Quote:
Originally Posted by Michael Mooney
According to the theory of space expanding faster than light (as per inflation cosmology), what is it exactly that is supposed to be expanding?
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On cosmic scales everything is expanding away from everything else. The simplest way to express such a thing is to put everything on a metric (or a grid, if you like), and to scale the metric. This accomplishes something very specific which I will describe.
If you consider a galaxy such as the Milky Way and measure the speed at which other galaxies are moving away from it then the further the galaxies are, the faster they will be moving away. That is a physical law of astronomy known as
Hubble's law.
It is sensible to assume that our position in the universe is not special so that any other galaxy out there also has galaxies receding from it in such a way that the recession velocity increases with distance. An easy way to show this (to model it) is to use a rubber sheet (think of a piece of paper made of rubber). You can mark a bunch of dots on the sheet representing galaxies.
If you have 4 people each grab a corner of the sheet and stretch it apart then you have effectively modeled the manner in which galaxies move away from one another. One of those dots on the rubber sheet has every other dot moving away from it and the further the other dots are from it, the faster they move away. This is true for any dot.
The expansion of the universe is very similar. Saying "space is expanding faster than the speed of light" means that the distance between two objects on the metric is increasing faster than c. This is an inevitable conclusion given two things:
- The further away one galaxy is from another the greater the rate at which their distance increases.
- The universe is infinite in size.
You're probably wondering what difference it makes if we say something is moving through space or if we say space is expanding between things. And, it indeed does make a difference which can only be revealed by measuring and understanding the apparent motion of cosmic objects. To give one example, if an object is moving away from us *through space* then we expect it to exhibit a redshift which can be calculated with and is due to special relativistic Doppler shift. If, however, space is expanding SR doppler shift will not give the correct redshift results. Cosmological redshift must be used. Wiki summarizes the difference:
Quote:
Hubble's law of the correlation between redshifts and distances is required by models of cosmology derived from general relativity that have a metric expansion of space. As a result, photons propagating through the expanding space are stretched, creating the cosmological redshift. This differs from the Doppler effect redshifts described above because the velocity boost (i.e. the Lorentz transformation) between the source and observer is not due to classical momentum and energy transfer, but instead the photons increase in wavelength and redshift as the space through which they are traveling expands. The observational consequences of this effect can be derived using the equations from general relativity that describe a homogeneous and isotropic universe.
Redshift - Wikipedia, the free encyclopedia
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That quote references
"the metric expansion of space" which is a good article at wikipedia. If you want to get a good understanding of just what astrophysicists mean by expanding space then that would be a good place to start.
I should probably also note that cosmic inflation which you mention is not any more fundamentally superluminal expansion than the expansion of space today. This is a common misconception which is addressed here:
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Inflation is sometimes described as “superluminal expansion” [App. B: 22–23]. This is misleading because any expansion described by Hubble’s law has superluminal expansion for sufficiently distant objects. Even during inflation, objects within the Hubble sphere (D < c/H) recede at less than the speed of light, while objects beyond the Hubble sphere (D > c/H) recede faster than the speed of light. This is identical to the situation during non-inflationary expansion, except the Hubble constant during inflation was much larger than subsequent values.
Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe (page 6)
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Quote:
Originally Posted by Michael Mooney
If space is not simply empty volume*, what then is it supposed to be?
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I don't think "volume" is that bad of a word to substitute for "space" in the setting of astronomy and cosmology. We could say that the volume between galaxies increases over time rather than saying the space between them increases. We would also say that the rate at which volume expands is proportional to the volume itself much in the way we would say the expansion of space between objects is proportional to the distance between them.
~modest
