Is the universe finite or infinite?

twofish-quant

One other thing, I'm a fan of going to the original papers. Here is the paper for BOSS

http://arxiv.org/pdf/1203.6594v1.pdf

Something to point out is that they put their data through six different parameterization, and then they explain why they do it.

The reason why is this

http://arxiv.org/pdf/0802.4407v2.pdf

Essentially, you can get very impressive looking numbers if you assume that the cosmological constant is constant. However, once you assume that dark matter changes then it becomes difficult to tell what is evolving dark energy and what is curvature.

Since the BOSS people are observationalists, they run their data through several models.

One reason I think this is worth looking at is that we have no clue what dark matter is, and if it turns out that it is evolving, that gets rid of the cosmic coincidence problem.

Other practical point is that the minimum curvature that we can measure is 10^-4 to 10^-5. Remember that in LCDM, the universe is not flat. It's wrinkly. If global curvature goes below 10^-5, then it gets lost in the wrinkles.

ViewsofMars

Twofish-quant, on the previous two pages we discussed a flat universe. I was wondering what you think about the comments by NASA Official: Dr. Edward J. Wollack
Page Updated: Monday, 04-02-2012- WAMP:

twofish-quant

I think he is doing a wonderful job of trying to simplify some very complicated topics.

A lot of the problems come in when you try to take something very complicated and then try to simplify things for popular consumption. If I have one or two pages to talk about cosmology, I'm not going to go into the messy details, because 99% of the people that read the press releases don't care about the messy details. He is trying to use some metaphors for what is going on. Those metaphors are somewhat inaccurate, but it's hard, maybe impossible to show the accurate version without a ton of greek symbols that will cause 99% of the readers to fall asleep. He is leaving out some important details, but putting in all of the details would give you a 100 page textbook, and most people reading it will fall asleep.

Press releases and popular websites are inherently misleading because they don't tell the full story, and they don't tell the full story because you can't tell the full story in two pages, and most people reading the sites don't care about the full story. That's why I like web links to original papers. Even if you can't totally understand everything in the papers, you can figure out some things that aren't obvious from press releases.

For example, one thing that becomes obvious when reading the BOSS paper is that getting good data is hard work. There are at least thirty pages listing all of the corrections that they made and justifying all of their decisions.

twofish-quant

Also, let me explain the problem with dark energy evolution and curvature.

Imagine a plot of possible dark energy evolution and curvature fits to data. It turns out that this looks like a long diagonal ellipse.

Now let's pretend that I assume that there is zero curvature. I slice the ellipse vertically at zero, and I get a very small error in DE evolution. Now let's pretend I assume that there is no dark energy evolution. I slice the ellipse horizontally at zero, and I get a very small error in curvature.

If I just look at the two errors, I can (incorrectly) assume that because I get a small error in curvature assuming zero DE evolution and a small error in dark energy evolution assuming zero curvature that both numbers are zero.

In fact the errors work out so that this isn't the case. The errors are huge, but it's just because of the way that I slice the error that it comes out small.

All of the quoted numbers that say that the universe is flat assume that dark energy is not changing, and since we have no clue what dark energy is, that's not a great assumption.

Again, if you look at the original WMAP and BOSS papers, it's obvious that everyone is aware of this problem, and trying to fix it. People don't mention it in popular summaries, not out of malice, but because you only have one page to explain something, so you have to leave out some messy details, and most people that read these sites really don't care.

ViewsofMars

Thanks! I'm an avid reader. I have a large library in my home. I'm especially fond of rare books. My computer has a large volume of of good stuff too.

So I can find the WAMP data through the Legacy Archive for Microwave Background Data Analysis (LAMBDA) at http://larnbda.gsfc.nasa.gov . (1.)

1. http://ntrs.nasa.gov/archive/nasa/ca...2008031489.pdf

I'll look at the BOSS papers. I'll get back to you on the Dark Energy.

My previous post from NASA stated, "The Age of the universe is controlled by the amount of the ingredients and the flatness of the universe." If I understand you correctly then the flatness of the universe is no longer necessary in determining the age of the universe. Is that correct? If so, please tell me how you would calculate the age of the universe. Thanks. I'm having fun.

Ken G

This language seems to completely overlook the reality of what models are in physics. We just don't make claims on reality when we make models, we make claims on the models (that they adequately describe the current observations of reality). We have no idea if a photon is massless, if c is constant, if dark energy is constant, if the universe is infinite, if curvature is zero. That's not the point at all. The point is to ask, can we adequately understand our current knowledge of reality using a model that uses massless photons, constant c, constant dark energy, and an infinite universe with zero curvature. That is the question that physics asks. And for the data we now have, the answer to that question is, "yes." Will that still be the answer 100 years from now? Probably not! But we have no idea which in that list will be the source of the "no", all we have, all we ever had, have, or will have, is the current best model.

Ken G

I mean ther terms "speculative" and "mainstream" in exactly the way the words are used in astronomy. Had I meant "crank", I would have said so.I don't agree. Yes there is a group of cosmologists who like to make anthropic arguments and refer to the multiverse, but that certainly does not make it part of the bulk of mainstream consensus that astronomers can comfortably refer to as "our current best understanding". What makes something mainstream is that a working astronomer could stand in front of an audience and say "we have observations that support the following view of things", without feeling like they had stepped well outside the realm of what can be empirically justified. You seem to be arguing that you could do that with a model of our universe that includes vastly many other universes we cannot see, but you could not do that with a model of the universe that was flat and infinite, since you have claimed the former is mainstream and the latter is not, whle I have claimed the opposite-- based on the actual observations that we have actually done, which is also why we see that kind of argument on the WMAP website and so forth (where we do not find multiverse arguments). I don't know any astromers who could comfortably stand in front of an audience and say that our best understanding is that our inverse is one of a gajillion unobservable ones, but I know plenty who would be perfectly comfortable saying our current best model of the universe is flat.
Actually, no. What I have is basically one person changing my words into things I did not say (like claiming that I equate speculation with crankism, or claiming that I said most cosmologists "assume" the universe is flat, either of which I would have actually said had I meant that), while failing to assail any of the core logic of my argument.

Thank you for repeating a point I have made myself countless times in this discusson!
Yup. Which is all part of the art in physics of "creating the best current model." Like I said, over and over.
No kidding. Really? Name one. I mean, one that I really said, not these absurd mischaracterizations.
No kidding! Goodness man, this is just how science works. We make our best models, based on what works. Then we do observations, and what works changes, so we change our best model. Are you now telling me that our best models can change based on new information? Is this supposed to come as some kind of surprise to me? You think that if we do the things on your list, that then we will be able to say we finally know that omega actually is 1? No, we won't be able to say that, we never get to say that. We never get to say that omega is actually 1, we never get to say that c is actually constant, we never get to say that protons never decay or that photons are exactly massles. All we get to do, all we ever get to do, is make the idealizations that work, given the current state of the observational data.

And when we do that in cosmology, we arrive at a flat universe, and it's OK to recognize that. This is all I have been saying, put away all the silly things you claim I have said.
Wrong, that's ridiculous. Why on Earth would physicists ever need to claim any model is exact? Are they fools, even after all these many centuries of doing physics?
Obviously. Like that's never happened!
So you think that claim becomes true when we think it is? If I think something is true about the universe, I can claim it, but if I'm skepical that it's true, I cannot, so I have to stop being a scientist when I form an opinion of how things are? If you take that approach, then you must either think that the whole enterprise of physics is hopeless (because the claims we make on the universe invariably get overturned later on), or else you have fallen into the all-too-common fallacy of thinking that our own current version is actually the correct one, despite all the clear evidence that this simply does not happen in physics.

Let me suggest a more workable option. We recognize that it doesn't make a hill of beans of difference what we personally "think is true" about the universe, what matters is the models we make, the simplifications this involves, the understanding this wins for us, and the observational data we can understand using those models. That's what physics is, would you not say?

twofish-quant

I'm making a claim on reality when I make my models. They might turn out to be incorrect claims, but they are claims nevertheless.

That's exactly the point.

We have experimental data that puts tolerances on those values. We then make physical theories that make statements about reality. Electroweak theory says that the photon is massless. Relativity says that c is constant. It could very well be that the standard model of cosmology in 2020 says that curvature is exactly zero.

Those are claims. If it turns out that the photon has mass, then electroweak theory is wrong. The standard electroweak theory in 1974 stated that the neutrino had zero mass. That turns out to be wrong. The standard cosmological model in 1995 stated that the cosmological constant was zero. That's also wrong. We make progress by making claims, and if those claims turn out to be false, then GREAT!!!!

The point of a theory is to go *beyond* current knowledge. Once you claim that the neutrino has zero mass, you can calculate the solar neutrino flux, and then you find that it's not what you think it was.

It's actually no. There are lots of things about the universe that don't make sense. LCDM falls apart once you start calculating power spectrum at galactic scales. Also, there's always a lot of noise in observations.

And any model is afraid to be wrong isn't very good.

Ken G

I know this debate is getting long, but this particular point is very important, so I must point out the logical fallacy in this argument. This is exactly the same as if we were playing poker against two opponents, and information had emerged that one of our opponents has a hand that fits into a highly unlikely class of poker hands. Now we make our best analysis of the other opponent's hand, and you say "we can't assume they have one of the more likely types of hands that fits with the data we have, because we already saw that fail once when we discovered the other opponent had an unlikely hand." No, we always expect a generic outcome, and getting a non-generic outcome once does not lead us to expect a non-generic outcome for something else, unless we expect some correlation between the outcomes. So your argument here is only logically accurate if there is some reason to expect a connection between the surprise that dark energy is just beginning to take over the large-scale dynamics of the universe, and that there would be barely observable curvature.

Now, should we expect such a connection? There is no evidence to suggest it. What we are doing is taking all the models we could imagine that have arbitrary amounts of dark energy and arbitrary post-inflation curvature, and we are throwing away all of them that are not consistent with the rather special amount of dark energy that we have observed we need. Then we analyze the surviving class of models, and ask, what is now the generic expectation for this class? Throwing away the models that are inconsistent with the dark energy requirements means we have models whose post-inflation curvature starts out very small, rapidly grows, and then begins to level off more recently. Going forward, the leveling off should turn over into falling curvature, or has already made that turn. Now we have the question, are models where the curvature just peaks up into what we can barely observe the generic class we should expect, or do they still seem highly non-generic, given the dark energy requirements we already have and any connections we expect between that and the post-inflation curvature?

I have argued the answer to that is "the latter," and not a single thing you've said contradicts that. Indeed, if we did observe curvature, it would be perfectly natural to immediately begin scrambling to find the connection between the amount of dark energy, and the very special post-inflation curvature, that made these seemingly independent "specialnesses" both occur together. Have I claimed that couldn't happen? Of course not, I've claimed we have no reason to expect that to happen, so we should not expect that to happen. It would be quite exciting if it did, so certainly we should look for it, we just shouldn't expect to find it, unless there is something very significant missing from our understanding of inflation.
Again, I would argue this is just not the correct connection between theory and observation in physics. The real reason people are skeptical of FTL neutrinos is that something going > c flies against a vast number of observations that we can understand with a theory that says things can't do that. The theory is nothing but a proxy for our understanding of that weight of observational evidence, that is all that is meant by "the theory is strong." So this is not at all a case of theory getting "ahead of observation", that is simply impossible in an empirical science. Instead, it is a case of a huge body of observations, unified and represented by a theoretical proxy, getting ahead of a single rather hard to interpret observation.

But as Einstein said, a single observation can indeed overturn an entire theory. It is all a matter of how certain we can be that the conclusions of that observation are correct, and there was not some subtle experimental error. We don't overturn our understanding of a vast number of experiments because of one uncertain and unconfirmed result, that doesn't mean the theory is "ahead of" the observations. We should certainly have gotten past the idea that a theory should be right because it sounds right to us!

twofish-quant

If someone tries to get a paper into Astrophysical Journal with young earth creationist arguments, then it's not science, and I can trash that paper. Anthropic arguments are sufficiently well accepted that you can write journal articles about them and have them pass peer review. If you don't believe me, go into the standard research databases and key in "anthropic."

It's a legitimate argument.

There's a difference between "mainstream" and "mainstream consensus." If we get ourselves into two or three different models which people scream at each other with, that's "mainstream" but it's not consensus.

And there is no consensus that omega=1.

You are trying to teach astronomy to astronomers, and cosmology to cosmologists.

Part of the reason I'm rather harsh toward you is because you keep doing that. It's fine if you make up your own philosophical rules, but once you start trying to argue that cosmologists should do this and shouldn't do that or astronomers should do this and shouldn't do that, then you need to realize that most scientists don't follow those rules.

Also, Stephen Hawking goes way out of things that are empirically justified. My beef with him isn't that he does that, my beef with him is that he does it and doesn't tell people he is doing that.

I'm claiming that your definition of "mainstream" is not a good one, and it's certainly not the one that I use. By "mainstream" I'm referring to arguments that are commonly used in writing theory papers, and assumptions that can be used within theory papers without having to justify them.

Steven Weinberg, Max Tegmark, Alan Guth just to name three.

Also you have this other habit of claiming sources without citing them. There's nothing wrong with being a minority opinion, and my claim is that you have philosophical beliefs that most astrophysicists don't share. Nothing wrong with that.

Well you seem to think that anything that is not observable is not scientific.

You are doing it again. Lecturing scientists about how science works.

No. You come up with random models without any clue if they will work or not. You then use observations to cross models off the list.

No. We see where we are at that point.

Yes we do. I make the claim that c is constant and photons are exactly massless. I can change my mind latter, but I make the claim now. If it turns out that omega is exactly one, then we start looking for symmetry mechanisms that would set omega to exactly one.

Because claiming that something is exact makes it easy to falsify. If I make the claim that photons are *exactly* massless or that omega is *exactly* one, that means that it's easy to come up with experiments to show that the model is wrong. If I come up with "waffle" statements, then it's harder to falsify things.

The goal of a theorist is not to be right. The goal of a theorist is to come up with something that is testable. A theory that says that the photon is *exactly* massless is much easier to test than one that has no predictions. Same with the speed of light.

The current theories of physics say that all electrons have *exactly* the same charge, and that particles and anti-particles have *exactly* the same mass. This means that you have models that are testable and falsifiable.

My big beef with string theory is that it hasn't come up with exact predictions. Even *stupid* predictions are better than no predictions.

There's too much psychology here. In my experience, one thing that makes a good theorist is not to have too many opinions about what is true or not. The job of a theorist isn't to "come up with true theories." The job of a theorist is go come up with theories and then have observationalists shoot them down.

For example, I can write a theory paper about the consequences of a universe with omega being *exactly* one. It doesn't mean that I think omega is one, I'm doing a what-if. Just because I claim that omega is one in a theory paper, doesn't mean that I believe it, since the point of a theory paper is to figure out consequences of assumptions.

Onward and upward.

What's interesting is going *beyond* current observational data. Physics is not just about "understanding observational data." A lot of it involves understanding things that we haven't observed.

I'd have less problem with your statements if you say "this is what I think physics is." Saying that "this is what physics is" or "this is what science is" implies that people who don't share your philosophical beliefs aren't doing science or aren't doing physics.

There is a lot of philosophical variation between physicists.

twofish-quant

In fact it does if you do Bayesian analysis. If you have a fair coin, and you flip it 50 times, and it always comes out heads, then the odds of the next flip coming out heads is 50:50. The trouble is that if you have even the slightly reason to suspect that the coin is unfair then it changes things considerably.

And there is reason to think there might be some connection.

I'm a theorist. I come up with new ideas which connect the two.

Also observationally dark energy and curvature are very closely connected and it can be hard to separate the two.

Strongly disagree. The thing about the theory is that you can tell "how bad things get" if the observation was correct.

But it's circular. Part of what makes you suspect that there is some experimental error is if you get weird results. If the observation was on something we didn't think we understood, then we wouldn't spend as much effort looking for experimental error.

Ken G

Which is exactly why I said "unless we expect some correlation between the outcomes." In the coin analogy, we obviously should, if we use the same coin, and we should not, if we use a different kind of coin. You have not offered any reason to expect that the presence of dark energy, and whatever is the post-inflation curvature, have any reason to be thought of as the "same coin." The only inflation theory I've seen that connects the two is "quintessence", but even that only connects the sources of dark energy and inflation, it doesn't have any reason to connect the magnitudes of an order-unity dark energy contribution with a barely-measurable curvature.
Which is....? We can only judge the strength of this claim on how well you can justify that reason.
Any theory that invokes two unknown variables will make it hard to observationally separate their values, that is not an argument that a non-generic outcome for one of the variables is evidence for a non-generic value (after accounting for our prior knowledge of the first) for the other. We should still expect the curvature to be generic, unless we have some specific aspect of the theory that suggests a connection between their values. I have not yet heard you give an argument that a 0.7 dark energy term in Omega suggests a non-generic curvature result that would make curvature measurable.
But here "weird" means "in contradiction with the way we understand all the other good observations we have done", not "in contradiction with our opinions of how we think the universe ought to work." The former is a perfectly valid way to contrast different bodies of observations and their relative uncertainties, the latter is a fallacy we have fallen into so many times we should really know better by now. But you are right when you object that I am actually describing a particular viewpoint about what science is or should be, and it is decidedly Popplerian, I just think this is so clearly the correct way to frame science that I'm not constantly prefacing it with "in my opinion". The point is I'm presenting an argument by evidence for why we regard those observations as weird, and it's not because the theory is "ahead" of observation, it is because the theory is supported by other observations.

twofish-quant

Dark energy causes curvature. But that's beside the point.

The point is that you are using a heuristic principle (i.e. observations producing coincidences should be rejected) that's known to have failed in one situation, and so there isn't any reason I can see that I should agree to using that principle in another situation.

Or maybe not. If you really believe that "reject coincidences" is a good principle, then it seems to me that you should conclude that there is curvature + dark energy evolution. If in fact there is a small amount of curvature and also some dark energy evolution, then that would get rid of the cosmic coincidence problem, and not generate any new coincidences that I can see.

That's not true. It just happens that the mathematics of the situation is such that current observations of the cosmological constant create this problem. There are ways around that problem.

There's an element of creativity and luck in doing theory. If someone comes up with useful theory, I really don't care how they do it. One thing that is interesting is that some of the most creative theorists also happen to be stubborn and pig-headed. Penrose, Newton, and Einstein for example.

In the case of "doing theory" there's no shame in coming up with a dozen silly ideas if you happen to come up with one that happens to have legs. The point of a theorist is not to be right. It's to be interesting. There's no way with pure thought to know if you are right or not. But with thought, you can come up with stuff that the observers might be able to figure out.

And part of the reason I'm arguing with you is that it's not.

There are some things that Popper IMHO got wrong. One is that there is nothing within the Popperian view for levels of certainty. There's also the problem that Popper has problems in situations where you have a model that's probabilistic (quantum mechanics). You also have problems when you deal with one time events (like the Great Depression or the Big Bang).

But a lot of those other observations are theory dependent.

The other thing is that there are very few observations of neutrinos, that's why they were doing that experiment in the first place. So there really are few observational reasons for arguing that "neutrinos will be different." Same for gravity waves. No one has observed a gravity wave. But we think that 1) they exist and 2) they travel at light speed. If the first experiments say that they are traveling faster than light, my reaction would be that they did their experiments wrong, not withstanding the fact that no one has ever observed a gravity wave.

ViewsofMars

I noted that you are a theorist on the previous page. I'd like you to answer my question found on the previous page (#73).

"We should stand firm and insist that genuine science is based on observational testing of plausible hypotheses. There is nothing wrong with physically motivated philosophical explanation: but it must be labeled for what it is. Overall: theory must be subject to experimental and/or observational test; this is the central feature of science." George F R Ellis, November 21, 2008, "Dark matter and dark energy proposals: maintaining cosmology as a true science?"
http://arxiv.org/PS_cache/arxiv/pdf/...811.3529v1.pdf

I'm interested in talking about science. Your comment leaves me drifting out in space with no spacecraft.

marcus

Nice quotes, VoM. I'll take them out of the context of your post #82 to have them accessible for mulling over.

Twofish: "The goal of a theorist is not to be right. The goal of a theorist is to come up with something that is testable."
http://physicsforums.com/showthread....84#post3944684

George Ellis: "We should stand firm and insist that genuine science is based on observational testing of plausible hypotheses. There is nothing wrong with physically motivated philosophical explanation: but it must be labeled for what it is. Overall: theory must be subject to experimental and/or observational test; this is the central feature of science." George F R Ellis, November 21, 2008, "Dark matter and dark energy proposals: maintaining cosmology as a true science?"
http://arxiv.org/PS_cache/arxiv/pdf/...811.3529v1.pdf

Twofish: "The point of a theorist is not to be right. It's to be interesting. There's no way with pure thought to know if you are right or not. But with thought, you can come up with stuff that the observers might be able to figure out."
http://physicsforums.com/showthread....98#post3944798

These strike me as very well chosen quotes. I'm not engaged in the discussion at least at present, but I'd like to mull them over and perhaps keep them handy. Here, for reference, is your post which afforded context.

Ken G

Actually, dark energy reduces curvature, so it does not cause it. We must not confuse the two meanings of curvature-- GR curvature, which is invariant, and spatial curvature, which is coordinate dependent everywhere but in cosmology (where we have the cosmological principle which picks out a very clear splitting between space and time). Dark energy reduces spatial curvature, and so does inflation-- they act on whatever spatial curvature is handed to us by our initial conditions. My point is that they reduce spatial curvature in unrelated ways-- or at least, no one has any theory to say why they should be related in the kind of special way that would be required to get a double-special value of both. That's pretty much my whole point.
Then I'll play poker with you any time-- since you've probably seen highly unlikely poker hands, and are therefore unable to expect my hand to be generic.

If I thought that was true, I would completely agree, but I don't see what you are basing that on. There's no value in turning two problems into one if you think you only had one problem in the first place.

twofish-quant

Depends on the type of energy.

1) You don't know that.
2) It's not crazy to think that the DE and inflation are part of the same quantum field. In that cause, the theorist would think about this and try to figure out something interesting.

I'm a theorist. The job of a theorist is to come up with theories. If there isn't a theory, then you make one up.

I don't know if you want to be a theorist, but one advice is that if you come up with an idea, then you should take it to it's logical conclusion. You've advanced the idea that "any theory that creates a cosmic coincidence should be rejected." Something that would be a useful paper would be to take that idea to it's logical conclusion and argue that the idea that we are seeing zero curvature and zero DE evolution is *wrong*.

Let's play logic chess.

I'm just trying to get you to take your claims to their logical conclusions. If you are arguing that "any theory that creates a cosmic coincidence *MUST* be wrong" and if you accept the standard interpretation of current observations, then logically you have a problem. You need to either reject your principle as a logical principle, or you must reject current observations.

If it's not a logical principle, then I don't see why it should apply to inflation. You can weaken your statement so it's a heuristic and not a logical principle, which is fine, You can also question current interpretations, which shows a lot of chutzpah, but it's cool if you turn out to be right (and if you aren't a jerk about it, no one will care if it's wrong).

1) Remember that the purpose of being a theorist is not to be right, but to be interesting, and being interesting often involves figuring out non-trivial consequences of ideas. I don't buy the "non-coincidence principle" because I know of one violation, but what if it's not a violation?

2) You are the person that quotes Popper. If you have *one* problem, that should falsify the principle, shouldn't it? However, it could be that the mathematics of the situation causes both problems to cancel out.

3) The whole *point* of much of science is to turn multiple problems into a single problem. It turns out that it makes the problem easier.