Big bang ordinary explosion, evidence for expansion of space?

[jostpuur]

The standard (popular) explanation about big bang goes like this: "We observe red shift in distant galaxies in all directions. One might think that this means that we are in the center of the universe, where some explosion occured, but there exists a more likely explanation: The universe is an expanding manifold, and it doesn't matter where you are, all galaxies always appear to move away from you."

For sake of considering alternative explanations, consider that a universe is a flat space, and somewhere there (in position $$x'$$) occurs an ordinary explosion, and in such manner that the speed distribution of projectiles depends linearly on the distance from the center of the explosion. Like this:

$$v(x) \approx a(x - x')$$

where $$a>0$$ is a real constant, and $$x\approx x'$$ is a point somewhere close to the explosion. This means that projectiles very close to the center are moving with small velocities, and projectiles farther away move with larger velocities. This is not impossible at all, if the explosion originally occurred as result of some repulsive interaction.

Well now too... IT DOESN'T MATTER WHERE YOU ARE IN THIS EXPLOSION, THE PROJECTILES (GALAXIES) AROUND YOU WILL ALWAYS APPEAR TO BE MOVING AWAY FROM YOU IN ALL DIRECTIONS! Because of this:

$$v(x + \Delta x) - v(x) \approx a\;\Delta x$$

for small $$\Delta x$$.

So when I'm been informed, that all galaxies around us are moving away from us, my first guess is that this is because an ordinary and large explosion has occurred long time ago, and we are somewhere among the projectiles of this explosion. But then I'm been told that this cannot be so. The red shift cannot be result of an ordinary explosion (meaning an explosion which has center somewhere in flat space), but this must be because the universe itself is a manifold that is expanding. What real evidence do you have for this claim?

Don't try to tell me that if we were in an ordinary explosion, and not in the center of it, then we would see red shift in one direction and blue shift in other one. That's not necessarily true.

 

[bcrowell]

The description you've given needs to be made into a mathematical model that is a solution of the Einstein field equations. That can be done, and the result is the Milne model: http://en.wikipedia.org/wiki/Milne_model As explained in the WP article, the Milne model is not compatible with current cosmological observations.

 

[mack_10]

I'm not sure that thinking of galaxies as projectiles is appropriate.

If I inflate a balloon all points on the surface of the balloon move away from each other trying to analyse each one of those points as a projectile from a common source of origin will give some interesting but false results.

Why assume an initial explosion at all why not a blanket casimir effect that continuously adds quanta of space/time and matter/energy per unit volume of extant space?

http://en.wikipedia.org/wiki/Casimir_effect

 

[jostpuur]

The description you've given needs to be made into a mathematical model that is a solution of the Einstein field equations. That can be done, and the result is the Milne model: http://en.wikipedia.org/wiki/Milne_model As explained in the WP article, the Milne model is not compatible with current cosmological observations.

From Wikipedia:

In particular it makes no prediction of the cosmic microwave background radiation nor the abundance of light elements which are hallmark pieces of evidence that cosmologists agree support Big Bang cosmology over alternatives.

How is the expanding manifold relevant in explaining CMB and other details of the hot past? Is it impossible for ordinary explosion that occurs in flat space to produce these same effects?

 

[jobigoud]

Don't try to tell me that if we were in an ordinary explosion, and not in the center of it, then we would see red shift in one direction and blue shift in other one. That's not necessarily true.

Hi,
I haven't done the maths, but instictively, I would say that the stuff that is at the same distance from the explosion than us, but on a different ray will not recede from us with the same speed than the stuff that is aligned on the same ray, but closer or farther from the explosion…

 

[Chronos]

OK, I've been into the eggnog so I can't resist. The big bang did not occur at a point embedded in some pre-existing spacetime continuum, it was the spacetime continuum. I do, however, encourage you to do the math. There is nothing instinctive about general relativity.

 

[zonde]

Hi,
I haven't done the maths, but instictively, I would say that the stuff that is at the same distance from the explosion than us, but on a different ray will not recede from us with the same speed than the stuff that is aligned on the same ray, but closer or farther from the explosion…

Take flat rubber sheet and stretch it without changing it's form and you will have the picture.

 

[zonde]

How is the expanding manifold relevant in explaining CMB and other details of the hot past? Is it impossible for ordinary explosion that occurs in flat space to produce these same effects?

These counterarguments from Wikipedia don't seem relevant.

But certainly accelerated expansion can't happen in such model without strong local effects and it can't happen smoothly. So there then should be some ad hoc solution for apparent acceleration of expansion.

 

[jobigoud]

OK, I've been into the eggnog so I can't resist. The big bang did not occur at a point embedded in some pre-existing spacetime continuum, it was the spacetime continuum. I do, however, encourage you to do the math. There is nothing instinctive about general relativity.

I wasn't applying my instinct to general relativity but at trying to refute the proposition that if there had actually been an explosion of matter into space the effects wouldn't necessarily be distinguishable from expansion of space itself.

This is what the original poster suggests, that the effects might not be distinguishable, and I think it may be true for 1D, but as soon as you move to 2D, you cannot mistake the two. Everything might also move away from you, but not at the same speed in all directions, even for small distances compared to the distance to the explosion center, and even if the speed increase linearly with explosion center distance…

 

[willem2]

From Wikipedia:



How is the expanding manifold relevant in explaining CMB and other details of the hot past? Is it impossible for ordinary explosion that occurs in flat space to produce these same effects?

With an ordinary explosion, the light from the CMB would expand in a spherical shell with distance ct from the center, and we would be inside this shell and so could never see the CMB.

 

[mack_10]

Surely if the CMB is due to a big bang then it would have radiated out from the source at the speed of light and only be detectable at the edge of the universe

 

[zonde]

Surely if the CMB is due to a big bang then it would have radiated out from the source at the speed of light and only be detectable at the edge of the universe

It is assumed that CMB is radiated out some time after big bang when intermediate space become transparent for photons.

 

[Vanadium 50]

That's if the universe expanded into pre-existing space. Since that's not what we see, we know that's not what happened.

 

[zonde]

That's if the universe expanded into pre-existing space. Since that's not what we see, we know that's not what happened.

Could you elaborate on this?
What would you expect to see in case of pre-existing space?

 

[Vanadium 50]

Nothing. The light would be "past us".

 

[jostpuur]

How is the expanding manifold relevant in explaining CMB and other details of the hot past? Is it impossible for ordinary explosion that occurs in flat space to produce these same effects?

This was my question, and I'm not seeing serious answers to it.

Isn't the origin of CMB something like this: Long time ago the space (or the region of space that we are familiar with) was filled with some hot fluid that was not transparent, and then it became transparent when it cooled, and left blackbody radiation into the transparent volume?

Why would this be in contradiction with ordinary explosion interpretation of the big bang?

How is the expanding manifold interpretation necessary in explaining the CMB?

 

[Ich]

Two things,

How is the expanding manifold interpretation necessary in explaining the CMB?

There is no "expanding manifold", it's "expanding space". Manifolds simply are, there's no dynamics.

Why would this be in contradiction with ordinary explosion interpretation of the big bang?

There's nothing wrong with the ordinary explosion, except that you get the details wrong.
We can see back to the inflationary era, and there absolutely no evidence to decide whether our visible universe started as a (very special) "explosion" in some preexisting universe or something entirely else.
The model we use is that of an infinite homogeneous space. This may be true, or (more likely?) it's rather some really, really huge region that underwent quite homogeneous expansion due to some as yet unknown "inflaton field". There's no observational difference between "infinite" and "huge", and the difference between explosion and expansion is rather a semantic one.
Just bear in mind that the simple "explosion" picture is ruled out by observational evidence. You need more complicated scenarios, including negative pressure (i.e. inflaton, dark energy) to model reality.

 

[Deuterium2H]

I wasn't applying my instinct to general relativity but at trying to refute the proposition that if there had actually been an explosion of matter into space the effects wouldn't necessarily be distinguishable from expansion of space itself.

This is what the original poster suggests, that the effects might not be distinguishable, and I think it may be true for 1D, but as soon as you move to 2D, you cannot mistake the two. Everything might also move away from you, but not at the same speed in all directions, even for small distances compared to the distance to the explosion center, and even if the speed increase linearly with explosion center distance…

I am not sure if anyone has clearly answered the Original Poster's (Jostpuur) question, and as further expanded upon by Jobigoud.

The simplest answer I can offer, to explain why the observed Universe and expansion of space could not have been the result of an "ordinary explosion of matter into a pre-existing space" is that this interpretation is both inconsistent with Observation evidence, but also violates the pillars of modern Cosmology: The Containment Principle, as well as the Cosmological Principle (all places in the Universe are alike).

If galaxies were moving outward from a single, centralized "explosion", then that means that there is an outermost shell of galaxies which are the farthest away from the "center". Those galaxies on the outer shell would be at an "edge", which violates the Containment Principle. Any observer in one of these outer shell galaxies would see a high density of galaxies in one direction, and...nothing in the other. This also violates the Location Principle, as this outermost observer would occupy a special, or "privileged" location. It also violates the Cosmological Principle as this observer would see a highly Anisotropic Universe. In one direction he sees a greater density of galaxies then in any other. We observe a highly Isotropic Universe, and combined with the Location Principle (that we do not occupy a special location), this implies a homogeneous Universe. Observed Isotropy and inferred homogeneity also means that no observer (wherever they are located) occupies any particular special or privileged location. All of these modern Cosmological principles would be violated if the observed Universe was the result of an "ordinary" explosion into a pre-existing space.

 

[Tanelorn]

In short there is no explosion just an expansion that happened everywhere at once. There is no edge or outermost shell of galaxies since the observable universe is pseudo infinite in size, far bigger than anything we can observe. eg. Penrose says 10 to power 30 times bigger.

 

[Tanelorn]

There is no edge or outermost shell of galaxies since the universe is pseudo infinite in size, far bigger than anything we can observe. eg. Penrose says 10 to power 30 times bigger.

Drop observable! :)

 

[Chronos]

The observable universe has a temporal edge called the surface of last scattering. It is located around z = 1100 from Earth [I am partial to redshift as a distance 'ruler' given 'proper distance' is virtually meaningless under GR]. z~1100 is the effective particle horizon of the universe in EM wavelengths. A neutrino telescope could peer beyond this veil, but, a neutrino telescope is highly impractical. A gravitational telescope is, however, very doable. We already have one up and running [LIGO] and another on in the works [LISA]. Gravity waves are so weak an immense detector is necessary.