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Age of the universe.

  1. May 9, 2003 #1
    When the media report on distant objects and state that the light from that distant object was emitted 10 billion years ago, for example, I wonder where the object is now, and more important, how long did it take the object to arrive at the place where it shined. How is the transit time from it's origination point considered in calculating the age of the universe?
     
  2. jcsd
  3. May 9, 2003 #2

    chroot

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    We cannot say where the object is now -- it is outside the bounds of the knowledge the universe gives us. In fact, relativity abolishes the very notion of "now" anyway -- the concept of "now" only exists in one place -- where the observer is.

    The distances to remote points of light is in fact not used in any way at all to calculate the age of the universe. We don't even have instruments yet that can detect the furthest things that are physically possible to observe (which should be approximately 13.3 billion light years away).

    The age of the universe has been calculated historically by looking at the redshifts of galaxies, and extrapolating Hubble's law backward to zero time. More recently, experiments like WMAP have provided even more accurate results, based on entirely different methods -- and they agree!

    - Warren
     
  4. May 9, 2003 #3
    WMAP?

    What is WMAP and how does it work? I have always been troubled with the red shift method because it doesn't seem to take into account the movement of our solar system in relation to the observed object. If we are moving in the opposite direction from the observed object that would increase the red shift over the actual speed of it's travel away from us. If we are moving in the same direction as the observed object, though slower, then it would decrease the red shift. If our movement is somewhat parallel to the object then the shift would seem to be even less even if it is of a great distance.
     
  5. May 9, 2003 #4

    russ_watters

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    Re: WMAP?

    You are exactly correct - but here's the neat thing: everywhere we look, nearly every object outside our galaxy is seen to be moving away from us at high speed.
     
  6. May 9, 2003 #5

    chroot

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    Re: WMAP?

    The following paragraphs will assume that you understand what the cosmic microwave background radiation is. If you do not, you may read up on it here:

    http://www.astro.ubc.ca/people/scott/cmb_intro.html

    The short story is that the cosmic microwave background radiation (CMBR) is a dull glow of radiation left over from when the universe was very hot and young. The photons which compose this radiation were in flight when the first atoms formed, and are still in flight today.

    WMAP (the Wilkinson Microwave Anisotropy Probe) is a satellite-borne microwave observatory that was launched a little over a year ago. All the information you could ever want on the project is available here:

    http://map.gsfc.nasa.gov/

    A cool photographic tour of some other ground-based CMBR experiments is available here:

    http://astro.uchicago.edu/cara/vtour/pole/darksector/cmbr/

    WMAP measured two spectrums of the CMBR: the temperature-temperature correlation angular power spectrum and the temperature-polarisation correlation angular power spectrum. Big words, I know. Here's how it works:

    The CMBR exhibits a near-perfect 2.725K blackbody spectrum, to better than 1 part in 100,000 (this is a much more perfect blackbody than can be made in a laboratory here on earth). The radiation is predominantly in the microwave (high frequency radio, essentially) region of the electromagnetic spectrum.

    However, as I said, it's a near-perfect 2.725K blackbody spectrum. It turns out that the most interesting physics deals with its very slight imperfections. There are, in fact, slight variations in the spectrum of the radiation if you look in different places in the sky. Such imperfections are known as anisotropies. If one measures the characteristic temperature of the CMBR in one direction in space, and then measures it in another, one will detect a very slight difference in characteristic temperature.

    Here is a map showing the characteristic temperature of the CMBR in every direction over the whole sky (in galactic coordinates):

    http://map.gsfc.nasa.gov/m_ig/020598/020598_ilc_640.jpg

    If you need help understanding what the map means, there's a simple page on it here:

    http://map.gsfc.nasa.gov/m_mm/mr_whatsthat.html

    Notice that there are cells of different temperatures -- some warmer, some colder, than average.

    If one then produces a graph that relates the sizes of regions with similar temperatures, a figured called the temperature-temperature angular power spectrum will be produced. (If necessary, you may need to review the 'power spectrum' concept -- it's a mathematical formalism related to the Fourier transform, but doesn't mean power, i.e. energy per unit time.) For example, if there are large “cells” of similar temperature, the angular power spectrum will show more power in large angles. If, however, the cells are very small, the angular power spectrum will show more power in small angles.

    An introductory page on the angular power spectrum (as well as a plot of it) is here:

    http://www.astro.ucla.edu/~wright/CMB-DT.html

    Keep in mind, when looking at the plots, that larger values of l refer to smaller angles. To determine the angular size of features at a given l, just divide 180 degrees by l.

    The inflationary Big Bang model predicts various shapes for this angular power spectrum, depending upon the choice of many free parameters. By measuring this angular power spectrum, many possible universes can be proven incorrect. By making repeated, very precise measurements, physicists can zero in on a single model that fits all the experiments.
    Our motion though local space (called 'peculiar motion') is largely irrelevant for redshift surveys. Galaxies show velocities (as determined by measuring redshift) of approximately 70 km/sec for every megaparsec (3.26 million light-years) they are distant. Since we're looking at galaxies all over the sky, and as far away as billions of light-years, we'd definitely notice something fishy about the distribution of velocities if our peculiar motion was large. We'd see galaxies on one half of the sky as moving faster than all of the galaxies in the other half of the sky. Since this isn't what we observe, we can conclude that our proper motion is negligible in comparison.

    In fact, the CMBR itself provides us with a frame of reference. If we were sitting perfectly still with respect to the CMBR, we'd see it being (almost) the same temperature in every direction. However, we don't! The CMBR is a little "hotter" in one direction and a little "cooler" in the opposite direction. This is usually called the 'dipole,' and looks like this:

    http://map.gsfc.nasa.gov/ContentMedia/990100b.jpg

    The CMBR is 6.706 mK hotter in one direction than in the other. This leads us to believe we're moving at roughly 600 km/s roughly towards the Virgo cluster. Keep in mind that I'm not talking about literal motion through space towards the Virgo cluster -- the Virgo cluster is in fact moving away from us! -- I'm just using the Virgo cluster a signpost to give you an idea of which way we're moving with respect to the CMBR.

    If you have any additional questions, please ask! If you can't tell, CMBR cosmology is one of my favorite topics. :)

    - Warren
     
  7. May 9, 2003 #6
    Thanks loads.

    Thanks to russ-watters and chroot. In one day you have cleared up a problem I have had for a long time. I am glad I found a forum with people of your caliber. So many times you get flippant answers from people that just want to "rag" you for not being as knowledgable as they think they are. I am a novice in this area but I can understand cogent explanations. To chroot; I visited the cited websites and they are great. Again, my thanks.
     
  8. May 9, 2003 #7

    chroot

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    Re: Thanks loads.

    You just made my day! Glad I could help.

    - Warren
     
  9. May 9, 2003 #8

    russ_watters

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    Re: Thanks loads.

    :smile:
     
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