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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Missing the target

Lately I've been seeing a lot of buzz on social media apropos of the Earth being hit by a killer asteroid.

Much of this appears to be wishful thinking.

Most of it seems to focus on the asteroid 2007 FT3, which is one of the bodies orbiting the Sun that is classified as a "near-Earth object" -- something with an orbit that crosses Earth's, and could potentially hit us at some point in the future.  It bears keeping in mind, however, that even on the scale of the Solar System, the Earth is a really small target.  This "deadly asteroid," we're told, is "on a collision course with Earth" -- but then you find out that its likelihood of its actually striking us on the date of Doomsday, March 3, 2030, is around one in ten million.

Oh, but there's "an altogether more sinister estimate" that 2007 FT3 could hit us on October 5, 2024, but the chances there are one in 11.5 million.  Why this is "altogether more sinister," I'm not sure.  Maybe just because it's sooner.  Or maybe the author of the article doesn't understand how math works and thinks that the bigger the second number, the worse it is.  I dunno.

Then there's the much-hyped asteroid 99942 Apophis, which was first thought to have a 2.7% chance of hitting the Earth in April of 2029 (more accurate observations of its orbit eliminated that possibility entirely), and then gets a second shot at us in April of 2036.  The 2036 collision depends on it passing through a gravitational keyhole during its 2029 close approach -- a tiny region in space where the pull of a much larger planet shifts the orbit of a smaller body in such a way that they then collide on a future pass.  Initially, the keyhole was estimated to be eight hundred kilometers in diameter, and this caused the physicists at NASA to rate Apophis at a four out of ten on the Torino Impact Scale -- the highest value any object has had since such assessments began.  (A rating of four means "A close encounter, meriting attention by astronomers.  Current calculations give a 1% or greater chance of collision capable of regional devastation.  Most likely, new telescopic observations will lead to reassignment to Level 0.  Attention by public and by public officials is merited if the encounter is less than a decade away.")  If it hit, the impact site would be in the eastern Pacific, which would be seriously bad news for anyone living in coastal California.

The close approach in 2029 [Image licensed under the Creative Commons Phoenix7777, Animation of 99942 Apophis orbit around Sun, CC BY-SA 4.0]

This, of course, spurred the scientists to try to refine their measurements, and when they did -- as the scale suggested -- they found out we're not in any danger.  The gravitational keyhole turns out to be only a kilometer wide, and Apophis will miss it completely.

In fact, there are currently no known objects with a Torino Scale rating greater than zero.

It's always possible, of course, that we could be hit out of the blue by something we never saw coming.  But given that we're talking about an unknown risk from an unknown object of unknown size hitting in an unknown location at an unknown time, I think we have more pressing things to worry about.  Sure, something big will eventually hit the Earth, but it's not going to happen in the foreseeable future.  NASA and the other space monitoring agencies in the world are doing a pretty good job of watching the skies, so maybe we should all just turn our attention on more important matters, like trying to figure out how nearly half of Americans think the best choice for president is a multiply-indicted, incompetent compulsive liar who shows every sign of incipient dementia.

In any case, I'm not concerned about asteroid impacts, and all the hype is just more clickbait.  So if you live on the West Coast and were planning on moving inland, or are considering cancelling your plans for a big Halloween bash this year, you probably should just simmer down.

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Relics of a lost planet

It took astronomers a good long while to figure out how the Moon formed.

Some initial working models were found, upon analysis to... well, not work.  One early idea was that what is now the Moon sheared away from the Earth while it was molten because of centrifugal force, but the viscosity of molten rock is too high (or the rotational speed of the Earth is way too low) for that to be feasible.  Another possibility was the gravitational capture of a pre-formed body, but that makes it hard to explain the Moon's nearly perfect circular orbit.  (Captured objects -- a likely candidate is Neptune's moon Nereid -- tend to have highly elliptical orbits and/or orbits not parallel to their host planet's rotation, because there's no reason to suppose that their capture occurred at any particular angle.)

A big clue came from isotopic analysis of lunar rocks, which found that the ratios of isotopes for several different elements were nearly identical to terrestrial rocks, arguing for a common source.  The prevailing theory is that the Moon formed when, about 4.5 billion years ago, the proto-Earth was struck by a Mars-sized planet -- named Theia, after the Greek Titan who was the mother of Selene, the goddess of the Moon -- which caused a blob of material to shear away, propelling it into orbit where it coalesced into what we see today on a clear night.

Artist's depiction of the collision between Theia and the proto-Earth [Image is in the Public Domain courtesy of NASA/JPL]

The reason the topic comes up is because of a paper that appeared this week in Nature that I found out about from a friend and loyal reader of Skeptophilia.  A team led by geophysicist Qian Yuan of Arizona State University took a look at two large low-velocity provinces (LLVPs) in the Earth's lower mantle -- dense regions where seismic waves slow down, and which are hypothesized to have a significantly higher iron oxide content than the rest of the mantle -- and their models support the astonishing idea that these are the remnants of Theia.

It's wild that there are still relics discernible, between the violence of the collision and the fact that 4.5 billion years have passed since it happened.  You'd think this would be plenty enough time to stir the mantle and homogenize the material Theia brought in with whatever was present in the proto-Earth.  But Yuan et al. think that the collision's energy was mostly dissipated into the upper mantle, allowing the remnants of Theia's core to sink into the lower mantle without mixing completely -- where the pieces are still detectable today.

Like all good science, the Yuan et al. paper raises some interesting questions, such as what effect the collision had on the rest of Earth's evolution.  "A logical consequence of the idea that the LLVPs are remnants of Theia is that they are very ancient," said Paul Asimow, of the California Institute of Technology and senior author of the paper, in an interview with Science Daily.  "It makes sense, therefore, to investigate next what consequences they had for Earth's earliest evolution, such as the onset of subduction before conditions were suitable for modern-style plate tectonics, the formation of the first continents, and the origin of the very oldest surviving terrestrial minerals."

So that's today's cool scientific research, which I can say without fear of contradiction is pretty close to earthshattering.  Think about that next time you see our companion's ghostly white light in the night sky -- that despite its tranquil appearance, it may well have been born from a collision of almost unimaginable violence, billions of years ago.

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Witness to a crash

Well, thanks to my friend, the brilliant writer Gil Miller, I now have another reason to huddle under my blankie for the rest of the day.

We've dealt here before with a great many cosmic phenomena that you would seriously not want to get too close to.  Some of these sound like Geordi-Laforgian technobabble from Star Trek, but I promise all of them are quite real:

• supernovae
• quasars
• blazars
• fast radio bursts
• magnetars
• fast blue optical transients
• Wolf-Rayet stars
• gamma-ray bursters
• false vacuum collapse

From this,  you might come to the conclusion that I have a morbid fascination with astronomical phenomena that are big and scary and dangerous and can kill you.  This is not entirely incorrect; I would only modify it insofar as to add that I am also morbidly fascinated with geological phenomena (earthquakes, volcanoes, pyroclastic flows, lahars) and meteorological phenomena (hurricanes, tornadoes, lightning, microbursts) that are big and scary and dangerous and can kill you.

Call it a failing.

In any case, thanks to Gil's eagle-eyed facility for spotting cool recent research in science, I now have a new astronomical one to add to the list -- a luminous fast cooler.  This one provides the added frisson of being (as yet) unexplained -- although as you'll see, there's a possible explanation for it that makes it even scarier.

The research that uncovered the phenomenon was done by a team led by Matt Nicholl, astrophysicist at Queen's University Belfast, using data from ATLAS, the Asteroid Terrestrial-Impact Last Alert System (speaking of scary phenomena) telescope network in Hawaii, Chile and South Africa.  The event they discovered was (fortunately) nowhere near our own neighborhood; it was spotted in a galaxy two billion light years away.

What happened is that a completely ordinary, Sun-like star suddenly flared up by a factor of a hundred billion.  The first thought, of course, was supernova -- but this explosion's profile was completely different than that of a supernova, and stars the size of the Sun aren't supposed to go supernova anyhow.  Then, as if to add to the mystery, it cooled just as fast, fading by two orders of magnitude in only two weeks.  A month later, it was only at one percent of its peak brightness shortly after detonating (still, of course, considerably brighter than it had been).

The first question, of course, is "if it wasn't a supernova, what was it?"  And the answer thus far is "we're not sure."  So the researchers started trying to find other examples of the phenomenon, and uncovered two previously unrecognized events that matched the recent explosion's profile, one in 2009 and one in 2020.

But that still doesn't tell us how a perfectly ordinary star can suddenly go boom.  Nicholl says that the team has come up with only one possible hypothesis -- and it's a doozie.

"The most plausible explanation seems to be a black hole colliding with a star," Nicholl said.

Well, that's just all kinds of comforting.

Artist's conception of a black hole devouring a star [Image is in the Public Domain courtesy of NASA/JPL]

So it's all very well to say cheerily, "Hey, at least the Sun's not gonna go supernova, and we don't have any Wolf-Rayet stars nearby, and the nearest gamma-ray burster isn't pointed in our direction, and false vacuum collapse is really unlikely!  We're sitting here happily orbiting a highly stable star still in the prime of life, in a quiet corner of the galaxy!  What could go wrong?"

Apparently, what could go wrong is that a black hole could come swooping in out of nowhere and make the Sun explode.

Now, mind you, there are no black holes near us.  That we know of.  And chances are, we would, because even though they're black (thus the name), their influence on the matter around them is considerable.  The great likelihood is if there were a black hole headed for a crash with the Sun, you'd know about it plenty in advance.

Not that there's anything you could do about it, other than the time-honored maneuver of sticking your head between your legs and kissing your ass goodbye.

So thanks to Gil for making me feel even tinier and more fragile than I already did, which led me to share this delightful discovery with you.

Have a nice day.

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