Cosmology Q&A

From Brendon’s mate Luke.

The Rev Dodgson said:

Cosmology Q&A

From Brendon’s mate Luke.

“Is everything getting bigger?
No, since then we couldn’t tell. There are two reasons
why some objects fail to expand in step with the universe
as a whole. On small scales, the universe is not perfectly
homogeneous. Clumps of matter experience the mutual
attraction of gravity; if sufficiently dense, the matter and
its spacetime ceases to expand. On very small scales, other
forces like electromagnetism will hold objects to be a constant
physical size.”

Which raises the question, how do we know that we aren’t getting smaller, with space staying the same size?

i think if that were the case then the SoL would have to vary.

if the universe started shrinking, what happens to the speed of light, radio waves for example too, and does everything that follows the inverse-square law do as it does presently.

the only difference is that distant galaxies would start to show a shift towards the blue end of the spectrum as opposed to the red end.

JudgeMental said:

the only difference is that distant galaxies would start to show a shift towards the blue end of the spectrum as opposed to the red end.

Why would they?

because the expansion is determined by the redshift of galaxies.

>because the expansion is determined by the redshift of galaxies.

rev might be suggesting we’re not at the centre of the universe

if the universe started shrinking, what happens to the speed of light, radio waves for example too, and does everything that follows the inverse-square law do as it does presently.

nothing about the above would change.

we are in regards the bit we can see.

rev might be suggesting we’re not at the centre of the universe

i believe PM put all those concerns to bed in a thread a while ago.

How big is the universe really?

We don’t know. GR tells us how spacetime is curved
(geometry), but not how it is connected (topology). If space
is positively curved, then it is finite. If space is flat or nega-
tively curved, then it could be infinitely large. Or it could
be finite. For example, the “flat torus”, as its name suggests,
has a flat geometry, finite size, and the topology of a do-
nut. If our assumption that the cosmological principle
holds everywhere in the universe is wrong, then all bets
are of. In any case, we can’t see more than the observable
universe, so we’re theorizing.

JudgeMental said:

because the expansion is determined by the redshift of galaxies.

We interpret the redshift as an expansion of space, with matter staying constant size.

Constant size space with shrinking rulers would also produce a red-shift.

JudgeMental said:

rev might be suggesting we’re not at the centre of the universe

i believe PM put all those concerns to bed in a thread a while ago.

I don’t believe he did, in fact I doubt that they are put to bedable.

But regarding the Earth being the centre of the greater universe, I don’t think anyone suggests that it is.

The universe can expand within known physics, but we don’t know that matter is able to shrink. So an expanding universe is less speculative and more economical.

(Also, there might be some known reason why matter can’t shrink)

Bubblecar said:

The universe can expand within known physics, but we don’t know that matter is able to shrink. So an expanding universe is less speculative and more economical.

(Also, there might be some known reason why matter can’t shrink)

Matter staying constant size is just an assumption. If it was assumed that “space” remains constant sized (which to me seems a more logical default assumption) then we would say:

“Matter can shrink within known physics, but we don’t know that space is able to expand. So shrinking matter is less speculative and more economical.

(Also, there might be some (un?)known reason why space can’t expand)

The Rev Dodgson said:

Bubblecar said:

The universe can expand within known physics, but we don’t know that matter is able to shrink. So an expanding universe is less speculative and more economical.

(Also, there might be some known reason why matter can’t shrink)

Matter staying constant size is just an assumption. If it was assumed that “space” remains constant sized (which to me seems a more logical default assumption) then we would say:

“Matter can shrink within known physics, but we don’t know that space is able to expand. So shrinking matter is less speculative and more economical.

(Also, there might be some (un?)known reason why space can’t expand)

We know that objects can move further apart while maintaining the same relative size. This remains true even if everything was shrinking. But we don’t know that matter can get smaller and smaller. In fact we’re usually told that if things could shrink, their properties would change drastically. And it wouldn’t just be matter that would have to shrink, but the energy associated with it.

the thing is that things in the universe aren’t getting bigger it is the metric expansion of space that is causing the expansion. so the reverse isn’t everything, us and our rulers, getting smaller.

also i’ll go back to my first point that if we, and our rulers, were shrinking then i think we’d observe a change in the speed of light over time.

Bubblecar said:

The Rev Dodgson said:

Bubblecar said:

The universe can expand within known physics, but we don’t know that matter is able to shrink. So an expanding universe is less speculative and more economical.

(Also, there might be some known reason why matter can’t shrink)

Matter staying constant size is just an assumption. If it was assumed that “space” remains constant sized (which to me seems a more logical default assumption) then we would say:

“Matter can shrink within known physics, but we don’t know that space is able to expand. So shrinking matter is less speculative and more economical.

(Also, there might be some (un?)known reason why space can’t expand)

We know that objects can move further apart while maintaining the same relative size. This remains true even if everything was shrinking. But we don’t know that matter can get smaller and smaller. In fact we’re usually told that if things could shrink, their properties would change drastically. And it wouldn’t just be matter that would have to shrink, but the energy associated with it.

But the expansion of space is different to things just moving apart, whilst maintaining the same size. The moving apart (or together) can continue whilst space is expanding, and/or matter is contracting.

JudgeMental said:

the thing is that things in the universe aren’t getting bigger it is the metric expansion of space that is causing the expansion. so the reverse isn’t everything, us and our rulers, getting smaller.

Well that’s the usual assumption, yes.

The question is, why is this the assumption, rather than the apparently at least equally logical assumption of shrinking rulers, rather than expanding space?

JudgeMental said:

the thing is that things in the universe aren’t getting bigger it is the metric expansion of space that is causing the expansion. so the reverse isn’t everything, us and our rulers, getting smaller.

If you apply the same argument to expanding space you will get the same variation in the speed of light.

Neil deGrasse Tyson: Is the Universe Expanding or Are We Shrinking?

http://www.youtube.com/watch?v=MFtoejrI1Gc

Bubblecar said:

Neil deGrasse Tyson: Is the Universe Expanding or Are We Shrinking?

http://www.youtube.com/watch?v=MFtoejrI1Gc

Hmm, not terribly enlightening :)

I remember threads on this topic growing long & fat in SSSF, and me leaving it to the experts to worry about :)

if we are shrinking then the constants and the forces we know about would also have to change. also if the speed of light was not a constant then standard candles would not be standard.

also i doubt quantum mechanics allows atoms to shrink.

JudgeMental said:

also i doubt quantum mechanics allows atoms to shrink.

How would it know the atoms are shrinking?

Bubblecar said:

Bubblecar said:

Neil deGrasse Tyson: Is the Universe Expanding or Are We Shrinking?

http://www.youtube.com/watch?v=MFtoejrI1Gc

Hmm, not terribly enlightening :)

At least he didn’t just dismiss it.

i doubt it would “know”. maybe the rules governing our understanding of qm don’t allow it.

I doubt that the Planck length is entitled to shrink.

Bubblecar said:

I doubt that the Planck length is entitled to shrink.

There seems to be an underlying assumption that distances have some absolute value on a small scale, but not on a large scale.

That seems odd to me.

why?

JudgeMental said:

why?

Why would they be different?

Surely distance is (like velocity) relative, or it is somehow fixed by some external reference (although I don’t know how we could know that).

Why should it be fixed at a small scale, but relative on a large scale?

How would that even work?

JudgeMental said:

why?

Because he’s an engineer.

i would imagine that quantum limits how small stuff can be but you can use lots of that small stuff, an infinite amount maybe, to make big stuff.

anyway i see this exercise in us getting smaller v the universe expanding as getting clunky. too many things seem to have to change for it to get up.

JudgeMental said:

i would imagine that quantum limits how small stuff can be but you can use lots of that small stuff, an infinite amount maybe, to make big stuff.

What does that have to do with the current discussion?

errrr everything.

JudgeMental said:

anyway i see this exercise in us getting smaller v the universe expanding as getting clunky. too many things seem to have to change for it to get up.

I see it as two different ways of looking at precisely the same phenomenon.

JudgeMental said:

errrr everything.

I don’t recall mentioning the amount of matter in the Universe.

maybe, but one is backed by observation, on a number of levels, and the other isn’t. otherwise it would be being discussed by the experts. and i don’t see that.

well matter is just one aspect. the closeness of that matter to other bits of matter can’t be ignored.

The Rev Dodgson said:

Bubblecar said:

I doubt that the Planck length is entitled to shrink.

There seems to be an underlying assumption that distances have some absolute value on a small scale, but not on a large scale.

That seems odd to me.

According to physicists, the Planck length is, in principle, the smallest measurable length. If it’s actually shrinking, then its length at any particular time would not be, in principle, the smallest measurable length. It would only be the smallest measurable length at that time.

JudgeMental said:

maybe, but one is backed by observation, on a number of levels, and the other isn’t. otherwise it would be being discussed by the experts. and i don’t see that.

Any observations that apply to one, also apply to the other.

Bubblecar said:

The Rev Dodgson said:

Bubblecar said:

I doubt that the Planck length is entitled to shrink.

There seems to be an underlying assumption that distances have some absolute value on a small scale, but not on a large scale.

That seems odd to me.

According to physicists, the Planck length is, in principle, the smallest measurable length. If it’s actually shrinking, then its length at any particular time would not be, in principle, the smallest measurable length. It would only be the smallest measurable length at that time.

????

If the rulers are also shrinking, why would it change?

how? how can you ignore quantum effects in a shrinking universe? remember an expanding universe isn’t one where stuff is getting bigger. you want our rulers to be getting smaller which isn’t the opposite.

>If the rulers are also shrinking, why would it change?

Because you claim that it’s shrinking, i.e., the rulers are not to be trusted. The smallest things they can measure are actually continually getting smaller.

a collapsing universe, ending in a big crunch, is the opposite and in that the laws of physics, as we see them, would remain the same.

We discussed this shrinking ruler approach back on old SSSF, when KJW was still around. Yes, it’s a valid viewpoint, but it’s a bit more complicated than the expanding universe model.

The expansion only creates an effect at a very large scale because at smaller scales the other forces dominate it. Even at the scale of galactic clusters feeble old gravity is enough to overcome expansion.

So a shrinking ruler model has to come up with a mechanism that explains why the shrinking is uniform at small scales but then becomes non-uniform at the very large scale.