Entropy of the initital state of the universe

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Discussion Overview

The discussion revolves around the concept of entropy in relation to the initial state of the universe, exploring theoretical implications, observational limitations, and interpretations of entropy in cosmology. Participants engage with ideas presented by Roger Penrose regarding the organization of the universe and the implications of the second law of thermodynamics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant references Roger Penrose's assertion that the universe's initial state was highly ordered, suggesting a mathematical representation of this order as 10^123.
  • Another participant questions the entropy value of a universal singularity, indicating uncertainty about its implications.
  • Some participants clarify the concept of the past light cone, explaining it as the observable region of the universe due to the finite speed of light and the expansion of the universe.
  • There is a discussion about the limitations of observing events immediately after the Big Bang, with questions raised about how knowledge of early universe events is obtained despite observational constraints.
  • One participant proposes a controversial view that the initial, current, and final entropy of the universe could be zero, suggesting a definition of entropy based on entanglement with the environment.

Areas of Agreement / Disagreement

Participants express differing views on the nature of entropy, the implications of the past light cone, and the understanding of the universe's early moments. No consensus is reached on the value of entropy at the universal singularity or the implications of Penrose's claims.

Contextual Notes

The discussion highlights limitations in observational capabilities regarding the early universe and the dependence on theoretical models to interpret entropy and organization. Some assumptions about the nature of the universe and entropy remain unresolved.

revo74
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I saw an old interview with Roger Penrose where at one point he was talking about the degree of organization the universe exhibited at it's initial state. He said the second law of thermodynamics tells us as time passes the universe is becoming more disorderly, which means if we were to go back in time the universe would become more and more ordered/organized. He said the degree of organization the universe was in was so special that the mathematical figure representing/describing it would be at least 10^123. Can someone please explain to me how he came up with this figure. It's a very large number obviously. How does this large number represent degree of order?

Link to the video (begin at 4:58): http://www.youtube.com/watch?v=pEIj9zcLzp0
His published paper: accelconf.web.cern.ch/accel...nf/e06/PAPERS/THESPA01.PDF

In the opening of this paper he mentions initial state numerous times. Later on he says this though:

"To deal with a spatially infinite universe, I shall assume that we need consider only, say, that comoving portion of the universe that intersects our past light cone. This contains something of the order of 10^80 baryons."

What time in the universes' history would the "portion of the universe that intersects our past light cone" be? Is he talking about present time? If not, when?
 
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About entropy, what value would you assign to a Universal singularity?
 
Doug Huffman said:
About entropy, what value would you assign to a Universal singularity?

I don't know. I am a laymen. Was hoping for some insight regarding my OP.
 
Not sure about the first question, but our past light cone refers to the region of the universe which we can observe due to signals from the universe having enough time to reach us since the beginning of time. The edge of this region is also known as the particle horizon, which is the farthest part of the universe which we can see. Let us assume for a moment that the entire universe is infinite in extent (might be wrong, but we don't know how big it is). We can only see a small region of space around us. As the universe ages, light from farther away will have time to reach us, and our observable universe is getting bigger all the time.

We can't say how many particles there are in the universe. We can only estimate the particles in the observable part of the universe, which is the space enveloped by our past light cone
 
The Eddington Number, NEdd, has good provenance.

As I recall, Barrows and Tipler review it in their The Anthropic Cosmological Principle.
 
Khashishi said:
Not sure about the first question, but our past light cone refers to the region of the universe which we can observe due to signals from the universe having enough time to reach us since the beginning of time. The edge of this region is also known as the particle horizon, which is the farthest part of the universe which we can see. Let us assume for a moment that the entire universe is infinite in extent (might be wrong, but we don't know how big it is). We can only see a small region of space around us. As the universe ages, light from farther away will have time to reach us, and our observable universe is getting bigger all the time.

We can't say how many particles there are in the universe. We can only estimate the particles in the observable part of the universe, which is the space enveloped by our past light cone

If we can only see as far back in time as light takes to travel to us then how do we know what happened a split second after the Big Bang? We are told all these things about what happened in the universe during the first few seconds after the universe underwent inflation and so forth, but how do we know this is we can't observe it? The CMB was roughly 380k years after the Big Bang, which suppose to be as far back as we can see. Is there any good reason to believe the universe is actually much older than 13.79 billion years old?
 
Paraphrasing E. T. Jaynes (Probability Theory: The Logic of Science) only cause and effect are constrained to the arrow of time, logic is not so.
 
revo74 said:
If we can only see as far back in time as light takes to travel to us then how do we know what happened a split second after the Big Bang?

We can only observe light from the point in the universe's history when the CMB was emitted. But we can make other observations that tell us about events that happened before that. Light is not the only way of observing.
 
Here is my glib answer... the initial entropy of the universe was 0, the current entropy of the universe is 0, the final entropy of the universe will be 0. BUT... since in QM entropy is not additive you can have non-zero entropies for parts of the universe which do not add to the entropy of the whole. I define entropy as a measure of entanglement with the environment and so the "universe as a whole" with "no environment" has no entropy.
 

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