Celestial smashup

Just about everyone with even a passing interest in astronomy knows that the universe is expanding.

Ever since Edwin Hubble realized back in 1929 that almost everything outside of our own galaxy is redshifted (moving away from us), and that the degree of a galaxy's redshift is proportional to its distance from us -- something that has since been named Hubble's Law -- we've known that space is getting larger.  So, Hubble and others reasoned, if you run the clock backwards, there must have been a time when everything was collapsed together into one colossally dense point, that then for some reason that is still unknown, began to rush outward.

In other words, the Big Bang, which seems to have happened about 13.8 billion years or so ago, give or take a day or two.

However, that doesn't mean that everything is moving apart.  Within our own galaxy, there's enough mutual gravitational pull from all the massive objects therein to overcome the expansion, at least for now.  (Whether that'll continue forever remains to be seen; hold that thought, I'll get back to it.)  Even outside of our own galaxy, the members of the Local Group are gravitationally bound, and in fact, the nearest galaxy to us, Andromeda, is moving toward us at the impressive speed of 110 kilometers per second, so the Milky Way and Andromeda will eventually collide.

There are two reasons you shouldn't fret about this.  The first is that it's not going to happen for something like three billion years.  The other is that usually when two galaxies collide, shifts in the gravitational field fling stuff around, but very few collisions are expected to occur between individual stars.  Galaxies are, in fact, mostly empty space; if the Sun was the size of a typical orange and was sitting in the middle of downtown Washington D.C., the nearest star (Proxima Centauri) would be a slightly smaller orange... in San Francisco.

So while the alterations in mass distribution during a collision might throw stuff around a bit, and certainly change the shape of both galaxies, it's unlikely that any intelligent civilizations in the new combined Andromilkyway would be otherwise perturbed by it.

Note, however, I said that this is the case when two galaxies collide usually.

A paper last week in Monthly Notices of the Royal Astronomical Society describes a collision that occurred in a cluster of galaxies called "Stephan's Quintet," located (fortunately) about 290 million light years from here.  Recall my saying that the Andromeda Galaxy and Milky Way are moving toward each other at 110 kilometers per second; this enormous wreck happened eight times faster than that, with a speed that has generated a tremendous shock wave akin to a sonic boom in space.

Stephan's Quintet, showing the region affected by the collision [Image credit: Arnaudova et al., University of Hertfordshire]

"Since its discovery in 1877, Stephan's Quintet has captivated astronomers, because it represents a galactic crossroad where past collisions between galaxies have left behind a complex field of debris," said Marina Arnaudova of the University of Hertfordshire, who led the research.  "Dynamical activity in this galaxy group has now been reawakened by a galaxy smashing through it at an incredible speed of over 2 million mph (3.2 million km/h), leading to an immensely powerful shock, much like a sonic boom from a jet fighter.  As the shock moves through pockets of cold gas, it travels at hypersonic speeds – several times the speed of sound in the intergalactic medium of Stephan’s Quintet – powerful enough to rip apart electrons from atoms, leaving behind a glowing trail of charged gas, as seen with WEAVE [the William Herschel Telescope Enhanced Area Velocity Explorer]."

Which actually spells "WHTEAVE," but the discovery is cool enough that we'll let that slide.

The shock wave also compresses that interstellar gas and causes it to emit radio waves, which confirmed Arnaudova's team's discovery.

So locally, stuff can certainly move together, sometimes violently, even though the overall trend of the universe is to expand.

But.

According to a recent study by the Dark Energy Survey Project, there's a possibility that the amount of dark energy has changed over the life of the universe -- and is changing in such a way that it will affect the universe's ultimate fate.  If the amount of dark energy per unit volume of space were constant, it would mean that its effects on expansion would increase over time (since matter is thinning out, and the gravitational pull of matter is what's holding things together).  Thus, its outward pressure would proportionally increase, eventually overcoming all other attractive forces and ripping everything apart down to the constituent atoms.

This has always seemed to me to be a rather dismal prospect, not that I'll be around to see it.  Everything spread out in a thin soup of subatomic particles, and that's that.

But the new data suggests that the amount of dark energy is actually decreasing over time, meaning that its effects will gradually diminish -- and gravity will win, resulting in a "Big Crunch."  Everything turning around, falling inward, and ultimately colliding in a colossal smashup that might perhaps rebound in another Big Bang, and a new universe that resets the dials and starts it all over.

I first ran into this "oscillating universe" model when I took an astronomy class in college, and I thought it was a pretty cool idea; certainly better than the "Big Rip" that's predicted if the amount of dark energy per unit volume of space is a constant.  The point is still being debated, and (much) more data is needed to determine which is correct; but I, for one, would love it if the laws of nature were such that the universe might go through an unlimited number of bounces, and the whole game would begin again.

Maybe, just maybe, with any sentient life forms that evolve in Universe v. 2.0 getting a shot at doing it better next time.

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