The size of the universe - a question.

[pErvinalia]

referring to my previous post... To put it another way, how can two points get physically further apart with expanding spacetime, if a metre remains constant? If you are going to say that a metre remains constant, then the question is still - what does it remain constant in relation to? You are going to say the speed of light, presumably. But then that requires that time remains constant. I.e. both space and time remain constant, yet at the same time are expanding. That seems to be a contradiction.

[pErvinalia]

Can you explain the concept of the universe not having a centre? And of course, what is spacetime expanding in relation to?

Well it's common in pop science to think of the early universe as point, but this point exists nowhere except on a graph drawn by people.

The truth is we don't actually know.

Sean Carroll posits the mind-bending scenario that the early universe could've been infinite, and infinitely dense. Then something set it off from this high entropy state to a lower one. In the process it loses density. Becoming less tightly packed. In all places at the same time. Infinity expanded is still infinity.

Something which is infinite can have no centre.

Well the universe isn't generally posited to be infinite in size, is it? And the accepted theory for the big bang was a singularity, isn't it?

[Animavore]

That's a circular argument. You are saying a metre won't change because the speed of light doesn't change therefore a metre doesn't change.

The real question, again, that you keep avoiding, is what is spacetime(?) expanding in relation to? In relation to that, distance (i.e. a metre) and time will change also. If the proposition is that spacetime expands (and in relation to something).

It's not circular. The speed of light is constant. That means it doesn't change. A metre is based on the distance the speed of light makes in a set fraction of a second. That distance can't change without the speed of light changing!

Your "in relation to what" question as your asking makes no sense. Galaxies move in relation to other galaxies, the space between pushed apart by a force we don't understand. The galaxies themselves don't stretch and expand. Metres and planets and people don't. They are held together by other forces.

[pErvinalia]

That's not really making sense. You are now saying that what's really expanding is the distance between things, not spacetime. I can explain an increase in measured distance between two objects without invoking an expansion of space time (two magnets of the same polarity, for example).

[pErvinalia]

I'll take a break from this thread for a while to see whether a comprehensivey description of how all points in space can see up to about 13 billion light years away can emerge. I'd like to see if anyone else has an explanation that comes across perhaps in a clearer way.

[Animavore]

Read a book.

[Animavore]

To understand what we actually have evidence for, you have to understand a little bit of the history behind big bang cosmology. It is a deep misunderstanding to think that the classic big bang theory has some kind of special evidential support over other contemporary theories. It doesn't, it simply came first and primarily for that reason, has been the most cited and most generally accepted theory for the early universe and it's origin.

http://en.wikipedia.org/wiki/History_of ... ang_theory
This gives a decent overview of the history of big bang cosmology, why it was invented in the first place and what it says. This is important, because understanding the history behind the different ideas tells you what evidence lead to their proposals.

Basically what happened was that two independent lines of evidence implied that the universe was smaller (not that it had an ultimate beginning) in the past, than it is now.

1) One was that Einstein's general relativity implied that space had to be either expanding or contracting, it would not remain static. Is this actually evidence that time began? Nope.

2) The other was the observation that galaxies are moving way from each other at a speed proportional to their mutual distance. Having observed this, Edwin Hubble all the way back in 1929, put the observed redshifts of distant galaxies in a logarithmic coordinate system and drew a straight line through them. The obvious implication of this line was that everything had to have been closer together in the past. How close together is of course the big question and it wasn't lost on anyone.

Basically what happened then, was that physicists went with Hubble's graph in one form or another (Hubble got the rate of expansion off by about 10%, which wasn't too bad), and simply extended the line so far back in time they could until they arrived at a point. Literally, everything would have had to have been collected into a single point infinitely small. Now obviously, the drawing of this line all the way back to a time when the universe would have been contracted into a single point, is fundamentally an extrapolation and nothing more. Extending your data points beyond what you observe is to extrapolate, by definition.

Anyway, having done this, physicists then went on to ask themselves: "supposing space was really fucking small, hot, and dense in the past, but subsequently expanded to the size we see today, what would we expect to see?".

They then went calculating, using their understanding of the physics of atoms and the very small (quantum mechanics and atomic physics) and arrived at two major predictions:
They would expect to see a specific distribution in the elements available in the early universe. See: http://en.wikipedia.org/wiki/Big_Bang_nucleosynthesis

They would expect to see a more or less uniform band of radiation, emanating from all directions, of a certain temperature in the microwave range. The cosmic microwave background radiation. See: http://en.wikipedia.org/wiki/Cosmic_mic ... background

These were at the time the two major predictions of ... what, exactly? A hot and dense state - hot and dense enough that at one point, all matter in the universe was in the form of high-energy radiation.

While initially the idea had been to extend Hubble's law all the way to a singularity (the infinitely small point of infinite temperature and density), there is no requirement that there ever was a singularity (this is still just an extrapolation), in order for us to be able to explain that the universe was hot and dense enough for all matter to have existed in the form of radiation.

We have no actual evidence that the universe was a singularity, that it was that small. We have no actual evidence of an ultimate beginning. The evidence we have is the cosmic microwave background radiation and the distribution of light elements in the early universe.

Now, since that time, other lines of evidence has been added, still supporting that the universe was hot and dense enough to create the CMBR and the distribution of light elements, such as early galaxy formation and the large-scale structure of the universe (how galaxies are arranged into clusters, which are spread out more or less in a "network" pattern). http://en.wikipedia.org/wiki/Big_Bang#O ... l_evidence

None of this evidence implies that the extrapolation all the way to a singularity is justified. Not to mention that to go a step further and suggest that at some point, there wasn't even any singularity at all, but a philosophical void of non-being, is to make shit up wholecloth.

What is even worse, we know that general relativity, the very theory of spacetime that predicts that the universe should not remain static, actually breaks down under sufficiently hot and dense states. This means general relativity is incomplete, something is missing. So we can't use general relativity to argue that there was a singularity, or that time has an ultimate beginning.

So when we're talking about the classic big bang theory, and the evidence for it, it's important to be specific and say what we mean. We can say there definitely was a big bang. What does this mean? It means that the universe was certainly extremely small at one point, and that all observed matter and energy was packed into this very hot and dense state - and subsequently expanded very rapidly while cooling off. Where it came from, whether it was even smaller still, we simply do not know. That last extension of the line in Hubble's coordinaty system, back to a supposed t=0, is an extrapolation. It isn't actually required for the theory to work, and there are many other suggestions that propose entirely different scenarios for the early universe. Early meaning the time before the formation of the CMBR and big bang nucleosynthesis.

Here comes the important point: The competing models to the classic bing bang theory (with it's singularity extrapolation) PREDICT THE SAME THINGS, POST-PLANCK-TIME, AS THE CLASSIC BIG BANG THEORY DOES:
1. The CMBR.
2. Distribution of light elements.
3. Large scale structures in the distribution of matter (galaxies, clusters, superclusters etc. etc.)
4. Galaxy formation etc.

So in this respect, they all have equal amounts of evidence for them. Because they all explain what we observe, at least to the degree of accuracy we have done the observations. There are differences between them, but they require even better and more accurate observations of the patterns in the CMBR and stuff like that, than current instruments have been able to detail.

So, since we have multiple different hypotheses that all explain the same set of observations, should we then really believe any particular one? No, we shouldn't. The classic bing bang model with an extrapolated singularity, was simply the first model on the scene. It made sense to do at the time, to draw the line through the data points and simply extend it all the way into a singularity and then ask that "what if?" question. But that is all it is, a line on a piece of paper. It is not itself evidence of a singularity. To have evidence of a singularity you'd have to derive predictions unique to a singularity and observationally confirm them. We have no such evidence.

So the state of modern cosmology is this: Shut up and calculate. Come up with theories, see if you can derive testable predictions. If you can, test them. We don't know what happened before the planck time, it's that simple.

But cosmologists are human beings, and some of them are religious and like everyone else, have their biases and their pet theories. And then there's religious apologists, who in their disengenous ways, like to quote cosmologists who support the classic big bang theory for whatever reason (either because it's the one that came first and therefore the one they first came to know about and all that, or... ), and through that try to present a false picture of some sort of consensus among cosmologists.

It's in effect an appeal to authority fallacy. But it's even worse, because their authority is synthetic, there is plenty of competition with the classic big bang extrapolation. Craig and company like to quote that shitty Barow and Tipler book endlessly, as if it constitutes an extant consensus on modern cosmology, and even worse, as if some kind of consensus on an extrapolation is evidence that that extrapolation is justified. Quick hint: it isn't. Either there is empirical evidence for it or there isn't, and there isn't.

So what should you believe? At this point in time, only that the universe was very small, hot and dense, about 13.8 billion years ago, and subsequently expanded very rapidly, cooled off, matter started forming etc. etc.
That's it. We don't know what happened before this, there are many competing models. Heck, we already know our current models all have major holes in them. Who predicted dark matter and dark energy? Noone did, which means our understanding of nature at cosmological scales is seriously lacking. So this isn't the time to start taking bets and believe hype by religous apologists. We have to wait for the evidence to decide.

That evidence won't be coming from philosophers. And it most certainly won't be coming from religious apologists. It is pretty obvious why people like Craig are so enamored with the classic big bang model, they think they can bend it to support their theistic views. Of course, even THAT I would take issue with, because even if the universe was a singularity, it doesn't follow it came from nothing. But that's a discussion for another time. I hope this clarifies the subject a little for you.

Edit: spelling.

[pErvinalia]

Read a book.

I've read heaps. They are by physicists, who are second only to mathematicians at being unable to explain things in regular speak.

[Animavore]

Read a book.

I've read heaps. They are by physicists, who are second only to mathematicians at being unable to explain things in regular speak.

Then read this one.

https://www.amazon.co.uk/Big-Bang-Impor ... 0007152523

It's by a science journalist.

[Alan B]

As I understand it, with my limited number of brain cells, we can observe the universe limits to be 13.7 billion light years in all directions. Thus giving the illusion that we are at the 'centre' of this universe (the God-botherers will love that one) and that the universe is 13.7 billion years old. If we are in fact not at the centre, but say, a third of the distance away and the observation limit is still '13.7 billion light years in all directions' then the questions become: What governs this limit? and How old is the universe, really?

If we observe that the universe edge is 'x' billion light years in one direction and 'y' billion light years in the opposite direction, then that would be an entirely different ball-game. We would know where we are in the universe - at the moment we haven't a clue.

[Scot Dutchy]

Is there only one universe? Why should there be?

[pErvinalia]

Because one is more than enough!

[Alan B]

Any observers in other universes might have the same problem...

[Scot Dutchy]

I have heard the number is infinite.

[pErvinalia]

You been talking to Galaxian?