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How can galaxies collide if the universe is expanding?

If the universe is expanding and galaxies are moving away from each other, why do astronomers say the Milky Way and Andromeda galaxies are going to collide?

Arp 302

The universe is expanding at an ever increasing rate but Arp-302 provides a textbook example of how gravity dominates in local space causing the two galaxies to collide, much like the Milky Way and Andromeda will in billions of years from now. (Source: X-ray NASA/CXC/IfA/D.Sanders et al; Optical NASA/STScI/NRAO/A.Evans et al)

I read that the universe is expanding, and thus galaxies are moving away from each other. But in 'Wonders of the Universe', Professor Brian Cox talks about the collision between Milky Way and Andromeda galaxies. How can they collide when they are moving away from each other?—Ay

Over the next five billion years a barely detectable fuzzy blob in the evening heavens will grow bigger and bigger until eventually it fills the entire sky.

The fuzzy blob is Andromeda, a galaxy containing about a trillion stars which is destined to collide with our own galaxy the Milky Way.

"When we look at nearby galaxies, some are coming closer because of gravity, but others are moving away," says Associate Professor Charley Lineweaver from the Australian National University's Mount Stromlo Observatory.

"The Milky Way and Andromeda are part of a bunch of galaxies known as the local group which are gravitationally bound, so the forces of gravity locally, are able to overcome the expansion of the universe."

"The local group in turn is being gravitationally drawn to a bigger bunch of galaxies called the Virgo cluster at a velocity of about 500 kilometres per second."

Lineweaver says that's a speed comparable with the rate of the expansion of the universe on the local scale.

"But if you go far enough away, that speed is dwarfed by the recession velocity due to the expansion of the universe."

The rate at which the universe is expanding is accelerating. This acceleration is caused by the mysterious, and little understood, force of dark energy. Most physicists believe dark energy pushes things apart as it causes the fabric of space-time itself to expand.

"So if we look at more distant galaxies, they're all moving away. And when we look at very distant galaxies, they're moving away even faster!"

"We'll get lonelier because galaxies beyond our super cluster will eventually disappear beyond the cosmic horizon leaving us as one lonely but massive super cluster," says Lineweaver.

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The ultimate end

The speed of recession, how fast the universe is expanding, depends on how far into the distant universe you look.

But dark energy has limits to its ability to make things expand, says Lineweaver, so it's unlikely the expansion of the universe will end in a big rip.

That's a very speculative form of dark energy that would eventually begin effecting matter on smaller scales down to the subatomic level, tearing apart atoms into protons, neutrons and electrons.

"The data suggests that it's not going to happen. When we look through telescopes and measure the expansion rate of the universe it's not happening in a way that would rip apart bound structures."

"In other words things that are bound like my hand or the eucalyptus leaf on the tree outside my window are bound by atomic forces and they are not susceptible to the expansion of the universe because they are just too strong."

Instead, the universe will ultimately run out of energy, he says.

"Our best current model indicates that in about 105 googol years the most massive black holes that ever formed will have evaporated into photons".

"Then the universe will reach a heat death."

"Heat death is when the entropy of the universe reaches its maximum which is called equilibrium. No stars, no planets, just photons."

Dr Charley Lineweaver is an astronomer with the Australian National University Mount Stromlo Observatory. He was interviewed by Stuart Gary.

Tags: astronomy-space, the-universe, physics, galaxies

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Published 23 August 2011