## Extreme Universe: The most distant object known! April 28, 2009

Posted by jcconwell in Extreme Universe, Gamma Ray Bursts.
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On April 23, 2009, the Swift satellite detected that explosion. This spectacular gamma ray burst was seen 13 billion light years away, with a redshift of z = 8.2, the highest ever measured.

Combined X-ray, UV image from Swift

The cataclysmic explosion of a giant star early in the history of the Universe is the most distant single object ever detected by telescopes.

The colossal blast was picked up first by Nasa’s Swift space observatory which is tuned to see the high-energy gamma-rays emitted from extreme events. Other telescopes then followed up the signal, confirming the source to be more 13 billion light-years away. Scientists say the star’s destruction probably resulted in a black hole.

“This gets us into a realm where we’ve never been before,” said Nial Tanvir, of the University of Leicester, UK.  This is the most remote gamma-ray burst (GRB) ever detected, and also the most distant object ever discovered.”

“We completely smashed the record with this one,” said Edo Berger, a professor at Harvard University and a member of the team that first measured the burst’s origin. “This demonstrates for the first time that massive stars existed in the early Universe.”

GRB 090423 Infrared afterglow as seen by Gemini North

The burst occurred some 13.1 billion years ago, or perhaps a bit more accurate, when the Universe was only 630 million years old, a mere one-twentieth of its current age. Astronomers like to use age rather than distance because when you get this close to the big bang, there are three ways (at least) of referring to distance.

There is a Luminosity distance which ASSUMES  a 1/ (distance squared) law, which works when the space in between in FLAT.

There is the way that you’ll see it referred to in the press, most of the time, since the light has been traveling for 13.1 billion years, the distance is 13.1 billion light-years. Not wrong, but it assumes no expansion.

Then there is….sound of can opener, opening up can of worms….

the proper distance… the distance you would measure if you could take into account all the extra real-estate the universe has added for 13.1 billion years, the expansion of the universe.

To give a little background in redshift and cosmology, a redshift is an increase in the wavelength of the light. There are three types of redshift. The first is Doppler caused by the motion of the source away from the observer. The second is a gravitational redshift caused by light climbing out from a strong gravitational field, like a black hole or neutron star. The third is what we see here the cosmological redshift, caused by the expansion of the universe.

All three are measured by a number called z. This number is the fractional change in the wavelength of the light, or

z = (λ-λ0)/λ0

Where λ0 is the wavelength emitted from far away and λ is what we see in our telescope. The new gamma ray burst had a z = 8.2, meaning an ultraviolet line of Hydrogen emitted at 121 nm. would be shifted all the way down to infrared at 996 nm, (visible is between 750 nm and 380 nm)

Now in cosmology, General Relativity gives a relation between the AGE  of the object, since the big bang, with t=o as the moment of the big bang and its redshift z. Using a model of the expansion of the universe, redshift can be related to the age of an observed object, the so-called cosmic time–redshift relation. This depends on the shape and density of the universe, if we denote a density ratio as Ω0:

$\Omega_0 = \frac {\rho}{ \rho_{crit}} \ ,$

with ρcrit the critical density dividing a universe that eventually crunches from one that simply expands. This density is about three hydrogen atoms per cubic meter of space. At large redshifts one finds, with H0 as the Hubble constant at the present time:

$t(z) = \frac {2}{3 H_0 {\Omega_0}^{1/2} (1+ z )^{3/2}} \ ,$

But finding the distance is a little more complicated.

Picture walking along a sidewalk to your friends house one block away. Now if you had a insane  construction crew adding sidewalk as you were walking, by the time you got to your friend’s house and looked back you might see a lot more than one block of sidewalk.

Well the mad construction crew of the universe can add a lot in 13.1 billion years, so that if you look back now to the gamma ray burst you might find it around 40 billion light years away.