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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Collision of galaxies

When I was an undergraduate at the University of Louisiana, I and several of my friends were blown away by the original series Cosmos, written and narrated by Carl Sagan.

Monday mornings, we gathered in the student lounge, eagerly discussing whatever mind-blowing filigree of physics had been the subject of that week's episode.  I still recall one of the ones that made the biggest impression on me -- the tenth episode, "The Edge of Forever," which included, among many other things, wonderful simulations of the motion of stars within a galaxy, and what happens when two galaxies collide.  (You can watch a clip of it here.)  The simulations were (at the time) state-of-the-art, and certainly enough to blow the mind of a sophomore physics student like myself; what struck me most was that galaxies aren't rigid, and their constituent stars don't "hang together," but move independently around the massive black hole at the galactic core.  This can settle down into a shape that seems pretty stable -- such as the spiral pattern of the Milky Way -- or it can destabilize, flinging stars out into space, exploding the galaxy and scattering its pieces across hundreds of thousands of light years.

Sagan, of course, put it best: "A galaxy is a fluid made of a billion suns, all bound together by gravity."

When galaxies collide, it disrupts both completely; at the same time, collisions between the stars themselves are extremely uncommon.  However big the stars are, they're still minuscule with respect to the galaxies that contain them.  It's like the atoms writ large, isn't it?  The seemingly solid matter around you is made up of tiny charged particles interacting through the force of electromagnetism, but in between those particles is... nothing.  Matter is mostly empty space, and only seems solid because you're feeling the mutual repulsion of the electrons in your fingers and the electrons on the surface of whatever you're touching.  Likewise, most of interstellar space is very close to nothing, and the galaxies themselves are made up of particles (stars) interacting through a different force (gravity), and separated by vast, empty voids.

Makes you almost think that the pagans might have been on to something with their dictum of "As above, so below."

Map of the Milky Way, as it would look from above the galactic disk [Image licensed under the Creative Commons 鄭興武和馬克 裡德（Mark J. Reid）銀河系棒和旋臂結構遺產性巡天(BeSSeL)項目組/南京大學/哈佛-斯密松天體物理中心., Milky Way large, CC BY-SA 4.0]

This topic, and my reminiscences of Cosmos, come up because of a paper in Nature Astronomy last week called "Evidence for a Vast Prograde Stellar Stream in the Solar Vicinity," by a team led by astronomer Lina Necib of the California Institute of Technology.  What this paper tells us is something stunning; there is a streamer of stars in the Milky Way that started out somewhere else, and collided with our galaxy.  Rather fortunately, apparently the angle and velocity with which the streamer hit were more or less the same direction the original galaxy was turning, so these stars simply got sucked in, like some bits of debris going down a whirlpool.

The streamer has been named Nyx, after the Greek goddess of the night.  250 stars have been identified as being part of Nyx.  "The two possible explanations here are that they are the remnants of a [galactic] merger, or that they are disk stars that got shaken into their new orbits because of a collision with the disk of the Milky Way," said study lead author Lina Necib, in an interview with CNN.  The likelihood, though, is the former, something that is expected to be confirmed by chemical analysis of the constituent stars.  "Galaxies form by swallowing other galaxies," Necib said. "We've assumed that the Milky Way had a quiet merger history, and for a while it was concerning how quiet it was because our simulations show a lot of mergers.  Now, we understand it wasn't as quiet as it seemed.  We're at the beginning stages of being able to really understand the formation of the Milky Way."

I think it's stunning that we can figure out this sort of thing at all -- that 250 out of the estimated 250 billion stars in the Milky Way started out somewhere else in the universe.  I think that's pretty damn impressive.  "This particular structure is very interesting because it would have been very difficult to see without machine learning," Necib said.  "I think we reached a point in astronomy where we are not data limited anymore.  This project is an example of something that would have not been possible a few years ago, the culmination of developments in data with Gaia, high resolution simulations, and machine learning methods."

How pleased and amazed Carl Sagan would have been.  He went a long way toward bringing the wonders of the universe, from the largest scales to the smallest, to laypeople.  He certainly blew the minds of me and my friends, and that was back in 1980.  Necib's comment, that we're still at the beginning of being able to understand the formation of galaxies, tells us that we have a long way still to go -- and that many, many more eye-opening discoveries are sure to come our way in the next years.

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This week's Skeptophilia book of the week is for anyone fascinated with astronomy and the possibility of extraterrestrial life: The Sirens of Mars: Searching for Life on Another World, by Sarah Stewart Johnson.

Johnson is a planetary scientist at Georgetown University, and is also a hell of a writer.  In this book, she describes her personal path to becoming a respected scientist, and the broader search for life on Mars -- starting with simulations in the most hostile environments on Earth, such as the dry valleys of central Antarctica and the salt flats of Australia, and eventually leading to analysis of data from the Mars rovers, looking for any trace of living things past or present.

It's a beautifully-told story, and the whole endeavor is tremendously exciting.  If, like me, you look up at the night sky with awe, and wonder if there's anyone up there looking back your way, then Johnson's book should be on your reading list.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]