Gamma Ray Bursts Place Limit on Quantum Gravity

The right-most panel shows the discovery image made using the UKIRT Wide Field Infrared Camera with the K filter (centred at 2.15 μm) at a mid-time of about 30 min after the burst. The other three images (Y, 1.02 μm; J, 1.26 μm; H, 1.65  μm) were obtained approximately 1.5 h after the burst using Gemini North's Near Infrared Imager and Spectrometer (NIRI). The main panels are 40 arcsec to a side, oriented with north to the top and east to the left. Insets, regions around the GRB, smoothed and at higher contrast. The absence of any flux in Y implies a power-law spectral slope between Y and J steeper than F_ν  ∝ ν^-18 and, coupled with the blue colour at longer wavelengths (J-H(AB) ≈ 0.15 mag), immediately implies a redshift greater than about 7.8 for GRB 090423. Image credit: Nature.

In June the Fermi telescope detected a distant gamma ray burst (GRB) at a redshift of 8.2 which was called at the time the most distant object ever observed.  Today Nature has published papers on some of the results from the Fermi team. Here is a quick look at them; first up is that distant object GRB 090423, A gamma-ray burst at a redshift of  z=8.2.

Long-duration γ-ray bursts (GRBs) are thought to result from the explosions of certain massive stars, and some are bright enough that they should be observable out to redshifts of z > 20 using current technology. Hitherto, the highest redshift measured for any object was z = 6.96, for a Lyman-α emitting galaxy. Here we report that GRB 090423 lies at a redshift of z  ≈ 8.2, implying that massive stars were being produced and dying as GRBs ~630 Myr after the Big Bang. The burst also pinpoints the location of its host galaxy.

Another paper published today explores a tantalizing prospect for physicists: the unification of general relativity and quantum field theory, which seem to be fundamentally incompatible.  The observation of a single photon at ~31 GeV from GRB 090510 places limits on quantum gravity which predicts that Lorentz invariance breaks down at the Planck length.  This would cause different energy photons to travel at different speeds.  Observations of varied speeds of photons from GRB 090510 were not convincingly seen putting a damper on quantum-gravity theories. The Fermi team has gone on to derive A limit on the variation of the speed of light arising from quantum gravity effect .

A cornerstone of Einstein's special relativity is Lorentz invariance—the postulate that all observers measure exactly the same speed of light in vacuum, independent of photon-energy. While special relativity assumes that there is no fundamental length-scale associated with such invariance, there is a fundamental scale (the Planck scale, l_Planck  ≈ 1.62 ~ 10^-33 cm or E_Planck = M_Planckc² ≈ 1.22 x 10¹⁹ GeV), at which quantum effects are expected to strongly affect the nature of space–time. There is great interest in the (not yet validated) idea that Lorentz invariance might break near the Planck scale. A key test of such violation of Lorentz invariance is a possible variation of photon speed with energy. Even a tiny variation in photon speed, when accumulated over cosmological light-travel times, may be revealed by observing sharp features in γ-ray burst (GRB) light-curves. Here we report the detection of emission up to ~31 GeV from the distant and short GRB 090510. We find no evidence for the violation of Lorentz invariance, and place a lower limit of 1.2E_Planck on the scale of a linear energy dependence (or an inverse wavelength dependence), subject to reasonable assumptions about the emission (equivalently we have an upper limit of l_Planck/1.2 on the length scale of the effect). Our results disfavour quantum-gravity theories in which the quantum nature of space–time on a very small scale linearly alters the speed of light.

And that isn't all!  There is yet another interesting Fermi paper just out, The Fermi Haze: A Gamma-Ray Counterpart to the Microwave Haze.  This has implications for WIMP dark matter models which would explain the observation, though this is only a speculation, but it is exciting to speculate...

UPDATE: This video really nails what is going on here: