Expansion of Universe but no expansion of galaxies

In summary, the conversation discusses the concept of cosmological expansion and its effects on the space between galaxies and within galaxies. It is stated that while the space between galaxies is increasing with time, the size of galaxies or gravitationally bound space remains constant. Some find this unnatural and question how the edge of a gravitationally bound system is defined and the role of dark energy. The conversation also touches on the influence of cosmological expansion on local systems and its potential impact on Earth's orbit and temperatures over Earth's history. References are provided for further reading.
  • #1
BOYLANATOR
198
18
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?
 
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  • #2
BOYLANATOR said:
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?

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
 
  • #3
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.
 
  • #4
BOYLANATOR said:
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?

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.
 
  • #7
Lino said:
Very much more than decent ...

Thanks. I didn't do it all by myself ... had lots of feedback by folks here on this forum.
 
  • #9
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.
 
  • #10
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.
 
  • #11
The Cooperstock paper confirms that cosmological expansion has a negligible effect on local systems.
 
  • #12
Thanks Chronos. What about the inconsistency with paleontology records?

Regards,

Noel.
 
  • #13
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_books/genrel/ch08/ch08.html#Section8.2 [Broken] (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.
 
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  • #14
  • #15
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.
 

1. What evidence supports the expansion of the universe?

The expansion of the universe is supported by several pieces of evidence, including the redshift of galaxies, the cosmic microwave background radiation, and the abundance of light elements. Galaxies appear to be moving away from each other, which is known as the redshift effect, and this can be observed through the use of spectroscopy. Additionally, the cosmic microwave background radiation is a remnant of the Big Bang and provides further evidence for the expansion of the universe.

2. How do we know that galaxies are not expanding with the universe?

The expansion of the universe refers to the overall increase in the distance between objects on a large scale. While individual galaxies may appear to be moving away from each other, this is due to the expansion of the space between them, not the galaxies themselves. This can be observed through the use of Hubble's Law, which states that the further away a galaxy is, the faster it appears to be moving away from us due to the expansion of the universe.

3. Can the expansion of the universe be explained by other factors?

While the expansion of the universe is currently the most widely accepted explanation for the observed redshift of galaxies, some alternative theories have been proposed. These include the possibility of a variable speed of light or a non-uniform distribution of matter in the universe. However, these theories have not been supported by substantial evidence and the expansion of the universe remains the most well-supported explanation.

4. How does the expansion of the universe impact the formation of galaxies?

The expansion of the universe does not directly impact the formation of galaxies. Galaxies are formed through the process of gravitational collapse, where matter is drawn together by its own gravity to form structures. While the expansion of the universe may influence the distribution of matter and the rate of galaxy formation, it is not the driving force behind their formation.

5. Is the expansion of the universe accelerating or slowing down?

The expansion of the universe was initially thought to be slowing down due to the gravitational pull of matter in the universe. However, in the late 1990s, observations of distant supernovae revealed that the expansion of the universe is actually accelerating. This was unexpected and led to the discovery of dark energy, a mysterious force that is believed to be driving the acceleration of the expansion of the universe.

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