What force is needed to overcome expansion?

In summary, objects that are gravitationally bound do not move apart with expansion. However, under a constant acceleration of expansion, a minimum gravitational force is required to keep them from moving apart. This is due to the presence of a cosmological constant, which acts as a repulsive force at cosmological distances. The distance at which this repulsion balances gravity is roughly the same as the Hubble radius. The minimum force needed depends on the mass and distance of the objects and can be further understood through the concept of Jeans Instability.
  • #1
rede96
663
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As I understand it gravitationally bound objects don’t move apart with expansion. So I was wondering, assuming constant acceleration of expansion, what is the minimum gravitational force required to keep objects from moving apart?
 
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  • #2
Are you talking about static universe model.
 
  • #3
RyanH42 said:
Are you talking about static universe model.

I'm not really well up on current models as I am just an interested layman. So was wondering if we assume the current Hubble constant of 70 m/s per mega parsec
 
  • #4
Albert Einstein added a positive cosmological constant to his equations of general relativity to counteract the attractive effects of gravity on ordinary matter, which would otherwise cause a spatially finite universe to either collapse or expand forever.This is the property of static universe.If you need numbers I can't help you.
 
  • #5
RyanH42 said:
Albert Einstein added a positive cosmological constant to his equations of general relativity to counteract the attractive effects of gravity on ordinary matter, which would otherwise cause a spatially finite universe to either collapse or expand forever.This is the property of static universe.If you need numbers I can't help you.

Ok, thanks for that. I was probably thinking more practically than that. I read that expansion does not apply any forces to objects. So if this was the case then it seemed that any force at all attracting / binding two objects would be enough for it to stop them receding with expansion. But that assumes a constant rate of acceleration.
 
  • #6
In a system of objects which are gravitationally bound they will in general remain gravitationally bound and not significantly affected by cosmic scale expansion.
I can imagine however that there could be edge cases where a very loosely bound object can become detached.
After all, the extent of an objects gravitational field is infinite, (is it not?), - although it becomes insignificantly weak far from the object.
So there is no place in the universe where gravity is totally absent, just extremely weak, and weak enough so that expansion could dissociate for example a tiny outlying galaxy from a loosely bound cluster.
This is not a theory, it just seems to me as if it could be possible, but probably someone can present me with impenitrable math which refutes the possibility, and I will happily take their word for it.
 
  • #7
There's a distinction between unaccelerated expansion, which as I understand it has no effect whatsoever, and a cosmological constant (leading to accelerated expansion) which is similar to a tiny repulsive force proportional to distance - the latter doesn't prevent the formation of gravitationally bound systems up to the scale of say galaxy superclusters, but it becomes significant at cosmological distances. I am not sure of this but if my recollection is correct, the distance at which this repulsion balances gravity is roughly of the same order as the Hubble radius.
 
  • #8
rede96 said:
As I understand it gravitationally bound objects don’t move apart with expansion. So I was wondering, assuming constant acceleration of expansion, what is the minimum gravitational force required to keep objects from moving apart?
Depends upon how massive the objects are and how far apart they are.

Read up on Jeans Instability if you want an in-depth analysis on how this works:
https://en.wikipedia.org/wiki/Jeans_instability
 

1. What is thermal expansion?

Thermal expansion is the phenomenon in which a material expands when heated and contracts when cooled. This occurs due to an increase or decrease in the average distance between its atoms or molecules.

2. What forces are involved in thermal expansion?

The main forces involved in thermal expansion are intermolecular or interatomic forces, such as the electromagnetic force, which holds atoms together in a material. As the material expands, these forces are overcome, resulting in a change in size or shape of the material.

3. How is thermal expansion measured?

Thermal expansion is typically measured using the coefficient of thermal expansion (CTE), which is the fractional change in size or volume of a material per degree change in temperature. It is usually expressed in units of parts per million per degree Celsius (ppm/°C) or parts per million per degree Fahrenheit (ppm/°F).

4. Can thermal expansion be controlled?

In some cases, thermal expansion can be controlled by using materials with low CTE or by designing structures with joints or gaps that allow for expansion and contraction. However, in most cases, thermal expansion is a natural and unavoidable process.

5. How does thermal expansion affect everyday objects?

Thermal expansion can affect everyday objects in various ways. For example, it can cause metal bridges to expand and contract, resulting in structural damage over time. It can also cause gaps in railroad tracks, leading to train derailments. On a smaller scale, thermal expansion can cause cracks in pavement and ceilings, and can even affect the accuracy of instruments such as thermometers and clocks.

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