Physics Forums

Physics Forums (http://www.physicsforums.com/index.php)
-   Cosmology (http://www.physicsforums.com/forumdisplay.php?f=69)
-   -   Origin of universe... (http://www.physicsforums.com/showthread.php?t=285100)

jobyts Jan15-09 03:22 PM

Origin of universe...
 
[Please move this question to appropriate form, if it does not fit here]

Many of my theist friends ask me, "if science tells you matter can neither be created or destroyed, how did the universe originate from nothing". I'm unable to answer them clearly . Can someone explain this in currently accepted scientific (not philosophical) theories? If it is too complex, please point me some urls.

Thanks,

mgb_phys Jan15-09 03:32 PM

Re: Origin of universe...
 
The glib answer is that there was no universe around to notice any laws being broken!
A slightly less glib answer is that if the universe was created at a singularity (an infinitely small point) and time was created at the same time (as it were) then the laws of physics didn't apply.

Another way of looking at it is, physics doesn't say that mass cannot be created or destroyed - it says that mass+energy cannot be created .... but they can be converted into each other (e=mc2).
So if the total energy of the universe (in the form of mass of all the stars etc) is balanced by the negative gravitational energy of the expansion - then the total energy of the universe is zero and no laws have been broken.

http://www.astrosociety.org/pubs/mer...2/nothing.html

DavidSnider Jan15-09 03:33 PM

Re: Origin of universe...
 
Nobody knows. That is in the realm of philosophy still.

Let's just say that a God is necessary to create something out of nothing. That says nothing about the validity of angels\demons, the trinity, sin\salvation, his 'plan' and all sorts of other weird theistic concepts.

DaleSpam Jan15-09 04:10 PM

Re: Origin of universe...
 
Also, a singularity is not "nothing". It is a lot of stuff all at the same point. So the big-bang is most definitely not creatio ex nihilo.

Chalnoth Jan16-09 12:11 AM

Re: Origin of universe...
 
There are three possible ways I know of approaching this issue:

1. We don't actually know that the universe had a start, so it's a bit premature to assume there was one. Regions like our own might well have been born from some other region, and that one from some other, and on back ad infinitum.
2. We actually do know of things that come from "nothing", as it were: quantum vaccuum fluctuations. What this means is that if it is possible for a particular type of matter to exist in a region of space-time, then it will necessarily pop in and out of the vaccuum in that region. Perhaps it is similarly possible for space-times to just pop in and out of nothing in the exact same way?
3. In a very real way, our universe is still nothing: its total energy is identically zero. This is clear if we look at the Hamiltonian formalism, which considers not only the energy stored in matter, but also the energy stored in gravitational potential energy. It turns out that the energy stored in gravitational potential energy is negative, and exactly cancels that stored in matter.

Point 3 is a very important point to make, as points 1 and 2 only deal with a singular event that started it all off. But point 3 shows that the whole "matter cannot be created or destroyed" idea is total bunk. Of course matter can be created and destroyed! The matter we see all around us was created when inflation ended. The energy that produced that matter was created as inflation progressed. And all of it will go away when the universe dies a heat death and becomes nothing but empty space.

marcus Jan16-09 01:31 AM

Re: Origin of universe...
 
Quote:

Quote by Chalnoth (Post 2036644)
3. ... It turns out that the energy stored in gravitational potential energy is negative, and exactly cancels that stored in matter.
.

I'm curious, have you actually done the calculation? I know people used to say that, e.g. back in the 1980s.
Alan Guth famously conjectured that the universe might be 'the ultimate free lunch' because negative gravitational energy might conceivably exactly cancel positive (matter etc.) energy.
But I haven't seen anybody maintaining that lately, at a scholarly level.

I see that MGB just gave a 2002 source derived from a 2001 textbook. It was aimed at a general audience and was not a rigorous statement. I know one of the authors. I kind of doubt he would say the same thing now in 2009.

So at the moment I don't have a source. Maybe you can provide one. I'm hoping for some recent scholarly article that maintains total energy = 0 and gives some solid reasoning. Something on arxiv dated > 2005 maybe? A page reference to Steven Weinberg's 2008 Cosmology text?

Hopefully whatever calculation will not depend on inflation, because inflation has not been proven. I'm guessing you would agree that inflation scenarios are just that, scenarios---based on any number of assumptions about exotic inflaton fields. All that supports them is that they explain a few puzzles, for which there may be alternative explanations, so nothing clinches the deal. But even an argument that depends on some inflation scenario would be a help.

====================
BTW Chalnoth, I certainly agree with your point #1!
I think most cosmologists would be quite surprised if it turned out that the universe began around the time of the big bang. The mainstream research community seems to have moved away from that supposition. I don't know of any scientific reason to imagine that the universe had a beginning and I like your characterizing the idea as "premature". :biggrin:

I'm leery of your point #2 because I dont think a prior universe in which such a quantum fluctuation might happen could properly be characterized as "nothing". I don't know any professional cosmologist who currently says our universe came from nothing. A lot of prominent people are working on what the pre-big-bang state could have been---detail, mathematics---and I don't know of any who describe it as "nothing".
Just like someone who knows what they're talking is not apt to say "it came from a point of infinite density".

hellfire Jan16-09 04:51 AM

Re: Origin of universe...
 
I agree with marcus that the claim of a zero energy universe is not rigorous. Specially because we do not have a definition of gravitational energy in a FRW universe.

As far as I know, this idea is suggested by an argument similar to this one:

Assume that the zero energy balance between energy stored in a mass and its gravitational energy can be written as:

[tex]m c^2 - \Sigma G \frac{m \, m_i}{r_i} = 0[/tex]

This means

[tex]\frac{G}{c^2} \Sigma \frac{m_i}{r_i} = 1[/tex]

Considering [tex]M_U[/tex] and [tex]R_U[/tex] as mass and radius of a homogeneous universe, this can be written as

[tex]\frac{G}{c^2} \frac{M_U}{R_U} \approx 1[/tex]

[tex]M_U \approx \frac{c^2}{G} R_U[/tex]

On the other hand, we know that

[tex]M_U = \frac{4}{3} \pi R^3_U \rho_U[/tex]

with a density equal to the critical density

[tex]\rho_U = \frac{3 H^2}{8 \pi G}[/tex]

thus

[tex]M_U = R^3_U \frac{H^2}{2G}[/tex]

equating both expressions for [tex]M_U[/tex]

[tex]R^3_U \frac{H^2}{2G} \approx \frac{c^2}{G} R_U[/tex]

[tex]R_U \approx \sqrt 2 \frac{c}{H}[/tex]

The total null energy arises for a radius of order of the Hubble radius.

granpa Jan16-09 05:14 AM

Re: Origin of universe...
 
I dont know what all that means but I do know that most of the energy in a field is within a few radii of the object that creates it.

Chalnoth Jan16-09 07:40 AM

Re: Origin of universe...
 
Quote:

Quote by marcus (Post 2036702)
I'm curious, have you actually done the calculation? I know people used to say that, e.g. back in the 1980s.
Alan Guth famously conjectured that the universe might be 'the ultimate free lunch' because negative gravitational energy might conceivably exactly cancel positive (matter etc.) energy.
But I haven't seen anybody maintaining that lately, at a scholarly level.

Not personally, no. But it's a well-known result. However, there is a caveat that I did not mention: the result only applies for a closed FRW universe:
http://www.springerlink.com/content/jg1350836415r215/

That aside, it's largely irrelevant, because it's only a heuristic tool anyway at this level of discussion. The fact of the matter is that gravity, when acting on the right kind of matter, will cause a very small region with a large energy density to become an incredibly large region with nearly the same energy density. And it doesn't matter what the overall curvature is for this fact to be true: as long as the right kind of matter is around, it works.

Bear in mind that energy just isn't a conserved quantity in the typical formalism of General Relativity. The conserved quantity is the stress-energy tensor, which, under a variety of conditions, forces energy to not be conserved.

Quote:

Quote by marcus (Post 2036702)
Hopefully whatever calculation will not depend on inflation, because inflation has not been proven. I'm guessing you would agree that inflation scenarios are just that, scenarios---based on any number of assumptions about exotic inflaton fields. All that supports them is that they explain a few puzzles, for which there may be alternative explanations, so nothing clinches the deal. But even an argument that depends on some inflation scenario would be a help.

While I will agree that the specific scenarios of inflation that we have devised so far may well not be accurate, and in fact may well be highly unlikely to be accurate, the general idea of inflation is essentially guaranteed to be accurate.

Basically, there needs to be an accelerated expansion of space in the distant past for there to be any large-scale correlations on the CMB whatsoever. If there isn't such an accelerated expansion, then the fact that the CMB is nearly uniform is a fundamental impossibility, as different regions of the CMB will not have had enough time to ever be in contact with one another. So we can be quite certain of an accelerated expansion in the distant past. Secondly, when the COBE and later WMAP data was released, we got confirmation that the distribution of correlations indicates a nearly constant scaling rate for the fluctuations, which is the scaling rate for an accelerated expansion driven by something like a large vacuum energy, precisely as inflation predicts.

Therefore we can be confident at least in these general properties. We don't yet know the mechanism that caused it, but we're working on that.

Quote:

Quote by marcus (Post 2036702)
I'm leery of your point #2 because I dont think a prior universe in which such a quantum fluctuation might happen could properly be characterized as "nothing". I don't know any professional cosmologist who currently says our universe came from nothing. A lot of prominent people are working on what the pre-big-bang state could have been---detail, mathematics---and I don't know of any who describe it as "nothing".
Just like someone who knows what they're talking is not apt to say "it came from a point of infinite density".

Well, the worry is that we just don't know how to describe "nothing" to begin with. Note that if it happened to be true that our universe came from "nothing", then it is trivially true that whatever this "nothing" is, it can contain, in a sense, a universe like our own. Given this fact, if we were ever learn how to describe this "nothing" properly, we might well discover that it demands that a universe be generated. But since we don't know how to describe "nothing", we can't say whether or not this is the case just yet.

marcus Jan16-09 08:59 AM

Re: Origin of universe...
 
Quote:

Quote by Chalnoth (Post 2036934)
Not personally, no. But it's a well-known result. However, there is a caveat that I did not mention: the result only applies for a closed FRW universe:
http://www.springerlink.com/content/jg1350836415r215/

That aside, it's largely irrelevant, because it's only a heuristic tool anyway at this level of discussion...
...
... forces energy to not be conserved.

Your reference is to a 1992 paper which is available for a price of $12.50.
==exerpt==
THE ENERGY OF A CLOSED UNIVERSE IN A REFERENCE FRAME
FORMED BY A SET OF FREELY MOVING CLOCKS
N. N. Gorobei and A. S. Luk'yanenko UDC 530.12:517.988.38
The Hamiltonian of a closed universe is found in a reference frame formed by a
set of freely moving clocks. In this physically defined gauge the energy of the
universe is positively defined.

In gravitational theory in the absence of a planar asymptote the Hamiltonian of a closed
universe reduces to a linear combination of couples of the first sort and vanishes on the surface of the couples. This serves as the basis for the assertion found in the literature that
the energy of a closed universe is equal to zero [1]...
==endquote==

This is the sort of thing that bothers me when people talk about the total energy being zero. In 1992 one typically did not consider a constant dark energy density. After 1998 we think of dark energy as over 70 percent. And the amount increases as the volume increases.

As you point out there is no Energy Conservation law for the universe.

It is far from certain that the total energy of our universe is even well-defined. If the spatial volume is infinite the energy might be infinite or simply not defined.

I fail to see how it could be considered as heuristic to study a spatial finite case with zero cosmological constant. It is too unreal. Too unrelated to the actual universe to serve as a reliable guide, I would think. But that is what these two 1992 authors seem to be doing with their closed FRW case.

Of course I can't be sure without seeing the article that they are assuming the cosmological constant is zero, but that was normal back then. It would help if you could find a paper that proves this result in the case of positive Lambda.

I remain unconvinced for the time being, but thanks for looking. It was interesting to see what you did come up with!

================================
You give a nice summary of the usual justifications for assuming one of the various inflation scenarios. You mention the horizon problem, you mention the scale-independence of the CMB power spectrum (which I think is one of the strongest supports), you didn't mention flatness though you might have. I agree that these are strong arguments, and one hears them recited over and over.

However there are other ways of addressing the horizon problem, for example. The various types of inflation are not the only answers to these puzzles. I believe I have the option to remain unconvinced at this point. It may be a matter of taste, and level of skepticism.

Quote:

the general idea of inflation is essentially guaranteed to be accurate.
I don't believe that. I like the idea of inflation very much! I don't however consider it guaranteed to be accurate.

Quote:

Basically, there needs to be an accelerated expansion of space in the distant past for there to be any large-scale correlations on the CMB whatsoever. If there isn't such an accelerated expansion, then the fact that the CMB is nearly uniform is a fundamental impossibility, as different regions of the CMB will not have had enough time to ever be in contact with one another.
No, that is not true. What you say is model dependent. In some models there would not have been enough time, in others there would have been enough time.

Quote:

So we can be quite certain of an accelerated expansion in the distant past.
Not on that grounds. Inflation is not the only way of addressing the horizon problem---the near temperature-uniformity of the CMB. Therefore we cannot be certain inflation occurred merely because the CMB is nearly the same temperature all over.

Quote:

Secondly... a nearly constant scaling rate for the fluctuations...
That is what I was referring to earlier when I mentioned the scale-independence of the CMB power spectrum! From my point of view that is pretty impressive. It is much stronger evidence for inflation than the horizon business--the temp uniformity. Temp uniformity too easy to explain by newer mainstream models.

Quote:

Therefore we can be confident... We don't yet know the mechanism that caused [inflation], but we're working on that.
I'm not sure who "we" is. Are you personally working on "inflaton" mechanisms? Or any other exotic physics that might be responsible for the fine-tuned behavior, the graceful exit, the decay, the re-heating...? I haven't seen much research about possible inflation mechanisms lately. If you know of anything recent, say in the past 2 or 3 years, please let me know---hopefully available at arxiv online.

Chalnoth Jan16-09 09:14 AM

Re: Origin of universe...
 
Quote:

Quote by marcus (Post 2036993)
This is the sort of thing that bothers me when people talk about the total energy being zero. In 1992 one typically did not consider a constant dark energy density. After 1998 we think of dark energy as over 70 percent. And the amount increases as the volume increases.

Perhaps, but it doesn't actually matter for the result. After all, if it works for an FRW universe with a radiation energy component, which has an energy density that scales as 1/a4, then I see no reason why it won't work for an energy component with arbitrary scaling. As near as I can tell, the paper makes no assumptions at all as to the contents of the universe, so it should work just fine with any form of matter/energy.

Quote:

Quote by marcus (Post 2036993)
I fail to see how it could be considered as heuristic to study a spatial finite case with zero cosmological constant.

I consider it as being heuristic because it's not necessary to show. There's no reason to bother about the Hamiltonian formalism when things work just fine in the Lagrangian formalism. It's just kind of superficially appealing because we're used to energy being conserved.

marcus Jan16-09 09:53 AM

Re: Origin of universe...
 
Quote:

Quote by Chalnoth (Post 2037014)
As near as I can tell, the paper makes no assumptions at all as to the contents of the universe, so it should work just fine with any form of matter/energy.
...

So you think their argument would go through even with dark energy, a positive cosmological constant?
Did you pay the $12.50 and read the article?
I just looked at the first page they give as a sample.

Chalnoth, you mention that radiation energy density decreases as 1/a4. That is true, and matter energy density decreases as 1/a3.
But what does that have to do with dark energy density? Dark energy density does not decrease as space expands. It is constant. So it behaves quite differently. It scales as 1.

I don't see how a 1992 argument, in that rather unrealistic case, could carry over to contemporary (positive Lambda) cases. Maybe we will get some help on this from someone more knowledgeable than myself who can explain, or cite a more up-to-date paper available online.

Chalnoth Jan16-09 09:57 AM

Re: Origin of universe...
 
Quote:

Quote by marcus (Post 2037052)
So you think their argument would go through even with dark energy, a positive cosmological constant?
Did you pay the $12.50 and read the article?
I just looked at the first page they give as a sample.

Haha, no, I grabbed it from the university. They'd actually charge me $34 for it here in Italy...

marcus Jan16-09 10:42 AM

Re: Origin of universe...
 
Quote:

Quote by Chalnoth (Post 2037055)
Haha, no, I grabbed it from the university. They'd actually charge me $34 for it here in Italy...

You are fortunate to be able to use the university connection. It is a scandal how much they charge for downloading just one paper. I am retired and it is more complicated for me---I have to go on campus and make a request with the librarian.

Chalnoth Jan16-09 10:48 AM

Re: Origin of universe...
 
Quote:

Quote by marcus (Post 2037105)
You are fortunate to be able to use the university connection. It is a scandal how much they charge for downloading just one paper. I am retired and it is more complicated for me---I have to go on campus and make a request with the librarian.

Yeah, I know. This is why I absolutely love the new move to free resources such as arxiv.org. It helps the science because it gets new information out there faster, and there remains little problem with the signal-to-noise ratio, despite the lack of general oversight.

Journals do, however, still serve a purpose as they provide a measure of prestige and also provide much-needed independent verification.

marcus Jan16-09 10:51 AM

Re: Origin of universe...
 
Quote:

Quote by Chalnoth (Post 2037106)
...Journals do, however, still serve a purpose as they provide a measure of prestige and also provide much-needed independent verification.

It's a dilemma. Peer-review is essential. Maybe the PLoS model will work out. The question is how to share the cost of that independent verification you refer to.


Quote:

Quote by hellfire (Post 2036815)
I agree..that the claim of a zero energy universe is not rigorous. Specially because we do not have a definition of gravitational energy in a FRW universe.

As far as I know, this idea is suggested by an argument similar to this one:
...
...
...

Hellfire thanks! I didn't see your derivation at first. Your post kind of settles the question for me, at least for the time being.

Chalnoth Jan16-09 11:59 AM

Re: Origin of universe...
 
Quote:

Quote by marcus (Post 2037110)
It's a dilemma. Peer-review is essential.

Well, the official peer review that goes into publishing is just the first step of peer review. It's only really effective in catching egregious errors. Quite a lot of crap still makes it through. Publication isn't really the end of peer review, it's just the beginning.

Quote:

Quote by marcus (Post 2037110)
Maybe the PLoS model will work out. The question is how to share the cost of that independent verification you refer to.

We shall see. I definitely hope that all of science moves towards free distribution.

marcus Jan16-09 12:29 PM

Re: Origin of universe...
 
Let's consider the question that Jobyts asked in the original post.

Quote:

Quote by jobyts (Post 2035994)
Many of my theist friends ask me, "if science tells you matter can neither be created or destroyed, how did the universe originate from nothing". I'm unable to answer them clearly . Can someone explain this in currently accepted scientific (not philosophical) theories? If it is too complex, please point me some urls.
...

Jobyts, your friends ask you "how did the universe originate from nothing?"

Conventional standard cosmology does not suggest that the universe originated from nothing.
Cosmology is changing with the recent appearance of some new models that do not experience a "singularity" and work back further into the past. But to keep things simple, let's just talk about the classic cosmology model. The one that has been standard for many decades.

This classic conventional model does not say anything about the universe "originating". The model is admittedly incomplete. It's domain of applicability does not reach back to the beginning of expansion---it breaks down at "t=0" and stops computing meaningful numbers.

The domain of validity of the traditional model only goes back to within a few Planck time-units of the "t=0" mark. From there on it is either not reliable or does not function at all.

So as a scientist one cannot say that the traditional model tells us that the universe had an origin. There is no scientific evidence of an origin---so according to the model traditionally used in cosmology, we must admit ignorance. The universe might have always existed. It might have been doing something else before "t=0".
We just don't know.

You might find it helpful to read the page of information called A Tale of Two Big Bangs at the Einstein Online website
http://www.einstein-online.info/en/s...ogy/index.html
The term "big bang" is used in two very different senses, which causes newcomers a lot of confusion. E-O is clear and up-to-date--straightens several things out. It is the public outreach website of a major research institution in Germany. The language is English.

=========================
Now suppose your friends ask you "how did the universe originate from nothing?"
What can you say to them? If you tell them the truth---that science does not say that the universe originated from nothing, and it would make no sense for a scientist to try to explain what he has no reason to believe---would your friends be able to accept that as an answer?

The trouble is that the public has been lied to by popularizers who say things like "the Big Bang theory is that the universe began from a singularity, a point of infinite density!" This is hogwash. Or they say "Big Bang theory says that Time did not exist before the Big Bang." This is baloney. We do not know these things and cannot rationally assert them. Standard cosmology (also known as the Big Bang theory) does not make those claims. On the other hand, authors who wanted to sell popular books and magazine articles have said things like this so much that the general public tends to be programmed to expect something like this. Many half-informed people are certain of it! :biggrin:

So what can you say? I want to leave the question open. What response will work with your friends? Can you yourself suggest something? It would be great to get suggestions from others as well! Perhaps you could respond to your friends by asking them a question back, like "How do you know that the universe originated from nothing? For that matter why do you think that it originated at all?"

What there is no good reason to suppose does therefore not require an explanation.


All times are GMT -5. The time now is 06:56 PM.

Powered by vBulletin Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
© 2014 Physics Forums