Time from big bang / size of universe

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Discussion Overview

The discussion revolves around the concepts of the age of the universe, the implications of its size, and the nature of the big bang. Participants explore theoretical frameworks, cosmological models, and the relationship between observable phenomena and the universe's expansion.

Discussion Character

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

Main Points Raised

  • Some participants question how the elapsed time since the big bang can be known without understanding the universe's full extent and whether a larger universe would affect the contraction towards a singularity.
  • Others clarify that the big bang should not be viewed as an explosion in pre-existing space, suggesting that cosmological models do not start finite and become infinite.
  • A participant introduces inflationary cosmology, proposing that a negative pressure energy field could have caused rapid expansion, leading to observable limits defined by a "cosmic horizon."
  • Another participant discusses the age of the universe as measured from different observational perspectives, noting that observers near black holes or moving at relativistic speeds could perceive different ages.
  • One participant mentions the cosmic microwave background radiation (CMBR) and its significance in understanding the universe's early state, emphasizing that the light we observe has traveled vast distances due to ongoing expansion.
  • A later reply expresses uncertainty about the determination of the universe's age, attributing it to properties of the background radiation.

Areas of Agreement / Disagreement

Participants express various viewpoints, and while some concepts are clarified, there is no consensus on the implications of the universe's size or the interpretation of the big bang. Multiple competing views remain throughout the discussion.

Contextual Notes

Limitations include the dependence on definitions of the universe's size, the implications of different observational frames, and unresolved questions regarding the nature of the big bang and cosmic expansion.

Pjpic
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How can the elapsed time from the big bang be known if it isn't known how far the universe extends past the ability to observe it? In other words, would a larger universe take longer to contract towards a singularity? Or, if the universe were infinitely large how could it have expanded from a big bang in less than an infinite time?
 
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You seem to be imagining the big bang as an explosion at a certain point in a previously empty space. That's not what it was. We have a FAQ about that: https://www.physicsforums.com/showthread.php?t=506991

Pjpic said:
Or, if the universe were infinitely large how could it have expanded from a big bang in less than an infinite time?
If it's infinite now, then it was also infinite at all previous times. Cosmological models don't start out finite and then become infinite.
 
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There is a type of cosmology called inflationary cosmology, where with a certain burst from a negative pressure energy field that pushes with a uniform energy out into the fabric of space, this energy field yields a vast amount of energy that could have multiplied the size of the universe millions of times in a fraction of a second, if this cosmic expansion was faster then the speed of light then light from the furthest reaches of space would not have reached us. therefore we can only see what is in our space, say let's call the radius of what we can see our "cosmic horizon", then that is all we can see
we can measure the size of this energy field and also the speed at which is must be expanding and accelerating at, meaning we can measure the size and the amount of time it must have been expanding for ;)
 
Here is an explanation I saved from a usually reputable source here...Crowell:

The age of the universe as usually discussed is for an observer who is at rest relative to the average motion of the matter and radiation in the universe (the "Hubble flow"), and is in the context of homogeneous models, which wouldn't include any structure such as black holes, etc. Yes, you're right, different observers can measure different ages of the universe on their clocks. You can't be "on" a black hole, but an observer hovering just outside a black hole's event horizon would say that according to her clock, the universe is very young. There is no limit on how young the universe could be according to such an observer. The same applies to an observer moving at nearly the speed of light relative to the Hubble flow


Here are some other questions and answers about the age of the universe...they are consistent with Crowell's post above...but may offer some different perspective:


http://www.astro.ucla.edu/~wright/cosmology_faq.html#ct2

You may be interested to know if you don't already that the most distant galaxies are receding from our vantage point (Earth) at faster than the speed of light...and always have been. And so are the most distant galaxies from all other similar vantage points throughout the universe! But nobody knows for sure if the universe is infinite or not...
 
Another perspective:

About 380,000 years after inflation concluded, what we now see as cosmic microwave background radiation (CMBR) could begin to get through early matter after all the initial high energy ionization subsided...we see it coming from everywhere..as Crowell implied...it all happened at once, everywhere...

We only get the CMBR light from some particular batch of early matter once and it passes by. Tomorrow we will get light from matter that is farther away than that batch whose light we got yesterday because the universe continues to expand. The source of the CMBR is NOW about 45b light yrs away but the light we get from it was emittied 41 million light years ago. It has taken so long to get here because the universe has expanded...by a factor of about 1080 times...

At the UCLA souce I posted previously, there is a "nedwright calculator"...you can plug in numbers and it will do calculations for you...

In essence, it makes no real difference how much bigger the universe is than the observable universe.
 
I think that the age of the universe was determined from the properties of its background radiation. Not sure though.
 

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