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canopuss
2010-Oct-25, 03:25 AM
How can we say galaxies that are 13 – 14 billion light years away are the ones that existed just few hundred million years after the big bang ?

http://www.nature.com/news/2010/101020/full/news.2010.552.html


Why the same analogy doesn’t apply to Sun which is few light minutes away, but it is 4 – 5 billion years old ? Same analogy doesn’t apply to Andromeda galaxy either !

Jeff Root
2010-Oct-25, 05:01 AM
I see light which left my computer monitor screen about two
billionths of a second ago. The monitor is a year old.

I see light that was reflected from the surface of the trilobite
fossil I'm holding about one billionth of a second ago. The fossil
is 450 million years old.

We see the light of the Sun which left the Sun's surface about
eight minutes earlier. The Sun is 4.6 billion years old.

We see the light from the newly-discovered extremely-distant
galaxy which left that galaxy about 600 million years after the
Big Bang, or 13.1 billion years ago (assuming that the redshift
measurement is correct). The galaxy was 100 million years old
when the light left it. The galaxy is "now" 13.2 billion years old
(assuming that "now" has meaning across such distances).

The galaxy we are in, the Milky Way, is probably about the
same age as that distant galaxy: 13.2 billion years old.

Although the light from that extremely distant galaxy traveled
13.1 billion light-years to reach us, it was emitted when the
galaxy was much closer to us than that, and the galaxy is "now"
much farther away than that.

-- Jeff, in Minneapolis

grant hutchison
2010-Oct-25, 07:33 AM
How can we say galaxies that are 13 – 14 billion light years away are the ones that existed just few hundred million years after the big bang ?

http://www.nature.com/news/2010/101020/full/news.2010.552.html


Why the same analogy doesn’t apply to Sun which is few light minutes away, but it is 4 – 5 billion years old ? Same analogy doesn’t apply to Andromeda galaxy either !From theory and observation, the Universe is believed to be 13.7 billion years old. From the observed red shift in the light from this distant galaxy, its light has been travelling for 13.1 billion years to get to us. That leaves only 600 million years at the start of the Universe for the distant galaxy to have formed and to emit the light we're currently seeing. So we can deduce that the image we see is of a very young galaxy, as it existed in the early Universe.
The sun's light has been travelling for eight minutes when we see it: that leaves 13.7 billion years for it to have formed and to emit the light we're currently seeing. The age of the sun is therefore pretty much unconstrained by our light-travel-time calculation, and we need to seek other evidence in order to calculate its age. The same logic applies to the Andromeda galaxy.

The longer our "look-back time", the more constraint there is on the possible age of the objects we're observing.

Grant Hutchison

canopuss
2010-Oct-26, 10:31 PM
Thanks for the replies Jeff & Grant.