It is not possible for the visible and UV spectrum of a galaxy, redshifted, to mimic a continuous blackbody spectrum. We know, from direct measurements, that it is an extremely accurate (to within one part in 100,000) blackbody radiation spectrum ( ) corresponding to a temperature of 2.72548РІВС’.00057 K (degrees above absolute zero). The cosmic microwave background is a separate phenomenon. We also observe, in those 'ultra deep field' campaigns, that the galaxies observed are discrete sources, easily distinguished from the continuous background. We can definitely observe galaxies from extremely far away, and those galaxies are observed in visible and infrared wavelengths, not in microwaves. If you want to understand them yourself, then you need to invest the time and effort to study and learn those mathematical theories, the existing experimental data which support them, and the experimental techniques which you can use to explore new areas.Р”В«y Michael Kelsey (not verified) on D: You are invoking 'Olbers' Paradox' ( ), which I invite you to read about. To understand those things, we use mathematical theories backed up by experimental test and verification.

You should not _expect_ to have a good intuition for the behaviour of small quantum systems, nor of the behaviour of extremely fast moving objects, nor of the behaviour of space and time in strong gravitational fields.

'If I can't understand X, then X must be wrong.' Your experience, which is what your intuition is based on, is limited to (a) large, non-quantum objects (b) extremely low velocities and (c) weak gravitational fields. Any directional difference in the spectral shape at the high end is going to be unmeasurable - consistent with no difference at all, within the Buffalo: The 'argument from ignorance' is a well-known fallacy. The uncertainty in the energy scale for the data points near the GZK cutoff is much larger than that. It blows my mind that those GZK protons have a truly macroscopic amount of energy: 50 EeV is 8 joules, or about the kinetic energy of a well-thrown tennis ball!Р”В«y Michael Kelsey (not verified) on It's not a pretty table (the sort of thing XKCD would have), but the Wikipedia article ( ) has some more or less interesting bullet points at the The CMB dipole corresponds to a motion of about 200 km/s. So above the 5 x 10^19 eV cutoff, the proton loses energy really quickly until it gets below pi0 threshold, then it coasts, losing energy slowly but linearly through pair production and inverse Compton scattering (gamma p -> gamma p). The difference between those processes is the proton's energy loss: brem only takes a couple of per mil of the proton's energy, but the Delta+ resonance eats something like 20%! The GZK cutoff does indeed arise from neutral pion production (via gamma p -> Delta+ -> p pi0), but that is not because the pi0 is the 'lightest particle you can create.' In fact, inverse bremsstrahlung (gamma p -> p e+ e-, where the proton is the 'beam' particle here) starts at a threshold of just a few x 10^17 eV, two orders of magnitude lower.

This follows the SI convention that base units are spelled out in lowercase, but the abbreviation is capitalized only when the unit is derived from a proper name (the non-SI 'litre', abbreviated 'L', being the only exception). The units of energy in HEP are abbrevated as 'eV', but they are written out as 'electron-volts', without a capital 'V'. one tiny nitpick, and one actual comment.

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