questions about big bang

[uniqueuponhim]

Just a disclaimer before I actually ask my questions:
I am NOT an IDer
I am NOT trying to poke holes in big bang theory and am NOT trying to prove it wrong. In fact, I hold no illusions that I could even if I did want to.
I am simply curious. I do not know a heck of a lot about big bang theory, and just had a couple of questions I would like to have answered. I'm currently a second year engineering student, and I have been fortunate enough to have the opportunity to spend the summer working on my own research project thanks to a government grant, supervised by a professor in my university's physics department, whom I now work for. As I am an engineering student, my knowledge of physics is limited to what I need to know for engineering and what I need to know for my work and my research project (which focus on making semiconductors and photovolatic cells, respectively, from organic molecules, rather than the more common silicon.) I have never taken any classes in astronomy, however I do hold a great deal of interest in the subject. You should also note that as an engineering student, and not a science student, my knowledge of the scientific method is fairly limited as well.

So, on to my questions:
First of all, what is it that separates big bang theory from all of the other hypotheses? What evidence have astronomers found that only big bang theory so far can explain? The extent of what I know is that astronomers have discovered that all galaxies (or clusters of galaxies, at least) are receding from each-other, and from that they have deduced that the universe is currently expanding. But my question is, why is it that the universe neccessarily had to begin as a singularity? Why could it not have been contracting at some point, eventually halting before becoming a singularity, and beginning to expand again?

Second, how are scientists able to look back and determine exactly what was going on as little as 10^-44(I think) seconds after the big bang happened? How are they able to deduce this, and how are they able to be certain that those deductions are accurate?

Third, and this one is a bit tricky - I understand what is meant when it is said that the universe is expanding, but only to a certain extent. Does it merely mean that the distance between any two clusters of galaxies which are stationary with respect to each-other will always be increasing and that there isn't actually a finite amount of space (ie. I could fly off in one direction, and after leaving the "universe" behind, I could just continue flying on into infinity, while the universe behind me continued to expand, and the space separating me from it along with it) or does it mean there is actually a finite amount of space which is ever-increasing, and the universe is actually enclosed. If it is the latter, what happens on reaching the "edge?" Would the universe simply loop in on itself, and you could just keep going in the same direction, and eventually return to the spot you left without changing direction (like the world map in final fantasy games) or is it warped in some other way so that you can never reach the edge, or is it something else altogether?

Finally, I had one last question: If the singularity from which the big bang initiated was purely energy, and all of the matter that existed today came from that energy, then how was the imbalance of matter and antimatter created? Shouldn't there have been exactly equal amounts of each? Where did all of the extra matter come from, or what happened to the antimatter that should have arisen along with the matter that does exist?

Thanks in advance for answering my questions.

[ToSeek]

First of all, what is it that separates big bang theory from all of the other hypotheses? What evidence have astronomers found that only big bang theory so far can explain?

See http://www.astro.ucla.edu/~wright/co...tml#BBevidence .
In short:
- the darkness of the night sky (not possible if the universe is infinite).
- the apparent expansion of the universe
- the cosmic microwave background

Why could it not have been contracting at some point, eventually halting before becoming a singularity, and beginning to expand again?

Conceivably, it could have. But you need to come up with a reason for it to do so and to demonstrate that your model is consistent with the observed evidence. Currently, the existing model of the Big Bang explains most of the appearances very well (though there are still issues to be worked).

Second, how are scientists able to look back and determine exactly what was going on as little as 10^-44(I think) seconds after the big bang happened? How are they able to deduce this, and how are they able to be certain that those deductions are accurate?

This is a matter of producing models based on the appearances and our current understanding of physics. There's no way of telling for sure, but it is possible to come up with scenarios that are largely consistent with both.

Does it merely mean that the distance between any two clusters of galaxies which are stationary with respect to each-other will always be increasing and that there isn't actually a finite amount of space (ie. I could fly off in one direction, and after leaving the "universe" behind, I could just continue flying on into infinity, while the universe behind me continued to expand, and the space separating me from it along with it) or does it mean there is actually a finite amount of space which is ever-increasing, and the universe is actually enclosed. If it is the latter, what happens on reaching the "edge?" Would the universe simply loop in on itself, and you could just keep going in the same direction, and eventually return to the spot you left without changing direction (like the world map in final fantasy games) or is it warped in some other way so that you can never reach the edge, or is it something else altogether?

There are three theoretical possibilities:
- The universe is open and negatively curved (akin to a saddle in two dimensions). Space is infinite.
- The universe is closed and positively curved (akin to the surface of a sphere). If you go long enough in one direction, you'll end up back where you started. Space is finite, but growing (as if the sphere is getting larger).
- The universe is flat and not significantly curved (like a sheet of paper). Space is still infinite. Based on the observations, this is the most likely possibility.

[uniqueuponhim]

First of all, what is it that separates big bang theory from all of the other hypotheses? What evidence have astronomers found that only big bang theory so far can explain?

See http://www.astro.ucla.edu/~wright/co...tml#BBevidence .
In short:
- the darkness of the night sky (not possible if the universe is infinite).
- the apparent expansion of the universe
- the cosmic microwave background

Alright... I suppose the next question would be, at least for the cosmic microwave background part, how does that support big bang theory? (It's fairly obvious for the other two, at least to me)

Why could it not have been contracting at some point, eventually halting before becoming a singularity, and beginning to expand again?

Conceivably, it could have. But you need to come up with a reason for it to do so and to demonstrate that your model is consistent with the observed evidence. Currently, the existing model of the Big Bang explains most of the appearances very well (though there are still issues to be worked).

But what evidence is there that is explained by big bang theory, but not that one? Surely there must be some.

Second, how are scientists able to look back and determine exactly what was going on as little as 10^-44(I think) seconds after the big bang happened? How are they able to deduce this, and how are they able to be certain that those deductions are accurate?

This is a matter of producing models based on the appearances and our current understanding of physics. There's no way of telling for sure, but it is possible to come up with scenarios that are largely consistent with both.

Doesn't tell me much, but I suppose that's the answer I expected to hear anyway.

Does it merely mean that the distance between any two clusters of galaxies which are stationary with respect to each-other will always be increasing and that there isn't actually a finite amount of space (ie. I could fly off in one direction, and after leaving the "universe" behind, I could just continue flying on into infinity, while the universe behind me continued to expand, and the space separating me from it along with it) or does it mean there is actually a finite amount of space which is ever-increasing, and the universe is actually enclosed. If it is the latter, what happens on reaching the "edge?" Would the universe simply loop in on itself, and you could just keep going in the same direction, and eventually return to the spot you left without changing direction (like the world map in final fantasy games) or is it warped in some other way so that you can never reach the edge, or is it something else altogether?

There are three theoretical possibilities:
- The universe is open and negatively curved (akin to a saddle in two dimensions). Space is infinite.
- The universe is closed and positively curved (akin to the surface of a sphere). If you go long enough in one direction, you'll end up back where you started. Space is finite, but growing (as if the sphere is getting larger).
- The universe is flat and not significantly curved (like a sheet of paper). Space is still infinite. Based on the observations, this is the most likely possibility.

So in other words, nobody really knows, but we think it's the latter? What observations are there which point to the latter one being true?
Also, if it were true, just as a sort of side-just-for-fun-question, at what distance from the universe(and by universe here, I mean the collection of matter which comprises it, not the space it encompasses) would it become just a star-like point to the naked eye, and how far from the universe would you have to be for it to be too faint to see at all with the naked eye?

[cran]

In short:
- the darkness of the night sky (not possible if the universe is infinite).

- ToSeek

It is possible, and astronomy proves that all the time... to differentiate 'light' from 'darkness' requires a minimum number of photons from source per unit time, per unit area of light receptor - for an infinite universe to appear as 'light' would also require an infinite time for reception ... longer time exposures by very high resolution receptors are finding more sources of light in what previously appeared to be 'dark' blank spaces...

The other two are based on plausible (though not necessarily unique) interpretations of observations...

But what evidence is there that is explained by big bang theory, but not that one? Surely there must be some.

- uniqueuponhim

It is central to the theory, and is based upon observations of apparent expansion, in turn based upon observation and measurement of redshift.

Second, how are scientists able to look back and determine exactly what was going on as little as 10^-44(I think) seconds after the big bang happened? How are they able to deduce this, and how are they able to be certain that those deductions are accurate?

This is a matter of producing models based on the appearances and our current understanding of physics.

- Q. uniqueuponhim; A. ToSeek

I think it's 10^-43 seconds (Planck era), and it's based on quantum-related theory and lab experiments.

So in other words, nobody really knows, but we think it's the latter? What observations are there which point to the latter one being true?

- uniqueuponhim

The interpretation of the sum of all large-scale astronomical observations... and yes, nobody really knows - that's what makes science fun

...at what distance from the universe(and by universe here, I mean the collection of matter which comprises it, not the space it encompasses) would it become just a star-like point to the naked eye, and how far from the universe would you have to be for it to be too faint to see at all with the naked eye?

- uniqueuponhim

That would depend upon whether one can define a limit to the matter of the universe - a year or so back, some workers announced a diameter of ~78GLy, if that's any help... from there, you need someone with reasonable skills in trig to answer your question.

[ToSeek]

Alright... I suppose the next question would be, at least for the cosmic microwave background part, how does that support big bang theory? (It's fairly obvious for the other two, at least to me)

The BBT calls for a plasma at one point during the formation of the universe that emits radiation at a near-perfect black-body spectrum. This spectrum is attenuated by the expansion of the universe and is detectable today as the CMBR. There is no other theory that I know of that can explain how the CMBR is a near-perfect (within parts in 10,000) blackbody spectrum.

But what evidence is there that is explained by big bang theory, but not that one? Surely there must be some.

I'm not sure. It may just be a matter of Occam's razor: the BBT is the simplest explanation. We see the universe expanding, and if we run the film backwards, it eventually goes down to a point beyond which we can no longer speculate. Additionally, we can model the formation of the universe forward from that point, and, given our current understanding of physics along with certain adjustments (like inflation), we get something resembling our current universe.

Doesn't tell me much, but I suppose that's the answer I expected to hear anyway.

Kind of hard to avoid: there have been books written about this that are a little hard to sum up within the span of a BB post.

So in other words, nobody really knows, but we think it's the latter? What observations are there which point to the latter one being true?

Astronomers have basically taken an inventory of all the matter they can see, directly or indirectly. The resulting total is pretty close to "1", if you define "1" as the exact amount of matter it takes to close the universe. Considering that it could just as well have been anywhere between 10^40 and 10^-40, most cosmologists conclude that odds are the number is exactly 1, else it's just too much of a coincidence that it's so close.

Also, if it were true, just as a sort of side-just-for-fun-question, at what distance from the universe(and by universe here, I mean the collection of matter which comprises it, not the space it encompasses) would it become just a star-like point to the naked eye, and how far from the universe would you have to be for it to be too faint to see at all with the naked eye?

I don't think we really know how big the universe is. We are limited by the age of the universe to seeing about 14 billion light years away in any direction, so the known universe is a sphere 28 billion light years in diameter. You'd have to use trigonometry and your personal definition of a point to decide how far away you'd have to be to diminish the universe to a point. My guess is that at that distance the universe would be invisible to the naked eye, since it's mostly dark empty space, after all.

[ToSeek]

It is possible, and astronomy proves that all the time... to differentiate 'light' from 'darkness' requires a minimum number of photons from source per unit time, per unit area of light receptor - for an infinite universe to appear as 'light' would also require an infinite time for reception ... longer time exposures by very high resolution receptors are finding more sources of light in what previously appeared to be 'dark' blank spaces...

Only because those sources of light are isolated. If the universe is infinitely old and infinitely large, then no matter where you look, there would be a star, and the sky would be a continuous field of them.

[Spaceman Spiff]

I don't think we really know how big the universe is. We are limited by the age of the universe to seeing about 14 billion light years away in any direction, so the known universe is a sphere 28 billion light years in diameter. You'd have to use trigonometry and your personal definition of a point to decide how far away you'd have to be to diminish the universe to a point. My guess is that at that distance the universe would be invisible to the naked eye, since it's mostly dark empty space, after all.

That's not quite right. If the expansion did indeed begin 14 billion years ago, then light from the first proto-galaxies has taken nearly that long to arrive here - let's say 13.7 billion years, if nature needed 300 million years for gravity to do its thing. With the expansion parameters (Hubble's parameter, etc) governing the mass/energy budget and so the expansion history of the "universe", this means that such an object is presently 35 billion light years away. The "distance" you quoted is the so-called "lookback time distance", or c x t_lookback, which can only be an underestimate of present cosmic distance - because the universe has been expanding all the while light has been traveling from "there and then" to "here and now".

And we cannot view the universe to have ever been a point - because we live in that same universe. With light, the best we can do is see to the look-out limit represented by the cosmic background radiation, corresponding to an epoch some 400,000 years after the initial expansion at which photons first began flying unfettered through space-time (instead of in constant interaction with the surrounding matter, as would have been the case prior to this epoch). This "horizon" now has a radius of something like 48 billion light years, but the universe as a whole is easily much larger and may even be infinite.

Have a look at the graph provided at the bottom of my page with cosmology links, and also this page.

[Nereid]

There are many, many sites on the internet which describe, explain, outline, summarise, and otherwise go on about the BBT.

ToSeek has already given one of the best (IMHO), though I'd've pointed to Ned Wright's tutorial rather than the FAQ.

Another good one is the Cambridge Cosmology site. The WMAP cosmology section gives a shorter, simpler summary. If you want to know all about the CMBR, and are comfortable with first year university level math and physics, Wayne Hu's site is excellent (it also covers other parts of cosmology, but not so deeply).

I should also mention the 'size' and 'recession speed' non-issue (in most models which fit the data well, all objects with a z above a certain value are 'receeding' from us at speeds in excess of c). There are several excellent materials on this, from Lineweaver and Davis (I seem to have misplaced those bookmarks just now).

If you have any questions that these resources don't answer for you uniqueuponhim, please ask here!

[iron4]

- The universe is flat and not significantly curved (like a sheet of paper). Space is still infinite. Based on the observations, this is the most likely possibility.

A flat Universe is not necessarily infinite in extent. Infinite is one possibility, but indeed, there are 18 possible topologies, some of them finite. This paper (appeared in Scientific American) talks about this fact

http://www.dushkin.com/text-data/art...31943/body.pdf

[snarkophilus]

Also, if it were true, just as a sort of side-just-for-fun-question, at what distance from the universe(and by universe here, I mean the collection of matter which comprises it, not the space it encompasses) would it become just a star-like point to the naked eye, and how far from the universe would you have to be for it to be too faint to see at all with the naked eye?

I'm not entirely sure that you can separate the matter and the space (but that's not the question). Here's the answer, assuming that from the outside (sic), the universe is a sphere (and that light travels outside, and that it's an Euclidian topology, yada yada yada).

Let's measure the size of the object as an angle, so you can decide how close to a point you want it to be. We'll call that angle theta.

First, there's a nice simplifying assumption we can make. Since we're talking about looking at a sphere that is almost a point, we know that the observer is going to be sufficiently far away from the object that we can consider the thing to be a 2-d circle, facing us. (The lines are almost parallel, and almost reach the "equator" if we are looking from one of the "poles.")

So, we get a nice isoceles triangle with base 2r and top angle theta. We want to solve for the height.

tan(theta/2) = r/h --> h = r/tan(theta/2)

Taking 2*r ~= 70 GLy, as cran suggested, and assuming 1' arc (1/60 degree) is pretty close to a point ( http://www.tedmontgomery.com/the_eye/acuity.html ), we get

h ~= 35 GLy/tan(1/120 degrees)
~= 24 000 GLy

24 000 GLy * 9.47*10^15 m/Ly = 2.3 * 10^29 m

That is a pretty big number.

(Does anyone else hate degrees, and just wish we did everything in radians? I do!)

[worzel]

- the darkness of the night sky (not possible if the universe is infinite).

Nitpick: the universe need only be finte, or expanding to resolve Olber's paradox.

[uniqueuponhim]

A flat Universe is not necessarily infinite in extent. Infinite is one possibility, but indeed, there are 18 possible topologies, some of them finite. This paper (appeared in Scientific American) talks about this fact

http://www.dushkin.com/text-data/art...31943/body.pdf

I just finished reading that article, and I do have a few questions about it:
First of all, why does it have to be either a cube or a hexagonal prism? Why not a dodecahedron or an isocahedron in which the opposite faces are glued together? That would allow for rotations of 72 or 144 degrees for the dodecahedron and 120 for the isocahedron. For that matter, why not a tetrahedron, the simplest of shapes, or a sphere in which every pair of opposite points are glued together, or a cylinder with the same properties for the sides of it as the sphere, and with the two ends simply glued together, or glued together with a certain rotation (and with any possible rotation, since they are circles.) Also, with the cubes, why is it that only one pair of faces could be glued together with a certain rotation? Why not two, or all of the pairs of faces? With that situation in there with two cubes on top of one-another, why could we not add more cubes and make it even more complicated?

Also, why are we automatically assuming that we will be able to see ourselves through one of those glued-together sides? After all, on earth, you cannot get a good telescope and look in one direction and see the back of your own head, and neither could you see the entire surface of the earth from any vantage point; an ant on a torus would not be able to see itself, or even the entire torus from any position. How do we know that the curvature of the universe into 5th+ dimensions that causes this shape to arise doesn't also limit our perception of it, in the same way that the curvature of the earth into the third dimension limits our perception of its surface?

[snarkophilus]

I just finished reading that article, and I do have a few questions about it:
First of all, why does it have to be either a cube or a hexagonal prism? Why not a dodecahedron or an isocahedron in which the opposite faces are glued together? That would allow for rotations of 72 or 144 degrees for the dodecahedron and 120 for the isocahedron. For that matter, why not a tetrahedron, the simplest of shapes, or a sphere in which every pair of opposite points are glued together, or a cylinder with the same properties for the sides of it as the sphere, and with the two ends simply glued together, or glued together with a certain rotation (and with any possible rotation, since they are circles.) Also, with the cubes, why is it that only one pair of faces could be glued together with a certain rotation? Why not two, or all of the pairs of faces? With that situation in there with two cubes on top of one-another, why could we not add more cubes and make it even more complicated?

You'll have to excuse me if this sounds a little rusty... it's been a while...

Anyway, you don't use other shapes because it doesn't make sense topologically. That is to say, other shapes reduce to one of the 18 characterized 3-manifolds, or else are non-Euclidean. It's the "Euclidean" qualifier that really limits how you connect things.

Take your cylinder example, minus gluing the opposite sides. No matter how much you twist it before connecting the ends, because it is continuous, it still ends up equivalent to an ordinary old torus. Gluing opposite sides together creates a different topology altogether. Now there are an infinite number of shortest paths from one point to another. It completely ruins the space, and isn't at all what we observe the universe to be like.

The particulars of how you glue things together normally make no difference. In math, it's the topological invariance that is important. Can you turn one shape into another only through stretching and bending, but without punching or patching holes? This article has added the "no stretching edges" rule to satisfy the constraint that the universe looks essentially Euclidean.

With the cubes, they did glue them together with varying rotations. That's why there are 18 such manifolds. I strongly suspect (though I haven't verified it) that stacking more than two cubes reduces to the two cube case.

In summary, um... it's just the way the math works out. Reading the papers by Nowacki and by Hanschze and Wendt is probably the best thing to do.


As an aside, I'm not so sure about the truth of the universe having a constant topology. It seems to me like the formation or dissolution of a black hole changes the topological character of the universe. Of course, that's just my understanding of what black holes are thought to be....

Also, why are we automatically assuming that we will be able to see ourselves through one of those glued-together sides? After all, on earth, you cannot get a good telescope and look in one direction and see the back of your own head, and neither could you see the entire surface of the earth from any vantage point; an ant on a torus would not be able to see itself, or even the entire torus from any position.

Because the Earth is not a good analogy, because it is not a topological space (if it had black hole-like gravity, and you were right on the event horizon, it would be!). An ant on a torus constructed in our space would not be able to see himself, but an ant in a space that was topologically a torus would be able to see himself. Interestingly, because it would take time for light to propagate around the torus, he'd be able to see himself as he was in the past. Doubly interesting, if he could see himself and count the number of occurrences he saw, he'd be able to determine the minimum age of his torus (unless it underwent a hyper-inflationary period, in which case some light might not have reached him yet). This is kind of like measuring the cosmic microwave background. In fact, he might be able to extrapolate and very accurately determine the age and expansion properties of his universe at present.

Actually, because of the way a torus is built, the number of copies of himself he sees should vary with the angle he looks at, and the distribution of those copies would tell him both the present size of his universe and also how it grew in the past. (Of course, the problem could end up being numerically unstable if the expansion is weird.)

These are very good questions! They provide a lot of food for thought.

How do we know that the curvature of the universe into 5th+ dimensions that causes this shape to arise doesn't also limit our perception of it, in the same way that the curvature of the earth into the third dimension limits our perception of its surface?

The assumption is that the universe is three dimensional. Or, at least, the visible space component of it is. Looking around and not seeing a fourth dimension, I'd say it's a decent place to start.