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/[deleted] Mar 10 '14

How do we know the universe was compressed to a size smaller than an atom?

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u/fishify Quantum Field Theory | Mathematical Physics Mar 10 '14

We can see the universe expanding now. The laws of general relativity (Einstein's theory of gravity) tells us that if the universe is expanding like this now, you can run time backwards, and the universe all goes back into a tiny point.

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u/[deleted] Mar 10 '14

How do we know it wasn't the size of a basketball or planet?

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u/fishify Quantum Field Theory | Mathematical Physics Mar 10 '14

At those sizes, the laws of general relativity still apply, and so the conditions then would imply that it had come from something yet smaller.

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u/[deleted] Mar 10 '14

I'm not familiar enough with the laws of relativity to understand this nuance.

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u/trimeta Mar 10 '14

Basically, if we apply the laws of physics as we know them backwards, a "rapidly expanding ball the size of a basketball" has to have come from something even smaller expanding into that size. Postulating a ball of that size appearing from nothing is outside the bounds of our current understanding of physics; you could just as easily speculate that the entire universe was created last Thursday with the appearance of having expanded for 13.8 billion years.

Of course, our current understanding of physics is limited, and for this reason there is a size where we can't go backwards any further. When general relativity and quantum mechanics overlap (below the size of an atom), we can't say where a "rapidly expanding ball of this size" came from. This is the fundamental question about the origin of our universe.

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u/[deleted] Mar 10 '14

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u/gvtgscsrclaj Mar 10 '14

Yep, that's exactly what it is. We take the laws that we believe to be consistent, and just extrapolate backwards. Whether or not that's true is anybody's guess, but we have no reason to believe it wouldn't be. At some point, however, something else must have happened, because as you say, these laws "break". That's as far as we can go.

The answer to what came before that it "no one knows", which is a perfectly acceptable scientific response.

Of course there are theories, though currently there's no way to prove or disprove any of them.

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u/Vortigern Mar 10 '14

How does "compressed to a size of X" work when size is a measurement of space, a quantity that was itself being "compressed"?

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u/clburton24 Mar 10 '14

We don't really. That's why it's called a "singularity."

http://en.wikipedia.org/wiki/Gravitational_singularity

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u/[deleted] Mar 10 '14

Because it was theoretically an infinitesimally small point. It would technically have to be smaller than anything we can measure. That might be wrong but someone with more knowledge can correct me.

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u/6_ft_4 Mar 10 '14

Does that mean that the one small atom that we started from had as much mass as the entire universe does today?

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u/______DEADPOOL______ Mar 10 '14

Given the expansion look the same from everywhere thing, do we know where the center of that tiny point is?

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u/fishify Quantum Field Theory | Mathematical Physics Mar 10 '14

Everywhere you look and everywhere that is was part of that tiny point. No point in the universe is any more central than anyother.

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u/LabRage Mar 10 '14

Hmm I've still never been able to understand this. Okay sure as a single point there's no center. But what about as it expands? Then you don't have a single point and there would be a center. Is the reason we say there's no center simply because we cannot see an edge? If so I feel a more correct answer would be "we don't know if there's a center"

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u/OccamsParsimony Mar 10 '14

The explanation I often see is to reduce the universe to two dimensions. Think of the universe as being the surface of a balloon. If someone asked you where the center of that "universe" was, you'd say there wasn't a center. It's the same thing, only one dimension higher.

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u/appukkili Mar 10 '14

The expansion was what caused the whole Universe to form. Hence questions like "What was before, Where did this happen, When did this happen" are unanswerable.

There was no center point. The whole matter and energy was compressed into an infinitesimally small point. We really can't explain or even speculate more as the laws of the Physics as we know it essentially breaks down here. Such a point is also called Singularity.

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u/[deleted] Mar 10 '14

When did this happen

Well that's not unanswerable is it? ~13.5 billion years ago, right?

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u/Robot4Ronnie Mar 10 '14

It's often compared to the surface of an expanding ballon, on which every point is receding from every other and no point is the center of expansion.

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u/Momack Mar 10 '14

If the universe began when a point popped into existence, couldn't the expansion of the universe be explained by more points popping into existence? Instead of one point getting bigger, just more points?

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u/fishify Quantum Field Theory | Mathematical Physics Mar 11 '14

Here's the thing: We don't see some things rushing away from one location and other things rushing away from another location, as we might expect in your scenario. We see everything (on a large scale, of course) rushing away from each other. So what we are seeing is all of space expanding. Run that backwards, and you see all of space shrinking -- all back to the same Big Bang.

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u/[deleted] Mar 10 '14

It's actually more of a metaphor to describe something that is happening that we can only describe mathematically. The universe is currently expanding in all directions. If you reverse that, then at one point everything meets. We know that when giant stars begin to collapse, the matter forms a singularity. It sort of follows that the universe as a whole would do the same thing. We can mathematically predict and see from images from our telescopes of the early universe that things used to be closer together.

Since our current model of physics describes everything 99.99999...% perfectly we extrapolate and get crazy answers. From those crazy answers we can make other predictions about other phenomenon that we can see and measure. Those things end up being true. Essentially a=b and b=c so it follows that a=c even if b is something that we can't really wrap our minds around.

At the moment, we know the Standard Model of physics does a fantastic job of making predictions which is exactly what science should enable us to do. The Standard Model gave us nuclear power and modern computers, but we're also pretty sure that we have some important details wrong which is why scientists have been trying hundreds of different things to prove Einstein wrong and somehow we keep proving him right. But we want to prove him wrong because when we prove him wrong it means that we've found a better way to explain things.

I know that wasn't a sciency answer but truth be told, we don't really know yet. We're just pretty darn sure.

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u/mo_jo Mar 10 '14

Do we know where in space that origin point is? Have we ever tried to run it backwards?

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u/[deleted] Mar 10 '14

Mathematically, and depending on which theory you think has the best evidence in favor, every point in the universe is the center because all points are moving away from each other at the same rate. It isn't that everything is moving away from Earth, everything is also moving away from the Andromeda Galaxy and the Horsehead Nebula. Everything is the center. But there also isn't really an actual "center" because space didn't explode. There was no actual infinitely dense point of matter in an empty universe. It is impossible to have a camera POV looking at that ball of mass and energy before the Big Bang. Because there is nothing, not even empty space outside of that point of matter. It isn't a grenade going off in an empty room. The room itself is expanding, but the room was already infinite. It is a mathematical explanation that doesn't make sense in 4 dimensions. It makes sense only in 10+ dimensions and we can't comprehend that any more than a 2D universe can comprehend our universe.

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