Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Finite/infinite universe

  1. Dec 22, 2007 #1
    Is it possible to conceive of a universe which had a beginning but is spatially infinite?
     
  2. jcsd
  3. Dec 22, 2007 #2

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    I have trouble with the beginning part. I find it difficult to imagine a universe with a beginning (whether spatial finite or infinite doesn't matter.) Difficult but not altogether impossible.

    If I can get past that, and imagine a universe with a beginning (somehow springing into existence in a state of infinite density) then it is not much harder to go further and picture it as spatially infinite at that moment when it jumps into existence.

    ===================

    Among cosmologists who think of the universe as having a beginning it is not unusual for them to picture it as beginning with infinite spatial extent. The standard LambdaCDM model that cosmologists use can be either spatial finite or spatial infinite, and the spatial infinite version has space be infinite all the way back in time, for as long as time has meaning.
    That is the most common model. So there has got to be no problem conceiving of it! It is the default version!

    ===================

    Personally I prefer a recently developed variation on the standard model which has a bounce instead of a singularity. So there is no beginning in the picture. Before our expanding phase there was a contracting one, which reached a very high (but not infinite) density, stopped contracting, and began expanding. It fits the data as well as the version with a singularity. One reason I find it easier to picture is that I don't have to imagine a beginning of time. The model results from quantizing the classical cosmology model---quantizing gets rid of the singularity. But that is just my personal preference.

    ====================

    A lot of cosmologists still use the classic unquantized LambdaCDM model, which hits a singularity as you go back in time, and you cant evolve back any further. And of them most still use the flat (infinite space) version. So they are conceptualizing exactly as you said! A beginning to time, and a spatial infinite universe as soon as time begins ticking.
    So the answer to your question has got to be YES it is definitely conceivable.

    Which is not to say that one personally one has to like it. There are alternatives.
     
    Last edited: Dec 22, 2007
  4. Dec 22, 2007 #3
    Thank you, Marcus.
    If that is the case, then could one also think that past a certain point the current universe is completely empty? And could that point be thought of as the "edge" of the universe?
     
  5. Dec 22, 2007 #4

    Garth

    User Avatar
    Science Advisor
    Gold Member

    Yes, the standard GR model when the average density is not above the critical density, [itex]\Omega_{total} \leq 1[/itex], is infinite and unbounded and would have expanded from a Big Bang.

    In pure unadulterated GR theory this would have been a singularity and may be described as a 'beginning'.

    Garth
     
    Last edited: Dec 22, 2007
  6. Jan 17, 2010 #5
    Sorry guys i dont mean to but in here. but It seems that the universe is simply an illusion of time itself.

    Lets just go into a little disscussion of how time works, and we will have a better understanding of how the universe was created, and the paridox of how it is impossible to have a start point or an end point for that matter. Due to human perception of time of working in a (Time-Frame) manner. We only preceive movment through time by movement of an object covering distance within a certain frame of time.
    When the reality of time is not the movement within a frame, but the movement and counter movement of the present frame of time.

    The Past is not a second ago, nor is it billions of years ago. The Past is not what has happend in a (Frame) of time before the (Present) Frame we currently are in. But the past is the sum total of everything that has happend within any present frame of time all at once. all in one frame. In other words Everything that has happend in the past happend at the same time. Even your life growing up up to the point where you read these words. they all happend In the start of time when the big bang happend.

    By the time you complete the reading of the words i type the universe in the future time frame has ended all at once, with as much activity of everything that has happend in the past and the present. It give many infinite outcomes resulting in everything that can possibly happen, However, the probility of nothing happining in the future, is impossible even if everything has been done once twice, or an infinate number of times.

    Making the present the most infinate possible. Time may move into the future, but the present is always the frame time remains in. Even if A particle is completly destroyed in the present it exsisted in the past, and also in the anti-future. Oh yes the anti-future... Time's Evil twin. think of Anti-Time as time moving in reverse just as Time moves into the future, Anti-Time moves into the past. They are two differnt forces. Yes Time is a force, The movement of anti-time works to keep everything in possibility to remain in the (Present/anti-Present) Frame of time.

    Every Action has an equil and opposing reaction, this does not just apply to movement but to Time as well. And through this results in even more possibilitys in the production of matter, and the destrustion of it also. And who knows the movement of time and anti-time might very well explain the highly active nature of the electron. that funny little massless particle that seems to be everywhere all at once.

    Past Present Future and Anti-past, anti-present, anti-future. They are those funny little dimintions of string theory, that when combined make the universe we know and love. Just have fun thinking that Travel through time is possible, becouse it is not going to happen, unless you can break the law of time. or the law of movement. and even if you could. travel back into the past be it 1 sec ago or a million years ago. will result in the same reaction. the present. now. right now in fact. same with the future. you will always end up right back where you started.

    On the flipside. oddly enough you can pull from the future into the present, or from the past into the present. but that might not be so plesent to pull from the past. Unless it is from the anti- past you might be ok there. but this is just speculation. I really dont know what the hell i am talking about.
     
  7. Jan 17, 2010 #6
    Poetry is in another section of this forum I believe.
     
  8. Jan 17, 2010 #7
    sorry, i guess i cant explain things very well :D

    I bet you cant prove me wrong.
     
  9. Jan 17, 2010 #8
    Correct. There is nothing to prove.
    Start from putting a list of experimentally verifiable predictions.
    Otherwise it is just pure lyrics.
     
  10. Jan 17, 2010 #9
    Let me work on that and get back to you.
     
  11. Jan 17, 2010 #10

    DevilsAvocado

    User Avatar
    Gold Member

    10 points :biggrin:

    (Hi Dmitry, I will get back to you on the Schwarzschild solution, but my brain is right now parallel processing the Hubble volume + FTL + N-body simulations, so it’s kinda 'system overload' right now...)
     
  12. Jan 17, 2010 #11

    DevilsAvocado

    User Avatar
    Gold Member

    Italian antipasti is delicious, the rest I don’t know...

    I think you’ve been watching Julian Barbour one (anti?)time to many.

    https://www.youtube.com/watch?v=<object width="480" height="385"><param name="movie" value="http://www.youtube.com/v/WKsNraFxPwk&hl=en_US&fs=1&rel=0&color1=0x006699&color2=0x54abd6"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/WKsNraFxPwk&hl=en_US&fs=1&rel=0&color1=0x006699&color2=0x54abd6" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="480" height="385"></embed></object>

    When Barbour launches his 'drying rack' as a scientific proof of Eternity – every red lamp in my universe starts flashing.

    [​IMG]

    Take an egg and throw it on the wall – In Julian Barbour’s world this egg can now be boiled for breakfast. In most other worlds – you take a new egg for breakfast, instead of waiting for the egg for ∞ time, to reassemble and bounce back into your hand.

    At the macroscopic level where we live the second law of thermodynamics guaranties the arrow of time. At the microscopic level there is definitely more freedom for bouncing eggs, but lack of throwers at this level make it hard to discuss... as a breakfast topic...
     
  13. Jan 17, 2010 #12
    Well i must ask you if time was only in affect to move forward. could that same time move backwards while moving forwards? I dont think something can move forward and backwards in time at the same time. So how is the concept of a differnt force that governs time moving backwards?

    the way i see it. and please forgive me. but Standard forward movement of time affects space, but a backwards movment could affect gravity. We have not found a particle for gravity. and i dont think a particle for space has been found.

    If Anti Time did in fact exsist. then it would govern the force of gravity. An example could be.

    The amount of movement needed moving forward in time to leave the earths gravity, would have to exceed the pull of gravity. And if anti-time does in fact govern gravity then the only way to move faster than anti time is to gradualy increase acceleration until you leave the area where Anti time is in effect. Becouse if you traveled at a constant speed that is not exceeding the natural Anti-time gravity, then Anti time would pull you back down. Law of motion every action has a = and opposite reaction.

    So how fast would one have to travel without accelerating to leave the pull of earths gravity?

    Sorry If This still sounds philosloppycal
     
  14. Jan 18, 2010 #13

    DevilsAvocado

    User Avatar
    Gold Member

    The Graviton is in the pipeline. Space is not a particle/energy/force – it’s just space.
    9.81 m/s2 = at free fall the speed increases by about 9.81 meters per second every second = you must accelerate, or else you go down.


    The only reason for time is so that everything doesn't happen at once -- Albert Einstein
     
    Last edited: Jan 18, 2010
  15. Jan 18, 2010 #14
    LoreSpade,
    Just FYI

    SR and GR strongly suggest the so called Block time, which is the same as Eternalism: http://en.wikipedia.org/wiki/Eternalism_(philosophy_of_time)

    I believe most of the physicists believe in Block Time, except the minority, like Smolin. So, first of all, there is no "flow of time", anti-blah blah, pulling to something.
     
  16. Jan 18, 2010 #15

    DevilsAvocado

    User Avatar
    Gold Member

    Does Block Time violate the thermodynamic arrow of time and/or the causal arrow of time? If so – what happens if I travel back in Block Time (4-dimensional spacetime) and kill my grandmother when she is 10 years old?

    Will I be dead before I travel back to kill grandma, or alive and then die when grandma dies? Or will grandma & I be stone dead all the (block) time? :wink:
     
  17. Jan 18, 2010 #16
  18. Jan 18, 2010 #17

    DevilsAvocado

    User Avatar
    Gold Member

    Well... I don’t know, but the Novikovs self-consistency principle seems a little too 'magical' in my taste...

    "The Novikov Principle does not allow a time traveller to change the past in any way, but it does allow them to affect past events in a way that produces no inconsistencies—for example, a time traveller could rescue people from a disaster, and replace them with realistic corpses seconds before it occurs. Providing that the rescuees do not re-emerge until after the time traveller first journeyed into the past, his/her motivation to create the time machine and travel into the past will be preserved."

    If we apply this on my grandma: I could go back in time, kill my grandma, and just seconds before she dies – I replace here with a 'living copy'!? Where should I get a living copy of my grandma from...?? I’m no magician...!?

    And furthermore; suppose someone is filming this event! When I come back to 'today', grandma is dead and that’s ok. But, in the family archive there’s a new film proving that grandma was replaced by a 'magic clone' at the age of 10 – and I myself is responsible for not being the 'real me'!?!?! HELP!

    Okay, maybe CTC’s are even more 'magical', so this is really not a big question... :rolleyes:
     
  19. Jan 18, 2010 #18
    Novikov principle tells that for some weird reason you wont be able to kill your grandpa if someone was filming the event so everybody would know that the murder was fake.
     
  20. Jan 18, 2010 #19
    Re. #2. If our universe is the result of a collapsing previous universe, then that universe had sufficient gravity to occasion the collapse. If that universe did not come into being in it's expanded form, then it too must have expanded outward. Therefore, collapse of our own universe is inevitable, preceded by slowing, which will cause the galaxies to move apart at an accelerating rate.
     
  21. Jan 18, 2010 #20

    DevilsAvocado

    User Avatar
    Gold Member

    Killing is not my favorite hobby, so this maybe doesn’t matter much...? :biggrin: Anyhow, Novikov principle feels a little like a Brand Unique Stunning Theory (BUST)... :wink:

    From my local horizon it looks like we have 3 options:

    A) Block Time is real and there is absolutely no difference between Past Present & Future in the 4-dimensional spacetime, but time travel is forbidden by the laws of physics.

    B) Block Time is real and there is absolutely no difference between Past Present & Future in the 4-dimensional spacetime, and time travel is possible.

    C) Block Time is not the whole answer to what time is. We have misunderstood some parts in SR & GR (which also explains why GR <> QM). Time travel is not possible.

    My personal favorite is C and then A.

    If B is true, it must lead to completely new physics, including mind control on time travelers (I never liked KGB or CIA :smile:). And where is Occam's razor in this case...?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Finite/infinite universe
Loading...