Undergrad The Universe is expanding with acceleration

[Ignat]

TL;DR

There are galaxies that are not moving away, but approaching the Earth.

According to Hubble's law, galaxies are moving away from Earth at a speed proportional to their distance. In other words, the further away they are, the faster they move away from the Earth. The speed of galaxies is determined by their redshift.

Based on this law, some scientists have come to the conclusion that the universe is expanding with acceleration. However, there are galaxies that are not moving away, but approaching the Earth, for example Andromeda. Now this approaching is explained by the proximity of the Milky Way and Andromeda and their attraction. But there is, for example, the Large Magellanic Cloud galaxy, which is also close to Earth, but moving away from the Milky Way.

 

[PeroK]

Andromeda and the Milky way are attracted by their mutual gravity. Like an apple falling to Earth.

Universal expansion applies on the largest cosmological scales - beyond that of local galaxy clusters. Where the expansion dominates gravity.

 

[Ignat]

Why is the Large Magellanic Cloud galaxy, which is also close to Earth, but moving away from the Milky Way?

 

[russ_watters]

Why is the Large Magellanic Cloud galaxy, which is also close to Earth, but moving away from the Milky Way?

"Gravity dominates expansion"(other side of the coin from what @PeroK said) doesn't mean that all objects within a certain distance will be moving towards each other, it just means that their gravitational interaction is a larger component of what causes the motion than space's expansion. I'm not sure there's a known reason why the LMC is moving towards the Milky Way instead of away, or if a "why" is even needed; locally the motion/interaction of objects can be pretty chaotic.

 

[PeroK]

Why is the Large Magellanic Cloud galaxy, which is also close to Earth, but moving away from the Milky Way?

I don't know anything specific about the LMC. A quick search suggests that its motion relative to the Milky Way is an active area of observation. Galaxies within a cluster have peculiar velocities relative to each other, which depend on their origin and local dynamics.

Universal expansion is determined by the homogeneous nature of the universe on the largest scales. The equations that predict expansion treat galaxy clusters as point masses separated by large distances. These equations simply do not apply to the local cluster of galaxies, as the density of matter is much higher than the universal average.

It's a common misconception that expansion applies at all scales. In the dynamics of the solar system, for example, the influence of the vacuum between planets is negligible, compared to the mass and gravity of the Sun and planets. At the other end of the the scale, the influence of the vacuum dominates. It's only on this scale that the universe is homogenous.

 

[PeterDonis]

The OP question has been answered. Thread closed.