PDA

View Full Version : Dark energy, big bang



stitt29
2010-Jul-13, 01:41 PM
Hi,

I'm just wondering about the expansion of the singularity at big bang. Is dark energy needed for this to have happened? Or is dark energy only needed for the expansion of when the Universe was hot and dense expanding to the bigger cooler Universe we have now (I think this is called inflation)?

Hornblower
2010-Jul-13, 02:12 PM
Hi,

I'm just wondering about the expansion of the singularity at big bang. Is dark energy needed for this to have happened? Or is dark energy only needed for the expansion of when the Universe was hot and dense expanding to the bigger cooler Universe we have now (I think this is called inflation)?

My simplistic inclination is to say that all of the cosmic energy at this point is "dark" in the sense that where our theory breaks down and gives us a singularity, we have no means of ascertaining its characteristics. As the expansion progresses, some of the energy differentiates into forms that are familiar to us. The remainder, the presence of which was unsuspected until about 1998 when the apparent expansion acceleration was discovered, still has the adjective "dark" attached to it as a placeholder.

I think my rudimentary remarks are in reasonable agreement with the latest mainstream theory, and I welcome any input from those who can expand upon them. I am here to learn as well as to attempt answering the questions of others.

Cougar
2010-Jul-13, 02:17 PM
I'm just wondering about the expansion of the singularity at big bang. Is dark energy needed for this to have happened? Or is dark energy only needed for the expansion of when the Universe was hot and dense expanding to the bigger cooler Universe we have now (I think this is called inflation)?

Inflation is thought to have lasted from 10−36 seconds after the Big Bang to about 10-33 seconds. That's not very long. There seems to be some connection between inflation and dark energy, but this is really unknown.

stitt29
2010-Jul-13, 02:22 PM
I thought dark energy was needed for inflation. Is the energy needed to expand the initial singularity also dark (unknown). Have you just answered this with a "yes"

Ken G
2010-Jul-13, 02:59 PM
My simplistic inclination is to say that all of the cosmic energy at this point is "dark" in the sense that where our theory breaks down and gives us a singularity, we have no means of ascertaining its characteristics.That seems like a pretty reasonable position to me. High-powered particle theorists often have models or pictures about how inflation could have occurred (they use language like "symmetry breaking" and "phase changes", but I've never heard a precise clarification of how these ideas fit into a predictive model), but it seems pretty speculative if not quite "dark." Perhaps there is always a tendency to want to imagine we know more than we really do.

Ken G
2010-Jul-13, 03:04 PM
I thought dark energy was needed for inflation. Is the energy needed to expand the initial singularity also dark (unknown). Have you just answered this with a "yes"I think we've answered with a "?". What is normally called "dark energy" is not necessarily involved in inflation, it might be something different that requires tons and tons of empty space (that gets called a "cosmological constant" approach to dark energy). We only recently have had enough empty space in the universe for this to matter, so inflation would have been something different. However, another model for dark energy is called "quintessence", which, as I understand it, does attempt to connect the acceleration we see now with (much) stronger acceleration during inflation. Quintessence says that there's always a kind of background tendency to accelerate the expansion, which was hugely important when gravity "decoupled" from the other forces (we don't even know if gravity was ever unified with them!), then submerged into relative unimportance, and is recently re-emerging as a significant effect.

I also got the sense from your original question that you were interested in the initial expansion, prior to inflation. Efforts to "explain" that are on even more speculative ground, so the only safe approach for now is to just say "it's an initial condition." That's a standard technique in physics to focus your attention on what you are trying to explain, and isolate that from what you don't consider to be your mission to explain (and which you don't even know is possible to explain).

stitt29
2010-Jul-13, 03:04 PM
Inflation is thought to have lasted from 10−36 seconds after the Big Bang to about 10-33 seconds. That's not very long. There seems to be some connection between inflation and dark energy, but this is really unknown.

Thanks. I had a quick look on wikipedia about this.

What was causing the expansion before inflation? i.e. from zero to 10^-36. And how are these precise fractions of time worked out?

Ken G
2010-Jul-13, 03:13 PM
What was causing the expansion before inflation? i.e. from zero to 10^-36. And how are these precise fractions of time worked out?I think my previous post on this crossed with yours. As for how the times are worked out, there is nothing precise about them, they are just order-of-magnitude estimates that come from the fundamental parameters. The time of inflation is supposed to be when gravity "decouples" from the other forces, and the expansion gives us a temperature as a function of time, so you just extrapolate backward in time until our theories can no longer work for both gravity and the other forces, at which point you conclude something has to give. Then you say "it's inflation." Once inflation starts, it takes a little time before things "straighten themselves out" and gravity is a different effect from the other forces. How they know how long that should take is a complete mystery to me-- the whole business seems to assume we can know a lot about the things we don't know about.

stitt29
2010-Jul-13, 03:23 PM
That's a standard technique in physics to focus your attention on what you are trying to explain, and what you don't consider to be your mission to explain.

sorry. I don't understand this sentence. have you missed out a word maybe? It seems you are saying some things can't be explained but we should accept them in order to move on. Is that what you mean?

stitt29
2010-Jul-13, 03:25 PM
yep, we keep crossing. Thanks for your replies so far Ken G

Nereid
2010-Jul-13, 03:30 PM
Thanks. I had a quick look on wikipedia about this.

What was causing the expansion before inflation? i.e. from zero to 10^-36. And how are these precise fractions of time worked out?(bold added)

You probably know this already, but just in case you don't ...

The two, highly successful, theories which are used to make estimates like this are general relativity (GR) and the standard model of particle physics (SM), extended to energies/temperatures far, far, far beyond any probed to date, in earthly labs.

Despite the fact of their incredible success, GR and the SM - which is based on quantum mechanics, or quantum theory - are deeply, and fundamentally, incompatible.

For all (so far) observable phenomena, this deep incompatibility doesn't matter; the physical conditions in which the incompatibility become observable have not yet been found, in our universe ...

... except for its (possible) origin.

The scale at which GR and the SM are mutually incompatible is called the Planck scale, and for most cosmological models that means something like 'earlier than ~10^-43 seconds'.

A corollary: unless and until we have a successful theory of quantum gravity - or other theory which addresses the fundamental mutual incompatibility of GR and the SM - we can saying nothing about the inferred state of the universe at the Planck scale (or in the Planck regime).

If you don't know what the fundamental mutual incompatibility is, please ask! I, for one, would be happy to try to explain this to you.

Ken G
2010-Jul-13, 04:00 PM
sorry. I don't understand this sentence. have you missed out a word maybe? It seems you are saying some things can't be explained but we should accept them in order to move on. Is that what you mean?Sorry, that was badly worded. I just mean that physics usually does not attempt to explain everything that happens to a system, it attempts to connect a final state to an initial state. The final state is only "explained" insofar as the initial state is "taken for granted." That approach really lets us focus our attention on what we are trying to understand (the evolution), and ignore what we are not trying to understand (the initial state).

There may even be cases where the initial state is simply not explainable, as may be true for the universe, though of course there's no harm in attempting to connect every state to some prior condition-- and models like that exist for the initial state of the universe (like brane collisions, or quantum fluctuations originating in pure vacuum). They all seem pretty speculative to me, and since physics is pretty much all about assuming an initial state, in any event the best you can ever do is "push back one more level" what you are going to have to simply assume.

KABOOM
2010-Jul-13, 04:03 PM
I thought that Dark Energy was the phenomena identified to explain how entities (galaxies, cluster of galaxies, clusters of stars, etc) stayed "together" given what we know about the forces of gravity and that absent something like Dark Energy many of these entities should theoretically no longer stayed "together" given the distances between underlying bodies within such entities.

Ken G
2010-Jul-13, 04:04 PM
You are thinking of "dark matter." The jargon is really pretty awful, but that's because we just have no idea what these things are.

icarus2
2010-Jul-13, 05:12 PM
Dark energy has a problem of phase transition.
Dark energy have a very big positive value(Inflation Energy) in early universe and very small positive value(Cosmological Constant) at now

trinitree88
2010-Jul-13, 05:53 PM
SNIPPET..... Quintessence says that there's always a kind of background tendency to accelerate the expansion, which was hugely important when gravity "decoupled" from the other forces (we don't even know if gravity was ever unified with them!), then submerged into relative unimportance, and is recently re-emerging as a significant effect.

I also got the sense from your original question that you were interested in the initial expansion, prior to inflation. Efforts to "explain" that are on even more speculative ground, so the only safe approach for now is to just say "it's an initial condition." SNIPPET......

Ken G. I don't usually snippet your stuff, a thousand pardons....
To the OP. There is a physics cartoon, with two physicists pointing at a blackboard covered with equations, followed by " and then a miracle happens", and the older one is telling the other..."I think you need to clarify the steps here...or...something to that effect".
It adequately summarizes Ken G's remarks. It doesn't really explain anything in terms we can justify based on what has been seen in a lab.
1.Why doesn't my arrow shot at targets during summer camp suddenly experience a small piece of inflationary spacetime, and accelerate at superluminal velocity, annihilating the target, a line of trees, and the entire mountain behind them both????
2. Which one of the ~ 10500 string theories would explain that coherently????
3. After annihilating the mountain, the arrow abruptly returns to it's normal, lab derived momentum and kinetic energy, and falls ballistically to the ground. Who turned the inflation off, just at the right time, so that the real estate agents could now sell the camp with an ocean view (nice)???? The "timing" of inflation just happens to match that needed to smooth out the microwave background, while leaving time for a universe to grow lumpy with galaxies. Who ordered that? (I.I. Rabi????)
4. Well, we can certainly live with a vacuum energy density result that is off by more than 10120 orders of magnitude. (That's a little bit bigger than my pendulum lab's allowable error for determining little "g"....~5%.
5. It's why finding funding for some fundamental physics projects requires delicate addressing of issues before the funding streams' committees.
pete

SEE:http://magicanimation.com/misc/SidneyHarris_MiracleWeb.jpg

Cougar
2010-Jul-13, 06:26 PM
2. Which one of the ~ 10500 string theories would explain that coherently????

There are only about 5 string theories (or maybe just one M theory that unifies them all). Most or all of them, however, seem to suggest 10500 possible solutions within their theoretical frameworks. This is rather a problem, since physicists were hoping to find a single, unique solution that would correspond to the single universe that we inhabit.

kevin1981
2010-Jul-13, 07:14 PM
until we have a successful theory of quantum gravity - or other theory which addresses the fundamental mutual incompatibility of GR and the SM - we can saying nothing about the inferred state of the universe at the Planck scale (or in the Planck regime).

If you don't know what the fundamental mutual incompatibility is, please ask! I, for one, would be happy to try to explain this to you.

I would like to know the answer to this please. I have known for a while that quantum physics and gravity do not go together, but i do not really no why.
I know that space/time is "smooth" in GR, compared to the bubbly discreetness of QM. But what is so hard about finding a quantum theory of gravity?

Also, if we do find an answer, how would that help us understand the first initial conditions of the big bang. Thanks

Ken G
2010-Jul-13, 07:22 PM
There is a physics cartoon, with two physicists pointing at a blackboard covered with equations, followed by " and then a miracle happens", and the older one is telling the other..."I think you need to clarify the steps here...or...something to that effect".
It adequately summarizes Ken G's remarks.Right, that cartoon is notorious for situations like this!


4. Well, we can certainly live with a vacuum energy density result that is off by more than 10120 orders of magnitude. (That's a little bit bigger than my pendulum lab's allowable error for determining little "g"....~5%.That's a particularly noisome problem, isn't it-- the current state of theoretical physics in regard to these issues is pretty much "we have a good idea how these things will work out, though we haven't worked them out in detail, but we are confident that 120 orders of magnitude will resolve itself once we do iron out the details." There may be a dose of wishful thinking there, I don't know.

Nereid
2010-Jul-13, 07:29 PM
You are thinking of "dark matter." The jargon is really pretty awful, but that's because we just have no idea what these things are.
Well, we do know some things about the darks!

For example, dark matter is cold, and non-baryonic.

And dark energy is, today, indistinguishable from lambda, the cosmological constant.

astromark
2010-Jul-13, 08:19 PM
The use of the words...should not prohibit understanding. What is said and understood may not be what was meant...

Dark Mass, or Dark Mater... its most of the mass of things as yet unknown. Gravity effected unknown mater or mass.

Dark Energy, is whatever that is, that is eccelorating the expansion of the Universe. Pulling it to bits...

Unconfirmed speculation regarding the facts as yet unknown is rife... Understanding the unknown, impossible.

Nereid
2010-Jul-13, 08:25 PM
The use of the words...should not prohibit understanding. What is said and understood may not be what was meant...

Dark Mass, or Dark Mater... its most of the mass of things as yet unknown. Gravity effected unknown mater or mass.

Dark Energy, is whatever that is, that is eccelorating the expansion of the Universe. Pulling it to bits...

Unconfirmed speculation regarding the facts as yet unknown is rife... Understanding the unknown, impossible.(bold added)

I'm pretty sure you did not mean this the way it comes across, astromark. For example, the causes of what we today call infectious diseases (such as the 'flu) was, once, unknown. Yet, today, we understand this unknown very well indeed. Once we had no clue as to what powers the Sun; today, we have much more than just a mere clue.

Perhaps, within our lifetimes, CDM (cold dark matter) and DE (dark energy) will become much, much better understood ...

astromark
2010-Jul-13, 08:49 PM
Yes 'Nereid' what you point to is a fact... in that all I have said is that I will not argue about what I do not know...

Its a cold dark morning so talking of cold dark mater... yes, I am qualified... :eh:Winter.

Bring on the warm enlightened summer nights... We seem to be missing a complete understanding as yet..

The unified theory...and 42. Still a joke with a pointy end. So much as yet unknown as strings and multi realities..

Yes we are getting closer to a understanding. I still do not know. To use the word know suggests tested and known..

its not there yet. Is it ? Thats all I have said. Its not much. Its a quest as yet not fulfilled... knowledge of the unknown is a quest, a trek.

With the information available to me at this time a complete understanding is impossible...today.

I trust that the science of it will be uncovered...

Ken G
2010-Jul-13, 09:18 PM
Well, we do know some things about the darks!

For example, dark matter is cold, and non-baryonic.
"Know" is a strong word. That is certainly the prevailing view, but alternatives like modifications to gravity continue to be worked on. There are also some who feel that you'll never get everything you want from cold dark matter, and you'll need some more neutrino-like hot dark matter. I was just referring to the fact that we have no idea what the stuff is, even though some progress has been made on its attributes.

And dark energy is, today, indistinguishable from lambda, the cosmological constant.True, but that may just be because the data is so limited. If you only have enough data to assert one statistic about the acceleration, you are likely to be able to fit that statistic with a theory that has one free parameter, and you're not going to be interested in entertaining theories with more than one. I'm not sure we are seeing anything beyond that in current dark energy studies, but it certainly isn't my area of expertise.

astromark
2010-Jul-14, 05:50 AM
Research, test, question, challenge... The godlike pillars of science... in which I trust. religiously... :eh:Mark.

Nereid
2010-Jul-14, 01:42 PM
"Know" is a strong word. That is certainly the prevailing view, but alternatives like modifications to gravity continue to be worked on. There are also some who feel that you'll never get everything you want from cold dark matter, and you'll need some more neutrino-like hot dark matter. I was just referring to the fact that we have no idea what the stuff is, even though some progress has been made on its attributes.True, but that may just be because the data is so limited. If you only have enough data to assert one statistic about the acceleration, you are likely to be able to fit that statistic with a theory that has one free parameter, and you're not going to be interested in entertaining theories with more than one. I'm not sure we are seeing anything beyond that in current dark energy studies, but it certainly isn't my area of expertise.
I have started a thread, in the Astronomy section, to continue discussion of this topic, as it is inappropriate to continue it here: Astronomy: what can we know? (http://www.bautforum.com/showthread.php/105908-Astronomy-what-can-we-know)

I invite all interested BAUTians to join that discussion.

stitt29
2010-Jul-14, 02:09 PM
from Nereid


The scale at which GR and the SM are mutually incompatible is called the Planck scale, and for most cosmological models that means something like 'earlier than ~10^-43 seconds'.



If you don't know what the fundamental mutual incompatibility is, please ask! I, for one, would be happy to try to explain this to you.

Thanks I looked up Planck scale. Still a bit bewildered. Your explanation on this would be much appreciated

Nereid
2010-Jul-14, 02:34 PM
from Nereid





Thanks I looked up Planck scale. Still a bit bewildered. Your explanation on this would be much appreciated
I'm working on it (and other BAUTians have, in the past, given good answers to this, or similar, question; so it doesn't have to be me)

Ken G
2010-Jul-14, 02:58 PM
It sounds like Nereid has some specific issues she wants to mention around that incompatibility, but I could start you off with some general issues. The basic incompatibility you have already mentioned yourself-- the treatment of space and time. GR treats them as a continuous and definite united manifold, whereas QM treats them differently-- location is an observable, treated as a mathematical "operator" that provides only a statistical connection between an input state and an output result, so is subject to all the uncertainties and vagaries of quantum mechanics (like a particle can be in a state of "superposition" of lots of definite position states, which you can think of as meaning it is in many places at once, or simply that reality is fundamentally noncommittal about its location). That doesn't happen in GR, objects have definite locations, but those "locations" are in spacetime, not space or time separately. Also, QM treats time as a parameter of the theory, not as an observable. We make the connection to our own concept of measurable time manually-- it is not within that theory to make that connection. Indeed, quantum mechanics requires us to consider the difference between what part of the physics is axiomatized in the theory, and what part we just apply manually based on common sense about how we want to cross the gap from the microscopic to the macroscopic. GR does not distinguish that difference, because it does not recognize any such gap.

There are many other differences as well, both in terms of the physical theory itself, and its apparent philosophical implications. Some feel that close attention to the philosophical differences provides a guide to understanding the differences in the physical theories and the difficulties in unifying them, others ignore the philosophy and just look at the predictions they make. Where the predictions get different is at the Planck scale-- in GR, I can talk about the gravitational interaction between two objects at a distance of a Planck length, and make predictions about what will happen. In QM, to specify the locations of two particles on that scale requires that we give those particles so much energy that their gravity will create a mini black hole when we return to the ramifications of GR. So whether or not we pass that situation through QM or not will alter our predictions of what happens. Worse, we can't even do the QM calculation, we can only see that it would be different if we could do it.

For a list of the philosophical differences of those theories, you will always find excellent material at the Stanford Encyclopedia of Philosophy:
http://plato.stanford.edu/entries/quantum-gravity/

Nereid
2010-Jul-14, 09:39 PM
from Nereid

The scale at which GR and the SM are mutually incompatible is called the Planck scale, and for most cosmological models that means something like 'earlier than ~10^-43 seconds'.

If you don't know what the fundamental mutual incompatibility is, please ask! I, for one, would be happy to try to explain this to you.
Thanks I looked up Planck scale. Still a bit bewildered. Your explanation on this would be much appreciated
Technically, the incompatibility has to do with "background dependence" vs "background independence", which Ken G has touched upon; crudely one can think of this in terms of there being some abstract canvas upon which stuff happens vs the canvas itself being inextricably wrapped into the stuff itself (perhaps a bad analogy). If you're interested in learning more about this aspect, I think we should get publius involved. More deeply, this comes from the mathematical structures of the two theories.

Another way of seeing this mutual incompatibility is to consider the Heisenberg Uncertainty Principle (HUP), and the energy-time pair in particular.

You probably know that virtual particle-antiparticle pairs can pop into existence and then vanish again, as long as the product of the time they exist and their energy is below a certain threshold (if you are unfamiliar with this, just holler). This has been tested extensively, and every prediction that is currently observable, in particle accelerators for example, has been shown to match the observations.

What happens, then, if the time is extremely short? The energies get bigger, and as mass is equivalent to energy, this is the same as saying there is a mass popping into, and out of, existence. Now mass tells spacetime how to curve (to quote Wheeler), per GR. But in the HUP spacetime is flat.

Not to worry, as long as the curvature is not too severe, there are ways of finessing this.

But not when the mass becomes so great that black holes form! You can consider the virtual black holes to rip the fabric, for time to become so warped that the delta-t in the HUP loses meaning (these are, of course, just analogies, and cannot be taken literally).

Or you can say that the HUP - and QM and quantum theories in general - cannot handle virtual black holes.

The scale - mass/energy, time - at which this incompatibility occurs is the Planck scale.

Please let me know if this makes sense; specifically, please let me know if it's too technical, or not technical enough.