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Originally Posted by Weltevreden SA
Steven wrote, "Space can expand faster than light." This is oft-said and not-so oft explained. Why should space have a property which is essentially nonphysical until something physical expands to occupy it? Why, too, did space abruptly expand at at specific point in time, achieve the expansion velocity that it did, and slow to approx its present rate at a point in time so briefly after it began? What set the initial and ending boundaries? These issues have been floating without definition in my awareness for some time and this is a good occasion to address them. They are also very relevant: I’m comparing data re today’s cosmic matter-energy inventory ( Fukugita et al) and Brian Lacki’s “CMD” of the energy sky (Fig 4), with a recent group of papers devoted to the properties of cosmic voids and filaments, Rieder, Alpaslan, Tempel, Libeskind. These introduce important issues, e.g., the large- to small-scale granularity of the products of inflation. But I note that all of these and others I’ve come across interpret space in terms of the interaction of mass density and energy density. Theirs is of course not the place to address what properties existed when there was space very high in potential energy density which endured an era 10 orders of magnitude in time in which no commensurate matter density existed. My question is not the hoary “Why is there something rather than nothing?” but “Why is space so small?” Steven, could you elaborate on some of these issues? It would help me no end. |
Dana,
Regarding space there are two issues here.
Masses travelling through space and masses being carried by expanding space.
To travel through space work is performed in moving a mass from point A to point B which requires the expenditure of energy.
To accelerate a mass up to the speed of light requires an infinite amount of energy hence the speed of light is the upper limit for masses moving through space.
If the mass is being carried by space no work or energy is being expended as the mass is stationary relative to space's frame of reference.
From our frame of reference as the observer, galaxies with red shifts greater than z=1.4 have recession velocities exceeding the speed of light. From our frame of reference it is the space that is expanding, not the galaxies moving through space.
A canoeist makes a good analogy. A canoeist being carried by the rapids is expending no energy and is stationary relative to the water but moving relative to an observer on the bank.
If the canoeist paddles through the water he is expending energy and is moving relative to the water.
Expanding space isn't limited to the speed of light but the question arises where does the energy come from and how does it cause space to expand.
We can use the canoeist analogy again. The energy that drives the river is the conversion of gravitational potential energy into kinetic energy or in layman's terms rivers flow downhill.
Space is inextricably linked with a vacuum which is a field of the lowest energy state.
The theory behind inflation is that the Universe existed in a
false vacuum.
The false vacuum has a higher energy state than a true vacuum.
Like the river analogy, the Universe dropped from this higher level to the lower energy vacuum state releasing energy and causing space to expand at an accelerated rate.
Inflation only accounts for a tiny piece of the space expansion mechanism, we have absolutely no idea about the current acceleration of the space through dark energy. Attempting to explain dark energy as another false vacuum effect has failed miserably.
What we do know however is how the behaviour of space expansion has changed in the past and is related to the interaction of gravity and dark energy on the Universe.
In the past when the Universe was smaller gravity had a greater influence than dark energy. At some point around 6 billion years ago, the Universe had become sufficiently large for dark energy to become the major player.
I'm not quite sure about your "Why is space is so small?" question, you will need to elaborate on the background behind the question.
Also I will need to study your links in detail before commenting.
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And oh yes, since we are on matters of great magnificence and enormity, could you enlighten this non-Ozzie what the dickens a “Gish Gallop” is? =Thanks, Dana in S A
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Alex has covered the topic nicely.
Regards
Steven