Universe receding?

[Paul-SunCoast]

I read that astronomers are united in stating that no matter where we are in the universe we would see the rest of the universe receding. However, science also tells me that our own galaxy, The Milky Way, and our nearest neighbour, Andromeda, are on a collision course albeit many millions of years hence. If that is the case why do we not 'see' Andromeda closing rather than receding. I understand the vast distances involved mean we cannot visually judge what is happening but astronomers are not using naked eye sighting to state the case for the universe in recession. And if Andromeda and The Milky Way are closing how does the 'receding universe' stand up to such an event?
I would appreciate any help understanding this and apologies to all concerned if I missing something everyone else sees as blindingly obvious.

[rustigsmed]

I believe andromeda galaxy shows blue shift whilst everything else is red shifted. Therefore we do 'see' it coming closer.

[pvelez]

Paul

these are not silly questions. There are 2 things going on here - the local and the global.

At a local level, M31 and the Milky Way are moving towards each other. This is simply a function of gravity although its not so easy to model as all of the other galaxies (and other matter) in the Local Group of galaxies also need to be taken into account. Similarly, the LMC and SMC are gravitationally bound and will make a close pass by the MW - as has possibly happened before.

On the large scale, the universe is expanding. Here's the interesting bit - it is not that everything is moving away from us, it is the space between everything is expanding. The classic explanation is to think of the universe as a balloon with all the galaxies represented by dots on the balloon's surface. As the ballon is inflated, all of the dots move away from each other.

So the space between us and M31 is expanding but the gravitational force between us is far greater than that of the "dark energy" that is driving us apart. Recognised that gravity is an inverse square law so if you double the distance between galaxies, the gravitational force between them drops by 1/4 not 1/2 if the scaling were linear. So objects that are close (relatively) like M31 and the MW have sufficient gravitational force to remain bound.

I hope that helps

Pete

[mithrandir]

Paul, it 4 billion years, not millions. M31 (Andromeda) is approaching at around 110km/sec and is around 2.5 million light years away.
Astronomers measure the red or blue shift of spectral lines in the light received. An approach velocity of 110km/sec means the light is blue shifted by a small but measurable amount - about 1 part in 27300.The gravitational attraction between the galaxies in the Local Group - primarily M31, M33 (Triangulum) and the Milky Way but with numerous smaller ones - is stronger than the dark energy driving the universe apart so they are staying together and may all coalesce one day.

[g__day]

Imagine you live somewhere in a really tall high rise and may move about freely, but every second every part of the high rise is growing - all the walls ceiling and floor are say growing 1mm further apart every hour. So if your 100 floors up the ground is getting 100mm apart every hour. Over time the rate is increasing - in a years time say the rate is 1.2mm per hour and so on (units not at all exact).

Imagine that highrise is your local galaxy, and you live in a city where all high rises are expand, but even quicker than the buildings getting bigger - the buildings themselves are moving in some direction but the ground between each building is itself expanding too (possibly faster than the buildings grow)!

This is what we interpret from the data. The fabric of spacetime itself is expanding (and the upper rate of the expansion of spacetime we believe is not bound by relativity (lightspeed) - unlike all the matter and energy moving in any local spacetime) This means that two very distant points in the Universe may even be stationary in their local spacetime but rapidly receding from each other because of expansion of the spacetime in between each other. This rate of recession can be a very high proportion (e.g. 70%) of the speed of light. If nothing slows this rate down then sometime in the far future the farthest seperated points of the universe could possibly be recessing at each other at a rate of multiples of lightspeed - without contradicting any of the laws of relativity that governs how enery and matter move through relativistic (normal energy density, normal gravitational) space time. So to my understanding the only parts of the Universe where spacetime is not relativistic at a macro scale is 1) inside the event horizon of a black hole or within a supernova where the energy threshold crosses the boundary so that the four fundamental forces might re-combine (which I think would require a collapse of a Sun several times larger than our Solar system / or possibly very briefly during the direct collision of two Galactic centre black holes. By this I mean if massive black hole in the Milkyway Galaxy had an almost perfect head on collision with the central black hole in the centre of Andromeda Galaxy - you might, just might for a small portion of time create an energy release of sufficient magnitude ( so energy densities greater than 10 ^ 19 Joules per m^3) - so temperatures of millions of trillions of Kelvin - theoretically capable of warping local spacetime into a non relativistic region of the Universe until it expands and cools!

Hope that mind boggling explanation and background information helps!

PS

Nice picture attached - size of known Universe = 93 billion light years, yet age of Universe = 13.8 Billion light years. Speed of light means the extra distance is due to cumulative expansion of spacetime, not recession of things in it!