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Universe Infinite

  1. Jan 15, 2004 #1
    Universe Infinite !!!

    I've heard many times that the volume or size of the universe is infinite!

    But, I don't know how they say it? Is there any mathematical proof???
  2. jcsd
  3. Jan 15, 2004 #2


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    There are lots of theories about the universe, but none, as far as I know have the universe of infinite size. In any case, the maximum we can see is about 14 billion light years. Moreover there are lots of figures (I can't quote them) which describe how much stuff there is in the unuverse.
  4. Jan 15, 2004 #3


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    Re: Universe Infinite !!!

    It is not surprising that you have heard that the universe is infinite in spatial extent, since (although it's not known for certain) this is often taken for granted by cosmologists as a working assumption.

    There is no mathematical proof that the universe is finite. Though it certainly might be! Neither case (finite/infinite) can be ruled out.

    A good idea is to look directly at professional journal articles by prominent cosmologists and see what they say about it based on the latest observations from the Wilkinson Microwave Anisotropy Probe (WMAP).

    Charles Bennet et al.
    http://arxiv.org/astro-ph/0302207 [Broken]
    see table 3 on page 33---"Best" Cosmological Parameters
    from the article
    "First Year WMAP Observations, Preliminary Maps and Basic Results"

    Charles Lineweaver
    http://arxiv.org/astro-ph/0305179 [Broken]
    "Inflation and the Cosmic Microwave Background"

    Michael Turner
    "Making Sense of the New Cosmology"
    http://arxiv.org/astro-ph/0202008 [Broken]

    Wendy Freedman and Michael Turner
    "Measuring and Understanding the Universe"
    http://arxiv.org/astro-ph/astro-ph/0308418 [Broken]

    The whole issue turns on how accurately they can measure a number called Omega. This is the first thing listed at the top of
    Bennett's Table 3.
    The current WMAP data say that Omega = 1.02 +/- 0.2
    which is tantalizingly close to one.
    If Omega is exactly one, then space is flat and infinite.

    However, if Omega is even slightly greater than one, then space may LOOK flat but on a very very very large scale (way greater than 14 billion LY) it may curve around on itself (analogous to a sphere surface) and be finite.

    Michael Turner, who is a world-renowned theoretical cosmologist, just goes right out and says "the universe is spatially flat" which is to say infinite. That is the way a lot of them think of it, because Omega has been measured so close to one. And there are some side reasons saying on theoretical grounds it ought to be flat and infinite. But based on observations, as of right now, WE CANNOT BE SURE it is infinite.

    So observational cosmologists like Bennett, who heads the WMAP team, along with Ned Wright and along with Lineweaver who was a leader in the earlier COBE satellite observations, tend to be more careful and guarded----they give you a figure with error-bounds, like
    1.02 plus or minus 0.02.

    It could go either way.

    Is Chittagong in Bangladesh?

    The WMAP satellite that is currently gathering data from the Microwave Background about the shape and extent of the universe is
    not even going around the earth.
    They put it a million miles further out from the sun. In its own orbit around the sun. Apparently it can work better out by itself than it can if it is close in to the earth.
    You see there is a lot riding on how accurate the Background can be measured! It would be great if in another year or two they could refine the figure for Omega some more and get better accuracy.
    Last edited by a moderator: May 1, 2017
  5. Jan 15, 2004 #4


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    Cosmology Resource Sticky Needed

    We could use a Cosmology Resource sticky thread
    to keep basic reference links handy like the above
    articles of Bennett, Lineweaver, Turner....

    Here is a link that PF-poster Nereid kindly provided about
    a wide-angle deep survey of the universe called GEMS

    GEMS covers a patch of sky as big as the full moon
    and took thousands of images in that patch
    and made a mosaic picture of that patch which is
    real deep, going way back in time, so you see
    galaxies forming and colliding and evolving.
    The article Nereid links to tells about it
    and shows a portion of the picture. The total GEMS
    picture has 3 billion pixels, the article says.


    Dark matter:
    Here's another Nereid link to a dark matter article (mapping it in a cluster by observing lensing)
    Neutrino astronomy:
    Has a big future potential
    in observational cosmology. Wolram, a PF-poster, provided these
    neutrino-related links:

    http://www.space.com/scienceastronomy/cosmic_neutrinos_030716.html [Broken]
    this gives the AMANDA2 neutrino sky map---the obseratory down near south pole.

    Basic facts/estimates about the cosmic neutrino background presented
    by Ted Bunn, one of the moderators on Usenet sci.physics.research.

    Two neutrino articles which surprised me, so I include them without
    being able to properly evaluate or interpret. Maybe someone else can.
    Observations purporting to have some bearing on the existence or
    non-existence of extra dimensions. If this is too wacky tell me and I
    will take it off this thread.
    Last edited by a moderator: May 1, 2017
  6. Jan 15, 2004 #5


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    more stuff for the cosmology resource sticky

    two good online cosmology calculators:

    Ned Wright's

    Siobahn Morgan's
    http://www.earth.uni.edu/~morgan/ajjar/Cosmology/cosmos.html [Broken]

    homepage for Siobahn in case you want to see who she is
    http://www.earth.uni.edu/smm.html [Broken]
    homepage for Ned in case you want to see who he is

    Martin Bojowald
    "Quantum Gravity and the Big Bang"
    General Relativity had a glitch and
    quantizing the theory fixed the glitch so
    it no longer predicts a moment of infinite
    density and curvature (a type of singularity).
    Evolution prior to big bang is shown in some
    of the articles cited in this brief survey.

    Labguy, another PF poster, provided news of a
    recent test of General Relativity
    (which GR passed with flying colors) a
    binary pulsar:


    The technical article about the binary pulsar
    and the most stringent verification of GR to date is:
    http://arxiv.org/astro-ph/0401086 [Broken]


    A great survey article about high energy cosmic ray observations
    (another window for observational cosmology to look thru)
    Floyd Stecker
    "Cosmic Physics: the High Energy Frontier"
    http://arxiv.org/astro-ph/0309027 [Broken]

    Last edited by a moderator: May 1, 2017
  7. Jan 16, 2004 #6


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    more stuff for a cosmology resource sticky (if we ever get one)

    Useful constants:
    One parsec = 3.857E16 meters
    Newton's G = 6.6742E-11 cub.meter/sq.second kg
    Best current estimate of Hubble parameter H = 71 km/s per Megaparsec
    Critical energy density derived from that = 0.85 joule per cubic km.

    ----discussion of the constants---

    If you put "best estimate" H into standard (SI) metric units it is H = 2.301E-18 per second, or 2.3 appropriately rounded off.
    The reciprocal, or "Hubble time" parameter is 4.3E17 seconds (roughly the same as the age of the universe, as it happens).

    Wendy Freedman led the Hubble Space Telescope "Key" Project to determine H with unprecedented accuracy and in 1998 they announced in effect that this (possibly the single most important) cosmological quantity, H, is 0.43 quintillion seconds.
    Except they dont say 0.43 quintillion seconds they either express the Hubble time in years (something like 13.8 billion) or they use traditional astronomer mongrel units and say it is "71 kilometers per second per Megaparsec."

    So what does this mean about the density of energy in the universe?
    It is good to have a rough idea of the extent of the universe but also an idea of the density of energy----how much joules per cubic kilometer, or cubic mile, or cubic lightyear is in it.
    How full is it of matter, and other forms of energy?

    Critical density (so-called "rho crit") is 0.851 joule per cubic kilometer, rounded off to 0.85 so as not to overstate the precision.

    You can probably calculate this for yourself with the standard formula for it

    "rho crit" = 3c2H2/8piG

    If you plug in the standard metric speed of light and the figure of
    2.3E-18 per second and the figure given at the beginning for Newton's G, then you will get 0.85 joule per cubic km.
  8. Jan 16, 2004 #7
    Re: Re: Universe Infinite !!!

    Yes! You are correct! Chittagong is in Bangladesh :) And I am also the only PF member of Bangladesh :(

    The idea just came while discussing with my friend...

    We all know from laws of thermodynamics:

    We also know that universe is full of mass, particles that means energy!

    And if the the universe is infinte, then, the total amount of energy in the universe is also infinte...which is not constant

    So, the problem arised [b(]

    Anyway! Thank you very much for you links! Those are huge links...it'll take at least a week for me! I'll read them one by one :)

    I am very much interested in cosmology :) Thanks again :)
  9. Jan 16, 2004 #8


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    Moni, I am so glad you stayed around. I was afraid you
    would not come back.

    Don't worry about reading ALL the links. The important thing is
    to find one good introduction to cosmology that works for you.

    I have found the most useful is Lineweaver
    "Inflation and the Cosmic Microwave Background"
    If you read through that one article you will already know a lot
    of cosmology!

    But several other people have said they like the
    "Cosmology Tutorial" and the "Cosmology FAQ" at the
    Ned Wright website.
    He is a professor at UCLA who teaches cosmology and
    also one of the leaders of the WMAP project.

    I do not think one needs to read BOTH Lineweaver and Wright.
    They are alternatives and the thing to do is find one that
    explains things in the right way for you to understand.

    Two women astronomers, Wendy Freedman and Siobahn Morgan, also
    have online surveys and related material. Nereid may know more.
    If you write a post requesting links to online introductory
    accounts of cosmology----Nereid might answer and give good links.
    Or some other people.

    I guess my two favorite things are Lineweaver's article and
    Ned Wright "Cosmology FAQ", but this might not be just right for you.

    An Australian woman, Tamara Davis, has written a paper together with Lineweaver which is about the confusion and trouble people have understanding the expansion of the universe. The article is called "Expanding Confusion". I will find the link in case you are interested.

    Dont work too hard. the thing is to find one article that works and read it patiently for a long time until it sinks in. Please keep asking questions! It helps keep the PF messageboard lively. We dont want Nereid to get bored and go away:wink:
  10. Jan 16, 2004 #9


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    Moni, the conservation of energy law does not have to apply to an entire finite universe. It can be applied to an isolated PART of the whole thing.

    If you can surround the system you are studying with an imaginary insulated box that does not let any energy in or out, or a kind of imaginary magic baggie that isolates the system, then you can apply the conservation law to what is inside.

    It may be true that the universe is finite, but it may also not be true. So to be safe we should probably remember the law as not applying to the whole universe but to an isolated finite piece of it.
  11. Jan 17, 2004 #10
    The radius of the Universe is exactly the magnitude in light-years that it's age is in years.
  12. Jan 17, 2004 #11


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    That is certainly not correct.

    - Warren
  13. Jan 17, 2004 #12
    It isn't? Why not?
  14. Jan 17, 2004 #13
    The particle horizon of the universe is believed to have a radius of 47 billion year lights. This is really greater that 13.7 billions
  15. Jan 17, 2004 #14
    yes, that is greater than 13.7 although wrong. The correct figure is closer to 13.2.
  16. Jan 17, 2004 #15


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    I get the distinct feeling you don't really know what you think you know. Sorry. meteor is correct.

    The reason the observable universe is larger than 13.7 Gly is simple: the universe wasn't always as large as it is today.

    - Warren
  17. Jan 17, 2004 #16
    A: Any data pointing to larger than 13.2 is skewed by spacetime curvatures(or just wrong) and
    B: the ability to see beyond the radius is also affecting the data.
  18. Jan 17, 2004 #17
    Sorry. meteor is correct.

    >maybe that is the belief, but not the fact
  19. Jan 17, 2004 #18


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    Warren (chroot) said: Sorry. meteor is correct.

    Polar replied: maybe that is the belief, but not the fact

    there are two issues here: what do the pros say? what is
    the consensus among mainstream astronomers about the radius of the
    observable universe? (often called "the particle horizon"
    which was the term Meteor used)

    and then there is the other issue: do you have to believe them?

    The clear answer to the second question is NO. You, PolarStarus, can believe it is whatever size. Everybody ought to feel free to imagine the universe however he likes.

    But on the other hand you should make an effort to understand
    the mainstream, and why they estimate the particle horizon at 47 billion LY at present. If you want to be unconventional, still try to understand the conventional view that you are differing from.

    BTW there is some "give" in the number 47, it depends on exactly which set of parameters you use to calculate it but in all events it is around 47 or in the neighborhood of 45-50. Meteor can probably cite you a standard source for his number.
    Warren teaches astronomy (I think he said as a sideline to his main carreer) so he may have more information bearing on this.
  20. Jan 17, 2004 #19


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    PolarS, here is something you might like to try:
    Ned Wright's or Siobahn Morgan's cosmology calculator.
    two good online cosmology calculators:

    Here are their homepages in case you want to see
    who they are and what they look like (snap shots)
    http://www.earth.uni.edu/smm.html [Broken]

    and here are their calculators

    Ned Wright's

    Siobahn Morgan's
    http://www.earth.uni.edu/~morgan/ajjar/Cosmology/cosmos.html [Broken]

    I just tried Siobahn's (which I find easier to use) and got a result surprisingly close to Meteor's 47.
    For very large, for practical purposes infinite, redshifts it said the present distance to the object is 46.1 or 46.2 etc billion LY.
    That is for redshift 10 thousand and 100 thousand---the calculator said the distance is just a tad over 46 billion LY.

    To get this kind of agreement you need to put in the current best estimates of the parameters.

    In Siobahn's calculator, put in 71 for H (she has 70)
    and put in 0.27 for Omega(matter)
    and put in 0.73 for Lambda(dark energy)

    after those preparations, whatever you put in for z (redshift)
    you will get the presentday distance to an object which is now being observed to have that redshift.

    the redshift of the cosmic microwave background is 1100.
    very distant quasars have z = around 6
    so those are things to plug in.
    Last edited by a moderator: May 1, 2017
  21. Jan 17, 2004 #20
    I like her Dr. Who page.
    One of the problems is this- Lambda isn't fixed, the accelration is decreasing in magnitude. The expansion rate will continue to increase but at an ever decreasing rate. The
    acceleration was much higher in the early universe and goes to zero
    as the age goes to infinity.
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