direction at 1/1000 of the speed of light you will see a doppler hotspot ahead of you where the measured cMB sky temperature is slightly warmer, by a tenth of a percent.
    And a doppler cold spot behind you, where the Background is cooler by 0.0027 Kelvin.

    "metric expansion of distances" essentially means that the distances between objects that are standing still is growing at a certain percentage rate.

    distance between a pair of stationary objects grows by such and such fraction of a percent per million years.
    Last edited: Feb 3, 2014
  7. Feb 3, 2014 #6
    Indeed. It all makes sense when you follow the observations to their logical conclusions. Even though the answer one ends up with isn't what they would expect setting out, that doesn't make it wrong. QM is even more like that it seems. You start from some observations and experiments that don't really make sense intuitively, and do your best to explain them. It's still hard to comprehend, but the reasoning to get there seems sound.
  8. Feb 3, 2014 #7
    Glad you found them of use, took a considerable effort to write them. As one that learned cosmology through diligent self study. I wasted several years following controversial topics trying to satisfy my own misguided perceptions of cosmology. Even when I tried to learn the real science from the web, I found it daunting to locate uncontroversial and accurate materials. It wasn't until I discovered this site and most importantly broke down and bought recommended text books did I realize how wrong I was.
    This was one of the primary motivators for writing the first two articles, as I truly hope they help prevent others from following false leads as I had gone through.
  9. Feb 3, 2014 #8
    It's unfortunate in that many who read an article on the internet suddenly feels they're an expert on the topic in question, and certainly makes it hard to track down reliable info. It's also one of the fields that people come into with *very* strong opinions on, and are frequently less than scientific in their understanding - not helped by the number or religious and philosophical texts that claim to have insight into the structure and workings of the universe. Good entry and mid level articles based purely on observation and sound science are very much appreciated, especially in fields with so much misinformation.
  10. Feb 3, 2014 #9


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Pragmatically (you mentioned being pragmatic, having a practical take on things) light defines what straight is.
    Massive objects of matter moving around makes light appear to bend. That is, background stars appear to be in a different place when a massive object is in front. That's just how it is.

    We have no right to expect that the distance between two stationary objects shouldn't change. What would keep it constant? If they aren't tied by string, I mean. Distances change, it's how it is.

    We have no right to expect that the straight line to a star should always be in the same direction, either. Straight lines of sight can be influenced by the distribution of matter, e.g. by massive objects. What would prevent them from being? The "R3" Euclidean model which makes us subconsciously regard geometry as a rigid block of glass, is not perfect.

    The whole thing is not to be bound by the prejudices built in verbal (English and other spoken) language or built in to the convenient math models like R3 that one has learned to use. No model is entirely right, they only approximate where appropriately applied.
  11. Feb 3, 2014 #10
    I fully agree with that sentiment Marcus. Their are often numerous and just as accurate ways to model the same thing in any scientific arena. As a self challenge I've been working on my own model to better understand the cosmological constant. I won't post any details however due to understandable forum regulations. I seriously doubt my personal model will work any better than what is already available, assuming it works at all, nor even a new idea. Its still a fun and challenging exercise.
  12. Feb 3, 2014 #11
    That line in particular is of interest to me; it's something that has become more and more prominent as I've learned more in the sciences: very very few problems have an exact mathematical solution. May or may not be too much into the metaphysics side of things to discuss here, but it's one of the things I've had the most difficulty grasping and accepting - almost everything we do is an approximation, and even if there is an exact value or answer to something, we can only ever obtain and arbitrarily close approximation.

    Fair enough. Seems like half the battle is unlearning the things one just assumes from everyday experience, rather than just purely learning what's right. Any kindergartner knows that "things fall down towards the ground", but as with your line of thinking, that's only an approximation, correct for everyday life, but never the less not useful for more general problems.
  13. Feb 3, 2014 #12


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    I agree FireStorm. You put it clearly and succinctly. Have fun with cosmology and ask plenty of questions!
  14. Feb 8, 2014 #13
    Expansion of the universe in real numbers. Hubble's Constant: H0 = 67.15 ± 1.2 (km/s)/Mpc from wikipedia calculate it for 13.7 billion light years and see how fast it is. It may surprise you
  15. Feb 8, 2014 #14


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    It will come out somewhat less than the speed of light. Let's see what distance you need to get the speed of light right on the nose. Paste this into google:
    "1/(67.15 km/s per Mpc)"

    You get 14.56 billion years. So the Hubble radius (if 67.15 were actually right) would be 14.56 billion lightyears.

    That would be the distance currently growing at speed of light.

    So the other distance you mentioned, 13.7 billion ly, would be growing at less than the speed of light, namely this fraction of c:

    It is not so surprising actually---largescale cosmic distances grow at speeds proportional to their length.

    Most of the galaxies we can see with telescopes are farther than 15 billion lightyears from us (in proper distance terms, i.e. if you could pause expansion at this moment to give time to measure) and so the distances to them are growing faster than c.
  16. Feb 16, 2014 #15
    About that... something I never see asked about when discussing space expansion and the observed large scale uniformity...

    Two distant objects in the past were closer together. Their light emissions would have been separated by some distance and geometrically by some angular amount. If things just get farther apart without space expansion, then those two objects at a later time will be further apart... but wouldn't the delayed reception of those earlier emissions indicate an apparent observed separation between the objects from an earlier time?

    If so, then the further distant and further back in time we observe, the greater would be the density of the observed objects, and the observed uniformity would not exist... the universe would look to increase density as we looked through further distance and observed earlier times.

    Isn't it the space expansion that preserves the uniformity observation by expanding the separation of the light paths in transit, matching the separation of their sources? Doesn't this result in making all the light paths radial, in the sense that the paths of objects moving away and the paths of their light emissions coinciding, so on the big scale, things move away and appear to send their light straight to us? Without expanding space to displace the light paths, how else would this happen?
  17. Feb 16, 2014 #16


    User Avatar
    Gold Member

    The term expansion is commonly used to describe what occurs with the universe, not space. It is the universe that's expanding, meaning its contents are becoming further apart. Uniformity is preserved as you described, because the expansion is equal in all directions, so distances increase between things, including frequency oscillations of light waves, but angles and relative positions stay the same. The light paths are stretching and separating at the same time (I think that's what you meant by radial) so when all reach us, I think the density of more distant objects should appear less than it was when the light began traveling to us.

    Wondering if I've missed something... Does this make sense?
    Last edited: Feb 16, 2014
  18. Feb 16, 2014 #17


    User Avatar
    Gold Member

    Yes, your post sounds exactly right to me.
  19. Feb 17, 2014 #18
    Yes, that's how I was trying to say it.
    The only snag is if the rate of expansion is changing, causing a mismatch between observed angles and the geometric alignment of the actual objects, this mismatch increasing with distance.

    With the distances involved, GR probably won't support the idea of distant objects having an "actual remote geometric distance between them" or the idea of "actual geometric lines between the remote objects and the observer's location to support a geometric angle of separation" between the objects from which the observed positions and angle of separation would deviate per this mismatch.
  20. Feb 17, 2014 #19
    Just an idle speculation:There is nothing is Einstein GR which predicts the accelerating expansion. But drawing an analogy with materials science. Assume that space is a "material." It does have a stress tensor and a stiffness and theoretically gravitational waves much like vibrations can travel in it. In this analogy does it have a "creep" function or an equivalence to plasticity? Much like a weight rotating on a metal bar, the tension caused by centripetal forces caused by rotation generates tension which causes creep and or plasticity and a accelerated weakening of the "material."

    Admittedly this is a very crude, far fetched analogy, but just bringing out the idea of treating space and gravity interacting like a material with very odd properties.
  21. Feb 18, 2014 #20
    Speculations are not supported in this forum.

    If you want a more accurate method to descibe what you posted. Google perfect fluid applications in Cosmology.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook