Notices
Results 1 to 12 of 12
Like Tree7Likes
  • 1 Post By johnzxcv
  • 2 Post By Napivo
  • 2 Post By tk421
  • 2 Post By Physicist

Thread: Why does the universe expand ?

  1. #1 Why does the universe expand ? 
    Senior Member
    Join Date
    Jul 2014
    Posts
    115
    I'm learning Cosmology. But I can't understand why does the universe expand?
    How do we know the galaxy is moving away from us with a speed that is faster than speed of light?
    The light from the galaxy will never reach us because it's moving away faster than speed of light.
    number8 likes this.
    Reply With Quote  
     

  2. #2  
    Senior Member
    Join Date
    Oct 2013
    Location
    UK
    Posts
    997
    There is a pressure there that causes is to expand. As for being able to see that galaxy, perhaps the light left quite some tine ago when it want going so fast
    Reply With Quote  
     

  3. #3  
    Junior Member
    Join Date
    Jul 2014
    Location
    Belgium
    Posts
    15
    Quote Originally Posted by johnzxcv View Post
    I'm learning Cosmology. But I can't understand why does the universe expand?
    Honestly no one knows as far as I know. And the real question is why does the universal expansion even speed up.

    Most mainstream cosmologists believe there was a huge explosion known as the Big Bang but no one ever was able to tell me whet exploded. It created a whole lot of stuff even more than we see in the universe now as both matter and antimatter was created.

    The matter and antimatter started annihilating each other (which of course created an even larger explosion) For some reason there was a bit more matter than antimatter created. The universe is the rest.

    So far that should explain why the universe is expanding. The explosion is still going on.

    And then we come to the realm of the unknown.

    It seems our universe is still accelerating it's expansion. (which should be impossible)
    So some smart guy decided there must be a driving force behind it and called it Dark Energy.

    I am not going to explain what I know of it but you can read it at wikipedia
    Dark energy - Wikipedia, the free encyclopedia

    While you are at it also read about dark matter.
    Dark matter - Wikipedia, the free encyclopedia

    Quote Originally Posted by johnzxcv View Post
    How do we know the galaxy is moving away from us with a speed that is faster than speed of light?
    The light from the galaxy will never reach us because it's moving away faster than speed of light.
    You probably have heard of the Doppler effect. that is why a sound is higher when it comes towards you and lower when it goes away.

    For light the same occurs but it is called Red Shift and Blue Shift

    Redshift - Wikipedia, the free encyclopedia
    Blueshift - Wikipedia, the free encyclopedia

    Now when Light comes from faster than light objects it decays even further and becomes radio waves. witch we can pick up via radio telescopes.

    I hope that helps a bit

    Farsight, proposed I read Expanding Confusion it is not an easy read but it explains a lot
    johnzxcv and number8 like this.
    "The only true wisdom is in knowing you know nothing."
    Socrates
    Reply With Quote  
     

  4. #4  
    Senior Member
    Join Date
    Jan 2013
    Posts
    427
    Quote Originally Posted by johnzxcv View Post
    I'm learning Cosmology.
    At what level? Undergraduate? Graduate? Knowing the answer would help us craft an explanation at the appropriate level. In the meantime, we'll just have to draw inferences from your questions.

    But I can't understand why does the universe expand?
    A short answer is that the Einstein Field Equations generally yield solutions that are not stable -- they predict either expansion or contraction, depending on certain factors. From the observational evidence, we inhabit a universe in which those "certain factors" result in expansion.

    How do we know the galaxy is moving away from us with a speed that is faster than speed of light?
    I don't know what you mean by "the" galaxy. But I'll ignore that part. The farther away one goes, the faster the recession velocity. Eventually you can get to a recession velocity that exceeds the speed of light. That occurs at a distance of about 14 billion light-years.

    The light from the galaxy will never reach us because it's moving away faster than speed of light.
    The observable universe is actually greater than 14billion light-years in radius, because the universe has been expanding for 14 billion years. In fact, the radius of the observable universe is about 47 billion light-years, give or take.

    Since you're just starting to study this subject, you would likely enjoy reading Ned Wright's cosmology tutorials and faqs (google for them).
    johnzxcv and number8 like this.
    Reply With Quote  
     

  5. #5  
    Senior Member
    Join Date
    Jun 2014
    Posts
    551
    Quote Originally Posted by johnzxcv
    I'm learning Cosmology. But I can't understand why does the universe expand?
    In the early history of the universe there was a period of where the universe underwent a phase called inflation (Inflation (cosmology) - Wikipedia, the free encyclopedia) where space itself underwent a hyper-accelerated expansion. The universe expanded at an extremely accelerated rate. It was almost as if there was antigravity pushing all matter apart. This was a result from either a positive cosmological constant (Cosmological constant - Wikipedia, the free encyclopedia).

    Now there's another period of inflation going on. We don't know right now if there was a period where there was no inflation. When inflation ends that doesn't mean the expansion ends. If this period of inflation ends there's no apparent reason that the universe should all of a sudden stop expanding. That'd be like the parts of an explosive coming to a stop after the explosion when in fact the parts keep going on forever (outside the atmosphere that is).

    Quote Originally Posted by johnzxcv
    How do we know the galaxy ...
    What galaxy are you referring to?

    Quote Originally Posted by johnzxcv
    ..is moving away from us with a speed that is faster than speed of light?
    That isn't true for all galaxies. Only for those further away than a certain distance. And the light does reach us. You can't readily compare things in general relativity with an expanding, curved spacetime with special relativity with flat spacetime.

    Quote Originally Posted by johnzxcv
    The light from the galaxy will never reach us because it's moving away faster than speed of light.
    That's not true. I've already given an example of when that's not true. See Superluminal motion - Wikipedia, the free encyclopedia

    The quasar 3C 273 is one such example. And it can be seen with a large amateur optical telescope.
    johnzxcv and FAiello like this.
    Reply With Quote  
     

  6. #6  
    Senior Member
    Join Date
    Jun 2014
    Posts
    551
    Quote Originally Posted by Jilan
    There is a pressure there that causes is to expand.
    That is only responsible for the acceleration of the expansion. And that is only a fairly recent discovery. We knew and can model an expanding universe with no pressure whatsoever.


    By the way. The pressure that is responsible for the expansion is negative pressure which is also known as tension.
    Reply With Quote  
     

  7. #7  
    Senior Member
    Join Date
    Jul 2014
    Posts
    115
    What cause that negative pressure?
    Reply With Quote  
     

  8. #8  
    Senior Member
    Join Date
    Jun 2014
    Posts
    551
    Quote Originally Posted by johnzxcv View Post
    What cause that negative pressure?
    Nobody knows. The name given to it is dark energy or quintessence

    Quintessence (physics) - Wikipedia, the free encyclopedia.
    Reply With Quote  
     

  9. #9  
    Senior Member
    Join Date
    Jul 2014
    Posts
    115
    The galaxy is moving away from us doesn't mean space between two galaxy expand.
    Reply With Quote  
     

  10. #10  
    Senior Member
    Join Date
    Oct 2013
    Location
    UK
    Posts
    997
    Which galaxy are you referring to? Most of them sure moving away from ours I think. Yes, the idea is that the space between them increases.
    Reply With Quote  
     

  11. #11  
    Senior Member
    Join Date
    Jun 2014
    Posts
    551
    Quote Originally Posted by johnzxcv View Post
    The galaxy is moving away from us doesn't mean space between two galaxy expand.
    That is correct. The fact is that all galaxies are moving away from us. In fact all galaxies are moving away from all other galaxies. We can tell from the light coming from these galaxies that its the space that's expanding and not that they're simply moving away from us. Don't worry. These cosmologists are ingenious. They've thought of everything.
    Reply With Quote  
     

  12. #12  
    Senior Member
    Join Date
    Jan 2014
    Location
    out there
    Posts
    306
    Can it be stated that the universe, on average, as a whole, is cooling?
    Can it be asked asked "where is this heat going?"?
    Can a thermodynamic and heat transfer perspective be useful?
    Reply With Quote  
     

Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •