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

The epansion of space, not matter

  1. Aug 19, 2013 #1
    Something occurred to me the other day when looking at pictures of the universe. This might be obvious to some, and probably should have been obvious to me, but I realised that the big bang describes the expansion of space, not matter. Because gravity causes matter to clump together, at some point in the younger universe matter must have been more widely and more evenly distributed. So why when we look at the sky do we see individual and unique pockets of matter? A pocket here, a pocket there, looooots of space in between. Why is there some hydrogen over here, some oxygen over there and no uniformity to the world we see around us? Wouldn't that suggest that space/matter was never uniform to begin with and that the big bang never occurred evenly everywhere at once? Surely there must have been some "events" which took place that caused regions of the universe to take on a unique identity, for instance one pair of particles interacting differently than another pair of particles, is there any indication as to what those events were? Perhaps random fluctuations?
  2. jcsd
  3. Aug 19, 2013 #2


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    One reason the sky map of the CMB temperature is studied so much is that it is a snapshot as of year 380,000 of the amount of fluctuation at various angular scales. Gross wide-scale fluctuation and little bitty small angle fluctuation.
    The temperature relates to the density of hot gas at the surface of last scattering when the CMB was emitted.

    The temperature of the CMB is uniform to within one thousandth of a percent. So there was approximate uniformity to within one part 100,000.

    I can't emphasize too much: BOTH the degree of approximate uniformity AND the precise power spectrum of the fluctuations are considered scientifically significant. How much amplitude there is in the fluctuations in each range of angular size.

    So the basic intent of what you are saying is right. THERE HAD TO HAVE BEEN FLUCTUATIONS (probably quantum fluctuations way back in the inflationary era, if there was one). These fluctuations have been the SEEDS of clumping, the seeds of "structure formation".

    We can measure the degree of structure formation by looking back in time to the early and not so early universe, to when the first galaxies were forming etc etc. This history of structure formation---the precise extent of it---has to agree with what we see in the CMB. It all has to fit together.

    There are some nice computer animations of simulated structure formation, stuff condensing into wispy cobweb stuff and galaxies. One batch of animations is part of a "TED" talk you can get if you google "smoot TED". It is laureate george smoot talking to an intelligent lay audience with some good slides and movie clips.

    Lot more to say about this. Maybe others will contribute more information about how history of structure formation is studied and modeled.
    Last edited: Aug 19, 2013
  4. Aug 19, 2013 #3
    However that uniformity didn't last, wouldn't it be accurate to say that something which is uniform to a fraction of a percent still isn't uniform regardless of how small the percentage is? We can observe just by looking in the sky that no two systems are alike, black holes are different sizes with different energies travelling in different directions collecting matter of all different shapes and sizes. Is there any indication or any ideas of what imperfection in the early universe caused the universe to develop in such a non uniform way?
  5. Aug 19, 2013 #4


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    The expansion of space

    Sure that is the point of computer modeling the growing degree of clumping. What starts with slight areas of overdense and underdense gas will begin to fall together into clumps. The schedule can be predicted if you know the degree of variation.

    It's a beautiful branch of study.

    they can actually model the SOUND WAVES in the hot gas of year 380,000. It is also possible that in future with finer observations of the CMB they will be able to see evidence of gravity waves in the geometry of that time as well. The world was a smaller place then. distances have increased 1100-fold since CMB was emitted, so that snapshot is of a world in which ordinary matter was a billion-fold denser. The physics of it can be modeled,
    Last edited: Aug 20, 2013
  6. Aug 20, 2013 #5


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Maybe I shouldn't say "sound"---the frequency is much lower than audible. These are very low frequency long wavelength pressure waves that they can see the imprint of on the CMB map, and the normal term is *acoustic* waves, not "sound". Too low frequency for us to hear.

    These waves (because they created regions of overpressure and under pressure) helped to seed the structure which we have today. A large region of overpressure, whatever the cause, will tend to collapse under its own gravity, and if not disrupted by some other events such might eventually contract to a density where stars would begin to form.

    Anyway, when we look at the CMB map we are seeing hot (3000 Kelvin) gas which, in year 380,000 when it emitted the light we're getting from it, was some 42 million lightyears from here (from the patch of hot gas that became our galaxy and the solar system and us---"our"matter in other words.

    We are seeing how the universe looked, how we looked etc. But the sample of gas whose light we are now getting was then a shell around us--it was the stuff that was then 42 million lightyears from us. (if you could have stopped expansion to measure).

    To check the distance then, and now (if you could stop expansion so as to measure) go here:
    You will see the 0.042 billion ly in the "Distance_then" column of the top row.
    The S=1090 is the factor by which distances and wavelengths have been enlarged since that time.

    You can get more decimal places accuracy in the Time column if you open "column selection and definition"menu. The blue dots supply explanations of the various columns and settings.

    Anyway the map's temperature variation is only 1 part in 100,000 but that all-important small variation in nearly uniform hot gas was enough (when the gas had expanded and cooled some) to seed the condensation of structure which life as we know it depends on.

    So your question is asking about something extremely interesting which is currently being figured out from the CMB observations. It's worth googling "Cosmic Microwave Background" and reading up about it in Wikipedia and the like.
  7. Aug 22, 2013 #6
    The matter in the universe is actually astonishingly evenly distributed compared to what it should be, this is explained by cosmic inflation which smoothed out the distribution of matter. But the reason we see "pockets of matter can be thought of as a result of quantum mechanical uncertainty and gravity. Particles acquire more random, uneven distributions due to quantum fluctuations. And particles "pocket" together because of the inverse square property of gravity, ie particles move towards closer particles, and are affected more by closer particles. So basically you have galaxies worth of particles moving closer and more randomly as they near each other, which only furthers the overall randomness; think 2nd law of thermodynamics.

    Also, as space expands outward it carries matter with it. Eventually this will cause galaxies to move apart faster than the speed of light and atoms to break apart.
  8. Aug 22, 2013 #7


    User Avatar
    Science Advisor

    If inflation theory is correct, then the ultimate source of the non-uniformity was quantum fluctuations, as Marcus said. Because of the uncertainty principle, quantum mechanics says that it is not possible for the initial universe to be perfectly uniform - each mode of oscillation had to have a minimum amplitude of hω/2. If this idea is correct, then the early universe was a uniform as it could possibly be given the constraints of quantum mechanics. These early fluctuations then grew with time to give the galaxies and galaxy clusters we see today.
  9. Aug 22, 2013 #8


    User Avatar
    Gold Member

    Yes, galaxies will (actually they already DO) recede from each other at much more than c.

    For the bolded part, you are talking about the "big rip" scenario which at best is highly speculative and as I understand it is deemed extremely unlikely by most physicists today.

    To state it as fact, as you have, is personal speculation that cannot be substantiated.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: The epansion of space, not matter