Expansion of Universe but no expansion of galaxies

BOYLANATOR

In my Cosmology course it is stated that the space between galaxies increases with time whilst the size of galaxies (or gravitationally bound space) does not increase.
To me, this seems quite unnatural.
I understand that the space between galaxies is much larger than the radius of a galaxy and that the increase in the radius of a galaxy may be imperceptibly small, however it seems odd that the space inside a galaxy obeys different physical laws to space outside.
And how do we define the edge of a gravitationally bound system, isn't the gravitational force infinite in range?
What is going on here?

phinds

There is nothing weird going on, unless you consider (as do many people) dark energy to be weird.

Inside galaxies, there is enough gravitational force to overcome dark energy, which is STAGGERINGLY weak on small scales (and for dark energy, a galaxy is a small scale).

Just google dark energy

Drakkith

When talking about what space is doing it is important to understand that gravity and expansion ARE spacetime. Meaning that both gravity and expansion are the manifestation of spacetime that is curved or warped a certain way. The amount/type of curvature inside galaxies and between nearby galaxies is generally enough to either completely stop expansion, or to overpower it by such a large margin that it is unmeasureable. The RULES don't change, it's that the environment changes when add lots of matter and energy.

Lino

Boylanator, Imagine that you had a big sheet of rubber and started to stretch it - every point stretches. Now imagine that you put a blob of really (really) strong glue at a spot in the center - this time when you stretch the sheet, the force that you use (still stretches the rubber but) is not strong enough to stretch the glue. This is (basically) what is happening in the universe: the rubber of space is stretching while the galactic glue is overcoming the stretching force and staying "bound". I'm just a beginner myself so this is not a very scientific analogy, but I hope that it helps.

Regards,

Noel.

phinds

There's a decent discussion here:

www.phinds.com/balloonanalogy/

Lino

Very much more than decent ...

phinds

Thanks. I didn't do it all by myself ... had lots of feedback by folks here on this forum.

Chronos

This may be helpful - The influence of the cosmological expansion on local systems, http://arxiv.org/abs/astro-ph/9803097v1

Chronos

The calculated value of about 11 meters per year increase in earth orbit due to cosmic expansion amounts to about 7 billionths of a percent per year. Sounds harmless enough until you crunch the numbers. Earth distance to the sun would double about every 100 million years at this rate - which is hugely inconsistent with paleontology records.

Lino

Chronos, I'm confused. I thought that you referenced the paper because you agree with it, but based on a brief review and your previous post, now I'm not so sure. I do value your insights so if you could clarify I would appreciate it.

Regards,

Noel.

Chronos

The Cooperstock paper confirms that cosmological expansion has a negligible effect on local systems.

Lino

Thanks Chronos. What about the inconsistency with paleontology records?

Regards,

Noel.

bcrowell

The Cooperstock paper predates the discovery that our universe has a positive cosmological constant, so its analysis needs to be modified somewhat. I have a treatment here: http://www.lightandmatter.com/html_b...tml#Section8.2 (subsection 8.2.10). The result ends up being qualitatively the same: there is an effect on local, bound systems, but it's much too small to measure.

Chronos

Here is a discussion of temperatures over earth history - The faint young Sun problem, http://arxiv.org/abs/1204.4449

Lino

Thanks Bcrowell and Chronos. It's a lot to take in but I look forward to trying.

Bcrowell, on a quick review of the contents and a read of some sections, this is a great easy read for intermediates. Thanks again.

Regards,

Noel.