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View Full Version : GRB - How far away is it now

2009-May-02, 04:38 PM
I'm sure many of you will have read about the newly detected gamma ray burst that occured when the universe was a mere 650 million years old. I understand that the light has taken about 13.2 billion years to reach us here on Earth.

My question is - how far away is this object (presumably now a black hole) now?

Alan

antoniseb
2009-May-02, 04:58 PM
In what frame of reference do you mean 'now'?
OK, I know that probably isn't helpful, but you are asking about something where the speed of light, and expansion of the universe are involved. The word now has a common sense meaning for us in our slow moving small space.

01101001
2009-May-02, 06:35 PM
[...] how far away is this object [...]

Wikipedia: Distance measures (cosmology) (http://en.wikipedia.org/wiki/Distance_measures_(cosmology))

With which method?

2009-May-02, 06:48 PM
i think he means- where is it now. literally.
not a very complicated question.

01101001
2009-May-02, 06:52 PM
i think he means- where is it now. literally.
not a very complicated question.

With which method?

Jeff Root
2009-May-02, 07:13 PM
i think he means- where is it now. literally.
not a very complicated question.

-- Jeff, in Minneapolis

speedfreek
2009-May-02, 09:58 PM
From Ned Wrights Javascript Cosmological Calculator

For Ho = 71, OmegaM = 0.270, Omegavac = 0.730, z = 8.000

* It is now 13.665 Gyr since the Big Bang.
* The age at redshift z was 0.651 Gyr.
* The light travel time was 13.014 Gyr.
* The comoving radial distance, which goes into Hubble's law, is 9144.0 Mpc or 29.824 Gly.
* The angular size distance DA is 1016.0 Mpc or 3.3138 Gly.
* The luminosity distance DL is 82295.9 Mpc or 268.415 Gly.

DrRocket
2009-May-02, 10:08 PM
In what frame of reference do you mean 'now'?
OK, I know that probably isn't helpful, but you are asking about something where the speed of light, and expansion of the universe are involved. The word now has a common sense meaning for us in our slow moving small space.

It gets even stickier when you consider that there is NO global reference frame in general relativity, so no universal clock. To develop a notion of a parameter that can serve as a universal time takes some slight of hand and significant assumptions. See this thread (http://www.bautforum.com/space-astronomy-questions-answers/87768-universe-flat.html) for some discussion.

trinitree88
2009-May-03, 03:56 PM
according to Yoda...it was long,long ago, in a galaxy, ..far, far away..:shifty::whistle::doh:

2009-May-03, 04:08 PM
Thanks for the replies.

Roll on October when I can (hopefully) commence studying for a degree in astronomy by distance learning with the University of Central Lancashire. I have a couple of the recommended text books and so far the principles discussed don't seem too difficult to understand. Living in Thailand, this is the only opportunity I have to take my interest in astronomy further. The big advantage is that there are no exams with passes being determined by assessments etc, which I suspect will be harder than a written 3 hour exam.

Alan

trinitree88
2009-May-03, 04:30 PM
Thanks for the replies.

Roll on October when I can (hopefully) commence studying for a degree in astronomy by distance learning with the University of Central Lancashire. I have a couple of the recommended text books and so far the principles discussed don't seem too difficult to understand. Living in Thailand, this is the only opportunity I have to take my interest in astronomy further. The big advantage is that there are no exams with passes being determined by assessments etc, which I suspect will be harder than a written 3 hour exam.

Alan

Alan. Good luck in your career. These are very interesting times in astronomy. You will find that a lot of the ideas in astrophysics have multiple points of view, and that the concensus usually gets it right.
You will also find that on rare occasion, the concensus doesn't get it right, which is where the rare opportunity to make a big difference comes in. Picking up on that opportunity will not be easy. pete

nutant gene 71
2009-May-03, 04:50 PM
From Ned Wrights Javascript Cosmological Calculator

For Ho = 71, OmegaM = 0.270, Omegavac = 0.730, z = 8.000

* It is now 13.665 Gyr since the Big Bang.
* The age at redshift z was 0.651 Gyr.
* The light travel time was 13.014 Gyr.
* The comoving radial distance, which goes into Hubble's law, is 9144.0 Mpc or 29.824 Gly.
* The angular size distance DA is 1016.0 Mpc or 3.3138 Gly.
* The luminosity distance DL is 82295.9 Mpc or 268.415 Gly.
So the answer to the original question "how far away is this object (presumably now a black hole) now?"... (beat)... it is 268.415 Gly.

Okay, so by this response one must presume that the 'physical' distance, such as measured with a ruler laid out in non-relativistic space from here to target (instantaneously) is 268+ billion light years away. So that 'black hole' target GRB is nearly 20 times as far in 'real' terms than our eye perceives it? Cool. :D Who'd a thunk it!

speedfreek
2009-May-03, 05:12 PM
Have a look at the Distance Scale of the Universe (http://www.atlasoftheuniverse.com/redshift.html).

Unfortunately, he luminosity distance is not very useful here - how dim an object looks is due to its light being spread over a larger area, so the relationship is not straightforward.

The most meaningful interpretation is usually taken to be the comoving radial distance, the one I bolded, 29.824 billion light years. This is the distance scale that expands with the universe, so even if there is no meaningful sense of "now" across the universe, that is the closest you'll get! ;)

The comoving distance is the one where the particle horizon, the coordinate from which the CMBR that we currently detect was emitted from, which is moving with the expansion of the universe, is 46 billion light years away.

cjl
2009-May-03, 10:17 PM
So the answer to the original question "how far away is this object (presumably now a black hole) now?"... (beat)... it is 268.415 Gly.

Okay, so by this response one must presume that the 'physical' distance, such as measured with a ruler laid out in non-relativistic space from here to target (instantaneously) is 268+ billion light years away. So that 'black hole' target GRB is nearly 20 times as far in 'real' terms than our eye perceives it? Cool. :D Who'd a thunk it!
Nope - luminosity distance is not the measurement being looked for here. The one that is closest to the question asked by the OP is the comoving radial distance, or 29.8 Gly

Luminosity distance simply means that in a flat, nonexpanding universe, the object would have to be 268 Gly away to have the same apparent brightness. This has nothing to do with its current actual distance. It is primarily useful in determining distance of an object if you know its true luminosity and apparent brightness, but know nothing else about it. It can also be used to determine the object's luminosity if you know the luminosity distance through other means, and you know the apparent brightness.