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Relativity and Cosmology

This is a Blog on relativity and cosmology for engineers and the like. My website "Relativity-4-Engineers" has more in-depth stuff.

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Regards, Jorrie

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New 'Blast-from-the Past' Record Confirmed

Posted October 03, 2011 6:00 PM by Jorrie

The latest confirmed redshift record for gamma ray bursts (GRBs)(1) has pushed the limit to z=9.4, making it the oldest one observed so far. Detected by SWIFT in 2009, it has the unassuming name of 'GRB 090429B'(2). Other telescopes have only lately confirmed its redshift. If our best model of the cosmos is correct, it puts the GRB at around ~ 13.2 billion years light travel time, about 100 million years farther than the previous record.(3)

There should be a galaxy where this GRB happened, but it has not been observed by Hubble (yet) and may be out of its range anyway. The James Webb telescope will probably be the one to observe it later (if it is ever launched?). According to the best models we have, that GRB happened around 500 million years after the BB, in the era where the first galaxies (and stars) are theorized to have formed - but they may be too far to observe at present.

From (2): "Gamma-ray bursts (GRBs) serve as powerful probes of the early Universe, with their luminous afterglows revealing the locations and physical properties of star forming galaxies at the highest redshifts, and potentially locating first generation (Population III) stars. Since GRB afterglows have intrinsically very simple spectra, they allow robust redshifts from low signal to noise spectroscopy, or photometry."

It seems that our present technology is reaching the limit as to how far back we can observe things (other than the CMB at z~1088, which is just light, not quite a 'thing'). It is always interesting to work out how far the host galaxy of this GRB is from us today. Difficult? Not if you simply use a cosmological calculator, e.g. the one linked to from my website.(4)

Answer: 31.2 billion light years. If you are interested in this sort of thing, you can also check how far from our location the host galaxy was when that GRB actually happened, if our best model is correct, of course.(5)

-J

(1) http://en.wikipedia.org/wiki/Gamma-ray_burst

(2) http://arxiv.org/abs/1105.4915

(3) http://cr4.globalspec.com/blogentry/8864/Blast-From-The-Past

(4) http://www.einsteins-theory-of-relativity-4engineers.com/cosmocalc_2010.htm

Just change the 1088 CMB redshift to 9.4 and click 'calculate'.

(5) We were obviously not around at that time, neither was our solar system.

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Anonymous Poster #1
#1

Re: New 'Blast-from-the Past' Record Confirmed

10/10/2011 6:43 AM

" you can also check how far from our location the host galaxy was when that GRB actually happened, if our best model is correct, of course."

You wrote "best model" more than once. How sure can we be that this model (the FLRW, I presume) is the best, or even remotely correct?

SL

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#2
In reply to #1

Re: New 'Blast-from-the Past' Record Confirmed

10/10/2011 7:44 AM

Yes, I was referring to the generic FLRW model (or perhaps rather metric), more specifically, the Lambda-Cold-Dark-Matter (ΛCDM) model, a special case of FLRW.

It is easier to say why it is the 'best model' than to say it is the 'correct model'. Best in the sense that it fits our current observations better than available rival models. It is however sometimes called a 'toy model', because it is certainly not quite correct. For one thing, it ignores the lumpiness of matter (cold and dark) on smaller scales. Secondly, the mysteriousness of dark matter and dark energy is not all that comforting.

To create a general, inhomogeneous model that satisfies both observations and Einstein's field equations (EFEs) is extremely difficult - there are no exact solutions known for such a case. All one can do is to add inhomogeneity on to the ΛCDM to approximate the 'real thing'.

One such approximation is Wiltshire's TimesScape model, on which the 'jury is still out'. If it survives, TimeScape may do away with dark energy (or may at least reduce its significance). It automatically gives some apparent acceleration of the expansion rate, without dark energy. Dark matter is however still required in order to make it compatible with observations.

However, when Occam's razor is applied, the ΛCDM model is probably still the simplest solution that fits observations.

-J

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Anonymous Poster #1
#3
In reply to #2

Re: New 'Blast-from-the Past' Record Confirmed

10/11/2011 11:38 AM

"However, when Occam's razor is applied, the ΛCDM model is probably still the simplest solution that fits observations."

Yes, but doesn't that model only fit observations by tuning it until the results agree with observations. It does not give the value of any of the parameters from first principles, not so? It is not really a solution to the problem.

SL

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Guru
Engineering Fields - Aerospace Engineering - Retired South Africa - Member - The Rainbow-nation Engineering Fields - Engineering Physics - Relativity & Cosmology Popular Science - Cosmology - The Big Picture!

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#4
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Re: New 'Blast-from-the Past' Record Confirmed

10/12/2011 3:09 AM

The ΛCDM model is a solution to Einstein's Field Equations, which is derived from 'first principles'. We know from experiment that Einstein was right, or at least very, very close to right. Every testable prediction agrees with observation within experimental uncertainty.

At a cosmic level, those uncertainties are relatively large. What is 'tuned' are the values of the parameters inside the model, as you hinted. Cosmologists fit the parameters statistically to various models and then choose the model and corresponding parameter set that agree best with a fairly large range of observations. So far, the ΛCDM model has won.

-J

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