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Age of the Universe and time duration

  1. Feb 12, 2014 #1
    We all very well know that, Earth has been made 4.5 billion years ago. Suppose, we are to explain the age of the star and we say that it is 27 billion years old. One year is the duration of time taken by the earth to revolve around the sun once. Before the origin of Earth, how could we explain the time duration because we don't have any point of reference for the time. Suppose a star took 'x' duration of time for its formation. How could we explain it on the basis of years because, the concept of years had come long after the formation of the star. So, I think, the time expressed in years (more than 4.5 billion years ago) is wrong.
  2. jcsd
  3. Feb 12, 2014 #2


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    A second is defined (in the metric system) as a certain number of vibrations of a certain type of atomic clock signal. A year is define as a certain number of seconds.

    When you hear that the most ancient stars imaged so far were formed around year 500 million, that's what the time interval "year" means. It does not refer to the orbit of this particular planet around the Sun, neither the Sun nor the Earth existed back in year 500 million.

    You've heard of the CMB ("cosmic microwave background"). That's the most ancient light that astronomers are currently able to detect. It originated from glowing hot gas around year 380,000. That is, around 380,000 years after the start of expansion. And of course there were no stars back then and no planets orbiting them. That is not what "year" means.

    Like I said, a year (for most of physics including early universe cosmology) is a certain number of atomic clock seconds. And cosmologists have to use a physical model to ESTIMATE the time it took for various things to happen, based on known physics. The basic model of expansion is called the Friedman equation. It's based on known physics (Einstein GR) and is checked and crosschecked every way people can think of. It brings us all the way up to the present and it gives a remarkably good fit to all the evidence so far.

    But obviously nobody was back there with a quartz crystal wristwatch, or an atomic clock, timing everything. They have to time the processes they CAN observe and date, and leverage that back into the past using the best-fit equation model.

    If you have more questions about this, keep on asking. There are other members who may come in and clear things up better than I can, and who have more detailed expertise, in some cases.
    Last edited: Feb 12, 2014
  4. Feb 12, 2014 #3

    Simon Bridge

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    I can look on my computer and see the date today and work out that I am 40-or-so years old. But my computer did not exist 40 years ago so how can I use my computer's clock to work out how old I am?

    Generally we do this sort of thing by comparing one length of time with another one.

    We don't have to stand there with a stopwatch the whole time to work out how long something took.

    The computer compares the vibrations of a crystal in it's works with the orbit of the Earth time, and knowledge of how long it has existed for, to make a calendar that I can use in comparison with the calendar on which my birthday was recorded ... to figure out the times of events before the computer, or even myself, even existed.

    With the star - we are comparing how old the star is with the length of time it takes the Earth to go around the Sun. This requires setting something to use as a calendar - since nobody was around to start any stopwatch.

    See: http://www.scientificamerican.com/article/how-do-scientists-determi/
    Last edited: Feb 12, 2014
  5. Feb 12, 2014 #4


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    On a related note, astronomers recently reported finding the oldest star discovered in the known universe, and it's only 6,000 light years away right here in the Milky Way. In this case the star's age - 13.6 billion years - is being estimated by the heavy element content such as iron (almost none) in its spectra. The star, SM0313, consists almost completely of hydrogen and helium.

    More here: Oldest star in known universe
  6. Feb 12, 2014 #5


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    So what? The milky way is already known to be about 13.2 billion years old. That is well within the error bars for the alleged age of SM0313, which is probably off by at least a billion years, given it is classified as a pop II star.
  7. Feb 13, 2014 #6


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    Some feel a significance to discovering a star this old. If the age estimate is accurate, SM0313 is older than the Milky Way. The iron content is reported to be lower than expected for a pop II star, possibly indicating lower energy supernova than previously thought for stars of this period.

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