My own conclusion is that today there is no interpretation of quantum mechanics that does not have serious flaws. This view is not universally shared… - Steven Weinberg

Under ordinary circumstances, visible light is absorbed or emitted when the electrons in an atom or molecule are excited into orbits of higher energy, or sink back into orbits of lower energy, respectively.

So what happens to the effective field theories of electroweak, strong, and gravitational interactions at energies of order 10¹⁵–10¹⁸ GeV? I know of only two plausible alternatives. One possibility is that the theory remains a quantum field theory, but one in which the finite or infinite number of renormalized couplings do not run off to infinity with increasing energy, but hit a fixed point of the renormalizable group equations. ... The other possibility, which I have to admit is a priori more likely, is that at very high energy we will run into really new physics, not describable in terms of a quantum field theory. I think that by far the most likely possibility is that this will be something like a string theory.

[T]he distance at present is<math>d_{\mathrm{max}}(t_0) = \frac{1}{H_0} \int_{0}^{1} \frac{dx}{x^2 \sqrt{\Omega_\Lambda+\Omega_K x^{-2}+\Omega_M x^{-3}}}</math>...[T]here may have been a time before the radiation-dominated era in which there was nothing in the universe but , in which case the particle horizon distance would... be infinite. But as far as telescopic observations... [<math>d_{max}(t_0)</math>] gives the proper distance beyond which we cannot now see.