Is Our Universe a Causal Patch Stretched by Inflation?

[Cerenkov]

Hello.
I'm not entirely sure how to go about asking these questions - so please bear with me if I trip up in the framing of them. Any helpful correction or advice is welcome, btw.

I've done some reading up on how inflationary theory tries to solve the horizon problem...

http://www.astronomynotes.com/cosmolgy/s12.htm
http://philosophy-of-cosmology.ox.ac.uk/horizon-problem.html
http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/cosmo.html

...and how a pre-inflationary causal patch of uniform temperature is postulated to do this.

So, is it reasonable to say that...

A.
Our universe can be considered to be this causal patch, stretched by inflation to a size far larger than any kind of cosmological horizon centered upon us?

B.
But our universe shouldn't be considered to be any causally disconnected regions that might have arisen from subsequent episodes of inflation?

Thanks,

Cerenkov.

 

[mathman]

Any description of the universe before inflation is highly speculative. your questions can only be answered by educated guesses.

 

[MikeinSpain]

Inflation was an idea to explain a problem with the big bang. The way I looked at it is that it was unnecessary.

If the BB expanded into complete nothingness, then there are no biases. As to the material of the original "singularity", that would have been uniformly spread too, so no biases there either.

As I see it, a homogenised Universe would naturally come about without inflation, and there is the problem. How do large objects like galaxies and supermassive black holes form so quicky?

Galaxy GN-z11 which has about a billion solar masses is believed to have formed just 400 million years after the big bang. And as we get better telescopes, we keep finding older objects so there may be even older galaxies.

SMBH's were first thought to have formed around 1400 million years after the big bang but the present record holder at 800 million solar masses is just 690 million years after the BB. A point here is that it might be thought that galaxies formed around SMBH's, so does GN=z11 have an SMBH?

 

[Bandersnatch]

A.
Our universe can be considered to be this causal patch, stretched by inflation to a size far larger than any kind of cosmological horizon centered upon us?
B.
But our universe shouldn't be considered to be any causally disconnected regions that might have arisen from subsequent episodes of inflation?

I think whenever people talk about 'universe', they indeed tend to refer to what can be described by Big Bang cosmology only.
In the context of inflation. I've seen the non-inflating patches described as 'bubble universes' and the larger inflationary landscape as the multiverse. But I'm not sure there is any definitive nomenclature here. One could also lean on the semantics of the 'universe' as being all there is, and insist that the non-inflating patches are just specific regions in the universe.
What one calls 'the universe' is context-dependent, and a bit of a personal choice, is what I'm saying.

 

[Bandersnatch]

Inflation was an idea to explain a problem with the big bang. The way I looked at it is that it was unnecessary.

If the BB expanded into complete nothingness, then there are no biases. As to the material of the original "singularity", that would have been uniformly spread too, so no biases there either.

One cannot arrive at the BB model unless one assumes homogeneity and isotropy, so in the model there can be no 'nothingness'. Insisting that there was is discussing something else than the BB model, and as such is outside the scope of this forum. In BB the singularity spans the entire space at time 0, and similarly, the material fills all of space.

The problem with BB which inflation explains is that without it there is no reason for unbiased, uniform spreading of material, if that material has always been causally disconnected. Since in BB everything starts out disconnected*, one patch of the universe could not 'know' what unbiased means - i.e. there was no physical process that could homogenise the universe, let alone to such precise extent as observed.

*with passing time the light cone of any observer always grows to encompass more and more material, starting from none at the singularity.

 

[MikeinSpain]

When I say expanding into nothingness, I am saying the singularity if that is what you wanted to call it was all there is, so when it expanded, there was nothing that could in any way impede any part of it and so create bias.

Surely in the BB evertything did start connected as it all came from the same singularity and everty thing was all created, as is, each at the same moment?

If one billion people each flip a coin one billion times each, they are going to balance out to pretty much exactly half heads and half tails. What they "know" has no bearing on the result.

Any difference in temperature would come about how? Any part of the big bang would expand faster than another part how? Remember, this is not an explosion.

 

[Bandersnatch]

Surely in the BB evertything did start connected as it all came from the same singularity and everty thing was all created, as is, each at the same moment?

No, everything was disconnected. See the following graph:

(source: Lineweaver & Davis, 2003)
At time 0 the light cone has zero extent. As time passes, neighbouring regions gradually become connected (before disconnecting again), but e.g. the regions opposite on the sky have never been in causal contact.
(the singularity on this graph is the entire x-axis extending to infinity in both directions)

 

[Bandersnatch]

If one billion people each flip a coin one billion times each, they are going to balance out to pretty much exactly half heads and half tails. What they "know" has no bearing on the result.

Let me address this bit too.
Let's assume for the moment that this is a good analogy for primordial matter distribution (it's not binary, but an infinite, or at least very large, range of values).
If one billion people flip a coin one billion times, then - even as the average will be 50-50 heads and tails - there will be islands of clustering, where e.g. 100 people, one next to another, flipped heads five times in a row.
Without a physical process accounting for equalising such clusters despite their causal disconnect, we should see them on the sky as regions with different CMBR temperatures.

Any difference in temperature would come about how? Any part of the big bang would expand faster than another part how? Remember, this is not an explosion.

What you did here is implicitly assume that the entire universe was homogeneous at t=0. You then ask what process could possibly account for deviations from this homogeneity, which is begging the question.

 

[PeroK]

Surely in the BB evertything did start connected as it all came from the same singularity and everty thing was all created, as is, each at the same moment?

Nothing can come from a singularity.

 

[Cerenkov]

"I think whenever people talk about 'universe', they indeed tend to refer to what can be described by Big Bang cosmology only.
In the context of inflation. I've seen the non-inflating patches described as 'bubble universes' and the larger inflationary landscape as the multiverse. But I'm not sure there is any definitive nomenclature here. One could also lean on the semantics of the 'universe' as being all there is, and insist that the non-inflating patches are just specific regions in the universe.
What one calls 'the universe' is context-dependent, and a bit of a personal choice, is what I'm saying."

Thank you for this help, Bandersnatch.

Btw, your graphic...

...looks interesting.

Could you please talk me through how I should go about understanding it properly?

Thanks again.

Cerenkov.

 

[Bandersnatch]

Could you please talk me through how I should go about understanding it properly?

I included annotated versions of these graphs in this post:
https://www.physicsforums.com/threa...increase-bc-of-expansion.912881/#post-5754083
See if it answers at least some of the questions you might have.

 

[Cerenkov]

Bandersnatch,

A quick glance at your linked info tells me that it's going to answer a LOT of my questions.

So, thank you very, very much indeed!

Cerenkov.