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stitt29
2008-Oct-16, 12:21 PM
Einstein's General theory of relativity came out in 1915. Hubble found the Universe to be expanding in 1921-22. GR didn't predict an expanding Universe. Big Bang theory states the universe expanded from a singularity. Why is big bang theory dominant when GR didn't predict expansion? What is the relationship between BBT and GR?

cjameshuff
2008-Oct-16, 12:49 PM
Einstein's General theory of relativity came out in 1915. Hubble found the Universe to be expanding in 1921-22. GR didn't predict an expanding Universe. Big Bang theory states the universe expanded from a singularity. Why is big bang theory dominant when GR didn't predict expansion? What is the relationship between BBT and GR?

GR predicts either expansion or contraction. Einstein originally added a "cosmological constant" in an attempt to make it static, something he later called his "biggest blunder". This got pulled when the universe proved to actually be expanding, and then reinstated to describe the acceleration in the expansion.

Metricyard
2008-Oct-16, 01:08 PM
Well, GR did predict the big bang. Credit to these hypotheses go to Alexander Alexandrovich Friedman (http://en.wikipedia.org/wiki/Alexander_Friedman),and Georges Henri Joseph Édouard Lemaître (http://en.wikipedia.org/wiki/Georges_Lema%C3%AEtre), among others.

stitt29
2008-Oct-16, 02:45 PM
Great, got it. GR did predict expansion (or contraction). What is expanding? Is it the distance between masses? Does it also calculate the mass of the Universe?

Ken G
2008-Oct-16, 03:33 PM
What is expanding? Is it the distance between masses?Yes, but only on the largest scales we see in the universe. On smaller scales, like the solar system say, there is no expansion in those distances, because local gravity effects are far more important and prevent said expansion. General relativity can handle gravity on all scales, so it does predict this basic dichotomy.

Does it also calculate the mass of the Universe?Like all theories, GR requires experimental input to determine the relevant parameters of the universe. So if you take observations, and fold them together with GR, you can infer the required mass density that would allow agreement between observations and theory (perhaps requiring dark matter and dark energy to achieve the desired consistency). If that's what you mean, then yes, and if not, then no.

Note also that some people strangely see it as a kind of "cheat" to allow GR to "look first" and then set its basic cosmological parameters in such a way as to achieve consistency, but that's exactly what science always does, it's how you do science.

dhd40
2008-Oct-16, 04:53 PM
(snip)
Like all theories, GR requires experimental input to determine the relevant parameters of the universe. So if you take observations, and fold them together with GR, you can infer the required mass density that would allow agreement between observations and theory(snip)
my bold!

I think thatīs the important point: GR doesnīt determine the mass of the universe but tells something about its mass density (depending on expanding, or not, etc)


Note also that some people strangely see it as a kind of "cheat" to allow GR to "look first" and then set its basic cosmological parameters in such a way as to achieve consistency, but that's exactly what science always does, it's how you do science.

Definitely! And itīs so easy to understand if, e.g., you look at some basic math: If I predict correctly that the correlation between x and y is linear then it doesnīt contradict my prediction if the values of the constants a and b in y = a + bx have to be adjusted to a specific situation. Thatīs not "cheating", itīs science!

stitt29
2008-Oct-16, 08:54 PM
As there are blueshifted galaxies does GR also predict that space is contracting (and expanding at the same time). I looked up blueshifted galaxies got this site
http://curious.astro.cornell.edu/question.php?number=75
and then a link to a NASA site
http://nedwww.ipac.caltech.edu/cgi-bin/nph-allsky?ra_constraint=Unconstrained&ra_1=&ra_2=&dec_constraint=Unconstrained&dec_1=&dec_2=&glon_constraint=Unconstrained&glon_1=&glon_2=&glat_constraint=Unconstrained&glat_1=&glat_2=&z_constraint=Less+Than&z_value1=0&z_value2=&z_unit=km%2Fs&ot_include=ANY&ex_objtypes1=Clusters&ex_objtypes1=Supernovae&ex_objtypes1=QSO&ex_objtypes2=AbsLineSys&ex_objtypes2=GravLens&ex_objtypes2=Radio&ex_objtypes2=Infrared&ex_objtypes3=EmissnLine&ex_objtypes3=UVExcess&ex_objtypes3=Xray&ex_objtypes3=GammaRay&nmp_op=ANY&out_csys=Equatorial&out_equinox=B1950.0&obj_sort=RA+or+Longitude&zv_breaker=30000.0
which states there are 6582 blueshifted galaxies. This does not seem to be consistent with an expanding universe. Any explanations?

Spaceman Spiff
2008-Oct-16, 09:56 PM
As there are blueshifted galaxies does GR also predict that space is contracting (and expanding at the same time). I looked up blueshifted galaxies got this site
http://curious.astro.cornell.edu/question.php?number=75
and then a link to a NASA site
http://nedwww.ipac.caltech.edu/cgi-bin/nph-allsky?ra_constraint=Unconstrained&ra_1=&ra_2=&dec_constraint=Unconstrained&dec_1=&dec_2=&glon_constraint=Unconstrained&glon_1=&glon_2=&glat_constraint=Unconstrained&glat_1=&glat_2=&z_constraint=Less+Than&z_value1=0&z_value2=&z_unit=km%2Fs&ot_include=ANY&ex_objtypes1=Clusters&ex_objtypes1=Supernovae&ex_objtypes1=QSO&ex_objtypes2=AbsLineSys&ex_objtypes2=GravLens&ex_objtypes2=Radio&ex_objtypes2=Infrared&ex_objtypes3=EmissnLine&ex_objtypes3=UVExcess&ex_objtypes3=Xray&ex_objtypes3=GammaRay&nmp_op=ANY&out_csys=Equatorial&out_equinox=B1950.0&obj_sort=RA+or+Longitude&zv_breaker=30000.0
which states there are 6582 blueshifted galaxies. This does not seem to be consistent with an expanding universe. Any explanations?

A small fraction of these in the latter list are classified as "galaxy" (your filter has the Object type set to ANY and does not exclude stars -- :doh:). Most of these are stars. So ~100 billion galaxies in the visible universe, you're concerned about ~100 nearby ones that have blueshifts? (distances known via independent means)

I believe this concept has been explained to you elsewhere (another thread).

stitt29
2008-Oct-16, 10:29 PM
Yes I remember. but the explanation is inadequate. Either GR is right or wrong. blueshifted galaxies means its wrong surely. Even if the objects are stars Why are they getting closer if the Universe is expanding?

Cougar
2008-Oct-16, 10:38 PM
Yes I remember. but the explanation is inadequate.

There are two things happening. (1) The universe is expanding (detectable only on very large scales), and (2) things within the universe are moving around relative to each other. It is (2) that causes some objects to be blueshifted relative to our vantage.

Note: These two things essentially have different mechanisms. (1) is termed cosmological redshift, and (2) results from doppler shift, i.e., from actual movement through space.

DrRocket
2008-Oct-16, 10:54 PM
Einstein's General theory of relativity came out in 1915. Hubble found the Universe to be expanding in 1921-22. GR didn't predict an expanding Universe. Big Bang theory states the universe expanded from a singularity. Why is big bang theory dominant when GR didn't predict expansion? What is the relationship between BBT and GR?

If you throw into the pot that the universe is observed to be expanding and that there is a certain amount of mass in the universe then the Big Bang does follow from general relativity. See Hawking and Ellis, The Large Scale Structure of Space-Time. I think you also find there a citation of a paper by Hawking and Penrose for that result.

Ken G
2008-Oct-16, 10:57 PM
Either GR is right or wrong. blueshifted galaxies means its wrong surely. Even if the objects are stars Why are they getting closer if the Universe is expanding?That was what I meant, just a few posts above, when I said the expansion is "only on the largest scales we see in the universe", and that "general relativity can handle gravity on all scales, so it does predict this basic dichotomy." It sounds like others have explained this to you as well, so it just doesn't sound like you are trying very hard here. If you already hold to a particular conclusion and are not willing to listen to people who know how the theory actually works, why bother asking?

The more I think about this, the more mystifying your stance becomes. Do you really think that your objection to general relativity (which, of course, is totally unfounded for exactly the reasons I gave, as well as others) is something that no one has thought of? A century of general relativity, involving minds like Einstein and Peebles, but they missed the simple fact that general relativity must be wrong if anything is blueshifted? Or perhaps you imagine some global conspiracy, where everyone knows general relativity fails this childishly simple test, but they are just hoping no one else notices? I can't even think of a single way to imagine your position as being anything but an example of completely uncritical thinking. Or perhaps you are joking? I'm seriously curious as to what you think is happening here.

DrRocket
2008-Oct-16, 11:40 PM
Yes I remember. but the explanation is inadequate. Either GR is right or wrong. blueshifted galaxies means its wrong surely. Even if the objects are stars Why are they getting closer if the Universe is expanding?

Yep. GR is either right or wrong.

It is probably wrong since it is completely deterministic and quantum mechanics seems to indicate that things ultimately depend on probabilities.

Quantum mechanics is also probably wrong since it seems to be incompatible with relativity in fundamental ways in which relativity seems to provide insight that nothing else does.

Newtonian mechanics and gravity are also wrong as demonstrated by the inability to predict phenomena that are better modeled and explained via relativity.

Classical mechanics and electrodynamics are wrong since they need to be modified by quantum field theories to predict the behavior of sub-atomic particles.

Quantum field theories are also likely wrong since they require the rather ad hoc procedure of renormalization to produce sensible answers and avoid infinities. Also they seem to be incompatible with general relativity.

Unbfortunately all of these "wrong" theories also produce exquisitely accurate predictions when used in appropriate situations. And they are absolutely the best theories that human minds have been able to develop.

Now what ?

Spaceman Spiff
2008-Oct-17, 02:23 AM
Yes I remember. but the explanation is inadequate. Either GR is right or wrong. blueshifted galaxies means its wrong surely. Even if the objects are stars Why are they getting closer if the Universe is expanding?

Wow. So you've learned nada. :sick: If the explanations were inadequate, is it because they are inadequate or because your understanding of them is? Or maybe, as the attitude of your questions seems to reveal, you already know what you believe.

I'll ditto Ken G:

If you already hold to a particular conclusion and are not willing to listen to people who know how the theory actually works, why bother asking?

So now consider the following:
Why aren't you expanding? Why isn't the Earth, Sun and solar system expanding (and becoming unbound)? Why not the Galaxy (and so becoming unbound)? Why are galaxies within the local group not (systematically and unbounded) moving away? Hmmm...something is wrong, but nothing you have said (and none of the aforementioned observational facts) have demonstrated that either GR or the big bang are wrong or inconsistent with one another.

Spaceman Spiff
2008-Oct-17, 02:28 AM
Einstein's General theory of relativity came out in 1915. Hubble found the Universe to be expanding in 1921-22. GR didn't predict an expanding Universe. Big Bang theory states the universe expanded from a singularity. Why is big bang theory dominant when GR didn't predict expansion? What is the relationship between BBT and GR?

For the record, Hubble & Humason's work on redshift and distance came out around 1928-29. Hubble did not initially attribute his finding to those of Einstein's.

stitt29
2008-Oct-17, 12:16 PM
yep, must have it wrong. Blueshift down to things moving around relative to each other. Everything created from a singularity 13.7 billion years ago

Ken G
2008-Oct-17, 01:39 PM
yep, must have it wrong. Blueshift down to things moving around relative to each other. Everything created from a singularity 13.7 billion years agoNow you are starting to get released from your misconceptions about the model, but there are a few misconceptions you still hold. You seem to now understand that saying "general relativity requires that all space must expand if any of it does" is a misconception-- actually the dynamical behavior of space depends on the dynamical behavior of the matter in that space, and the latter plays out rather differently in a solar system, in a galaxy, and in a galaxy cluster, compared to on the scale of the universe as a whole. So much for "cosmological redshifts" (the latter) and "regular Doppler shifts" (the former). So what misconception remains in your thinking?

The remaining misconception is that the "Big Bang model" says the universe "started in a singularity". You are to be forgiven for this misconception, because it is repeated everywhere. But that is not the scientific theory we call the Big Bang in astronomy, that is the popularized version that has had its science removed and all that remains is a creation myth. The fact is, science is terrible at origins, but good at cause-and-effect connections, so therefore science can only tackle origins when they can be seen as an effect of something else, not a "first cause".

Thus, the scientific version of the Big Bang model that you hear being referenced in a science forum is an evolution model-- it tells us what the universe did after it came into being. No physics has the vaguest idea what originated the universe, and no physical theory makes any claims to the contrary. What's more, no physical theory claims anything is "really" ever a singularity, what "singularity" means is "something else happens that we don't know anything about". So it is a very interesting aspect of the Big Bang model that it contains an explicit reference to its own limitations. That's all the term "singularity" is intended to convey-- there is no reason or justification in imagining that the "whole universe was once a single point". No scientist has the least idea what those words are intended to mean.

One final point: I detect an attitude in your tone that you somehow think that all we want is your lip-service acquiescence to some kind of dogma, as if that meant a thing to us. We are not a "Big Bang believers" club, like "the flat Earth society". In actual fact, it is the "Big Bang deniers" who function like such a club, based in a prevailing ignorance on the very topic they purport to reject (and you must confess you are a pretty good example of this phenomenon). As for us, we haven't the least care in the world what mantra you are willing to repeat. If you have a sincere desire to understand the Big Bang model in ways that you currently do not, we are happy to help you gain that understanding. Blind allegiance to either position, and whether it is sincere or pretended, is of no concern to us whatsoever.

Bakihtar
2008-Oct-17, 02:04 PM
redshift blueshift think of the dopler effect

also think of cars traveling down a freeway not all are going the same direction or the same speed

if you were to see the spectrum of the cars some would be blue some red

traveling galaxy's arent that hard to understand just dont throw wrenches into the works and defeat yourself

Spaceman Spiff
2008-Oct-17, 02:20 PM
Bakihtar -

May I suggest changing your slogan at the bottom of your posts:

WE did go to the moon and it's my belief the streets should run red with the blood of the nonbelievers :eek:

Streets running with blood isn't a civil concept, and relating that to "non-believers" represents the very antithesis of the BA forum. Not that conformity is required, but civility is.
Moderator?

Bakihtar
2008-Oct-17, 02:26 PM
hummmm didn't think like that ill change it asap

stitt29
2008-Oct-17, 03:40 PM
Ken G, thanks for the reply. I wouldn't think of myself as a Big Bang denier. But I do think it doesn't answer a lot of things.
1.All matter created out of nothing.
2. Universe was Xm^3 with a temperature of y degrees Kelvin
now it is 1000Xm^3 with a temp of y/1000 degrees Kelvin = 2.7 degrees Kelvin (i have used 1000 but this is obviously another unknown)
3 expanding from a point or even a primeval atom (which you have told me isn't actually the theory). What is it then please? (honest question)
4 The Vacuum of space has an energy that pushes galaxies apart (faster than the speed of light) but does not expand between galaxies because they are gravitationally bound. This bit sounds a bit like aristotle and the arrow, we need some force because the arrow is moving, Gallileo put him straight with the law of inertia. Especially since dark energy is never measured just assumed to be there.

If you can set me straight on some of these assumptions I could perhaps understand things better. I can see my tone seems derogatory (not intentional) but it's because I just don't get the theory. There seems to be a fudge for everything that doesn't make sense.

dhd40
2008-Oct-17, 03:55 PM
Bakihtar -

May I suggest changing your slogan at the bottom of your posts:
:eek:

Streets running with blood isn't a civil concept, and relating that to "non-believers" represents the very antithesis of the BA forum. Moderator?

Youīre right in principle. But I donīt think Bakihtar meant his slogan to be taken literally.

Nereid
2008-Oct-17, 04:55 PM
[...]

3 expanding from a point or even a primeval atom (which you have told me isn't actually the theory). What is it then please? (honest question)

[...]
No doubt someone will be along shortly with links to detailed explanations, which will range in complexity, depth, etc from quite simple to full-blown grad-level textbooks, and others will write some answers directly ...

Before that happens, I'll take a shot at just this one.

In most expositions* of cosmological models*, these last few years, the observable universe (little or nothing said about parts of the universe not observable) can be modelled as being very hot and very dense ~13.7 billion years ago (the actual 'age' is model-dependent).

Depending on which models you are considering, or which parts of the Standard Model of particle physics (SM) you include^, or what SM extensions you include, or which 'new physics' ideas you add, or ... the size, temperature, density, and composition can be specified; all models are silent on what the observable universe was like 'before' then.

AFAIK, with the possible exception of models (or proto-models) that incorporate new physics which claims to address the intolerable mutual incompatibility of GR and the quantum theory that underlies the SM, no models assume (or produce, as 'predictions') either "a point or even a primeval atom". IOW, there is no such assumption.

I started a Q&A thread recently on what the 'axioms' of the BBT are; you might be interested to read it and, perhaps, join in the discussion: What are the "axioms" of the Big Bang theory? (http://www.bautforum.com/questions-answers/79820-what-axioms-big-bang-theory.html)

* yes, plural; there are many different models; 'Big Bang Theory' is a misnomer.

^ AFAIK, all models include General Relativity (GR), though some may do so with modifications.

Ken G
2008-Oct-18, 02:16 AM
Ken G, thanks for the reply. I wouldn't think of myself as a Big Bang denier. But I do think it doesn't answer a lot of things. It isn't supposed to answer all things, only what we have access to with observations. Sometimes that theory is a bit oversold, in terms of what it is actually saying compared to what some people claim it is saying.

1.All matter created out of nothing.That's a classic example of what I mean-- the Big Bang theory says no such thing. It simply has no answer to where the matter came from, nor was it ever its charge to say that.


2. Universe was Xm^3 with a temperature of y degrees Kelvin
now it is 1000Xm^3 with a temp of y/1000 degrees Kelvin = 2.7 degrees Kelvin (i have used 1000 but this is obviously another unknown)Yes, that is more like what the theory actually says-- since those numbers X and y can be extracted from the observations using the theory.

3 expanding from a point or even a primeval atom (which you have told me isn't actually the theory). What is it then please? (honest question)Nobody has any idea. Perhaps the most important aspect of the Big Bang theory is that it points very clearly to its own limitation-- instead of pointing to a universe whose attributes and laws have always been the ones we see now, it points to a universe that was at some point totally different from anything we could even recognize, or our greatest laboratories could simulate. That is the real meaning of "the singularity", nothing more.


4 The Vacuum of space has an energy that pushes galaxies apart (faster than the speed of light) but does not expand between galaxies because they are gravitationally bound. This bit sounds a bit like aristotle and the arrow, we need some force because the arrow is moving, Gallileo put him straight with the law of inertia.But it stems directly from general relativity. It's not any kind of added postulate to that theory, it is that theory.


Especially since dark energy is never measured just assumed to be there.
No one likes additional parameters in a theory, but when you need them, you need them. When the quark model was found to help understand protons and neutrons, we might think "I liked it better when there were just protons and neutrons, now we have even more fundamental particles to learn about", but so it goes in the progress of science. We don't get the universe we'd like, we get the universe we get.


I can see my tone seems derogatory (not intentional) but it's because I just don't get the theory.Your tone wasn't really so bad. It's more that we've all seen people who didn't like the Big Bang but didn't really know much about it, so we lose patience.

mugaliens
2008-Oct-19, 10:30 AM
Hubble found the Universe to be expanding in 1921-22.

Was the universe not expanding in years other than 1921 and 1922?

:whistle:


GR didn't predict an expanding Universe. Big Bang theory states the universe expanded from a singularity. Why is big bang theory dominant when GR didn't predict expansion? What is the relationship between BBT and GR?

GR (http://en.wikipedia.org/wiki/General_relativity)describes gravity as a function of mass-energy and linear momentum of matter and radiation. While it's often thought to be limited to snap-shots of the universe as it is, the summation of all GR equations as applied to all matter and energy in the universe should backwards extrapolate to the singularity of the Big Bang.

However, one major stumbling block has been the reconcilation of gravitation (http://en.wikipedia.org/wiki/Gravitation)with the electronuclear force, or GUT (Grand unification theory (http://en.wikipedia.org/wiki/Grand_unification_theory)). The LHC's (http://en.wikipedia.org/wiki/Large_Hadron_Collider)search for the Higgs boson (http://en.wikipedia.org/wiki/Higgs_boson)is another step towards a Theory of Everything (http://en.wikipedia.org/wiki/Theory_of_everything#Modern_physics). However, linking at the GUT level requires energies of 1016, which is far greater than anything that could be reached by any Earth-based collider, and unification of GUT with gravity would require energies in the Planck range, roughly 1019.