I know the farthest things from earth are quasars, or at least the farthest we’ve detected. I’ve heard they have been observed at about 15 billion light years away–making the universe’s age to be about 15 billion years. My question is, if we observe these quasars at 15 billion light years away, doesn’t that mean that the light going into the telescope is 15 billion years old? That would put the quasars WAAAAAY out there by now. Has anyone taken into account the Dopler Effect? Are these 15 billion year estimates calculated already taking into account that the light is old and the Dopler Effect is changing the perception? Any ideas?
I think this might be useful…
When talking about the distance of a moving object, we mean the
spatial separation NOW, with the positions of both objects specified
at the current time. In an expanding Universe this distance NOW is
larger than the speed of light times the light travel time due to the
increase of separations between objects as the Universe expands. This
is not due to any change in the units of space and time, but just
caused by things being farther apart now than they used to be.
What is the distance NOW to the most distant thing we can see? Let’s
take the age of the Universe to be 10 billion years. In that time
light travels 10 billion light years, and some people stop here. But
the distance has grown since the light traveled. Half way along the
light’s journey was 5 billion years ago. For the critical density
case (i.e., flat Universe), the scale factor for the Universe is
proportional to the 2/3 power of the time since the Big Bang, so the
Universe has grown by a factor of 22/3 = 1.59 since the midpoint of
the light’s trip. But the size of the Universe changes continuously,
so we should divide the light’s trip into short intervals. First take
two intervals: 5 billion years at an average time 7.5 billion years
after the Big Bang, which gives 5 billion light years that have grown
by a factor of 1/(0.75)2/3 = 1.21, plus another 5 billion light years
at an average time 2.5 billion years after the Big Bang, which has
grown by a factor of 42/3 = 2.52. Thus with 1 interval we get 1.5910
= 15.9 billion light years, while with two intervals we get
5(1.21+2.52) = 18.7 billion light years. With 8192 intervals we get
29.3 billion light years. In the limit of very many time intervals we
get 30 billion light years.
If the Universe does not have the critical density then the distance
is different, and for the low densities that are more likely the
distance NOW to the most distant object we can see is bigger than 3
times the speed of light times the age of the Universe.
I didnt come up with this, haha, not so smart, but your question made me go find an answer for my own curiosity… I had no recolection of an answer from old astronomy courses… though im sure they covered it! found it here… http://www.faqs.org/faqs/astronomy/faq/part9/section-11.html
“Boy, wouldja get a load of the cloaca on that one”? -Cecil Adams, october 8 1999
The universe was born in 4004 BC. The light just looks old.
I tend to think that it is, because we are using instruments that we made here on Earth and we have not gotten very far out into space to take a good look around. Like one or two publications have printed reports on how we can actually see the borders of the universe, or the end of it.
That makes me laugh, because we are only seeing as far as our instruments allow and our devices do not have infinite range. I figure that once we manage to get people out beyond our solar system that we will have to rework our conclusions.
“Think of it as Evolution in action.”
I dunno. How old do “we” think it is?
There are ways of estimating the age of the universe other than saying “The furthest things we can see are about 15 billion LY away, so the universe must be 15 billion years old.” Somewhere on the Hubble telescope site they’ve got a press release giving an estimate of 12-14 billion years along with their reasoning, which uses a completely different method.
(the Hubble site is http://www.stsci.edu/ if you’re interested)
The press release you want is the one from May 25, 1999 entitled “Hubble Completes Eight-Year Effort to Measure Expanding Universe”.