Gamma-Ray Photon Observations Indicate Space-Time Is Smooth

There are multiple distance measures in cosmology - they are all in principle exact (at least, if you know all your cosmological parameters), but they differ significantly once you start getting above about 1 billion light years. Much above that, and they can differ incredibly much. Some of these measures are based on idealized measurements, others on the physics directly.

Some measures used in cosmological work are,

- proper motion distance (the distance a parallax measurement would give you)
- luminosity distance (the distance you would infer from the apparent brightness of a standard candle)
- angular diameter distance (the distance you would infer from the apparent angular size of a standard sized object). The angular diameter distance is notorious for getting smaller if you get far enough away in many cosmologies (including, apparently, the one we live in).
- look back distance (if you imagine that everyone has a clock synchronized at the big band, the difference between your time and the time you would read on the remote clock, if you could read it). This is also called the light travel time.
- proper distance (what some long yardstick would read).
- comoving distance (the proper distance divided by the scale factor - 1 plus the redshift, z - for the remote observer, to get a distance that doesn't change with cosmological time).

And, finally, each cosmological model will have a coordinate distance (the difference between the coordinates of two different places), which need not have a simple relation to any of the above.

It is fair to say that one of the easiest ways to make a fool of yourself in cosmology is to mix up distance scales. (As an additional cause of mixups, only proper distances can be subtracted - for the rest, the distance between A and B is NOT the difference of the distance to A and the distance to B, even if A and B are on a straight line as seen from the Earth.)

In this case, the Gamma Ray Burst 090510A was at a red shift of 0.897. Go to the Cosmology Calculator [ucla.edu] and you find that that

For Ho = 71, OmegaM = 0.270, Omegavac = 0.730, z = 0.897

It is now 13.666 Gyr since the Big Bang.
The age at redshift z was 6.376 Gyr.
The light travel time was 7.290 Gyr.
The comoving radial distance, which goes into Hubble's law, is 3053.8 Mpc or 9.960 Gly.
The angular size distance DA is 1609.8 Mpc or 5.2505 Gly.
The luminosity distance DL is 5793.1 Mpc or 18.895 Gly.

The proper distance is (1+z) times the comoving distance, or 18.89 Gly.

Source: http://rss.slashdot.org/~r/Slashdot/slashdotScience/~3/bkFstMcMSPk/gamma-ray-photon-observations-indicate-space-time-is-smooth

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