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How can the universe be big enough to fit all the stars when it's not that old?

  1. Jul 10, 2009 #1
    The universe is around 13.5 to 14 billion years old. That means that if everything started from one singularity which was the big bang, and nothing can travel faster than the speed of light, the size of the universe must be at most 14 billion light years in size.

    But even that is an extremely large overestimation since most things don't even go near the speed of light. The sun for example, orbits the milky way, at 150 miles per second, the speed of light is 186,282 miles per second.

    That's more than 1000 times slower than the speed of light. So given these calculations, either objects can travel faster than the speed of light, or the universe is tiny.

    but there are 10 billion galaxies, each with 100 billion stars out there, and they're not close together at all. The closest star to the solar system is 4 light years away. If that is the average distance between stars, and we're not even going to calculate the average distance between galaxies, how can so many stars and galaxies fit in so small a space.

    I'm no scientist so I'm sure I'm sure I've misunderstood some of the concepts of physics, but something here doesn't seem to make sense.
  2. jcsd
  3. Jul 10, 2009 #2


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    The universe can (and does) expand faster than light. The observable universe is now around 90Bn lyr across.
  4. Jul 11, 2009 #3
    if the universe is expanding faster than the speed of light, does that also mean the distance between the Earth and the Sun must also be expanding, if so at what rate?
  5. Jul 11, 2009 #4


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    No within systems that are gravitationally bound, like the solar system or even groups of galaxies then gravity wins and holds them together.
  6. Jul 11, 2009 #5

    mmm interesting... does that basically mean that everything in the universe has a natural inertia to repel from each other, and gravity is the counteracting force to keep them together.

    Like if the universe had no matter, then it would just get bigger infinitely. And anyone from any point to another, would be moving away from each other at exponentially faster velocities? [ based on the distance they are away and the space between them ]
  7. Jul 12, 2009 #6


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    Kind of. You're close, but your terminology is just bugging me. There is no intrinsic repulsion between objects in the universe, the space between them is simply expanding as a result of the initial inflationary event. The part about inertia is a good analogy though. In a sense, everything got an initial "kick" of expansion in the beginning, and in some places gravity takes all the energy out of the kick and so expansion no longer takes place. These are the places mgb_phys is talking about, and such systems are said to be gravitationally bound.

    Also, imagining a universe in the absence of matter (I assume you mean devoid of everything since matter and energy and matter are two sides of the same coin) is pointless. There is no points of reference and then metaphysics takes over as to if there is anything at all. Not very interesting, imo.

    That said, not all systems are gravitationally bound! In our universe the distance between, say, galactic clusters is increasing due to the expansion and will continue to do so. The gravity simply isn't strong enough to overcome the expansion, and your description is essentially correct. Expansion velocity is proportional to distance, and as the galaxies move farther apart, the distance increases, velocity increases, etc.

    Side note: This is all on a local scale. The question of whether there is enough mass to make the universe a gravitationally bound system is in the realm of cosmology, which is besides the scope of this discussion. Google fate of the universe if this interests you.
  8. Jul 12, 2009 #7


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    The observable universe is as big as observed - about 13.7 billion light years in all directions from earth. The objects that emitted all those photons are presently more distant due to expansion.
  9. Jul 12, 2009 #8
    ok cool interesting...

    one more question - why is the observable universe the same light years as the age of the universe? Wouldn't it be less? Given that if the light was emitted 13.7 billion years ago, but during that time the space in between has expanded, so shouldn't it in theory take longer than 13.7 billion years for the light to travel here.
  10. Jul 13, 2009 #9


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    Actually, the observable part is only about 13.3 bly from us. At that point you reach the surface of last scattering where no EM radiation is emitted. The objects we now observe as being at the 'edge' of the universe were much closer to us when the photons were emitted. The photons are just now reaching us. The 13.3 bly is the light travel time. See Ned Wright cosmic calcutor for how distance [then and now] is computed. Those same objects are far more distant than 13.3 bly 'now' due to expansion. The 13.7 bly figure is [at present] the maximum distance at which any form of emission is possible to detect. For example, in theory we could detect neutrino emiitted only seconds after the big bang.
    Last edited: Jul 13, 2009
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