asterroc: (Astro - H-alpha)
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A FB comment just reminded me my head was totally blown yesterday in cosmology.

When I teach Astro 101 I tell my students that we know about the Big Bang because we see that things are moving apart from each other right now, so this means they once had to be closer together in the past. Kinda like if we're watching a fireworks show, and we look away for a moment and turn back to see a firework after it's already exploded. We see the glowing bits moving apart from each other, and we know that perviously they all came from a big explosion. That's just like the Big Bang.

Turns out I'm wrong.

The rest of this post is based around two graphs, one which I already knew about and one which blew my mind. The text will extensively explain these graphs, hopefully sufficiently well for non-typically visioned people as well. Continue on if you want to learn some cosmo.

Here's a graph I already knew about, with an explanation.
Messy graph, explained in accompanying text
So there's many versions of this graph out there, but I believe this is the first and was from Permutter (maybe with an et al.) 1998. It was a huge deal when it came out, along with some other graphs I'm not going to bother with in this post. Let me explain...

On the horizontal axis is ΩM, pronounced aloud as Omega-M. This is a measurement of how much normal (baryonic) matter plus dark matter exists in the universe - you can think of it as how much stuff with mass there is. If we had ΩM=0 (and nothing else in the universe), that would mean there's no stuff with mass at all (which obviously can't be true). If we had ΩM=1, that would mean we had a "sweet spot" where it was just enough that the universe was flat (triangles' angles would add to exactly 180º) and would barely expand forever. If we had ΩM>1 that would mean the universe would have geometry like the surface of a sphere (triangles would be more than 180º). If we had ΩM between 0 and 1, the geometry would be like a saddle (triangles would be less than 180º) and the universe would definitely expand forever. Right now we think ΩM is approximately 0.3.

On the vertical axis is ΩΛ, pronounced aloud as Omega-Lambda, or sometimes it's written as just Λ, pronounced Lambda. (Here they write it as λ, lowercase lambda, but that's not common today.) This is how much dark energy there is - this is the stuff that we think is making the universe expand faster and faster. How much of it we have would modify what I said above about the universe's shape or geometry, and also its future expansion or collapse. We currently think ΩΛ is approximately 0.7. (The particularly good news is that ΩMΛ=1, approximately, which means the universe is flat, making math easier, and is pretty close being habitable for the longest possible time, yay!)

Now the convenient thing about this graph is the shaded areas and the lines. The red, yellow, and periwinkle indicate the results of different experiments trying to measure both ΩM and ΩΛ; the hot pink is where they all intersect and describes what we think (as of 1998) the Universe really is like. (Modern versions just tighten up that area, to approximately ΩM~0.3 and ΩΛ~0.7, as mentioned before.) The line going diagonally down indicates the size and geometry of the universe: closed means sphere-like geometry and finite in size, open means saddle-like geometry and infinite in size, on the line is flat geometry and barely infinite in size. The line going mostly horizontal but a little up indicates the future/fate of the universe: below the line is labeled recollapse (currently called the Big Crunch), above the line is labeled expand forever and in fact faster and faster (currently called the Big Rip), on the line itself would be barely expanding forever but slowing down (currently called the Big Chill or the Heath Death of the Universe). Based on where the hot pink is, we think our universe is flat and the Big Rip will be the "end" (though it's infinite in time, just no normal matter could survive).

The last region I didn't mention in the upper left is bright green and is labeled "NO Big Bang". I always thought before now that meant that that sort of universe was impossible - the Big Bang never happened, the Universe never formed.

I was wrong.

Here's the second graph which blew my mind, with an explanation.

Scale factor with time for different cosmologies
This one's from "An Introduction to Cosmology", Ryden, 2002. Horizontal axis is time, with 0 being right now (where by "right now", we mean plus or minus a few thousand or maybe million years, so all of humanity's existence certainly counts as "right now", and maybe all of life on Earth). Vertical axis is relative size of the universe, with 1 being the current size and 0 being Big Bang (if earlier than now) or Big Crunch (if after now), so where those curves all touch is right here right now. (And yes, you can have a larger or smaller infinite universe. Size really means... No, you know what, this entry is long enough, I'm not going to explain that part. Deal with it. Look up "scale factor" if you can't deal.)

Looking at the curves from bottom right to top left, we start with an upside down U (or a concave down parabola-ish) indicated with the long dashes. If our universe's expansion is going to slow down, turn around, and end in a Big Crunch, that's what it'd look like (because the size gets smaller again as time goes on). Note that where the curve hits the bottom edge to the left of now tells you how long ago the Big Bang took place. The dotted line is for a Universe starting in a Big Bang and ending in a Big Chill or a Big Rip (going into the future the size gets bigger and bigger, but this particular graph doesn't distinguish between Rips and Chills). Note how in this curve the Big Bang took place a little longer ago than in the Big Crunch curve. Then there's a dot-dash curve, which does a similar thing in the future (a Big Chill or Rip, though the dot-dash line's hidden by the solid line), but if you look at the past you can see that the line is almost flat for a long while ago before it dips down to a Big Bang longer ago. Since this curve is flat for a while, this sceneario is called the Loitering Universe (there was a Big Bang, then the size just kinda stuck around for a while, before it got bigger when approaching today).

The last curve is solid black and is a right side up U (or concave up parabola-ish). This describes the "NO Big Bang" in the bright green area from the previous graph. In the future this ends in a Big Rip/Chill, but if you look back in the past you can see the Universe had a minimum size, but it didn't shrink down to nothing (Big Bang) and instead before that minimum size it was actually bigger. In this scenario, the Universe has existed forever, though long ago it was shrinking, then changed its mind and is now growing.

This is kinda like seeing a firework, except that it never exploded - it used to be a huge big ball of glowing sparks, then they shrank, and then got bigger again. This makes absolutely no sense, so I guess we're kinda lucky that that's not what we're finding that the Universe is actually doing.

So yeah, that was my big HOLY SHIT moment yesterday, realizing what that "NO Big Bang" meant.

Hope you learned a bit of cosmo there. :)

Originally posted on Dreamwidth. comment count unavailable comments there. Comment here or there.

Date: 2013-10-19 03:46 am (UTC)
From: [identity profile] sildra.livejournal.com
This makes absolutely no sense, so I guess we're kinda lucky that that's not what we're finding that the Universe is actually doing.

Does the universe always having existed really make less sense than there being a hard t=0 with "before that" being a concept that can't even exist? I am not in any way questioning any cosmological claims. I'm just saying in terms of basic making sense when conceptualizing a hypothetical universe, neither of those things strike me as particularly making more or less sense than each other.

Date: 2013-10-19 04:01 am (UTC)
From: [identity profile] zandperl.livejournal.com
Maybe it's just me that's lucky then, since I've internalized the Big Bang so much that it makes sense to me. Certainly if we're comparing the Universe to a firework, the only scenario that makes sense is the Big Bang and Big Chill model, since that's what fireworks essentially do.

I realized after posting this (and after writing waaaaay too much) that we're actually in something akin to a loitering universe. If we look at this famous WMAP universe size graphic, there's an inflection point in the size about a third of the way from the dark ages to the present day, and IMO that's a key characteristic in the loitering models even if this one doesn't actually get fully flat.

Date: 2013-10-19 04:35 am (UTC)
From: [identity profile] sildra.livejournal.com
Yeah, I'd kind of wondered about that in your description--the graphs I've seen of the Hubble constant as a function of time (in sophomore-level astro, and department colloquia on cosmology) sounded more like your description of a loitering universe.

Date: 2013-10-19 04:46 am (UTC)
From: [identity profile] zandperl.livejournal.com
Hey, I'm just glad you didn't find a bazillion errors in what I wrote. Not that I'd mind if you pointed them out, I mean I'm glad that they might not actually be there. :-P

Date: 2013-10-19 04:51 am (UTC)
From: [identity profile] sildra.livejournal.com
I'm mostly not comfortable enough in cosmology to assume I'm right and you're wrong, considering you actually study astro, although if anything had been really glaring I would have at least had enough confidence to think to check Wikipedia for backup. I know just enough to explain to friends and relatives what the current big, exciting mysteries in the field are, and not a whole lot more. Nothing in your post particularly stood out as being different from the bits of cosmology I've passively picked up, in any case.

Date: 2013-10-19 04:57 am (UTC)
From: [identity profile] sildra.livejournal.com
Although my naive guess of what "no big bang" would mean is no beginning to the universe, or some other type of beginning to the universe, rather than no universe at all (I would expect conditions where the universe is impossible to be explicitly labeled as that, since the big bang is a theory and the universe is empirical fact, so to me "no big bang" just means some alternate theory instead). But it might just be that my (mostly unschooled) interest in philosophy of physics happens to bias me toward guessing in the right direction in this case.

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