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Physicists Debate Hawking’s Idea That the Universe Had No Beginning

quantamagazine.org

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Re: Physicists Debate Hawking’s Idea That the Universe Had No Beginning

#51

Post by deepsun »

djsumdog wrote: post #10

This seems different than the concept of infinite time, where prior to the big band there was something the collapsed into a ball of energy and maybe someday our universe will collapse back again (which has lost merit since we've seen matter in the universe isn't slowing down in spreading apart, but accelerating .. meaning in a few trillion years, atoms may be so far apart that light can't reach anything .. the event…

> what, if anything, is outside the observable universe

The answer is similar to the time. Remember, in cosmology there's no really separate space and time, there's only space-time.

So when you're looking at the edge of the observable universe, you really looking back in time. So the answer to the question "what is outside" is the same as "what was before".

Or, in simpler words, the question is meaningless, as there's no space to talk about "outside". Not sure the last phrase is correct, as I've just talked about separate space from time.

Re: Physicists Debate Hawking’s Idea That the Universe Had No Beginning

#52

Post by svachalek »

gavanwoolery wrote: post #48

I have very little knowledge of astrophysics, but I thought it would be an interesting idea if the universe constantly recycled itself. That is, every so often it collapsed in on itself and produced another big bang. Theoretically some parts of the universe would have already achieved escape velocity, but the rest of it is gradually gravitating towards some center of mass. Is such a thing even feasible?

A few decades ago, that was widely believed as the most likely state of the universe. However, it's now known that the big bang is essentially still accelerating and this is blamed on an unknown force called "dark energy". It doesn't have much effect within galaxies, but galaxies themselves are blowing away from each other at ridiculous speeds.

Re: Physicists Debate Hawking’s Idea That the Universe Had No Beginning

#54

Post by Balgair »

djsumdog wrote: post #10

This seems different than the concept of infinite time, where prior to the big band there was something the collapsed into a ball of energy and maybe someday our universe will collapse back again (which has lost merit since we've seen matter in the universe isn't slowing down in spreading apart, but accelerating .. meaning in a few trillion years, atoms may be so far apart that light can't reach anything .. the event…

Wikipedia's 'Timeline of the Far Future' is a great article on the posibilities of our universe's fate [0]. Of note are the entries near the end of the list.

Year 10^(10^(50)) : Estimated time for a Boltzmann brain to appear in the vacuum via a spontaneous entropy decrease [1]

Year 10^(10^(120)) : High estimate for the time for the universe to reach its final energy state, even in the presence of a false vacuum.

Year 10^(10^(10^(56))) : Around this vast timeframe, quantum tunnelling in any isolated patch of the vacuum could generate, via inflation, new Big Bangs giving birth to new universes. Because the total number of ways in which all the subatomic particles in the observable universe can be combined is 10^(10^(115)) a number which, when multiplied by 10^(10^(10^(56))), disappears into the rounding error, this is also the time required for a quantum-tunnelled and quantum fluctuation-generated Big Bang to produce a new universe identical to our own, assuming that every new universe contained at least the same number of subatomic particles and obeyed laws of physics within the range predicted by string theory.

Suffice to say, our understanding of physics and the universe is still in an infant state when it comes to predictions of the universe's future. Much more funding is needed :P

[0] https://en.wikipedia.org/wiki/Timeline_of_the_far_future

[1] https://en.wikipedia.org/wiki/Boltzmann_brain

Re: Physicists Debate Hawking’s Idea That the Universe Had No Beginning

#55

Post by BurningFrog »

djsumdog wrote: post #11
weego wrote: post #8

Earlier quoted context omitted.

A stars position can't be defined as being south of somewhere. South exists as a logical construct of our magnetic poles. Once you lose that you have no relative measure to work with. The same is possibly true of time.

But the trouble with the South Pole and Shuttlecock metaphors is that you assume those objects are literally everything; all matter. If you define the Universe as Everything there is (seen and unseen; where what we see is the "Observable Universe") there is no outside location from which to observe it in entirety. That's where the analogy really breaks down, but I guess it's still a useful thought experiment so long…

There can still be an unobservable (to us) universe outside ours.

If you assume the universe started N billion years ago, there is an event horizon expanding at the speed of light that fit this definition of an "observable universe" for us while allowing for an infinite number of equal universes, some of which are partially shared with ours.

Re: Physicists Debate Hawking’s Idea That the Universe Had No Beginning

#56

Post by XorNot »

svachalek wrote: post #52
gavanwoolery wrote: post #48

I have very little knowledge of astrophysics, but I thought it would be an interesting idea if the universe constantly recycled itself. That is, every so often it collapsed in on itself and produced another big bang. Theoretically some parts of the universe would have already achieved escape velocity, but the rest of it is gradually gravitating towards some center of mass. Is such a thing even feasible?

A few decades ago, that was widely believed as the most likely state of the universe. However, it's now known that the big bang is essentially still accelerating and this is blamed on an unknown force called "dark energy". It doesn't have much effect within galaxies, but galaxies themselves are blowing away from each other at ridiculous speeds.

Well it's more that space itself is expanding. It doesn't affect small scales because other forces collapse everything back together fast enough to overcome it. If the increase in dark energy is consistent though, eventually it'll be expanding so fast not even our atoms can hold themselves together.

Re: Physicists Debate Hawking’s Idea That the Universe Had No Beginning

#57

Post by eigenschwarz »

gavanwoolery wrote: post #48

I have very little knowledge of astrophysics, but I thought it would be an interesting idea if the universe constantly recycled itself. That is, every so often it collapsed in on itself and produced another big bang. Theoretically some parts of the universe would have already achieved escape velocity, but the rest of it is gradually gravitating towards some center of mass. Is such a thing even feasible?

It's definitely an interesting an idea that has been considered before. Hopefully the Wikipedia page on cyclic models can be a starting point of interest to you! https://en.wikipedia.org/wiki/Cyclic_model

Re: Physicists Debate Hawking’s Idea That the Universe Had No Beginning

#58

Post by deepsun »

gavanwoolery wrote: post #48

I have very little knowledge of astrophysics, but I thought it would be an interesting idea if the universe constantly recycled itself. That is, every so often it collapsed in on itself and produced another big bang. Theoretically some parts of the universe would have already achieved escape velocity, but the rest of it is gradually gravitating towards some center of mass. Is such a thing even feasible?

Not really, because you again assuming that there's an independent time-line, where a ball of universe expands and collapses.

Our current understanding of the Universe is that _time itself_ "expands and collapses". You cannot talk about time "before", because there was no time "before" the big bang, just as there's no space "outside" the ball, to talk about its boundaries.

Re: Physicists Debate Hawking’s Idea That the Universe Had No Beginning

#60

Post by onion2k »

chii wrote: post #31
onion2k wrote: post #21

Earlier quoted context omitted.

That's only disturbing if you believe everything is simple enough that humans could comprehend it straight away. If you accept that some stuff is beyond our understanding, and will be for quite a while yet, everything is much less bothersome. Accept that you are fallible and the universe is fine. I'm not saying this is easy though.

> If you accept that some stuff is beyond our understanding isn't that just an elaborate way to say 'give up'? I asked religious friends of mine about what god _is_, and the common answer is that it's beyond human understanding. I don't believe that people should take things on faith, and accept that anything is beyond understanding.

isn't that just an elaborate way to say 'give up'?

Accepting that we don't understand something is a first and very necessary step to even realising there's something to understand. Believing that we have the answer (eg "God made the universe!") is exactly what shuts down scientific inquiry and makes people 'give up'. I'm saying literally the opposite of that - we have to realise there's something to out there to learn in order to try and learn it.

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