How large is the universe?

[Puppycow]

I’m happy to entertain the notion that space is an autonomous thing that can expand while nothing else does. In this context however space is a thing not a distance.

This is above my pay grade too, but I think that the idea is that while the space expands, things in the space, such as a meter stick, would stay the same size. Thus distances between objects in the expanding space as measured by the meter stick would get larger.

 

[ynot]

This is above my pay grade too, but I think that the idea is that while the space expands, things in the space, such as a meter stick, would stay the same size. Thus distances between objects in the expanding space as measured by the meter stick would get larger.

I will let others confirm if that’s correct or not.

 

[rjh01]

This is above my pay grade too, but I think that the idea is that while the space expands, things in the space, such as a meter stick, would stay the same size. Thus distances between objects in the expanding space as measured by the meter stick would get larger.

There are two possible answers
1. The meter stick would stay the same size
2. The meter stick would expand at the same rate as the universe expands.

In case 2. it would expand 7,690 billionths of a meter per year. This is how much the universe expands each year (if this is wrong please substitute the right answer). This would be hard to measure. If the ruler was a solid object then the number of atoms would not change so the average distance between them would increase. This would mean that several other constants would also change. I do not think that these constants change over time. So I suggest a meter ruler would stay the same size.

Not sure what happens about objects like planets or stars that orbit stars. I think gravity would be much stronger than anything else.


But I am not an expert on the issue so if anyone can provide evidence that I am wrong I would love to see it. Ditto if I am right.

 

[Janadele]

infinite.

 

[Beerina]

Just basic cosmology allows things to be 14 bilion light years going in two directions makes the Universe at least 28 bly across. But then, that goes against the concept the Universe has no center and no edges.

I think the center is past time and the edges are future time but that's an against the mainstream concept and probably off topic.

That's all Im going to contribute at this point because I don't know much about cosmology.

Apparently the visible edge is about 30 LY away, even though the universe is only ~14 billion years old, the max distance light could possibly have traveled, because it is expanding.

Ooh, the far edges, including stars we can still see, are actually flying away faster than the speed of light; that limit is on stuff "in" the universe, not the fabric of spacetime itself. Presumably it is old, redshifted light, and anything "new" since it exceeded the speed of light is forever lost to us beyond a horizon of sorts.

 

[h.g.Whiz]

Apparently the visible edge is about 30 LY away, even though the universe is only ~14 billion years old, the max distance light could possibly have traveled, because it is expanding.

Ooh, the far edges, including stars we can still see, are actually flying away faster than the speed of light; that limit is on stuff "in" the universe, not the fabric of spacetime itself. Presumably it is old, redshifted light, and anything "new" since it exceeded the speed of light is forever lost to us beyond a horizon of sorts.

Sorts?

 

[Beerina]

Sorts?

Uhhh, it's left as an exercise for the reader?

 

[h.g.Whiz]

Uhhh, it's left as an exercise for the reader?

 

[Puppycow]

If the expansion of the universe is accelerating and some parts are far enough away from each other that they are moving away from each other at faster than the speed of light, then assuming that the expansion continues to accelerate forever the light would never be able to reach us because the distance it would have to travel to get here increases faster than the speed of light.

Does that explain it?

 

[ynot]

There are two possible answers
1. The meter stick would stay the same size
2. The meter stick would expand at the same rate as the universe expands.

In case 2. it would expand 7,690 billionths of a meter per year. This is how much the universe expands each year (if this is wrong please substitute the right answer). This would be hard to measure. If the ruler was a solid object then the number of atoms would not change so the average distance between them would increase. This would mean that several other constants would also change. I do not think that these constants change over time. So I suggest a meter ruler would stay the same size.

Not sure what happens about objects like planets or stars that orbit stars. I think gravity would be much stronger than anything else.


But I am not an expert on the issue so if anyone can provide evidence that I am wrong I would love to see it. Ditto if I am right.

If expanding space expands a metre stick then it would do so uniformly (in all dimensions) not just by length. All other matter in the Universe would also be expanded uniformly by the same amount. From a perspective point of view it would be as if everything had stayed the same size.

 

[GrouchoMarxist]

This big.

 

[IMST]

When a fruit loaf cooks and the dough expands between the pieces of fruit, do the pieces of fruit move apart at any speed, or is it merely apparent movement at no speed?

My understanding is that, while nothing can be said to be motionless, the apparent movement of very distant objects can be pretty much attributed to the expansion and any actual movement of the objects is insignificant at those distances.

As far as the objects themselves expanding, my understanding is that nothing that is close enough to be tied by gravity expands away (for example Milky Way and Andromeda should see no expansion of space between them), and physical are close enough to be electromagnetically tied, so there should be any expansion there either.

 

[rjh01]

If expanding space expands a metre stick then it would do so uniformly (in all dimensions) not just by length. All other matter in the Universe would also be expanded uniformly by the same amount. From a perspective point of view it would be as if everything had stayed the same size.

However if you measure the meter by counting the number of waves of a specific wavelength of light you might find that there are more waves per meter.

Yes I do agree that objects will expand in all dimensions.

 

[Wolfman]

I had it explained to me once like this:

Imagine a long, flat rubber band. Draw two dots on it, one at each end, exactly 10 inches apart. Then have an ant start walking from the first dot, towards the second dot, at a constant rate of speed; at the same time, start slowly stretching the rubber band.

The distance from point A to point B when the ant starts its journey is 10 inches; the total distance the ant travels will be larger than that; and when the ant reaches point B, the total distance between points A and B will be even larger than the distance traveled by the ant.

Thus, for example, the beginning distance could have been 10 inches; the distance traveled by the ant 15 inches; and the final distance 20 inches. If the ant represented light leaving the first point, then the ant would represent light from 10 light years away, that traveled 15 light years to reach us, and that shows a place that is now 20 light years distance from us.

I'm sure that there's something wrong with this, but as a layman's explanation, I think it works.

 

[IMST]

I had it explained to me once like this:

Imagine a long, flat rubber band. Draw two dots on it, one at each end, exactly 10 inches apart. Then have an ant start walking from the first dot, towards the second dot, at a constant rate of speed; at the same time, start slowly stretching the rubber band.

The distance from point A to point B when the ant starts its journey is 10 inches; the total distance the ant travels will be larger than that; and when the ant reaches point B, the total distance between points A and B will be even larger than the distance traveled by the ant.

Thus, for example, the beginning distance could have been 10 inches; the distance traveled by the ant 15 inches; and the final distance 20 inches. If the ant represented light leaving the first point, then the ant would represent light from 10 light years away, that traveled 15 light years to reach us, and that shows a place that is now 20 light years distance from us.

I'm sure that there's something wrong with this, but as a layman's explanation, I think it works.

This reminds me of reading about how surprisingly complex the question "where is it?" really is.

It can be answered with 3 different answers: Where was it when the light was emitted, where does it look like it is, or where is it now.

 

[Farsight]

Thanks for the correction. I had to look up quintessence because I actually thought it was an outdated concept from the ancient Greeks and medieval alchemists, i.e., the "fifth element" to go along with earth, air, fire and water. I now realize that there is a new use for the word.

My pleasure Puppycow. There's quite a few things like that. Aether is another one, see arXiv for papers with aether in the title.

rjh01: see the physicsworld article Changes spotted in fundamental constant featuring work by Webb et al:

"Billions of years ago the strength of the electromagnetic interaction was different at opposite ends of universe. That's the surprising conclusion of a group of physicists in Australia, who have studied light from ancient quasars. The researchers found that the fine-structure constant, known as α, has changed in both space and time since the Big Bang."

There's also Can GPS find variations in Planck's constant?

"Physicists in the US say that they have used publicly available data from global positioning system (GPS) satellites to put a limit on how much Planck's constant might vary from place to place. Their technique involves analysing the tiny corrections that are applied to the atomic clocks used in GPS satellites – corrections that are made to account for relativistic effects caused by the orbits of the satellites. However, not all physicists agree that the analysis is meaningful."

All:

As far as I know the metre stick doesn't expand as the universe expands, because it's electromagnetically bound.

This is a good paper: Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe by Tamara Davis and Charles Lineweaver. This is the interesting bit:

"We show that we can observe galaxies that have, and always have had, recession velocities greater than the speed of light".

Sounds impossible, but it isn't. Think about Wolfman's ant, and imagine you've got an elastic rope 100cm long. There's an ant on the other end of it, crawling towards your face:

After 0 seconds the ant is 100.00cm away from you and you stretch the rope 1cm. The ant is now 100.0 x 101/100 = 101.00cm away from you.
After 1 seconds the ant is 100.00cm away from you and you stretch the rope 1cm. The ant is now 100.0 x 102/101 = 100.99cm away from you.
After 2 seconds the ant is 99.99cm away from you and you stretch the rope 1cm. The ant is now 99.99 x 103/102 = 100.97cm away from you.
After 3 seconds the ant is 99.97cm away from you and you stretch the rope 1cm. The ant is now 99.97 x 104/103 = 100.94cm away from you.
After 4 seconds the ant is 99.94cm away from you and you stretch the rope 1cm. The ant is now 99.94 x 105/104 = 100.90cm away from you.
After 5 seconds the ant is 99.90cm away from you and you stretch the rope 1cm. The ant is now 99.90 x 106/105 = 100.85cm away from you.
After 6 seconds the ant is 99.85cm away from you and you stretch the rope 1cm. The ant is now 99.85 x 107/106 = 100.79cm away from you.

Anyway, you get the picture. Even though the far end of the rope is receding from you at 1cm per second and the ant is crawling towards you at 1cm per second, the distance between it and you is reducing faster and faster. It keeps on chugging towards you, and it will eventually reach you.

 

[Dancing David]

When a fruit loaf cooks and the dough expands between the pieces of fruit, do the pieces of fruit move apart at any speed, or is it merely apparent movement at no speed?

Does fruit loaf have the negative energy of gravity and dark energy?

Does it have anything like c in it.

One analogy too far.

 

[Dancing David]

I’m happy to entertain the notion that space is an autonomous thing that can expand while nothing else does. In this context however space is a thing not a distance.

Yes, 'space' in this context includes the 'vacum energy'

 

[Farsight]

DancingDavid: the raisins-in-the-cake is a well known analogy for the expanding universe. Google on raisin cake galaxy for examples. As regards c, there's a lot of people who think it varies over cosmological time, see arXiv. John Moffat and Jo Magueijo took the lead on this, see Comments on “Note on varying speed of light theories”. Note though that it's subtle, like the way the coordinate speed of light varies in a gravitational field. You can't measure it locally and directly because if c varies the rate of electromagnetic and other processes varies too.

 

[sol invictus]

This is above my pay grade too, but I think that the idea is that while the space expands, things in the space, such as a meter stick, would stay the same size. Thus distances between objects in the expanding space as measured by the meter stick would get larger.

That's correct. Objects are held together by internal forces (that's the reason they don't fall apart when you pick them up), so the very weak force from the expansion of space can't pull them apart. By contrast, very large structures (like clusters of galaxies) are very weakly bound together, and therefore expand more or less uniformly.