If the universe is expanding then why ....

[anonymous-user]

... are some galaxies colliding with other galaxies?

I thought the universe was expanding evenly in all directions so why would two galaxies collide?

I am sure there is a very simple reason; kindly educate me please.

Thank you.

[Beati Dogu]

Gravity pulls them together if they're near enough to each other.

[Eric Mc]

The universe is expanding on the cosmological scale. At the more "local" level, gravity will still draw things together and affect neighbouring objects.

If you throw a ball into the air, it falls back down again - even in an expanding universe..

[Einion Yrth]

Eric Mc said:

If you throw a ball into the air, it falls back down again - even in an expanding universe..

Depends how hard you throw it.

[Tony1963]

Einion Yrth said:

Depends how hard you throw it.

He said "you", so how hard can YOU throw it?

[Einion Yrth]

Tony1963 said:

Einion Yrth said:

Depends how hard you throw it.

He said "you", so how hard can YOU throw it?

But he wasn't talking to me...

[maffski]

Eric Mc said:

If you throw a ball into the air, it falls back down again - even in an expanding universe..

Doesn't it travel in a straight line? (Spacetime curvature)

[Toaster]

maffski said:

Eric Mc said:

If you throw a ball into the air, it falls back down again - even in an expanding universe..

Doesn't it travel in a straight line? (Spacetime curvature)

Hmmm I would have thought Eric would have known there is no 'air' in a vacuum what I think is being described is that it is thought that nearby Galaxies collide due to gravitational attraction (No balls are being thrown and no Air is involved)

[annodomini2]

MikeStroud said:

... are some galaxies colliding with other galaxies?

I thought the universe was expanding evenly in all directions so why would two galaxies collide?

I am sure there is a very simple reason; kindly educate me please.

Thank you.

1. Galaxies move.
2. The rate of expansion (currently) is less than the speed of light.
3. Gravitational attraction between 2 large bodies.

[anonymous-user]

Ok thank you all for that very simple explanation... gravity.

[4x4Tyke]

maffski said:

Eric Mc said:

If you throw a ball into the air, it falls back down again - even in an expanding universe..

Doesn't it travel in a straight line? (Spacetime curvature)

Actually no, not even if you threw it directly up. The Moon, the Sun, every other planet, the milky way and every other galaxy in the universe would distort space time slightly. Expanding space time would make path upwards and the path back down subtlety different. You would get different paths at midnight, midday and dawn and dusk.

Nothing is really straight, not even a 'beam' of light.

[4x4Tyke]

MikeStroud said:

Ok thank you all for that very simple explanation... gravity.

Put simply, yes, sort of. There is another important element, do not think of galaxies as thrown outwards like an explosion, instead think of spacetime as stretching like the surface of an balloon inflating. Galaxy movement is akin to brownian motion on the surface.

Edited by 4x4Tyke on Saturday 24th March 10:23

[Moonhawk]

Think of it like 100 ants walking on the surface of a semi inflated balloon.

If you inflate the balloon some more, the average distance between ants will tend to increase - but any two (or more) ants still have the ability to move around the surface and interact with each other.

In terms of galaxies, this interaction in the result of their motion and the gravity acting on them.

[Krikkit]

4x4Tyke said:

maffski said:

Eric Mc said:

If you throw a ball into the air, it falls back down again - even in an expanding universe..

Doesn't it travel in a straight line? (Spacetime curvature)

Actually no, not even if you threw it directly up. The Moon, the Sun, every other planet, the milky way and every other galaxy in the universe would distort space time slightly. Expanding space time would make path upwards and the path back down subtlety different. You would get different paths at midnight, midday and dawn and dusk.

Nothing is really straight, not even a 'beam' of light.

Well, it does, but the curvature of spacetime means that from our idealised Euclidian reference frame it doesn't appear it.

[Bullet-Proof_Biscuit]

But once all the visible matter collapses into black holes, and then all those black holes collapse into one big black hole, will the big bang happen again?

[annodomini2]

Bullet-Proof_Biscuit said:

But once all the visible matter collapses into black holes, and then all those black holes collapse into one big black hole, will the big bang happen again?

One theory known as "the big crunch", the other is "the big rip" where basically space time rips itself apart.

[HaiKarate]

Bullet-Proof_Biscuit said:

But once all the visible matter collapses into black holes, and then all those black holes collapse into one big black hole, will the big bang happen again?

Black holes evaporate (Hawking radiation) over time so in the very distant future there will be zero black holes.

[Gandahar]

MikeStroud said:

... are some galaxies colliding with other galaxies?

I thought the universe was expanding evenly in all directions so why would two galaxies collide?

I am sure there is a very simple reason; kindly educate me please.

Thank you.

Current velocity / direction being far greater than outside influences.

Nuff said. People do like to over egg the pudding when it comes to astronomy.