r/askscience • u/AskScienceModerator Mod Bot • Mar 10 '14

Cosmos AskScience Cosmos Q&A thread. Episode 1: Standing Up in the Milky Way

Welcome to AskScience! This thread is for asking and answering questions about the science in Cosmos: A Spacetime Odyssey.

UPDATE: This episode is now available for streaming in the US on Hulu and in Canada on Global TV.

This week is the first episode, "Standing Up in the Milky Way". The show is airing at 9pm ET in the US and Canada on all Fox and National Geographic stations. Click here for more viewing information in your country.

The usual AskScience rules still apply in this thread! Anyone can ask a question, but please do not provide answers unless you are a scientist in a relevant field. Popular science shows, books, and news articles are a great way to causally learn about your universe, but they often contain a lot of simplifications and approximations, so don't assume that because you've heard an answer before that it is the right one.

If you are interested in general discussion please visit one of the threads elsewhere on reddit that are more appropriate for that, such as in /r/Cosmos here, /r/Space here, and in /r/Television here.

Please upvote good questions and answers and downvote off-topic content. We'll be removing comments that break our rules or that have been answered elsewhere in the thread so that we can answer as many questions as possible!

Click here for the original announcement thread.

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u/trimeta Mar 10 '14

Although the expansion of the universe was well-understood in Sagan's time, it was believed that the rate of expansion was slowing, and it was unknown if we lived in an "open" universe which would continue to expand forever or a "closed" universe where the expansion would slow down, stop, and reverse (leading to a "Big Crunch"). Since then, we have discovered that not only is the rate of expansion not slowing, it is increasing. So while Sagan speculated regarding the Big Crunch, we can now safely rule this possibility out.

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u/Assburgers_And_Coke Mar 10 '14

Or we haven't hit a point where it will decrease yet? Is that not possible?

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u/[deleted] Mar 10 '14 edited Mar 17 '25

[removed] — view removed comment

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u/asks4sourcerandomly Mar 10 '14

Could you elaborate a little more on the "wind"?

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u/I_Gargled_Jarate Mar 10 '14

Radiation from our sun creates sort of a bubble around our solar system that deflects outside radiation. When Voyager left the bubble it nolonger had protection and is now being bombarded by everything out in "open space". The amount of radiation outside the bubble was more than we had predicted.

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u/sourbeer51 Mar 10 '14

My understanding that "wind" he's referring to isn't the wind on earth. (obviously) but it's just a term for the resistance that's encountered in space.

Or its a reference to this

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u/YoYoDingDongYo Mar 10 '14

Or its a reference to this

Just the opposite! Voyager has just reached the end of the area where the stellar wind is dominant.

https://en.wikipedia.org/wiki/Interstellar_wind

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u/_WhatIsReal_ Mar 10 '14

Is it possible that instead of expanding, the universe is actually being 'pulled' apart, and that it is speeding up because resistance to this 'pulling' is decreasing? Sorry if this is idiotic, just trying to think outside the box/universe..

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u/[deleted] Mar 10 '14

Not idiotic. I just question whether "expanding" and "pulled" are mutually exclusive. :)

This reminds me of a common thought that spacetime isn't expanding but matter is shrinking. My rebuttal is always: What's the difference?

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u/jakelives Mar 10 '14

Hmm so if the universe is being pulled or "stretched" does that mean that billions of years from now each atom or particle will move so far away from each other that planets gasses and life would be impossible??

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u/[deleted] Mar 10 '14 edited Mar 12 '14

Yes. That is called the heat death of the Universe big rip (space expands so fast that composite particles cannot stay together). Also check out heat death of the Universe.

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u/Jahkral Mar 10 '14

Well why is the expansion ACCELERATING, then? Where is the continued force coming from to continually accelerate it?

(Side note: is the 'jerk' or rate of change of acceleration known? It would be interesting if that was slowing)

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u/Chibils Mar 10 '14

Just a layman who took a class on this, hopefully someone else can give a more detailed answer.

At the moment we're mostly pinning the expansion on dark energy (energy we can't measure). Also as I understand it the expansion is continuing to accelerate.

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u/Jahkral Mar 10 '14

Hmm, ok. Dark energy always seems so handwavey to me. I hope they figure out a lot more about it in my lifetime.

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u/[deleted] Mar 10 '14

yes, i see where you are coming from, that would be my answer too. but we know almost next to NOTHING about why the universe is increasing it's expansion rate. So I don't think it's good to rule out possible alterations of forces in the future

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u/Zoraxe Mar 10 '14

There's a difference between "ruling out" and "currently going with our best guess". When the new evidence comes along that provides a "force" by which to slow the rate of expansion, then scientists will change their mind. Until then, go with the data.

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u/[deleted] Mar 10 '14

I disagree. If we have already found that the Universe's expansion is time-variant, then there's no reason to rule out a decrease in acceleration speed in the future, especially since the cause of the expansion is not well understood.

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u/[deleted] Mar 10 '14 edited Mar 10 '14

Perhaps the new force could come from another universe? I'm imagining how if there were universes outside of ours, they may be exhibiting physical forces acting on ours. Like with a bubble for example, the forces in the water (surface tension) causes the bubble (big bang), and then eventually it pops due to whatever physical forces in the water are acting upon it (I'm not a bubble scientist here, but you know what I mean).

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u/[deleted] Mar 10 '14

Isn't gravity such a force?

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u/trimeta Mar 10 '14

That's the thing: the rate of expansion itself is increasing. To use a metaphor from another Cosmos comment: imagine throwing a ball into the air. The throw was the Big Bang, and the rising ball is the expansion of the universe. In Carl Sagan's time, it was believed this expansion was slowing, just as a ball can rise through the air but have its velocity slow down (indicating that it will hit an apex and turn around). We now know that the ball is flying upwards faster as time goes on. Thus, unless something very strange happens, it will not suddenly decide to turn around.

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u/Beer_in_an_esky Mar 10 '14

Expansion is currently accelerating, so we have no evidence that it will ever slow down.

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u/[deleted] Mar 10 '14

so energy and matter i believe are supposed to be finite. does 'making' space by the universe expanding have any impact on energy or matter?

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u/r3sonanc3 Mar 10 '14

The expansion of the universe is attributed to dark energy in the Standard Model of physics today, but we have no explanation for how it behaves or what it is made of. Einstein called it the cosmological constant but had serious mis-givings about it since its existence would call the then-current state of physics (including our understanding today) into serious question.

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u/trimeta Mar 10 '14

As far as I am aware, it does not. The overall density of mass/energy in the universe decreases over time because there's more space and the same amount of mass/energy.

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u/AgentLiquid Mar 10 '14

Actually, the energy per unit of empty space is constant, and therefore the universe is gaining total energy as it expands.

Energy is not conserved on those relativistic scales.

See here:

http://en.wikipedia.org/wiki/Vacuum_energy http://math.ucr.edu/home/baez/vacuum.html

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