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13.5 billion year age versus 93 billion due to spatial expansion?

  1. Apr 5, 2010 #1
    I was watching a video on the scale of the observable universe. At the end, it shows the famous WMAP imagery and at the bottom it reads that the age of the observable universe is about 13.5 billion years old, but that due to the expansion of space the actual age is closer to 93 billion years. I hadn't heard this before and can't quite follow that logic. Could anyone help clarify that point?
     
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  3. Apr 5, 2010 #2

    nicksauce

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  4. Apr 5, 2010 #3
    If the "observeable" universe is 93 billion light years in diameter whereas the *actual* age of the universe is 13.5, how then can we observe those parts that must have obviously moved - if "moved" is even the right word - away at velocities greater than C?
     
  5. Apr 5, 2010 #4

    marcus

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    Nick's answer covers it, but maybe I can fill in some extra detail.
    A common way for astronomers to talk about the distance now to some matter that we are seeing amounts to imagining that you can freeze the expansion process right now and then time a light signal going from here to there (with everything frozen in place).
    That's called the proper distance and it is the type of measure used with the Hubble law: the standard way to describe expansion.

    It is in that sense that we say that the distance now to the farthest matter we can see is about 46 billion lightyears. We are currently receiving light which that matter emitted some 13.7 billion years ago, not long after the start of expansion. The matter was much closer then.

    Notice the difference between measuring time and measuring distance. years is a measure of time, lightyears is a measure of distance (which because of expansion doesn't need to correspond to travel time in any simple way.)

    The 46 billion lightyear radius of the portion of the universe we currently can observe can be thought of as the combined effect of what distance the light could have traveled on its own, without expansion, plus the added effect of expansion. By itself, in a static universe, the light could have only traveled 13.7, but in fact the oldest light comes from source material which is now around 46 from us.
     
  6. Apr 5, 2010 #5

    marcus

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    When you hear the word "recession rate" it is not strictly about the ordinary motion idea of traveling from point A to point B in a static geometry. To understand the Hubble law you really need to include the cosmic microwave background CMB in your picture, or what was earlier known as the Hubble flow before the CMB was observed.

    So you are right to be cautious and say "if moved is even the right word". It's good to be aware that the word can be applied to different processes basically because largescale geometry is not static.

    Most of the galaxies which we are now looking at with telescopes have redshift greater than 1.4 and any such galaxy is receding faster than c. So that raises a very interesting question. Most of the galaxies we can see in the sky are receding faster than c, so how do we see them? How could the light have gotten here?

    I think this is at the heart of your question. It's a good question to be asking. We've answered it many times here at PF cosmo forum, and sometimes just by referring to the 2005 Scientific American article by Lineweaver Davis, that explains it with a very simple picture. Nicksauce already gave a link to something in the Lineweaver SciAm---it's a great article. I keep the link to the main article in my sig for quick reference.
    You could have a look at it, see if you then understand how light can have gotten here from a galaxy that is now receding > c, and then if you aren't fully satisfied come back with a further question.

    Please see if you can find the Lineweaver link in my sig, Faust. Ask if there's any problem.
     
  7. Apr 5, 2010 #6
    Thanks for the responses everyone, makes a lot more sense now.
     
  8. Apr 5, 2010 #7
    F...ing damn right there is a problem!

    I have come to believe that C is a constant. A speed limit. NOTHING can move faster than C. The explanations I have heard are all about a conversion principle between energy and mass, the "treacle" of the Higgs field, and all that jazz. Which I have struggled to comprehend and make sense of. But this may be limited to mass and physicality but not structure and "spatial distance" in an expanding "nothingness". I can imagine how "space" can expand faster than C (if "faster" is even an operative concept here). I have heard about "dark flow" and the almost incomprehensible ideas that the "new" cosmologists are sporting... but I still fail to see how we can observe (and determine) that there are objects in this universe that accellerate away from us at > C velocity. It just doesn't make any sense. Which of course may be just because I don't understand what's going on (which is indeed very likely)... so... are we talking about a "ripple in space" kind of thing that has nothing to do with "mass" as we know it? And if so, how did they find out?
     
  9. Apr 9, 2010 #8
    I also don't understand how we can pass c. Can someone explain this to us?
     
  10. Apr 10, 2010 #9

    Chronos

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    You cannot surpass c locally. Globally, that is a different matter. Relativity does not forbid superluminal global expansion.
     
  11. Apr 10, 2010 #10

    Ich

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    That's very easy: we can't.
    We can observe objects that are said to be moving away with >c. But we can't determine that their speed is greater than c by any - more or less - direct measurement.
    This "speed" is defined in a certain coordinate system. Without going into details, GR - contrary to SR - allows for arbitrary coordinate systems, and in arbitrary systems you can have arbitrary velocities. Such coordinate velocities don't have the strict physical meaning that speed in SR has. They can mean something different, with more or less similarity to the speed concept of SR.
    For example, the cosmological "recession velocity" in a flat spacetime (an empty or almost empty universe) is not the speed of receding "test particles", but their rapidity. So you can have "velocities" >c even in flat spacetime, just by a redefenition of coordinates.

    Cosmologists use such strange coordinates because, in curved spacetimes, rigid SR coordinates simply don't work. There is also no well defined concept of relative velocity for objects that are far apart.
    So one uses coordinates that do work, along with these coordinate velocities. That's ok, but much too often these concepts get confused with the standard SR ones. For some reason many cosmologists seem reluctant to address this point.
     
  12. Apr 10, 2010 #11

    marcus

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    It's generally most efficient if anyone with a question about Hubble law recession rates takes a look at the Lineweaver Scientific American article first and THEN asks questions.

    Sounds like diagopod may have read Lineweaver SciAm. I keep the link in my signature because it has been referred to many times over the years at Cosmo forum. E.g. see Nicksauce's reference earlier in this thread.


    Max, your reply doesn't indicate that you read Lineweaver. Did you happen to, by any chance?

    Here's the link to that SciAm article. A good introduction.
    http://www.astro.princeton.edu/~aes/AST105/Readings/misconceptionsBigBang.pdf [Broken]

    It is a princeton.edu link because they use it at Princeton as supplemental reading in their introductory Astro course. The author, Charlie Lineweaver, was one of the leaders in the mapping of the cosmic microwave background (COBE mission). He is a world class authority and moreover is good at explaining things nontechnically at a wide audience level.

    My advice would be to read this easy illustrated article first, and then ask questions.
     
    Last edited by a moderator: May 4, 2017
  13. Apr 10, 2010 #12
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    Hi Diagopod. Here is a little bit of information about the age of the Universe from CERN:"CERN, the European Organization for Nuclear Research, is one of the world’s largest and most respected centres for scientific research. Its business is fundamental physics, finding out what the Universe is made of and how it works. At CERN, the world’s largest and most complex scientific instruments are used to study the basic constituents of matter — the fundamental particles. By studying what happens when these particles collide, physicists learn about the laws of Nature." :smile:
     
  14. Apr 15, 2010 #13

    EnumaElish

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    The gif picture from Sci. Am. makes sense to me. Suppose I throw you a ball, which takes 10 seconds to reach you. During those 10 seconds I take a tiny step away from you. When the ball reaches you, the distance between us is 10 ballseconds + one tiny step. However, it would be incorrect to conclude that I moved faster than the ball during those 10 seconds.
     
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