The light from distant stars, millions of light years from us, takes millions of years to reach our planet. Therefore, we see the past.
Wilkinson Microwave Anisotropy Probe (WMAP) is a spacecraft that, from 2001 to 2010, measured the temperature differences in the sky in a cosmic microwave background (relict radiation) left after the Big Bang.
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WMAP measurements have played a key role in creating the modern standard model of cosmology – Lambda-CDM.
Below is an image of the CMB temperature obtained using various astronomical observatories. This is the most detailed picture by the Space probe Planck operated by the European Space Agency from 2009 to 2013.
The average photon energy of the CMB is 2.7 Kelvin (that is, above absolute zero). The Planck image uses false colors to show changes in the range from -200 million Kelvin (blue) to +200 million Kelvin (red).
These characteristics of CMB are one of the most important pieces of evidence used to verify and validate our current model of the Universe (Lambda-CDM).
According to the Lambda-CDM, the Big Bang occurred 13.7 billion years ago with an error of just around 0.037 billion years.
379,000 years after the Big Bang, the Universe cooled down enough to become transparent, according to calculations.
Photons, that is, visible light, from the hot plasma that filled the Universe at that time, have traveled ever since and are now represented by redshift in the microwave range.
Thus, the farthest point in time, which is currently actively observed: 13.7 ± 0.037 billion years is 379,000 years.
Using photons, it is impossible to look further than up to 379,000 years after the Big Bang, since until that time it was opaque to them.
However, if someday humanity manages to invent and correctly use the neutrino telescope to measure cosmic neutrinos with very low energy (which today seems impossible), then we would see the picture of what was happening in just a few minutes or even seconds after the Big Bang.
In addition, it is theoretically possible to return to the level of approximately 10⁻³⁰ seconds after the Big Bang, if one could measure the probable gravity waves at the end of the inflationary period of the event.
In fact, it is likely that the primary gravitational waves could leave an imprint on the CMB image above. But to detect these traces, a new very sensitive satellite will be required.