# Isn't space expansion expectable?

[SOLVED] Isn't space expansion expectable?

Since Einstein showed that space and time are in fact entwined as a single entity spacetime, it makes me wonder if it's not just natural that we see space expanding with time.

As we examine spacetime forward in time (looking at regions where "the quantity of seconds" is increased -regions where time has expanded-), it may only be natural that we find also that "the quantity of meters" is also increased -space has expanded as well- ?

I know I'm wrong, because I know our physical theories allow scenarios where even when time continues to increase space shrinks, but I'd like someone to explain why the type of entanglement between space and time does not have anything to do with both increasing or decreasing together.

And now that we talk about expansion, an even more silly question: Has anybody played with the hypotesis that from a relative perspective, it could be that instead of space expanding, matter and energy could be shrinking into a fixed size space?

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## Answers and Replies

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Gerinski said:
Since Einstein showed that space and time are in fact entwined as a single entity spacetime, it makes me wonder if it's not just natural that we see space expanding with time.

As we examine spacetime forward in time (looking at regions where "the quantity of seconds" is increased -regions where time has expanded-), it may only be natural that we find also that "the quantity of meters" is also increased -space has expanded as well- ?

I know I'm wrong, because I know our physical theories allow scenarios where even when time continues to increase space shrinks, but I'd like someone to explain why the type of entanglement between space and time does not have anything to do with both increasing or decreasing together.
You are confusing the expansion/contraction of space with space-time. Space-time is static, if you visualize it you simply are looking at a 'block' universe, a static continuum, any change of space-time as you looked would require time to pass, but time is already in the model, as part of space-time. It is a slice of this space-time, space, that expand/contracts within it. The space slice does not have to expand or contract at all - at least not until you describe it with Einstein's equations of GR.
And now that we talk about expansion, an even more silly question: Has anybody played with the hypotesis that from a relative perspective, it could be that instead of space expanding, matter and energy could be shrinking into a fixed size space?
Not such a silly idea, it depends on what you use to measure things with; you can replace an expanding universe and fixed rulers with a fixed (static universe) and shrinking rulers - the masses of individual atoms would then increase not shrink with time. Fred Hoyle and Jayant Narlikar pioneered such ideas in their work on conformal gravity in the 1970's and I have used it in my work on Self Creation Cosmology, if your interested you will find threads about that subject here on GA&C.

Garth

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Garth said:
You are confusing the expansion/contraction of space with space-time. Space-time is static, if you visualize it you simply are looking at a 'block' universe, a static continuum, any change of space-time as you looked would require time to pass, but time is already in the model, as part of space-time. It is a slice of this space-time, space, that expand/contracts within it. The space slice does not have to expand or contract at all - at least not until you describe it with Einstein's equations of GR.

That's quite what I meant. By the assertion that "the universe is expanding", we mean that as time elapses, we find that more distance separates the objects which inhabit the universe. If we move in spacetime towards more time, we find that there is more space.

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Science Advisor
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Gerinski said:
Garth said:
That's quite what I meant. By the assertion that "the universe is expanding", we mean that as time elapses, we find that more distance separates the objects which inhabit the universe. If we move in spacetime towards more time, we find that there is more space.
That depends on the overall geometry or even topology of the universe. Consider space-time to be a cone, where the axis of the cone is the time direction and the circumference the space dimension. (I have suppressed two space dimensions so we can visually conceive it). If the time axis runs away from the cone's apex then as you move in a positive time direction there is more space, the circumference increases. however if actually the geometry is the other way round and the time axis runs towards the apex then the opposite is true, more time means less space!

In fact in the standard theory of cosmology space-time is shaped by gravity, determined by the amount and distribution of the matter and energy within the universe, according to Einstein's GR field equation.

I hope this helps,

Garth

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Einstein made the case for a universe that must either expand, or contract. There is no stable, static solution to the field equations. Einstein was very surprised by this result and hand inserted the famous 'cosmological constant' [CC] to explain why the universe was not collapsing. Of course, being a pretty bright fellow, he was quick to realize this was like trying to balance a pencil on its point and expecting it to stay put. He recanted CC, calling it his 'greatest blunder', as soon as Hubble revealed the actual direction the universe is taking. Ironically, his retraction of CC now appears to be his 'greatest blunder'.

xPAGANx
Perhaps the increasing expansion of the universe is simply the result of time speeding up.

As the mass of the universe becomes more spread out, wouldn't time speed up since the gravity of the universe is more spread out?

Enos
If the expansion of space is accelerating wouldn't time be slowing down?

xPAGANx
Maybe it isn't accelerating, Maybe it just appears to be because time is speeding up.

Science Advisor
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xPAGANx said:
Maybe it isn't accelerating, Maybe it just appears to be because time is speeding up.
Time "speeding up"? Time "slowing down?" - relative to what?

When statements like this are made you have to ask yourself: "How do you measure it?" The only way statements about time speeding up or slowing down make sense is when one clock is compared to another.

The two famous examples are the time dilation observed by a stationary observer with a clock observing a fast moving (relative to her) clock; and the second is when an observer with a clock far away from a gravitating object observes a second clock deep down within the gravitational potential well.

In both cases you have to think about how the observation itself, the exchange of photons, is made from one clock to the other.

The third example that you can ponder over is the time dilation implicit in cosmological (Hubble) red shift. Events seen at the "far side of the universe", i.e. at high cosmological red shift, are observed to happen more slowly than those nearby.

Garth

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cscott
Chronos said:
Ironically, his retraction of CC now appears to be his 'greatest blunder'.

Is a non-zero CC now used to fix whether the universe will eventually collapse or not instead of keeping the universe static?

Science Advisor
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Since 1998 [re: Perlmutter, et al], supernovae redshift data indicates the expansion of the universe slowed down for the first several billion years after the BB [as expected], and then decided to speed up [totally unexpected]. This led to the dark energy hypothesis [which is analogous to Einstein's cosmological constant].