gravity Double copy theory Group field theory Wheeler–DeWitt equation Geometrodynamics Hořava–Lifshitz gravity MacDowell–Mansouri action Noncommutative geometry Path-integral based models of quantum cosmology Regge calculus Shape Dynamics String-nets and quantum graphity Supergravity Twistor theory Canonical quantum gravity Experimental tests As was emphasized above, quantum gravitational effects are extremely weak and therefore difficult to test. For this reason, the possibility of experimentally testing quantum gravity had not received much attention prior to the late 1990s. However, in the past decade, physicists have realized that evidence for quantum gravitational effects can guide the development of the theory. Since theoretical development has been slow, the field of phenomenological quantum gravity, which studies the possibility of experimental tests, has obtained increased attention. The most widely pursued possibilities for quantum gravity phenomenology include gravitationally mediated entanglement, violations of Lorentz invariance, imprints of quantum gravitational effects in the cosmic microwave background (in particular its polarization), and decoherence induced by fluctuations in the space-time foam. ESA's INTEGRAL satellite measured polarization of photons of different wavelengths and was able to place a limit in the granularity of space that is less than 10−48 m, or 13 orders of magnitude below the Planck scale. The BICEP2 experiment detected what was initially thought to be primordial B-mode polarization caused by gravitational waves in the early universe. Had the signal in fact been primordial in origin, it could have been an indication of quantum gravitational effects, but it soon transpired that the polarization was due to interstellar dust interference. See also De Sitter relativity Dilaton Doubly special relativity Gravitational decoherence Gravitomagnetism Hawking radiation List of quantum gravity researchers Orders of magnitude (length) Penrose interpretation Planck epoch Planck units Swampland (physics) Virtual black hole Weak Gravity Conjecture Notes References Further reading External links "Planck Era" and "Planck Time" (up to 10−43 seconds after birth of Universe) (University of Oregon). "Quantum Gravity", BBC Radio 4 discussion with John Gribbin, Lee Smolin and Janna Levin (In Our Time, Feb. 22, 2001) General relativity Physics beyond the Standard Model Theories of gravity