Talk:Big bang

"distant galaxies can actually move apart faster than the speed of light"

I admit that I am not a physicist, but this really surprises me. Is this actually true?

Yes - nothing can travel "through" space faster than the speed of light. However if the space itself is expanding then two objects which are at rest (relative to their local environment) can move away from each other at speeds greatly in excess of the speed of light. To conceptualise this - imagine a balloon with dots drawn all over it. Blow up the balloon. Now none of the dots have moved relative to the balloon itself, yet they are now further apart from each other. Read Lawrence Krauss's book "The Physics of Star Trek" - he explains this beautifully (exploitation of this was the justification for how the warp drive worked) - MMGB

In the observable universe (that sphere around us of radius n light years, where n is the age of the universe in years) I do not think it is possible for two objects to recedes faster than the speed of light. Due to the initial inflation the universe is much larger than the observable universe and perhaps the example here is the relative motions of galaxies that are farther apart than n light years and so are not mutually observable -- however such objects can never be seen from each other and in generaal it is impossible for either to have any influence on the other ever (I think). --Eob

Eob - you're absolutely correct. Take Galaxy A (at the edge of our sphere of light perception, and receding at just below c, relative to us) and Galaxy B (likewise at the edge of our sphere of light perception, but diametrically opposed to Galaxy A, with the Earth at the centre of the imaginary line connecting all three). The light from Galaxy A and B is travelling at c and hence we can just perceive each. But relative to each other, they are receding at the linear addition of their recession velocities". They would not ever be able to perceive each other. Hence the "observable universe" of Galaxy A would have the earth at the extreme fringe of it (or "where the Earth was going to be when it formed" relative to their timeline) and nothing beyond it. Galaxy B would not exist, nor would it have ever existed, in their reference frame. Likewise, there is probably a Galaxy C beyond Galaxy B that we cannot perceive, and never will.

Even if C keeps moving away from us faster than c, there is a priori no reason to assume that we will never see C. Think of the light emanating from C as an ant walking towards us on a rubber band extending from us to C. The ant is walking at speed c relative to the rubber band, the rubber band is stretched, but the ant will still reach us eventually (unless you keep stretching faster and faster, in an exponential way, which is not what galaxies do). --AxelBoldt

The definition of our "observable universe" is "all the stuff that is moving away from us at less than the speed of light, hence we can still see it. Some versions of the inflationary hypothesis speculate that we can only see 1 millionth to a billionth of the real cosmos. Not that it matters, we cannot (by definition) ever verify it one way or the other (barring the discovery of wormholes, and let's NOT go there). - MMGB

No, objects in the observable universe can be receding faster than the speed of light. It works because what we see lies in the distant past - just because the objects are too far away for light to reach us now, doesn't mean they were when the light was emitted.

I see. But what does "now" mean? Under relativity there is no concept of simultaneity.

Simultaneity is irrelevant - we are only considering our reference frame. "Now" means "our now".

Given our now, it still appears to me to be impossible to talk about galaxies receding faster than the speed of light. I still think that this statement should be removed from this article, unless we have an astrophysisist in the house who can clarify this point.

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Again I have to prefix this with "I am not a physicist and I don't play one on TV", but...even with the "balloon" analogy, I am having a hard time believing that it is possible for the dots on the balloon to move apart faster than the speed of light under the constraints of relativity. The fact that they are locally "at rest" doesn't really mean anything anyway, does it? After all, in Einsteinian space-time there is no absolute sense in which anything is at rest anyway.

I am not saying that it is wrong that galaxies can move apart faster than the speed of light--but it is completely alien to my (admittedly limited) understanding of relativistic physics. Egern

I'm not sure of the which is right, but perhaps this issue is being conflated with the apparent superluminal velocity of distant galaxies? [1] (http://www.public.iastate.edu/~physics/sci.physics/faq/superluminal) -- DrBob

Nah, it's quite definite that some galaxies are receding at true superluminal velocities. This works because limits on relative velocities - you're quite right that local rest doesn't come into it - only apply to objects starting at the same point in space and time. The possible paths of an object there make up a cone, called the point's future, and contain no relatively superluminal paths. In flat static spacetime all cones are oriented the same way, so the same applies, but with the expansion of the universe the cones are relatively tilted. Remember relativity is a local theory. -- Josh Grosse

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Replaced the following

Although the Big Bang Theory is widely accepted, it probably will never be proved; consequently, it leaves a number of tough, unanswered questions.

with

The Big Bang Theory is now the accepted scientific view of the origin of the Universe.

because it gave the wrong impression -- no scientific theory is ever fully "proved". Statements like this just gives fodder to the creationists and their ilk. (Woops, stepping a little outside NPOV there). Perhaps the original author could elaborate on the "number of tough, unanswered questions" -- Eob

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I changed the wording a bit. I'd say that the big bang model is accepted by about 98% of the cosmologists out here, but there are a few here and there who don't accept it, and there are a few models here and there which attempt to challenge big bang. I don't think that they will get anywhere, but they exist. I also added some text about cbr. -- Chenyu

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Here is a reasonably well-written (though VERY biased) web page regarding "opposition" to the Big Bang theory. The author is well informed (which is not to say correct). Even though I personally think he's way off-target, it still merits reading and critical consideration. [2] (http://www.angelfire.com/az/BIGBANGisWRONG/index) - MMGB

For the record, the stars older than the universe thing was more or less resolved a while ago. The linked page doesn't appear that well informed, I'm sure we can do better.

OH absolutely, but it provides a good launching point for addressing the criticisms. The pages presents an appallingly distorted view of the evidence. - MMGB

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Why is this article at Big_bang rather than Big_Bang? The article itself consistently writes "Big Bang". --Zundark, 2001 Nov 20

Half an explanation - I wrote most of the article, and used "Big Bang" because that seemed proper to me. I'll ask LMS to make the call. - MMGB

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Someone called for an astrophysicist?

O.K. It's possible for two galaxies to be moving with respect to each other faster than the speed of light. Imagine a string of galaxies

A B C D E F G H I J

Now suppose that A and B are expanding from each other at 0.2c. Then A and C are going to expanding from each other at 0.4c. A and D are going to be expanding from each other at 0.6c. Eventually, you will have two galaxies that are expanding from each other at more than c. Another way of thinking about this is to use the Hubble law.

   velocity = Hubble constant * distance

If distance is large enough, velocity will be more than c.

This does not violate special relativity. Special relativity says that if you can send information faster than light, then you it is possible to have a path in space time that is a loop. This causes problems since it means that you can meet your father and shoot him before he has kids. This isn't a problem with Hubble expansion since if two galaxies are travelling faster than the speed of light with respect to each other, it means that a signal from one will never reach the other.

This actually causes problems with the big bang model and is the reason that people now believe that there was a hyper-rapid expansion of the universe at an early stage. The problem is that if galaxies A and H can't sense information to each other than how do galaxies A and H manage to have the same temperature and emit the same amount of cosmic background radiation. Answer: at some distant time in the past A and H were close to each other so that they could exchange information and end up with the same temperature. Then there was a rapid expansion and A and H could no longer communicate with each other. You can then play some more games and estimate the amount of lumpiness in the cosmic background radiation.

-- Chenyu

When you just add speeds together, you are using Newtonian mechanics for computing the totals of velocities as objects move apart. Based on what I've been able to find on the subject, in relativistic mechanics, you don't do a simple addition of speeds. I did a little lookup on the net to find an example of the math involved, and came across the following page: http://www.public.iastate.edu/~physics/sci.physics/faq/velocity . I haven't done this kind of math since my college days, so I'm pretty rusty, but the implications are clear even to me--you can never add the speeds of receding objects together such that they will move apart faster than the speed of light.

Yes, but they objects are not "moving apart at the speed of light" as such. However the space between them is expanding at a rate determined by the Hubble Constant (which is not yet precisely known, but is somewhere between 50 and 70 kms per second per 3.26 million light years.) There is a (bad) habit of saying that the objects are "moving". They are only "effectively moving, relative to our position, due to space expansion". Relative to their own reference frame, the galactic objects are moving at the (trivial by comparison) speed of several hundred kms per second.

Everything you are saying about addition of velocities under SR is correct, but SR goes out the window in this case, we have to consider GR. I will repeat - Special Relativity does not apply in this case. Special relativity deals with space as being "flat" (hence it is called "special relativity" as it deals with a special case). To be fair. one needs to examine a region of space of over a billion light years in diameter before SR even begins to get fuzzy. (Space is expanding at somewhere between 1000 and 1400 kms per second at this distance, and so the GR effects start to become noticable). - MMGB

One other nit. I get really annoyed when people say that special relativity proves you can't travel faster than the speed of light. The situation is

1) if special relativity is correct, and 2) if it turns out that you can send information faster than the speed of light,

then

3) from someone's point of view you are sending information back in time

Sending information back in time is a bad thing since it leads to all sorts of theoretical problems (what happens if you send a message to kill your father)? Unless we have evidence that someone is sending messages back in time, we'd rather avoid the problemby assuming that you can't do (3). Special relativity *seems* to be a good description of reality so we want to keep (1). That means the only thing left is to kill assumption (2). Of course, reality might have other ideas.

One consequence of this is that you can have "things" move faster than the speed of light, provided that those "things" don't exchange information.

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But I think special relativity also says that massive objects cannot travel faster than c, since their energy approaches infinity as they reach speed c.

Also, when you say above that A and H cannot send information to each other: is that really true? Couldn't a light beam emitted from H eventually reach A, like in my "ant-on-a-rubber-band" example above? --AxelBoldt

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What does it mean to have "emerged spontaneously"? I just looked up spontaneous in the dictionary and I'm not sure which definition would apply to the emergence of the universe. My guess is that, there might be a better word for what is meant here. - Tim

Sometimes human language with its inherent notions of past-present-future makes it hard to understand concepts of space-time. One way to think of it is to use an analogy in lower dimensions: Imagine the universe as a one-dimensional curve. We can move back and forth along this curve like beads on a string. The observable universe is a long segment of this curve with us at the center, but on an even larger scale this curve forms a closed horizontal circle. (The objects shuttling back and forth on the curve have no way of moving "sideways" into the two-dimensional horizontal plane.) As time evolves this curve gets larger as the universe expands. Now think of time as a third dimension going up perpendicular to the circle. Now you can think of the circle as a horizontal slice through a three-dimensional surface that is like a big cup that gets wider as you move up. Objects moving back and forth on the circle appear as lines zig-zagging up the curve of the cup. The beginning of time is the curved bottom of the cup. The Big Bang is just the lowest point on this curve. There is no concept of any thing "before" the Big Bang, the bottom point is just one point in a self-consistent space-time cup structure. --Eob

As far as I know this model Eob mentions was developed by Hartle & Hawking. It is explained in depth in Paul Davies book "The Mind of God, pp 63-66. As a simplification of Eob's description, we are so convinced that time moves in a linear fashion that we forget that this is not necessarily so. With the intense gravitational effects of the early moments twisting all definitions of space and time, it becomes mathematically meaningless to talk of time having "A beginning". Think of time as behaving in a 1/x fashion, approaching zero ever more slowly. (This is not what Hawking is saying, but I find it helps to use simple mathematical analogies).- MMGB

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Claiming that the energy or mass of an object increases to infinite is a bad way to explain special relativity. A better way is just to say that light is always moving at the speed of light, so no matter what you do, light is always moving at the speed of light away from you.

Similarly light is always moving from the galaxy at the speed of light, and if that galaxies is moving away at 1.3 c, that light will never reach you.

-- Chenyu

That's not true. The light is moving away from the galaxy at c, but will still be moving towards you at c in your reference frame, so will end up reaching you.

Sorry - it is true. The reason that the galaxy is moving away from you at supra-light speeds has nothing to do with the galaxy itself (which is moving slowly, relative to its own reference frame), it is because (relative to our position) the intervening space is expanding at supra-light speeds. Hence the distance the light must cross is also expanding at supra-light speed, and the light will thus never close the gap. People keep trying to include Special Relativity in this discussion but it is irrelevant, as SR only explains how objects move through space, and has no bearing on the geometry of space itself. - MMGB

The mathematics does not distinguish between objects moving through space and objects carried along by space, it simply describes how the observed distances and times change in each frame. The idea that the light can't close the gap doesn't take into account the expansion is proportionately smaller when you get closer to us. I'm pretty sure that there are Friedmann models where you can see objects receding at superluminal speeds.

I think we can settle this if you cite a literature reference to a Friedmann model where you can see superluminal speeds. Offhand, I don't see how this would work..... - Chenyu

Nov 21 - I've posted to sci.astro regarding this - MMGB

I had that same question once and also posted to usenet, and here's what I remember:

• special relativity constrains local speeds: you can never see any massive body whizzing by faster than the speed of light, c.
• this does not contradict the fact that some distant galaxies are moving away from us faster than c, something that is predicted by some cosmological models in general relativity. The space in between expands.
• light's speed is equal to c locally, which means if a photon whizzes by, everybody will measure its speed to be c.
• this does not contradict the fact that a photon emitted by a galaxy which moves away from us faster than c can eventually reach us. Locally it will always travel with speed c, the space between the galaxy and us expands, so the photon is exactly in the situation of the "ant-on-a-stretched-rubber-band" that I described above. Now, it depends on the particular cosmological model; in some models, some photons shot in our direction will never reach us. In the cosmological models that are currently en vogue, for every galaxy, there is a time when we will see it, we just have to wait long enough. The observable universe constantly enlarges and captures more and more galaxies as we speak. Note that this does not mean that there will be a time when we can see all galaxies at once.

Here are the Google links: http://groups.google.com/groups?hl=en&threadm=xzqoh1dtok9.fsf%40uni-paderborn.de&rnum=19 and http://groups.google.com/groups?hl=en&threadm=AXEL.98Jan20204156%40euler.uni-paderborn.de&rnum=17 --AxelBoldt