Comparable size of the observable universe immediately after inflation

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Discussion Overview

The discussion revolves around the size of the observable universe immediately after the inflationary period in cosmology. Participants explore various estimates of the universe's size post-inflation and the nature of expansion rates during and after this epoch, touching on theoretical implications and models.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant inquires about the size of the observable universe immediately after inflation, suggesting it could be comparable to the size of the Milky Way Galaxy.
  • Another participant estimates that the universe was about the size of a grapefruit after inflation.
  • A different participant notes the difficulty in quantifying the size of the universe post-inflation, mentioning various estimates ranging from the size of a marble to much larger scales, depending on initial assumptions about the universe's size before inflation.
  • Concerns are raised about the nature of expansion after inflation, questioning whether the expansion rate remained aggressive or slowed down significantly once inflation ceased.
  • One participant explains that the expansion rate transitioned from accelerating to decelerating after inflation, influenced by changes in the dominant energy components of the universe.
  • Another participant clarifies the distinction between metric expansion of space and the growth of the cosmological horizon, emphasizing that the observable universe's size did not grow significantly during inflation.
  • There is a discussion about the scalar field responsible for inflation and its behavior after the inflationary epoch, with a mention of the Hubble constant's decline over time.
  • A participant expresses agreement with a previous point regarding the change in expansion rate after inflation.

Areas of Agreement / Disagreement

Participants express a range of estimates regarding the size of the universe after inflation, indicating a lack of consensus. There is also a discussion about the nature of expansion rates post-inflation, with differing interpretations of how aggressive the expansion remained.

Contextual Notes

Participants acknowledge the uncertainty in quantifying the size of the universe immediately after inflation, highlighting the dependence on initial conditions and assumptions. The discussion also reflects the complexity of understanding the transition from inflation to subsequent expansion phases.

Herbascious J
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Regarding scale. How big was the region of the universe we now understand to be the observable universe, immediately after early inflation? I'm tyring to understand the scale. I know our sphere of observation is very approximately 45 billion light years or something, I don't know for sure, but I am curious to get a feel for how much expansion took place after inflation (according to scientists like Alan Guth). Was it the size of the Milky Way Galaxy? Thank you in advance.
 
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Pretty certain that after inflation it was about the size of a grape fruit...
 
It is very difficult to quantify the size of the observable universe after inflation ended. We do not know how 'big' it was when inflation started, how rapidly it doubled in size ['inflated'], or how long the inflationary period persisted. Estimates of size after inflation vary wildly. Alan Guth guestimated it was around the size of a marble. Lineweaver estimates the universe grew by a factor of ~10E30 during inflation - re: http://arxiv.org/abs/astro-ph/0305179v1. By that standard, if you assume the observable universe was a Planck length prior to inflation, you end up with a size of about 1.6E-05 meters after inflation. If you assume it was the size of a proton, you get a universe of about 1.6E15 meters, or around 1/10 the size of our galaxy.
 
Wow, that is a wide range... Ok, so what I see then is that the region was appreciably smaller than the observable universe now. So, the velocity of expansion was quite large, but now things have cooled off so to speak. So my question is; did this 'escape' velocity or expansion remain as aggressive and just continue out into the current Hubble flow, or, once inflation stopped, did the velocities drop significantly at that moment, like a braking? Thanks for the explanations.
 
The expansion rate changed quickly after inflation: it went from accelerating to decelerating, passing from a vacuum dominated spacetime, through a brief period of matter domination, then to radiation. As the dominant energy component of the universe changes, so does the expansion rate.

I'm confused by your question regarding the "expansion remain[ing] as aggressive and just continu[ing] out into the current Hubble flow" -- the expansion is uniform throughout space and is responsible for the Hubble flow. A key distinction that is important for understanding all this is that between: the metric expansion of space and the growth of the cosmological horizon -- are you comfortable with the difference between these ideas? During inflation, the amount of expansion that the universe underwent was huge (points in space that were 1 'unit' apart at the start of inflation ended up something like 10^26 'units' apart at the end). But the horizon, marking the size of the observable universe, scarcely grew at all during inflation.
 
Yes, I think I'm clear. I was only using the term 'observable universe' to get a feel for the scale, not to inquire about how it changes over time. Putting things into units like you do above ("points in space that were 1 'unit' apart...") is much more clear. I think I was just trying to ask if the expansion is halted to some degree once inflation stops. I don't think it does, but I wasn't sure.
 
Under current models, the scalar field responsible for inflation flattened out after the inflationary epoch ended and the universe entered into what is now called slow roll inflation. The Hubble constant was very large during that time, but, has since declined.
 
Herbascious J said:
Yes, I think I'm clear. I was only using the term 'observable universe' to get a feel for the scale, not to inquire about how it changes over time. Putting things into units like you do above ("points in space that were 1 'unit' apart...") is much more clear. I think I was just trying to ask if the expansion is halted to some degree once inflation stops. I don't think it does, but I wasn't sure.
The rate of expansion changed after inflation: importantly, it went from accelerating (in the sense that the scale factor, a(t), had a positive second derivative wrt time, \ddot{a} > 0) to decelerating.
 
I agree with bapowell.
 

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