Several observable effects arise from the interaction between CMB photons and the large-scale structure they crossed during their journey. One of the most intriguing is gravitational lensing, the deflection of light as it travels in the vicinity of massive objects such as galaxies and galaxy clusters. Similar to what happens when light rays pass through a glass lens, the trajectories of photons are bent as they encounter these 'gravitational' lenses: as a result, the image that an observer sees when looking at a cosmic object, such as a galaxy, through a gravitational lens is distorted. In the most spectacular cases, the same galaxy can be imaged more than once, or its shape can be twisted and appear as a giant arc.
"In the case of the CMB, gravitational lensing due to cosmic large-scale structure creates tiny, additional distortions to the mottled pattern of the CMB temperature fluctuations. The effect is extremely small, but we have been able to isolate it exploiting Planck's combination of high sensitivity and angular resolution over the entire sky," comments Jan Tauber, Planck Project Scientist at ESA.
Cosmic voyage video 13
"The CMB consists of the most ancient photons emitted in the Universe's history: we can think of them as the ultimate 'cosmic storytellers'. The tale these photons are telling us begins with the epoch when they originated, 380,000 years after the Big Bang, when the cosmos consisted of an almost smooth fluid of particles: but that is only the start," explains Tauber. "Thanks to the detection of gravitational lensing experienced by CMB photons, we can use Planck to explore the subsequent epochs in the history of the Universe, during which the invisible dark matter built up the 'cosmic web' and stars and galaxies started to form in the densest nodes of this intricate network of structure." 2ff7e9595c
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