Time has a beginning?

[Skwinty]

I'm thinking there's one problem with this analogy. Stretching the sheet would change the size and shape of the impressions made by the objects. Which in the analogous situation would imply gravity was changing.

Question: If the energy density of the rubber sheet does not change, then would this argument hold true?

After all, the energy density of the false vacuum does not change during expansion.

 

[Belz...]

The whole point of this conversation - specifically, your OP - is that no one (not even wikipedia) knows what the metric was at very early times. Steinhardt and Turok would like to suggest a specific possibility - one totally unsupported by any experimental evidence, and probably theoretically inconsistent. You like that because you think it "solves" a "problem". Well, what I'm telling you is that there are many far simpler resolutions of that apparent problem.

One is that there is an infinite amount of time, as measured using the appropriate dynamics, between (say) 10^-37 seconds and "t=0". Without knowing the metric and the dynamics we cannot determine whether or not that is the case.

Another is that there is a finite amount of time, but we should excise the point t=0. Then causality in all three senses I mentioned is perfectly safe.

A third is that there is a finite amount of time, including t=0, but it makes no sense to ask about the "cause" of that single event (because as you yourself said, without time one cannot have causality; hence, the beginning of time had no cause).

A fourth is that time in the naive sense we think of it ceases to exist near t=0. I know several mathematical models for how that can happen; the simplest is the Hartle-Hawking instanton. I would say that unless some (unknown) dynamics prevents one from getting near the singularity, this is the most generic possibility. Both time and space believed to be quantities that exist and obey the rules we're used to only on average. When one gets down to very small size and very high energy, those averages cease to be even close to a good description.

A fifth is that there was indeed some kind of bounce, but it looked nothing like the one Steinhardt and Turok like so much (since in fact there is very little motivation for their particular choice).

I could probably think of more...

Isn't physics grand ? Thanks, again, for a very interesting post.

 

[Cuddles]

I'm thinking there's one problem with this analogy. Stretching the sheet would change the size and shape of the impressions made by the objects. Which in the analogous situation would imply gravity was changing.

That would be true for a real rubber sheet, where stretching it in one direction affects how hard it is to stretch it in another. For the analogy you have to assume an infinitely stretchable rubber sheet, so that stretching it doesn't alter how much force is needed to stretch it further.

Although in all fairness I should point out that there are far more than just one problem with this kind of analogy.

 

[Tubbythin]

That would be true for a real rubber sheet, where stretching it in one direction affects how hard it is to stretch it in another. For the analogy you have to assume an infinitely stretchable rubber sheet, so that stretching it doesn't alter how much force is needed to stretch it further.

Although in all fairness I should point out that there are far more than just one problem with this kind of analogy.

Fair points.

 

[Perpetual Student]

Could emotional attachment and historical inertia be a major factor in preferring the inflationary model over the cyclic model? Steinhardt and Turok make the case that their model is less cumbersome, with no ad hoc piecing together of unrelated theories. In contrast, the cyclic model has an overarching principal based on dark energy throughout cosmic evolution.
So we have: a cyclical universe vs. a universe that has a beginning out of nothing and goes on to dissipate into nothingness for eternity. That certainly is an interesting contrast. Which one makes more sense?

 

[sol invictus]

Could emotional attachment and historical inertia be a major factor in preferring the inflationary model over the cyclic model?

No, absolutely not.

First of all, cyclic models were proposed long before inflation was. They go way, way back, and they have never succeeded in capturing much attention because they do not have any explanatory power. As for emotional attachment, no one likes inflation much - it's just the best option available, it's supported very strongly by data, and it's made many falsifiable predictions that were confirmed by experiments done much later.

Steinhardt and Turok make the case that their model is less cumbersome, with no ad hoc piecing together of unrelated theories.

You'll find that very, very few professionals agree with them.

So we have: a cyclical universe vs. a universe that has a beginning out of nothing and goes on to dissipate into nothingness for eternity. That certainly is an interesting contrast. Which one makes more sense?

Your question is absurd. First of all, your premise (the way you describe inflation) is wrong on just about every level. Secondly, one cannot evaluate those two possibilities on the basis of which one makes more sense to us, because our ideas about what is sensible are based on experiences with almost no relevance to this question.

But if you want a "common sense" reason why a cyclic universe is impossible, here's one: it's a perpetual motion machine.

 

[Perpetual Student]

First of all, cyclic models were proposed long before inflation was. They go way, way back, and they have never succeeded in capturing much attention because they do not have any explanatory power. As for emotional attachment, no one likes inflation much - it's just the best option available, it's supported very strongly by data, and it's made many falsifiable predictions that were confirmed by experiments done much later.

The model proposed by the Steinhardt and Turok seems to have little to do with older models since theirs is based on the fairly recent discovery of dark energy. The authors claim their model is just as consistent with observations as is the inflationary model, so I'm not sure why inflation is "the best option available."

You'll find that very, very few professionals agree with them.

That is why I have speculated that the preference for inflation may be due to historical precedence and emotional ties.

Your question is absurd. First of all, your premise (the way you describe inflation) is wrong on just about every level. Secondly, one cannot evaluate those two possibilities on the basis of which one makes more sense to us, because our ideas about what is sensible are based on experiences with almost no relevance to this question.

You seem to think many things I say are absurd. My premise (as you call it) is the premise of Steinhardt and Turok. Write them a letter; perhaps they can tell you why your comments are absurd. Why are you so defensive?

But if you want a "common sense" reason why a cyclic universe is impossible, here's one: it's a perpetual motion machine.

Hmm, perpetual motion vs. something appearing out of nothing?
Interesting choice!

 

[sol invictus]

The model proposed by the Steinhardt and Turok seems to have little to do with older models since theirs is based on the fairly recent discovery of dark energy. The authors claim their model is just as consistent with observations as is the inflationary model, so I'm not sure why inflation is "the best option available."

A claim does not suffice. The underlying reason is that their model has somewhere between very little and zero explanatory power.

That is why I have speculated that the preference for inflation may be due to historical precedence and emotional ties.

That really makes no sense, as I just explained. Cyclic models are the ones with historical precedence.

My premise (as you call it) is the premise of Steinhardt and Turok. Write them a letter; perhaps they can tell you why your comments are absurd.

There are hundreds of papers published on the topic, many of which expose serious problems with their claims. There is no need to "write them a letter".

Hmm, perpetual motion vs. something appearing out of nothing?
Interesting choice!

That's a false dichotomy, resulting from the fact that you don't understand what you're discussing. Inflation is NOT a theory of the origins of the universe - it's a theory of what happened AFTER the big bang, bounce, or whatever came before inflation. The cyclic model is perfectly compatible with inflation, in the sense that inflation could have occurred after the "bounce". But the success of inflation doesn't require any such assumption about what happened at the would-be singularity.

The debate is whether the cyclic model can account for the data without a period of inflation, and if so whether it is as or more predictive than inflation. As for inflation, it doesn't care whether there was a bounce, singularity, instanton, or what - that's what's so powerful about it. Regardless of origins of the universe, inflation almost always produces universes of a specific type - which just happens to match the one we live in. That's an extremely non-trivial claim, and that's what makes inflation successful.

The cyclic model, on the other hand, must be carefully tuned by hand to make its predictions match the available data.... and even having done that, it is very unclear whether the many assumptions going into it are even mathematically self-consistent. It really isn't good science, at least not so far.

 

[Perpetual Student]

That's a false dichotomy, resulting from the fact that you don't understand what you're discussing. Inflation is NOT a theory of the origins of the universe - it's a theory of what happened AFTER the big bang, bounce, or whatever came before inflation.

Now you're showing your ignorance -- and arrogance. The authors use the phrase "inflationary model" to refer to the complete big bang scenario, which includes the big bang, inflation, and all subsequent cosmological history -- in other words the theory of the "origin and history of the universe," which includes an inflationary era. It contrasts with their cyclic theory, which does not require an inflationary era.

 

[sol invictus]

Now you're showing your ignorance -- and arrogance. The authors use the phrase "inflationary model" to refer to the complete big bang scenario, which includes the big bang, inflation, and all subsequent cosmological history -- in other words the theory of the "origin and history of the universe," which includes an inflationary era.

They may use terms however they like. The fact remains that the predictions of inflation do not depend on the existence of a singularity before it started. As I said before, one could have a cyclic theory which included a phase of inflation after the bounce, and its predictions would be essentially identical to those of a more conventional model with inflation after a bang. That's one reason inflation is a good theory and the cyclic hypothesis unnecessary.

It contrasts with their cyclic theory, which does not require an inflationary era.

So they claim. You will find many, many papers in the literature pointed out very serious problems. I already mentioned the simplest: the whole scenario violates the laws of thermodynamics.

 

[Skwinty]

It contrasts with their cyclic theory, which does not require an inflationary era.

Perhaps you and Sol misread what S & T say about inflation.

The cyclic model includes a period of dark energy dominated, accelerated expansion, but we have recently come to understand that this does not play a key role in making the universe homogeneous and isotropic after the bounce.

 

[JoeTheJuggler]

Ragnarok is having a problem with the idea that galaxies can move within space relative to each other, while space itself is expanding.

That's my main objection to the balloon analogy. A galaxy drawn on the surface of the balloon would expand just as the balloon does.

As space expands, the galaxies themselves are not getting any larger, because gravity is holding them together. The Andromeda Galaxy is moving towards our own galaxy, despite the fact that the space between them is expanding, because it is being attracted by our galaxy's gravity.

I don't think this is true. I'm pretty sure expansion only happens out between galaxy clusters (not even between the Milky Way and Andromeda).

 

[Skwinty]

Is the cyclic model a perfectly efficient engine?
No. This question is motivated by the picture in which infinite branes bounce at regular intervals forever, a seemingly dissipationless scenario.
However, the picture is naive. It describes large regions of space for long periods of times, but it does not capture the full global picture. In the cyclic model, there are various sources of dissipation, which lead to flaws in the regular cycles in localized regions. For example, black holes formed during a cycle of structure formation are too massive to evaporate before the next brane collision. A black hole distorts the space-time in its vicinity, effectively traversing the two branes and gluing them together. Close to the black hole, the gravitational field is strong enough to prevent the regular periodic collision of the branes in that region. The build-up in the number of black holes from cycle to cycle is a form of dissipation.
What mitigates this effect is the expansion of the universe during the dark energy dominated phase and the contraction of the universe after the dark energy dominated phase ends. The expansion spreads the black holes apart and the contraction shrink the observable universe to a diameter much smaller than the spacing between black holes. Hence, there are more black holes overall after a cycle, but their mean separation is larger than the horizon size and so they have no effect on a typical observer.)
Over time, then, in a comoving picture of the two branes (a map in which you constantly rescale coordinates so that there is no apparent expansion), regions far from black holes are regularly colliding; but, since more and more black holes are produced as the cycles continue, less and less of the comoving volume is cycling. This is steady, long-term dissipation. A corollary is that, standing at any one location, cycling ends after a finite period due to the tiny but non- zero chance that, after many cycles, a black hole in the vicinity will end the cycling in the particular region. The probability can be estimated based on the fractional volume occupied by black holes today. Assuming 10 billion galaxies with million solar mass black holes at the core we find the fractional volume to be 10^(-30). In other words, cycling ends in any given region after 10^30 cycles, although an exponentially large amount of new volume is created for each black hole, so cycling continues overall.
http://www.physics.princeton.edu/~steinh/cyclicFAQS/index.html

 

[sol invictus]

That's my main objection to the balloon analogy. A galaxy drawn on the surface of the balloon would expand just as the balloon does.

That's why people often add coins to the analogy: you should imagine some coins glued to the surface of the balloon. As the rubber stretches the coins remain the same size, just as galaxies do (apart from their own evolution).

If everything expanded, then so would we and our "rulers", so expansion would be meaningless.

True - but it doesn't. The expansion of the universe has no effect on the size of hydrogen atoms, or of "small" gravitationally bound structures (like a galaxy or the solar system).

I don't think this is true. I'm pretty sure expansion only happens out between galaxy clusters (not even between the Milky Way and Andromeda).

I think you're in violent agreement. arthwollipot's point was that the expansion of the space is too slow to counterbalance the attraction. (That's not a very precise way to think of things, but it works at least roughly.)

 

[Perpetual Student]

Perhaps you and Sol misread what S & T say about inflation.

I am sitting in my chair with the book by my side. Please indicate a section that demonstrates that I have misread the book.

 

[sol invictus]

In other words, cycling ends in any given region after 10^30 cycles

While I don't agree with the physics that went into producing that estimate, never mind - let's accept it for the sake of argument. So suppose that's true. Then the "cyclic" universe is neither cyclic nor eternal! Oops - back to square one... now instead of solving any "problem" with the big bang and the beginning of time, you're in exactly the same situation and have to explain what came before.

And since to get there you've introduced all sorts of wild assumptions, new exotic forms of energy, etc., you're at more like square minus one.

 

[Perpetual Student]

They may use terms however they like. The fact remains that the predictions of inflation do not depend on the existence of a singularity before it started. As I said before, one could have a cyclic theory which included a phase of inflation after the bounce, and its predictions would be essentially identical to those of a more conventional model with inflation after a bang. That's one reason inflation is a good theory and the cyclic hypothesis unnecessary.



So they claim. You will find many, many papers in the literature pointed out very serious problems. I already mentioned the simplest: the whole scenario violates the laws of thermodynamics.

Let's see where we are here. The authors, who are highly respected theoretical physicists, have put their reputations on the line by presenting some new ideas in cosmology. You, on the other hand, remain anonymous, credentials unspecified, while making broad negative comments about a book you have not read (and based on comments you have made) don't understand.
Many physicists, including Stephen Hawking and Brian Greene have made favorable comments about the cyclic theory (without indicating that they agree or disagree with it). Alan Guth spoke quite respectfully about the cyclic model at a recent forum, where he debated Steinhardt. Guth obviously favors the inflation model but does not appear to dismiss Steinhardt out of hand as you do.
As a layman I have no idea which is the better approach. I do know that science has had many minority opinions move on the become accepted theories over time. Although I do find any model that does not have the universe and time beginning out of nothing more acceptable, I cannot say that Steinhardt and Turok have found a better answer.
My suggestion to you Mr. sol, is to try to show a little more respect for those seeking knowledge and a healthy dialogue. Your arrogance makes you look small and petty.

 

[sol invictus]

Let's see where we are here. The authors, who are highly respected theoretical physicists, have put their reputations on the line by presenting some new ideas in cosmology.

Ah, the old chestnut: argument from authority.

You, on the other hand, remain anonymous, credentials unspecified, while making broad negative comments about a book you have not read (and based on comments you have made) don't understand.

I understand the model rather well, actually. It's true that I have not read their book, but I have read some of their research articles. More importantly, I understand the physics and can work things out for myself.

My suggestion to you Mr. sol, is to try to show a little more respect for those seeking knowledge and a healthy dialogue. Your arrogance makes you look small and petty.

If you don't like my posts, don't read them. There's a perfectly good ignore function available.

If you'll look back to the OP, you started this thread with a whine and a confrontational assertion (which you never backed up despite my explicit request). As for my posts in the thread, I've skimmed through it and I stand behind them completely. Reading it over, it's quite clear who has something substantive to say, who has some vague and unsupported feelings about the way things ought to be, and who squirms and twists when pinned down or challenged on any of them.

 

[Reality Check]

Let's see where we are here. The authors, who are highly respected theoretical physicists, have put their reputations on the line by presenting some new ideas in cosmology. You, on the other hand, remain anonymous, credentials unspecified, while making broad negative comments about a book you have not read (and based on comments you have made) don't understand.
Many physicists, including Stephen Hawking and Brian Greene have made favorable comments about the cyclic theory (without indicating that they agree or disagree with it). Alan Guth spoke quite respectfully about the cyclic model at a recent forum, where he debated Steinhardt. Guth obviously favors the inflation model but does not appear to dismiss Steinhardt out of hand as you do.
As a layman I have no idea which is the better approach. I do know that science has had many minority opinions move on the become accepted theories over time. Although I do find any model that does not claim to have the universe and time beginning out of nothing more acceptable, I cannot say that Steinhardt and Turok have found a better answer.
My suggestion to you Mr. sol, is to try to show a little more respect for those seeking knowledge and a healthy dialogue. Your arrogance makes you look small and petty.

Hi Perpetual Student:
I think that you will find that sol invictus is working from the published papers of the authors which will have the complete information about their theory.

This is in contrast to the book which by definition will be "dumbed down" for the intended audience. That includes using the non-standard term of "inflationary theory" for the concordance model of cosmology when that term is usually just applied to the inflationary period of the early universe.

Your point about "anonymous, credentials unspecified" applies to everyone on this forum - including you and me. If sol invictus turns out to be Stephen Hawking, Brian Greene or Alan Guth would that change your mind?

In my (limited) experience, scientists usually speak well of other scientist's theories. There are exceptions of course (certain discussions about string theory have got heated in the past).

As far as I can see sol invictus has not been arrogant and in fact has been presenting one side of a healthy dialog. Whether you present the other side of the dialog or not is up to you. His point about the laws of thermodynamics is quite valid.

 

[Perpetual Student]

His point about the laws of thermodynamics is quite valid.

So, explain why the creation of the universe out of nothing does not violate the laws of thermodynamics.