Geography of all the known things
June 18, 2013 4:25 PM   Subscribe

Cosmography of the Local Universe. From the comments: "Best video display of our Universe and our exact position in it to date....

...As co-leader of the NASA/IPAC Extragalactic Database of galaxy Distances (NED-D), I see and hear everything I can on the galaxies beyond our own Milky Way, especially regarding how they are distributed in 3D space as determined by precise, redshift-independent estimates of their distances. In terms of moving - pardon the pun - pictures, this is by far the best I have seen among numerous motion pictures showing where we are at, literally and figuratively, in the biggest picture of all. Congratulations to the Extragalactic Distances Database (EDD) team on a job superbly done."
posted by slappy_pinchbottom (29 comments total) 22 users marked this as a favorite
Does anybody what the axes measure? SGX, SGY, and SGZ in km/sec?
posted by benito.strauss at 4:35 PM on June 18, 2013

SGX, SGY, SGZ are in Supergalactic coordinates, I believe. Distance => redshift => velocity, so some astronomers use km/s as an unit of distance. Ugh, I know.
posted by RedOrGreen at 4:46 PM on June 18, 2013 [1 favorite]

In particular, those are the X, Y and Z axes of the Supergalactic Cartesian Coordinate System. The km/sec units are measurements of velocity as determined by redshift, one of the mechanisms used to estimate distance (objects further away appear to us to be moving faster due to the expansion of universe). A portion of the video shows how redshift is related to 3D position.
posted by RichardP at 4:49 PM on June 18, 2013 [1 favorite]

Ah, thanks. I was having trouble playing it and will give it another stab later.

Also, Wow.
posted by benito.strauss at 4:59 PM on June 18, 2013

So this is what Sagan was hinting at. Could not stop watching, even on a phone.
posted by halfbuckaroo at 5:03 PM on June 18, 2013

This whole rock we're on - heck, our entire solar system - doesn't even show up as a rounding error. The word "awesome" should be reserved for stuff like this, because it really, truly evokes a deep sense of awe and wonder. But then again, I spend way too much time in the Exoplanet app, never fails to take my breath away.
posted by dbiedny at 5:06 PM on June 18, 2013 [1 favorite]

This is very cool. If you have disposable income there is a company that has a pretty cool glass sculpture in a four inch cube of visible galaxies for fifty bucks or so. Google pulled the UK version for me but I can't see where the US version is.
posted by bukvich at 5:22 PM on June 18, 2013 [1 favorite]

I had read some of the terms in this video, like the various voids and the Great Attractor, but had never quite put together what it means that there are flowing streams of galaxies, like rivers and we can make maps of them.

I wish Edwin Hubble could have lived to see this.
posted by localroger at 5:23 PM on June 18, 2013 [1 favorite]

bukvich, there's no US version because they are produced and sold directly by a UK artist. They do ship globally though. Should be about USD$85 with shipping.
posted by localroger at 5:29 PM on June 18, 2013

Ok, I was a skeptic, but after seeing this I totally see how every Morman gets his own personal planet, there are so many extra galaxies, let alone a whole buncha planets, no one in the galactic headquarters would even notice.

Well, flip aside it's just so odd that we can envision so much from just the bits of light from just a few telescopes. And that the paths of galaxies (clusters of galaxies?) can be tracked. The mind world astronomers must live in is astonishing, every day being reminded you're vastly smaller than an ant (proportional to some astro artifact). We so need to get off the planet and tweak these delicate bodies to be able to get across the emptiness.
posted by sammyo at 5:54 PM on June 18, 2013

Distance => redshift => velocity, so some astronomers use km/s as an unit of distance. Ugh, I know.

Basically, the universe is expanding. The rate that it's expanding is known as the Hubble constant, which is now determined to be (though this is likely to change somewhat as our instrumentation gets better) 69.32 ± 0.80 (km/s)/Mpc (or 21.25 ± 0.25 (km/s)/Mega-lightyear) aka ~70 km/sec per megaparsec...a parsec is around 3.76 light years, so if you were 3.76 million light years away, you'd be moving away from me at 70km/sec.
It gets more complicated. See, the Hubble constant isn't exactly constant per se, but an average determined by well, basically the video you just saw...tracking the proper motion of every galaxy we can see (and there's many we can't see, due to being blocked by the milky way itself. That's the 'zone of avoidance' they're talking about) and averaging them together. The later parts of the video mainly covered the super-local(?) area as that is known in greater detail, but the Hubble constant is determined by galaxies further away (shown when they zoomed out further) as well, though that data is 'fuzzier' as the only way to really measure out that far is using the Hubble constant itself, by looking at the red-shifting of the light. :/
posted by sexyrobot at 6:42 PM on June 18, 2013

So yeah, the scale is basically 'F*cking far'
posted by sexyrobot at 6:44 PM on June 18, 2013

Also, as you can see from the later animated parts of this video, what's basically going on well, everywhere, is that these giant voids are expanding (we don't know why. dark energy? what's that made of? nobody knows.) and squishing all the galaxies together like the soap between the bubbles in some sort of gigantic, very weird, foam. And the arrangement of galaxies and the filiaments of dust, gas, and loose stars between them (not shown in this video) is structurally almost identical to the arrangement of neurons and dendrites in the human brain, though that is slowly changing as the universe evolves.
posted by sexyrobot at 6:57 PM on June 18, 2013 [1 favorite]

The glass sculpture map bukvich linked has a Guinness record certificate for being the smallest scale 3D map ever created, at a scale of 1 centimeter = 1 billion light-years.
posted by localroger at 7:10 PM on June 18, 2013

Also, distance = speed. But...but...okay, fine. It's not that important. They aren't there anymore anyway. I knew that. I used to lie on my saddle blanket, looking at the night sky, and get absolutely goofy thinking about it. It still gives me the shivers.

Is there somewhere (out there) that's not actually moving? Or, maybe time doesn't work that way anyhow, and non-movement would just be an illusion found at event horizons of black holes, or for the entertainment of neutrinos and such, if they can be said to have any sense of time's passage.

make it quit make it quit make it quit
posted by mule98J at 7:28 PM on June 18, 2013

Even things that aren't moving are still getting farther apart.

I know, right?!

So am I right in guessing that they report distance in km/sec to reduce the number of implicit assumptions? I.e. redshift is measured directly (as a difference in wavelength), and that yields speed of recession with just (Relativity and the speed of light), but converting that to distance requires using a value for the Hubble constant, which has all the issues pointed out by sexyrobot.
posted by benito.strauss at 7:41 PM on June 18, 2013

Is there somewhere (out there) that's not actually moving?

All movement is relative. In the later stages of the video they define a reference point that is kind of an average for everything within a few billion light years as the reference point which is why it shows the Milky Way in motion, but really everything is moving with respect to everything else and everything is at rest with respect to itself.
posted by localroger at 7:43 PM on June 18, 2013

Is everything visible in the night sky contained within the Milky Way?
posted by cacofonie at 7:58 PM on June 18, 2013

No. Andromeda, for example, is visible with the naked eye.
posted by axiom at 8:03 PM on June 18, 2013 [1 favorite]

Messier Objects are popular to look for, and many are other galaxies.
posted by OHenryPacey at 8:58 AM on June 19, 2013

benito.strauss: Am I right in guessing that they report distance in km/sec to reduce the number of implicit assumptions? I.e. redshift is measured directly (as a difference in wavelength), and that yields speed of recession with just (Relativity and the speed of light), but converting that to distance requires using a value for the Hubble constant, which has all the issues pointed out by sexyrobot.

Yes, redshift is the directly measured quantity - you take a spectrum of a distant source, identify a series of spectral lines, say "Hey, that looks like the Hydrogen Lyman Alpha (for example) lines, but they are at the wrong wavelength", and calculate the implied Doppler shift which gives you a recession velocity. (Again, not stuff I personally do - and I've heard *very* experienced astronomers describe the process of identifying spectral lines as similar to sexing chickens - you know them when you see them. A story for another time.)

To go from velocity to distance does involve knowing the Hubble constant. Fortunately, we know that to rather high precision now, thank in part to the key project of the Hubble Space Telescope, although other astronomers pointed out that they were effectively using an incorrect meter stick. That's why the value of H_0 bounced around for a while between 60 and 75 km/sec/Mpc, and people made sarcastic plots showing the time evolution of the Hubble constant over publication year. But the value right now is *rather* solid.

There's a subtler issue - the value of the expansion "constant" might have been different in the past. (See, e.g., dark energy.) That's the sort of can of worms that keeps cosmologists in the big bucks (kidding) and leads to observers throwing up their hands and just quoting km/sec.

Hence my cryptic "Distance => redshift => velocity" last night, sorry.
posted by RedOrGreen at 9:08 AM on June 19, 2013 [1 favorite]

No need to apologize. It sent me down some interesting thoughts about what might make one constant more constant that another constant.

Maybe the only thing to worry about is that it might be more correct to say "redshift => velocity => distance", or possibly exactly reversed.
posted by benito.strauss at 1:06 PM on June 19, 2013

The Hubble constant isn't known quite so precisely as sexyrobot and RedOrGreen suggest. Depending on who you ask, the best value would be 68 (from the new Planck measurements), 69 (WMAP, the value sexyrobot gave) or 74 km/sec/Mpc (from the best measurements with the Hubble Space Telescope). Uncertainties in each of these values are large enough that they are in tension with each other but don't quite disagree. Of course, when I was in grad school there were still advocates of values of 50 and of 100, so we've come a long way.

Astronomers have an easy way of dealing with that uncertainty, though: most cosmologists will give distances in units of Mpc / h, where h is the value of the Hubble constant divided by 100 km/sec/Mpc (Oleg Gnedin tried to get people to call this unit a "CHIMP", standing for Comoving H Inverse MegaParsec, but regrettably failed). Using such units future-proofs all your work: the numbers you give will be correct whatever the actual value of H_0 turns out to be.

Older work in the field, which was only focused on our local neighborhood, tends to give distances in km/sec, but that gets less useful beyond a redshift of 0.03, which corresponds to recession velocity of around 10,000 km/sec (Brent Tully's work has been almost entirely in that regime, which is probably why he uses km/s units). The reason we don't continue to use km/s further than this is that at higher redshifts, you have to take into account general relativity to relate observed redshift to distance ; i.e., the Hubble relation (that recession velocity = the Hubble constant times distance) breaks down as redshift/distance increases (the relationship of recession velocity to distance also is no longer as simple as redshift increases due to special relativity).

The mapping of redshift to distance gets more complicated at higher redshifts, but it still turns out to be inversely proportional to the Hubble constant always (you do need to make assumptions about how the Universe has expanded over time -- equivalent to assuming the amounts of matter, dark energy, etc. -- to do the calculations, so you still don't get a definitive answer).

TL;DR: Measuring distances in astronomy is hard.
posted by janewman at 4:40 PM on June 19, 2013 [1 favorite]

Sorry to threadsit, but this is my jam...
So...they're not really kidding about this being 'local' space as well (as is noted in the title)...what you see here is only a tiny fraction of the entire universe.
What's represented here (when they zoom out to the full radius of 8000km/s) is a total diameter of about 200million light years.
Doing my back-of-the-envelope map and referring to the wikipedia page on the observable universe...if the observable universe is about 90Billion ly in diameter and you're looking at this on a (conservative laptop) monitor about a foot across, then the observable universe at this scale would be around 450-500 feet, or about 1 1/2 football fields in diameter.
HOWEVER...we now know, due to the flatness of the cosmic background radiation as measured by the WMAP probe, that the actual universe is at least (at least(!!!)) 10^23 times that volume...corresponding to a diameter of (math...math...exponents) of about 900 quadrillion light years, or at this scale, about 1 million miles, or 5 times the distance to the moon. (although many/most scientists agree that it is probably infinite...but we'll probably have to wait for further, more detailed measurements of the cosmic background to say this with the 7-sigma confidence which is the 'gold standard' of science. These measurements are currently only limited by the quality of our instruments...and our optics are always getting better...humans excel at optics.). It is assumed that this volume is filled with galaxies arranged pretty much as seen here (and throughout the observable universe), and with the same overall density/quantity...though the WMAP has uncovered a rather odd 'dent' and 'lump' on one side of the cosmic background (one large patch is slightly hotter, and one slightly cooler. This doesn't affect the overall flatness of the cosmic background, as they cancel out in the overall average). This hints that there might be even larger galactic structures (super-mega clusters!) lurking just outside our observable volume (the reason we can't see that far, beyond the 'observable', is that at a certain velocity (corresponding to greater distance x hubble constant), even the most potent of gamma rays get red-shifted off the bottom of the spectrum...and those lower-energy photons are also more easily absorbed by intergalactic and interstellar dust.)
posted by sexyrobot at 10:56 PM on June 19, 2013

Can you give a basic (college sophomore's) outline of 1)what "flatness" means in the context of CBR. Is it the same as uniformity? And 2) how does that determine/constrain the size of the Universe? ... Maybe just name-check the theories/equations involved? ... Cause I do dig this jam.
posted by benito.strauss at 11:12 PM on June 19, 2013

benito.strauss: When talking about the Universe, "flatness" actually refers to its geometry: in a flat Universe, Euclidean geometry is right - parallel lines never meet, the sum of the angles of a triangle is always 180 degrees, etc. General relativity allows space to (on average) be curved towards or away from itself instead, so that the sum of the angles of a triangle is more or less than 180 degrees, etc. See this image from the WMAP team for an illustration of how geometry works in two-dimensional curved spaces (space for us is three-dimensional, but it's really hard to visualize the three-dimensional surface of a four-dimensional sphere, which is the sort of geometry we're talking about).

If space were curved towards itself, by measuring how quickly it curves near us we could determine the radius of curvature, and hence the overall volume. Think of the surface of a balloon: if you measure how curved the surface near you is, and it's spherical, you can derive the radius of the balloon. If the geometry of the Universe is perfectly flat or curving away from itself everywhere, it generally has to be infinite. Since the Universe is measured to be pretty flat, that implies the radius of curvature must be big if not infinite, leading to the sorts of numbers sexyrobot is talking about.

If you want to see the actual calculations, they start with the Friedmann equations (which basically are the application of General Relativity to a universe that is assumed to be constant in composition everywhere) and go from there.

PS Though the edge of the observable Universe indeed corresponds to infinite redshift, as sexyrobot describes, I think an easier way of thinking about it is how far away from us a region whose light left it at the beginning of the universe (13.8 billion years ago) but is just reaching us now actually is today. That's not the same as 13.8 billion light years because those regions used to be closer to us (the Universe is expanding, after all). The farther away we look the further back in time we're looking too (due to the finite speed of light). . .
posted by janewman at 11:31 PM on June 19, 2013 [1 favorite]

PPS sexyrobot: we could never establish that the Universe is in fact flat with any confidence (7 sigma or otherwise). Basically, we can measure the curvature of the Universe as a fraction of its overall density; for a flat universe, it should be zero. However, if we measure 0 +/- 0.01 (the order of magnitude of measurements now), or 0 +/- 0.000000001, you can't exclude the possibility that the curvature is greater than zero, or less than zero, so you can't definitively show that the geometry is flat (you could demonstrate that it's NOT flat, however; for instance, if you measured this parameter to be 0.01 +/- 0.0001, you'd exclude the possibility of a flat universe as well as one curved away from itself).

Indeed, the (highly successful) theory of inflation suggests that ultimately, the Universe is curved one way or the other; it just grew so fast a fraction of a second after the Big Bang that the radius of curvature became ginormous and so it appears flat to any measurement we could actually make (just as the surface of the Earth appears flat in our everyday lives - it's because the Earth's radius of curvature is so much bigger than your local neighborhood, so the surface is effectively flat even though not truly so).
posted by janewman at 11:40 PM on June 19, 2013

If the universe was very, very small at one point, does this imply that the Planck constant was different? If not, what stopped everything from happening at once?
posted by Joe in Australia at 6:45 AM on June 20, 2013

Joe in Australia: we expect the Planck constant to stay the same over all time (though there are people who try to test for fundamental constants changing).

I'm guessing maybe you're wondering about what happens if the Universe is finite and its size is smaller than a Planck length (the smallest length that makes any sense in current theories, basically - you can divide by the speed of light and get a Planck time, which we think is the smallest sensible interval of time). The answer is that all bets are off - all our current theories break down in that domain (we'd have to have a theory that unifies quantum physics and general relativity to even guess at what happens). Similarly, at times less than a Planck time after the Big Bang, we have zero clue how physics works.

Of course, if the Universe is infinite in extent today, it was infinite in the past too, so 'very small' may be meaningless. There is no doubt the Universe was a lot smaller after the Big Bang than it is today, but it still may have been infinite in size even then. . . In that case, worrying about the Planck length doesn't make much sense, but the Planck time still does.
posted by janewman at 9:14 AM on June 20, 2013

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