On Hubble's constant, the expansion of the Universe and vacuum energy

  • #1
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Summary:

Creating the link between the expansion of the universe and the quantum fluctuations
Recently I was thinking about the Hubble's constant (which, actually, is not Hubble's and not constant...) and wondering: if the universe is expanding at 70 km/s each Mpc, then there's possible to calculate some expansion of space, say from me to a person 1 meter away from me (theoretically).

Given that, I invite you to look at some of my questions:

1. How ridiculous would it be to try to create a device to measure the expansion on earth?
2. Same question, but in space with no massive objects around - or free space.
3. We know about quantum fluctuations and the vacuum energy... isn't it and dark energy the same thing? On different scales?
4. How does the expansion influence time itself?
5. How does gravity influence on the perceived expansion? Would you need an additional dimension to describe space-time? Or did this question go too far? What theories approach this, and what kind of calculations need to be done to simulate this?
6. In question 5 I mentioned other dimensions - it is because, on a big scale, I imagine the universe's force fields and its objects like particles in a fluid, and gravity (kind of) increases viscosity. So for me the existence of an additional dimension comes naturally, this way the expansion may flow from somewhere, and it is not an intrinsic property of space-time.
 

Answers and Replies

  • #2
mathman
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Within galaxies and galaxy clusters gravity dominates space expansion. There is no way to measure expansion on earth.
 
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  • #3
HankDorsett
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Summary: Creating the link between the expansion of the universe and the quantum fluctuations

Recently I was thinking about the Hubble's constant (which, actually, is not Hubble's and not constant...) and wondering: if the universe is expanding at 70 km/s each Mpc, then there's possible to calculate some expansion of space, say from me to a person 1 meter away from me (theoretically).

Given that, I invite you to look at some of my questions:

1. How ridiculous would it be to try to create a device to measure the expansion on earth?
2. Same question, but in space with no massive objects around - or free space.
3. We know about quantum fluctuations and the vacuum energy... isn't it and dark energy the same thing? On different scales?
4. How does the expansion influence time itself?
5. How does gravity influence on the perceived expansion? Would you need an additional dimension to describe space-time? Or did this question go too far? What theories approach this, and what kind of calculations need to be done to simulate this?
6. In question 5 I mentioned other dimensions - it is because, on a big scale, I imagine the universe's force fields and its objects like particles in a fluid, and gravity (kind of) increases viscosity. So for me the existence of an additional dimension comes naturally, this way the expansion may flow from somewhere, and it is not an intrinsic property of space-time.
I was wondering about gravity and expansion of space myself. How do we know universe expansion is constant? If mass or it's gravity can bend space, could it also create a "drag" slowing down universe expansion?

Disclaimer: I have no education in this and some of my thoughts are a result of consumption.
 
  • #4
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I was wondering about gravity and expansion of space myself. How do we know universe expansion is constant?
The universe expansion is not constant, it is actually accelerating. So there is no constant value of the Hubble constant, and it's rate of change is not known neither. A lot more data is needed.

If mass or it's gravity can bend space, could it also create a "drag" slowing down universe expansion?
Yes. Actually, when making his theory of general relativity, Einstein had to introduce a cosmological constant, so that the universe didn't have to collapse into itself due to gravity. All the mass in the universe is attracting each other, so there had to be something else, some force or energy, that opposed to this collapse. Einstein introduced the constant to have a static universe, but then, when observations showed that it is actually expanding, he called it his biggest blunder. Now this blunder is a subject of active research.
 
  • #5
HankDorsett
Gold Member
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The universe expansion is not constant, it is actually accelerating. So there is no constant value of the Hubble constant, and it's rate of change is not known neither. A lot more data is needed.



Yes. Actually, when making his theory of general relativity, Einstein had to introduce a cosmological constant, so that the universe didn't have to collapse into itself due to gravity. All the mass in the universe is attracting each other, so there had to be something else, some force or energy, that opposed to this collapse. Einstein introduced the constant to have a static universe, but then, when observations showed that it is actually expanding, he called it his biggest blunder. Now this blunder is a subject of active research.
I may have used the wrong term when I said constant. I was wondering how we new the universe was expanding equally throughout the Universe. Every megaparsec expanding 67, 72 or 74, whatever the current belief is, throughout the Universe.
 
  • #6
Bandersnatch
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and it's rate of change is not known neither
Why would you say that? The evolution of the Hubble parameter is what the Friedmann equations trace. It's the basis of the entire big bang theory.

1. How ridiculous would it be to try to create a device to measure the expansion on earth?
2. Same question, but in space with no massive objects around - or free space.
3. We know about quantum fluctuations and the vacuum energy... isn't it and dark energy the same thing? On different scales?
1. As ridiculous as any other pointless endeavour, because you can't. It's not even a matter of sufficiently accurate technology - it's just that bound systems do not undergo expansion, so there's nothing to measure.
2. One can imagine a universe completely devoid of matter and radiation, that is expanding in accordance with Friedmann equations. This model universe is called Milne universe. In such universes the expansion remains steady forever. Finding out how fast does 1 metre expand is then just a matter of dividing the adopted value of the Hubble constant by however many metres are there in a megaparsec.
3. I don't know enough about it to say anything worthwhile, but here's an informative article on the topic: http://math.ucr.edu/home/baez/vacuum.html

The remaining questions are terribly vague.
 
  • #7
Bandersnatch
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I may have used the wrong term when I said constant. I was wondering how we new the universe was expanding equally throughout the Universe. Every megaparsec expanding 67, 72 or 74, whatever the current belief is, throughout the Universe.
No, you used the correct meaning of the 'constant' in 'Hubble constant' - it indicates constancy across space at a given epoch.
In the broad sense that it can't vary much, it's an assumption that went into obtaining the equations governing the expansion of the universe. Seeing how the equations are generally successful at describing the observed universe, it gives weight in support of the assumption being correct.
But it's not expected to be perfectly the same everywhere, because the universe is not filled with a perfect fluid - another assumption in the equations. On cosmologically intermediate scales there are overdensities and voids, which should cause the evolution of the Hubble parameter to slightly vary from place to place. This is being explored as one of the reasons for the recent discrepancy in its measurements made at different epochs.
 

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