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My question is about furthest object in the observable universe except Сosmic microwave background.

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Sort of hard to answer, as it changes as more and more things leave the observable universe (as everything moves away from everything else we will eventually be all alone in the galaxy) –  RhysW Oct 23 '13 at 18:50
This is yesterday's article on Universe Today that is directly related and straight from the start discusses how short-lived is our definition of "the furthest observable Universe". ;) –  TildalWave Oct 24 '13 at 6:11
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2 Answers

About 13.1 billion light years.

The z8 GND 5296 Galaxy mentioned in the article linked by TildalWave, mere 700 millions years old (after Big Bang) would be the mentioned distance from us for the light to reach us at indicated speed, considering the 13.798 bln years old universe. As we push technological barriers, we keep finding more distant objects.

The age of the universe is the hard limit in light years of distance of any observable object - there was nothing to emit light more than 13.798 bln years ago, or respectively, visible from more than 13.798 bln light years away. visible from distance that what was 13.798 bln light years 13.798 bln years ago - or, after adjusting for universe expansion over that time, currently 46.6 billion light years away.

Of course, as RhysW mentions, this limit changes over time as the universe ages and grows. The exact rate of growth is 74.2km/sec per megaparsec, or about 0.000000007% per year. I'd say, comparing to our life and current technological progress, an insignificant factor.

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The first year it grows by 1/13.768 billionths the second year it grows by 1/13.768 (+1 year) billionths, the third year 1/13.768 (+2 years) billionths. Percentage wise, it grows by a smaller and smaller amount each year as the universes compounded size increases. For it to stay at 0.000000007% increase a year the rate at which light travels would have to be increasing! Just a small note I thought was worth making –  RhysW Oct 24 '13 at 14:43
@RhysW: Yes, the rate is not constant. But the rate of change of that rate (the second differential) is even smaller and even more negligible. There is another hard limit lowering the 13.798 bln by roughly 150mln. though: first stars formed roughly 150mln years after beginning of universe. There will be nothing visible to be observed earlier/further, no bodies emitting light or radiation. –  SF. Oct 24 '13 at 15:03
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There are few objects that might have even higher redshift. Googling I've found three accepted papers:

  1. two are about a $z\approx10$ galaxy. One published in Nature and the other one in Astrophysics Journal Letters

  2. is about a strong lensed candidate at $z\approx11$ published on Astrophysical Journal

There are also few gamma ray burst associated with objects at large redshift. This database containts:

  • z=8.26 GRB090423
  • z=9.2 GRB090429B. The redshift if photometric, which means that the error on the determination is likely large

There might be few other around

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